Fig. 7. Generalized geologic map of Palo Duro Canyon State Park.
The Paleozoic Era has been divided into seven periods of geologic time. With the oldest at the bottom of the list, these periods and the source of their names are:
The Carboniferous Period in Europe includes the Mississippian and Pennsylvanian Periods of North America. Although this classification is no longer used in the United States, the term Carboniferous is found in many of the earlier geological publications and on many of the earlier geologic maps.
The periods of the Mesozoic Era and the source of their names are:
The Cenozoic periods derived their names from an old outdated system of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use:
Although the units named above are the major divisions of geologic time and of the geologic column, the geologist generally works with smaller units of the column called geologic formations. A geologic formation is a unit of rock that is recognized by certain physical and chemical characteristics. A formation is generally given a double name which indicates both where it is exposed and the type of rock that makes up the bulk of the formation. For example, the Beaumont Clay is a formation consisting of clay deposits that are found in and around Beaumont, Texas. For convenience in study, two or more successive and adjoining formations may be placed together in a group. Thus, the Tecovas and Trujillo Formations have been placed in the Dockum Group. Likewise, a formation may be subdivided into smaller units such as members, which may also be given geographic or lithologic (rock type) names.
As noted above, all of the rocks which crop out in Palo Duro Canyon are sedimentary in origin. They represent four different geological periods: the Permian, Triassic, Tertiary, and Quaternary (fig. 12).
Although these rock formations differ considerably in composition and age, they do not tell the whole geologic story of the area. Long spans of geologic time are not represented by rock units because the region was undergoing erosion or no sediments were being deposited during certain portions of geologic time. Rocks that had formed during one geologic period were removed by erosion during a later period. Thus, segments of the geologic record were destroyed or never recorded. For this reason, much of the geologic history of the Palo Duro area is unrecorded and must be inferred from fragmentary evidence borrowed and pieced together from adjacent areas. Even so, an interesting story can be assembled from the rocks that remain in the canyon today.
In general, the following descriptions of the formations exposed in Palo Duro Canyon State Park follow the procedure that most geologists use in presenting the results of their geologic investigations. The more distinctive characteristics of the rock units are described in order that they may be more easily recognized, and the ways in which the rocks were formed are also considered. With this background it is then possible to review the geologic history recorded in the bedrock of the canyon. A simplified geologic map is presented in figure 7; this shows the distribution of the major rock types in the canyon. The reader will find it helpful to refer to this map when reading the descriptions of the various formations.
The oldest formation exposed in the canyon is the Quartermaster Formation of Permian age (see fig. 6) which is named from exposures along the banks of Quartermaster Creek in Roger Mills County, Oklahoma. One of the more colorful formations in the park, the Quartermaster is composed primarily of brick-red to vermilion shales which are interbedded with lenses of gray shales, clays, mudstones, and sandstones. Averaging about 60 feet thick where exposed in the park, the Quartermaster forms the floor and lower walls of the canyon.
The rocks of this formation are easily examined at many places throughout the canyon and in them can be seen a number of interesting geologic phenomena. Probably the most noticeable of these features are the shining white veins of gypsum that lace the face of the red shale outcrops (fig. 8). A soft, transparent to translucent mineral that can be scratched by a fingernail, gypsum is hydrous calcium sulfate (CaSO₄·2H₂O). Three varieties of gypsum are found in the canyon: (1) satin spar, a fibrous variety with a silky sheen; (2) selenite, a colorless, transparent variety which commonly occurs in sheet-like masses; and (3) a fine-grained massive variety called alabaster. Satin spar is the most common variety of gypsum present and it commonly occurs in thin bands interbedded with the mudstones and sandstones. It is much more noticeable in the shales, however, for it is typically seen in narrow veins which criss-cross the surface of the outcrop and intersect the bedding planes at various angles. Although normally white, some of the satin spar has a soft pink or bluish hue due to the presence of impurities in the mineral.
