Geology 345
Lake Mead Field Report
T. Saylor IV
The structural geology class of 2012 took a field trip to Lake Mead, Nevada and studied the
geology and historical stress states of the surrounding area. The trip took us to a complex area
which has experienced multiple periods of tectonic activity and includes influence from the Sevier
orogeny, the Laramide orogeny and currently, the spreading of the Great Basin and Range. In order
to better understand how tectonics helped to create this varied topography, the class was
introduced to different geological structures at several separate locations. These include the
Rogers Spring strike-slip fault, the Pinto Ridge strike-slip fault, a section of the Lovell Wash and the
Valley of Fire State Park. Each of these stops had different types of structural features which
formed due to different states of stress, different stages of deformation and varying orientations of
the major stress fields. To say that this area is geologically complex would be an understatement.
The first part of the Lake Mead story begins with the geologic units found in this section of
the basin. The oldest unit seen in the field was the Mississippian Monte Cristo, a dark limestone
which was deposited in a marine environment. Above it is the Pennsylvanian Bird Spring
Formation, another marine limestone. The Pennsylvanian ended with the deposition of the Red
Beds / Hermit Shale. Sitting on top of that is the Permian aged Kaibab and Toroweap Limestone.
These two layers, which in other areas are uniquely separate, are undifferentiated in the Lake Mead
basin. The Triassic has two units to itself; the first being the Moenkopi Formation and then the
Chinle formation. Possibly the most noticeable unit in the Lake Mead area is the Aztec sandstone,
a counterpart to the famous Navajo sandstone of Colorado and Utah. Above the Aztec lays the
Cretaceous Baseline Sandstone. The last unit seen is the Tertiary Horst Spring formation, which is
comprised of the Rainbow Gardens, Thumb, Bitter Ridge Limestone and Lovell Wash members. All
of these units have been folded and faulted, deformed and thrust all over the place and have
created a gorgeous landscape of painted deserts, huge mountains and impressive valleys.
The core of structural field work can be broken into three sections: geometry, kinematics,
and mechanics. The first thing that needs to be done when interpreting structural features is to
describe their form. What type of feature is it? What is its strike and dip? What is the size and
magnitude of the feature? Are there any specific characteristics of the feature? Utmost detail
needs to be used when describing a specific features geometry. It is important to accurately
describe a feature seen in the field because this forms a basis for later interpretation.
When those questions have been answered, it is time to look for indications of kinematics,
or in other words, how did the feature move? This can be done in several ways: you can look for
displacements along its length. When looking at a fault, is there any indication of how much
movement occurred? In what direction did it move? Other things a geologist can look at are shears.
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Shears indicate sense of motion and are broken up into several categories, but the main ones seen
in the studied field areas include Riedel, conjugate Riedel and P-shears. These form in an en
echelon manner, where a left stepping pattern indicates dextral motion and a right stepping
pattern indicates sinistral motion. Folds can also have en echelon geometry but when a right
stepping fold system is seen, it indicates right lateral motion, and vice-a-versa for left stepping.
Thrust faults and normal faults can also occur in this en echelon pattern. Horsetail cracks represent
relative motion as well. A joint with tail cracks oriented clockwise off of it mean that the relative
motion the rock experienced was right lateral. Counter-clockwise tail cracks are indicative of left
lateral motion. An interesting side note, large strike-slip faults can have subsidiary normal faults
form on their flanks which when seen from a regional scale look like large tail cracks. They form
due to the surrounding strata relieving pressure which the main fault (or joint) has imposed. A large
scale normal fault tail crack system can be seen at the Pinto Ridge field location. Some of the best
kinematic indicators are slickenlines. These features form on the plane of a fault as a result of
grains crushing against each other during motion. They leave grooves in the surface which show the
exact sense of motion which the fault has had. They do not however show how much displacement
has occurred. In order to figure that out specific piercing points, such as matching beds or fold axes
need to be positively identified.
