NEW GEOLOGIC MAPPING IN THE SOUTH-CENTRAL PART OF THE PANGUITCH 30'x60' QUADRANGLE, SOUTHWEST UTAH

The Panguitch 30'x60' quadrangle is the focus of a new multi-year STATEMAP effort to provide regional and selected 1:24,000-scale geologic maps of an extraordinary part of southwest Utah. The quadrangle includes the high plateau country between Cedar Breaks National Monument on the west and Bryce Canyon National Park and Grand Staircase-Escalante National Monument on the east. It also includes much of Parowan and Sevier Valleys, and so provides a transect between the Basin and Range and Colorado Plateau Provinces at the south edge of the Marysvale volcanic field. Currently, new mapping is focused on the south-central part of the quadrangle, including the Henrie Knolls, Panguitch Lake, and Haycock Mountain 7.5' quadrangles.

Our mapping continues efforts of the old U.S. Geological Survey BARCO Project, which made important advances in this area in the late 1980s and early 1990s. Included in our current map area are numerous Pliocene to Quaternary basaltic and latitic lava flows, three of which – the Dry Valley, Miller Knoll, and Panguitch Lake lava flows – are of possible Holocene age and the subject of current dating efforts. We are developing a robust geochemical database to aid correlation of these lava flows, as well as correlation of Oligocene to Miocene regional ash-flow tuffs and local volcanic strata. Collectively, the lava flows record an incomplete and as yet not fully understood history of drainage development and stream capture in the Panguitch Lake area. This area also includes the south margin of the Markagunt megabreccia. Previously undescribed erosional outliers of the megabreccia on the south side of Haycock Mountain reveal pulverized and re-lithified Baldhills Tuff Member of the Isom Formation with well developed basal slickenlines – underlain by a distinctive diamictite and cut by clastic dikes of the same composition – that rests on Brian Head Formation or locally on pre-Needles Range(?) volcaniclastic conglomerate. These exposures document NW to SE transport of the megabreccia at this location, and hint that the entire Isom caprock at Haycock Mountain may be part of the upper plate of an enormous, north-dipping toe thrust, although further mapping is needed to investigate this interpretation.