Comparison of Quartz and Carbonate Shallow Marine Sandstones, Fredericksburg Cretaceous, Central Texas

Abstract

The lower Fredericksburg Cretaceous of central Texas contains two shallow marine sandstone deposits of similar shape but of contrasting lithologic character, genesis, and porosity trend. The lower sandstone (Paluxy Formation) is part of the initial clastic phase of the lower Fredericksburg depositional cycle and is composed predominantly of quartz sand and clay. It occurs as a tongue which projects southward from the main body of the formation. The Paluxy tongue was deposited by longshore currents, and modified shoreward by wave swash and tidal action in a coastal nearshore marine environment. The trend of the tongue is controlled by the position of the shoreline and by the configuration of the sea floor. The upper sandstone (Whitestone Limestone Member of the Walnut Formation) is the terminal phase of the lower Fredericksburg cycle and is composed entirely of carbonate grains. The Whitestone is an elongate, mound-shaped body of lime-grainstone trending northwest; it was deposited in an agitated, offshore, shallow marine environment by northwest-southeast-trending marine currents which were modified locally by surge channels normal to this trend. The trend of the mound is controlled by linear shoal areas.