Evolution and Diagenesis of Pennsylvanian Phylloid Algal Bioherms in the Paradox Basin of Southeastern Utah

Jonas P. Gournay

Three lower Ismay (Paradox Formation) bioherms, were examined at Honaker Trail, Soda Basin, and Eight Foot Rapids along the San Juan River. Approximately 150 samples were collected at approximately 0.5 m vertically spaced intervals. Samples of unique lithologies were also collected.

The lower Ismay bioherms studied lie along an east-west traverse with respect to each other and display slight differences in internal facies composition. Phylloid algae generally dominate the bioherms, but associated biota include: fusulinids, crinoids, bryozoans, ostracods, minor trilobites, and rugose corals. Facies changes and the presence of numerous stylolitized layers within the bioherms suggest distinct growth phases. Additionally, the phylloid-rich layers appear to exhibit vertical variations in total micrite. Variations in facies and micrite composition may be controlled by sea level fluctuation or may reflect microbial activity. Consistent variation in porosity and permeability within bioherms may reflect high order cyclical events, possibly sixth order.

Throughout the bioherms studied, variations in porosity and permeability correspond to variations in lithology. The bioherms are homogenous laterally, but heterogeneous vertically and the porosity zones reflect this. The zones are characterized by moldic porosity, increased abundances of phylloid algae, and each is capped by a mud-rich unit. Many of the porous zones are about 2 m thick with two or three associated smaller bands. The smaller bands are completely compartmentalized from the major bands. This information is of significant importance to production from phylloid algal reservoir systems.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California