GEOLOGY OF THE ALLENS RANCH 7.5' QUADRANGLE, WEST-CENTRAL UTAH

The Allens Ranch quadrangle lies at the northern end of the East Tintic Mountains west of Utah Lake near the eastern margin of the Great Basin of central Utah. Geologic mapping of the quadrangle is focused on improving the understanding of the Paleozoic stratigraphy, volcanic stratigraphy, and structural geology of the region. Mapping is supported by funding from the USGS EDMAP program. An earlier open-file map of the quadrangle by Proctor (1986) was made available by the Utah Geological Survey.

Paleozoic rocks form a thick section of mostly shallow marine sedimentary units ranging in age from the Upper Cambrian Cole Canyon Dolomite to the Pennsylvanian Butterfield Peaks Formation. They accumulated on a generally subsiding passive continental margin, but the succession of carbonate rocks is broken by siliciclastic units related to minor orogenic events. The Paleogene volcanic section consists of a suite of high-K extrusive rocks. Rhyolite, trachydacite, and trachyte ignimbrites along with latite lavas and block-and-ash flows dominate the sequence (~35 to ~32 Ma—Packard Quartz Latite to the Laguna Springs Volcanic Group). Volcaniclastic deposits cap this extrusive sequence and contain clasts of latite and shoshonite. The ignimbrites are believed to have eruption centers to the south and north and were emplaced in paleovalleys carved into the folded Paleozoic rocks. The intermediate to silicic sequence was followed by erosion, deposition of clastic sediments and lake deposits, and eruption of a mildly alkaline Miocene basaltic lava flow which we correlate with the Mosida Basalt that vented in the Soldiers Pass region.

The Paleozoic sedimentary rocks are highly faulted and folded. Major thrust faults are apparently related to the Sevier Orogeny. Tight folds that verge eastward and strike approximately north-south are related to these thrusts, as are some E-trending strike-slip faults. Other N-trending faults are related to later Basin and Range extension. However, large offset, range-bounding faults are buried by valley fill throughout the quadrangle and no young fault scarps have been identified cutting Lake Bonneville deposits.