USING SR ISOTOPES IN BASIN ANALYSIS: PALEOCENE-EOCENE FLAGSTAFF FORMATION, UTAH

Comparisons between 87Sr/86Sr ratios in continental basin deposits and their source rocks and rivers present an opportunity for examining long-term controls on chemical weathering and sedimentation. This study focuses on the Paleocene-Eocene Flagstaff Formation (central Utah), a lake deposit in a foreland basin system associated with the Sevier Fold-Thrust Belt. Freshwater carbonates comprise a large portion of the lowermost Ferron Mountain and uppermost Musinia Peak Members, whereas gypsum and carbonates occur in the middle Cove Mountain Member. Previous research attributed gypsum deposition in middle Flagstaff time to climate change or unroofing of diapirs of Jurassic gypsiferous carbonates. To examine more closely the influence of climate vs. tectonics in lower, middle, and upper Flagstaff sedimentation, we collected rock samples from the three Flagstaff members east of Ephraim and Manti, UT, presumably deposited in the central foredeep depozone. We also collected stream water and rock samples from the source area along the Wasatch Front where minor Paleozoic carbonates and Jurassic gypsiferous carbonates crop out, in addition to Tertiary volcanics and Mesozoic to Cenozoic siliciclastics. 87Sr/86Sr ratios and elemental concentrations were measured for all samples. 87Sr/86Sr ratios of source rocks range from .7071 to .7095. In contrast, both parametric and non-parametric tests demonstrate that all samples from the Flagstaff Formation represent one population of 87Sr/86Sr ratios (.7099 ± .0003) at the 95% confidence level. This is similar to the concentrated-weighted stream water 87Sr/86Sr ratio of .7088. 87Sr/86Sr ratios measured in stream water running through carbonates of Permian (.7110) and Jurassic (.7079) age suggest contributions of ~67% and 33%, respectively, to the Flagstaff Formation Sr isotope composition. These data indicate little change in input from source rocks during the entire deposition of the Flagstaff in the study area. Streams draining a thrust belt normally retain their location over long periods. During middle Flagstaff time, either a change to drier climate limited water input and/or drainage hydrology became closed. Changes in the eastern foreland basin margin could be linked to Laramide uplift of the San Rafael Swell, influencing local climate as well as regional drainage.