Paper No. 76-8
Presentation Time: 9:00 AM-5:30 PM
PALEOECOLOGY AND GEOCHEMISTRY OF EARLY CAMBRIAN MICROBIAL-ARCHAEOCYATHAN REEFS FROM THE WESTERN USA AND MONGOLIA
Cambrian reef ecosystems experienced a rapid increase of skeleton-producing organisms driven primarily by the origination of archaeocyathans. Archaeocyathans are an extinct class of carbonate producing sponges that were responsible for some of the earliest metazoan reef ecosystems. The buildup of carbonate skeletons from these organisms allowed for more complex habitats and created more niche space for additional organisms. However, at the end of Series 2 of the Cambrian, archaeocyathan diversity declined during the Sinsk extinction. This extinction removed the major framework organisms within early reef ecosystems and led to microbial communities flourishing. Microbial communities do not generally produce complex environments that additional organisms can inhabit and this change represents a major transition in the evolution of early marine life. The goal of this project is to test the hypothesis that the extinction of framework building organisms in Cambrian reef ecosystems caused a dramatic change in reef taxonomic and functional diversity. Using point count analysis on thin sections from the western Basin and Range, as well as contemporaneous material in western Mongolia, changes in biodiversity and dominance can be calculated. Stable carbon isotope analysis from biogenic carbonate and organic carbon will also be used to reconstruct the environmental conditions prior to, during, and after the extinction of framework builders. Some of the oldest microbial-archaeocyathan reefs of the western Basin and Range region occur in the lower Cambrian Montenegro Member of the Campito Formation in the form of mounding patch reefs. From outcrop observations, archaeocyathan richness gradual increased through the overlying lower Poleta Formation before being locally replaced by oolites from a higher energy environment. The uppermost Harkless Formation was observed to have minimal faunal richness with the exception of thin reef-bearing beds. Harkless Formation reefs were the only reefs to contain coralomorphs. All of the reefs in the region were within clotted microbialite. Future work will include additional point count analysis, stable carbon isotope analysis, and observations of Mongolian reefs to assess global patterns.