Abstract

The Lower Ordovician Ellenburger Group was named for outcrops in the Ellenburger Hills of the Llano region in central Texas. It is equivalent to the productive Ellenburger of the Permian Basin, but in central Texas outcrops are not completely dolomitized and hence can provide more information about the original depositional environments and cyclicity of the formation. Mineral exploration cores were cut in a number of shallow wells on the north and west sides of the Llano uplift in the 1950’s and 1960’s. Some of these wells cut continuous core through the entire Ellenburger Group. They have been stored at the Bureau of Economic Geology in Austin. Until recently they were unslabbed and unstudied. Currently, efforts are underway at University of Texas of the Permian Basin to describe these cores and build a modern depositional and sequence stratigraphic framework for the central Texas Ellenburger. Despite a strong karst diagenetic overprint in portions of the core, we have identified two types of cycles that will be on display. One type shallows up to tidal flats and exposure, is generally less dolomitized, contains algal laminations, fenestral textures and mudcracks, and is relatively less porous and permeable. The other type shallows up to high energy grainstone shoals, is highly dolomitized in the shoal caps, contains cross bedding, and is relatively more porous and permeable. Slabbed core examples of both Ellenburger cycle types will be on display during the poster session.