HomeMy Public PortalAboutRES-CC-2018-44Resolution #44-2018
A RESOLUTION ADOPTING UTAH GEOLOGICAL SURVEY SPECIAL STUDY 162 AND
THE GEOLOGIC HAZARDS OF THE MOAB QUADRANGLE SET OF 13 HAZARD -
SPECIFIC MAPS
WHEREAS, Utah Geological Survey (UGS) regularly produces geologic hazard maps for use by
local regulatory agencies;
WHEREAS, UGS produced a special study report and maps identifying and generally locating
thirteen geologic hazards within the Moab quadrangle;
WHEREAS, geologic hazards have the capacity to cause harm to the City's inhabitants and
damage to their property;
WHEREAS, the City, in its regulatory capacity, can utilize said maps as best available
information when reviewing development proposals, infrastructure projects, or any other work
subject to City review;
NOW, THEREFORE, BE IT RESOLVED BY THE GOVERNING BODY OF THE CITY OF
MOAB, UTAH THAT THE CITY COUNCIL HEREBY ADOPTS THE ATTACHED
SPECIAL STUDY AND THIRTEEN GEOLOGIC HAZARDS MAPS.
PASSED AND ADOPTED in open Council by a majority vote of the Goveming Body of the
City of Moab this 9ch day of October, 2018.
By:
Emily S. Niehaus, Mayor
Resolution #44-2018
Attest:
Rachel E. Stenta, City Recorder
Page 1 of 1
GEOLOGIC HAZARDS OF THE MOAB
QUADRANGLE, GRAND COUNTY, UTAH
by Jessica J. Castleton, Ben A. Erickson, and Emily J. Kleber
UTAH
DNR
SPECIAL STUDY 162
UTAH GEOLOGICAL SURVEY
a division of
UTAH DEPARTMENT OF NATURAL RESOURCES
2018
GEOLOGICAL SURVEY
Blank pages are intentional for printing purposes
GEOLOGIC HAZARDS OF THE MOAB
QUADRANGLE, GRAND COUNTY, UTAH
by Jessica J. Castleton, Ben A. Erickson, and Emily J. Kleber
Cover photo: Image of northern Moab —Spanish Valley with Jurassic Navajo Sandstone in the foreground and veg-
etated Quaternary alluvial deposits on the valley floor. In the background, the Colorado River carves a path through
the uplifted Triassic and Jurassic deposits bounding the valley to the west. Photo by Emily Kleber.
GEOLOGICAL SURVEY
ISBN: 978-1-55791-945-8
SPECIAL STUDY 162
UTAH GEOLOGICAL SURVEY
a division of
UTAH DEPARTMENT OF NATURAL RESOURCES
2018
STATE OF UTAH
Gary R. Herbert, Governor
DEPARTMENT OF NATURAL RESOURCES
Michael Styler, Executive Director
UTAH GEOLOGICAL SURVEY
Richard G. Allis, Director
PUBLICATIONS
contact
Natural Resources Map & Bookstore
1594 W. North Temple
Salt Lake City, UT 84116
telephone: 801-537-3320
toll -free: 1-888-UTAH MAP
website: utahmapstore.com
email: geostore@utah.gov
UTAH GEOLOGICAL SURVEY
contact
1594 W. North Temple, Suite 3110
Salt Lake City, UT 84116
telephone: 801-537-3300
website: https://geologv.utah.gov
Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah
Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use, and does not
guarantee accuracy or completeness of the data. The Utah Department of Natural Resources, Utah Geological Survey, shall
not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to
claims by users of this product. Geology intended for use at 1: 24, 000 scale.
CONTENTS
ABSTRACT 1
INTRODUCTION 1
Purpose and Scope 1
Previous Work 3
Setting 3
Geology 4
GEOLOGIC HAZARDS 6
Shallow Groundwater 8
Salt Tectonics -Related Ground Deformation and Faulting 9
Flood Hazards 12
Landslide Hazards 14
Rockfall Hazards 16
Radon Hazard 18
Collapsible Soil Susceptibility 20
Expansive Soil and Rock Susceptibility 21
Soluble Soil and Rock 23
Corrosive Soil and Rock 24
Piping and Erosion 25
Wind -Blown Sand 26
Shallow Bedrock 27
MAP LIMITATIONS 28
ADDITIONAL INFORMATION AND GUIDELINES 28
ACKNOWLEDGMENTS 29
REFERENCES 29
FIGURES
Figure 1. Location of the Moab quadrangle 2
Figure 2. Lithological column of geologic units 4
Figure 3. Color variation in the Paradox Formation 5
Figure 4. Formation of salt -dissolution -related faulting over time in the Moab —Spanish Valley area 9
Figure 5. Diagram of a graben formed by two normal faults 10
Figure 6. Colorado River flooding into the mouth of Courthouse Wash in 1917 13
Figure 7. Flooding over Arches National Park service road near park entrance 13
Figure 8. Rockfall diagram 16
Figure 9. Components of a characteristic rockfall-path profile 17
Figure 10. Typical structural damage to a building from expansive soil 21
Figure 11. Subsurface void formation due to shrink -swell of soils having a high clay content 23
Figure 12. "Popcorn" texture with evaporite precipitation in soils derived from the Chinle and Paradox Formations 23
Figure 13. Representation of sinkhole formation due to salt dissolution near a subsided well 24
Figure 14. Evaporite precipitation and corrosion on concrete masonry unit wall 25
Figure 15. Gully erosion in slope underlain by Chinle Formation 26
Figure 16. Piping erosion diagram 26
TABLES
Table 1. Summary of known geologic -hazard fatalities in Utah 7
Table 2. FEMA FIRM panel information and effective dates 14
Table 3. Recommended requirements for site -specific landslide -hazard investigations in the Moab quadrangle 15
Table 4. Recommended requirements for site -specific rockfall hazards investigations to protect life and safety 18
Table 5. Soil geologic factors that contribute to radon hazard potential 19
Table 6. Radon hazard potential mapping criteria and indoor radon potential 19
Table 7. Correlation between geotechnical tests of soils and expansive -soil susceptibility 22
PLATES
Plate 1. Shallow groundwater potential map
Plate 2. Salt tectonics -related ground deformation hazard map
Plate 3. Flood hazard map
Plate 4. Landslide susceptibility map
Plate 5. Rockfall hazard map
Plate 6. Radon hazard potential map
Plate 7. Collapsible soil susceptibility map
Plate 8. Expansive soil and rock susceptibility map
Plate 9. Soluble soil and rock susceptibility map
Plate 10. Corrosive soil and rock potential map
Plate 11. Piping and erosion susceptibility map
Plate 12. Wind -blown -sand susceptibility map
Plate 13. Shallow bedrock potential map
GEOLOGIC HAZARDS OF THE MOAB
QUADRANGLE, GRAND COUNTY, UTAH
by Jessica J. Castleton, Ben A. Erickson, and Emily J. Kleber
ABSTRACT
The Moab 7.5-minute quadrangle is located in south-central
Grand County, Utah. Currently, the area is experiencing rapid
development and population growth that is expected to con-
tinue for the foreseeable future. As urbanization expands into
areas less suited for development, geologic hazards become of
increasing concern in the planning, design, and construction of
new facilities and infrastructure. This geologic -hazard study
of the Moab quadrangle uses available geologic, hydrologic,
soil, and geotechnical information to identify where geologic
hazards may exist and where detailed, site -specific, geotech-
nical/geologic-hazard investigations are necessary to protect
health, welfare, and safety. This study provides maps and in-
formation for 13 geologic hazards: shallow groundwater, salt
tectonics -related ground deformation, flooding, landsliding,
rockfall, radon gas potential, collapsible soil, expansive soil
and rock, corrosive soil and rock, soluble soil and rock, piping
and erosion, wind-blown sand, and shallow bedrock. Histori-
cally, the most widespread annual hazard in Utah is flooding.
Flooding is of special concern because it occurs frequently,
can cause significant damage to property and infrastructure,
and can be life threatening. The surface and near -surface soils
and rocks of the Moab —Spanish Valley are commonly salt -
rich and have highly soluble minerals (i.e., easy to dissolve in
water). The addition of water from development, mainly due
to landscape irrigation and poor surface -runoff management,
to previously dry areas will increase hazards related to soluble
soils, other problem soils, and landslides. Landslides, debris
flows, and rockfalls are of growing concern as development
encroaches near and onto steep hillsides adjacent to cliffs,
where development is often favored due to scenic vistas and
aesthetics. With the exception of flooding, rockfall, and ra-
don, geologic hazards identified in the Moab —Spanish Valley
region are typically localized and are rarely life threatening.
However, all geologic hazards are potentially costly when not
recognized and properly addressed in project planning, de-
sign, construction, and maintenance.
INTRODUCTION
This study provides maps and information on 13 geologic
hazards in the Moab 7.5-minute quadrangle, in south-central
Grand County, Utah. The quadrangle encompasses the City of
Moab municipality and the northern part of the Moab —Span-
ish Valley (figure 1), a northwest -southeast -trending graben
formed from the collapse of a salt anticline. The Colorado
River runs northeast to southwest through the northern part
of the quadrangle. The Moab —Spanish Valley area has experi-
enced rapid population growth in residential and commercial
areas and is expected to see increasing growth in the coming
decades. As the population grows and tourism increases in
the area, urbanization will increase; therefore, comprehensive
geologic information available early in the planning and de-
sign process is critical to avoid or reduce the risk from geo-
logic hazards and protect public health, welfare, safety, and
the local economy.
Purpose and Scope
Geologic -hazard mapping is a multidisciplinary, dynamic
process that uses a variety of available data to create an in-
tegrated product intended for multiple uses. This study pro-
vides geotechnical engineers, engineering geologists, design
professionals, building officials, developers, and the public
with information on the types and locations of geologic haz-
ards that may affect existing and future development in the
Moab quadrangle (figure 1). This mapping is best applied
when used in conjunction with the Utah Geological Survey's
(UGS) Guidelines for Investigating Geologic Hazards and
Preparing Engineering -Geology Reports, with a Suggested
Approach to Geologic -Hazard Ordinances in Utah (Bow-
man and Lund, 2016). We compiled the data and created the
maps for this study at a scale of 1:24,000 (1 inch = 2000 feet)
using a geographic information system (GIS). This approach
resulted in geologic -hazard maps that incorporate data and
2
Utah Geological S trvey
Explanation
1
City of Moab Boundary
Arches National Park
05 2
3 9
0 05 1 2 3 4
1410meters
-109 62'
-109.8' -109 56'-109.53'-109.5'
3867
Figure 1. Location of the Moab 9ttadrangle showing principal geographic features, including the boundary of the City of Moab and major
transportation routes (base map from Utah Automated Geographic Reference Center, 2016).
methods from a variety of scientific disciplines, including
engineering geology, geotechnical engineering, geomor-
phology, imagery analysis, GIS technology, and geologic
field mapping and reconnaissance.
The geologic -hazard maps (plates 1 through 13) are designed
as aids for general planning to indicate areas where detailed,
site -specific geotechnicallgeologic-hazard investigations are
recommended. The maps should not be enlarged for use at
scales larger than 1:24,000, and are not a substitute for site -
specific geotechnicallgeologic-hazard investigations. The
maps are based on a geologic -hazard analysis of the Moab
quadrangle. The geologic hazards addressed are shallow
groundwater, salt tectonics -related ground deformation,
flooding, landsliding, rockfall, radon potential, collapsible
soil, expansive soil and rock, corrosive soil and rock, soluble
soil and rock, piping and erosion, wind-blown sand, and shal-
low bedrock. Other hazards may exist.
In the state of Utah, counties and municipalities are encour-
aged to develop geologic hazard ordinances (Utah State Code,
2016). As of the writing of this report, no geologic hazard or-
dinances exist in Grand County or the City of Moab. Grand
County does have an ordinance for flood damage prevention
that applies to areas having a special flood hazard designa-
tion zone identified by the Federal Emergency Management
Agency (FEMA) (Grand County, 2014).
Areas in Utah that have geologic hazard ordinances include
Salt Lake, Utah, and Iron Counties. The Salt Lake County
Geologic Hazards Ordinance (Salt Lake County, 2017) re-
quires, at minimum, investigation of surface -fault -rupture,
liquefaction, debris -flow, landslide, and snow avalanche
hazards prior to development. The Utah County Natural
Hazards Overlay Zone (NHO) (Utah County, 2017) re-
quires, at minimum, assessment of known special hazard
areas including rockfall, debris -flow, landslide, and surface -
fault -rupture hazards prior to development. Iron County has
incorporated geologic conditions into the zoning section of
the county code (Iron County, 2016). These counties also
address flood zoning in the ordinances.
Geologic hazards of the Moab quadrangle, Grand County, Utah
3
The scope of work for this study consisted of (1) identify-
ing and reviewing geologic, hydrologic, and soils information
available for the quadrangle; (2) digitizing relevant geologic,
hydrologic, and soils information; (3) compiling a digital geo-
technical database incorporating test data, borehole logs, and
other information from existing geotechnical/geologic-hazard
reports; (4) field reconnaissance and mapping; and (5) pre-
paring this report and accompanying maps describing each
geologic hazard. Other hazards not identified, quantified, or
mapped may be present within and near the quadrangle that
may affect existing and future development.
Previous Work
Hylland and Mulvey (2003) completed a geologic -hazard
study for the Moab —Spanish Valley area that included maps
and a comprehensive report. Their maps included part of the
Moab quadrangle and presented information on expansive
soils, gypsiferous soils, alluvial -fan flooding, debris flows,
collapsible soils, rockfall, shallow groundwater, fractured
rock and subsidence associated with salt dissolution, and soils
susceptible to piping and erosion. Other previous studies rel-
evant to geologic hazards in the Moab quadrangle include:
• understanding subsurface brines and their movement
in the Moab area (Mayhew and Heylman, 1965),
• radon -hazard potential (Sprinkel and Solomon, 1990),
• earthquake potential and seismic hazards of the Para-
dox Basin region (Wong and others, 1996),
• the earthquake potential of the Moab fault (Olig and
others, 1996),
• modeling flooding of the Colorado River in relation to
a large uranium mine tailings pile adjacent to the river
(Kenney, 2005), and
• paleoseismic study of a normal fault attributed to salt -
dissolution faulting on the southwest flank of Moab —
Spanish Valley (Guerrero and others, 2014).
In addition, UGS geologic mapping (Doelling and others,
2002) and engineering consultant geotechnical/geologic-haz-
ard investigations have increased our understanding of the
area's geology and hazards.
Setting
The Moab quadrangle is in Grand County, Utah, which cov-
ers approximately 3694 square miles (9567 square kilome-
ters [km2]) in southeastern Utah. The quadrangle contains the
City of Moab and the main entrance to Arches National Park.
