RECOGNITION OF A LATE PLEISTOCENE BRAIDED FLUVIAL TUFA SYSTEM ASSOCIATED WITH GROUNDWATER DISCHARGE DEPOSITS, UPPER LAS VEGAS WASH, NEVADA

Comprehensive geologic mapping in the groundwater discharge deposits of the upper Las Vegas Wash has revealed an extensive groundwater-fed, tufa riverine system of late Pleistocene age. These microbially mediated, ambient temperature tufas exhibit a distinctive morphology, resembling an anastamosing fluvial network, closely mimicking recent and early Holocene flow systems in the upper Las Vegas Wash. Although tufa had been previously noted in the area, the recognition of a braided fluvial system, with its complementary microfacies has never been described. The tufas display external morphologies, such as phytoclasts, oncoids, cyanoliths, stromatolites, and resurgence features. Although braided fluvial tufas predominate, paludal and lacustrine tufas associated with pooling water behind tufa barrages are also noted. Radiocarbon dates establish the chronology and provide stratigraphic control in the post full-glacial section of these deposits, which entomb one of the most significant late Pleistocene vertebrate assemblages from the American southwest.

Within the regional stratigraphic framework, the fluvial tufas are the latest deposits in a long (225+ ka) sequence of late Pleistocene ground water discharge. The tufas allow a firm chronology of the transition from the last glacial maximum climatic regime and redefine the timing of the latest glacial spring discharge. In the earliest generation of tufa precipitation, spring-fed stream channels cut into dissected marsh deposits following the collapse of the full-glacial climate, then filled with sands, silts and tufa. Multiple black mats and other organic material, intercalated with the tufa, yielded dates of 16,820–16,300 ¹⁴C ka at the base, allowing precise chronologic constraint of the hiatus. Several generations of tufa persist to at least 12,430 +/- 60 ¹⁴C ka.

A braided fluvial tufa system is regionally unique, since it is characterized by flowing streams emanating from groundwater discharge loci and lacks any association with tufa mounds/towers proximal to Pleistocene basins containing pluvial lake deposits. Combining current precise chronology with stable isotope analyses will provide a new source of high-resolution data regarding paleoclimate and paleoenvironmental conditions during the late Pleistocene in the southern Great Basin.