ENVIRONMENTAL CONTROLS ON MICROBIALITE BUILD-UPS IN THE LOWER FILLMORE FORMATION, UTAH

The most widely-held hypothesis for the decrease in microbialite dominance following the Precambrian-Cambrian transition (PCT) is that the evolution of grazers led to the relegation of microbialites to environments that are less hospitable to animals. An alternate interpretation for the microbialite decline is that ocean chemistry became less favorable to microbialites during the PCT (Grotzinger & Knoll, 1999). Although microbialite fabrics are found in reef settings throughout the Phanerozoic, larger microbial build-ups in post-Ordovician marine strata are rare except in the aftermath of mass extinctions. During the Late Cambrian and Ordovician, however, microbialites experienced a resurgence as dominant reef-builders (e.g., Rowland & Shapiro, 2002) following the Cambrian Radiation and during the onset of the Great Ordovician Biodiversification Event. Resurgence of microbialites during an interval of metazoan diversification has important implications for the grazer hypothesis.

Here we report early results of our paleoecologic investigation of microbialite build-ups from the Lower Ordovician Fillmore Formation (FF) of the House Range in Utah. The first 120m of the FF consists of poorly-exposed siltstones interbedded with intraformational conglomerates. Two intervals within this section contain multiple microbialite-bearing beds. The microbialites occur on coarse-grained hardgrounds and are bounded and capped by fossiliferous, coarse-grained material. In some cases, the tops of mounds exhibit erosional truncation. Sponge fossils are associated with most of the build-ups--those lower in the section occur in the spaces between the build-ups whereas those higher in the section occur within and on the microbialites. Gastropods, nautiloids, and echinoderms are commonly found in between the build-ups; nautiloids are rare elsewhere in the section.

The sulfur isotopic composition of carbonate associated sulfate is elevated throughout the section, suggesting a long-term interval of deep ocean anoxia. However, low organic carbon and total sulfur abundances coupled with fossil data suggest that the microbialites were not formed under anoxic conditions. Preliminary rare earth elemental results reveal differences between microbialites and non-microbialite-bearing strata.