Abstract

The early Paleogene Flagstaff Formation on the Wasatch Plateau of central Utah preserves a ca. 300-meter-thick sequence of palustrine and lacustrine limestone and dolostone. These rocks formed during a climatically dynamic interval of Earth's history, and contain important sedimentological and geochemical records of early Paleogene greenhouse climate. The Flagstaff carbonates also document a multiphase history of diagenetic alteration that must be clearly differentiated in order to extract a paleoenvironmental record. We have studied the stable carbon and oxygen isotope geochemistry of sample suites from three local stratigraphic sections in conjunction with observations from field studies, petrography, and x-ray diffraction analysis. Our results provide evidence for a paragenetic sequence including primary carbonate deposition in a shallow freshwater to saline lake system, primary or early secondary formation of micritic dolomite, episodic exposure and pedogenesis, and late-stage pressure solution and cementation. Stable-isotope data indicate that the isotopic composition of cements is distinct from, but largely determined by, that of the host micritic carbonate, and suggest that isotopic data from micrites and dolomicrites preserve records of lateral and temporal paleoenvironmental changes in Lake Flagstaff.

Integration of lithological and isotopic evidence indicates the presence of a persistent lateral paleoenvironmental gradient among our study sites and a secular drying trend represented in all three study sections. Existing age constraints indicate that the desiccation of Lake Flagstaff occurred during a multi-million-year interval of global climate warming that was punctuated by extreme, transient (~ 200 kyr) warmth during the Paleocene–Eocene Thermal Maximum (PETM). Our analysis suggests that changes in the areal extent and chemistry of Lake Flagstaff can be attributed largely to climatic changes affecting the lake basin and document a multi-million-year reduction in precipitation: evaporation ratios similar to that previously reported from sites in Wyoming to the north. Although the correlation of the PETM within the Flagstaff Fm. remains unclear, existing constraints are consistent with this event having corresponded to the maximum lowstand of Lake Flagstaff, perhaps providing evidence for regional climatic drying during this episode of global warmth.

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