The Colorado River, which flows from northeast to south-
west, bisects the northern part of the quadrangle (figure 1).
Grand County has a population of about 9516 (U.S. Census
Bureau, 2016), and is projected to grow to 13,098 by 2050
(Utah Foundation, 2014). The City of Moab is the most popu-
lated area within the county and has about 5235 people (U.S.
Census Bureau, 2016). The Moab quadrangle consists of pri-
vate, state, and federal property, and the largest land managers
are the U.S. Bureau of Land Management, the National Park
Service, and the U.S. Department of Energy (DOE). Due to
the spectacular geology and unique setting, the Moab area re-
ceives more than one million tourists per year visiting Arches
and Canyonlands National Parks, among other local and state
parks and attractions (Kem C. Gardner Policy Institute, 2016).
Mining exploration, extraction, and remediation are part of the
past and current economic activities in the area. Uranium and
vanadium were prospected in Moab and the surrounding area
in the 1910s and 1920s. Potash, manganese, oil, and natural gas
have been extracted from geologic deposits in the Moab area
over the past century. A notable mining -related deposit in the
area is a large uranium mine tailings pile adjacent to the Colo-
rado River. The Atlas Minerals Corporation created the tailings
pile as part of a uranium -ore processing facility from the mid-
1950s to 1984. This poses a significant contamination threat to
the more than 50 million downstream users of Colorado River
water. Atlas filed for bankruptcy in 1998, and in 2001, the U.S.
Congress transferred the responsibility of site cleanup to the
DOE (Shenton, 2016). As part of the Moab Uranium Mill Tail-
ings Remedial Action (UMTRA) project, the DOE has removed
approximately 52%of the estimated 16 million tons ofcontami-
nated mining tailings by railcar to a disposal site --30 miles (48
km) to the north (Shenton, 2016). The tailings pile area within
the Moab quadrangle geologic -hazard maps is labeled as "not
mapped." This is due to the ongoing nature of the UMTRA proj-
ect and studies pending in the area. For more information about
the remediation history, progress, and hazards associated with
the UMTRA tailings pile, visit www.moabtailings.org or www.
gjem.energy.gov/moab.
In the Moab quadrangle, the Moab —Spanish Valley is bounded
by --500 to 1000-foot (-175-300 m) near -vertical cliffs of
Pennsylvanian -age to Jurassic -age sedimentary rocks. Eleva-
tion in the quadrangle ranges from approximately 6315 feet
(1925 m) along the southwestern rim of Moab —Spanish Val-
ley to 3917 feet (1194 m) along the Colorado River. The area
is characterized by low precipitation; large daily temperature
changes; cold, dry winters; and hot, dry summers. Average
annual precipitation in the Moab area from January 1893 to
June 8, 2016, is 9 inches (23 cm) (Western Regional Climate
Center [WRCC], 2016). Precipitation in the Moab area falls
fairly equally throughout the year, only varying within —0.6
inch (1.5 cm) from month to month (minimum: 0.42 inch
[1.07 cm] in June; maximum: 1.03 inches [2.62 cm]). Sum-
mer precipitation is primarily from monsoonal patterns that
bring high winds and moisture from the Gulf of Mexico and
the Pacific Ocean. Summer temperatures in the area common-
ly exceed 90°F (32.2°C); the January 1893 to June 8, 2016,
average maximum temperature for July is 98.2°F (36.7°C),
4
Utah Geological Survey
and the January 1893 to June 8, 2016, average maximum
temperature for January is 42.4°F (5.7°C) (WRCC, 2016).
We present the distribution of geologic hazards using a
U.S. Geological Survey Moab quadrangle 1:24,000-scale
topographic base map published in 1997, which conforms
to the North American Datum of 1983 (NAD 83).
Geology
The Moab quadrangle is within the Colorado Plateau
physiographic province, which overall is characterized
as a broadly uplifted region of relatively undeformed,
"layer -cake" sedimentary strata. Geologically, the quad-
rangle is in the fold -and -fault belt of the asymmetric
Paradox Basin, which was made structurally complex
by faulting and salt-diapir movement and salt dissolu-
tion. The Paradox Basin developed in mid -Pennsylvanian
through early Permian time along the southwest flank of
the Uncompahgre Uplift (Stevenson and Baars, 1986). As
the Uncompahgre Uplift rose, the basin subsided along
northwest -trending normal faults while strata of the Penn-
sylvanian Paradox Formation which is rich in evaporite
minerals, including halite, potash, and magnesium salts —
were deposited. Salts have low density, and under pres-
sure from layers of younger strata deposited above, salt in
the Paradox Formation moved upward along fractures and
faults as large diapirs, creating long, northwest -trending
salt -cored anticlines. The Moab —Spanish Valley is part
of a series of northwest -trending valleys in southeastern
Utah and southwestern Colorado formed from collapsed
salt anticlines in the Paradox Basin. Dissolution of the
salt core in the anticlines caused collapse of the overlying
strata. Timing of anticline collapse is poorly constrained,
but is thought to have occurred in the late Pliocene to
early Pleistocene (Doelling, 2001). The Moab —Spanish
Valley is about 17 miles (27 km) long and 0.5 to 1.5 miles
(0.8-2.4 km) wide, and is bounded by steep cliffs ofPenn-
sylvanian-age to Jurassic -age sedimentary rocks.
The geology of the Moab 7.5-minute quadrangle is com-
plex, thus, a detailed discussion of the regional geology
of the area is beyond the scope of this report. Detailed
information on the geology of the greater Moab —Spanish
Valley area can be found in Doelling (1985, 2001), Doel-
ling and others (1988, 2002), Huffman and others (1996),
Doelling and Morgan (2000), and Doelling and Kuehne
(2013a, 2013b, 2013c). The following descriptions of
geologic units in the quadrangle are modified from Doel-
ling (2001), Doelling and others (2002), and Hylland and
Mulvey (2003) (figure 2).
The oldest rock unit exposed in the quadrangle is the
Middle Pennsylvanian Paradox Formation, which is
dominated by evaporite minerals but also contains car- Figure 2. Lithological cohtmn of geologic units exposed in the Moab
bonate rocks, silty sandstone, and black shale (figure 2). quadrangle (after Doelling and others, 2002).
wcn
�
v}i
w
w
CO
FORMATION
AND MEMBERS
THICK-
NESS
feet
(meters)
o
m
�
LITHOLOGY
Q
m
a
Surficial and basin -fill deposits
u to450±
(up to 137±)
Q
-- -
-
Subsurface only
Morrison
Salt VVash Mbr.
30+(9+)
Jms
ck
Red marker
Fm,
ayrg ,.,.F,rt
4D-50(12-15)
Jmt
CurtisFm
Moab Mbr
60-100(18-3o)
Jctrp
_'•; :;.,:.•
�-�
j • •
Commonly jointed
J-3 unconformity
Eolian cross -beds
•o
Entrada
Slick Rock Mbr
250±(76±)
Jes
CermelFm
Dewey Bridge Mbr.
90-110(273a,
Jcd
- -,,-
J-2 unconformity
• Forms arches
Navajo Sandstone
300-700
(91--213)
Jn
Jnl
•-
High -angle cross -beds
.. •, -•
Highly jointed
.'•:�.•
.:::::
` •' Thin limestone beds
• .
` a'
3
b
Kayenta Formation
2_50-400
Jk
4 t
Eolian marker bad
� '--
Ledge and(76-122) -. bench
VVingate Sandstone
250-400
(76 122)
)yJ
. Prominent cliff former
J-0 unconformity
(.i
QA-3
F-
a
D
Chinle Formation
100
(30-213) 13)�,
�,<-
Thick beds at top
"Black ledge"
c,..,.,-
Local unconformities
>S
Moenkopi Formation �I
(0 229)
'tim
unconformity
"'^"
}
}LI Ripple marks
L
-r
D
.. A-1 unconformity
ceJ
a
Cutler Formation
(0-1,524) 0-5 000
Pc
ei. ..1.
•,��-'•;
Arkosic sandstone
rm
�
Unconfomrity
a
m
�
a
o
HonakerTrai)
Fonnabon
0-2.700
(0-823)
Ph
Fossilferous
•-
Subsurface only
c
N
c
->
�
m
-o
2
_
Paradox
Formation
9.000+
(912,743+) -
{91-743+)
i�//iQ
7/,?-
1/
Gypsum and shale
�, caprock
-t-+ +
Salt beds
.--V
+}+
Subsurface only
Geologic hazards of the Moab quadrangle, Grand County, Utah
S
The Paradox outcrops appear pale yellowish gray, pale greenish
gray, and grayish white to light gray with patches of dark gray
(figure 3). Within the Moab -Spanish Valley, the Paradox For-
mation is exposed in two discontinuous bands along the north-
eastern and southwestern margins. In the northern part of the
quadrangle, the Upper Pennsylvanian Honaker Trail Formation
and the overlying Lower Permian Cutler Formation crop out
south of the Arches National Park visitor center. The Honaker
Trail Formation is composed of grayish sandstone, siltstone,
and limestone. The Cutler Formation forms cliffs and slopes
composed of red -brown and maroon cross -bedded sandstone
and conglomerate with thin siltstone and limestone beds.
The Lower Triassic Moenkopi Formation forms steep slopes
and ledges above the Cutler Formation and consists of brown,
micaceous sandstone, siltstone, mudstone, and shale (figure
2). Overlying the Moenkopi is the Upper Triassic Chinle For-
mation, also a slope -forming unit. The Chinle is gray -red to
red -brown sandstone, siltstone, conglomeratic sandstone, and
mudstone (figure 2). Capping these formations are cliffs of the
Figure 3. Paradox Formation exposures on the margins of the Moab -Spanish valley that have varying degrees of color appearance.
(A) Example of light to dark gray Paradox Formation. (B) Example of pale yellowish gray, and pale greenish gray Paradox Formation.
6
Utah Geological Survey
Lower Jurassic Wingate Sandstone and Kayenta Formation.
The Wingate is composed of fine-grained, well -sorted sand-
stone and forms the massive, dark -brown cliffs south and west
of Moab, and along the Colorado River north of Moab. On top
of the Wingate is the Kayenta Formation, a ledge and step -like,
lavender -gray and dark -brown sandstone (figure 2). The Kay-
enta Formation caps many of the cliffs bordering Moab —Span-
ish Valley. The Lower Jurassic Navajo Sandstone overlies the
Kayenta, forming an irregular surface of pale -orange to light -
gray sandstone fins, hills, and swales on the northeastern and
southwestern sides of Moab —Spanish Valley.
Overlying the Navajo Sandstone is a Middle to Upper Jurassic
sequence of mostly sandstone units exposed in and near Arches
National Park. These rocks include the Dewey Bridge Member
of the Carmel Formation, the Slick Rock Member of the Entra-
da Sandstone, the Moab Member of the Curtis Formation, the
Summerville Formation, and the Tidwell and Salt Wash Mem-
bers of the Morrison Formation (figure 2). Most of the arches
in Arches National Park are formed in sandstone of the Dewey
Bridge, Slick Rock, and Moab Members. Strata of the Summer-
ville and Morrison Formations, exposed in only a small part of
the study area within Arches National Park, generally consist of
red to brown sandstone and siltstone and gray limestone over-
lain by pale yellow -gray sandstone interbedded with green and
red mudstone and siltstone (figure 2).
The floor of Moab —Spanish Valley is composed of Quater-
nary fill and surficial deposits derived from the nearby La
Sal Mountains and local valley slopes and transported by the
Colorado River, Dry Creek, Pack Creek, and smaller, local
drainages. Debris flows and rockfalls from the cliffs bound-
ing the valley produce prominent colluvium and talus slopes.
Downslope of these deposits are alluvial fans derived from
erosion of upstream channel deposits and slope sediments.
The alluvial -fan deposits interfinger with stream alluvium of
Mill and Pack Creeks and the Colorado River in the interior
of the valley. The valley -bounding cliffs give way to broad,
flat plateaus of highly jointed and deeply eroded sandstone.
Quaternary eolian sand deposits are present on the valley floor
and the upper plateau areas.
GEOLOGIC HAZARDS
Early recognition and mitigation of geologic hazards can re-
duce risk to life, property, and the economy. Since 1847, an
estimated 5797 fatalities have occurred due to geologic haz-
ards in Utah (Bowman and Lund, 2016) (table 1). Radon gas
exposure and subsequent lung cancer has been Utah's most
deadly geologic hazard, with over 5372 fatalities (data from
1973-2012), followed by landslide hazards with 337 docu-
mented fatalities, and flooding hazards with 101 documented
fatalities (Bowman and Lund, 2016). As debris flows are both
a landslide and flooding hazard, fatalities are listed in both
hazard categories. Hazard mapping is essential to identify ar-
eas that need further investigation to determine hazard extent,
risk, and mitigation measures. In almost all cases, it is more
cost effective to identify and characterize geologic hazards
and then implement appropriate mitigation in project design
and construction, rather than rely on additional maintenance
over the life of the project (Bowman and Lund, 2016).
On an annual basis, the most common and damaging geologic
hazard in Utah, and the Moab quadrangle, is flooding. Be-
cause of their potentially wide distribution, frequent occur-
rence, and destructive nature, floods will likely be the prin-
cipal geologic hazard in the quadrangle that planners, land
owners, and others will have to address in the future.
The Moab quadrangle has significant gypsiferous, corrosive,
expansive, and collapsible soil and rock, and piping and ero-
sion potential due to the mineralogy of surficial geologic units
and their weathering by-products. Significant hazard potential
exists from the proximity and exposure of the Paradox, Moen-
kopi, and Chinle Formations to surface and groundwater. The
Paradox, Moenkopi, and Chinle Formations contain various
amounts of soluble minerals. The mostly salt -based minerals
such as gypsum, potash, and halite present collapse, piping,
corrosion, and erosion issues. The dissolution of subsurface
soluble -mineral deposits can create underground voids. De-
pending on the location and size of voids in relation to the
ground surface, they can present a significant collapse hazard.
Small voids may coalesce over time, creating larger voids and
forming sinkholes. These units also contain sulfates which
can degrade unprotected construction materials over time, and
uranium which decays to dangerous radon gas.
