Effects of Long-Term Accommodation Change on Short-Term Cycles, Upper Paleozoic Platform Limestones, West Texas

Abstract

Upper Paleozoic limestones were studied in the subsurface on the eastern side of the Central Basin platform west Texas to determine the effect of long term change in accommodation on shon term cycles Approximately 87 cycles are present in five stratigraphic intervals that are from bottom to top the Strawn Canyon Cisco Wolfcamp detrital interval and Wolfcamp reef interval The lower three intervals are Middle to Upper Pennsylvanian and the two Wolfcamp intervals are Lower Permian The average cycle thickness is 3 7 rn Most cycles are bounded by sub aerial exposure surfaces and each cycle apparently represents a glacio eustatic fluctuation of sea level at an average frequency of approximately 163 000 yr per cycle Characteristics of cycles change upward from the base of the Strawn well depth of 2950 m 9700 ftj to the top of the Cisco depth of 2730 m 8950 ftj and those changes includea general shallowing of depositional facies more grainstones a decrease in average cycle thickness an average of 5 2 decreasing to 1 9 m an increase in amount of shale and adecrease in 15IC and 15 0 values of bulk rock samples These patterns are interpreted to be aresponse to a long term decrease in rate of accommodation The changes upward through the Wolfcampian between depths of 2620 and 2730 m 8600 and 8950 ftj however include a general thickening of the depositional cycles an average of 2 increasing to 4 5 m an increase in 1513C and 15 0 values and more deep water facies more wackestones and mudstones apparently in response to a long term increase in accommodation A major drowning and backstepping of the carbonate platform margin occurred at the top of the Wolfcamp reef inter val The time represented by each cycle includes 1 time when the sediment surface was below sea level accumulating carbonate sediment and 2 time when the sediment surface was subjected to meteoric diagenesis above sea level and no carbonate sediment accumulated Meteoric diagenesis apparently caused the stable carbon and oxygen isotope compositions of entire limestone cycles to become lighter with more prolonged subaerial exposure Low 15 C and 15 0 values in thin cycles suggest that the cycles were subjected to progressively longer subaerial exposures as the rate of accommodation decreased finally amajor unconformity was created at the top of thePennsylvanian This scenario suppons the idea that compared to thin cycles thick cycles spent more time below sea level accumulating sediment and less time above sea level being subjected to meteoric dia genesis The most widespread porosity occursin cycles of intermediate or greater thickness 2 m that were subaerially exposed for brief to mod erately long periods estimated at 5 00050 000 yr of freshwater diagenesis upper part of the Strawn lower and middle parts of the Canyon and Wolfcamp reef intervals Thin cycles 2 m thick upper part of the Cisco and lower pan of the Wolfcamp detrital interval have little porosity because I longer subaerial exposure resulted in more cements filling most matrix pores and 2 vugs fractures fissures and caverns that formed during prolonged subaerial exposure were filled by shale