COMPARATIVE STRATIGRAPHY AND PALEOHYDROLOGY OF CHANNEL-FILL DEPOSITS OF THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION OF UTAH

The Lower Cretaceous Cedar Mountain Formation consists of 30 to 150 m of olive gray to light purple shale and siltstone, pedogenic calcrete horizons, and fine- to coarse-grained lenticular sandstone bodies. The formation is interpreted to represent an alluvial floodplain system dominated by meandering or anabranching channels. We characterized channel-fill sandstones within the Ruby Ranch Member of the Cedar Mountain Formation in two localities: in the vicinity of Dinosaur National Monument, and near Green River, UT. We noted channel body thickness, paleocurrent orientations, lateral continuity in down-current and cross-channel dimensions, bedform geometry, and grain size distributions within channel fill. In both localities, we recognize the channel deposits as single-story ribbon sandstones, with channel widths of 100-400 meters and channel depths of 0.3 to 3.6 meters. Most exposures reveal a lag of pebbles and cobbles transported at the base of the channels, which consist of locally derived pedogenic calcrete nodules and far-traveled chert clasts.

Six sites identified for paleohydrologic study are characterized by a simple incised channel with a single-story fill, and nearly complete exposure of the entire channel cross-section. We measured the longest exposed axis of 200 to 500 clasts from each of the six channels, using a random walk sampling method to characterize grain-size distribution. The D50 and D90 particle sizes average 4.5 and 15.3 mm for chert clasts and 13.4 and 43.6 mm for calcrete clasts. Using empirical equations, the chert data suggest flow velocities of 0.4 to 0.5 m/s, and critical shear stresses of 1 to 2 N/m2, were required to transport the bed material. Incorporating the calcrete data, we estimate flow velocities of 0.9 to 1.2 m/s, and critical shear stresses of 7 to 8 N/m2.

Because the chert clasts are not locally derived, estimates of flow velocity and critical shear stress based on this lithology likely underestimate actual conditions. Estimates based on the locally derived calcrete nodules more likely represent minimum values of flow velocities and critical shear stresses necessary to effectively transport the bed material. These data, along with paleocurrent data, are used to evaluate regional paleoslope in the Cretaceous Sevier foreland basin.