One of the main focuses of the field trip was to locate, describe and interpret faults and
fault systems. We looked at a variety of fault styles, including strike-slip, normal and thrust. Each of
these formed due to different tectonic activities. The thrust faults we saw were formed due to the
Sevier orogeny, which took rocks from the Cambrian age up to the Monte Cristo, Bird Spring and
Kaibab / Toroweap formations and thrust them over the Aztec sandstone. This motion occurred
during the Cretaceous when the accretion of terranes from the West pushed the land Eastward.
This thrust, known as the Muddy Mountain thrust created an impressive mountain chain. Later,
during the Miocene, the basin and range system seen today began to form. This created large
strike-slip faults and subsidiary normal faults which proceeded to chop up the landscape creating
alternating mountain ranges and valleys.
Faults form due to applied stresses. New faults may form in response to tectonic activity
and utilize a pre-existing fault already located at that site. This means that a strike-slip fault may
form where there once was a normal fault. This would occur due to changes in the stress
orientations. For a normal fault σ1 would have originally been vertical to the plane and σ3
perpendicular horizontally, but as the stress state changed, σ1 would need to form at an angle of
near 30° horizontal to the fault plane with σ3 90° off of that (still horizontally) for it to be
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reactivated as a strike-slip fault. Fault growth can also be influenced through joint systems which
already exist in beds. As a fault grows, it can mechanically feel the gaps made by the joints. Since it
is easier for a fault to grow through an opening that is already there rather than break through solid
rock, it will utilize these pre-existing features.
Faults are great fluid controls. They can either be a super highway or they can impede fluid
flow completely. Springs can form on the edges of faults, creating oasis’ in arid areas such as
Nevada. The water eventually finds an outlet, and localized areas grow large amounts of
vegetation. The first major fault the class conducted research at was the Roberts Spring fault,
which created an oasis of its own. Roberts Spring is a massive sinistral oblique slip fault, which is
the dominate form of strike-slip motion that faults in the Southern Nevadan basin have
experienced. The fault strikes at 090° and has a dip of 66°. Slickenlines were apparent on a
section of the exposed fault plane (Figure 1) and had a pitch of 80°east which is indicative of
largely dip-slip rather than strike-slip motion. For strike-slip motion to occur along a fault, an
optimal stress field with a σ1 oriented 30° to the fault plane would need to be present. This means
that σ1 would need to be oriented approximately NE-SW. Later on in our field work, we would find
that a stress field with this basic orientation holds true for most of the basins structural features,
but not all. The Rogers Spring fault bisected a large section of Mississippian Monte Cristo, but
towards the Western edge, a large section of Quaternary Alluvium was situated next to it. When
one looked closely at the Alluvium, a deformation band could be seen. If a fault strikes through a
bed which is currently in a state of deposition, and has a deformation band oriented similarly to it
inside of its lithologic section, then one can assume that the fault has been active since the most
recent period of its deposition. In the case of the Roberts Spring fault, this would be during the
Quaternary which makes it a currently active fault.
The topographic feature known as Pinto Ridge was created due to a large fault striking at
222° with a dip of 90°. This fault, named the Pinto Ridge fault, is made of Permian Kaibab a
heavily silicified layer with large chert nodules all along its beds. When the fault was examined
closely from a map view, rather than a cross-sectional view, Riedel and P shears could be seen.
Their orientations were at roughly 025° and 232°, respectively. With their right stepping sense of
geometry, a history of left-lateral motion could be assumed. Pinto Ridge is a large anticlinal fold
with the beds folded on each side of the fault. This means that folding occurred first, and was later
followed by faulting. With the age of the folded beds, an approximate age on the folding event can
be determined. Since the youngest unit seen is the Moenkopi, which is of Jurassic age, then folding
must have occurred after its deposition. This means that folding is post-Jurassic and with the
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knowledge that spreading occurred in the Miocene (Dubendorfer et al, 1994), the fault must be of
similar age, but older that the folding. When looked at from a regional scale, numerous other
faults can be seen striking in a clockwise manner off of the central Pinto Ridge fault. These are all
normal faults which formed in a horsetail manner with the relieving of pressure within the rocks.
