NEW GEOLOGIC MAP OF THE FRANICS 7.5’ QUADRANGLE IN THE WASATCH BACK OF NORTHERN UTAH: A STORY OF VOLCANIC ROCKS, LANDSLIDES, AND RIVER CAPTURE

A primary goal of modern geologic mapping is to create detailed, 1:24,000-scale geologic maps of areas experiencing high population growth rates. New mapping of the Francis 7.5’ quadrangle in Summit and Wasatch Counties, Utah, addresses a region experiencing a large influx of housing developments extending from Park City to Heber City. Communities within the mapping area include the Town of Francis in Kamas Valley to the northeast, and a part of Heber City to the southwest. Our work is the first geologic map of the Francis quadrangle created at this scale, making this project valuable for land-use planning and various geologic resource assessments.

Most of the Francis quadrangle is covered by Paleogene Keetley volcanic rocks. Brecciated lahar and debris flow deposits are capped by resistant lava flows, including the latite Francis Flow and trachyandesite Coyote Canyon Flow. Chaotic blocks within debris avalanche units are sourced from various Paleozoic and Mesozoic deposits and are interpreted to be flank collapse deposits on the margin of stratovolcanoes. Lineaments covered with vegetation, likely demarcating jointing patterns, are common across the brecciated units. The Keetley volcaniclastic deposits have a fine, ashy matrix conducive to landsliding. One-meter resolution lidarslopeshade maps reveal landslide features not easily identified in field investigations. Landslides are mapped based on a hummocky textured surface continuing downslope from a vegetated scarp. These areas have primarily been mapped as a mixed depositional unit to include mass wasting and colluvium (Qmc). If the deposit culminated in a distinct toe, it was identified as a mass wasting unit (Qms).

The quadrangle is bisected by the Provo River. The modern river flows west out of the Uinta Mountains towards Utah Lake, but ridge-and-swale topography of a glacial outwash deposit north of the Provo River suggests it once flowed north as the south fork of the Weber River. The mechanism and timing of river capture is the topic of ongoing research. In addition to this new research, our geologic mapping of the Francis quadrangle provides the foundation for geotechnical, resource, hazard, environmental, and hydrological studies, which are important for sustaining growth.