Fig. 8. Veins of selenite gypsum (top arrow) in Quartermaster Formation. Notice diagonal joint to left of geologist’s hand (lower arrow).
The presence of gypsum in the Quartermaster red beds is of special significance to the geologist, for it provides valuable information about the geologic history of the Palo Duro area. It is known, for example, that when a landlocked body of sea water in an arid climate becomes separated from the ocean, one of the most common salts to precipitate is hydrous calcium sulfate, or gypsum. Gypsum may also be precipitated when a lake without an outlet evaporates in an arid climate. Geologic evidence suggests that the sediments which gave rise to the rocks of the Quartermaster Formation were deposited in a landlocked arm of the sea during the latter part of the Permian Period. As evaporation continued and the sea water was reduced to approximately one-third of its original volume, gypsum was precipitated. There must have been periodic influxes of silt- and mud-bearing waters entering the ancient Permian sea, for layers of shale and mudstone are interbedded with the gypsum.
It is believed that much of the satin spar and selenite gypsum was originally anhydrite (CaSO₄). Unlike gypsum, anhydrite does not contain water, but it can be changed to gypsum in the presence of moisture. There are two lines of evidence that indicate an anhydrite origin for the Quartermaster gypsum. First, microscopic examination of gypsum samples reveals the presence of residual anhydrite crystals embedded in the gypsum. Second, many of the gypsum beds have been squeezed into rather gentle folds. These consist of small anticlines, upfolds or arches, and synclines, downfolds or troughs (fig. 9). It has been suggested that this folding took place as the anhydrite underwent hydration, or took on water. As hydration occurred and the anhydrite was converted to gypsum, the gypsum expanded, thereby exerting both lateral and vertical pressure on the beds around it. This produced the crumpled, wave-like folding so characteristic of certain of the gypsum beds. However, there is not complete agreement that the folding in the gypsum is due to the hydration of anhydrite. Certain geologists attribute this deformation to slumping caused by solution cavities, for gypsum is relatively easily dissolved in water. As the gypsum was dissolved and carried away in solution, the removal of the supporting layers of gypsum permitted slumping and consequent deformation in the overlying shales and mudstones. Although some geologists believe that the folds were caused by expansion due to the hydration of anhydrite and others support deformation related to the removal of soluble gypsum, there is general agreement that the folding is local and not related to regional or widespread deformation.
Fig. 9. Sagging beds of Quartermaster Formation have produced this gentle syncline, or downfolding, in the rocks. The “dome” on Capitol Peak can be seen in the background.
Not all of the red Quartermaster shales are uniformly colored. Some of them contain gray-green, circular spots called reduction halos (fig. 10). These spots, which in places give the red shales a distinctive polka-dot appearance, have been produced as the result of chemical change of certain minerals within the shale.
As noted earlier, sediments are usually laid down in horizontal layers. However, in certain environments, sediments may be deposited in such a way that the layers are inclined at angle to horizontal (fig. 11). This structure, called cross-bedding or cross-stratification, is found in certain sandstones and other coarse-grained or fragmental sedimentary rocks. Cross-bedding typically consists of rather distinct inclined layers separated by bedding planes (the surface of demarcation between two individual rock layers). Bedding of this type commonly occurs in sedimentary rocks formed in rivers, deltas, and along the margins of lakes or oceans. The cross-bedding in the Quartermaster and certain of the Triassic formations is believed to have been developed under similar conditions. Although cross-bedding is also common in certain rocks of eolian origin (deposited by wind) none of the cross-bedding in the canyon’s rocks is due to the action of wind.
In addition, some of the Quartermaster strata have ripple marks on their surfaces. These features are common in certain sedimentary rocks and were formed when the surface of a bed of sediment was agitated by waves or currents. The size, shape, and cross section of the ripple marks can be used to tell whether the marks were produced by waves or currents. The ripple marks in the Quartermaster appear to have been formed by the action of waves on a shallow sea floor.
