EXTENSIONAL TECTONIC AND CLIMATIC SIGNALS IN MIOCENE-AGE FOSSIL LAKES OF THE LAKE MEAD (NEVADA) REGION: NEW ISOTOPIC DATA FROM THE HORSE SPRING FORMATION

A fundamental problem in basin analysis and tectonics is separating climatic from tectonic signals in the stratigraphic record. There are few examples where this has been successfully accomplished in extensional tectonic settings—particularly in the Basin and Range province—yet it is crucial if we are to understand both paleoclimate and crustal evolution in extensional regions at the level of resolution that sedimentary basin fills can potentially allow. Fossil lake sequences, because of their ability to provide detailed paleoenvironmental records, provide key data that can help address this problem. Miocene- to Pliocene-age lacustrine limestone sequences in the Lake Mead (Nevada) region, spanning the 6-20 Ma time period, may be particularly useful in this effort because they (a) are spectacularly well-exposed; (b) include numerous ashes suitable for dating; (c) embrace a period of on-going extension; and (d) bracket a time of dynamic global climate change, the mid-Miocene climatic optimum. These lake sequences show a depletion in δ¹⁸O values through time, suggesting a trend toward cooler/wetter conditions coincident with cooling coming off of the Miocene climatic optimum. This depletion is not, however, smooth. Between 14 and 15 Ma there is an abrupt 4-5‰ negative shift coincident with inferred changes in rates of extension that may represent changes in water sources or Rayleigh fractionation of precipitation due to increased tectonic relief. Furthermore, detailed isotopic trends within these sequences are far from regular or smooth. One 500 m-thick outcrop of the Bitter Ridge Limestone Member of the Horse Spring Formation shows nearly a 10‰ positive shift in δ¹³C, from ~2-4‰ to ~15‰; this is accompanied by two dramatic positive shifts in δ¹⁸O. We interpret these isotopic values analogous to those found in S. American Altiplano lakes that are highly evaporative and have significant thermal spring input. Lithofacies evidence supports this interpretation. Stratigraphic variation in the correlation between O- and C-isotopes suggests changes from open to closed lake conditions that we interpret to be tectonically induced, although climate forcing cannot be ruled out.