GEOLOGY OF THE LYMAN 7.5’ QUADRANGLE, WAYNE COUNTY, UTAH: TECTONICS AND MASS WASTING AT THE COLORADO PLATEAU MARGIN

The Lyman 7.5’ quadrangle is located in Wayne County, Utah, at the western edge of the Colorado Plateau. Thousand Lake Mountain forms the eastern boundary of the quadrangle and the Fremont River flows southward across the alluvial Rabbit Valley at the quadrangle’s western edge. The Thousand Lake Fault (TLF) marks the boundary between the stable Colorado Plateau block and the extended terrane of the High Plateaus of Utah to the west.

Our research and 1:24k scale mapping focus on: 1) understanding the kinematics and timing of movement on the TLF, 2) delineating the stratigraphy of the volcanic rocks, and 3) defining the origin of the enigmatic Rabbit Valley topographic salient. The main strand of the TLF dips westward and strikes N-S in the southern part of the quadrangle, angling NE-SW farther north. Kinematic evidence indicates down-to-the-west normal slip on the TLF and total offset of 800-1000 m. Although the TLF experienced a protracted slip history, Quaternary boulder fan deposits are down-faulted against the Jurassic Navajo and Carmel formations and provide evidence for movement on the fault within the last ~200k years.

Volcanic rocks are well exposed in the NW part of the quadrangle and include a ~250 m thick sequence of late Oligocene to Miocene (26-23 Ma) ash-flow sheets. From bottom to top, the sequence includes interlayered welded trachyte and volcaniclastic sediment, a distinctive porphyritic trachyandesite, a fine-grained trachyte, and the biotite-rich Osiris trachyte.

The Rabbit Valley salient protrudes W from the flank of Thousand Lake Mountain and is underlain by tilted strata of the Cretaceous to Paleocene North Horn and Flagstaff formations. These strata are overlain by tilted biotite-bearing ashy sandstones and poorly lithified conglomerates; this sequence is collectively capped by trachyandesite ash flows. A series of NE- and NW-striking normal faults cut rocks in the Rabbit Valley salient.

Surficial deposits dominate the western slope of Thousand Lake Mountain. Multiple complex mass movement events including slumps and debris flows have brought volcanic boulders and sedimentary rocks down from the mountain flank. Boulder-covered slopes and boulder fans overlie bedrock and older alluvial fans. Multiple fluvial terraces occur 10 to 120 m above the modern Fremont River in the NW part of the quadrangle.