Geologic map of the Logan 7.5' quadrangle, Cache County, Utah

The Logan, Utah 7.5' quadrangle is underlain by Paleozoic sedimentary rocks of the western Bear Ri ver Range, and Tertiary rocks and Quaternary sediments deposited in the Cache Valley. Paleozoic rocks are predominantly limestones and dolostones of Late Cambrian through Pennsylvanian and Permian(?) age. Clastic sedimentary rocks make up the rest of the section. Paleozoic rocks record deformation during the Cretaceous Sevier orogeny. In the central two-thirds of the quadrangle, the Providence Canyon thrust fault places Ordovician through Devonian rocks, which are overturned to the west, on Devonian and Mississippian rocks that form the west limb of the Logan Peak syncline. Tertiary deposits are only present in the subsurface in the Logan quadrangle. They consist of Eocene(?) Wasatch Formation conglomerate, and Miocene-Pliocene(?) Salt Lake Formation conglomerate, siltstone, and tuffaceous sandstone that are up to 8,000 feet (2,440 m) thick in Cache Valley. Quaternary units were deposited in the deltas, shorelines, and lake bottom of Lake Bonneville. Prominent units were deposited during the Lake Bonneville high stand (approximately 16,000-15,000 years ago) at a present-day elevation of -5,100 feet (1,555 m) and during the Provo shoreline still-stand (-14,500-13,500 years ago), when the lake was at an elevation of 4,780 to 4,790 feet (1,457 to 1,460 m). Other Quaternary deposits include alluvial stream and fan deposits, landslides, slumps and/or flows, and colluvial deposits. The mountain front of the Bear River Range formed due to slip along the East Cache fault, an active normal fault with about 16,400 to 21,000 feet (5.0 to 6.4 km) of offset. Fault scarps along the East Cache fault are in Lake Bonneville deposits and in post-Provo-phase alluvial deposits near the mouth of Logan Canyon. Excavations across fault scarps indicate that two surface rupturing earthquakes occurred along the East Cache fault in the past 15,000 years. The East Cache fault lies at the very base of the Bear Ri ver Range in most of the map area. The most valuable mineral resource in the quadrangle is Quaternary sand and gravel. Geologic hazards in the quadrangle include flooding, mass wasting, and earthquakes.