LOWER-MIDDLE CAMBRIAN THROMBOLITES FROM THE CARRARA FORMATION, SOUTHERN GREAT BASIN, CALIFORNIA: PALEOECOLOGICAL IMPLICATIONS RELATIVE TO BIOMERES

Microbialite buildups were relatively rare in shallow marine carbonate environments in the Early Cambrian. The reef-building archaeocyaths instead dominated these environments in association with calcimicrobes. Following the disappearance of archaeocyaths and olenellid trilobites at the Early-Middle Cambrian boundary, microbialite buildups increased in abundance and remained prominent until the Early Ordovician. This ‘microbial rebound’ is reflected by thrombolites reported from Lower and Middle Cambrian beds of the Carrara Formation in the southern Great Basin of eastern California and western Nevada. Did the extinction of archaeocyaths and trilobites prompt the resurgence of thrombolites in the Middle Cambrian?

To test this hypothesis, the paleoecology of thrombolite occurrences was examined in the southern Great Basin. Thrombolites have thus far been reported in three of the members of the Carrara Formation: the Pyramid Shale, Red Pass Limestone, and Jangle Limestone members. Here we present some preliminary results on thrombolites collected from the Pyramid Shale Member in the Southern Nopah Range and the Jangle Limestone Member exposed at Eagle Mountain in California in an effort to understand their paleoecological position during and following the Early-Middle Cambrian transition. The macrostructure and mesostructure (sensu Shapiro, 2000) are well preserved, but petrographic analysis does not reveal the presence of calcimicrobes.

The Pyramid Shale member includes the L-M Cambrian extinction boundary that coincides with a trilobite biomere boundary. The Red Pass and Jangle Limestone members, on the other hand, contain two trilobite zone boundaries. The stratigraphic positions of these thrombolites relative to zone and biomere boundaries are important in evaluating their role as post-extinction disaster forms and is the basis for ongoing research.