  The age of the earth is calculated in terms of billions of years. For convenience, geologic time is divided into units, called PERIODS, of different lengths (Figure 1). Sediments deposited during each period and the fossilized remains of animals and plants found in these sediments give a partial record of the events and life of that period. A nearly homogeneous (uniform composition) rock layer may be identified as a FORMATION. Formations are usually spread over a wide area like a large blanket and are stacked on top of each other with the oldest at the bottom and the youngest at the top. When viewed in the walls of the canyon, they resemble a huge layer cake and may be traced along the canyon walls. Figure 1. GEOLOGIC MAP OF PALO DURO CANYON Figure 2. Stratigraphic section at Palo Duro Canyon with Geologic Time-scale for reference.
The lowest and oldest exposed formation in Palo Duro Canyon is the Quartermaster. It was deposited near the edge of a shallow sea that occupied a wide area in Texas some 280 to 230 million years ago (Figure 3). This was during the Permian Period, the last period of the Paleozoic Era. The sediments in the park area were carried from the east and deposited in a nearshore environment. Sedimentary structures, such as crossbeds and ripplemarks, are present throughout the Quartermaster Formation. Halite casts suggest that there was a high rate of evaporation as sedimentation occurred. Gypsum (altered anhydrite) is also interpreted to be an evaporite deposit. The gypsum is now seen as horizontal white layers of alabaster and satin-spar varieties within the Quartermaster Formation. The Quartermaster Formation is mostly siltstone and shale, and is commonly a distinctive red color. This red color is the result of combining oxygen from the air with the iron in the sediments (oxidation) much as a nail rusts after it has been exposed for a long period. The bedded gray zones represent times when there was enough fresh water from the land to offset temporarily the oxidation process. Smaller circular gray areas have organic nuclei that produced local areas of chemically altered iron by a process called reduction. At the close of the Permian Period and the Paleozoic Era, the Panhandle region was uplifted and a period of widespread erosion followed. Consequently there are no sediments in this area to represent the early or middle portions of the Triassic Period. Breaks, such as this, in the sedimentary record are called UNCONFORMITIES. They may have been caused by a lack of deposition in the area or by an interval during which erosion removed earlier sediments. Figure 3. Paleogeographic map of the Permian Period (240 m.y.) The Tecovas Formation was deposited in swamps, lakes, and streams approximately 200 million years ago during the Late Triassic (Figure 4). The Tecovas is mostly purplish lavender, yellow, orange, and buff siltstone and shale. The bright-colored shale of the Tecovas Formation is easily followed for many miles. Amarillo (the Spanish word for yellow) got its name from Amarillo Creek where the yellow bed of the Tecovas Formation crops out far from Palo Duro Canyon. The shale of the Tecovas forms the less steep portions of the canyon walls and often is covered by talus (weathered, broken rock) or vegetation. The uppermost Tecovas is usually mantled with boulders from the overlying sandstone of the Trujillo Formation. The Tecovas Formation contains numerous concretions or irregularly shaped, weathered rocks. The unusual shape of a concretion is the result of the hardening of the sediments around a nucleus. As the rock weathers, the resistant material surrounding the nucleus remains. Most of the concretions are composed of limonite, hematite, manganite or calcite. Some of the calcite concretions are a variety termed “septarian.” These concretions have calcite ridges in a honeycomb pattern throughout the rock. Some of the concretions are simply nodular or spherical aggregates. Also in the Tecovas, geodes filled or lined internally with calcite crystals are found. Figure 4. Paleogeographic map of the Triassic Period (181 m.y.) Overlying the Tecovas is the Trujillo Formation. It was deposited by streams that probably originated in an ancient highland southeast of the present Panhandle. These streams were flowing more than 181 million years ago. The sandstone contains some alternating layers of shale and marl-pebble conglomerate. The Trujillo Formation is a resistant formation and forms some of the upper portions of the canyon walls. The steep portions are, in part, the result of a persistent fracture system common in the Trujillo Formation. The sandstone and conglomerate of the Trujillo characteristically exhibit well-developed crossbedding. Their gray color is sometimes obscured by a crust of red mud or iron stain. The uppermost red shale contains mineralized wood. The gray micaceous sandstone has many round sandstone concretions. Some of these are septarian concretions with cracks filled by calcite. Others may contain leaf imprints. There is no evidence that Jurassic sediments were ever deposited in the region. Cretaceous rocks are also missing in this area although water-worn fossil oysters occur in the gravel at the base of the overlying Ogallala. These fossils indicate that marine Cretaceous sediments were deposited nearby and possibly covered the Triassic deposits in the region. The rocks were then eroded away some time between the end of the Cretaceous Period and the beginning of the Pliocene Epoch, a span of about 50 million years. During the Pliocene Epoch, approximately 2-10 million years ago, the Rocky Mountains were again uplifted. Sediments in streams and floodplains were deposited on the erosional surface of the Trujillo Formation (Figure 5). These stream-deposited sediments are the Ogallala Formation. The Ogallala Formation, which forms the upper part of the sequence of rocks exposed in the canyon, is present throughout most of the Panhandle. The formation is important as it is the principal aquifer of the Panhandle and supplies many farms and cities in the region with water. The Ogallala is a siltstone and sandstone that has, in places, been cemented by silica which came from groundwater. The formation contains many pockets of common opal and the basal part is in many places almost a chert. There are also some thin gray shale lenses. Figure 5. Paleogeographic map of the Pliocene (10 m.y.) Scattered over the Ogallala are Late Pliocene and Pleistocene playa lake deposits up to 3 million years old. Some of these are fresh water lake deposits of silt, limestone, and wind-transported sediments or loess. Below these sediments is a layer of caliche which was deposited by evaporation of groundwater rich in calcium carbonate during Late Pliocene and Pleistocene time. Less than one million years ago, during the Pleistocene Epoch of the Quaternary Period, the Prairie Dog Town Fork of the Red River began eroding headward into the Llano Estacado (Figure 6). The caprock escarpment is the result of differing resistance to erosion. The faster erosion of softer layers under the more resistant Ogallala and Trujillo formations forms the steep slopes of the escarpment. The Prairie Dog Town Fork of the Red River is the small stream flowing in Palo Duro Canyon. Throughout the past million years it has been slowly excavating the canyon. The rate of change has been slow but continuous, carving the steep colorful walls of Palo Duro Canyon, an area of geologic interest and great scenic beauty. |
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