FLUVIAL ARCHITECTURE OF EXHUMED AND INVERTED PALEOCHANNELS OF THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION OF EAST-CENTRAL UTAH

Sinuous ribbons of sedimentary rock in the Early Cretaceous Ruby Ranch Member of the Cedar Mountain Formation have been previously interpreted as topographically inverted paleochannel-fill deposits of a meandering stream system. The fluvial style was largely based on the elevated channel planform and grain size parameters, while largely ignoring the internal lithofacies architecture of the deposits. Although paleocurrent data matches reasonably well with the sinuous outline of the ridges, examination of the internal architecture reveals complex multi-storey sequences with little evidence for lateral accretion, but instead dominated by downstream accretion.

The 4-10 m thick deposits consist of 4 lithofacies, dominated by trough-cross bedded medium- to coarse-grained sandstone, with abundant conglomerate, and lesser amounts of fine-grained ripple cross-laminated sandstone and green mudstone. Cross beds are 10’s cm thick, rarely up to 2 m. Conglomerate beds are typically < 1 m (up to 2 m) thick. Most clasts are carbonate, <10 cm size (up to 50 cm), and also chert and quartz pebbles. Mudstone may contain thin sandstone beds and abundant bioturbation. The ridges consist of 2-4, fining upward, 2-3 m thick, sequences. Grain size varies significantly along the length of the channel. In a N-S segment, there are two distinct storeys, separated by cm to m of green mudstone. In this area there is a greater percentage of sandstone relative to conglomerate than in the eastern end of the channel. In the W-E segment there are 2 or 3 fining- upward sequences capped by discontinuous mudstone and separated by erosional surfaces. The top of one fining upward sequence contains mudstone with pedogenic carbonate concretions (<10 cm). The overlying conglomerate contains clasts of the mudstone with concretions.

Multi-storey channel deposits reflect repeated channel deposition in the same location, but separated by significant time intervals. Mudstone beds reflect floodplain overbank and lacustrine deposition; paleosol development indicate significant time between channel deposition in a particular area. Resistant cementation of the uppermost channel deposits has preserved the sequences; it is not clear if the sinuous shapes to the ridges reflect the true nature of the channel planform of the Ruby Ranch fluvial systems.