NEW GEOLOGIC MAP OF THE LOWER DIRTY DEVIL RIVER-HITE CROSSING AREA, GLEN CANYON NATIONAL RECREATION AREA, SOUTHERN UTAH

For several years, the Utah Geological Survey, in cooperation with the National Park Service, has been working on a series of detailed geologic maps covering Glen Canyon National Recreation Area (GCNRA). Accurate, detailed geologic maps are essential to the management of the fragile desert lands because of their proximity to Lake Powell, which receives about 1.8 million recreation visits per year. We recently completed a geologic map of the largest remaining insufficiently mapped part of GCNRA, the Dirty Devil River-Hite Crossing area in the northern part of the recreation area. The map covers an area of about 400 km2 near the north end of Lake Powell where the Colorado River enters the lake (depending upon lake level). Exposed strata range from the Pennsylvanian Honaker Trail Formation, exposed in the bottom of lower Cataract Canyon, to the Middle Jurassic Page Sandstone, which caps a few remote mesas. Exposures are unusually good, permitting detailed examination of the 3-dimensional relationships of map units. The southwestern pinch-out of the Permian White Rim Sandstone and the western pinch-out of the Triassic Hoskinnini Sandstone are both within the area, presenting the opportunity to understand how these units correlate with adjacent strata. The Triassic Chinle Formation is extensively involved in and mantled by massive landslide complexes that extend the length of the outcrop belt. Nevertheless, sufficient exposures reveal rapid lateral variations over distances of just a few tens to hundreds of meters. These changes, which include lithologic changes, channeling, and pinch-outs, make mapping of the Chinle members challenging, but also reveals much about the terrestrial depositional setting of this formation. The area is cut by a northwest-trending fault zone consisting of over a dozen small fault splays, most having less than 5 m of displacement, that trend oblique to the major structural fabric of the area. This zone has been the focus of vertical hydrocarbon migration as evidenced by bleached zones and “dead” interstitial oil residue.