PETROGRAPHY AND DIAGENESIS OF PALUSTRINE CARBONATE BEDS IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, EASTERN UTAH

Palustrine carbonate beds in the Early Cretaceous Cedar Mountain Formation of Eastern Utah capture the Aptian-Albian record of carbon isotope excursions associated with Oceanic Anoxic Event 1b, indicating a coupling between marine, atmospheric, and terrestrial carbon reservoirs during a dynamic period of global change. Peak d¹³C values greater than –3 ‰ VPDB, as compared with background values ranging between –8 to –7 ‰ VPDB, are believed to have tracked changing carbon isotopic compositions of atmospheric CO₂ and terrestrial C3 plants through the precipitation of pedogenic calcite. In order to better understand the processes responsible for the encoding of this chemostratigraphic record, we have undertaken petrographic and diagenetic investigations of selected carbonate beds in our stratigraphic sections. Evidence for ephemeral ponded water is provided by the common occurrence of ostracode and gastropod shells, and charophytes indicating emergent aquatic vegetation. Brecciated fabrics are ubiquitous, suggesting that pedogenic overprinting was common to all samples. The carbonates consist of complex mixtures of micritic calcite, microspar, intergranular spars in intraclastic grainstones, and vein-filling spars in septarian and later tectonic fractures. Some beds are partially to completely dolomitized. Calcitic rock components have two dominant diagenetic trends in carbon and oxygen isotope space: 1) meteoric calcite lines (MCLs) recording early meteoric phreatic precipitation, and 2) positive linear convariant trends (PLCT) recording influences of evaporation and CO₂ degassing in meteoric vadose environments. Intergranular spars with PLCT display meniscus and pendant cement fabrics that also evince precipitation in vadose settings. MCL d¹⁸O values range between –9.4 to –2.7 ‰ VPDB, indicating major changes in the oxygen isotopic composition of local meteoric precipitation during the Aptian-Albian.