MIDDLE CAMBRIAN MICROBIAL COMMUNITIES ALONG A BATHYMETRIC GRADIENT

Microbial communities within the uppermost part of the Middle Cambrian (Bolaspidella Zone) Wheeler Formation make up a distinctive and laterally-continuous horizon in the Drum Mountains of west-central Utah. These communities were originally described by Rees (1984). The microbial communities in the Drum Mountains display pronounced bathymetric-related changes in morphology and composition. Along a 2.5 km, NW (shallower) to SE (deeper) traverse, the following assemblages occur: 1) Laminar stromatolitic mats. 2) Tall (1 - 2 m), narrow (10 - 15 cm) stromatolitic columns. 3) Short (0.5 m), thick, teardrop-shaped (15 - 20 cm by 30 - 60 cm) stromatolitic columns showing pronounced current orientation with intercolumnar areas filled with oncoids, ooids and bioclastic debris. 4) Very large (10 m diameter, 3 m thickness) biohermal mound complexes with up to 1 m synoptic relief. These comprise a variety of microbialites, including the calcimicrobes Renalcis and Epiphyton, as well as thrombolites and stromatolites. In places the bioherms are cut by steep-sided tidal channels filled with ooids, oncoids and bioclastic materials. Small (10 - 20 cm diameter) thrombolite mounds occur at a stratigraphically-equivalent, deep ramp position 30 km to the SW in the more distal House Range. These microbial communities appear to have developed during the earliest transgressive stage of the carbonate platform and are overlain by deeper-water, late transgressive - early highstand facies. Thus, they occur just above a sequence boundary, roughly corresponding to the contact between the Wheeler Formation and the overlying Marjum/Pierson Cove Formations