1. What is the depositional nature of the rocks in the study area and how does their lithologic compositions and sedimentary structures record changing water depths, environments of deposition, and paleogeography? Big bend national park has complex rocks that are subsided in two seas hundreds of millions of years ago. Fossil data from the mid to Upper Cretaceous period predicts the sequence of rock strata in Big Bend National Park to be from bottom to top: limestone, shale and clay, limestone again, and sandstone and clay.
The earliest layer was deposited in the relatively deep marine environment, then the ocean regressed to produce the next layer, then transgressed to produce the last. Different types of sediments are associated with varying depths of ocean water. Sediments accumulate over time and are deposited by water, air, or both. Santa Elena: It’s a time of mid cretaceous around 100my ago when the oldest rocks of study area start deposit. The Santa Elena formation is in the deep oceanic environment, this formation consists of the mid-Cretaceous rock types cherty limestone and marl.
The rocks exist in this formation are thick, cherty limestone mixed with marly limestone. The rocks in this some finely grained rocks due to organic material from shell and inorganic calcite. The formation also contains many small marine fossils from that period. The Santa Elena Formation has been cut through by the Rio Grande to form the Santa Elena Canyon. Del Rio: As the oceans regressed to a shallow ocean depth, approximately 97 mya, layers of shale and clay formed the Del Rio Clay. They contain remains of marine organism and animal fossils. The bedding of Del Rio formation with shale extends all directions.
The beds record the wet or dry season circle after the sediment is deposited. The lithological and paleontological data suggested that is was deposited in a shallow marine environment yellow green to yellow brown clay. Buda Formation: Buda is 95 my ago mid cretaceous, which consist of light colored, brittle limestone mixed with marl in deep ocean water environment. I observed some Buda in the bed of wash. The beds are clean and horizontal. The top of beds where the water don’t reach are covered with alluvium and soil. The shells, sand, and mud were deposited at the bottom of the ocean and solidified into rock.
The ocean change its depth to more depth due to transgression due to which formation of lime stone start happen a fossils present in this age. Boquillas Formation: The youngest formation was about 90my age of mid cretaceous period and consists of grey chalk mixed with flaggy limestone and marl with very thin inter bedded layer in deep oceanic water. To show its deposition information of the organism found in the cretaceous age, which were bivalves, we use straight column. The straight column shows the different thickness of each formation and all the fossils we found.
The multi-layered strata of the Boquillas Formation have been lifted into the ridge of Big Bend and have a gnarled form that erodes fairly easily. Boquillas formation is sub-divided into lower and upper boquillas. Lower boquillas is more limestone and upper boquillas is more siltstone. Typical Boquillas Outcrop | | Buda-Del Rio-Santa Elena contacts 2. What fossils are present in the formations mapped in the study area and how do they reflect changing water depths and environments of deposition? The fossils my group found in Big Bend are Ammonites, Gastropods, and Bivalve, Inoceramus.
The Ammonites are the most common fossils we found in field area. These creatures lived in the shallow water between 240 – 65 million years ago, and they moved through the water by jet propulsion expelling water through a funnel like opening to propel themselves in the opposite direction. Bivalves were found from the deepest depths of the oceans. They can burrow into the sediment or live on the ocean floor. Some can even move around through the water by snapping their shell open and shut to swim. According to the research, fossils can be found in Big Bend include ripple marks, fossil shells, and current structures.
There should be some dinosaur fossils found in shallow marine and non-marine deposits in Big Bend. Unfortunately, I didn’t see any in my field area. Beginning in the upper Cretaceous, among fish and shellfish fossils, mosasaur bones have been discovered. These specimens are found in the lowlands of the central part of the park. Most of fossils I found were in Buda formation. These are (respectively) limestone interbedded with shale; and clay with some thin sandstone interbeds, both of which are of the lower late Cretaceous. This is Inoceramus in Boquillas formation.
Ammonite 3. What type of folds are present in the study area, what is their orientation, and how do they compare with the regional pattern of folding? I saw several small folds over there, I draw two plunging anticline and one plunging syncline. The Dragger Mountain is an anticline plunging. The measurement of fold axial plan is N30W, 80NE The younger rocks are in the center and the older rocks are exposed on the top of the mountain. The folds were created during Quachtia and Laramide deformation. Some folds were caused by the same Laramide-age thrust fault.
Based on the notes, the direction of Santa Elena formation, Del Rio formation, Buda formation, and the Boquillas formation don’t orient horizontally. The dips I measured over there are in a range between 20-30 degrees. Anticline in the Boquillas I found this picture on online which shows really clear fold. 4. What is the nature of faulting in the study area (type, orientation, magnitude of displacement, and relative ages) and how does it compare to the regional pattern of faulting? In Big Bend field area, I saw two normal faults and one reverse fault. Normal fault is caused by tensional stress.
The faults over there occurred after the mountain building episodes that created the Rocky Mountain. Due to fault, the half areas of Dagger Mountain drop down eastern side of mountain. A lots stresses stretch the earth’s crust at that time, large rocks went downward along the active faults. The central mass of Big Bend National Park, including the Chisos Mountains, from the Sierra del Carmen to the east to the Mesa de Anguila to the west comprises such a block of rocks dropped downward by faulting. I found this pic online, which is normal fault in Big Bend. Reverse Fault 5.
What is the nature of magmatism in the study area, how did it form, and how does it correlate to other magmatic events in the region? Based on the notes what I took, the nature of magmaism is intrusive igneous. And the intrusion is younger than all the formations over there. This park was affected by volcanic activity 46 and 2 million years ago. At that time, due to the continued elevation of the region, alluvial fan was deposited. Big Bend’s desert landscape itself is a study in contrasts—mesas, mountains, and dikes formed by volcanic activity, limestone ridges and cliffs formed 100 – 200 million years ago when shallow seas covered the area.
Volcanic activity was not continuous during these eruptive cycles. Periods of hundreds of thousands or perhaps millions of years passed between eruptions. During the quiet interludes the forces of erosion carved new landscapes, many of which were destined to be buried under layers of ash and lava from later eruptions. When magma, cools and hardens below the surface of the earth it is call an igneous intrusion. The magma seeps or intrudes, into cracks and faults in the rock and never reaches the surface.
Eventually, over millions of years, the softer rocks will weather and erode away leaving the harder igneous rocks to remain exposed on the surface. Dikes and sills, and laccoliths are above the surface rock formations created by igneous intrusions millions of years ago. 6. Given all the information compiled above, what is the sequence of events recorded in the rocks and structures of the study area and what are the major tectonic events those have controlled these events? About 70 and 50 million yeas ago, mountain-building events created folds and faults in Big Bend. 6 and 28 million years ago, the park environment was affected by volcanic activity and produced ubiquitous igneous rocks. Later on, about 25 million years ago, the park was affected by the crustal extension of the Basin and Range province activity. Finally, due to erosion, all the formations and other canyons formed. The oldest rocks found in the Big Bend are only about 500 million years old. Known for its complex rock formations, Big Bend’s rocks were actually formed when two seas flowed into the area millions of years ago, leaving thick deposits of limestone and shale.
Volcanic activity helped create the land, especially the Chisos Mountains. According to the research from Kana college, it says the Laramide orogeny, which affected much of western North America between 70 and 50 million years ago as the North American and Farallon plates collided. This mountain-building event produced large folds and faults in Big Bend. Both can be seen on Persimmon Peak at the north entrance to the park. The region including the park continued to rise in altitude.