PALEOECOLOGY AND GEOCHEMISTRY OF EARLY CAMBRIAN CALCIMICROBIAL-ARCHAEOCYATHAN REEFS FROM THE WESTERN BASIN AND RANGE, USA

The early Cambrian was an interval of substantial ecological innovation. In particular, reef ecosystems became more diverse as organisms with robust skeletons were incorporated into microbialite structures. Specifically, archaeocyathan sponges served as some of the first reef-building organisms during this period. Calcimicrobial-archaeocyathan reefs from lower Cambrian strata in the western Basin and Range of California and Nevada provide insight into the establishment of reef ecosystems. Here we report on the sedimentological, paleoecological, and geochemical conditions during the early establishment and development of reefs in Laurentia. Biodiversity and carbonate production changes, as well as sedimentological conditions, were determined from point counts in 105 petrographic thin sections. Geochemical parameters were assessed from stable carbon isotope and trace element data from 85 samples.

This region preserves 2-3 shallowing upward parasequences of near shore environments and exhibits sedimentological processes (e.g. cementation and growth) common to reefs. Shannon’s diversity increased from 0.463 to 1.163 when archaeocyathan and coralomorph framework builders were added to reef environments. However, additional organisms within the reefs (e.g. trilobites, echinoderms) appear unchanged and their diversity was not correlated to framework builder diversity. Elevated stable carbon isotope ratios occurred in one microbial reef suggesting that even without metazoan framework builders these reefs can still be productive, albeit less diverse. Increases in sodium concentrations from trace element data also indicate a shallow upwards sequence. Furthermore, Sr/Mn ratios average 2.20 throughout the study interval, which suggest minimal diagenetic alteration. Principle component analysis reveal that several different framework-building assemblages exist in the early Cambrian due to a combination of microbial, archaeocyathan, and coralomorph framework builders. The distribution of these assemblages was largely controlled by sea level. Overall, early Cambrian reefs express sedimentological processes common to other Phanerozoic reefs, however, diversity remained low until additional organisms evolved to inhabit them.