Dakota Sandstone and Associated Rocks Adjacent to San Juan Sag Near Gunnison, Colorado: ABSTRACT

Abstract

The stratigraphy and depositional systems of the Dakota Sandstone and associated rocks were studied in outcrop at the northeastern margin of the San Juan volcanic field in southern Gunnison and northern Saguache Counties, Colorado. This study fills in a major gap in regional Mesozoic stratigraphy and provides a last view of these rocks before they are concealed to the south by the volcanic cover of the San Juan sag, a frontier hydrocarbon basin. Locally, the Burro Canyon Formation is interpreted as a dominantly meandering fluvial system formed under oxidizing conditions similar to the underlying Morrison Formation. The Burro Canyon Formation pinches out along a roughly east-west line just south of US Highway 50 and is missing for about 15 mi south to the edge of the continuous volcanics. The Dakota Sandstone consists of a lower, low-sinuosity fluvial system abruptly overlain by an upper, mostly nearshore marine sequence which grades upward into the offshore Mancos Shale. While the total Dakota thickness is relatively constant, the fluvial system thickens markedly to the south where marine rocks are quite thin. To the north, marine rocks thicken irregularly and fluvial rocks are thin. In this area, the Mancos Shale can be readily subdivided intomore » Graneros, Greenhorn, Carlile, Juana Lopez, and upper Niobrara formations, although the Fort Hays limestone is locally missing. The upper Mancos Shale and Mesaverde Group are also missing, presumably due to late Eocene prevolcanic erosion.« less