The area of the Trans-Pecos Province is approximately 47,000 square miles. Small areas of the pre-Cambrian basement rocks are exposed but larger areas are underlain by more than 15,000 feet of sedimentary rock that is mainly marine in origin. The estimated average thickness of the sedimentary rocks is 1.7 miles and the gross volume is 80,000 cubic miles. The sedimentary rocks include representatives of every system from the Cambrian to the Quaternary. The Triassic, Tertiary, and Quaternary rocks are non-marine.

The following tabulation presents a summary of the partly successful results of exploration in the Trans-Pecos Province. The entire productive area is in the Delaware basin, and practically all the oil is from the top of the Delaware Mountain group (top of Guadalupe; Permian).

Parts of six major tectonic provinces of different age intersect in the Trans-Pecos region (Table III). Each is related to distinct facies of the sedimentary succession. These provinces are chronologically distinct but overlap areally. A single area may thus form a part of several provinces and may contain several facies one above the other in its stratigraphic column.

In the Trans-Pecos Province the rocks of the pre-Permian basins may be summarized as follows.

The tectonic history, lithofacies, and oil possibilities of different areas of the Trans-Pecos Province are summarized in Table III.

In Pennsylvanian time the Marathon area was the site of actively growing mountains. Thick wedges of sediments on the north flanks of these mountains are now represented by sandstone and siltstone that grade northward