A number of interesting geologic features in the canyon have been formed in part in the Quartermaster Formation. These include the multi-hued Spanish Skirts (fig. 26), the Devil’s Slide (fig. 35), Capitol Peak (fig. 32), and Catarina Cave (fig. 27). The latter is a rather unusual cave in that it has developed in a large mass of landslide debris divided by projecting bedrock of the Spanish Skirts. The cave has been formed by suffosian, a process whereby water enters the landslide debris on the upper slopes and follows buried channels in the landslide removing rock debris as it passes through. The flood water exits at the base of the landslide by means of Catarina Cave. The plan of the cave closely resembles the drainage patterns of surface gullies.
Rocks of the Triassic System (fig. 6) are well represented in Palo Duro Canyon and consist of the Tecovas and Trujillo Formations. These formations are part of the Dockum Group of Late Triassic age.
Having a total thickness of about 200 feet, the Tecovas (which is named from exposures found on Tecovas Creek in Potter County, Texas) consists largely of multicolored shales. Also present are thin layers of soft sandstone, which are disseminated throughout the shales, and a more prominent bed of white sandstone, which marks the middle of the formation. The Tecovas shales overlie the Quartermaster Formation, and the lower zone of lavender, gray, and white shales forms a relatively smooth slope that is easily distinguished from the steeper slopes of gullied red-and-white-banded shales beneath them (fig. 12).
Fig. 10. Chemical reactions in certain of the red Quartermaster shales have produced reduction halos (p. 19) which give the rocks a polka-dot appearance.
Fig. 11. This boulder, located near the foot of Triassic Peak along the Sad Monkey Railroad track, exhibits the cross-bedding typical of the Trujillo sandstones.
But the contact zone between the Tecovas and Quartermaster shales involves more than a mere change in color. Here is one of the missing “chapters” in the geologic history of the canyon, for part of the Late Permian record and all of the record of Early and Middle Triassic time are missing from the geologic column. Such gaps in the column are represented by unconformities in the rocks. Here the unconformity is an ancient erosional surface between the Tecovas Formation of Late Triassic age and the Late Permian Quartermaster Formation, and there are many millions of years of earth history represented in this missing “chapter” in the geologic story of Palo Duro Canyon. During this vast span of time, thousands of feet of sediments were probably deposited, converted into rock, and then later removed by erosion.
Near the middle of the Tecovas Formation there is a bed of white, crumbly (friable) sandstone. Averaging about 15 feet in thickness, this sandstone contains many joints (small crack-like fractures) along which no appreciable movement has taken place (fig. 8). There are two distinct sets of these joints which intersect each other at right angles. The distinctive joint patterns, the color, and the friability of this sandstone clearly differentiate it from the harder, darker, and more coarse-grained sandstones of the overlying Trujillo Formation (p. 22).
The upper part of the Tecovas consists of a layer of orange shale which overlies the middle sandstone unit and is in contact with the lower part of the Trujillo Formation.
Fig. 12. Taken from the northwest rim near Coronado Lodge, this photograph shows the four major rock units exposed in the park: (1) The Quartermaster Formation which forms the lower wall and canyon floor; (2) Tecovas Formation; (3) Trujillo Formation which caps the mesas; and (4) Ogallala Formation.
The fossils which have been found in the Tecovas Formation suggest that these rocks were derived from sediments deposited in swamps and streams. Unlike the marine deposits of the Quartermaster, the rocks of the Tecovas were formed from continental deposits laid down on the land. Fossils found in the canyon include the bones and teeth of the extinct semi-aquatic reptiles known as phytosaurs (fig. 13) and bone and skull fragments of a primitive amphibian called Buettneria (fig. 14). Coprolites (the fossilized excrement of animals), pieces of petrified wood, and the teeth and bones of lungfish have also been reported from the Tecovas.
Fig. 13. The skull of this crocodile-like creature called a phytosaur is typical of the reptiles that inhabited the Palo Duro area during the Triassic Period. (Photograph courtesy Panhandle-Plains Historical Museum.)
