DIGITATE STROMATOLITES IN THE EARLY CAMBRIAN EAGLE MOUNTAIN SHALE MEMBER OF THE CARRARA FORMATION, EAGLE MOUNTAIN, CALIFORNIA

Small digitate stromatolite clusters occur at the top of one of several oncoid-bearing limestone beds in the Early Cambrian Eagle Mountain Shale Member (EMSM) of the Carrara Formation (CF) on the west side of Eagle Mountain, Inyo County, California. The limestone beds occur 40 m above the contact with the Zabriskie Quartzite within a 17 m stratigraphic interval. In adjacent ranges the limestone beds are thicker, contain thinner shale interbeds, and are recognized as the Thimble Limestone Member of the CF. The limestone bed containing the stromatolites (44 m above the base of the EMSM) is 12 cm thick and is the only oncoid bed containing stromatolites at this locality.

The stromatolites are exposed on the upper surface of the limestone bed and appear as ovate clusters irregularly distributed on the bedding plane where they are exposed in the troughs of ripple marks. The microbial structures have an average long axis dimension of 8 cm with the largest cluster observed being 20 cm by 14 cm. In cross-section, the stromatolites are 1.5 to 4 cm high and contain mm scale laminae. The bases of the stromatolites are small domes approximately 2 cm high from which .5 cm wide columns rise to the top of the bed. Red-brown shale typical of the EMSM overlies the carbonate bed.

The sequence of lithologies, from the base to the top of the bed, is intraclast conglomerate, oncoid-bearing grainstone, stromatolite boundstone, and finally fossilerous grainstone wave ripples suggesting a depositional sequence representing changing energy levels near the intertidal-subtidal boundary. The restricted occurrence of the stromatolites at the top of the oncoid bed suggests that the microbialites were able to establish themselves only when movement of the oncoids ceased and the substrate became stabilized. Continued environmental change to one of more wave motion caused reworking of the sediments and created the ripple marks that moved over and eroded the stromatolites. Finally, carbonate deposition ceased when an influx of siliciclastic sediments occurred. This same sequence occurred several times in this stratigraphic interval although stromatolites only developed at the top of one of the beds, perhaps because sea level change was slow enough for the microbialites to populate the stabilized oncoid surface.