PROVENANCE AND STRATIGRAPHIC CORRELATION OF THE OLIGOCENE WHITE RIVER GROUP IN THE BIGHORN MOUNTAINS, WYOMING AND BLACK HILLS, SOUTH DAKOTA, USA

Clastic deposits of the Oligocene White River Group are preserved irregularly across northern Wyoming and western South Dakota. Laramide uplifts and possibly magmatic centers supplied sediment to broad, low valleys beginning around 40 Ma. Flat lying strata of the White River Group in the elevated Bighorn Mountains and Black Hills and rest unconformably on Precambrian-Paleozoic rocks on a surface of moderate relief. U-Pb ages were obtained for detrital zircons from two samples. A pilot sample (n=99) from the southern Bighorn Mountains (44.1468°N, -107.1972°W), from a 10-m-thick, matrix-supported gravel comprised of crystalline clasts yields a peak age of 2904 Ma, which reflects the age of the local basement rocks. Six grains are Proterozoic in age, which may reflect recycling of local Paleozoic strata. A second sample (n=144) was collected near Whitewood Peak, east of Deadwood (44.387521°N, -103.696535°W), from a 20-m-thick conglomerate in the Mountain Meadow Fm. that rests unconformably on Paleozoic strata. Clasts are dominantly aphanitic volcanic and pelitic metasedimentary rocks derived from local basement rocks. The zircon age suite for this sample is diverse, ranging from Archean to middle Cenozoic. Cenozoic-age zircons comprise about 70% of the age suite and yield a peak age of 54 Ma, consistent with the age of local intrusive rocks. The youngest zircon in the Black Hills sample is 33.7 Ma, consistent with an Oligocene maximum depositional age. The balance of the age suite is Precambrian in age and likely derived from recycling of Paleozoic strata. The samples from both localities originate from local uplifts and were likely deposited in proximal fluvial systems and are not part of a regionally consistent depositional system. The sediment provenance history and regional stratigraphic correlation of Oligocene deposits is key to understanding the nature of Cenozoic exhumation and burial of Laramide ranges and adjacent basins, respectively.