A number of minerals including hematite, an iron mineral, and psilomelane, a barium-magnesium oxide, occur in the Tecovas. Hematite is an ore of iron and psilomelane a manganese ore, though neither of these is present in commercial quantities in the canyon.
The Tecovas also contains a number of concretions which range from a fraction of an inch to as much as 6 inches in diameter. These spherical masses are generally harder than the fine-grained shaly sands in which they are found and were thus left behind when the surrounding rock was eroded away. Some of these concretions are marked by cracks or veins filled with the mineral calcite. Concretions bearing this type of structure are called septaria, or septarian concretions.
Geodes are also found in the Tecovas Formation. These are rounded concretionary rocks with a hollow interior that is frequently lined with mineral crystals. Well-formed crystals of clear calcite have been found in many of the geodes from the Tecovas.
Among park landmarks that are characterized by the multi-hued Tecovas strata are the middle portion of Triassic Peak (fig. 25), the upper part of the Spanish Skirts (fig. 26), Capitol Peak (fig. 32), and the Devil’s Slide (fig. 35).
Named from rock exposures on Trujillo Creek in Oldham County, Texas, the Trujillo is easy to distinguish from the underlying Tecovas Formation. The contact is quite distinct and lies between the top of the orange Tecovas shale and the base of the massive-bedded, cliff-forming Trujillo sandstone (fig. 25). Although generally fine grained and thickly bedded, there are local concentrations of pebble-sized rock fragments in the Trujillo. The weathered surface of the lower sandstone is stained red or dark brown by iron oxides. However, a fresh, unweathered surface is typically gray or greenish gray in color, and careful examination of the unweathered rock reveals the presence of tiny flakes of mica.
The basal Trujillo sandstone is one of the most conspicuous rock units in the canyon and forms many of the prominent benches and mesas so typical of the Palo Duro landscape. In places the sandstone is cross-bedded (p. 20) and contains channel deposits of coarse sand which suggest that the sediments from which it was derived were deposited in ancient stream beds.
Red, maroon, and gray shales overlie the basal sandstone member of the Trujillo, and these shales are overlain by cross-bedded, coarse-grained sandstone. Another interval of varicolored shales separates the middle sandstone bed from the upper sandstone member. The middle sandstone unit is a conspicuous ledge- or cliff-forming rock and is medium to coarse grained and commonly cross-bedded. In most localities, the upper sandstone is overlain by a section of red and green shales which mark the uppermost limits of the Trujillo Formation. In places, however, this shale section has been removed by erosion and rocks of Tertiary age directly overlie the sandstone.
Although fossils are not common, the remains of Buettneria (fig. 14), leaf imprints, pieces of mineralized wood, and the scattered teeth and bone fragments of reptiles and amphibians have been found. Phytosaur remains, especially teeth, have also been collected from the Trujillo sandstones.
The Indians who formerly inhabited the Palo Duro area (p. 3) put the rocks of the canyon to a number of uses. This appears to be especially true of the rather coarse-grained Trujillo sandstones, which were commonly used for constructing primitive rock shelters. The abrasive surface of the sandstone was especially well suited for grinding grain, and mortar holes have been found in a number of places. One of these (fig. 15) can be seen along the tracks of the Sad Monkey Railroad (p. 35) near the foot of Triassic Peak. The Indians also used the clays of the Quartermaster, Tecovas, and Trujillo Formations to make pottery, and iron and copper minerals such as hematite and malachite were used to make red and green pigments for decoration and war paint.
The Trujillo shales and sandstones can be seen in a number of Palo Duro’s more spectacular geological oddities. These erosional remnants are best developed where blocks of erosion-resistant sandstone protect underlying pedestals of softer shale (fig. 15). This type of differential weathering (p. 31) has produced a number of interesting and unusually shaped pedestal rocks or “hoodoos” (figs. 16 and 20). The most spectacular erosional remnant—and one that has come to be the “trademark” of Palo Duro Canyon—is the Lighthouse (fig. 31). The great jumble of boulders called the Rock Garden (fig. 34) is also composed largely of massive blocks of dislodged Trujillo sandstone. These boulders accumulated on the canyon floor as a result of landslides. In addition, the rock profile known as Santana’s Face (fig. 28) is a naturally sculptured profile in the Trujillo sandstone that forms the cap of Timber Mesa.
