COMBINED CA-ID-TIMS AND CARBON ISOTOPE CHEMOSTRATIGRAPHY TO CORRELATE CRETACEOUS CONTINENTAL STRATA OF THE RUBY RANCH MEMBER OF THE CEDAR MOUNTAIN FORMATION

Determining the age of continental strata is vital to understanding the timing of climate changes and the development of ecosystems during the Cretaceous Terrestrial Revolution. The combination of geochronologic data with widespread carbon isotope chemostratigraphy provides time constraints for a lower Cretaceous continental unit, the Cedar Mountain Formation. Specifically, a unique palustrine-lacustrine sequence of the Ruby Ranch Member reveals a bulk organic carbon isotope record that ranges between -32.3‰ and -21.1‰ and averages -26.3‰ vs. VPDB. A positive C-isotope excursion occurs that is interpreted to represent the C10 carbon isotope excursion (Bralower et al. 1999; Ap12-15 and Al1 segments of Herrle et al. 2004) that has been identified in other locations worldwide (Herrle et al. 2004). An ash layer within the positive isotope excursion provides an estimated eruption age from CA-ID-TIMS of ca. 116 Ma. This is slightly older than the interpolated age of the base of the Hypacanthopilites jacobi Ammonite zone, thus providing a chemostratigraphic and geochronologic correlation to other biostratigraphically constrained Aptian-Albian units. This also firmly places the time of deposition for the Ruby Ranch Member of the Cedar Mountain Formation within the Aptian to Albian.