AMSBURY, DAVID L.¹ and O. T. HAYWARD^2
1128 Homestead, Kerrville, TX
²Dept. of Geology, Baylor University, Waco, TX

Abstract: Quaternary Erosional Landscapes Of Central and North Texas

Two stratigraphic surfaces punctuate late Neogene erosion, based on reconnaissance: a pre-Pleistocene plain, and a midPleistocene landscape similar to today's.

The earliest preserved landscape ("Time 0 Surface") is marked north of the Colorado/Brazos divide by cobbles and boulders on interfluves. Gravels are vein quartz and quartzites from the New Mexico Rockies. Gravel lags along the divide occur 60-70 m above the base of the late Miocene Ogallala Formation and may represent the widespread mid-Eocene, pre-volcanic erosional surface of the southern Rockies. The "Time 0" surface in the Lampasas Cut Plain can be contoured. Substrates range in age from early Cretaceous to mid-Eocene. "Time 0" gravel sheet(s) and a stripped plain on the Edwards Limestone formed a nearly featureless late Miocene surface of at least 250,000 sq km.

The first gullies working headward along the incipient Balcones Escarpment intersected water-satu rated, uncemented, early Cretaceous sand. Exumation of the paleo Colorado and paleo Brazos valleys would have been near-catastrophic. Valley-widening by pedimentation then excavated a minimum of 70,000 cubic km of material from these two valleys during the short-cycle stage of the early to mid-Pleistocene. Incision ranged from 100m (Brazos) to 200m (Rio Grande). Pecos and Rio Grande integration occurred after the major episode of canyoncutting, about 600,000 my. This mid-Pleistocene landscape was preserved by the Stage III/IV Reynosa Caliche, probably marking the top of the coastal Lissie Formation, the Pecos Mescalero Plain, and the Rio Grande La Mesa Surface.

Later incision during 100-120,000-year glacial cycles cut 15-30 m into the mid-Pleistocene surface, excavating enough material to form the widespread Beaumont alluvial -deltaic plains.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas