CLIMATIC CONTROL OF FLUVIAL-LACUSTRINE CYCLICITY IN THE CRETACEOUS CORDILLERAN FORELAND BASIN, WESTERN UNITED-STATES

Tectono-stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomi tant sedimentological response as manifested by change in thickness, c omposition or depositional environment of sediment accumulating in adj acent basins. Lacustrine limestone units within the early Cretaceous f luvial/lacustrine Gannett Group of western Wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipit ation and evaporation, indicating that lakewater chemistry was control led by variation in regional climate. Change in proportion of allochth onous terrigenous elastic vs. autochthonous carbonate deposition, as w ell as carbonate Mg/Ca ratio and stable isotopic composition, occurs a t two scales. Metre-scale alternation of micritic limestone and argill aceous marl is accompanied by mineralogical and isotopic variation wit hin individual beds, indicating preferential carbonate accumulation du ring intervals of decreased regional meteoric precipitation relative t o lake-surface evaporation. Limestone deposition began during interval s of maximum aridity, and decreased as increased meteoric precipitatio n-driven flux of terrigenous elastic sediment overwhelmed sites of car bonate accumulation. Similar upsection variation in limestone mineralo gy and isotopic composition at a scale of tens of metres reflects the multiple processes of long-term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenous sediment, across- basin fluvial-deltaic progradation, and renewed accumulation of riveri ne terrigenous units. Such trends suggest that formation-scale alterna tion between fluvial elastic and lacustrine carbonate deposition was c ontrolled by climate change.