Claire Johnson

Mentor: Katherine Marenco


This project is an investigation into the structure, morphology and fossil composition of an Early Ordovician (~485-470 million years before present) reef unit in the Fillmore Formation of western Utah. The establishment of the reef coincided with both the most significant biodiversification event in the history of life (the Great Ordovician Biodiversification Event) and the most prolonged “metazoan reef gap” in the Phanerozoic, ~40 million years in which reef building was primarily carried out by microbial communities rather than animals. Most reef-like structures built during the Early Ordovician were small (1-2 meters in height/diameter), isolated mounds. Based on my research, these Fillmore Formation exposures represent a true reef, built by microbial communities and sponges, that extended laterally for tens of kilometers and was resistant to high-energy currents.

In 2014, I studied one mound structure within the reef in detail. My results indicated that the mounds found in this interval are steep-sided structures up to 2.5 meters wide that were built upon a stabilized grainstone substrate. Each mound is composed of a stromatolitic core surrounded by non-laminated microbialite that contains metazoan fossils and carbonate mud. Data that I collected this summer indicate that the overall reef structure is at least three meters thick, is made up of interconnected mounds, and is likely supported and cemented by the microbial fabrics that surround the stromatolitic cores. Each mound within the reef is bounded by shelly grainstone-filled channels that vary in width, reflecting episodes of high current energy that deposited larger grains into the topographic lows between the rigid mounds. In outcrop, the grainstone channels appear as lenses between the larger interconnected mounds, which can be inferred in three dimensions as having an egg carton-like morphology.

Building on my earlier research, this study includes a broader investigation into the composition and morphology of this reef interval, while focusing in greater detail on the types of both metazoan (sponge) and microbial life within the mound fabric, and using the fossils found within the grainstone as a proxy for the animal community that lived in association with the reef.