Microbial, Algal, and Metazoan Dominated Highstand Carbonate Buildups and the Link to Lowstand Evaporite Distribution, Pennsylvanian of the Paradox Basin, SE Utah

In addition to water depth and accommodation space variation, water quality appears to have been a major control on dominant faunal components of carbonate buildups in the carbonate/evaporite system of the Pennsylvanian (Desmoinesian) of the Paradox basin, in southeastern Utah. Throughout the several million year history of alternating transgressive/highstand carbonate deposition on the basin margins and highs, and lowstand evaporite deposition in the basin, carbonate buildup thickness and faunal dominance can be related to basin geometry and the resultant spatial distribution of lowstand evaporites. Exposures of the basin’s western shelf along the San Juan River in the Goosenecks region reveal a distinctive vertical progression of carbonate buildup types. In the lowest 3rd order sequence of the shelf, the oil bearing buildups are limited to two stacked 4th order sequences; the first with 2-4m thick phylloid algal biostromes, which is overlain by a sequence with bryozoan and tubular foraminifera packstones capped by complex Chaetetes (a calcareous sclerosponge) heads and framestones. Stromatolites encrusted the top of Chaetetes heads before the mound became entombed in evapo-moldic silty dolomite. In the overlying 3rd order sequence of the lower Akah, 4th order sequences are capped by microbial boundstones forming progressively thicker deposits in two stacked sequences, ultimately culminating in mounds >5m thick. Carbonate buildups in 3/5 of the 4th order sequences within the upper Akah sequence return to high energy Chaetetes dominated mounds. In the overlying 3rd order Desert Creek sequences there are no buildups in outcrop, but phylloid algal, bryozoan, and coralline-algal buildups occur on and adjacent to topographic highs such as the giant Aneth field (Chidsey et al., 1996). Spectacular and well studied phylloid mound complexes are exposed in outcrops along the San Juan River in the 3rd order sequence of the lower Ismay.

In these Desmoinesian sequences, microbial buildups are limited to the tops of 4th order sequences, during a portion of a basin evolution (Akah sequences) when evaporites where expanding farther onto the shelf on all sides of the almost filled basin. This unique association suggests that hypersalinty may have favored microbial carbonate buildup development. Shallow open marine conditions favored either phylloid buildups with higher energy settings dominated by Chaetetes, and deeper settings dominated by bryozoa.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California