MICROBIAL MAT TEXTURES IN LITHOFACIES OF THE PENNSYLVANIAN PARADOX FORMATION, PARADOX BASIN, UTAH

The Paradox Basin is an asymmetric basin that subsided in the Pennsylvanian and Permian, and contained an inland saline lake during the Pennsylvanian. Marine waters likely entered the basin by seepage through porous clastic and (or) carbonate units that formed a southeastern barrier to the ocean, and by periodic storm and seasonal high-tide washovers. This study identified lithofacies within the Pennsylvanian Paradox Formation based on description of two subsurface cores. A typical vertical succession of lithofacies consists of irregular laminated anhydrite, selenitic anhydrite, halite-sylvite, evenly laminated anhydrite, silty dolomite, and gray to black mudstone. Microbial mat textures were recognized in each lithofacies, including the organic-rich black mudstone. This idealized lithofacies sequence represents a complex interplay of regional climatic, and local and global sea level changes.

The irregular laminated anhydrite is interpreted to have been deposited in a gypsiferous, arid, tidal-flat environment with little clastic or freshwater input. Anhydritic lithofacies transition upward into the halite-sylvite facies, which represents a shallow salina environment and hyper-saline conditions. Microbial and thin-shale laminae occur within the halite-sylvite facies. The evenly laminated anhydrite formed above the halite-sylvite facies as additional water input decreased lake salinity. Alternating dark- and light- colored laminations in the two anhydrite facies are interpreted as microbial mat laminations and anhydrite, respectively. The silty dolomite facies represents increased rainfall and clastic input to the basin, and it includes ripped-up microbial mat fragments. The gray to black shale facies was deposited in a perennial meromictic lake during periods of salinity stratification. In thin section, peloidal, clotted fabrics are locally recognized. Linear pyrite stringers are also present, and in some cases the linear stringers are folded back on themselves, suggesting rolled-up microbial mat fragments. The recognition of microbial mat textures in all lithofacies in the Paradox Formation encourages further research into the controls on mat development because of their potential importance in understanding climate fluctuations and the deposition of source rocks.