Structure and Stratigraphy of San Francisco Mountains, Western Utah

Abstract

Listen

Translate article

The San Francisco Mountains are part of a fault-block range in the Basin and Range physiographic province in west-central Utah. The rock succession in this range was mapped as a normal stratigraphic sequence by B. S. Butler. Recent investigation, however, indicates that older rocks rest on younger rocks along an extensive overthrust, as first suggested by Nolan and Hintze. The Morehouse quartzite, presumed by Butler to be of Ordovician-Silurian(?) age, is recognized to represent the Precambrian(?) to Lower Cambrian Prospect Mountain Quartzite. The Grampian assigned by Butler to the Cambro(?)-Ordovician, is identified as including Middle(?) and Upper Cambrian carbonate rocks, Ordovician Pogonip Group, Lower Mississippian(?) limestone, and Pennsylvanian Ely L mestone. The Prospect Mountain Quartzite has overridden the Paleozoic carbonate rocks. The thrust plate underlies the crest of the San Francisco Mountains north of a large quartz monzonite stock, plunges northward, and forms the bulk of the Cricket Mountains. The Prospect Mountain Quartzite normally is overlain by younger Cambrian strata in the Cricket Mountains. Two small klippen of the quartzite remain south of the intrusive body. Boulder conglomerates of probable early Tertiary age unconformably overlie the thrust plate; therefore, the age of the thrusting is post-Early Pennsylvanian and pre-early Tertiary. The direction of movement at the thrust is not evident from the local record, but regional relationships indicate an easterly to southeasterly direction. The displacement at the thrust is greater than the 4-mile width of the range and probably is of large magnitude, as the thrust has no root within the range. Misch postulates that both the Wah Wah thrust described by Miller and the San Francisco thrust of this report resulted from the frontal breakthrough of the Snake Range decollement. If so, the displacement has been several tens of miles. The Paleozoic strata underlying the San Francisco Mountains were folded and faulted prior to large-scale overthrusting. Fluviatile boulder conglomerates, probably formed as a result of the Laramide orogeny, covered the deformed and eroded Paleozoic beds. A large outpouring of volcanic flows and pyroclastic rocks followed in middle Tertiary time. Cenozoic deformation has been superposed on the earlier structures. This period of tectonism included intrusion of a quartz monzonite stock and Basin-and-Range faulting which outlined the present range. In late Pleistocene, Lake Bonneville flooded the adjacent intermontane basins and during retreat left numerous shoreline deposits along the western range flank.