STABLE ISOTOPIC EVIDENCE FOR EOCENE UPLIFT OF THE WESTERN U.S. CORDILLERA

We present a Cenozoic stable isotope record for terrestrial carbonate and authigenic smectite from the Carlin-Piñon Range area of northeastern Nevada. The Carlin-Piñon Range area hosts a ~4500m thick sequence of fluvio-lacustrine sedimentary rocks that were deposited throughout the Cenozoic in what is now the northern Great Basin region. Petrographic analysis indicates that none of the samples collected were exposed to significant burial diageneis or hydrothermal alteration. The carbonate and smectite stable isotope records exhibit two major isotopic shifts during the Cenozoic. 1) a ~10‰ shift from higher to lower d-values is observed in lacustrine carbonates during the Eocene; 2) a ~5‰ shift from lower to higher d-values is observed in authigenic carbonate and smectite between the late Miocene and Pliocene. Temperature effects alone are not sufficient to explain the observed isotopic shifts, and carbonate trace element ratios suggest evaporation was minor during most of the Cenozoic. We suggest the observed Eocene shift are the result of regional uplift during the Eocene, and combined temperature and paleotopographic effects during the late Miocene.