Cenozoic pisolitic limestone and caliche near San Angelo, West Texas



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A distinctive "surficial" limestone consisting of pisolites with crenulated laminae, set in a matrix of brown fine-grained, secondary calcite, occurs widely in the region of San Angelo in western Texas. The material occurs in a one- to eight-foot thick zone as indurated, rounded masses three inches to six feet in diameter. Stratigraphically it occurs beneath zero to four feet of surficial loam, and generally overlies sandy friable caliche. The pisolitic limestone was found beneath two distinct, relatively flat topographic surfaces: (1) the High Plains-Edwards Plateau, where it rests on Cretaceous Edwards Limestone, and (2) east of the High Plains on stream terraces and the Lipan Flats, where it overlies the Middle Pleistocene Leona Formation. Only one zone of pisolitic limestone was found on either surface, and that on the High Plains has exactly the same appearance as that on the terraces and Lipan Flats. Laboratory data, particularly X-ray diffractograms and insoluble residues, could not distinguish between them. The pisolitic limestone probably formed by soil-forming processes in Middle Pleistocene time (probably Sangamon Interglacial) beneath relatively flat, stable surfaces underlain by porous and permeable carbonate material (chiefly limestone gravel). In the study area the pisolitic limestone does not mark the top of the Pliocene Ogallala Formation, as is considered to be the case farther to the north in Kansas and adjoining states. Although the Ogallala Formation is not present in the study area, pisolitic limestone undistinguishable from that described as occurring at the top of the Ogallala is widespread on top of Middle Pleistocene gravel in the study area.