MAXIMUM DEPOSITIONAL AGE AND PROVENANCE OF NEOPROTEROZOIC DIAMICTITE UNITS IN CENTRAL UTAH

Glaciomarine-bearing strata in the Sheeprock, Deep Creek, and central Wasatch ranges of Utah record two episodes of glaciation, which are distinguished by a change in detrital zircon provenance (U-Pb ages) from regionally extensive sources to more localized sources and by the presence of syn-depositional volcanic grains in the upper interval. In the Sheeprock Range, a lower diamictite interval (Otts Canyon Formation) has a dominant population of Grenville-age (950 – 1200 Ma) zircon grains and contains abundant quartzite clasts. The upper diamictite interval (Dutch Peak Formation) has a dominant Archean population and contains large granitoid clasts, reflecting a more local basement source. The upper diamictite contains basalt, trachyte, and rhyodacite volcanic clasts and a distinct population of ca. 680 Ma zircon grains. In the Deep Creek Range, a lower diamictite interval (unit T3 of Trout Creek sequence) has a dominant population of Grenville-age zircon grains and a minor population of ca. 800 Ma grains. An upper diamictite interval (unit T5) has a well-defined Paleoproterozoic population and contains large granitoid clasts, reflecting local basement sources in the Mojave Province. A single thinner interval of glacial strata is found eastward in the central Wasatch Range (Mineral Fork Formation). Diamictite here contains a mixture of Grenville, Paleoproterozoic and Archean grains, interpreted to record recycling of detrital zircons from the underlying Big Cottonwood Formation and correlatives; one diamictite sample contained two ca. 700 Ma zircon grains. Similar patterns are found in northern Utah (Perry Canyon formation) and SE Idaho (Pocatello Formation), where an upper diamictite interval is marked by influx of locally derived basement material and contains young (ca. 680 Ma) volcanic material. Regional relations indicate: 1) a lower diamictite interval was derived from regionally extensive sources including Grenville-age material; 2) an upper diamictite interval was derived from more local basement sources and deposited ca 680 Ma and 3) the change in source areas between lower and upper diamictite intervals likely records development of local basement uplifts during an early phase of rifting along the western margin of North America, synchronous with volcanism and glaciation.