Sea Level and Climate Controls on Lithofacies of the Bead Mountain Sequence

Ryan Lellis
Texas Tech University

The Bead Mountain “sequence” consists of the lower Valera Shale and overlying Bead Mountain Limestone (lower Permian) and is part of the Wichita-Albany Group, of the Eastern Shelf of the Midland Basin of north-central -Texas.

The Bead Mountain "sequence” consists of a lower mudstone- dominated interval with thin carbonate beds and an upper carbonate - dominated interval with thin carbonaceous shale partings. In places, the mudstones of the lower interval contain abundant plant material, conchostracans (fresh water arthropod), and weakly developed paleosols. In addition, spectral gamma-ray analysis reveals that the lower system has a relatively high concentration of thorium and potassium, and a low concentration of uranium. The carbonate beds in the lower system are generally skeletal packstones/peloidal wackestones.

A significant question for the Bead Mountain interval is the origin of the limestone/shale couplets. The carbonate and shale couplets of the lower system may represent changes from wet to dry climate. Carbonate deposition could have become dominant during dry periods due to low terrestrial sediment influence. In contrast, a wet climate would supply more siliciclastic material and fresh water into the system, which might hamper carbonate production. Another possibility is that the couplets may be driven by higher order relative sea level fluctuations. The higher order driver is then imprinted on a larger scale relative sea level oscillation which may account for the switch from clay to carbonate dominated system. Continued outcrop description, gamma ray profiling, geochemical analysis, and thin sections will help determine the dominant driver for Bead Mountain couplets.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas