WHEN CARBONATES SPEAK: RECONSTRUCTING MID-CRETACEOUS PALEOCLIMATE AND PALEOENVIRONMENT FROM CARBONATES IN THE RUBY RANCH MEMBER OF THE CEDAR MOUNTAIN FORMATION

The mid-Cretaceous (Aptian-Cenomanian) climate is considered to be one of the warmest in recent Earth History, with atmospheric CO₂ three to six times higher than today, and temperatures 6-12ºC warmer than present. The terrestrial Cedar Mountain formation (CMF) preserves a valuable record that can be used to reconstruct the paleoclimate and environment of the mid-Cretaceous using stable isotope geochemistry and sedimentology. The Ruby Ranch member of the CMF in the vicinity of the Price River 2 dinosaur quarry, East Central Utah consists of stacked variegated gray (reduced) or red (oxidized) paleosols, punctuated by thick carbonate units and abundant carbonate nodules. Thick carbonate units appear to be pedogenic in origin (calcretes) and are bright red indicative of oxidizing conditions. Nodules occur mostly in clustering masses not associated with paleosols and have radiating fibrous calcite fabrics. Isolated nodules are also found suspended in the gray paleosols, and none have been found in association with oxidized paleosols. Carbonate carbon and oxygen stable isotopic data for two distinct calcrete layers cluster at δ18O values of - 5.1 ± 0.25 ‰ V-PDB and δ13C values of - 4.1 ± 0.12 ‰ V-PDB. Carbon and oxygen stable isotopic data for two nodules have significantly lower δ13C values and greater variability. The data for these nodules define two similar but distinct meteoric calcite lines at d18O of -6.5 ±0.38 ‰ V-PDB and -6.2±0.25 ‰ V-PDB. Isotopic composition of “calcretes” and nodules do not appear to be affected by strong evaporative enrichment as would be expected for calcrete development in arid regions. Possible alternative that requires additional isotopic and petrographic evaluations is that calcrete are developed on palustrine/lacustrine carbonates. Nodular carbonates are suspected to be formed by discharging groundwater, perhaps small springs.