Abstract
Miocene lake deposits of southern Nevada provide an exceptional opportunity to study well exposed microbial carbonates. The Lovell Wash Member of the Horse Spring Formation comprises carbonates, evaporites, tuffs and sandstones that formed in a rift basin setting. Thick-bedded limestones with evidence of dessication and devoid of macrofauna indicate that shallow alkaline lakes once dominated the area. Recent mapping in the Lovell Wash Member has shed light on a new hub of microbial activity that provides insight into the formation of microbialites in an ancient spring system. Diverse carbonate structures-including domal and stratiform stromatolites, thrombolites, pisolites, and ooid grainstones-occur in a 60 x 40 m area. In this same site we identified a unique sedimentary feature that seems to be a central vent structure of a spring. This feature is a depression shaped like a meter-scale inverted cone that cross cuts thinly laminated lacustrine limestone, with a 3-4 cm-thick lining. This, in turn, is partially filled by an accumulation of granular limestone over- and underlain by large voids. A similarly textured limestone with higher porosity caps the depression as a 10 cm-thick mound. The thinly laminated lacustrine limestone abuts and overlies the mound, effectively encasing the entire structure. We mapped this structure using four distinct textural classes that we petrographically analyzed. The crustal lining contains minor radial fibrous ooid fragments, opaque filaments, micrite and 25 percent porosity. The overlying, irregular mound shows 35-45% concentric and fragmented radial fibrous ooids ranging from 0.2-1mm with micrite and sparry calcite lining the voids. Microporosity is approximately 15 percent, but this unit has 0.50-2 cm macropores locally. Pore spaces with isopachous cement suggest travertine began to grow around the ooids and grains. The thinly laminated limestone surrounding the structure shows clotted grains, ooids and micrite, with stratiform stromatolitic morphologies as well. We hypothesize that the spring was subaqueous and conducive to the formation of the diverse microbial forms at this site.