DEVELOPMENT OF MIDDLE JURASSIC MICROBIAL BUILDUPS IN BIGHORN BASIN, WYOMING

The Jurassic was an important time of extensive global reef development and these reefs were often completely different in composition and biota to modern reefs. Middle Jurassic microbial buildups from the Gypsum Spring Formation in the Big Horn Basin of northern Wyoming represent some of the earliest known occurrence of Jurassic microbial reefs in North America.

Middle Jurassic microbial buildups are examined in Celotex Quarry near Cody, Wyoming. The microbial outcrops exhibit a succession of well-exposed and diverse bioconstructions. These microbial buildups have a diameter of approximately 2.5 m and a thickness of 30 cm. The buildups are comprised of several layered “heads,” each cemented by clotted microbial fabric (thrombolite). Petrographic analysis indicates that these structures include blue-green algae (cyanobacteria), coralline red algae, green algae, encrusting foraminifera, encrusting bryozoa, sponges, corals, and mollusks.

Initial environmental interpretations of these microbial buildups suggest that they developed in shallow water (< 1 meter water depth), which was possibly brackish. Evidence for shallow water include (1) the presence of dinosaur tracks on neighboring micritic laminae (probably also microbial in origin) and (2) characteristic benthic fauna.