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.
[Illustration: 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.
[Illustration: 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.