LACUSTRINE CARBONATE DEPOSITION WITHIN TERTIARY VOLCANIC EVENTS, BREWSTER COUNTY, TEXAS

Non-marine limestone is interbedded with the Eocene-age Crossen Trachyte and Sheep Canyon Basalt, south of Alpine, Texas, in the Trans-Pecos Magmatic Province. The limestone, up to 60 m thick, was originally described by Collinsworth in 1984, and consists of mudstone, bioclastic packstone and wackestone, stromatolitic travertine, and uncommon oncoids. Individual flows of Sheep Canyon basalt within the limestone sequence can be distinguished. The lake(s) was probably formed by flows blocking the existing drainages.

Recently a new quarry was established in the limestone, exposing a nine-meter thick section on the wall. The continuous, well bedded limestone shows cyclic deposition. The bottom of each cycle begins with peloidal grainstone or packstone at the base and stromatolitic travertine at the top of each carbonate cycle. The peloidal beds are 10-18 cm thick and the travertine beds are 15 to 38 cm thick. Within many of the cycles styolites are present. Each cycle is separated by a thin orange clay layer. Fossils present include stromatolitic algae, gastropods, bivalves, and biochemically induced casts of grass and reeds. Although some of the other lacustrine beds are silty, the limestone from the quarry is 98.9% CaCO3 in the peloidal mudstones and 99.1% CaCO3 in the stromatolitic travertine. The lack of clastic input to a lake surrounded by lava flows suggest some of the water may have entered as springs emerging from the flows at the edge of the lake. The cycles are interpreted as shallowing-upwards events representing shallow water (bioturbated peloidal limestone) followed by exposure (travertine and clay). Beds in the quarry wall are uniform and no buildups are present. Sources of carbonate may include the Cretaceous limestone underlying the volcanics.