NEW GEOLOGIC MAPPING IN THE GLEN CANYON DAM - LEES FERRY AREA OF GLEN CANYON NATIONAL RECREATION AREA, SOUTHERN UTAH AND NORTHERN ARIZONA

For several years, the Utah Geological Survey, in cooperation with the National Park Service, has been working on a series of geologic maps covering Glen Canyon National Recreation Area (GCNRA). Accurate, detailed geologic maps are essential to the management of these fragile desert lands because of their proximity to Lake Powell, which receives about 1.8 million recreation visits per year. We recently completed 1:24,000-scale geologic maps of the popular Glen Canyon Dam and Lees Ferry area of northern Arizona and southern Utah. The maps cover an area of about 800 km² near the southern end of Lake Powell and along the Colorado River below Glen Canyon Dam. Exposed strata range from the Permian Hermit Shale near the head of Marble Canyon to the Cretaceous Dakota Formation that caps small isolated mesas. Much of the area is eroded to the level of the Middle to Lower Jurassic, providing extensive exposures of these strata, including the type section of the Page Sandstone. New age control from samples obtained during this mapping will help us understand the correlation between the Page and lower members of the Carmel Formation.

The Lees Ferry map was produced primarily from a published USGS map of the area (USGS Bulletin 1137, published in 1963). The Wahweap–Page–Navajo Canyon area consists of new mapping done in the field on 1:24,000-scale color stereo aerial photographs and in the lab using VROne CAD software and 1 meter high-resolution ortho-imagery. Both maps were completed as geodatabases using Arc/GIS software. The higher precision and increased detail refined placement of bedrock and surficial units, and permitted improved structural contouring (Page/Carmel contact as the primary datum), significantly revising placement of subtle but significant folds that trend across this gently warped terrain. These maps are being released as plot files and in GIS databases, and will be incorporated into a full map of GCNRA.