GEOLOGIC MAPPING OF EXHUMED, MID-CRETACEOUS PALEOCHANNEL COMPLEXES NEAR CASTLE DALE, UTAH INDICATE THE LOCAL DAKOTA SANDSTONE IS THE FLUVIAL FACIES OF THE MUSSENTUCHIT MEMBER OF THE CEDAR MOUNTAIN FORMATION

East of Castle Dale, Utah, an array of well-preserved, exhumed paleochannels in the Morrison, Cedar Mountain and Dakota formations consist primarily of point bar complexes and scattered, low sinuosity channels. An area of ~140 km² was mapped at 1:24,000 scale focusing on exhumed point bar complexes and channel sandstones of the Cedar Mountain and “Dakota” formations. The Cedar Mountain Fm. consists, from bottom to top, of 2.5-10 m of Yellow Cat/Buckhorn Conglomerate equivalent units, ~80 m of the Ruby Ranch Mbr., and ~30 m of the Mussentuchit Mbr. The Dakota Sandstone consists of conglomeratic to sandy channel fills within the Mussentuchit Mbr.

The Ruby Ranch-Mussentuchit contact is diagnosed as the top of a laterally extensive, mappable, ~10 meter thick paleosol which is a maroon color with calcrete horizons and root traces. Exposures of the contact can be poor and display a shift in color from maroon mudstone to a white-green, silty mudstone that weathers to a popcorn texture, derived in part from weathered volcanic ash, like the balance of the Mussentuchit fines.

In the southern third of the mapped area, Ruby Ranch sandstones are thin, discontinuous channel segments surrounded by floodplain deposits. In the middle to northern area, point bar complexes dominate, some of which are amalgamated. Highly variable flow direction data was obtained from 4 well-preserved complexes and 1 channel segment with an average NE flow. The variability, also displayed in radius of curvature data, is accredited to high channel sinuosity.

“Dakota”channels within the Mussentuchit Mbr. are more laterally continuous, with common overprinting of point bars, than in the Ruby Ranch, but the flow data shows a shared NE flow. The “Dakota” channels were deposited in an area with less accommodation space than those in the Ruby Ranch Mbr. The stratigraphic position of the lowermost sandstone varies notably, but all lie above the Ruby Ranch-Mussentuchit contact.

Detailed 3-D mapping clearly indicates the Dakota Sandstone is not a stratigraphically justifiable unit in this area because the sandstones consist of numerous channel complexes at various positions within the Mussentuchit Mbr. of the Cedar Mountain Formation.