THE BROWNS PARK FORMATION IN NORTHWESTERN COLORADO AND SOUTH-CENTRAL WYOMING: IMPLICATIONS FOR LATE CENOZOIC SEDIMENTATION AND PALEOGEOGRAPHY

The Browns Park Formation in northwestern Colorado and south-central Wyoming centered around the Elkhead Mountains consists of 1) a basal conglomerate up to 300 ft. (93 m) thick unconformably overlying all older rocks and deposited as coalescing alluvial fans locally derived from the adjacent Park Range/Sierra Nevada and Flat Tops area, and 2) an overlying sandstone member up to 2200 ft. (670 m) thick deposited mainly as eolian dunes and loess plus minor fluvial deposits. Cross-bedding measurements (n=250) from all environments indicates eolian and fluvial transport directions to the NNE. Mineralogy of the sediments indicates a complex plutonic, metamorphic and sedimentary source area to the south and southwest. A fine-grained volcaniclastic component represents far-traveled air-fall material probably sourced from the Great Basin and San Juan Mts region to the southwest. Age dates from several sources indicate the Browns Park Formation was deposited during the late Oligocene -Miocene (~ 30 – 7.2 Ma). The Formation is intruded by and overlain by volcanics in the region ranging in age from 11.4 – 6 Ma.

Deposition of the Browns Park Formation represents a major regional period of late Oligocene and Miocene aggradation that filled basinal areas throughout the study area and vicinity with several thousand feet of sediment and buried the flanks of Laramide ranges up to the current elevations of about 10,000 feet (3050 m). The Browns Park sandstones and volcaniclastics evidently were picked up by the wind and carried north across an inferred continental divide and deposited by wind and alluvial processes. This depositional setting could have represented a series of ponded and internally drained basins throughout the study area and vicinity, or alternatively the drainage system might have connected north to the ancestral Platte River system. These late Oligocene-Miocene aggrading systems predated the current late Miocene – Recent period of regional uplift and erosion and the establishment of the present day Colorado River system including the cutting of the Grand Canyon.