Our class ventured into one of these normal faults to look closer. It was on the South Eastern side
of the Pinto Ridge fault and striked at 175° and had a dip of ~ 50° across its length. With Pinto
Ridge striking at 222°and being sinistral in its motion, then an approximate orientation for σ 1 and
σ3 can be determined. Since σ1 needs to be at an angle of near 30°to a fault plane in order for it to
slip, then the shear system which created Pinto Ridge would lie about NE to SW for σ1 and NW to
SE for σ3.
A third major fault system which the class studied was the large Las Vegas Valley Shear
Zone. To get there, we hiked through a section of the Lovell Wash on a day remembered as el
marzo de muerte by some. Before reaching the main section of the shear zone, we mapped out
numerous faults all trending in an East to West manner. Most of them turned out to be sinistral
strike-slip faults, but a few normal faults with apparent oblique motion were found. Kinematic
indicators such as Riedel shears informed us that the most recent period of motion has been leftlateral (Figure 2). However, these shears were crosscut by longer left stepping shears. Using the
rules of crosscutting relations, the longer shears had to have occurred first, followed by the shorter
shears beginning and ending inside of them. This means that our faults had to have two separate
periods of motion, the first being right-lateral, and the second being left-lateral. This story was
found all along Lovell Wash in a string of faults. When the class reached the Las Vegas Valley Shear
Zone, we studied the outcrop on the Southern side of the fault. Although it was heavily weathered
Moenkopi limestone, a shear sense could still be discerned in the outcrop. Again, it was found that
long left stepping Riedels cross cut short right-stepping Riedels. If there are two separate motion
events, then the stress orientations have changed at least twice. This means that an older paleostress was oriented at NW to SE for σ1 and NE to SW for σ3, which allowed the fault to move
dextrally. At a later point in time, σ1 and σ3 swapped and allowed for left lateral motion. This new
σ1 orientation at NE to SW is the same as it was for the creation of the Pinto Ridge fault. Pinto
Ridge possibly developed around the same time the Las Vegas Valley Shear Zone began moving
sinistrally but only after it had finished moving dextrally.
While mapping in the Lovell Wash area, several interesting features which were not faults
were noticed. The first interesting thing was a section of an old stream bed which had ripples along
its length. The ripples were asymmetric and show the direction of flow. By using stereographic
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projection (attached) where the tilted bed is back rotated to horizontal, the direction of the original
stream flow was ascertained. The stream is located in the Thumb member and in that area strikes
at 317° with a dip of 79°. The current ripple direction was marked 55° off of West along the
bedding plane. When the bed was back rotated to horizontal, a paleo-current direction was found of
012° NE. Along the length of a small thrust fault found higher up in the Wash, a fault propagation
fold could be seen on the hanging wall while a fault bend fold had formed on the footwall. The
folding could be seen in a dark green shale layer. It is uncommon but not unheard of for these
features to form in tandem, but it would be more likely to have occurred in the opposite sense, or
on the other wall. At the end of the Lovell Wash, several layers of parasitic folds were found within
a larger fold. Another stereographic projection found their original axes orientation before they had
been tilted.
The final studied area was the Valley of Fire State Park. The entire park is made up of the
gorgeous aeolian Aztec sandstone. Due to the nature of its surrounding stress field, the rocks are
all massively deformed. Numerous structures can be seen in the park, including plumose and
growth lines on exposed joint surfaces (Figure 3). If one looks at specific outcrops, compaction
bands can be seen. There is a large amount of these compaction bands, each with their particular
wavy geometry. At one outcrop two main sets of nearly parallel bands can be seen. The oldest
bands formed at approximately 165° - 170° and crosscut a younger set oriented at 020° - 040°.
