LATE CENOZOIC VOLCANISM IN CURLEW VALLEY, SEVIER RIFT VOLCANIC ZONE, NORTHWEST UTAH

Late Miocene to Pleistocene volcanism in Curlew Valley (north of Great Salt Lake) was concentrated in northeast trending zones along the east and west margins of the valley, and in the center of the valley. The linear distribution of volcanism indicates a structural control imposed by regional Basin and Range tectonics. The zones also differ in terms of the composition, landforms, and geochronology associated with the volcanic rocks. Lavas erupted along the valley margins are basaltic; along the western margin they occur as dissected flows that probably had associated cinder cones, whereas along the eastern margin they form a series of shield volcanoes ranging from 2-6 km in diameter. Lavas erupted in the central zone are bimodal and include 1) basalt flows (4.9 and 3.6 Ma; K/Ar) that form a narrow, low escarpment, and appear to have erupted from a linear vent system, and 2) rhyodacite flows (4.4 Ma; K/Ar) and a rhyolite dome (2.2 Ma; ⁴⁰Ar/³⁹Ar) that together form the Wildcat Hills. A combination of published and unpublished geochronology data consisting of eleven K/Ar analyses and two new ⁴⁰Ar/³⁹Ar analyses suggest that volcanic activity migrated from west to east across the valley. Basalt flows on the western margin yielded dates ranging from 8.6-5.0 Ma, the bimodal lavas in the central zone range from 4.9-2.2 Ma, and the eastern margin basalts range from 1.16-0.44 Ma. Eruptions appear to have been dominantly effusive; however, nearby ash deposits may have originated from Curlew Valley vents. A biotite-rich ash with chemistry similar to that of the rhyolite dome was recently discovered northwest of Tremonton, Utah (approximately 70 km east of the dome). Similarly, the Hansel Valley Ash (a basaltic ash found in over a dozen locations in the basal part of the Lake Bonneville marl section in northern Utah) has chemistry similar to that of the shield volcanoes. If cogenetic, these deposits indicate that there was an explosive component to the eruptive history. Granular, perlitic glass around the margins of the rhyolite dome, possibly due to high-temperature hydration, and weakly developed pillows in an adjacent basalt flow suggest that a marshy or shallow lacustrine environment may have existed in the valley when some of the eruptions occurred. Such environmental conditions would have enhanced the explosive nature of the eruptions.