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

The Jangle Limestone Member (JLM) of the Carrara Formation (45-50 m thick) is composed primarily of stromatolitic boundstone, ooidal and oncoidal grainstone, and ribbon rock indicating depth variations from subtidal to supratidal. Microbial deposits (thrombolites/dendrolites) are abundant in the uppermost limestone bed of the JLM and vary considerably over a 50 km distance from Emigrant Pass in the Nopah Range to Eagle Mountain near Death Valley Junction. The macrostructure of the microbial build-ups vary in scale and shape and include small domes and columns, larger elongate domes, and a large zoned reef (230 m long). A major question is what factors control the location and variation of these microbial structures. This paper will address the conditions that favored the formation of the microbialite reef at the top of the JLM at Eagle Mountain. At this locality, the uppermost continuous bed in the JLM is an orange dolomite containing planar stromatolites that is overlain by thin-bedded, nonresistant, chocolate colored mudstones of the basal Desert Range Limestone Member (DRLM) of the Carrara Formation. At regular intervals, small microbial mounds (10s of cm in diameter and up to 10 cm high) are present above the orange dolomite overlain by an interval of rip-up clasts of the planar stromatolites grading upward into the mcrobialites. The sequence is completely different beneath the reef where the orange stromatolitic unit is overlain by the following sequence: ooidal grainstone, ooidal and oncoidal grainstone containing isolated microbial structures, more ooidal grainstone, and finally a massive reef core approximately 4 m thick. The basal cross-bedded ooidal grainstone only occurs under the reefal buildup and extends several meters beyond the reef on the northwest end and ends at the reef margin to the southeast. These observations suggest that the relief created by the ooidal shoal was a favorable site for the microbial structures after a previous abortive attempt represented by the underlying bed containing more isolated forms. The reef apparently thrived in the high energy, more oxygenated environment created by shoal area and ultimately was destroyed as water deepened and it was inundated by the influx of siliciclastic sediment of the basal DRLM.