Allocycles, Autocycles, and Dolomite Distribution in Albian Peritidal Successions of the Upper Glen Rose, Austin, Texas
Driving mechanisms of cycle-scale stratigraphy in greenhouse systems remain controversial, yet significant emphasis is often placed on the lateral continuity of shallowing upward vertical facies repetitions in broad carbonate shelves. The implementation of stratigraphic models in which meter-scale cyclicity is laterally continuous for tens of kilometers has profound implications on reservoir production methods and furthers sedimentologic theories advocating the dominance of Milankovitch orbital cyclicity in carbonate systems. We examine these principals through sequence stratigraphic analysis of the final 80 meters of shallow subtidal to intertidal high-frequency cycles in the Glen Rose composite sequence.
Lower Albian peritidal cycles of the Upper Glen Rose Formation exposed in roadcuts along Highway 360 in Austin, Texas provide a superb example of Lower Cretaceous shelf interior vertical facies repetitions. High-frequency cycles, cycle sets, and sequences are correlated along the 12 km dip-oblique transect of roadcuts. Analysis of detailed measured sections indicates that shallowing upward vertical facies patterns in the tectonically stable Comanche Shelf are representative of both autocyclic and allocyclic sedimentation. Our results suggest that autocyclicity is an inherent aspect of peritidal sedimentation, and autocyclicity is prevalent in late highstands of composite sequences. Using multiple measured sections in similar stratigraphic intervals, we identify autocyclic facies cycles and employ Fisher plots of allocyclic, laterally correlative high-frequency cycles to compile a relative sea level curve for the Upper Glen Rose.
Prominent trends are noted regarding potential reservoir facies distributions in the highstand systems tract. Progressing toward the top Glen Rose sequence boundary, a distinct increase in dolograinstone and dolopackstone facies are observed. These facies possess significant inter-crystalline or moldic porosity, and are often capped by algal-laminated dolomudstones indicative of intertidal sedimentation. Toward the sequence boundary, cycle sets are increasingly capped by decimeter scale collapse breccias thought to be representative of former evaporite nodules lost by dissolution during early burial diagenesis. Relatively argillaceous, non-reservoir facies are concentrated in maximum floods of high-frequency sequences and would provide considerable barriers to flow in similar depositional settings of subsurface reservoirs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009