ENVIRONMENTAL CONTROLS ON MICROBIALITE DISTRIBUTION IN THE EARLY/MIDDLE CAMBRIAN CARRARA FORMATION, INYO COUNTY, CALIFORNIA

Microbialites (stromatolites, thrombolites, dendrolites, and oncoids) are present in all carbonate members of the Carrara Formation (CF) and are particularly common at carbonate/mudstone contacts. Three examples of microbialite occurrence in the CF will be described and related to changing environmental conditions.

Small digitate stromatolite clusters are present in one of several oncoid-bearing limestone beds in the Eagle Mountain Shale at Eagle Mountain. The stromatolites are exposed on the upper surface of the bed and appear as ovate clusters (cm scale) in the troughs of ripple marks. The restricted occurrence of the stromatolites to the top of the oncoid bed suggests that the microbialites developed only when movement of the oncoids ceased and the substrate stabilized. Subsequently ripple marks migrated over the stromatolites causing cessation of growth.

The uppermost carbonate bed (1.7 m) in the Pyramid Shale at Emigrant Pass is composed of five distinct microbialite intervals. Basal planar stromatolites are overlain by laterally-linked hemispheroids up to 15 cm high. Large laterally-linked hemispheroids (70 m high) capped by thrombolites are next in the sequence. The upper two intervals consist of small hemispheroids and planar stromatolites. Comparison with microbialite distribution in modern environments indicates that this one interval records a minor transgression and regression within the Sauk Sequence transgression.

Microbialites (thrombolites/dendrolites?) are abundant at the top of the uppermost limestone bed of the Jangle Limestone and vary considerably along strike. The microbialites occur in a variety of shapes and scale including small domes and columns (cm scale), elongate domes (m scale), and a large zoned reef (100s of m long). These microbialites are overlain by mudstones of the basal Desert Range Limestone recording increasing water depth and siliciclastic influx that the microbial communities could not tolerate.

These three microbial occurrences in the CF suggest that microbial communities were able to thrive during times of optimum growth conditions related to rate of sea level change, water depth, substrate stability, bathymetry, and water clarity. In all cases, growth of the microbial communities was terminated by rising sea levels and an influx of siliciclastic sediment.