ORGANIC CARBON CHEMOSTRATIGRAPHY AND X-RAY DIFFRACTION (XRD) MINERALOGY IN LACUSTRINE STRATA OF THE RUBY RANCH MEMBER WITHIN THE CEDAR MOUNTAIN FORMATION NEAR MOAB, UTAH

The Cedar Mountain Formation (CMF) represents the earliest deposition of terrestrial Cretaceous strata in the United States and records significant changes in biota (sauropod dominated Jurassic fauna to the ornithiscian fauna of the mid to late Cretaceous) and climate (onset of greenhouse conditions). Improved stratigraphic and chronostratigrapic constraint is vital to understanding the timing and processes of these changes.

The “Lake Carpenter” site is a well exposed lacustrine sequence in the Ruby Ranch Member of the CMF near Moab, Utah was sampled from four trenches at 25 cm increments for geochemical analyses, specifically carbon isotope chemostratigraphy. In addition, XRD analyses improved the lithologic description. Results indicate silicate mineralogy consists of clays and quartz. Carbonate mineralogy fluctuates between calcite, high magnesium calcite, and dolomite. The presence of high Mg calcite and dolomite are consistent with previous interpretations of arid conditions represented by the Ruby Ranch Member.

The δ13C of sedimentary organic carbon ranges from a three point average of -28.8 ‰ (VPDB) to -23.4‰ (VPDB). Tentatively, these results appear to correlate with the carbon isotope excursions Ap10, Ap11, C9, Ap12, Ap13, Ap14, and Ap15 during the Late Aptian (C-isotope segments defined by Bralower et al., 1999 and Herrle et al., 2004). This is also supported by previous lithostratigraphic and chemostratigraphic work within the CMF that correlates with these Aptian segments.