Southwest Section

The Sierra Diablo Mountains of west Texas contain world-class exposures of Lower Permian (Leonardian) platform carbonates. As such, these outcrops offer key insights into the products of carbonate deposition in the transitional icehouse to greenhouse setting of the early to middle Permian that are available in few other places. They also afford an excellent basis for examining how styles of facies and sequence development vary between inner and outer platform settings. We collected detailed data on the facies composition and architecture of lower Leonardian high-frequency cycles and sequences from outcrops that provide more than 2 mi (3 km) of continuous exposure. We used these data to define facies stacking patterns along depositional dip across the platform in both low- and high-accommodation settings and to document how these patterns vary systematically among and within sequences. Like icehouse and waning icehouse successions elsewhere, Leonardian platform deposits are highly cyclic; cycles dominantly comprise aggradational upward-shallowing facies successions that vary according to accommodation setting. Cycles stack into longer duration high-frequency sequences (HFSs) that exhibit systematic variations in facies and cycle architectures. Unlike cycles, HFSs can comprise symmetrical upward-shallowing or upward-deepening facies stacks. High-frequency sequences are not readily definable from one-dimensional stratigraphic sections but require dip-parallel two-dimensional sections and, in most cases, HFS boundaries are best defined in middle platform settings where facies contrast and offset are greatest. These studies demonstrate that HFSs are the dominant architectural element in many platform systems. As such, the lessons learned from these remarkable outcrops provide a sound basis for understanding and modeling carbonate facies architecture in other carbonate-platform successions, especially those of the middle to upper Permian.