RECONSTRUCTING ARCHAEOCYATHAN REEF SEAFLOORS OF THE LOWER POLETA FORMATION IN ESMERALDA COUNTY, NV

The appearance of laterally continuous, wave-resistant archaeocyathan reefs in the early Cambrian fundamentally altered the relationship between reefs and their environments.

Once confined to small and patchy bioherms that grew in quiet water, over the course of ~10-15 million years, archaeocyaths began to form framework constructions that were more robust, more complex, and more disruptive to local sedimentation patterns than their reef mound counterparts. This development in Cambrian reef morphologies was recognized in the Botomian lower Poleta Formation of Esmeralda County, NV, a unit described as a wave-resistant, laterally continuous, thick (<70 m-thick) archaeocyathan-microbial buildup. We offer a more nuanced view of the macroscale geometry of this unit, documenting large but laterally isolated archaeocyathan reefs within the lower Poleta Formation at a site NE of Gold Point, NV.

Here we present a 1:24,000 scale geologic map of a 3 km² area NE of Gold Point, as well as composite regional stratigraphy of the Poleta through lower Mule Springs formations. These units are low-grade, but are deformed by high angle, conjugate fault sets associated with Basin and Range extension. It is largely due to this deformation that this area has been left understudied. Based on 11 stratigraphic sections measured over 2 kms, we document dramatic facies change within the lower Poleta Formation. Over short, km-scale distances, this unit transitions from <70 m of archaeocyathan-microbial buildups to >7 m of wackestone and grainstone facies. In addition, we document at least two previously unrecognized exposure surfaces within the lower Poleta Formation, demarcating regional stratigraphic sequences. Reefs are present within each of these sequences. Finally, we use the measured sections of the lower Poleta to spatially reconstruct the paleoenvironments of the Cambrian seafloors in which these reefs once thrived. In doing so, we can begin to assess the level of control archaeocyathan reefs exerted on their environment in this region, contributing to our understanding of the evolution of metazoan reefs through time.