COOL WATER TUFAS AND BLACK MATS: INDICATORS OF PALEOENVIRONMENTAL CHANGE IN LATE PLEISTOCENE SPRING DISCHARGE DEPOSITS OF THE UPPER LAS VEGAS WASH, NEVADA

Renewed interest in the Rancholabrean age fossiliferous paleo-spring deposits of the upper Las Vegas Wash has resulted in a wealth of new information from detailed geologic mapping, establishing stratigraphic control and elucidating paleoenvironmental change through time. These fine-grained sediments, indicators of past ground water discharge, closely track hydrologic change through at least the last two glacial maxima and entomb one of the most significant late Pleistocene vertebrate assemblages from the American southwest. The regional stratigraphic framework of these deposits, first described in the Tule Springs area of the upper Las Vegas Wash, comprises stratigraphically ascending units, A through G. Unit D, the last full glacial deposit, is marked by extensive wetlands associated with previously reported radiocarbon dates of >26,300–17,770 14C ka. Unit E (14,780 – 7,400 14C ka) is the latest glacial spring discharge event. There is a widely recognized paucity of dates in this interval. Newly recognized tufa deposits allow a firm chronology of the transition from the last glacial maximum climatic regime to the latest glacial spring discharge to be documented, as well as the establishment of the base of Unit E. The tufas exhibit a distinctive morphology that follows a braided fluvial system. In all observations, they are associated with stream channels cut into the dissected marsh deposits of Unit D, following the collapse of the full-glacial climate, then filled with sands, silts and tufa of Unit E. Multiple black mats and other organic material, intercalated with the tufa, yielded radiocarbon dates of 16,820– 16,300 14C ka at the base of earliest Unit E to allow precise chronologic constraint of the hiatus. Precipitation of tufa favors humid, warm, temperate climatic episodes, and so constitutes an important paleoenvironmental signal; for the first time, the temporal relationship with the underlying full glacial deposits has been revealed.