--> Abstract: Sea Level and Climate Controls on Lithofacies of the Bead Mountain Composite Sequence, by Ryan Lellis and Peter Holterhoff; #90089 (2009)

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

Ryan Lellis and Peter Holterhoff
Department of Geosciences, Texas Tech University

The Bead Mountain composite sequence consists of the underlying Valera Shale and overlying Bead Mountain Limestone (lower Permian) of the Wichita-Albany Group of the Eastern Shelf of the Midland Basin of north-central -Texas. The Bead Mountain composite sequence appears to be of late Artinskian age, straddling the Wolfcampian – Leonardian boundary.

The Bead Mountain composite sequence is divided into two sequences based upon the recognition of lowstand siliciclastic packages containing evidence of subaerial exposure. Internally, these sequences consist of two sets of major facies couplets. Each sequence consists of a lower couplet set dominated by siliciclastic mudstones with thin carbonate interbeds and an upper couplet set dominated by carbonates with thin carbonaceous shale partings. Mudstone dominated intervals are generally drab in color (black to medium gray green), contain plant material, conchostracans (fresh water arthropods), and weakly developed paleosols in the lower sequence as indicated by clay slickensides and red and yellow mottles. The associated carbonate beds are generally thin skeletal packstones or peloidal wackestones. These muddy intervals are interpreted as overbank or lacustrine facies, and contain potential sequence boundaries. Carbonate dominated intervals are thick packages of bioturbated mollusk packstones and grainstones which are interbedded with thin, laterally discontinuous, carbonaceous shales. A horizon of crinoid fossils near the top of the upper sequence signifies the most open marine facies in the Bead Mountain composite sequence.

Gamma ray spectrometry reveals that the relative abundance of thorium and calculated volume of shale in these carbonate-dominated intervals decreases towards the top of each package. This trend indicates a reduction of detrital input into the carbonate platform system upward through each sequence. These carbonate intervals are interpreted as deepening upwards shallow marine facies with the potential maximum flooding surfaces of each sequence located near their top.

A significant question for the Bead Mountain package is the origin of the limestone/shale couplets. The carbonate and shale couplets 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, in turn hampering carbonate production. Continued outcrop description and correlation, spectral gamma ray profiling, geochemical analysis, and thin section examination will help determine the dominant driver for Bead Mountain couplets.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009