Print this pageSequence Stratigraphy and Porosity in Late Paleozoic Limestones, Southwest Andrews Area, West Texas, USA
SALLER, ARTHUR H., J. A. D. DICKSON, and TOSHIKAZU EBATO
Porosity trends can be related to cycle stratigraphy, stable isotope compositions, and duration of subaerial exposure in Middle Pennsylvanian to Lower Permian limestones (Strawn to Wolfcampian; 2620-3000m) on the eastern Central Basin platform, west Texas. Approximately 90 depositional cycles, most bounded by subaerial exposure surfaces were recognized. Individual small-scale cycles represent eustatic sea-level fluctuations of 20-100 m at frequencies of 100,000 to 400,000 years/cycle. A large-scale, upward shallowing of depositional environments and thinning of cycles occurs between the Strawn and top of Cisco (2950 to 2720m), in response to a decrease in long-term rate of accommodation. Depositional cycles in the Wolfcampian thicken and contain more deep water facies upward, recording a long-term transgression which culminated in a major drowning and backstepping of the carbonate shelf at the end of the Wolfcampian. Bulk rock delta{13}C and delta{18}O values generally decrease as depositional cycles thin, indicating that thinner cycles were subjected to longer, more intense subaerial exposure prior to the major unconformity at the end of the Cisco. Two major factors apparently contributed to lower porosity in thinner cycles. (1) Longer subaerial exposure caused more cementation. (2) Shales, which are more abundant in intervals with thin cycles, decreased porosity by filling early vuggy to cavernous pores and accelerating pressure solution. Porosity is highest in thick cycles which, underwent brief subaerial exposure.