MORPHOLOGY AND FLOW CHARACTERISTICS OF EXHUMED PALEOCHANNELS IN THE UPPER JURASSIC BRUSHY BASIN MEMBER OF THE MORRISON FORMATION NEAR CASTLE DALE, UTAH

The architecture of fluvial paleochannels can record the influences of regional tectonics. To gain insights into these influences we are studying changes in fluvial architecture in strata from the upper Jurassic through middle Cretaceous on the west flank of the San Rafael Swell. Here, we present preliminary results of a study of exhumed palostream channels in the Brushy Basin Member of the Morrison Formation east of Castle Dale, Utah. The foundation of this study is our 3D geologic map of 18 km² of the member which includes more than130 distinct channels and 37 point bars. The paleochannel fills consist of fine- to medium-grained sandstones and minor pebble chert conglomerates which are moderately- to well-cemented and weather out in relief as sandstone ridges (isolated channels) and benches (meanders) on/within overbank deposits of silty mudstones overprinted with paleosols.

Preserved paleochannel widths range from 7-70 m. Paleoflow measurements of 26 paleochannels reveal an average NE paleoflow direction. Calculations using meander wavelength of 16 representative channels indicate low sinuosity (1.8), low paleoslope (.00068), and average flow volume of 200 cubic m/s. The paleochannels are largely isolated, vertically and laterally, showing little evidence of significant lateral migration. Vertical distribution of paleochannels is surprisingly constant throughout the member.

Previous studies of the Brushy Basin Mbr. noted the prevalence of low sinuosity streams flowing to the northeast along with evidences of significant accommodation space. Our study provides new information based on a large dataset that indicates (1) that channels were isolated laterally and vertically, (2) that lateral migration was minimal, and (3) throughout Brushy Basin time there was no notable change in fluvial architecture indicating basin subsidence and fill rates were balanced and discharge through the basin did not change appreciably.