SKINNER PEAKS QUADRANGLE GEOLOGIC MAP AND VOLCANIC STRATIGRAPHY ON SOUTH MARGIN OF EAST TINTIC VOLCANIC FIELD, CENTRAL UTAH

Detailed geologic mapping of the Skinner Peaks quadrangle, located along the Basin and Range-Colorado Plateau transition in central Utah, delineated about 8500 feet (2600 m) of exposed Mesozoic and Tertiary bedrock, and overlying Quaternary and Tertiary surficial deposits. Key aspects of the quadrangle include its location along the Levan segment of the active Wasatch fault zone, southern exposures of the Levan culmination within the Sevier fold-thrust belt, proximity to the central Utah oil play and East Tintic volcanic field, and exposures of volcanic rocks previously included in the Goldens Ranch Formation (GRF). Our mapping and geochemical and geochronologic analyses allowed us to refine the volcanic stratigraphy and suggest regional correlations. We informally separated the volcanic rocks into the Chicken Creek Tuff Member of the revised GRF and members 1 through 5 of the formation of Painted Rocks. These rocks overlie the Eocene Green River Formation and consist largely of lahars interlayered with dacite to rhyolite tuffs that span a time from slightly before 39 Ma to slightly after 30 Ma based on 40Ar/39Ar analyses. This stratigraphy and other geologic mapping data allow for correlation northward to the East Tintic volcanic field (~35 to 33 Ma) of the East Tintic Mountains and Long Ridge. The Chicken Creek Tuff (38.6 Ma) lies near the bottom of the volcanic section and is mapped northward into Sage Valley; its source vent is not known. Member 1 may correspond to the Hall Canyon Conglomerate Member of the GRF and to volcanic conglomerate units A and B of the volcanic rocks of Sage Valley. Member 2 appears to correlate with the Fernow Quartz Latite (34.9 Ma) present in Sage Valley and in the southern East Tintic Mountains, where the Fernow caldera is thought to exist. Overlying tuffs dated at 34.0 Ma (member 4) and 29.8 Ma (part of member 5) have no apparent direct correlatives to the north. A northward source for members 2 and 3 is probable, but a source is not known for the other members. The Levan monzonite intrusions (24 Ma) are younger and thus not a source. Our work suggests the East Tintic volcanic field may encompass a larger area and spanned a longer time period than previously thought, and/or other unknown or concealed sources may also exist.