Abstract: Oligocene Volcanic Sedimentary Apron in Trans-Pecos Volcanic Field of Texas

Anthony W. Walton

In the Oligocene Tascotal Formation of Trans-Pecos, Texas, a volcanic-sedimentary apron more than 100 m thick, consists mostly of repetitive, fining-upward sequences 0.5 to 2 m thick. Above a basal scour surface or sharp grain-size change, the sequences include medium to coarse sandstone or granule conglomerate, which is structureless or low-angle trough cross-stratified. Gradationally above this is structureless or thinly bedded fine sandstone or siltstone. Tracked horizons, mud drapes, or subtle grain-size changes delineate bedding surfaces and demonstrate the episodic nature of deposition. Major channel deposits, up to 15 m thick, fill depressions scoured into the sequences. Most channels contain parallel-bedded or low-angle cross-stratified fine sandstone to granule c nglomerate but in their lower part have beds of cobble to boulder conglomerate one or two clasts thick. Airfall-tuff beds are fine grained and contain much biotite or sanidine, but few VRF's. Above tuff-beds, the rock is anomalously fine grained, but the sediment grades upward into sequences like those below the tuff within several meters.

The apron developed by processes analogous to those which build alluvial fans. Sediment was introduced directly to the apron surface by airfall as well as being transported by streams from source areas. The channel deposits indicate main transport routes; the lower part of the repetitive sequences record minor distributaries, and upper parts represent top-stratum or bar deposits. Contemporaneous volcanic activity produced lava flows, welded tuffs, and much fine pyroclastic material. Because the grain size of available sediment was small, the apron's gradient was low.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC