Isotopic and elemental records of authigenic calcite from lacustrine deposits in the intraforeland basins of Utah were analyzed in an effort to reconstruct the regional paleoclimate, paleohydrology, and paleotopography of the early Cenozoic central North American Cordillera. Isotopic profiles for Paleogene Lakes Uinta, Flagstaff, and Claron show relatively large oxygen isotopic shifts that are diachronous among basins with an ~7‰ decrease in δ18Ocalcite values at ca. 45 Ma in Lake Flagstaff, an ~5‰ decrease in δ18Ocalcite values between ca. 42 and 35 Ma in Lake Claron, and an ~6‰ decrease in δ18Ocalcite values between ca. 44 and 43 Ma in Lake Uinta. We interpret these negative oxygen isotopic shifts to be the combined result of increased hypsometric mean elevation of basin catchments and related freshening associated with basin infilling. The basins studied also have undergone periods of intense evaporation during periods of hydrologic closure, which, for example, produced an ~7‰ increase in δ18Ocalcite values in Lake Uinta beginning at ca. 51 Ma. Hydrologic closure in the Uinta Basin probably resulted from growth of local topography that diverted previously substantial inflows from low elevation regions within the foreland.
This study adds to the growing body of evidence that suggests a pattern of along-strike variations in the timing of topographic development and dissection of the Cordilleran landscape during the early Cenozoic. We favor an interpretation that calls for the middle Eocene rearrangement of regional drainage patterns such that intraforeland basins that once received waters from far-flung foreland river systems became dominated by inflows of low δ18O waters from catchments with higher hypsometric mean elevations that drained the adjacent hinterland and/or basin-bounding uplifts. This drainage reorganization is analogous but subsequent to the large-scale integration of catchments in the northern Cordillera that has been recognized on the basis of isotopic and sedimentological evidence in Montana and Idaho at ca. 50–47 Ma, with rivers flowing southeast into Lake Gosiute at ca. 49 Ma and then for a time reaching Lake Uinta, causing a prominent highstand in that lake at 48.6 Ma. The negative oxygen isotopic shifts presented herein, which occurred between ca. 45 and ca. 35 Ma in the intraforeland basins of Utah, may reflect the north-to-south progression of drainage integration in the Cordillera as magmatism and related topography swept southward through the hinterland and increased the hypsometric mean of catchments that fed subjacent intraforeland basins.