The Ogallala Formation is named from exposures around Ogallala in Keith County, Nebraska. There is a major unconformity between the Trujillo Formation of the Triassic and the overlying Ogallala Formation of Pliocene (Late Tertiary) age. Missing here is the geologic evidence for what may have been some of the more exciting chapters in the canyon’s history. There is no record, for example, of the Jurassic and Cretaceous Periods which together encompass almost 120 million years of earth history. Also missing is any evidence of what transpired during more than 90 percent of the Tertiary Period, for no rocks of Paleocene, Eocene, Oligocene, or Miocene age are exposed in the canyon. Together these four epochs comprise approximately 47 million years of earth history. It is impossible, of course, to determine how many geologic formations may have been formed and later eroded during the 167 million years represented by this unconformity. However, our knowledge of present-day deposition and erosion suggests that the missing geologic record undoubtedly represents many thousands of feet of rock.
Fig. 14. The skeleton of Buettneria, a large amphibian, found in Upper Triassic strata in the canyon. (Photograph courtesy Panhandle-Plains Historical Museum.)
The lower portion of the Ogallala Formation is composed of a reddish-brown, fine- to medium-grained sandstone that contrasts sharply with the underlying red and green shales that are exposed in the top of the Trujillo Formation. Much of this sandy rock is characterized by pebbles consisting of a variety of igneous, sedimentary, and metamorphic rocks. Because it consists of rock and mineral fragments of varied composition and size, this kind of sedimentary rock is called a conglomerate. The type of rock fragments found in basal Ogallala conglomerates suggests that they were transported to the Panhandle-Plains area by streams flowing southeastward from the Rocky Mountains. As these streams deposited their loads, they left behind a wide spread blanket of sand, gravel, and mud which formed an extensive alluvial plain that extended from western Nebraska to northwest Texas. Although it is less than 100 feet thick in Palo Duro Canyon, in places this great mantle of fluvial (stream-deposited) sediments is as much as 900 feet thick.
Fig. 15. The depression in this boulder is a mortar hole believed to have been used by the Indians for grinding corn.
Fig. 16. This pedestal rock, located near the Lighthouse, is capped by a slab of weather-resistant Trujillo sandstone.
Most of the Ogallala Formation consists of a mixture of diverse rock types such as conglomerate, sandstone, siltstone, clay and marl. But the upper part of the formation is characterized by thick caliche deposits. A dull, earthy calcite deposit, caliche typically forms in areas of scant rainfall. It is believed to originate when ground moisture, containing dissolved calcium bicarbonate, moves to the surface where the moisture steadily evaporates leaving a calcium carbonate crust on or near the surface (fig. 17).
Caliche, which derives its name from the Latin calix, meaning “lime,” may be firm and compact or loose and powdery. It is also commonly found mixed with other materials such as clay, sand, or gravel. Caliche commonly occurs in the Trans-Pecos, southwestern Gulf Coastal Plain, and the High Plains area of Texas (see fig. 5, p. 8). In the latter area it typically makes up the “caprock.” Caliche is commonly quarried in these parts of Texas where it is used as road material and as an aggregate.
Good exposures of Ogallala caliche can be seen on the surface around the overlook at Coronado Lodge on the northwest rim of the canyon (fig. 17). Ogallala strata also crop out along the upper reaches of Park Road 5 as it starts to descend into the canyon. But probably the most spectacular exposures of the Ogallala are exposed in the precipitous face of the Fortress Cliff (fig. 33) which forms part of the eastern rim of the canyon.
Also located within the Ogallala Formation is a very important aquifer—a porous, water-bearing rock formation. This fine-to coarse-grained sandstone is very porous and permeable and is the most important single water-producing formation in the Panhandle-Plains area.