Cenozoic history of northeastern Chispa quadrangle, Trans-Pecos, Texas

Tertiary beds of tuff and related sedimentary deposits with intercalated lava flows, comprising an aggregate thickness of slightly more than 1,000 feet, rest with an angular unconformity upon Cretaceous and Permian. This volcanic sequence is placed in the McCutcheon group (Eifler, 1951), and no names are proposed for the individual members. The early flows are found in a basin-like depression with a very thick water-laid tuff section directly above, but the remaining flows and tuffs are found over extensive areas, and are less affected by erosion immediately following deposition of the individual units. After deposition of the Tertiary volcanic rocks, faulting occurred, and the Quaternary formations developed. Much variation of climatic conditions is revealed by numerous changes in Quaternary sediments. Three intrusions are present in the quadrangle, and, in the Oat Mountain intrusion, large blocks of Permian and Cretaceous rocks were brought up and exposed on the north and south ends of the Mayfield Hills. The major structural features are the southward and eastward-trending faults which divide the quadrangle into two distinct units: horst mountains, and the corresponding graben, Michigan Flat