A MIDDLE CAMBRIAN ZONED MICROBIALITE REEF, JANGLE LIMESTONE MEMBER OF THE CARRARA FORMATION, EAGLE MOUNTAIN, CALIFORNIA

A zoned microbialite reef crops out at the contact between the Jangle Limestone and the Desert Range Members of the Carrara Formation on the west side of Eagle Mountain. The presence of the reef is atypical of this contact, not only at Eagle Mountain but also in the nearby Resting Spring and Nopah Ranges where smaller microbial structures are common at or just below the contact. At all localities studied, the basal Desert Range Member is composed of yellowish brown mudstone. The reef is approximately 230 m long with 8 m of relief. Microbial boundstone comprises the uppermost 4 m of the reef. The reef is zoned with smaller domes occurring at the southern end with more robust columnar and fan-shaped forms to the north. The southern end of the reef is marked by a structureless microbial unit that grades into well-defined domal forms with heights of 10 cm and spacing between the crests of 30 cm. This distinctive zone grades laterally into and is overlain by a zone of poorly defined larger domes with average heights of 20 cm and maximum widths of 20 cm. These forms contain darker rims of more concentrated microbial clusters. This is the most extensive zone in the reef and extends for 70 m under talus to a northern outcrop characterized by fan-shaped forms in two stratigraphic intervals. A lower unit containing structures that are 45 cm high with maximum widths of 15 cm is overlain by prominent 85 cm high features with widths of 22 cm at the base and 65 cm at the top. Reef breccia occurs in scattered outcrops at the northern end of the zone. The maximum dip of the margin of the reef is approximately 50 degrees. The mesostructure of the reef is composed of irregular patches of branching microbial clusters alternating with areas of silty grainstone composed predominantly of trilobite fragments and intraclasts. Crossbedded oncoid- and ooid-bearing grainstone occur stratigraphically below the microbialites suggesting that the initial topography of the grainstone created a favorable substrate for microbial growth. The presence of the larger microbial forms and reef breccia at the northern end of the outcrop suggest an energy source from that direction which is generally consistent with a northeast/southwest trending Cambrian shoreline.