Landslides, rockfalls, alluvial -fan flooding, and debris flows
are of growing concern as development increases on hillsides,
where development is often favored due to scenic vistas. Some
bedrock units in the quadrangle contain a high percentage of
clay and are correspondingly weak and susceptible to land-
slides, especially when wet. Existing landslides in the quad-
rangle, especially older ones, can be difficult to recognize, and
their stability remains suspect. Landslide identification and
proper accommodation in project planning and design is criti-
cal to avoid slope -stability problems. One landslide was iden-
tified in the Moab quadrangle (plate 4). New landslides could
develop if groundwater conditions on slopes change due to
human- or climate -induced conditions, such as landscape ir-
rigation, wastewater disposal fields, infiltration basins, and/or
increased precipitation. Conditions conducive to rockfall are
present along the valley -forming cliffs. Damaging rockfalls
are a hazard in many locations in the quadrangle. Damaging
events are likely to increase as development moves into those
areas, unless effective hazard -reduction measures are imple-
mented. Alluvial -fan flooding occurs when a concentrated
amount of water, usually from a cloudburst rainstorm, is cap-
tured in a drainage or slot canyon, picks up debris in turbulent
flow, then deposits debris on an alluvial fan due to the increase
in surface area, shallowing of the slope angle, and slowing of
Geologic hazards of the Moab quadrangle, Grand County, Utah
7
Table 1. Summary of known geologic -hazard fatalities in Utah (from Bowman and Lund, 2016).
Geologic Hazard
Fatalities
Landslide Hazards
Landslides'
4
1.2%
337
5.7%
Rockfall
15
4.5%
Debris Flowsz
15
4.5%
Snow Avalanches3
303
89.8%
Earthquake Hazards
Ground Shaking
2
100%
2
< 0.1 %
Flooding Hazards
Flooding
81
80.1 %
101
1.7%
Debris Flowsz
15
14.9%
Dam and Water Conveyance Structure Failure'
5
5.0%
Problem Soils
Radon Gas4
1973-2001
14605
--
5372
92.6%
2002-2011
38166
2012
96s
Total:
5797
'Because of uncertainty in event initiation, three fatalities are listed in both the"Landslides"and "Dam and Water Conveyance Struc-
ture Failure"categories.
2Debris flows are both a landslide and flooding hazard.
3The majority of post-1950 snow avalanche fatalities are in the backcountry from human -induced avalanches; however, many have
occurred near or in developed areas where appropriate mitigation measures should be used.
"Limited data are available and contain various assumptions; exact number of fatalities is unknown.
sBased on World Health Organization general estimate that 14% of lung cancer cases are attributable to radon gas (Sasha Zaharoff,
Utah Department of Health, written communication, 2015) and data from http://epht.health.utah.gov/epht-view/query/result/
ucr/UCRCntyICD02/Count.html.
6Utah Environmental Public Health Tracking Network (2015).
the flow. These occur most often on mapped Holocene alluvial
fans. Debris flows in the Moab quadrangle can be caused by
precipitation that falls far away from the deposition area, trav-
eling great distances at fast speeds.
Geologic and geomorphic mapping and seismic interpreta-
tion indicate the presence of fault scarps in the Moab —Span-
ish Valley that are attributed to subsurface movement associ-
ated with salt tectonics (Olig and others, 1996; Guerrero and
others, 2014). In Utah, most earthquakes are associated with
the Intermountain Seismic Belt (ISB) (Smith and Sbar, 1974;
Smith and Arabasz, 1991; Bowman and Arabasz, 2017), an
approximately 100-mile-wide (160 km), north -south -trending
zone of earthquake activity extending from northern Montana
to northwestern Arizona; however, the Moab —Spanish Valley
is outside that zone. Most earthquakes in Moab —Spanish Val-
ley cannot be attributed to movement on known faults and
have regionally been smaller than magnitude 5 (Wong and
others, 1996). Along the margins of the Moab —Spanish Val-
ley, some fault scarps have been investigated to determine
their movement history (Guerrero and others, 2014). The fault
scarps most likely formed from diapirism and/or collapse due
to salt dissolution (Doelling and others, 2002). Fault locations
and displacements were likely influenced by the extent of the
underlying Paradox Formation, the mechanical strength of
overlying layers of rock, and changes in hydrostatic base level
over time (Guerrero and others, 2014). The periodicity and
magnitude of earthquakes that likely produced the fault scarps
in the Moab —Spanish Valley are poorly understood due to the
complexities of salt -related tectonics. In western Colorado, a
paleoseismic study of the Hogback monocline (--140 miles
northeast of Moab) and adjacent faults showed their move-
ment was caused by salt dissolution and diapirism, and mod-
eling showed the faults able to cause a significant earthquake
(M, 6, [moment magnitude]) with a rupture area as large as
77 square miles (200 km2) (Gutierrez and others, 2014). We
did not complete a ground -shaking -hazard analysis or map
for the Moab quadrangle. The origin of mapped faults cannot
8
Utah Geological Survey
currently be discerned between tectonic- or salt -dissolution -
related faulting due to the lack of requisite information, so any
ground -shaking model would not be accurate.
Shallow groundwater, wind-blown sand, and shallow bed-
rock are typically localized in nature. While potentially costly
when not recognized and properly accommodated in project
planning, design, and maintenance, these hazards are rarely
life threatening. By contrast, hazards posed by rockfall, flood-
ing, and elevated levels of indoor radon gas can be life threat-
ening. Breathing radon gas over time significantly increases
the risk of lung cancer, but effective techniques are available
for reducing indoor radon levels in existing construction and
preventing dangerous levels in new construction (U.S. Envi-
ronmental Protection Agency [U.S. EPA], 2010).
Shallow Groundwater
Groundwater is in saturated zones beneath the land surface in
soil and rock at various depths. Shallow groundwater levels are
typically dynamic and fluctuate in response to a variety of con-
ditions; groundwater levels may rise or fall in response to long-
term climatic change, seasonal precipitation, irrigation, and the
effects of development. Most development -related groundwa-
ter problems occur when water is within 10 feet (3 m) of the
ground surface. Shallow groundwater can flood basements and
other underground facilities, damage buried utility lines, and
destabilize excavations. Groundwater inundation of landfills,
waste dumps, and septic-tank/wastewater disposal systems can
impair the performance of these facilities and lead to ground-
water contamination. Groundwater can change the physical and
chemical nature of rock and soil, cause soils and rocks suscep-
tible to expansion and collapse to activate, and can be a con-
tributing factor to slope instability (Wieczorek, 1996; Ashland
and others, 2005, 2006). During moderate to large earthquakes,
groundwater within approximately 50 feet (15 m) of the ground
surface can cause liquefaction in sandy soils.
Groundwater may exist under unconfined (water table) or con-
fined (artesian/pressurized) conditions, in regional aquifers,
and/or as local perched zones. The deep unconfined and con-
fined aquifers are commonly grouped together and called the
principal aquifer (Thiros, 1995). Artesian pressure can force
groundwater from the principal aquifer upward to the ground
surface where it is discharged through springs and seeps. A
shallow unconfined aquifer is typically present where confin-
ing layers overlie the principal aquifer (Thiros, 1995). Perched
groundwater develops where water from precipitation, irriga-
tion, and/or urban runoff percolates through thin, permeable,
unconsolidated surface deposits and collects above less -perme-
able underlying layers.
Surficial deposits in the Moab quadrangle are highly vari-
able and range from impermeable to moderately permeable
bedrock and soils (clay, silt, sand, and gravel) (Doelling and
others, 2002). Groundwater data in the quadrangle are limited
outside areas of recent development; therefore, perched water
or unknown groundwater conditions may extend outside of
the mapped zone of shallow groundwater (plate 1). Perched
groundwater and seasonally shallow groundwater may locally
contribute to development problems in areas not having persis-
tent shallow groundwater. Areas of localized perched shallow
groundwater may result from the addition of water from land-
scape irrigation and stormwater control. The addition of post -
development water may cause sinkholes by soil piping or the
dissolution of subsurface evaporite minerals and contribute to
damage from collapsible and expansive soils. Groundwater in
the Moab —Spanish Valley area is contained in two aquifers, the
Glen Canyon aquifer and the unconsolidated valley -fill aquifer
(Lowe and others, 2007). The shallow -groundwater -potential
map does not differentiate between aquifers and is not intended
to model the deeper regional aquifer; instead it indicates the
potential for shallow groundwater resulting from soil drainage
capacity, geology, and hydrology.
To evaluate shallow groundwater potential (plate 1), we used
six main sources of data: (1) UGS geologic mapping (Doelling
and others, 2002), (2) a geotechnical database of information
from consultant geotechnical and geologic hazard reports com-
piled by the UGS, (3) previous groundwater studies, (4) water -
well drillers' logs on file with the Utah Division of Water Rights
(UDWR, 2009), (5) private industry water -well data, and (6)
the Natural Resources Conservation Service (MRCS) Soil
Survey Geographic (SSURGO) Database for Grand County,
Utah Central Part (UT624); Canyonlands Area, Utah —Parts
of Grand and San Juan Counties (UT633); and Arches National
Park, Utah (UT687) (MRCS, 2016a, 20166, 2016c).
We obtained groundwater -level data from geotechnical/geolog-
ic-hazard studies and water -well logs and incorporated the data
into a geotechnical database. The shallow groundwater map-
ping is based on geologic unit using NRCS data and geotechni-
cal data as modifiers. The NRCS maps the occurrence of wet
or potentially wet soil conditions. Wet conditions are defined
by the NRCS as soils in which depth to groundwater is less
than 60 inches (152 cm), and potentially wet soil conditions are
defined as poorly drained, fine-grained soils that may develop
shallow groundwater locally when rates of water application
exceed the soil's drainage capacity. Geotechnical data that in-
dicate where depth to groundwater was observed to be shallow
(less than or equal to 10 feet [3 m]) was obtained from geotech-
nical borehole and water -well logs. The NRCS and geotech-
nical data were overlain with the geologic map to determine
the shallow groundwater potential of each geologic unit, and
NRCS soil unit boundaries were used to modify the geologic
unit where determined necessary. To account for temporal and
seasonal fluctuations in groundwater, we used the most conser-
vative (shallowest) depth to groundwater reported in an area.
Our shallow -groundwater -potential map on plate 1 is not in-
tended to provide numerical depths to groundwater, but rather
to indicate where shallow groundwater may affect develop-
ment and contribute to other geologic hazards. We created
three shallow -groundwater -potential categories to identify soil
Geologic hazards of the Moab quadrangle, Grand County, Utah
9
and rock units that are either naturally wet or have the potential
to develop wet conditions. Areas mapped as bedrock are gen-
erally not considered to have shallow groundwater; however,
some bedrock units can be highly weathered and fractured, and
contribute to shallow groundwater conditions. The categories
define the conditions under which shallow groundwater may
occur, but the categories do not represent relative severity rank-
ings, or actual depth to groundwater.
The shallow -groundwater -potential categories shown (plate 1)
are approximate and mapped boundaries are gradational. Lo-
calized areas of higher or lower groundwater are likely to exist
within any given map area, but their identification is precluded
because of the generalized map scale, relatively sparse data, un-
identified areas of perched shallow groundwater, and non -geo-
logic factors such as landscape irrigation and stormwater control.
SW
Salt Tectonics -Related Ground Deformation
and Faulting
The Moab -Spanish Valley is a graben formed by the collapse
of a salt -cored anticline that was created by diapirism (figure
4) in the salt -rich Paradox Formation (Guerrero and others,
2014). The valley formed due to the dissolution of salt de-
posits at depth, causing the rock above to collapse or subside
downward, forming a valley (Doelling and others, 2002). Pro-
cesses such as salt diapirism—the upward movement of salt
due to its low density and plastic nature --and salt dissolution
have resulted in ground deformation, including the develop-
ment of fractures, folds, joints, grabens, and faults along the
valley margins of the Moab -Spanish Valley. The resulting
displacement of the ground surface may also produce ground
cracking, surface warping, and multiple, complex scarps. De-
MOAB-SPANISH VALLEY
FAULTS
FORMING
SALT DISSOLUTION
PARADOX FORMATION
CONTINUING SALT
DISSOLUTION
1
ALLUVIUM
MOAB FAULT
CONTINUING FORMATION
OF FAULTS
CONTINUING SALT
DISSOLUTION
1
NE
MOAB ANTICLINE
A
Figure 4. Formation of salt -dissolution -related faulting over time in the Moab -Spanish Valley area. Cross section trends southwest -
northeast across the northern Moab -,Spanish Valley area and the Moab anticline (modified from Baars and Doelling, 1987).
10
Utah Geological Survey
pending on the magnitude of subsurface movement, scarps
can range from a few inches to several feet high and extend
for many miles along a fault trace or deformation zone. Local
ground tilting and graben formation (figure 5) by secondary
gravitational faulting may result in a deformation zone along
the fault trace that can be tens to hundreds of feet wide. Sur-
face fault rupture related to gravitational faulting can cause
damage similar to that of tectonic -related surface fault rupture
and can have serious consequences for structures or other fa-
cilities that lie along or across the rupture path. The extent of
the underlying Paradox Formation, the mechanical strength
of geologic strata, and changes in hydrostatic base level over
time (Guerrero and others, 2014) affect subsurface displace-
ment and subsequent surface faulting. Unpredictable failure
rates related to underground salt movement make the hazard
very difficult to quantify.
To evaluate the salt tectonics -related ground -deformation haz-
ard (plate 2), we used five main sources of data: (1) UGS geo-
logic and hazard mapping (Doelling and others, 2002; Hyl-
land and Mulvey, 2003), (2) the Utah Quaternary Fault and
Fold Database (UGS, 2017), (3) the Guidelines for Evaluat-
ing Surface -Fault -Rupture Ha=ards in Utah (Lund and others,
2016), (4) aerial photography interpretation, and (5) a recent
paleoseismic investigation (Guerrero and others, 2014).
Compared to tectonically generated earthquakes and asso-
ciated fault systems, there is significantly less research and
literature about the magnitude of earthquakes created by salt -
dissolution faulting. Due to the unpredictable nature of salt
tectonics, we mapped an area of ground deformation based on
analysis of geologic units, mapped faults, and existing ground
deformation. Ground deformation in this area may be severe.
Continued ground deformation, subsidence, possible surface
fault rupture, and other hazards, such as sinkhole formation,
ground cracking, differential settlement, and widespread sub-
surface erosion (piping), can occur in the ground deformation
zone. Geologic mapping by Doelling and others (2002) shows
potentially active salt -dissolution normal faults (where the
hanging wall has moved down relative to the footwall) along
which additional salt -tectonic -related surface faulting and
movement may occur. Due to the unknown nature of these
faults, we show mapped faults on the salt tectonics -related
ground- deformation hazard map (plate 2) and categorize
them as well defined, concealed, or approximately located, in
accordance with the Guidelines for Evaluating Sztrface-Fault-
Rupture Ha=ards in Utah (Lund and others, 2016). We con-
sider a fault well defined if its trace is clearly detectable by a
trained geologist as a physical feature at the ground surface
(Bryant and Hart, 2007) and UGS 1.24,000-scale mapping
(Doelling and others, 2002) shows them as solid lines, indicat-
ing that they are recognizable as faults at the ground surface.
