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.