25

A younger fauna is of early Ordovician age. In a collection made by C. L. Baker on Simpson Springs Mountain Ulrich has identified the following species:

• Obolus rotundatus (Walcott).
• Lingulella pogonipensis (Walcott).
• Schizambon typicalis (Walcott).
• Eoorthis desmopleura (Meek).
• Symphysurina spicata n. var. (Ulrich).
• Symphysurina 2 n. sp.
• Conokephalina inexpectans (Walcott).
• Apatokephalus finalis (Walcott).
• Hungaia? sp.

From East Bourland Mountain Josiah Bridge has obtained various species of Symphysurina and other fossils similar to those in the preceding list. Ulrich notes that the fauna is like that in a well-marked zone of the lower part of the Pogonip limestone of Nevada. The Pogonip limestone is classified by the Geological Survey as of Lower Ordovician age, but this part of the formation is assigned by Ulrich to the upper part of his Ozarkian system.

It is probable that the erratic blocks in the Woods Hollow shale are of a facies foreign to the Marathon geosyncline. Their lithology is quite unlike that of indigenous rocks of the same age, and their faunas have not been revealed by the most diligent collecting from ledges in place. Massive limestones of Upper Cambrian and early Ordovician age are not known to crop out at any place in trans-Pecos Texas. They are found farther east, in the Llano-Burnet uplift (Central Mineral Region). Probably a western extension of these limestones lies beneath the surface not far north of the Marathon folded belt.

Rocks of Ordovician age were reported to occur in the Marathon Basin by J. A. Udden, who collected a few fossils, considered to be of Trenton age by Schuchert, during the course of his journey to the Chisos country, "along the wagon road near Ridge Spring and at different points south from this place for a distance of 10 miles."

Important stratigraphic work in the area was done by Baker and Bowman in 1915. They collected fossils and made brief studies of the Cambrian and Ordovician succession, and their fossil collections were studied by E. O. Ulrich. During field work in 1929 and 1930 the writer restudied the Ordovician rocks of the Marathon Basin and divided the section into five formations.

The Ordovician system in the Marathon Basin, with a maximum thickness of about 2,000 feet, is brought to the surface in the Marathon and Dagger Flat anticlinoria (pl. 23). The higher members also appear in anticlines of less height between the anticlinoria and to the south of them. The system is now divided, in ascending order, into the Marathon limestone, the Alsate shale, the Fort Peña formation, the Woods Hollow shale, and the Maravillas chert. The Lower, Middle, and Upper Ordovician are all represented. All the formations contain fossils in greater or less quantity, but collecting is nearly always difficult, as it entails the splitting of great quantities of slaty shales and thin-bedded limestones that bear little or no surface indication of the presence of organic remains. The best fossils are obtainable in finely granular limestone, where there has been little compression of the shells.

The Ordovician section at Marathon is composed of relatively thick beds of shale, muddy limestone, and chert. These beds are intercalated with thinner layers of conglomerate, boulder beds, and sandstone. The faunas are mostly of a specialized facies, with plentiful floating and attached graptolites, associated with linguloid and oboloid brachiopods, pteropods, and trilobites. A very different contemporaneous facies is found in exposures 100 miles to the northwest, where the rocks are nearly all dolomitic limestones (fig. 16) and contain faunas characterized by orthoid brachiopods, cephalopods, corals, and sponges. The differences between the two sections are, however, more apparent than real, for several fossils and a few faunal groups are found in both regions. There is no suggestion that the strata were deposited in separated seaways. It is probable that the more or less clastic Ordovician strata at Marathon were deposited on or near muddy shores, in agitated water, and that the limestones with the gastropod-cephalopod assemblages were deposited farther from shore, in clean and quiet water.

During the course of the present field work 10 stratigraphic sections of Ordovician rocks were measured, of which only 5 extend entirely from the Cambrian to the probable Devonian (pl. 2). At four places in the area the stratigraphic sequence is particularly well exposed and is relatively free from structural complications :

• (1) On the south side of the road to the Roberts ranch, 3 miles west-southwest of old Fort Peña Colorada;
• (2) between Woods Hollow and Little Woods Hollow, on the old Louis Granger place, 6 miles southeast of Marathon;
• (3) on the south side of the Woods Hollow Mountains, 3 miles northeast of "

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  Bridge, Josiah and Dake, C. L., Faunal correlation of Ellenburger limestone of Texas: Geol. Soc. America Bull., vol. 43, pp. 725-748,1932.

  Udden, J. A., Sketch of the geology of the Chisos country: Texas Univ. Bull. 93, pp. 18-20, 1907.

  Baker, C. L., and Bowman, W. F., Exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, pp. 79-101, 1917. A preliminary statement, containing the first published descriptions of their formations, is given in Review of the geology of Texas: Texas Univ. Bull. 44, 1st ed., 1916.

  King, P. B., Pre-Carboniferous stratigraphy of the Marathon uplife: Am. Assoc. Petroleum Geologists Bull., vol. 15, pp. 1066-1076, 1931.