A UNIQUE SNAPSHOT IN TIME OF AN ANCIENT LAKE: THE SPECTRUM OF LACUSTRINE CARBONATE MORPHOLOGIES AND TEXTURES FROM A SINGLE STRATUM

One, 0.5 to 5 meter-thick stratum within the Miocene Lovell Wash Member of the Horse Spring Formation (exposed north of Lake Mead, Nevada) contains an amazingly diverse suite of morphologies and textures that include tufas, travertines, and microbialites. This stratum is laterally continuous for over 6 km and provides a snapshot of an ancient, spring-fed lake. Indeed, fossil spring vents are preserved in this stratum. Our research group has traced this unit over its entire outcrop belt and documented over 100 macrostructures, including spring vents, bioherms, biostromes, and others. These macrostructures are, in turn, comprised of 230 mesostructures, including a variety of stromatolitic, thrombolitic, oncolitic, and other textures. We have examined over 120 thin sections from these mesostructures for microtextural characterization. One outcome of this work is an exceptionally thorough characterization of the textural building blocks of these lacustrine limestones that were likely extant in the lake at approximately the same time. We also find that facies that might be interpreted as tufas or travertines contain admixtures of stromatolitic and thrombolitic textures, interpreted as microbial in origin. Similarly, some microbial bioherms contain evidence of tufa and travertine textures. Our work suggests that tufas, travertines, and microbialites are not mono-genetic and that their shared and complexly-constructed fabrics show evidence of contemporaneous microbial and spring activity within any given macrostructure. We suggest that, as with clastic sedimentary rocks, we develop and apply a non-genetic terminology for the building blocks of lacustrine carbonates, in an effort to clarify the complex origins of these composite macrostructures.