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).
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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).