ECOLOGICALLY ZONED MICROBIAL REEF AT EAGLE MOUNTAIN, CALIFORNIA

The Middle Cambrian Jangle Limestone Member of the Carrara Formation contains a zoned microbial reef at Eagle Mountain in southeastern California near the Nevada border. The reef is located at the contact with the overlying Desert Range Limestone Member. Six different lithofacies were used in the field to define reef zones: oolite with trilobite fragments, massive dendrolite, organized thrombolite, columnar thrombolite, discontinuous microbial grainstone, and channel-filling sediments. The oolitic grainstone underlies the reef at its basal section. Columnar thrombolites dominate the northern section of the outcrop and are displaced by organized thrombolites in a semi-laterally continuous band. Massive dendrolites are common at the southern reef flank and in a distinct channel. The channel is filled by lime mud and clastic terrigenous sediment. The northern section of the outcrop is truncated by a drainage and the reef briefly crops out on the other side with both columnar and organized thrombolites. The focus of this study is on the southern section where ecological zonation is apparent. Petrographic analysis shows a poorly defined but recognizable microbial texture dominated by Renalcis. The Carrara Formation is rich in trilobites and thin-sections show detrital trilobite fragments. Some shell material was reworked and incorporated into the microbial micrite. Wave energy, water depth and light dispersion may have controlled reef ecology. The influx of terrigenous sediment may have changed conditions for this reef as the Carrara Formation shows several clastic to carbonate sedimentary cycles with the alternation of shale and limestone finally terminating in the thick limestone cliffs of the overlying Bonanza King Formation. In this regard, the fully microbial reef at Eagle Mountain is analogous to metazoan reefs which range from the Early Cambrian to the present.