Part 11

Geology

Geologic studies of the State of Texas have indicated the presence of rocks formed during every era and period of geologic time. These range from the Precambrian granites of the Llano uplift to the Quaternary gravels of the High Plains.

One of the best ways to become acquainted with the geology of Texas is to study the geologic map of the State (Pl. 10). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On Plate 10, colors are used to show the distribution and geologic age of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and formation contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings.

PRECAMBRIAN ROCKS

The Precambrian rocks of Texas are composed of igneous and metamorphic rocks and some sedimentary rocks. Most of the Precambrian outcrops are in the Llano uplift and El Paso and Van Horn regions.

Alterations produced by vast amounts of time, heat, and pressure have obliterated any trace of fossils that may have been present in these rocks. With the exception of some questionable primitive plants collected in the Van Horn region, no Precambrian fossils have been reported from Texas.

PALEOZOIC ROCKS

Rocks of Paleozoic age are widespread in Texas, and rocks of each period are well exposed. Outcrops are found in the Llano uplift, North-Central Plains, and Trans-Pecos region. The most extensive exposures are of Pennsylvanian and Permian age, and the former are highly fossiliferous in parts of the North-Central Plains.

Cambrian

Rocks of late Cambrian age are exposed in the Llano, Marathon, and Solitario uplifts, and the Franklin Mountains near El Paso. These are sedimentary rocks consisting of conglomerates, sandstones, shales, limestones, and some dolomites.

Some of these formations are relatively fossiliferous, but the specimens are commonly fragmental and very poorly preserved. Fossils that are apt to be found in the Cambrian rocks of the Llano uplift include brachiopods, gastropods, trilobites, and small rounded objects believed to have been formed by algae (primitive one-celled plants). In other parts of the State, Cambrian rocks are sparsely fossiliferous and the fossils consist primarily of fragmental brachiopods, trilobites, and algae.

Ordovician

Ordovician outcrops are present in the Llano uplift of central Texas and in the Marathon, Solitario, El Paso, and Van Horn regions of Trans-Pecos Texas. These are sedimentary rocks and consist largely of sandstones, cherts, limestones, and dolomites.

Although some of the Ordovician formations are fossiliferous, they are seldom collected by amateur paleontologists because they are exposed in relatively inaccessible places and the fossils are usually poorly preserved. Ordovician fossils reported from Texas include sponges, corals, brachiopods, gastropods, cephalopods, and trilobites. In addition, the Marathon formation of the Marathon uplift contains large numbers of well-preserved graptolites (fig. 24, p. 86).

Silurian

The Silurian of Texas is poorly represented in surface exposures, and only one formation, the Fusselman, has been described. The Fusselman crops out in the El Paso and Van Horn regions where it is a white dolomitic limestone. Fossils are not abundant in this formation, but brachiopods and corals have been collected at a few localities.

Devonian

Devonian rocks are best developed in Trans-Pecos Texas, especially in the Marathon, El Paso, and Van Horn regions. In addition to the Trans-Pecos exposures, there are minor outcrops of Devonian rocks in the Llano uplift of central Texas.

Fossils are rare and fragmental in the Trans-Pecos exposures and consist primarily of radiolarians and brachiopods. The Devonian rocks of central Texas are predominantly calcareous and, although the material is usually poorly preserved, many fossils have been collected from them. These include bryozoans, corals, brachiopods, gastropods, and trilobites. Conodonts and fragments of primitive armored fishes (Pl. 37) have also been reported.

Mississippian

Mississippian rocks are exposed in the Llano region and in the Hueco Mountains of the Trans-Pecos area. The Trans-Pecos rocks primarily contain brachiopods with some bryozoans and gastropods.

The central Texas Mississippian rocks are much more fossiliferous and some of the material is well preserved. Fossils reported from this area include brachiopods (Pl. 17), crinoids, gastropods, cephalopods, trilobites, and ostracodes.

Pennsylvanian

Pennsylvanian rocks are well represented in Texas and are exposed in the Llano uplift, north-central Texas, and Trans-Pecos Texas.

In Trans-Pecos Texas fossiliferous rocks crop out in the Hueco and Diablo Mountains. Fossils found in this area are algae, fusulinids, corals, brachiopods, pelecypods, gastropods, cephalopods, and crinoids. There is also a thick section of Pennsylvanian rocks in the Marathon uplift, but only one formation, the Gaptank, is very fossiliferous. It contains many fossils including fusulinids, sponges, corals, bryozoans, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Certain Pennsylvanian strata in the Llano region are very fossiliferous, and the material is well preserved. The more abundant forms are fusulinids, corals, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Probably the best Pennsylvanian collecting areas are to be found in north-central Texas. Here the thick marine limestones and shales contain large numbers of well-preserved invertebrate fossils, and the terrestrial or shallow marine strata have yielded an abundance of plant fossils. Invertebrate fossils are apt to be found along the banks of streams and gullies and in railroad and highway cuts. Many of the limestones bear large numbers of fusulinids or crinoid stems, and the shales may contain many corals, brachiopods, and mollusks. The best collecting will, of course, be found where the rocks have been sufficiently weathered.

Typical invertebrate fossils are foraminifera (principally fusulinids), corals (especially the solitary or “horn” corals), brachiopods, bryozoans (the lacy and branching types are most common), pelecypods, gastropods (exhibiting a variety of coiling), cephalopods (nautiloids and goniatites predominate), and crinoids, which in many areas are found in thick crinoidal limestones (fig. 8). Some typical Pennsylvanian fossils are illustrated in Plates 14, 15, 17, 18, 19, 20, 21, 24, 32, and 35.