Abstract

The term “Ellenburger Limestone” was first introduced by Paige (1911) to describe the lower Ordovician carbonate rock outcrops in the Ellenburger Hills of the Llano region in central Texas. The Ellenburger was elevated to group status and subdivided into three formations (Tanyard, Gorman, and Honeycut) by Cloud and others in 1943 (1945). In 1948, Cloud and Barnes published an extensive compilation of central Texas Ellenburger outcrop data. The Ellenburger of central Texas is equivalent to the productive Ellenburger dolomites of the Permian Basin and nonproductive El Paso group in far west Texas. The central Texas Ellenburger rocks are not completely dolomitized and hence can provide more information about the original depositional environments and cyclicity than the subsurface dolomitized Ellenburger in the Permian Basin.

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. This poster presentation will feature the description and correlation of three cores that cut the entire Ellenburger section: Johanson (McCullogh County), Glaze (San Saba County), and Rasco (Mills County).

Despite a strong diagenetic overprint in portions of all of the cored wells, we have identified several types of depositional cycles. 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 second 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. The third type consists of rip-up clast capped cycles that are generally limestone but can be dolomitized, and are less porous and permeable.

The Johanson, Glaze and Rasco cores can be correlated from west to northeast. The Cambrian San Saba/Ordovician Ellenburger contact is used as a datum upon which to hang the cores. Cloud and Barnes (1948) indicated that Ellenburger outcrops thin westward due to erosion of the uppermost Honeycut formation and thinning of the Gorman formation. We have observed the same westward thinning in central Texas Ellenburger cores. However, the basal Ellenburger in the Johanson core contains the most open marine deposits observed to date, suggesting that the ocean deepened westward from the Llano uplift in Early Ordovician time. Numerous trilobites, brachiopods, and crinoids occur in this core. As you move to the north and northeast, Ellenburger cores display restricted depositional cycles with abundant oolite shoals and algal laminated tidal flats.