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PRE-CAMBRIAN ROCKS
No rocks of pre-Cambrian age crop out in the Marathon Basin. The floor of basement rocks in the region has been deeply buried by Paleozoic strata of the Llanoria geosyncline. To the north and south of the geosynclinal area pre-Cambrian rocks lie nearer to the surface and have been discovered in a few exposures and a few deep wells.
Pre-Cambrian rocks north and south of Marathon
About 50 miles northeast of Marathon, near Fort Stockton, a well drilled by the Shell-Humphreys Companies on university land penetrated old rocks below the Permian at a depth of 4,750 feet. The basement rocks are considered to be granites by J. T. Lonsdale, who found that the abundant minerals in the cuttings are quartz, microcline, albite, hornblende, and biotite. Accessory minerals include magnetite, zircon, apatite, and calcite. This is probably a part of the foreland area north of the Llanoria geosyncline. The granite probably has the same relation as the pre-Cambrian rocks that crop out at the surface near Van Horn, 100 miles northwest of Marathon. At that place strata of Permian age overlap across the older Paleozoic rocks and rest locally on the pre-Cambrian on the crests of pre-Permian uplifts.
About 80 miles south of Marathon C. L. Baker has found schists beneath the Cretaceous on the crest of an anticline in the Sierra del Carmen, east of the village of Boquillas. This exposure is probably a part of the land that bordered the geosyncline on the southeast.
Fragments of crystalline rocks in the Paleozoic sediments at Marathon.-The land area of Llanoria, southeast of the geosyncline, appears to have been composed largely of crystalline rocks and probably stood as a highland or mountain area during a large part of Paleozoic time (fig. 16 and pl. 20 A). For the most part the former highland is now buried beneath Cretaceous and younger strata, and the hypothesis of its former existence is based largely on evidence supplied by the composition of the Paleozoic sediments in the geosyncline.
"Van der Gracht (The Permo-Carboniferous orogeny in the south-central United States: K. Akad. Wetensch. Amsterdam Verb., Afd. Natuurk., deel 27, no. 3, table Va and elsewhere, 1931) notes a reported discovery of pre-Cambrian rocks in the similar district of the Solitario uplift, southwest of Marathon. This report has proved to be erroneous, according to a letter from E. H. Sellards, June 1932.
Sellards, E. H., op. cit. (Texas Univ. Bull. 3232), p. 52. The well has also been noted by E. L. Jones and R. C. Conkling (Basement rocks in the Shell-Humphreys well, Pecos County, Tex: Am. Assoc. Petroleum Geologists Bull., vol. 14, pp. 314-316, 1930) and by P. B. King (Geology of the Glass Mountains, part 1: Texas Univ. Bull. 3038, p. 117, 1930). Jones and Conkling regarded the rocks as metamorphosed sandstones, penetrated by igneous dikes.
Quoted by Bose, Emil, Vestiges of an ancient continent in northeastern Mexico: Am. Jour. Sci., 5th ser.. vol. 6, p. 133, 1923. See also Kellum, L. B., Imlay, R. W., and Kane, W. G., Evolution of the Coahuila Peninsula, Mexico; Part 1, Relation of structure, stratigraphy, and igneous activity to an early continental margin: Geol. Soc. America Bull., vol. 47, pp. 972-977,1936.