STABLE ISOTOPE INTERPRETATIONS OF TERTIARY PALEOTOPOGRAPHY IN THE EASTERN GREAT BASIN

The links and feedbacks among climatic, tectonic, and topographic conditions in ancient orogenic systems are not well known. This gap in our geological knowledge stems from a lack of robust paleoelevation records for many of the world's major mountain belts and plateaus. For example, the topographic and elevational evolution of the western U.S. Cordillera is debated in the geological literature, with numerous publications supporting widespread late Cenozoic surface uplift and some recent publications supporting late Cretaceous to early Cenozoic uplift of portions of the Cordillera. Resolving this debate is important to advancing our understanding of how tectonic processes influenced long-term regional climatic change during the Cenozoic.

Here we present a stable oxygen isotope record for Tertiary authigenic calcite bearing formations exposed near the Clover Mountains in eastern Nevada, proximal to the eastern margin of the Great Basin. Stable isotope stratigraphies were constructed from sampled outcrops of the Eocene-Oligocene Claron Formation and Miocene Muddy Creek Formation. These stable isotopic records were used as a proxy for paleoelevation change using a -2.8‰/km oxygen isotope lapse rate. δ¹⁸O values of calcite range from -20‰ to -25‰ (PDB) in late Eocene deposits and exhibit a gradual shift to isotopically heavier values (~17‰) towards the late-Oligocene, suggesting that a regional paleotopographic adjustment was concurrent with sedimentary deposition in the Claron basin. Moreover, the carbonate samples from the younger Muddy Creek Formation yielded more positive δ¹⁸O values that range from -9‰ to -11‰, suggesting continued topographic lowering (~3 km elevation decrease) contemporaneous with crustal extension during the Neogene.

A conspicuous shift in the stable isotope record to more negative δ¹⁸O values ~-30‰ correlate with the emplacement of late Eocene/early Oligocene volcanic units in the stratigraphic section, suggesting 1-2 km of surface uplift occurred during magmatic development. The results of this study agree well with published proxy interpretations of spatial and temporal changes in Tertiary paleoelevations of the western U.S Cordillera. Further aspects of this study will examine sample thin-sections to determine clastic sediment sources in the outcrops of interest.