The document summarizes the geologic features and history observed in Big Bend National Park based on field mapping. During the Cretaceous period, limestone formations like the Santa Elena were deposited in the region as sea levels rose and fell. These formations were later folded and faulted during the Laramide Orogeny as the Farallon plate collided with North America. Normal faults and igneous intrusions were also observed that provided insight into the tectonic activity after deposition and folding of the layers. Analysis of the rock units, fossils, and structural features helps reconstruct the geologic changes in the area over the past 100 million years.

1. Jacob Williams
Geologic Field Methods
April 23, 2014

2. Big Bend Term Paper
Introduction
Big Bend National Park is home to many geologic features. In just our mapping area I
was able to observe normal faults, small scale synclines and anticlines, igneous intrusions, and
fossils. All of these features combined can be used to figure out the geologic history of the
region. You can tell how the sea level was changing or what tectonic activity was occurring.
During the Cretaceous the sea level underwent transgression and regression, causing the water
level to rise and deposit the limestone forming sediments in what is now west Texas. After the
deposition of the Santa Elena, Del Rio, Buda, and Boquillas formations, the rocks were then
folded by the Laramide Orogeny.
Tectonic setting
The northern tip of the Dagger Mountain area currently ranges from 2600 to 3400 feet
above sea level and is located ~430 miles to the west of the coastline of Texas at its minimum
distance. During the Lower Cretaceous the same area was under water due to the rise of sea level
allowing the limestone to be deposited (Figure 1). The increase in sea level was caused by
increased mid ocean ridge spreading and volcanism. The rise in sea level can be used to assume
there was little to no ice on Earth during this time. The change in rocks also tell the height of the
water. Rocks that are fine grained are deposited in deeper water and rocks that are clays or coarse
grained are deposited in shallow water.
The rock formations at Big Bend National Park were originally deposited in horizontal
layers but now they are folded and faulted. The cause of this is an orogenic event, specifically
the Laramide Orogeny. The Laramide Orogeny is the result of the Farallon Plate colliding with

3. the North American Plate (Liu et al 2010). The Farallon Plate was forced under the North
American plate which would thicken the crust causing it to rise in elevation due to isostasy.
There are also two normal faults near the northern tip of dagger mountain in between the two
formations of Santa Elena (Figure 3. These faults were formed by a divergent boundary where
two landmasses are pulled apart. This resulted in a range and basin faulting, where the land mass
in the middle sunk down in between the two faults (Figure 2).
During an Orogenic event magmatic activity is likely to happen. In Big Bend I observed
three igneous sills exposed at the surface in the Boquillas formation. These occurred after the
deposition of the rocks and have been dated at 64-17 Ma, making these igneous intrusions
younger than the folding (Cullen et al 2013).
Rock Units
The majority of the rocks in west Texas are Cretaceous Limestone deposited around 100
million years ago. The oldest layer of sedimentary rocks at the surface, is the Santa Elena
formation. The Santa Elena is a micritic limestone that is resilient to weathering and is abundant
with bivalve fossils. These stronger fossils and abundance of calcite(Figure 6) likely helped the
Santa Elena to form massive cliffs. The Santa Elena is brown/gray in color and display tear pants
weathering. It is 750 feet in thickness and was formed during the time when the sea level was
higher in the Early Cretaceous (Cullen et al 2013). The micritic cherty limestone it is today and
the types of bivalve fossils tell us that the Santa Elena was deposited when the sea level was
raised, 100 Ma. Stratigraphically the Santa Elena is older than the Del Rio Clay.
The Del Rio is a clay that was formed after the Santa Elena when the sea level regressed
97 million years ago (Henry et al 1998). It is reddish brown in color and is the host of many
fossils belonging to the foraminiferal genus (Figure 4). The Del Rio Clay is very easily

4. weathered and valleys are usually located where outcrops of Del Rio used to be, like between the
Buda Limestone and the Santa Elena Limestone. After the Del Rio was deposited the sea level
rose and lead to the deposition of the Buda Limestone.
The Buda Limestones micritic texture tells us it was formed after the sea level rose again
95 Ma. It is resistant to weathering and displays tear pants weathering like the Santa Elena but
does not form large cliffs. It is gray in color and is home to Scleractinian Fossils. After the Buda
Formation was deposited the sea level rose once more which allowed the formation of the
Boquillas limestone.
The Boquillas Limestone is a flaggy limestone that shows laminated bedding. It was
deposited in deeper water than the Buda Limestone 90 Ma. It is not as resistant to weathering as
the Buda and Santa Elena. Inoceramus fossils can be found in the buda (Figure 5). The Boquillas
is white/tan in color and has a micritic texture.
The Dagger Mountain area is also home to Cenozoic aged Igneous sills. The Laramide
Orogeny is likely the cause of these intrusions. These sills are made up of mafic minerals with an
aphanitic texture which means they are classified as a Basalt (Figure 7). Near the surface the
basalt was subjected to weathering, which made it break apart easily. The basalt found at lower
depths was much stronger. Isotopic dating of the basalt has it ranging from 64-17 Ma. (Cullen et
al 2013)
When rock fragments break off due to weathering or gravity, they become alluvium. The
alluvium deposits found in Big Bend are Quaternary aged. The Boquillas Limestone makes up
most of the alluvium, as well as pieces Santa Elena, Buda, Del Rio, Fossils, and Calcite.
Alluvium deposits are a good indication of where a rock layer began, like the Del Rio Clay. The
only evidence left, on the surface, of the clay is in the form of alluvium deposits. As you walk up

