Sedimentology and petrography of deltaic facies in the Aguja Formation, Brewster County, Texas

Abstract

The Upper Cretaceous Aguja Formation, northwest of McKinney Hills in the eastern part of Big Bend National Park, Texas, was deposited in a deltaic depositional environment. Prodeltaic, distributary mouth bar, distributary channel, interdistributary bay (including levee, crevasse splay and crevasse channel), and abandonment facies can be distinguished.

Two delta progradational episodes are recorded. The basal sandstone member was deposited during the initial progradation (Early Campanian) of a lobate delta complex. The delta system was abandoned and transgressed depositing marine shale of the McKinney Springs tounge of the Pen Formation. Progradation of a slightly elongate delta complex resumed in Late Campanian time depositing the Terlingua Creek Sandstone member and upper shale member. Paleocurrent analysis indicates that the deltas prograded eastward across the Coahuila Platform.

The primary source terrane for the deltaic sediments was volcanic but also included plutonic igneous, metamorphic, and sedimentary rocks. It is possible to distinguish the deltaic sub-environmentspetrographically. Distributary channel deposits tend to be richer in rock fragments and abandonment deposits richer in quartz than other deltaic facies. Sandstone porosity has been occluded by quartz overgrowths, authigenic clays, and carbonate cements including calcite, ferroan calcite, and ankerite.