Abstract

The Elkhead Region centers on the volcanic Elkhead Mountains in northwestern Colorado and south central Wyoming, an area well suited to a study of the late Cenozoic history of part of the Rocky Mountains. Here the late Cenozoic volcanics of the Elkhead Mountains (flows and shallow intrusions) overlie and intrude a thick section of the Late Oligocene-Miocene Browns Park Formation.

The Browns Park Formation accumulated throughout northwestern Colorado and vicinity as part of late Oligocene through Miocene sedimentation in the central and southern Rocky Mountains. In the Elkhead Region the Browns Park Formation consists of a basal conglomerate member and an overlying sandstone member. The basal conglomerate ranges from 0–300 feet (91m) in thickness and overlies a prominent unconformity that truncates all older rocks. The conglomerate consists mainly of Precambrian lithic clasts derived from the adjacent Park and Sierra Madre Ranges, and was deposited as coalescing alluvial fans.

The overlying sandstone member has thicknesses that reach up to 2200 feet (670 m) preserved beneath and adjacent to the volcanics of the Elkhead Mountains. Sedimentary textures and structures suggest that much of the sandstone is eolian. A coarser-grained facies (very fine- to medium-grained) characterized by large-scale festoon or trough cross bedding (up to 30–50 feet, 9–15m, high) was deposited in extensive dune fields. A massive finer-grained facies (very fine-grained sandstone to siltstone) along the eastern part of the area is interpreted to be loess. In addition, coarser-grained sandstone with smaller-scale cross bedding, plus massive to laminated siltstone, suggests local fluvial channel and floodplain environments. Analyses of cross-bedding directions from all environments (250 measurements) indicate an overall eolian and fluvial transport direction to the NNE (mean N 19 E). Mineralogy of the sediments indicates a complex source area to the south and west consisting of plutonic, metamorphic, volcanic and sedimentary rocks, probably derived from the Precambrian-cored Laramide uplifts of the southern Rocky Mountains, the San Juan-West Elk Mountains volcanic province, and the Mesozoic sedimentary rocks of the Uncompahgre Plateau region. Volcanic ash occurs throughout the rocks and probably was derived from contemporaneous volcanism in the adjacent Basin and Range province to the southwest.

Sediments from these sources evidently were picked up by the wind from an apparently ineffectual Colorado River drainage system and carried north across an inferred paleo-continental divide and deposited in an aggrading basin by northeast-flowing eolian and fluvial processes. This drainage probably connected north to the ancestral Platte River system. Deposition of the Browns Park Formation represents a major period of late Oligocene-Miocene aggradation throughout the central Rocky Mountains, which filled basinal areas with several thousand feet of sediment and buried the flanks of the Laramide ranges up to current elevations of about 10,000 feet (3050 m).

The age of the Browns Park Formation in the region has been well established in the literature as latest Oligocene through Miocene, with dates from both fission track and radiometric methods ranging from 24.8 to 7.2 Ma. Four K-Ar ages determined as part of this project from the Elkhead Mountains igneous rocks range from approximately 7.8–11.4 Ma. As these rocks intrude and overlie the Browns Park Formation, they help confirm the minimum age for the formation as late Miocene.