These two band trends create a conjugate set in map view. A feature not commonly found in rocks,
but everywhere in the Aztec of the Valley of Fire is a shear-enhanced compaction band. Different
from normal compaction bands, these have a zigzagging pattern which occurs when there is an
almost equal component of shear as well as compaction (Eichhubl et al, 2009). Compaction bands
also commonly form in the Navajo sandstone, the Aztec’s counterpart in Arizona, Utah and
Colorado. Compaction bands are excellent aquatards. Understanding in which orientation these
bands form can help us interpret how fluid flow is controlled in the rocks. Since a compaction band
is impermeable due to the grain cataclasis which creates it, liquids will flow along their trends
(Fossen et al, 2011). At one outcrop in the Valley of Fire, fluid was seen slowly seeping through the
rocks on the outer edges of several bands.
There are a number of large strike-slip faults within the Valley of Fire. The class spent some
time at one fault system and endeavored to interpret its geometry, motion, and past movement
history (Figure 4). A large amount of gouge was present in the basin of one of the faults. It was fine
and powdery which implies a large amount of motion occurred along its length. This particular fault
had Riedel evidence for left lateral motion and trended N-NE. A second fault was connected to the
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first one and had shear evidence for right-lateral motion. It trended N-NW. The angle between the
two faults was approximately 60°and defined a conjugate set for a specific stress state. In this
case, σ1 trended at 335° but later rotated about 50°to create subsidiary horsetail cracks which
could be seen in the outcrop. Our field interpretation agrees with the research that Flodin and Aydin
conducted on the Valley of Fire in 2004. This trend of a North Eastern σ1 follows the same stress
state as the Pinto Ridge and Rogers Spring faults and places their age in a similar time frame. In
the case of these faults, this puts them sometime after the Sevier orogeny but before the Miocene
aged tectonics. Overall, the story told by the faults keeps this stress orientation for the most recent
strike-slip motion in the Lake Mead region.
The field trip to Lake Mead hammered in the processes of structural field work. Describing
geometry, locating kinematic indicators, and interpreting the mechanics of an area lead to a better
understanding of what happened in the past. Getting up close and personal with structural features
outside is the best way to learn about them. The region of land around Lake Mead has had an
incredible tectonic influence. Features of all shapes and sizes abound in the rocks which make it an
excellent location to learn the basics of structural geology. Set all of that information in a scenic
locale close to sin city and you have a perfect classroom.
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Figure 1: The exposed fault plane at Rogers Spring fault. Note the
slickenlines which show movement direction. Photo by Simon
Kattenhorn
Figure 2: A fault plane from the Lovell Wash. Students are
locating kinematic indicators. Photo by Simon Kattenhorn
Figure 4: A large left lateral fault with high amounts of
gouge in its center. Photo by Simon Kattenhorn
Figure 3: Plumose features and growth lines shown nicely on an
exposed joint surface. Photo by Simon Kattenhorn
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References
• Dubendorfer, Ernest M., and David A. Simpson. "Kinematics and Timing of Extension in
the Western Lake Mead Region, Nevada." Geological Society of America Bulletin(1994):
1057-073. Print.
•Eichhubl, Peter, John N. Hooker, and Stephen E. Laubach. "Pure and Shear-enhanced
Compaction Bands in Aztec Sandstone." Journal of Structural Geology 32.12 (2010): 1873886. Print.
• Flodin, Eric A., and Atilla Aydin. "Evolution of a Strike-slip Fault Network, Valley of Fire
State Park, Southern Nevada." Geological Society of America Bulletin 116 (2004): 42-59.
Print.
• Fossen, Haakon, Richard A. Schultz, and Anita Torabi. "Conditions and Implications for
Compaction Band Formation in the Navajo Sandstone, Utah." Journal of Structural Geology
33.10 (2011): 1477-490. Print.
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