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rocks. The youngest beds exposed belong to the lower part of the Magdalena limestone, but beneath them are shales and limestones from which a fauna of Smithwick (early Pennsylvanian) age has been collected. The limestones associated with the shales have a close resemblance to those of the transported blocks of early Pennsylvanian age in the boulder-bed member of the Haymond formation. The fauna of the transported blocks suggests also that they are of about the same age.

East of these localities there are no other outcrops of Pennsylvanian rocks until central Texas is reached, but a few hundred feet of limestones of middle Pennsylvanian age have been encountered by deep borings in the Big Lake oil field, in Reagan County, northeast of the Marathon Basin.

.-The Pennsylvanian section at Marathon, although unlike most of the sections nearby in trans-Pecos Texas, is similar to more distant sections to the east and northeast, which also lie in or near the site of the Llanoria geosyncline. In these regions, as in the Marathon district, there were disturbances and mountain-making movements at several times in the Pennsylvanian epoch, and the deposition of clastic sediments was closely related to this orogeny.

In parts of central Texas, southern Oklahoma, and western Arkansas, where the stratigraphic sequence resembles that in the Marathon region, there are three main masses of clastic rocks. One, represented by the Stanley shale, Jackfork sandstone, and Springer formation, of early Pennsylvanian age, is found only in the Ouachita Mountains and Ardmore Basin. Another is represented by the Strawn group of Texas and the Atoka and overlying formations north of the Ouachita Mountains. The third is represented by the Cisco group in Texas and by the Pontotoc group and some underlying beds in Oklahoma. In central Texas the clastic rocks are separated, as well as overlain and underlain, by more calcareous strata. Beneath the Strawn group are the Smithwick shale and Marble Falls limestone. In parts of southern Oklahoma their equivalent, the Wapanucka limestone, separates the Springer and Jackfork clastic rocks from the clastic rocks of the Atoka. Between the Strawn and Cisco groups in central Texas is the Canyon group, which contains abundant and conspicuous beds of limestone. In the southern part of the central Texas area the Cisco group is overlain by the predominantly calcareous beds of the Wichita group.

Times of movement in these regions are suggested by several unconformities. Between the Smithwick shale and the Strawn group in central Texas there is a well-marked break, and the Strawn overlaps westward on the older beds. A similar break has been recognized in Oklahoma between the Wapanucka limestone and the Atoka formation. Miser suggests that nearly contemporaneous movements gave rise to the uplifts that furnished the source for the boulders in the Johns Valley shale (between the Jackfork and Atoka formations in the Ouachita Mountains). Less conspicuous unconformities are reported to occur higher in the section in Texas and Oklahoma, but the next pronounced break in the Arbuckle Mountain area is at the base of the Vamoosa formation, high in the Pennsylvania section. This and the succeeding formations, including the Pontotoc group, overlap all the older formations of the Arbuckle Mountains. In central Texas these later Pennsylvanian unconformities are not so marked, but Plummer and Moore report several breaks in the Cisco group.

It has been the writer's opinion for many years that the Tesnus formation of the Marathon Basin is correlative with some part of the lowest mass of clastic deposits in Oklahoma and Arkansas (Stanley, Jackfork, and Springer formations). The fossil evidence is meager, and the fossils are poorly preserved. The plants of the Stanley and Jackfork formations are now considered to be of very early Pennsylvanian age, but White believed the plants collected near the top of the Tesnus formation to be slightly younger. No fossils have been found in the lower part of the Tesnus. The stratigraphic position of the Tesnus is similar to that of the Stanley and Jackfork, as it lies between novaculites of Devonian (?) age and limestones considered to be of Marble Falls and Wapanucka age. It also resembles the Stanley and Jackfork formations in lithology : all three consist of chloritic sandstone, dark slaty shale, and white quartzite.

The character of the Dimple limestone, a calcareous layer in a predominantly clastic succession, and its widespread occurrence within the area of exposure suggest that it may be of the same age as the Marble Falls and Wapanucka limestones, which occupy a similar position and cover wide areas in central Texas and southern Oklahoma. The few fossils in the Dimple limestone are of a somewhat different facies from those in the two formations to the east, but this difference may have been brought about by conditions "

Arick, M. B., Occurrence of strata of Bend age in Sierra Diablo, Culberson County, Tex.: Am. Assoc. Petroleum Geologists Bull., vol. 16, pp. 484-486, 1932.

Sellards, E. H., Bybee, H. P., and Hemphill, H. A., Producing horizons in the Big Lake oil field, Reagan County: Texas Univ. Bull. 3001, pp. 149-203, 1930.

Plummer, F. B., and Moore, R. C., Stratigraphy of the Pennsylvanian formations of north-central Texas: Texas Univ. Bull. 2132, p. 60, 1921.

Miser, H. D., Carboniferous rocks of Ouachita Mountains: Am. Assoc. Petroleum Geologists Bull., vol. 18, pp. 1008-1009, 1934.

See references in Plummer, F. B., and Moore, R. C., op. cit.; Morgan, G. D., Geology of the Stonewall quadrangle, Okla.; Oklahoma Geol. Survey Bull. 2, pp. 19-20, 1924; and Dott, R. H., Overthrusting in Arbuckle Mountains, Okla.: Am. Assoc. Petroleum Geologists Bull., vol. 18, pp. 583-588, 1934.

White, David, Age of Jackfork and Stanley formations of Ouachita geosyncline, Arkansas and Oklahoma, as indicated by plants: Am. Assoc. Petroleum Geologists Bull., vol. 18, pp. 1010-1017, 1934.