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By PHILIP B. KING

This report describes the geology of the Marathon region, in trans-Pecos Texas. The Marathon region lies on the edge of the Mexican Highlands province, where that province merges into the Great Plains on the east. Structurally, the region is a broad dome of Cretaceous rocks, from whose central part the Cretaceous cover has been stripped away, leaving an area of low country in the center, the Marathon Basin. Here strongly folded Paleozoic rocks are exposed. The Monument Spring and Marathon quadrangles, described in detail in this report, extend across the basin area.

The Paleozoic rocks exposed in the basin and in the Glass Mountains, which flank it on the northwest, have a thickness of 21,000 feet. The greater part of them were laid down in a subsiding area, the Llanoria geosyncline. The oldest rocks are Upper Cambrian sandstones and shales, whose base is not exposed. Overlying them are 2,000 feet of Ordovician rocks, composed of shaly limestone and shale, with some beds of chert, whose chief fossils are graptolites. The Ordovician is overlain by the Caballos novaculite, possibly of Devonian age, which reaches 600 feet in thickness. This white siliceous rock is the chief ridge maker in the Marathon Basin.

The Caballos novaculite is overlain by a great series of clastic rocks of Pennsylvanian age, as much as 1200 feet thick in the southeastern part of the area but much thinner in the northwest. Two of the lower formations are a mass of arkosic sandstone and shale and are separated by a widespread thinner limestone formation. The two formations contain few fossils other than land plants. The upper of the two contains a remarkable layer of mudstone, in which are embedded large blocks of older rocks. The blocks are believed to have been derived from the erosion of advancing thrust sheets and to have marked the fist strong uplift in the region; they may have been transported to their present positions either by glaciers or by mud streams. The uppermost Pennsylvanian formation consists of conglomerate and sandstone derived from the erosion of rising folds and contains abundant upper Pennsylvanian marine fossils.

The strong deformation to which the Paleozoic rocks of the Marathon Basin have been subjected apparently culminated E after the deposition of this uppermost formation of Pennsylvanian age. The Permian rocks of the Glass Mountains, to the northwest, rest, at least in places, with great angular unconformity on the disturbed older beds. The structural features seen in the basin consist of close folds, trending northeast and overturned to the northwest which are broken by numerous thrust faults. The faulting culminated on the northwest in the nearly flat-lying Dugout Creek overthrust, with a known displacement of more than 6 miles. Farther southeast are other great thrusts, also with miles of displacement, some of which are folded and therefore older than the frontal fault. The folds of the Marathon region are a part of a system of structural features formed from the rocks of the Llanoria geosyncline, which extends northeastward in sinuous courses to the Ouachita Mountains of Oklahoma and Arkansas. Northwest of the geosyncline and folds of the Marathon region, during Paleozoic time, there was a foreland area, which was gently folded at the same time as the movements at Marathon. Southeast of them was a region underlain by pre-Cambrian crystalline rocks. Both these areas are now mostly concealed by Cretaceous and Tertiary rocks.

The Permian rocks of the Glass Mountains, 5,000 feet or more thick, consist of limestones, siliceous shales, clay shales, and sandstones, which interfinger in a most complex manner. The most striking stratigraphic features of the series as exposed in the mountains are limestone reefs, constructed in large part by limesecreting organisms. The reefs apparently had a marked influence on the development of the other lithologic facies. The Permian rocks contain marine fossils, in places very abundantly. Most of the faunas are similar to the Guadalupian fauna originally described by Girty from northern trans-Pecos Texas. The Permian rocks are tilted to the northwest, away from the Marathon Basin, and are apparently in greater part younger than the folds in the basin.

The Cretaceous rocks that surround the Marathon Basin have a maximum thickness of about 1,200 feet and are mostly limestones. They were laid down on the eroded edges of the folded or tilted Paleozoic rocks, whose surface had been reduced to a peneplain during Triassic and Jurassic time. Over the Cretaceous west of the Marathon region lie lavas and tuffs of early Tertiary age. Within the region small masses of igneous rock, in part of alkalic composition, have intruded the Paleozoic and Cretaceous rocks.

The Cretaceous rocks dip gently away from the Marathon dome on its north, east, and south sides. On the west side they are sharply buckled and locally overthrust toward the west. The structural features on the west side of the Marathon dome are in part older and in part younger than the early Tertiary lavas. All the rocks of the dome are broken by normal faults that are younger than post-Cretaceous folds and probably of later Tertiary age.

No rocks younger than the Tertiary igneous rocks exist in the vicinity of the Marathon region except gravel deposits that cover part of the lowlands. These were deposited on various surfaces of erosion. The oldest stands several hundred feet above the present streams, and the gravel on it is probably of Pleistocene age.

The rocks of the Marathon region contain relatively few materials of economic value. Locally there are some metallic minerals, chiefly near the igneous intrusions. The hard siliceous novaculites of the Marathon Basin may be of use for whetstones or road metal. The jointed bedrock of the basin and its cover of gravel contain a supply of underground water. The area does not seem to be favorable for the accumulation of oil or gas.