Stromatolites of the Lower Cambrian Deep Spring Formation; Mount Dunfee, Esmeralda County, Nevada
The Middle Member of the Lower Cambrian Deep Spring Formation at Mount Dunfee, Nevada contains a diverse assemblage of stromatolites that formed on a tide- and storm-dominated, siliciclastic-influenced, carbonate shelf. The stromatolites formed in a shallow subtidal environment within and near active oolite shoals under strongly focused currents. The stromatolites occur in four different lithofacies: (1) bioherms of digitate stromatolites, (2) bioherms and biostromes of inclined stromatolites, (3) isolated forms of massive and hemispheroidal stromatolites, and (4) a biostrome of cryptomicrobial boundstones. The first three lithofacies are interpreted as microbial reefs. They had topographic relief, formed in active agitated waters, and exerted a physical control over their environment. The stromatolites incorporate an average of 16% very-fine grained detrital quartz within their microstructures. These quartz-bearing stromatolites are intermediate between "pure" quartzose and "pure" calcareous stromatolites. Few examples of such quartz-rich stromatolites have been documented from the ancient rock record. Stromatolites of the Deep Spring Formation illustrate the "generalistic" nature of microbial organisms and their ability to form organic sedimentary structures in a wide range of depositional environments.