Note on Paleozoic rocks of central Texas

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Abstract
THE writer had the opportunity the past season to make a hurried reconnoissance of a portion of the Paleozoic area of Central Texas: the chief object in view being the study of the Cambrian section and the collecting of fossils from the Texas Potsdam hOl•izon.At all localities where the base of the Potsdam was observed, it rests, unconformably, on a great formation that is stratigraphic• ally the equivalent, of Powell's Grand Canon series (Grand Canon and Chuar group;l).*In the Grand Canon of the Colo• rado the latter are overlaid by the Tonto group, a series of rocks, in both lithologic and paleontologic characters, singu.larly like the Texas Potsdam group.For this series of Pre• Potsdam strata the local name of Llano group is proposed from the best exposures of the group occur• ring in the county of Llano.Outcrops also occur in Burnet, Mason, San Saba, Blanco and Gillespie counties.The finest exposure seen by the writer, in dil'ect contact with the base of the Texas Potsdam group, is along the west• ern base of Packsaddle mountain, in Llano county.Here the massive reddish colored sandstones of the Potsdam strike north and south with a slight dip to the eastward, and rest on alterna• ting beds of shale, sandy shales, sandstone, limestone and schists that strike east and west, dipping south 15° to 40°.The stmta exhibit but little evidence of metamOl'phism, beingindll' rated but little more than the beds of the overlying Potsdam and Carboniferous.The section shows the Llano and Potsdam groups unaffected by changes subsequent to the consolidation of the Potsdam sediments.Across the valley of Honey creek, four miles west of Pack• saddle mountain, the strata of the Llano group have been more metamorphosed, plicated, and broken b.y intrusive dykes of granite.This is along the eastern base of a ridge of Potsdam Silurian and Carboniferous rocks that stl•ike eastward with a dip that increases from 10° at the north end of the ridge to 40° at the south end.The movement producing this position, as compared with the Potsdam beds of Packsaddle mountain, appears to have been at the close of the Paleozoic, and to have caused, in part, the metamorphism of the strata of the Llano group, but was not accompanied by the intrusive rocks, as they are Pre•Potsdam-in part cotemporar.y with the deposition of the sediments of the Llano group, but more large• ly the result of extrusions of granite at or near the close of the *
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Authors Charles D. Walcott
Journal american journal of science
Year 1884
DOI
10.2475/ajs.s3-28.168.431
URL
https://doi.org/10.2475/ajs.s3-28.168.431
Keywords Keywords not found

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