SEARCHING FOR FRAMEWORK BUILDERS AFTER THE REGIONAL EXTINCTION OF ARCHAEOCYATHAN REEFS IN THE CAMBRIAN MULE SPRING LIMESTONE NEAR SPLIT MOUNTAIN, NEVADA, USA

Reef ecosystems of the early Cambrian were supported by a framework of microbial organisms (Renalcis- and Epiphyton-group) and archaeocyathan sponges. However, a combination of anoxia and sea level regression resulted in the regional extinction of archaeocyaths in western Laurentia by the end of Stage 4 of the Cambrian. Throughout the Phanerozoic, mass extinctions are frequently followed by thrombolite reefs built by microbial ‘disaster taxa’. In Laurentia, metazoan reefs do not reappear until the late Cambrian when lithistid sponges are found in dendrolites. Here, we explore the Mule Spring Limestone, which overlies archaeocyath-bearing formations, for microbial disaster taxa or replacement metazoans to determine the timing of reef restructuring after this extinction event. We analyzed 49 petrographic thin sections from three sections within the Mule Spring Limestone with point count analysis to determine carbonate contributions. Section 1 (East Hill) contains mudstones and peloid/bioclast packstones. Section 2 (East Hill 2) has wavy laminated microbial mudstones. Section 3 (West Valley) contains mostly peloid wackestones and packstones with occasional oncoid grainstones. This section also includes two possible thrombolite beds, however, they did not create significant topographic relief. Overall, the Mule Spring Limestone is composed of 64.9% micrite, 15.0% peloids, 5.8% microbial material, and only 0.8% bioclastic grains.

The presence of wavy laminated mudstones suggests a shallow intertidal, carbonate-rich environment and signifies a setting that could potentially support reef ecosystems. While some possible metazoan framework builders may have been present, poor preservation, and sparse occurrences make definitive identification difficult. Furthermore, lack of distinct relief by thrombolite layers suggests minimal reef building activity occurred. Thus, the late early Cambrian in this region is distinct from other post-extinction intervals in that there appears to be a lag between loss and reestablishment of novel reef building communities. Further research into the geochemical conditions will be performed to determine if persistent anoxia also occurred during this interval and may have prevented the establishment of new framework builders.