The Indians said that after making the Earth, the Great
Spirit simply dumped all the leftover rocks on the Big Bend.
Big Bend Brochure (NPS)
570 MYA - 245 MYA
North America is covered by a shallow ocean belt that
trends northwest-southeast from the Gulf of Mexico to the Pacific Northwest.
In Big Bend, the sediments deposited during the Cambrian were primarily
sandstones, limestones, and dolomites. Marine life included trilobites,
brachiopods, bryozoans, snails, clams, and sponges. The Cambrian Period
marks the first time that multicellular organisms are known to have existed,
but none of these organisms (or the rocks containing them) are presently
exposed at the land surface.
Ordovician, Silurian, Devonian, Mississippian --
Shallow seas continue to cover the Big Bend region. Limestone and chert
beds were deposited, some of which contain brachiopod, coral, cephalopod,
and gastropod fossils. The numbers and diversity of corals increase
dramatically, and the first land plants appear.
Big Bend is still covered in shallow seas, but the
character of sedimentation changes slightly to include shales (in addition
to chert, limestone, and sandstone deposition).
The Devonian Period is
known as the "Age of Amphibians" since this is when the first amphibians
evolved. Marine life also showed great leaps in evolution, with the
development of more complex and numerous species of fish. Occasionally
the seas receded somewhat, and land plants thrived in a marshlands environment
-- the most common plants were ferns, horsetail rushes, and huge decidious
trees. Fern forests and amphibians thrived on land, and the seas (which
still covered portions of Big Bend country) were filled with a
multitude of developing organisms.
Pennsylvanian -- During most of the
Pennsylvanian, Big Bend was covered by shallow seas. However,
towards the end of the Pennsylvanian Period (approximately 300 MYA), the
cataclysmic collision of North America with South American and Africa
produced the Ouachita Mountain chain that ranges across the eastern seaboard
and the Gulf states. In southwest Texas, the collision caused the entire Big
Bend region to rise above sea level, exposing much of it to a land environment
for the very first time. In the modern Big Bend region, remnants of the
Ouachita Mountains are visible only at Persimmon Gap at the northern
entrance to the park. Land organisms thrived during the Pennsylvanian, The Age of Reptiles.
Permian -- The Permian Period left no geologic
record in Big Bend. We know from studies in other parts of the world that
precursors to the dinosaurs were developing during the Permian, but Big
Bend is missing the strata (rock layers) from this time period. A
gap such as this one in the geologic record is known as an unconformity.
245 MYA - 66 MYA
|Triassic, Jurassic -- There is no record of Triassic/Jurassic aged rocks in Big Bend National Park. When there appears to be a gap in the rock record, it is called an unconformity.|
Cretaceous (early) --
During the early Cretaceous, the Big Bend area was covered by ocean.
Ammonites, mollusks, and other shellfish are predominantly found amongst
the sandstones and limestones deposited during this time period. During
the beginning of the Triassic, a thick layer of limestone known as the
Glenrose formation was deposited in a deep sea environment. Following the
deposition of the Glenrose formation, a very thin (3 meter) layer of
sandstone known as the Maxon formation was deposited in a shallow sea
environment. Because the sediments show that the ocean was first
fairly deep then less deep, we can infer that the ocean was receding
toward the end of the early Cretaceous.
|Mid-Cretaceous -- For most of the mid-Cretaceous, Big Bend was covered by ocean. The beginning of the mid-Cretaceous in the park is characterized by thick (750-850 ft) beds of cherty limestone, the Santa Elena Canyon formation. The next layers of rocks deposited in the park indicate that the ocean levels were receding, although small transgressive/regressive cycles can be found. Later on, it is evident from coal beds, mudstone, and thick layers of continental clay that the Big Bend area had a swampy environment. Dispersed throughout the clay and mudstone layers are dinosaur fossils.|
|Late Cretaceous -- The late Cretaceous was a time of uplift in which the ocean receded. Vast swampland with dense vegetation marked the beginning of the of the late Cretaceous, depositing continental clays and with it dinosaur fossils. The Javelina Formation is one such layer of clay and fossils. It was during this time also that flowering plants first grew in the park. The end of the Cretaceous is marked by the extinction of all dinosaurs.|
66 MYA - Present
Paleocene -- The Paleocene epoch was the calm before
the volcanic storm struck. The oceans receded to close to where they presently
are. Terrestrial vegetation continued to evolve, and mammals appeared for
the first time. Sandstone and
clay with thin interbedded layers of conglomerate are deposited throughout the park during this
Eocene -- The beginning of the Eocene epoch is marked with strata
very similar to that which was deposited in the Paleocene. However, this peaceful landscape
was shortly transformed into an ash covered wasteland after a major
volcanic eruption in the
area. After all the ash and debris settled, rain and erosion occurred, leaving behind tuffaceous
sandstones and tuffaceous clays. These layers are now known as the
Toward the end of the Eocene another volcanic eruption marked the Big Bend region, throwing ash and pouring thick layers of lava over the landscape. Most of the extrusive igneous features seen in the park were formed at this time. These lava and ash flows coupled with their sedimentary counterparts are known as the Chisos formation and can be found scattered throughout the central and south sides of the park.
Oligocene -- The Oligocene marks another violent period of time in
the history of Big Bend. During this time, yet another volcanic episode occurred. The resulting
strata, the South Rim formation is a massive lava flow combined with ash beds, some sandstone,
and conglomerate. The South Rim formation is exposed on the top of the Chisos mountains and is
also present atop Burro Mesa.
Miocene, Pliocene, Pleistocene, Holocene (Quaternary Period) -- Since the formation
of the South Rim Formation, the only depositional features that are really worth noting are alluvial features.
Talus slopes formed and are continuing to form around the massive and resistant volcanic features
of the park. Loads of sand and silt are carried from areas of high elevation to lower elevations
by rainfall and are eventually swept to the Gulf of Mexico by way of the Rio Grande. Desert
vegetation and sparse grasslands now cover the Big Bend area. Bears, javelina, rabbits, coyotes,
and tourists inhabit the Big Bend.
Big Bend's geologic history is quite unique, a mixture of placid ocean deposition, murky swamps with large reptiles, and violent earth-shaking volcanic eruptions. Now that you know a bit about the geologic history of the park, visit the links on the image-map below to learn more about the geologic processes that occurred throughout the history of the Big Bend.