SEDIMENTARY FACIES ANALYSIS OF THE EARLY CRETACEOUS POISON STRIP AND RUBY RANCH MEMBERS OF THE CEDAR MOUNTAIN FORMATION, EAST-CENTRAL UTAH

Excellent 3D exposures of sandstone bodies in the Poison Strip and Ruby Ranch Members in the Klondike Bluffs area near Arches National Park were studied using measured sections and photomosaics obtained with GigaPan and drone technologies. Lithofacies were mapped in detail and, along with paleocurrents data, were used to interpret fluvial styles.

The multistoried Poison Strip Member eroded into green mudstone; the base of the lower storey consists of lenses of cm-scale mudstone rip-ups, up to 40 cm thick, and medium-grained sandstone with decimeter-scale load casts. The majority of the unit consists of 20-50 cm thick sets of trough cross-bedded sandstone; laterally sets are truncated by a 4-5 m thick set of low-angle cross-bedded sandstone. The upper meters of the unit consist of massive sandstone beds with uniformly dispersed chert granules. The upper storey lies above a discontinuous thin green mudstone overlain by a cm-size mudstone rip-up conglomerate, which are overlain by several m of 10-30 cm-thick trough cross beds and meter-scale lenses of ripple laminated sandstone. The storey top contains pebbly massive sandstone with well-defined, 1 m relief, channel margins that cut obliquely into the cross-bedded sandstone. The ‘Klondike Bluffs’ sandstone in the Ruby Ranch Member, 23 m above the Poison Strip Member, also eroded into green mudstone with overlying conglomerate containing rip-up clasts (up to 5 cm). The Klondike Bluffs sandstone is a pebbly sandstone and chert conglomerate with large-scale (up to 3 m wide) trough cross-bed sets. Paleocurrent measurements of cross beds indicate northeast to northwest transport direction for all units.

Abundant trough cross bedding indicates most of the Poison Strip and Klondike Bluffs sandstones were deposited by dunes in high–energy bedload-dominated channels. A narrow range of paleocurrent directions indicate low-sinuosity rivers. Erosive-based massive sandstones are interpreted as deposits of hyperconcentrated flows that developed during extreme events. The massive units that cap storeys may reflect flood events that led to avulsion and abandonment of the channel belt. The load casts indicate rapid channel emplacement into a wet floodplain area during avulsion. The stratigraphic change to the coarser Klondike Bluff may represent rejuvenation of the source area.