Tertiary shorelines, Texas coastal plain

Abstract

The position of the present shore line of the Gulf of Mexico in Texas is taken as one reference line and the area of outcrop for each series as another. At the outcrop Paleocene Midway deposits are marine; Eocene deposits are alternately continental and marine; Oligocene, Miocene and Pliocene sediments at the surface are continental. A single exception is the presence of a brackish-water near-shore deposit of Middle Miocene age in far eastern Texas. The position of the shoreline during the Paleocene would be north of the present area of outcrop of the Midway Group. During the Eocene, the shore line would have fluctuated; at times it was north, at times within, and at other times south of the area of outcrop. The Oligocene, Miocene, and Pliocene are continental so the shore line must have been south of the area of outcrop, and form subsurface information, fluctuated on either side of the position of the present shore line. A post-Early Pliocene pre-Middle Pleistocene erosion surface underlies widespread high-levels sands (Willis of eastern Texas), gravels (Uvalde of central Texas, Seymour and others of north central Texas) and caliche (Reynosa of southern Texas). From their position on the interstream divides, the aforementioned sands, gravels, and caliche would have been deposited prior to the incision of the present day river valleys and therefore antedate all river terraces. The valley incisions were probably associated with the first lowering of sea level during the Pleistocene.