Although not well expressed at the surface, approximately
located or buried faults (Doelling and others, 2002) may still
represent a significant ground -deformation and surface -fault -
rupture hazard and should be evaluated prior to development.
Approximately located faults are shown as a dashed line, and
buried faults are shown as a dotted line. Also mapped is the
potential for valley floor subsidence that can cause tilting and/
or damage to structures due to differential settlement, lateral
earth pressures, ground cracks or displacements in fractured
rock, and/or ground collapse, including sinkhole formation.
The plateaus and canyon areas are subject to regional and lo-
cal subsidence potential resulting in fracturing and displace-
ment of rock. Fractures weaken the rock and can lead to unsta-
ble conditions in road cuts and tunnels, increase potential for
aquifer contamination, and increase susceptibility to rockfall
and slope instability.
Figure 5. Diagram of a graben formed by two normal faults showing the relative movement of the hanging wall and footwall.
Geologic hazards of the Moab quadrangle, Grand County, Utah
11
Paleoseismic investigations in two trenches by Guerrero and
others (2014) in the Moab —Spanish Valley indicate nine fault-
ing events over the past —4500 years. Closed -interval slip
rates from the paleoseismic data are highly variable; vertical
slip rates range from 1.0 to 15.5 mm/yr (average 3.07 mm/yr),
and recurrence intervals range from 73 to 815 years (average
316 years) (Guerrero and others, 2014). For comparison, the
range of mean, Holocene, closed -interval slip rates for each
of the five central segments of the Wasatch fault zone is 1.3-
2.0 mm/yr (Working Group on Utah Earthquake Probabili-
ties, 2016). Using statistical analyses from paleoseismic data,
Guerrero and others (2014) indicate faults in the Moab —Span-
ish Valley that ruptured the surface do not behave like tec-
tonically driven faults, having comparatively higher slip rates
and higher slip per event in relation to their length. Although
not tectonically driven, faults in the Moab —Spanish Valley can
produce damaging earthquakes and surface rupture on a rela-
tively short timescale (Gutierrez and others, 2014).
In addition to the graben -bounding normal faults, the trace
of the Moab fault is inferred to trend northwest -southeast
down the middle of the Moab —Spanish Valley (Doelling and
others, 2002). Unlike other faults in the region, this fault is
believed to be primarily related to Tertiary extensional tec-
tonics but shows no evidence of movement related to exten-
sional tectonism in the Quaternary (Olig and others, 1996).
The last period of major tectonic activity on the Moab fault
occurred during the Laramide orogeny (>35 Ma; Solum and
others, 2005). The Moab fault is unlikely to be a source of
significant modern earthquakes.
The salt tectonics -related ground deformation hazard map
(plate 2) shows potentially active faults in the Moab quad-
rangle along which salt tectonic -related ground deformation
and surface faulting may occur. The UGS recommends a
site -specific ground deformation and surface -fault -rupture -
hazard investigation be performed in the areas identified as
having severe ground -deformation potential prior to devel-
opment. Because of the lack of paleoseismic data and the
poorly understood mechanisms of Quaternary salt -dissolu-
tion -related faulting in the Moab quadrangle, we based the
zone of concern on the mapped soluble geologic units at the
surface and where they are likely within 50 feet or less from
the surface (Doelling and others, 2002). Valley floor and
plateau subsidence can occur where soluble rock is present
beneath the surface. The inferred trace of the Moab fault is
shown on plate 2, but does not include a special study zone
due to the pre -Quaternary age of latest major fault activity
(Olig and others, 1996; Solum and others, 2005), based on
the limited data available.
Deformation due to salt can occur anywhere within the
quadrangle, but the margins of the valley have the potential
for the most damaging deformation. Pre -development in-
vestigations present many challenges. Traditional geotech-
nical investigations could worsen the risk associated with
shallow subsurface soluble deposits. Drilling, test pits, and
trenches may introduce water at depth, increasing the risk
of salt tectonic -related ground deformation, including sur-
face rupture from faulting, as well as sinkhole formation
and piping and erosion, all discussed in detail within this
report. We recommend that completed borings be appropri-
ately grouted to prevent conducting water into the subsur-
face. Test pits should be excavated with caution, but if nec-
essary, non-native fill of geologic material without soluble
salts and other minerals should be used as backfill and ap-
propriately compacted. Geophysical investigation methods
to determine displacement, voids, and salt tectonics -related
structures are recommended to limit potential water expo-
sure to the subsurface.
The most conservative approach to prevent damage in these
areas is avoidance of development. Where avoidance is not
possible, disclosure of the hazard and the associated risk
should be mandatory. Currently, no investigational method-
ology can determine the frequency or extent of risk to de-
velopment from salt tectonics. This risk should be clearly
defined to potential land owners and disclosed.
The areas of ground deformation, valley floor subsidence,
and plateau subsidence related to salt tectonics shown on
plate 2 are approximate and mapped boundaries are based on
an estimated deformation zone determined by interpreting
geologic units on the surface and at depth, and the distance
from mapped faults. Localized areas of deformation where
surface rupture may occur are likely to exist anywhere within
the quadrangle, but their identification is precluded because
of the generalized map scale and relatively sparse data.
Salt tectonics -related faulting and ground deformation
does not necessarily preclude development in the area, but
it should cause significant concern for the design and con-
struction of structures and facilities. A high amount of risk
is associated with development in these areas and should be
addressed in site -specific geotechnical/geophysical investi-
gations, engineering design, and communication with poten-
tial land users. Disclosure of the risks associated with devel-
opment and limited mitigation options should be mandatory.
If the risk is understood and assumed by land users and own-
ers, then site -specific geotechnical (with precautions noted
above) and/or geophysical investigations are recommended
to determine the extent of subsurface damage, voids, and
salt tectonics -related structures until the processes involved
in salt -dissolution -related faulting are further understood.
Engineering design should account for potential differential
movement. The extent of vertical displacement is unknown
at this time, except for fault scarp heights, and investigations
should be focused on determining possible displacement on
salt tectonics -related faults in the area. In these areas, other
salt -dissolution and diapirism-related hazards include solu-
ble soil and rock (plate 9), which can cause sinkholes and
subsidence; corrosive soil and rock (plate 10); and piping
12
Utah Geological Survey
and erosion (plate 11). These additional hazards should also
be addressed in a comprehensive geologic hazard/getotech-
nical investigation.
Flood Hazards
Flooding is the overflow of water onto lands that are normal-
ly dry and is the most commonly occurring natural hazard
(Keller and Blodgett, 2006). Damage from flooding includes
inundation of land and property, erosion, deposition of sedi-
ment and debris, and the force of the water itself, which can
damage property and take lives (Utah Division of Homeland
Security, 2008). Historically, flooding is the most prevalent
and destructive (on an annual basis) hazard affecting Utah.
The flood hazard map (plate 3) shows areas in the Moab
quadrangle that may be susceptible to flooding. Within the
quadrangle, several drainages are known to be capable of
flooding and include the Colorado River, Courthouse Wash
(figure 6), Pack Creek, Mill Creek, North Fork Mill Creek,
Moab Canyon, Kane Springs Canyon, Grandstaff Canyon,
and Pritchett Canyon. Several small ephemeral drainages
contribute to the flood hazard as well. Seasonal weather pat-
terns that deliver moisture to southeastern Utah, particularly
during the late summer monsoon season, also contribute to a
high flood hazard and flash flood hazard. Types of seasonal
floods that typically occur are riverine (stream) floods, flash
floods/debris flows, and sheet floods. Flash flooding in the
area can be very localized when intense rain accumulates on
the plateaus and quickly floods slot canyons and overwhelms
infrastructure in the valley (figure 7). The potential for flood-
ing is increased by human activities, such as placing struc-
tures and constrictions in floodplains, active alluvial fans,
or erosion -hazard zones; developing without adequate flood
and erosion control; and the unintentional release of water
from an engineered water -retention or conveyance structure
(such as a dam or canal).
To evaluate flood hazard (plate 3), we used six main sources
of data: (1) FEMA National Flood Insurance Program Flood
Insurance Rate Maps (table 2) (FIRMs) (FEMA, 2016),
(2) UGS mapping (Doelling and others, 2002; Hylland and
Mulvey, 2003), (3) aerial photography interpretation, (4)
10-meter National Elevation Dataset (NED) (USGS, 2016a)
and 0.5-meter lidar data (Bowen Collins & Associates, Inc.,
2015) where available, to examine past and present drain-
age patterns, (5) the National Hydrography Dataset (NHD)
(USGS, 2016b), and (6) a geotechnical database compiled
by the UGS that includes unpublished consultant's reports
having updated flood mapping (Bowen Collins & Associ-
ates, Inc., 2016).
Geologic mapping is critical to determine the distribution
of geologically young flood -related deposits, which aids
in identifying flood -prone areas and evaluating their rela-
tive susceptibility to flooding and/or debris flows. Because
of many variables contributing to flood hazard, including,
but not limited to, precipitation intensity and duration, soil
conditions, and topography, the geologic unit itself is not an
absolute indicator of flood hazard susceptibility but rather
a relative indicator. Geologic units assigned a flood hazard
category in the Moab quadrangle will likely demonstrate
different flood susceptibility in other locations. Flood haz-
ard categories were modified in geologic units where field
observations, topographic and aerial photographic analysis
warrant. Small ephemeral drainages and slot canyons may
be mapped as low flood hazard; however, these drainages
have a high flash -flood hazard. The NHD delineates streams
in drainages using GIS modeling based on 30-meter NED
data (USGS, 2016b). These data were added to the map to
indicate a high flood potential in drainages that have been
identified by the NHD as having permanent or ephemeral
flowing streams. Determining the actual extent of flooding
is beyond the scope of this study and should be conducted
as part of site -specific geologic hazard investigations. Small
individual drainages were not mapped due to topographic
complexities and scale limitations of the map.
Debris -flow and alluvial -fan deposits are likely to occur in
the very high and high categories and can occur anywhere
in the quadrangle (plate 3). Debris -flow hazard is highly
dependent on rainfall and snowmelt as well as sediment
supply; therefore, debris flows may occur in areas mapped
as moderate or low, and not only in areas with mapped ac-
tive or historical debris -flow deposits. Post -wildfire flood
hazard is considered high in areas having slopes greater
than 17 degrees (30%), based on the Salt Lake County
Geologic Hazards Ordinance (Salt Lake County, 2017),
and a debris -flow site -specific investigation should be per-
formed. The potential for flooding is significantly increased
by wildfires. Wildfire increases flood potential by decreas-
ing saturation of water into the ground. The flood hazard
may be mapped as very low or low in many areas with
slopes greater than 17 degrees (30%); however, exposed
bedrock and sparse vegetation can increase the flood haz-
ard in these locations.
Flood hazard associated with shallow groundwater was con-
sidered where data are available. Areas of potential shallow
groundwater (< 10 ft [3 m]) were mapped as high flood haz-
ard potential.
Site -specific geotechnical/geologic-hazard flood investiga-
tions can resolve uncertainties inherent in the generalized
hazard map (plate 3) and help ensure safety by identifying
the local flood and debris -flow hazard. Chapter 5 of UGS Cir-
cular 122, Guidelines for the Geologic Investigation of De-
bris -Flow Hazards on Alluvial Fans in Utah (Giraud, 2016),
recommends minimum standards for performing debris -flow
investigations in Utah.
FEMA-designated flood zones delineated on the FIRMs are
overlain on our mapped hazard categories (table 2, plate
3). FEMA, through its National Flood Insurance Program
Geologic hazards of the Moab quadrangle, Grand County, Utah
1.3
Figure 6. Colorado Riverflooding into the mouth of Courthouse Wash in 1917, looking to the east. Photograph from Dan O laurie museum collection,
Museztm of Moab, used with permission.
Figure 7. Flooding over Arches National Park service road near park entrance. (photo: July 19, 2017).
14
Utah Geological Survey
Table 2. FEMA FIRM panel information and effective dates.
DFIRM ID1
FIRM IDz
Panel
Suffix3
FIRM Panel
Effective Date
Scale
LOMR Date4
49019C
49019C_269
1759
D
49019C1759D
4/2/2009
6000
49019C
49019C_293
1758
D
49019C1758D
4/2/2009
6000
49019C
49019C_296
1766
D
49019C1766D
4/2/2009
6000
49019C
49019C_295
1767
D
49019C1767D
4/2/2009
6000
11/14/2016
49019C
49019C_294
1754
D
49019C1754D
4/2/2009
6000
49019C
49019C_76
1775
D
49019C1775D
4/2/2009
24000
11/14/2016
'Digital Flood Insurance Rate Map (DFIRM) ID is the digitized version and consolidation of existing FIRM data.
zlnsurance Rate Map (FIRM) ID is the panel reference of specific areas used since the 1970s.
3Suffix indicates the number of times a panel has been revised.
4Letter of Map Revision (LOMR) Date indicates official modifications to the FIRM.
(NFIP), makes federally subsidized flood insurance available
to individuals residing in participating communities. Not all
areas on the Moab quadrangle have been mapped by FEMA,
and FEMA may designate flood zones in the future. FIRMS
are legal documents that govern the administration of the
NFIP. Property owners should consult the appropriate FIRM
directly when considering the purchase of NFIP flood insur-
ance (FEMA, 2016). Flood insurance can also be purchased by
landowners outside of mapped zone A designated by FEMA.
The flood -hazard -potential categories shown on plate 3 are ap-
proximate and mapped boundaries are gradational. Localized
areas of higher or lower flood hazard are likely to exist within
any given map area, but their identification is precluded be-
cause of the generalized map scale and non -geologic factors
such as climate change, wildfire, removal of vegetation and/or
topsoil, modification of waterways and/or the ground surface,
unidentified areas of perched shallow groundwater, landscape
irrigation, and stormwater control.
Landslide Hazards
Landslide is a general term that refers to the gradual or rapid
movement of a mass of rocks, debris, or earth down a slope
under the force of gravity (Neuendorf and others, 2005). The
term covers a wide variety of mass -movement processes,
and includes both deep-seated and shallow slope failures.
The moisture content of the affected materials when a slope
fails can range from dry to saturated. However, high mois-
ture content reduces the strength of deposits susceptible to
landslides and is often a contributing factor to landsliding.
Three broad factors, acting either individually or in combi-
nation, contribute to landsliding (Vannes, 1978; Wieczorek,
1996): (1) an increase in shear stress, (2) low material strength,
and (3) a reduction of shear strength. Common factors that in-
crease shear stress include adding mass to the top of a slope,
removing support from the toe of a slope, transient stresses as-
sociated with earthquakes and explosions, and the long-term
effects of tectonic uplift or tilting. Low material strength in
rock or soil typically reflects the inherent characteristics of
the material or is influenced by discontinuities such as joints,
faults, bedding planes, and desiccation fissures. Factors that
reduce shear strength include both physical and chemical
weathering, and the addition of water to a slope, which in-
creases pore -water pressure and reduces the effective inter -
granular strength within the slope materials.
Although one or more factors may make a rock or soil mass
susceptible to landsliding, a trigger is required for landslid-
ing to occur (Vames, 1978; Cruden and Varnes, 1996). A
trigger is an external stimulus or event that initiates landslid-
ing either by increasing stresses or reducing the strength of
slope materials (Wieczorek, 1996). Landslide triggers may
be either static or dynamic. Static conditions include intense
rainfall or prolonged periods of above -normal precipitation,
rapid snowmelt, added water from irrigation or improper
drainage, improper grading, and rapid erosion. Dynamic con-
ditions include earthquakes and other ground shaking. Al-
though frequently obvious, some triggers are subtle and not
readily apparent. For example, a nearly imperceptible combi-
nation of weathering and gradual erosional undercutting can
eventually cause landsliding.
Landslides are categorized based on how they move: topple,
fall, slide, spread, or flow (Cruden and Varnes, 1996). In the
Moab area, the common types of landslides are fall and topple.
Fall and topple movements are due to exposed rigid bedrock
being affected by slow erosional processes; fractures and joint-
ing of rock faces are precursors to these types of events. Falls
are associated with weakened rock detaching from cliff faces or
overhangs and falling or sliding to the valley floor. Topples are
like falls, but have a rotational aspect. When a weakened rock
column dislodges, it rotates away from the rock face, tumbling
down the slope. Falling rock attributed to fall and topple move-
ments are addressed on the rockfall hazard map (plate 5).
Geologic hazards of the Moab quadrangle, Grand County, Utah
15
Flow, spread, and slide movements are possible in Moab —
Spanish Valley area, but require high subsurface water con-
tent to trigger movement. Due to the arid climate, these types
of movement are less likely, except around perennial rivers
and creeks. Flow-, spread-, and slide -type landslides typically
undergo either rotational and/or translational movement. Ro-
tational slides move on a concave sliding surface, resulting
in back -tilted areas at the head of the slide, and they can be
shallow or deep seated and can move very slowly or rapidly.
Translational slides form on planar surfaces and slide out over
the original ground surface. The sliding surface can form on
bedding planes, faults, joints, or other discontinuities.
To evaluate landslide susceptibility (plate 4), we used five
main sources of data: (1) UGS geologic and hazard mapping
(Doelling and others, 2002; Hylland and Mulvey, 2003), (2)
aerial photography interpretation, (3) 10-meter NED (USGS,
2009) and 0.5-meter lidar data (Bowen Collins & Associates,
Inc., 2015) where available, (4) analysis of mapped land-
slides in similar geologic conditions, and (5) field mapping
and reconnaissance. We classify landslide susceptibility as
high, moderate, or low. High landslide susceptibility consists
of mapped landslides, geologic units that have experienced
previous landsliding elsewhere in Utah as identified by geo-
logic mapping, and that underlie slopes that equal or exceed
a determined critical slope angle. Moderate landslide sus-
ceptibility consists of areas having steep slopes in a geologic
unit with material that may be susceptible to landsliding but
has no prior landslides, and in geologic units with material
that is highly susceptible to landsliding where the slope is
slightly lower than the critical angle. Low landslide suscepti-
bility consists of areas having slopes below the critical angle
in units not likely susceptible to landsliding.
In the Moab quadrangle, we applied critical slope angles of
10 and 22 degrees based on analysis of landslides in south-
ern Utah within similar geologic units. To determine these
slope angles, we used GIS to calculate the average slope of
each mapped landslide included in the Landslide Maps of
Utah (Elliott and Harty, 2010) in southern Utah. The land-
slide slopes were then exported to a spreadsheet based on
geologic unit, and the average slope angle for each geologic
unit was determined. Using mean landslide slope plus or mi-
nus one standard deviation, we assigned critical angles to
geologic units in the Moab quadrangle. Similar methodology
has been used in other landslide evaluation and susceptibil-
ity investigations in similar geologic units to define critical
slope (Hylland and Lowe, 1998; Giraud and Shaw, 2007;
Lund and others, 2008; Knudsen and Lund, 2013; Knudsen
and others, in review). We assigned a critical angle of 10 de-
grees for Quaternary deposits along the Colorado River and
its tributaries (Pack Creek and Mill Creek), smaller tributar-
ies (Courthouse Wash and Grandstaff Canyon), and ephem-
eral drainages. We assigned a critical angle of 22 degrees to
the Chinle and Moenkopi Formations and deposits originat-
ing from them. Geologic units that were not determined to be
landslide prone were not assigned a critical angle.
Although earthquake -induced ground shaking increases the
potential for landsliding in susceptible material, the rela-
tive landslide susceptibility of the slope material does not
change. For example, slopes mapped as having moderate
landslide susceptibility are more likely to fail during an
earthquake than under static conditions; however, slopes
having moderate landslide susceptibility are less likely to
fail than slopes having high susceptibility under static or dy-
namic conditions.
The landslide -susceptibility map (plate 4) shows areas of
relative landslide susceptibility where site -specific slope -
stability conditions (material strength, orientation of bed-
ding and/or fractures, groundwater conditions, and erosion
or undercutting) should be evaluated prior to development.
A valid landslide -hazard study must address all pertinent
conditions that could affect, or be affected by, the proposed
development, including earthquake ground shaking, perched
or irrigation -induced groundwater, and slope modifications.
This study can only be accomplished through the proper
identification and interpretation of site -specific geologic
conditions and processes. Chapter 4 of UGS Circular 122,
Guidelines for Evaluating Landslide Ha. ards in Utah (Beu-
kelman and Hylland, 2016), recommends minimum stan-
dards for performing landslide -hazard evaluations in Utah.
The guidelines outline a phased approach to slope -stability
investigations, beginning with a geologic evaluation and
progressing through reconnaissance and detailed geotechni-
cal-engineering evaluations as needed based on the results
of the previous phase. Table 3 summarizes minimum UGS
recommendations for site -specific investigations for each
landslide -susceptibility category in the Moab quadrangle;
see Beukelman and Hylland (2016) for more information.
Table 3. Recommended requirements for site -specific landslide -
hazard investigations in the Moab quadrangle; see Beukelman and
Hylland (2016) for more information.
Landslide
Susceptibility
Recommended Site -Specific Study
High
Detailed engineering geologic and
geotechnical-engineering study
necessary.
Moderate
Geologic evaluation and
reconnaissance -level geotechnical-
engineering study necessary;
detailed engineering geologic and
geotechnical-engineering study may
be necessary.
Low
Geologic evaluation and
reconnaissance -level geotechnical-
engineering study necessary; detailed
geotechnical-engineering study
generally not necessary.
16
Utah Geological Survey
Some local governments in Utah have created and maintain
geologic -hazard -specific ordinances to limit the impact of
landslides. Salt Lake County's Zoning Ordinance Code pro-
hibits development, including clearing, excavating, and grad-
ing, on slopes exceeding 17 degrees (30%) and sets aside these
areas as natural private or public open space (Salt Lake Coun-
ty, 2010). Also, all roads are restricted from crossing slopes
steeper than 17 degrees (301)/0) unless they meet specific re-
quirements and gain authorization (Salt Lake County, 2017).
While it is possible to classify relative landslide hazard in
a general way based on material characteristics and critical
slope inclinations, landslides ultimately result from the ef-
fects of site -specific conditions acting together to drive the
slope toward failure. For that reason, all development in areas
of sloping terrain, where modifications to natural slopes will
be significant or where landscape irrigation or onsite waste-
water disposal systems may cause groundwater levels to rise
(Ashland, 2003; Ashland and others, 2005, 2006), require a
site -specific geotechnical/geologic-hazard study to evaluate
the effect of development on slope stability and recommend
appropriate design and mitigation measures.
The landslide -hazard -susceptibility categories shown on plate
4 are approximate and mapped boundaries are gradational.
Localized areas of higher or lower landslide susceptibility are
likely to exist within any given map area, but their identifica-
tion is precluded because of the generalized map scale and
non -geologic factors, such as modification of slopes, unidenti-
fied areas of perched shallow groundwater, landscape irriga-
tion, and stormwater control.
Rockfall Hazards
Rockfall is a natural mass -wasting process that involves the
dislodging and downslope movement of individual rocks
and small rock masses (Varnes, 1978; Cruden and Varnes,
1996). Rockfalls are a hazard because dislodged rocks trav-
eling at high speed can cause considerable damage. Rock-
falls can damage property, roadways, and vehicles, and pose
a significant safety threat. Rockfall hazards occur where a
rock source exists above slopes steep enough to allow rapid
downslope movement of dislodged rocks by falling, roll-
ing, and/or bouncing (figure 8). Most rockfalls originate on
slopes steeper than 35 degrees (Wieczorek and others, 1985:
Keefer, 1993), although rockfall hazards may be found on
less -steep slopes.
Rockfall-hazard potential is based on a number of factors, in-
cluding geology, topography, and climate. Rockfall sources
Figure 8. Steep cliffs are a rockfall source and dislodged rocks fall, roll, and/or bounce down steep slopes below the source. Rocks traveling
at high speed can cause significant damage and injury or death. (photo: Moab Rim trail east of the Colorado River).
Geologic hazards of the Moab quadrangle, Grand County, Utah
17
include bedrock outcrops or boulders on steep mountain-
sides or near the edges of escarpments, such as bluffs, ter-
races, and ancient shorelines. Talus cones and scree -covered
slopes are indicators of a high rockfall hazard, although oth-
er areas are also vulnerable. Rockfalls may be initiated by
talus cones and scree -covered slopes are indicators of a high
rockfall hazard, although other areas are also vulnerable.
Rockfalls may be initiated by thermal cycling (solar heating
of the rock, Collins and Stock, 2016), frost action, rainfall,
weathering and erosion of the rock or surrounding material,
and root growth, though in many cases a specific triggering
mechanism is not apparent. Rockfalls may also be initiated
by ground shaking. Keefer (1984) indicates earthquakes as
small as MH, 4 can trigger rockfalls.
The rockfall hazard map (plate 5) shows areas in the Moab
quadrangle that may be susceptible to rockfall. Where no haz-
ard is mapped, rockfall hazard is either absent or too local-
ized to show on a 1:24,000-scale map. Each hazard category
includes three components (figure 9): (1) a rockfall source, in
general defined by geologic units that exhibit relatively con-
sistent patterns of rockfall susceptibility throughout the study
area; (2) an acceleration zone, where rockfall fragments de-
tached from the source gain energy and momentum as they
travel downslope—this zone often includes a talus slope,
which becomes less apparent with decreasing relative haz-
ard and is typically absent where the hazard is low; and (3)
a runout zone, including gentler slopes that may be covered
discontinuously by scattered large boulders that have rolled or
bounced beyond the base of the slope.
To evaluate rockfall hazard (plate 5), we used four main
sources of data: (1) UGS geologic and hazard mapping (Doel-
ling and others, 2002; Hylland and Mulvey, 2003), (2) aerial
photography interpretation, (3) 0.5-meter lidar data (Bowen
Collins & Associates, Inc., 2015) where available, and (4)
field mapping and reconnaissance.
We assigned a hazard designation of high, moderate, or low
based on the following rockfall-source parameters: rock type,
joints, fractures, orientation of bedding planes, and potential
clast size, as determined by mapping (Doelling and others,
2002; Hylland and Mulvey, 2003) and field reconnaissance, as
well as slope angle, acceleration zone, and a shadow angle of
20 degrees. The shadow angle was determined through field
reconnaissance and aerial photography analysis of rockfall
deposits and their source. The shadow 20-degree angle was
calculated for boulders originating in the Curtis Formation,
the Entrada Sandstone, and the Wingate Sandstone. Rockfalls
originating in the Navajo Sandstone and the Kayenta Forma-
tion have larger shadow angles, around 28 to 30 degrees. To
be conservative we applied the 20-degree shadow angle across
the map area. We evaluated slopes below rockfall sources for
slope angle, vegetation, clast distribution, clast size range,
amount of embedding, and weathering of rockfall boulders.
Table 4 summarizes our recommended requirements for site -
specific geotechnical/geologic-hazard investigations related
to rockfall hazards to protect life and safety. Chapter 7 of UGS
Circular 122, Guidelines for Evaluating Rockfall Ha=ards in
Utah (Lund and Knudsen, 2016), recommends minimum
standards for performing rockfall-hazard evaluations in Utah.
Figure 9. Components of a characteristic rockfall path profile (after Lund and others, 2008).
18
Utah Geological Survey
Table 4. Recommended requirements for site -speck rockfall hazards investigations to protect life and safety.
Hazard
Potential
Classification of Buildings and Other Structures for Importance Factors1
I
II
III
IV
one- and two-
family dwellings
and townhouses
all other buildings
and structures,
except those
listed in groups II,
III, and IV
buildings and
other structures
that represent a
substantial hazard
to human life in the
event of failure
buildings and
other structures
designated as
essential facilities
buildings and
other structures
that represent
a low hazard to
human life in the
event of failure
High,
Moderate
Yes
Yes
Yes
Yes
Noz
Low
Yes
Yes
Yes
Yes
Noz
None
No
No
No
No
No
'Risk category from International Code Council (2014a).
zProperty damage possible, but little threat to life safety.
The rockfall-hazard-potential categories shown on plate 5
are approximate and mapped boundaries are gradational.
Localized areas of higher or lower rockfall potential are
likely to exist within any given map area, but their identi-
fication is precluded because of the generalized map scale
and non -geologic factors such as cuts, fills, or other modi-
fications to the natural terrain.
Radon Hazard
Radon is an odorless, tasteless, and colorless radioactive
gas that is highly mobile and can enter buildings through
small foundation cracks and other openings, such as utility
pipes. The most common type of radon is naturally occur-
ring and results from the radioactive decay of uranium,
which is found in small concentrations in nearly all soil
and rock. Although outdoor radon concentrations rarely
reach dangerous levels because air movement and open
space dissipate the gas, indoor radon concentrations may
reach hazardous levels because of confinement and poor
air circulation in buildings and other confined spaces.
Breathing any level of radon over time increases the risk
of lung cancer, but long-term exposure to low radon levels
is generally considered a small health risk. Smoking great-
ly increases the health risk due to radon because radon
decay products attach to smoke particles and are inhaled
into the lungs, greatly increasing the risk of lung cancer.
The U.S. Environmental Protection Agency (EPA, 2009)
recommends that action be taken to reduce indoor radon
levels exceeding 4 picocuries per liter of air (pCi/L) and
cautions that indoor radon levels less than 4 pCi/L still
pose a significant health risk.
Indoor radon levels are affected by several geologic factors
including uranium content in soil and rock, soil permeability,
and groundwater. Granite, metamorphic rocks, some volcanic
rocks, shale, hydrothermally altered rocks, and soils derived
from these rocks are generally associated with elevated urani-
um content contributing to high indoor radon levels. Soil per-
meability and groundwater affect the mobility of radon from
its source. If a radon source is present, the ability of radon
to move upward through the soil into overlying buildings is
facilitated by high soil permeability. Conversely, radon move-
ment is impaired in soils having low permeability. Saturation
of soil by groundwater inhibits radon movement by dissolving
radon in the water and reducing its ability to migrate upward
through the soil (Black and Solomon, 1996). However, if the
source of the radon gas is above or within the groundwater
table, shallow groundwater may not reduce the movement of
radon. Long-term exposure to water with dissolved radon is
also dangerous (drinking, etc.).
Along with geologic factors, non -geologic factors also influ-
ence radon levels in a building or other confined space. Al-
though the influence of geologic factors can be estimated, the
influence of non -geologic factors, such as occupant lifestyle
and structure construction and maintenance, are highly vari-
able. As a result, indoor radon levels fluctuate and can vary in
different structures built on the same geologic unit; therefore,
the radon level must be measured in each structure to deter-
mine if a problem exists. Testing is easy, inexpensive, and may
often be conducted by the building occupant, but professional
assistance is available (for more information visit https://ra-
don.utah.gov). Evaluation of actual indoor radon levels in the
quadrangle was beyond the scope of this study.
Geologic hazards of the Moab quadrangle, Grand County, Utah
19
To evaluate the radon -hazard potential (plate 6), we used four
main sources of data to identify areas where underlying geo-
logic conditions may contribute to elevated radon levels: (1)
soil permeability data from the NRCS Soil Survey Geographic
(SSURGO) Database for Grand County Utah —Central Part
(UT624); Canyonlands Area, Utah —Parts of Grand and San
Juan Counties (UT633); and Arches National Park, Utah
(UT687) (MRCS, 2016a, 2016b, 2016c), (2) depth -to -ground-
water mapping (this study), and (3) UGS geologic and hazard
mapping (Doelling and others, 2002; Hylland and Mulvey,
2003) and (4) U.S. Geological Survey (USGS) National Ura-
nium Resource Evaluation (NURE) Hydrogeochemical and
Stream Sediment Reconnaissance Data (USGS, 2004). Using
the geologic factors of uranium content, soil permeability, and
depth to groundwater, we classified soil and rock units using
a three-point system (table 5) into high (3 points), moderate
(2 points), and low (1 point) hazard categories based on their
potential to generate radon gas and the ability of the gas to mi-
grate upward through the overlying soil and rock (after Black
and Solomon, 1996). Points were assigned based on the shal-
low groundwater mapping (plate 1), permeability, and relative
uranium content of mapped rock units in the Moab quadrangle
which were summed together to report indoor radon hazard
potential (table 6).
Saturation of soil by shallow groundwater (less than ap-
proximately 30 feet [9 m]) inhibits radon movement by dis-
solving radon in the water and reducing its ability to migrate
upward through subgrade and foundation soil (Black, 1993).
Our groundwater mapping focused on the principal aquifer
where it is shallow and unconfined or artesian, and on lo-
cally unconfined or perched aquifers 30 feet (9 m) or less
below the ground surface. Even in areas with very shallow
groundwater, the source of radon may be above the water ta-
ble or introduced from imported material. If the radon source
was determined to be above the water table, then shallow
groundwater no longer contributes to the inhibition of ra-
don gas and we assigned a higher point value to the shallow
groundwater factor.
Geologic mapping is important for identifying geologic
units having high uranium content, particularly outside
of areas covered by previous investigations where radio-
metric data are limited. In the Moab quadrangle, the most
uranium -rich bedrock units are the Permian Honaker Trail
and Cutler Formations (Black, 1993; Doelling and others,
2002; Hylland and Mulvey, 2003), the Triassic Moenkopi
Formation (Black, 1993; Hylland and Mulvey, 2003), the
Mossback and Shinarump Members of the Triassic Chinle
Formation (Finch, 1954; Black, 1993; Doelling and others,
2002; Hylland and Mulvey, 2003), and the Tidwell and Salt
Wash Members of the Jurassic Morrison Formation (Finch,
1954; Mohammad, 1986; Doelling and Kuehne, 2013). All
alluvium and colluvium from locally derived uranium -
bearing geologic units (Doelling and others, 2002), as well
as alluvium interpreted to be from the intrusive igneous La
Sal Mountains, were assigned a point value of 3 for their
undetermined, but possible, uranium content (Hylland and
Mulvey, 2003). Any areas where uranium ore or waste
products have been stored warrant a detailed site -specific
study; these areas can emit very high concentrations of ra-
don, even in open air.
Table 5. Soil geologic factors that contribute to radon hazard potential. Soil permeability from NRCS data. Groundwater depth from shallow
groundwater mapping in this study. Uranium data from unpublished reports and NURE (modified from Black and Solomon, 1996).
Factor
Point Value
1
2
3
Uranium (ppm, estimated)
<2
2-3
>3
Permeability (Ksat, in/hr)
Low
0.06-0.6
Moderate
0.6-6.0
High
6.0-20.0
Groundwater depth (feet)
<10
10-30
>30
Table 6. Radon hazard potential mapping criteria and indoor radon potential (from Black and Solomon, 1996).
Category
Point range
Potential indoor radon concentration (pCi/L) estimate*
Low
3-4
< 2
Moderate
5-7
2-4
High
8-9
> 4
*Indoor radon concentrations are highly dependent upon structure design and construction.
20
Utah Geological Survey
The NRCS reported hydraulic conductivity (Ksat) values of
saturated soil for their soil units based on testing performed at
selected locations (MRCS, 2016a, 2016b, 2016c) and assigned
permeability classes to their soil units based on the hydraulic
conductivity of the unit (table 5). The hydraulic conductivity
values of non -soil map units (water, borrow pits, and other
artificial units as mapped by the NRCS) are reported as zero;
however, they do not necessarily represent impermeable sur-
faces. Therefore, the hydraulic conductivity of adjacent units is
assumed to apply to non -soil map units.
The map of radon -hazard potential (plate 6) is intended to pro-
vide an estimate of the underlying geologic conditions that may
contribute to the radon hazard. The map does not characterize
indoor radon levels because they are also affected by highly
variable non -geologic factors. The map can be used to indicate
the need for testing indoor radon levels; however, we recom-
mend testing be completed in all existing structures and other
confined spaces. If professional assistance is required to test for
radon or reduce the indoor radon hazard, a qualified contractor
should be selected. The EPA provides guidelines for choosing a
contractor and a listing of state radon offices in the Consumer's
Guide to Radon Reduction (EPA, 2010). The Utah Department
of Environmental Quality (DEQ, 2017) provides information on
radon mitigators and ordering test kits on their website at https://
deq.utah.gov/Programs Services/programs/rad iation/radon/.
The tailings pile area within the Moab quadrangle is labeled on
the geologic -hazard maps as "not mapped." This is due to the
ongoing nature of that project and studies pending in the area.
For more information about the remediation history, progress,
and hazards associated with the tailings pile, visit www.moab-
tailings.org or www.gjem.energy.gov/moab.
The radon -hazard potential map (plate 6) is not intended to in-
dicate absolute indoor radon levels in specific structures. Al-
though geologic factors contribute to elevated indoor -radon -
hazard potential, other highly variable factors, such as building
materials, construction methods, and foundation openings, af-
fect indoor radon levels; therefore, indoor radon levels can vary
greatly between structures located in the same hazard category.
The hazard -potential categories shown on plate 6 are approxi-
mate and mapped boundaries are gradational. Localized areas
of higher or lower radon potential are likely to exist within any
given map area, but their identification is precluded because of
the generalized map scale, relatively sparse data, and non -geo-
logic factors such as variability in building construction. The
use of imported fill for foundation material can also affect radon
potential in small areas because the imported material may have
different geologic characteristics than native soil at the site.
Collapsible Soil Susceptibility
Collapsible soils are relatively dry, low -density soils that de-
crease in volume or collapse under the load of a building or
infrastructure when they become wet. Collapsible soils may
have considerable strength and stiffness in their dry natural
state, but can settle up to 10 percent of the susceptible deposit
thickness when they become wet for the first time following
deposition (Costa and Baker, 1981; Rollins and Rogers, 1994;
Keaton, 2005) causing damage to property, structures, pave-
ments, and underground utilities.
Collapsible soils are present in the Moab quadrangle and
are typically geologically young materials, chiefly Holocene
debris -flow sediments in alluvial fans and Pleistocene to Ho-
locene colluvial deposits (plate 7). Collapsible soils typically
have a high void ratio, a corresponding low unit weight (<80
to 90 Ib/ft3; Costa and Baker, 1981), and a relatively low
moisture content (<15 percent; Owens and Rollins, 1990),
all characteristics that result from the initial rapid deposition
and drying of the sediments. Alluvial fans are an example of
this depositional environment and, in many cases, have a high
collapsible soil hazard. Intergranular bonds form between
the larger grains (sand and gravel) of a collapsible deposit;
these bonds develop through capillary tension or a binding
agent such as silt, clay, or salt. Characteristically, collapsible
soils consist of silty sands, sandy silts, and clayey sands (Wil-
liams and Rollins, 1991), although Rollins and Rogers (1994)
identified collapse -prone gravels containing as little as 5 to
20 percent fines (U.S. Standard #200 sieve) at several loca-
tions in the southwestern United States. Later wetting of the
soil results in a loss of capillary tension or the softening of
the bonding material, allowing the larger particles to slip past
one another into a denser structure. Naturally occurring deep
percolation of water into collapsible deposits is uncommon
after deposition due to the arid conditions in which the de-
posits typically form and the steep gradient of many alluvial
fans. Therefore, soil collapse is often triggered by human ac-
tivity related to urbanization, such as irrigation, wastewater
disposal, and/or surface drainage changes.
To evaluate collapsible -soil susceptibility (plate 7), we used
two main sources of data: (1) UGS geologic and hazards map-
ping (Doelling and others, 2002; Hylland and Mulvey, 2003),
and (2) a geotechnical database compiled by the UGS. First,
we evaluated test data from the geotechnical database; swell/
collapse tests (SCT), dry density, and moisture tests were all
used to determine collapse potential. Next, we integrated geo-
logic -unit descriptions from UGS geologic and hazard map-
ping (Doelling and others, 2002; Hylland and Mulvey, 2003)
with the geotechnical data to assign a susceptibility category to
mapped geologic units. We classified unconsolidated geologic
units into four categories based on their collapse potential.
Where geotechnical data provide evidence for high collapse
susceptibility, as indicated by SCT results exhibiting collapse
potential equal to or more than 3 percent (Jennings and Knight,
1975), we assigned two susceptibility categories: highly col-
lapsible soil, where SCT tests indicate collapse potential equal
to or more than 5 percent, and collapsible soil A, where SCT
tests indicate collapse potential over 3 percent and less than
Geologic hazards of the Moab quadrangle, Grand County, Utah
21
5 percent. Where geotechnical data are lacking, we assigned
geologic units that have a genesis and texture conducive to col-
lapse to the collapsible soil C category. Finally, where older
geologic units (Pleistocene) are mapped with no available geo-
technical data, but that have a genesis or texture permissive
of collapse, we assigned them to the collapsible soil D cat-
egory. Geologic units in which other geotechnical information
(chiefly low density and moisture content) provide evidence
for potentially collapsible soils, would be delineated as col-
lapsible soil B; however, there is no collapsible soil B mapped
in the Moab quadrangle. All susceptibility categories represent
geologic units having a potential for collapse. Geologic units
with SCT results indicating a demonstrated high percentage of
collapse dictate that the geologic units containing the SCT test
data are elevated above other similar geologic units lacking
geotechnical test data.
The collapsible -soil -susceptibility categories shown on plate
7 are approximate and mapped boundaries are gradational.
Localized areas of soil having higher or lower collapse po-
tential are likely to exist within any given map area, but
their identification is precluded because of the generalized
map scale, relatively sparse data, and non -geologic factors
such as disturbed land, changes in drainage and water runoff
patterns, landscape irrigation, and wastewater control. All
mapped susceptibility categories may potentially exhibit a
high percentage of collapse; therefore, site -specific investi-
gations should be performed at all locations to resolve un-
certainties inherent in the maps.
Expansive Soil and Rock Susceptibility
Expansive soil and rock swells as it gets wet and shrinks as it
dries out. These changes in volume can cause cracked foun-
dations and other structural damage to buildings (figure 10),
structures, pavements, and underground utilities, heaving and
cracking of canals and road surfaces, and failure of wastewater
disposal systems. Expansive soil and rock contains a signifi-
cant percentage of clay minerals that can absorb water directly
into their crystal structure when wetted. When clay content
is greater than approximately 12 to 15 percent, the expansive
nature of the clay dominates, and the soil is subject to swell.
Some sodium-montmorillonite clay can swell as much as 2000
percent upon wetting (Costa and Baker, 1981). The resulting
expansion forces can be greater than 20,000 pounds per square
foot (Shelton and Prouty, 1979) and can easily exceed the
loads imposed by many structures. Expansive soils are chiefly
derived from weathering of clay -bearing rock formations and
may be residual (formed in place) or transported (usually a
short distance) and deposited in a new location. The principal
transporting mechanisms are water or wind, but soil creep and
mass -wasting processes can play important roles locally.
Figure 10. Typical structural damage to a building from expansive soil (after Black and others, 1999).
22
Utah Geological Survey
To evaluate expansive soil and rock susceptibility (plate 8),
we used three main sources of data: (1) UGS geologic and
hazard mapping (Doelling and others, 2002; Hylland and
Mulvey, 2003), (2) a geotechnical database compiled by the
UGS, and (3) the NRCS Soil Survey Geographic (SSURGO)
Database for Grand County, Utah Central Part (UT624);
Canyonlands Area, Utah —Parts of Grand and San Juan
Counties (UT633); and Arches National Park, Utah (UT687)
(NRCS, 2016a, 20166, 2016c). We classified soil and rock
units into three categories based on their potential for volu-
metric change: high, moderate, and low (table 7).
The NRCS (2016a, 2016b, and 2016c) assigned a linear ex-
tensibility value to soils. Linear extensibility is an expres-
sion of volume change that represents the change in length
of an unconfined clod as moisture content is decreased from
a moist to a dry state. We compared the ratings presented by
the NRCS with the laboratory test results in our geotechnical
database. Correlations between the NRCS information and the
geotechnical test data are generally good, but some discrepan-
cies exist locally. Where geotechnical testing data show el-
evated levels of swell potential, we used geologic -map data
to modify the boundaries between susceptibility categories.
