The landscape beneath the Chihuahuan Desert in the Big Bend National Park region has a long and varied geologic history. The description below is a brief overview of the large-scale events that affected the development of this unique landscape. But first:
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Click here to see a general stratigraphy chart for Big Bend National Park.
The Ouachita Orogeny
The park's oldest rocks are highly deformed sedimentary rocks of late Paleozoic age; the formed more than about 300 million years ago. These rocks are exposed only in the northeast corner of the park in the vicinity of Persimmon Gap and Dog Canyon , but they also occur deep in the subsurface throughout much of the park region. These rocks formed from sediments deposited an ancient ocean basin, but were thrust upward into part of an extensive mountain system late in the Paleozoic Era. This mountain building period was called the Ouachita Orogeny. This orogeny is associated with a period when the ancient South American Plate collided with North America, about 280 to 260 million years ago. The Ouachita mountain belt extended from the Southern Appalachian region across the Mississippi Valley region, all the way across Texas into the Big Bend Region, and southward into Mexico . This all happened before the Modern Gulf of Mexico began to form when the two tectonic plates split apart again (to become North and South America). After the climax of the orogeny, the mountain system gradually eroded away, and after many millions of years the Big Bend region was eventually buried by marine sediments during the Permian Period and again later in the Mesozoic Era.
The Permian Basin
Ocean waters once again flooded into a basin that developed as the South American continental plate split away from the North American Plate in the late Paleozoic era. In the West Texas region, great barrier reefs developed along the margins of an inlands seaway. The basin existed through middle to late Permian time, about 280 to 250 million years ago. The basin eventually filled with sediments and salt (evaporites) and was buried beneath terrestrial "red bed" sediments during the Triassic Period. The Permian Basin is host to major to major oil and gas fields in the West Texas region called the Permian Basin. However, rocks of Permian and Triassic age are not preserved in the Big Bend National Park area; but they occur through the region north of the park extending northward into New Mexico. In the park area Permian-age sediments may never have been deposited or preserved, or were stripped away by erosion, or may possibly be locally buried by younger sediments.
The Western Interior Seaway
Starting in Early Cretaceous time, about 140 million years ago, the surface of the land throughout many of the western states sank below sea level, as a shallow seaway flooded onto the continent. At its maximum extent, this broad shallow seaway extended across much of North America from the Gulf of Mexico all the way to Alaska, and was as wide as the distance between Nevada and Minnesota. The Western Interior Seaway, as it is known, existed for nearly 80 million years. It was bounded on the east by continental lowlands in the Mississippi Valley region, and to the west by the Cordilleran mountain belt along the Pacific side of the continent. During its history sea level rose and fell and the shorelines advanced and retreated along the margins of the seaway. Evidence of this ancient seaway system can be seen in the massive sections of fossiliferous limestone and shale rock formations of Early to Late Cretaceous age exposed throughout Big Bend National Park, such as those exposed in the massive cliffs of Santa Elena Canyon, and in Boquillas Canyon and throughout the Sierra Del Carmen.
The Laramide Orogeny
Beginning about 70 million years ago, the Rocky Mountain region began to experience major tectonic upheaval. Large mountain belts began to rise. Likewise, intervening basins began to sink and fill with sediments derived from the rising ranges. These changes began along the western margin of the Western Interior Seaway and gradually spread eastward. As this mountain orogeny proceeded, the seaway gradually retreated eastward and eventually completely vanished at about the same time that dinosaurs went extinct around 65 million years ago. In the Big Bend region, large faults and tectonic folds gradually deformed the Earth's crust beneath the Big Bend landscape. Some of the geologic features that exist today in the Big Bend region developed, or began to develop, in the Laramide orogenic period that extended into the early to middle Tertiary (middle Eocene, about 45 million years ago). Deformation from Laramide Orogeny was not as intense as other regions in the Rocky Mountains. The great folds (anticlines) associated with the Sierra Del Carmen, Mariscal Mountain, and Sierra San Vicente are examples of Laramide structures. Most Laramide structures were deformed later tectonic events, or subsequent burial by volcanic and sedimentary rocks.
