A COMPARISON OF THE FLUVIAL SYSTEMS OF THE SALT WASH AND BRUSHY BASIN MEMBERS OF THE LATE JURASSIC MORRISON FORMATION

The Morrison Formation of the Colorado Plateau is famed for its dinosaurs and uranium, but it also provides crucial insights into the Late Jurassic tectonics of western North America. It was deposited in the nascent foreland basin of the Sevier Orogeny preceding the eastward migration of the forebulge. More than 200 exhumed fluvial paleochannel segments exposed in the Morrison across 35 km² near Castle Dale, Utah on the flanks of the San Rafael Swell were mapped in 3D. These channels provide insights into the paleotectonics and paleoenvironment of the western margin of the extensive Morrison basin, and they can be used as terrestrial analogues for raised, sinuous features on Mars.

The study compares the fluvial architecture of the Salt Wash and Brushy Basin members of the Morrison Formation to better understand shifts in the fluvial systems and depositional environments through time. About 100 channels were mapped in each member. Fluvial deposits in both members are similar in several aspects, including lithology, paleofluvial parameters, and a NE paleoflow direction. Paleochannel fills consist of 0.3-4.8-m-thick, moderately- to well-cemented, fine- to medium-grained sandstones with minor conglomerates. The paleochannels form ridges and benches through differential weathering of surrounding mudstone overbank deposits. Lack of fines within channel fills suggest the streams were perennial. Paleofluvial parameters were calculated using meander wavelength or preserved channel width. Both members show a low average paleoslope (0.0007) and moderate sinuosity (1.8). Average bankful width is 34 m. The two members differ in thickness, channel occurrence, and morphology. The Salt Wash member is 45 m thick, with a 1.3 mud/sand ratio. Superimposed channels are common, and 50% of exposed channels represent point bars. The Brushy Basin member is 115 m thick, with a 3.2 mud/sand ratio, and thin, laterally extensive caliches. Channels are more isolated and 33% represent point bars. These results indicate that, while the paleoslope did not change between the members, the Brushy Basin (1) preserves more fines, (2) its floodplains were exposed longer in a drier environment, and (3) its channels migrated less than channels in the Salt Wash member.