Correlation of Tertiary rock units, West Texas

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Maxwell, Ross A.

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University of Texas at Austin. Bureau of Economic Geology


Tertiary rocks, including sandstone, conglomerate, shale, pyroclastics, tuff, and lava, are preserved in Big Bend National Park and in a much larger area to the west and northwest. Some of the rocks have distinctive characteristics that enable recognition by their lithology. Others are distinctly dissimilar, although they may have been deposited about the same time. This dissimilarity may be in part due to a different source of material, as the normal behavior of the volcanic rock is to thin and disappear away from the source; also the volcanic rocks disappear by nondeposition and in other places by erosion, so that the casual observer would be unable to develop an orderly stratigraphic sequence over a large area. It was natural that the early investigators named and described rock units from a physical feature within the area where they worked, and commonly, due to lack of time, they did not examine the rocks in adjacent areas. This has led to much confusion in stratigraphic correlation. Multiple names have been applied to the same rock unit in different areas; the same name has been applied to different rock units because of lithologic similarity. Some rock units have distinctive lithological characteristics and can be traced for tens of miles along the outcrops. Other formations have distinctive vertebrate fossils and, to a less extent, invertebrate fossils. Also, some lavas have distinctive paleomagnetic characteristics. It is granted that these differences are only tools with which to work, but by using all the criteria a reasonable correlation can be formulated. Plate I is a possible correlation of the Tertiary volcanic rocks in West Texas.


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