Subsurface Lower Cretaceous Formations of South Texas

Abstract

ABSTRACT Evidence is presented: (1) that the Hosston and Sligo formations extend from eastern to southern Texas, that they do not crop out on the Coastal Plain, and that they represent the Neocomian and lower Aptian, respectively; (2) that the Travis Peak formation of the outcrop area is represented in the subsurface by a shaly sequence, herein named the Pearsall formation, which includes basally the Pine Island shale member, medially the Cow Creek limestone member, and at the top the Hensell shale member; (3) that the tops of (a) the Cow Creek limestone member of the Travis Peak and the Pearsall formations, (b) the James limestone, and (c) the Dierks limestone mark approximately the Aptian-Albian boundary; (4) that the Ultima Thule gravel member of the Holly Creek formation of Arkansas, which passes basinward into the basal part of the unit known informally as the Hill sandy lentil of the Rodessa, may mark the position of a minor disconformity, and is probably correlative with the conglomeratic beds at the top of the Hensell sand member of the Travis Peak formation; (5) that the Glen Rose formation of South Texas passes northward in the subsurface into the Paluxy, Mooringsport, and Ferry Lake formations and the upper part of the Rodessa formation; (6) that some silty shaly beds at the top of the subsurface Glen Rose limestone of South Texas are probably equivalent to the Paluxy sand of the outcrop; (7) that the Edwards and Comanche Peak limestone of South Texas are considerably thicker than equivalent beds in East Texas; (8) that the surface of the Edwards limestone in the area of the San Marcos arch was subjected to considerable erosion prior to Fort Worth time; (9) that the Kiamichi formation is represented in South Texas by a sequence of black limestone and shale containing considerable anhydrite and, locally, salt; (10) that the Kiamichi formation throughout most of the subsurface of South Texas grades into the overlying Georgetown limestone, but near its northern boundary pinches out in a short distance and is overlapped disconformably by the Georgetown; (11) that the surface and subsurface Georgetown limestone of South Texas west of Uvalde County grades into a thicker rudistid facies.