A NEW ARRAY OF EXHUMED PALEOCHANNELS IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, UTAH

Exhumed channels have long been known from the Cedar Mountain Formation. Here, we report on approximately 30 additional exhumed paleochannel segments in the basal Cedar Mountain Formation extending roughly from Castle Dale, UT to ~20 km south of Green River, UT. Data presented here are derived from aerial stereophotograhs and preliminary field work in the area mentioned above. The channels are located in areas lacking the Buckhorn Conglomerate. The majority of the channels are located ~20 m above the Cedar Mountain-Morrison Formation contact and are underlain and flanked by silty to sandy mudstones. Preserved conglomeratic to sandy channel segments range in length from 0.1 to 2 km with widths of 2-20 m, and show a general northeastward flow direction. The longest preserved paleochannels are lightly to moderately sinuous. Several tight-radius point bar (gooseneck) complexes are exceptionally preserved and consist of nested sandstone crescents eroded to ridge and swale topography. Despite the preservation of point bars, the thalweg of the gooseneck channels are not preserved, possibly because once abandoned, the channels became oxbow lakes which filled with mudstones that were lost to erosion during exhumation. Thus, although the preserved channels suggest a dominance of low to moderately sinuous streams, it is likely that there is a bias against the intact exhumation and preservation of highly sinuous channels (oxbow lakes).

In the westernmost portion of the study area, the sandstone channels are located tens of meters above the base of the formation, are few in number, small, isolated, and surrounded by fine grained floodplain deposits. In contrast, to the east, near Green River, UT and on to Moab, UT the sandstone channels are relatively close to the base of the formation, closely spaced to amalgamated, thicker and called the Poison Strip Sandstone. We attribute these east to west fluvial architectural shifts to greater accommodation space in the western area as a function of higher subsidence rates.