Chemostratigraphic analysis of palustrine carbonate beds in the Cedar Mountain Formation of Eastern Utah has yielded carbon and oxygen isotope data that 1) capture a carbon isotopic excursion curve that is believed to coincide with Oceanic Anoxic Event 1b described by existing oceanic core data, which demonstrates a link between marine, atmospheric, and terrestrial carbon reservoirs, and 2) give us a picture of what was happening to the environment during the Aptian and Albian Ages (121-99 Ma) of the Early Cretaceous, a dynamic period of global climate change. Through petrographic and diagenetic analysis of pedogenic calcite precipitated during this time period, we are able to track the change in d13C isotope compositions of atmospheric CO2 and terrestrial C3 plants. Pedogenic overprinting demonstrated by the brecciated fabrics found throughout all the carbonate rock samples collected, along with intensive microsampling of different forms of calcite, let us follow the progression of d13C and d18O isotope change in the environment. The different forms of calcite sampled include micritic calcite, microspar, intergranular spar in intraclastic grainstone, vein-filling spar in septarian and later tectonic fractures. Two dominate diagenetic trends can be seen in the carbon and oxygen isotope data: 1) meteoric calcite lines (MCLs) recording early meteoric phreatic precipitation, and 2) positive linear covariant trends (PLCTs) recording evaporation and CO2 degassing in meteoric vadose environments. The presence of PLCT’s in the carbonates suggests that the environment was becoming arid, and that the lake that deposited the calcium carbonate was desiccating. This interpretation is supported by the presence of ostracodes, gastropod shells, and charophytes suggestive of an ephemeral ponded water environment, as well as meniscus and pendant cement fabrics found in the intergranular spar of intragranular grainstones.
Andrew Sorenson, ’04 Algona, IA
Majors: Geology, History
Sponsor: Rhawn Denniston