This work attempts to establish the age and chronologic sequence of mid-Cenozoic tectonic events in Trans-Pecos Texas through the use of radiometric dates, and new or revised structural, lithostratigraphic, and vertebrate biostratigraphic information. Late Mesozoic to early Cenozoic Laramide faulting, folding, and jointing superimposed on older trends, established the fabric governing younger structures. Late Oligocene events, occurring about 28 to 26 m.y.B.P., include right-lateral divergent wrench faulting, local compression, and the last episodes of silicic intrusion-extrusion. The major period of basin-and-range faulting began about 19 m.y.B.P., accompanied by late and mafic minor intrusion. This paper follows the sequence of tectonic events established by Muehlberger in 1980 and DeCamp in 1981 for Trans-Pecos Texas.

A widespread low relief surface was cut across Laramide structures as deformation decreased in the Eocene. Integrated, perennial streams flowing southeastward began extensive laterally continuous aggradation in the area south and southwest of the stable Diablo uplift. Mainly fine, volcaniclastic sediments accumulated on surfaces of continued low relief. Initially, sediment sources were distant but became progressively more local. Episodic ignimbrites and flows covered large areas with increasing frequency. Eocene climate in Trans-Pecos Texas was humid and subtropical, but an irregular trend toward increasing dryness was evident by 31 m.y.B.P.

Sedimentary bodies younger than middle Oligocene have little lateral continuity. Deposited under semiarid conditions, as destructional volcanic sediment aprons, alluvial fans, or bolson fills, these units show progressive divergence from depositional styles of early Tertiary sediments. Early Arikareean (early late Oligocene) right-lateral divergent wrench faulting interrupted long-established drainage patterns. The faulting, dated by intrusions and biostratigraphy at about 28 to 26 m.y.B.P., closed the interval of laterally continuous, and preceded that of discontinuous, deposition. The irregular Terlingua monocline, long considered a Laramide structure, is re-interpreted as another example of Trans-Pecos Texas linear east-west tectonic elements discussed by Dickerson in 1981. The str cture is a large monocline cut by a set of an echelon-left normal faults and smaller monoclines, modified by compression. Formation of the large monocline involved rocks as young as late Eocene-early Oligocene. The Terlingua monocline provides clear evidence of the sequence of events and some indication of timing. Later Arikareean sediments lie on uneven eroded older rocks, disturbed by early stages of this wrench faulting. About 23 m.y.B.P. downfaulted basins began to retain bolson/alluvial fan sediments. Deposition may have resulted from progressive deformation of the change to mafic volcanism. Increasing aridity may also have been a factor.

Basin-and-range faulting affected Trans-Pecos Texas during the Hemingfordian (early Miocene), and continues. This tectonism faulted the later Arikareean-Hemingfordian alluvium by reactivating old faults and creating new ones. Basin-and-range faulting shifted, deepened, and more completely restricted basins of deposition by forming a series of northwest oriented grabens which received great thickness of later Miocene and younger alluvium. (See Figure on page 615).

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