5. the slope you can tell where the Del Rio used to be by the rock fragments. Alluvium can give us
a recent weathering history of the rocks by the sorting, shape, and size. Large pieces of alluvium
that have traveled a long distance would be evidence of a high energy form of travel, like a
glacier or a fast moving river.
Structural Geology
According to the law of original horizontality, all sedimentary rocks are deposited in
horizontal layers and then folded later on due to compression or decompression of the plates or
igneous intrusions. The bed layers of the rock formations were all dipping to the west and ranged
from a 5° dip to a 57° dip. This means at some point there was tectonic activity going on that
caused the bed layers to deform after they had been deposited. There were two types of folds I
observed in the field, a small scale anticline and syncline (Figure 8). The anticline was found
adjacent to the igneous intrusion, which could be the cause for the folding. The syncline was
found between the two normal faults, which could be the cause for that since it is such a steep
syncline.
There were two major faults in the Dagger Mountain area. Bother were two normal faults
located in the north eastern part of our mapping area (Figure 3). The western fault is dipping in
the east direction and the eastern fault is dipping in the west direction (Figure 2). This type of
faulting is known as a Graben, which is german for grave. The Graben section of the fault is the
hanging wall and the Horst part is located on the footwall (Figure 2). The normal faults were
caused by the diverging plates.
Discussion

6. At the beginning of the Cretaceous 138 Ma., the western part of Texas was above sea
level. Shortly after the sea level began to rise due to mid ocean ridge activity and warming of the
planet. The Santa Elena formation was deposited ~100 Ma. after the sea level had risen far above
what it is currently. Marine life inhabited the reefs formed by the cherty micritic limestone and
their fossils can still be found preserved in the Santa Elena. The water underwent regression ~97
Ma. leading to the deposition of the Del Rio Clay. This was not a reef building limestone like the
Santa Elena, so the fossils content changed to Foraminifera, a calcareous shelled fossil that is
responsible for most of the calcium carbonate found in the Del Rio. ~95 Ma. the sea level
transgressed once more and began the deposition of the Buda Limestone. The Buda, also being a
reef former, contains many fossils. ~90 Ma. as the sea level continues to rise the Boquillas was
formed and was home to Inoceramus, a bivalve fossil. After the ocean regressed the Laramide
Orogenic event occurred and folded the rock formations as the Farallon Plate subducted
underneath the North American Plate. This is likely the cause of the igneous sills that can be
found intruding the Boquillas Limestone. The faulting likely occurred after the Laramide
Orogeny since the rocks were already folded when they fell down. The features we observe
today can tell us an abundance of information of what happened hundreds of million years ago
and are key to figuring out what will happen in the future.

7. Figure 1: The sea level increased during the Cretaceous due to an increase in mid ocean ridge
activity and warming of the earth.
100Ma 75Ma
Figure 2: This illustrates the folding and faulting caused by the Laramide Orogeny.

8. Figure 3: The boxed area indicates the location of the normal faults caused by the Laramide
Orogeny.
Figure 4: Foraminifera fossils found in the Del Rio Clay
Figure 5: Relatively unaltered Inoceramus fossil preserved in the Boquillas Limestone. You can
also see the laminated bedding.
.

9. Figure 6: Calcite found in the Santa Elena Limestone showing perfect cleavage.
Figure 7: Basalt
Figure 8: Anticline and Syncline.
References

10. Liu, L., Gurnis, M., Seton, M., Saleeby, J., Muller, D., Jackson, J. 2010, The role of oceanic
plateau subduction in the Laramide orogeny, Nature Geoscience, Vol. 3, Issue 5, pg 353-357
Cullen, J., Knox, N. K., Crouch, J., Satterfield, J. L., 2013 Polyphase Laramide Structures and
Possible Folded Tertiary(?) Sills at Dagger mountain, Big Bend National Park, Texas, The
Compass: Earth Science Journal of Sigma Gamma Epsilon, Vol. 85, Issue 3, Article 3
Henry, C.D., Tyler, N., 1998, Geology of Big Bend Ranch State Park, Texas: Texas Parks and
Wildlife Press, 72 pp.