Using geotechnical data in our database, we evaluated liquid
limit (LL), plasticity index (PI), SCT tests, and expansion in-
dex included in the NRCS data (NRCS, 2016a, 2016b, 2016c)
for swell potential. SCT tests are the most reliable indicator
of swelling potential; we used them as the primary indicator
of swell potential, and LL and PI tests in the absence of SCT
data. Table 7 shows the correlation between these geotechni-
cal tests and expansive soil and rock susceptibility.
Chen (1988) recognized that while PI is an indicator of ex-
pansive potential, other factors also exert an influence, and
therefore reported a range of PI values that categorize a soil's
Table 7. Correlation between geotechnical tests of soils and
expansive -soil susceptibility.
Test
Susceptibility Category
Low
Moderate
High
Swell -Collapse
(SCT)
0-2%
2-3%
> 3%
Liquid Limit
(LL)
0-30
20-50
> 45
Plasticity Indexz
(PI)
0-15
10-35
> 20
Expansion Index3
(El)
0-50
51-90
> 91
1 Jennings and Knight (1975)
z Chen (1988)
3 Nelson and Miller (1992)
capacity to shrink or swell. Chen (1988) presented a correla-
tion between swell potential and PI that illustrates the use of
PI as an indicator of swelling potential (table 7). The use of PI
values can assist in selecting samples for swell/collapse test-
ing. Chen (1988) placed the lower bound of soils with high
swelling potential at a PI of 20, but also included soils with
a PI between 20 and 35 in the moderate category. The 2015
International Building Code (IBC) and International Residen-
tial Code (IRC) (International Code Council, 2014a, 201413),
adopted in Utah, which use PI as one of four criteria to deter-
mine if soils are considered expansive, include soils having a
PI of 15 or greater in the expansive soil category. In general,
PI values equal to or more than 20 can serve as a rough indica-
tor of high swell potential and can be used to select samples
for more extensive swell/collapse testing.
The Unified Soil Classification System (USCS) uses LL data
when classifying fine-grained soils. The USCS classifies soils
having an LL greater than 50 as highly plastic (capable of be-
ing permanently deformed without breaking); such soils typi-
cally contain expansive clays. The USCS classifies soils hav-
ing an LL less than 50 as having low or medium plasticity.
We identified geologic units containing expansive clay miner-
als by examining geologic unit descriptions and geotechnical
test data from the units. We classified units as having moder-
ate or high swell potential depending on geotechnical test data
from the unit and its corresponding NRCS classification. Due
to the scale of our mapping, individual sites within any sus-
ceptibility category (high, moderate, low) may exhibit a high
percentage of swell. The expansion of material may lead to
underground void spaces where further erosion will increase
void volume and tunneling (figure 11). Over time, a shrink -
swell cycle can erode potentially large subsurface caverns and
result in collapse (Dunne, 1990). A key indicator of surficial
expansive material is the textural change when water is intro-
duced and then removed. When the expansive material swells
with water then shrinks after water removal, the surface be-
comes disturbed over several cycles; the result is clod aggre-
gation at the surface, resembling popcorn (figure 12) (Hylland
and Mulvey, 2003). In the Moab quadrangle, both the Chinle
and Paradox Formations are susceptible to expansion (as evi-
dent in surface texture [figure 12]), near -surface cracking, and
subsurface voids (figure 11).
The expansive -soil -and -rock -susceptibility categories shown
on plate 8 are approximate and mapped boundaries are grada-
tional. Localized areas of soil and rock having higher or lower
expansive susceptibility are likely to exist within any given
map area, but their identification is precluded because of the
generalized map scale, relatively sparse data, and non -geo-
logic factors, such as disturbed land, changes in drainage and
water runoff patterns, landscape irrigation, and wastewater
control. All mapped susceptibility categories may potentially
exhibit a high percentage of collapse; therefore, site -specific
investigations should be performed at all locations to resolve
uncertainties inherent in the maps.
Geologic hazards of the Moab quadrangle, Grand County, Utah
23
Dry season to early
wet season
Late wet season
Shrinkage
crack
Erosion of
carck walls
Crack closed
by swelling
Collapsing of
wall and roof
Tunnel/void space
Figure 11. Subsurface void formation due to shrink -swell of soils
having a high clay content. voids may continue to enlarge in the
subsurface and propagate to the surface, creating a sinkhole hazard
(modified from Dunne, 1990).
Figure 12. "Popcorn" texture with evaporite precipitation in soils
derived from the Chinle and Paradox Formations (photo: base of
cliffs on the southwest side of the Moab valley).
Soluble Soil and Rock
Soluble soil and rock are subject to dissolution and reduced
soil and rock strength, which can cause considerable dam-
age to structures, foundations, and infrastructure. Soil and
rock containing salt, gypsum, and limestone are susceptible
to dissolution, which is associated with karst, sinkholes,
and subsidence. Gypsum (CaSO4.2H20)-bearing soil and
rock are highly soluble. Changes in surface -water flow and
groundwater can quickly dissolve gypsiferous material, re-
sulting in cavities that can collapse, either propagating to
the surface or causing local or regional subsidence. Where
the amount of gypsum is greater than 10 percent, dissolution
can result in localized land subsidence and sinkhole forma-
tion (Mulvey, 1992; Muckel, 2004; Santi, 2005). Gypsum
dissolution can be greatly accelerated by application of wa-
ter from sources like reservoirs; septic -tank and wastewater
drain fields; street, roof, or parking -lot runoff; and irrigation
(Martinez and others, 1998). Care should be taken in areas
of gypsiferous materials to avoid surface -flow- and ground-
water -regime changes. Surface flow should be directed to
areas where it will not percolate into the material below.
Landscape irrigation is discouraged, and storm drain infra-
structure should be regularly maintained to prevent leaks
and sealed pipes should be considered. Gypsum is a weak
material that has low bearing strength and is not suited as
subgrade or foundation soil.
Other evaporite minerals with high salt content, including ha-
lite (NaCI), anhydrite (CaSO4), carnallite (KMgC13•6H20),
and sylvite (KCI), are common in the Moab quadrangle and
surrounding area (Mayhew and Heylman, 1965). These min-
erals are highly soluble and are intermixed with gypsum in
the Paradox Formation.
Limestone and rock made up of mostly calcium carbonate
(CaCO3) and soils derived from them are moderately sus-
ceptible to dissolution. Karst terrain is common in areas
of limestone rock. Climate, water, and human activity are
factors in chemical weathering resulting in limestone dis-
solution. The arid climate of east -central Utah contributes
to slow rates of limestone dissolution. However, changing
surface -flow and groundwater regimes and/or increased pre-
cipitation due to climate change could accelerate dissolution
of limestone -bearing rocks in the Moab area.
To evaluate soluble soil and rock (plate 9), we used three
main sources of data: (1) UGS geologic and hazard mapping
(Doelling and others, 2002; Hylland and Mulvey, 2003),
(2) a geotechnical database compiled by the UGS, and (3)
the NRCS Soil Survey Geographic (SSURGO) Database
for Grand County Utah —Central Part (UT624); Canyon-
lands Area, Utah —Parts of Grand and San Juan Counties
(UT633); and Arches National Park, Utah (UT687) (NRCS,
2016a, 2016b, 2016c).
24
Utah Geological Survey
L L Salt
L L L
We classified soil and rock units into eight categories based
on their potential for dissolution: highly soluble rock (HSR);
highly soluble soil (HSS); gypsiferous rock (GR) A, B, and
C; gypsiferous soils (Gs); and limestone rock (LR) A and B.
HSR and HSS categories include the Paradox Formation
which contains significant amounts of gypsum and other salts
and alluvial -fan deposits. The depth to the Paradox Formation
along the eastern and western margins of the Moab —Spanish
Valley is not well constrained and caution should be taken
during development to limit the removal of surface material
along the valley margins, and to limit the addition or change
the rate of water application. The Paradox Formation poses
a dissolution hazard, even at depth, as cavities can form and
propagate to the surface creating sinkholes and subsidence
(figure 13). Additionally, varying layers of unconsolidated
deposits can conceal gypsum and salt -bearing material. The
thickness of the unconsolidated deposits can range from a thin
veneer to several hundred feet (Doelling and others, 2002).
The concealed gypsum -salt deposits contribute to sinkhole
susceptibility and are exposed or are at shallow depth below
the surface primarily along the valley -edge areas.
The classification system for soluble soil and rock that can
contribute to dissolution and collapse hazard is a relative
susceptibility ranking as opposed to a hazard -severity rank -
Subsided well
Original Ground Surface
L L LLL
L LLL
LLLL L LLL
LLL
L L L L L L L LLL
L L L L L LLLLL L
L LLL L L L L LLL
LLLLL L L
L L L L L L
LLLLLL L
L L L
L LLL L L LLL
L L L L L L L L
L LLLLLLL L
L L L
L L
LLLLL
Salt
L LLLL L L L L L L L LLLLLLL
L L LLLLL LLLLL L LLLLLLL
Figure 13. Representation ofsinkhole formation due to salt dissolution
near a subsided well. Dissolution can occur in a similar manner due
to groundwater dissolution and percolating surface water (modified
from Dunrud and Nevins, 1981).
ing. Soluble rock and soil hazard category GRA poses a sig-
nificant hazard due to dissolution. The Chinle Formation
along the valley margins has been deformed due to uplift in
the salt -cored anticline region of the Moab —Spanish Valley.
This deformation has incorporated gypsum and salt from the
Paradox Formation into the Chinle, increasing the dissolution
and collapse hazard. Category GRB may contain gypsum and
other soluble salts locally and has a significant potential for
dissolution and collapse. Category GRc includes talus and al-
luvial material that may be composed of units with significant
gypsum and salt content. This category represents a thin cover
above rock and soil units that pose a significant dissolution
and collapse hazard. Caution should be taken when removing
soil or surficial deposits in this category, as it could expose
soil or bedrock below that has an increased dissolution and
collapse hazard. Category GS includes soils lacking signifi-
cant geotechnical data, but have been identified by the NRCS
(2016a, 2016b, and 2016c) as gypsum -bearing soils.
Geologic units consisting of limestone or interbedded car-
bonate rocks are mapped as LRA and LRB. The solubility of
these units is relatively lower than the solubility of gypsum -
and salt -bearing units; however, the potential for dissolution
and collapse is still present and could increase from land -use
modification, introducing and concentrating surface water,
and groundwater -regime changes due to development.
The soluble -soil -and -rock -hazard categories shown on plate 9
are approximate and mapped boundaries are gradational. Lo-
calized areas of higher or lower soluble soil and rock hazard
are likely to exist within any given map area, but their identifi-
cation is precluded because of the generalized map scale, rela-
tively sparse data, and non -geologic factors such as landscape
irrigation and stormwater control.
Corrosive Soil and Rock
Corrosion of Portland cement concrete (PCC) occurs from a
chemical reaction between a base (concrete) and a weak acid
(sulfate, sodium, or magnesium in soil or water) (Muckel,
2004). Soil and rock with high gypsum content is associated
with corrosion of concrete. Gypsum is soluble and along with
associated sulfates, such as sodium sulfate and magnesium
sulfate, can dissolve in water to form a weak acid solution that
is corrosive to concrete and metals in areas where the amount
of soil gypsum is one percent or greater (Muckel, 2004). Sul-
fate -induced corrosion of unprotected concrete slabs, walls,
masonry blocks, and buried infrastructure is widespread in
arid regions of Utah (figure 14). Corrosion of steel (metals)
occurs from an electrochemical process that results from con-
tact between steel (metals) and soluble chloride salts found in
soil or water (White and others, 2008).
To evaluate corrosive soil and rock (plate 10), we used three
main sources of data: (1) UGS geologic and hazard mapping
(Doelling and others, 2002; Hylland and Mulvey, 2003), (2)
Geologic hazards of the Moab quadrangle, Grand County, Utah
25
Figure 14. Evaporite precipitation and corrosion on concrete masonry unit wall (photo: northeast valley margin, February 23, 2017).
a geotechnical database compiled by the UGS, and (3) the
NRCS Soil Survey Geographic (SSURGO) Database for
Grand County Utah —Central Part (UT624); Canyonlands
Area, Utah —Parts of Grand and San Juan Counties (UT633);
and Arches National Park, Utah (UT687) (NRCS, 2016a,
2016b, 2016c).
We classified soil and rock units into five categories based
on their potential for corrosion of concrete and metals: cor-
rosive rock A (CRA), corrosive rock B (CRB), corrosive soil
A (CSA), corrosive soil B (CSB), and buried or concealed cor-
rosive soil or rock (CSC). Site -specific investigations prior to
development should include testing for sulfate and gypsum
content and pH of soils. Other testing may be required; how-
ever, specialized corrosion engineering consultants are rec-
ommended. It is important to include testing for sulfates in
geotechnical investigations, as sulfates can degrade concrete
over time. Concrete masonry unit walls, foundations, and
other structures, where high sulfate levels are found, should
follow applicable American Concrete Institute, IBC, and IRC
standards, such as the use of Type V (sulfate resistant) cement.
The corrosive -soil -and -rock -potential categories shown on
plate 10 are approximate and mapped boundaries are grada-
tional. Localized areas of soil having higher or lower corro-
sive potential are likely to exist within any given map area,
but their identification is precluded because of the generalized
map scale and relatively sparse data. All mapped categories
may exhibit corrosive potential; therefore, site -specific inves-
tigations should be performed at all locations to resolve uncer-
tainties inherent in the maps.
Piping and Erosion
Piping and erosion can cause significant damage to roads,
canals, earth -fill dams, structures, bridges, culverts, and
farmland. Rapid erosion may occur when susceptible materi-
als are exposed to running water or wind. Monsoonal storms
typically bring intense rainfall and high winds. Heavy rain
can quickly erode fine-grained sediment. Slope runoff that
becomes channelized can form gullies (figure 15) and erode
steep banks of streams and rivers. Erosional gullies can con-
tribute to the piping hazard. Piping, also referred to as tunnel
erosion, is subsurface erosion by groundwater that moves
through permeable, non -cohesive layers in unconsolidated
materials and exits at a free face (figure 16). Fine-grained
sand, silt, and clay particles are removed by the subsurface
flow of water, creating void space. An exit point at a free
face may not always be obvious. Entrained silt and clay can
travel with the subsurface groundwater flow for long dis-
tances, enter the regional groundwater regime, and exit as
seeps and springs or into streams and rivers.