Middle Tertiary Volcanism
Beginning about 45 million years ago and lasting for about 20 million years, the Big Bend region experienced massive periods of volcanism. An ancient chain of volcanoes eventually extended across the Big Bend region from Mexico northward into New Mexico and beyond. Eruptive activity was episodic and included both massive eruptions that blanketed the landscape with volcanic ash and debris, and periods when lava flow poured over large sections of the landscape. Between eruptive periods, streams eroded the upland areas and deposited reworked volcanic sediments on broad floodplains. Eventually this period of volcanism ended, and the ancient volcanoes eroded away. The Chiso Mountains and the many other features of volcanic origin in the Big Bend region are only remnants of the ancient volcanic mountain chain that once existed.
Basin and Range Tectonics
Around 20 million years ago, a new period of tectonic activity began that even continues to influence the geologic activity of the region today. Due to changes in plate tectonic motion along the western edge of the North American continent, the region in the western states began began to gradually pull apart in a process called crustal extension. Basically, the California region began to gradually detach and migrate north and west away from the core of the North American continent in the central states and Mexico. This has resulted in the development of large fault-bounded blocks in the upper crust that have slipped and rotated, forming the long, narrow mountain ranges, with intervening basins, that define the landscape throughout the Great Basin Region (extending from Nevada, across southern Arizona and New Mexico, and into West Texas and Mexico, including the Big Bend region). In addition to block faulting and extension, the entire region has gradually risen (this includes the entire Rocky Mountains and Great Plains regions). Basin and Range faults, and their associated uplifts and basins in the region, are aligned mostly in a northwest-to-southeast orientation. Many of these faults affected older structures associated with the earlier periods of mountain building and volcanism.
The Great Denudation
As the landscape has risen, erosion has worn down the landscape. In late Miocene time (about 10 million years ago) the region encompassing Big Bend National Park may have had very subdue relief (having minimal elevation differences between the hilltops and stream valleys). As the land has risen, particularly in the past 5 million years, the rivers became entrenched in their valleys. Many thousands of feet of Cretaceous- and Tertiary-age sediments were stripped away, especially during the recent period of the Ice Ages of the Quaternary Period. This rapid erosion and lowering of the landscape surface resulted in the formation of the three spectacular, narrow canyons along the Rio Grande River (Santa Elena, Mariscal, and Boquillas canyons). It also resulted in the exposure of the great volcanic stocks and plutons that form the core of the Chisos Mountains and other volcanic centers in the region. It also re-exposed many of the great folds and fault bounded mountains that formed earlier during the Larimide Orogeny and Basin and Range extension.
The Modern Landscape
The modern landscape reflects ongoing processes that are shaping the landscape. Most noteworthy are perhaps the rugged, eroding spires and ridges in the upland areas, the expansive alluvial fan surfaces that extend down slope from upland sediment source areas, the badlands forming in barren, soft sediments, and the migrating stream channels in the floodplains of the major streams and along the Rio Grande River. Throughout the Ice Ages the climate varied considerably, ranging from the dry conditions that are typical of the modern Chihuahuan Desert, to cool and wetter conditions when all of North America was affected by the chill conditions of an ice age. These fluctuations in climatic conditions also caused regional ecosystems to change. During the cool and wet periods, the extensive plains around Big Ben may have been dominated by pine and oak forests, and during drier periods the landscape was dominated by grasslands, and like today, desert flora and fauna. In this manner species migrated into the region only to later become isolated in small pockets relative to their earlier range. The Chiso Mountains are a classic example of a "land island" - an isolated upland area that has a host of unique habitats with species that are isolated by great distances from their closest living relative species.
The fluctuating climate also influenced the rate of erosion and deposition along streams, and especially the Rio Grande where during the wettest periods the river's discharge probably expanded dramatically. Regional ground cover also influences the amount of sediment that could be moved across the surface and into stream channels. It is likely that the greatest volume of sediments move during brief and episodic large storm events within the extended dry periods when plant cover is sparse. The existance broad flood plain and elevated terrace gravels along sections of the Rio Grande River suggest that the river and its tributaries episodically cut or back fills their valleys with sediments.
The landscape is still changing, both from natural and human-induced effects. For instance, intense overgrazing in the period shortly before the land was incorporated into the park cause extensive soil degradation in many portions of the park. Recovery of the park soils and associated ecosystems has varied depending on local conditions. Today, much of the Big Bend landscape is highly protected and remains mostly isolated wilderness.
Click here for a list of selected references (reports and maps) for the Big Bend region.
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