REEVALUATION OF THE NEOPROTEROZOIC JESSE EWING CANYON FORMATION, EASTERN UINTA MOUNTAIN GROUP, NORTHEASTERN UTAH

The Uinta Mountain Group (UMG) holds key information regarding active processes and paleogeography of NE Utah and NW Colorado during Neoproterozoic time. The basal Jesse Ewing Canyon formation is exposed in an ~56km2 area, records the initiation of basin formation in the area, and is thus important for understanding basin evolution during this time. Previously interpreted as an alluvial fan system, alternative interpretations of the Jesse Ewing Canyon Formation are likely based on detailed mapping (1:12,000 scale), measured sections and facies analysis.

Three main facies were mapped within the Jesse Ewing Canyon Formation. The conglomerate-breccia facies (10's m thick) contains interbedded darkreddish-brown to dark gray conglomerate and breccia, containing clasts primarily derived from the underlying Archean-Paleoproterozoic(?) Red Creek Quartzite, interbedded with considerable amounts of quartz arenite and maroon shale. The coarse-grained facies is overlain at most localities by a finer-grained facies of interbedded sandstone and dominantly maroon shales with subordinate pebble conglomerate and breccia (100+ m thick). A laterally-equivalent shale-dominated facies (~10's m thick) contains organic-rich gray to green shale that grades upward into the coarser crossbedded sandstone of the overlying undifferiented Uinta Mountain Group. Mapping results show no clear evidence for intraformational faulting, though the nature and distribution of facies suggest local high relief, likely generated by pre- and syndepositional tectonism. Preliminary facies analysis suggests that not all facies within the Jesse Ewing Canyon formation record alluvial fan processes. Rather, only the basal section of the formation potentially records alluvial fan processes and the two upper finer-grained facies record deposition in a low energy body of water that was possibly marine.