Soil and rock susceptible to piping and erosion are preva-
lent in the Moab quadrangle. The Chinle Formation and the
Paradox Formation are highly susceptible to piping and ero-
sion, as well as other geologic units that have high silt and
clay content or high evaporite mineral content. Soil and un-
26
Utah Geological Survey
Figure 15. Gully erosion in slope underlain by Chinle Formation
(photo: northeast valley margin).
Outlet generated by seepage
erosion after water percolates
from end of a macropore
Figure 16. (A) Piping erosion caused by water entering cracks as
expansive soils dry. (B) Clay particles are suspended and evaporites
dissolve in solution and move with subsurface water flow creating
void spaces. Outlets may not be obvious as water may carry sediment
a significant distance through a network of tunnels or into the larger,
regional groundwater flow (modified from Dunne, 1990).
consolidated rock with high shrink -swell potential are also
highly susceptible to piping and erosion. Clay shrinkage
cracks allow water to easily penetrate below the surface
(figure 16); as the soil hydrates and swells, the cracks can
close leaving subsurface voids that form near -surface sink-
holes and ground subsidence.
To evaluate piping and erosion (plate 11), we used three
main sources of data: (1) recent UGS geologic and hazard
mapping (Doelling and others, 2002; Hylland and Mulvey,
2003), (2) a geotechnical database compiled by the UGS,
and (3) the NRCS Soil Survey Geographic (SSURGO) Da-
tabase for Grand County Utah —Central Part (UT624);
Canyonlands Area, Utah —Parts of Grand and San Juan
Counties (UT633); and Arches National Park, Utah
(UT687) (NRCS, 2016a, 2016b, 2016c).
We classified soil and rock units into four categories based
on their potential for piping and erosion: highly susceptible
rock (HSr), highly susceptible soil (HSs), susceptible rock
(Sr), and susceptible soil (Ss). The presence of these units
in and of themselves does not create a piping and erosion
hazard. However, a change in conditions brought about
naturally or through human activity, such as cut -and -fill
construction techniques, can create the conditions neces-
sary for piping to occur. While susceptible to erosion, these
units are generally stable in their natural, undisturbed state,
but can quickly erode if disturbed or if surface -water drain-
age conditions change in an uncontrolled manner.
The piping -and -erosion -susceptibility categories shown
on plate 11 are approximate and mapped boundaries are
gradational. Localized areas of soil with higher or lower
piping and erosion susceptibility are likely to exist within
any given map area, but their identification is precluded
because of the generalized map scale and relatively sparse
data. All mapped susceptibility categories may potentially
exhibit piping and erosion; therefore, site -specific inves-
tigations should be performed at all locations to resolve
uncertainties inherent in the maps.
Wind -Blown Sand
In southeast Utah, there are significant amounts of loose
sand and seasonal winds that make wind-blown sand a vi-
able hazard in the Moab quadrangle. Even a few inches of
sand on a road can be very dangerous (Stipho, 1992). In
the Moab quadrangle, wind-blown sand can cause damage
to infrastructure, by burial and/or sandblasting effect, and
create unsafe driving conditions. Dust storms commonly
occur due to wind associated with the monsoonal season
that affects southern Utah and northern Arizona. The most
important factors for wind-blown sand hazard are (1) a
source of sand grains, (2) a source of wind at a threshold
speed, and (3) proximity to areas likely to be affected by
wind -blown -sand hazard.
Geologic hazards of the Moab quadrangle, Grand County, Utah
27
In desert regions, up to 98 percent of dry, non -cohesive sand
grains picked up by winds can travel up to 1 meter (3 feet)
above the surface (Stipho, 1992). Studies in other arid re-
gions (Stipho, 1992; Sherman and Nordstrom, 1994) indi-
cate the threshold shear velocity, or wind speed needed to
initially move a sand grain, is a function of the sand grain
diameter. According to these studies, a wind speed of 0.5
mph (0.2 m/s) is required to initiate movement of a 0.2 mm
sand grain, the standard sand size in NRCS reports (NRCS,
2016a, 20166, 2016c). To transport sand within 3 feet (I me-
ter) of the surface, wind speeds more than 9 mph (4 m/s) are
required (Sherman and Nordstrom, 1994).
The average annual wind speed (1998-2006) from the Moab -
Canyonlands airport, about 14 miles north ofthe City of Moab,
is 6.3 mph (2.8 m/s) (WRCC, 2006). Monitoring from 1992
to 2002 indicates that western wind directions are dominant
throughout the year, with the exception of June through Au-
gust when more southern and eastern winds prevail from sum-
mer monsoon storms (WRCC, 2006). The average monthly
wind speed for the period of 1996-2006 varied from 3.7 mph
(1.6 m/s) in December to 9.2 mph (4.1 m/s) in April (WRCC,
2006). Monthly average wind speeds indicate local transpor-
tation of sand grains is possible, given that the initiation wind
speed for sand grain movement is 0.5 mph (0.2 m/s). This may
indicate a more significant local hazard of wind-blown sand
for mapped eolian deposits close to infrastructure or develop-
ment. However, wind -blown -sand deposits that interfere with
infrastructure most often occur from storm -related winds,
which are typically above average wind speeds, and are the
most likely to cause more massive sand migration and adverse
road conditions for drivers.
To evaluate wind -blown -sand susceptibility (plate 12), we
used four main sources of data to identify areas where geolog-
ic and historical environmental conditions may contribute to
elevated wind -blown -sand hazard susceptibility: (1) percent-
age of soil sand (<0.2 mm grain size) data from the NRCS Soil
Survey Geographic (SSURGO) Database for Grand County
Utah —Central Part (UT624); Canyonlands Area, Utah —
Parts of Grand and San Juan Counties (UT633); and Arches
National Park, Utah (UT687) (NRCS, 2016a, 2016b, 2016c),
(2) historical wind -speed and direction data (WRCC, 2006;
National Renewable Energy Laboratory [NREL], 2012), (3)
UGS geologic and hazard mapping (Doelling and others,
2002; Hylland and Mulvey, 2003), and (4) studies from other
desert (Stipho, 1992; Lund and others, 2008) and coastal areas
(Sherman and Nordstrom, 1994). Using the geologic factors
contributing to the supply and distribution of potential wind-
blown sand, we classified soil and rock units into high, moder-
ate, and low susceptibility hazard categories.
The NRCS data were particularly useful because of the de-
tailed sand percentages available for the different soil map
units. However, it should be noted that these data are only
representative of the top 5 feet (1.5 m) of soil. Any removal
of soil for development or construction should be reassessed
for wind -blown -sand hazard. In coastal environments, wind-
blown sand hazard is often mitigated by stabilizing deposits
with vegetation (Sherman and Nordstrom, 1994), but this
may not be applicable in deserts where water is scarce and
should not be added to the surface. Armoring deposits with
appropriate sized and graded gravel, cobbles, and/or boul-
ders may be needed.
We evaluated sand source areas within and adjacent to the
Moab quadrangle. The highest contribution to sand in the
Moab quadrangle is from geologically young eolian de-
posits and areas that have been disturbed, including talus
slopes, landslides, and developed areas. A majority of this
sand is derived from the Navajo Sandstone. Soil units with
high concentrations of sand, as reported in NRCS (NRCS,
2016a, 2016b, and 2016c) and geologic -map data (Doelling
and others, 2002), include geologically young and modern
eolian and dry -alluvium deposits, which have greater than
50 percent sand (<0.2 mm sized particles). We mapped talus
slopes and sandy geologic units having a low quantity of
sandy soil (<50% sand with <0.2 mm sized particles), and up
to 30 percent of fines (0.125 mm [0.005 in] to 0.2 mm [0.008
in]) as moderate for wind -blown -sand susceptibility. Areas
having low soil sand quantity (<50%) and on mapped sand-
stones, which are mainly on the broad, flat Navajo Sandstone
exposed at higher elevations in the quadrangle, were classi-
fied as low susceptibility to wind-blown sand.
Although distal sources of sand were not mapped, there is
still a possibility that sand and dust could be transported
from regional active dune fields, dried playas in southern
Utah, and other areas of small, easily mobilized sediment. In
western Utah, dust from dried playas, agricultural lands, and
other barren and/or disturbed areas can contain bacteria, vi-
ruses, or fungi (Hahnenberger and Nicholl, 2014). Evaluat-
ing dust and related biological hazards was beyond the scope
of our mapping.
The wind -blown -sand -hazard susceptibility categories shown
on plate 12 are approximate and mapped boundaries are gra-
dational. Localized areas of higher or lower wind-blown sand
susceptibility are likely to exist within any given map area,
but their identification is precluded because of the general-
ized map scale, relatively sparse data, and non -geologic fac-
tors, such as variability in building infrastructure and design.
The use of imported fill for foundation material can also affect
wind -blown -sand susceptibility in small areas, because the
imported material may have different geologic characteristics
than native soil at the site.
Shallow Bedrock
Exposed bedrock is abundant in the Moab area. Less obvi-
ous are areas of shallow bedrock within the Moab —Spanish
Valley, where bedrock is overlain by a thin cover of young-
28
Utah Geological Survey
er unconsolidated deposits. Bedrock formations that are
not significantly fractured and are strong and stiff usually
have satisfactory bearing -capacity and settlement charac-
teristics; however, large loads may exceed the rock bear-
ing capacity and specialized rock mechanics engineering
will be required (Goodman, 1980). The principal prob-
lem related to shallow bedrock is difficulty of excavation,
particularly in highly resistant bedrock units, which often
require blasting. Shallow bedrock makes excavations for
basements, foundations, underground utilities, and road
cuts difficult, can cause areas of perched groundwater,
and create problems for wastewater disposal. Not ac-
counting for shallow bedrock in project design may lead
to excessive, unaccounted for construction cost, contract
change orders, and project delays.
To evaluate shallow -bedrock potential (plate 13), we used
four main sources of data: (1) UGS geologic and hazard
mapping (Doelling and others, 2002; Hylland and Mulvey,
2003), (2) the NRCS Soil Survey Geographic (SSURGO)
Database for Grand County Utah —Central Part (UT624);
Canyonlands Area, Utah Parts of Grand and San Juan
Counties (UT633); and Arches National Park, Utah
(UT687) (MRCS, 2016a, 2016b, 2016c), (3) a geotechni-
cal database compiled by the UGS, and (4) field mapping
and reconnaissance. We classified shallow bedrock as hard
or soft where exposed at the surface, and identified areas
of buried shallow bedrock (less than 10 feet [3 m] below
the surface).
We used UGS geologic and hazard mapping (Doelling and
others, 2002; Hylland and Mulvey, 2003) to identify areas
where bedrock is exposed at the ground surface, and qual-
itatively classified bedrock units based on geologic unit
descriptions. After identifying bedrock outcrops, we used
the restrictive layer data reported by the NRCS (2016a,
20166, and 2016c) soil survey to identify areas of poten-
tially shallow bedrock. The NRCS restrictive layer column
identifies areas where bedrock is found less than 6.5 feet
(2 m) below the surface.
We also used geotechnical borehole logs in the UGS
geotechnical database to help identify areas of shallow
bedrock. We compared the borehole logs with geologic
mapping, NRCS soils mapping, and geotechnical testing
information to confirm the existence of shallow bedrock
where it was identified by the NRCS and to identify other
potential areas of shallow bedrock. Correlations between
the borehole logs, geologic mapping, geotechnical data,
and NRCS information are generally good, but some local
discrepancies may exist.
The shallow -bedrock -potential categories shown on plate
13 are approximate and mapped boundaries are gradation-
al. Localized areas of shallow bedrock are likely to exist
within any given map area, but their identification is pre-
cluded because of the generalized map scale, relatively
sparse data, and limited subsurface data.
MAP LIMITATIONS
The geologic -hazard maps accompanying this report are de-
signed to provide geotechnical engineers, engineering geolo-
gists, design professionals, planners, building officials, devel-
opers, and the general public with information on the geologic
hazards that may affect existing and future development in
the Moab quadrangle. Information provided herein includes
the type and location of critical geologic hazards, and rec-
ommendations for site -specific investigations to confirm the
presence of, investigate in detail, and develop mitigation for
the hazards. Additionally, the maps can aid local governments
in developing geologic -hazard elements in their general land -
use plans for development, re -development, planning, regula-
tion, and design in Utah. We mapped 13 geologic hazards in
the Moab quadrangle; however, other hazards may exist that
could affect existing and future development.
We recommend performing site -specific geotechnical/geolog-
ic-hazard investigations for all development in the quadrangle
using the guidelines presented in UGS Circular 122 (Bowman
and Lund, 2016). Site -specific geotechnical/geologic-hazard
investigations can resolve uncertainties inherent in these gen-
eralized hazard maps and help ensure safety by identifying
the need for hazard mitigation and/or special construction
techniques. As with all maps, these geologic -hazard maps
have limitations. The maps are not for use at scales other than
1:24,000, and are not a substitute for site -specific geotechni-
cal/geologic-hazard investigations. The maps are based on
limited available geologic, geotechnical, and hydrologic data.
The quality of each map depends on the quality of the data,
which varies by hazard throughout the quadrangle. Conse-
quently, geologic hazard boundaries shown on the maps are
approximate and subject to change with additional informa-
tion. Small, localized areas of geologic hazards may exist in
the quadrangle, but their identification may be precluded due
to limitations of data availability and/or map scale.
ADDITIONAL INFORMATION AND
GUIDELINES
In addition to the information contained in this report, the UGS
Earthquakes and Geologic Hazards web page at https://geol-
ogy.utah.gov/hazards/ provides links to general information on
geologic hazards in Utah. The UGS web page for consultants
and design professionals (https://geology.utah.gov/about-us/
geologic-programs/geologic-hazards-program/for-consul-
tants-and-design-professionals/) provides links to recommend-
ed guidelines for geotechnicaUgeologic-hazard investigations
and reports, UGS geologic -hazard maps and reports, geologic
Geologic hazards of the Moab quadrangle, Grand County, Utah
29
maps, groundwater reports, historical aerial photography, and
other sources of useful information. The UGS advises following
the recommended guidelines when preparing site -specific engi-
neering -geologic reports and conducting site -specific geotechni-
cal/geologic-hazard investigations in Utah (Bowman and Lund,
2016). Typically, geologic -engineering and geologic -hazard
considerations would be combined in a single report or included
as part of a geotechnical report that also addresses site founda-
tion conditions and other geoengineering aspects of the project.
ACKNOWLEDGMENTS
We thank Phillip Bowman with the City of Moab for provid-
ing critical data for this project. We thank Bill Lund (UGS, re-
tired) and Tyler Knudsen (UGS) for their work in developing
the methods that this study incorporates. We also thank Steve
Bowman, Mike Hylland, Stephanie Carney, Greg McDonald,
Adam McKean, and Tyler Knudsen (UGS) for their thorough
review of the report and maps.
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