DETRITAL ZIRCON GEOCHRONOLOGY OF LOWER CRETACEOUS CONGLOMERATES, SAN RAFAEL SWELL AND WASATCH PLATEAU, CENTRAL UTAH AND ITS IMPLICATIONS FOR CONGLOMERATE CORRELATION AND DETRITAL SOURCES

U-Pb ages of detrital zircon grains collected from four samples of Lower Cretaceous Cedar Mountain (Neocomian-Albian) and San Pitch (Aptian-Albian) Formations in central Utah exhibit discrete, distinctive peaks on age-probability plots. Paleoproterozoic grains (2475-1651 Ma; 62 of 100 grains) dominate the basal Cedar Mountain Formation in Salina Canyon. In contrast, the type Buckhorn Conglomerate on the San Rafael Swell and Member C of the San Pitch Formation in Salina Canyon contain mostly Mesoproterozoic grains (1579-1000 Ma; 61 and 64 of 100 grains, respectively). A population of Ordovician-Mississippian grains in the type Buckhorn Conglomerate indicates substantial contributions from late Paleozoic and/or Mesozoic source rocks uplifted in the thrust belt. Our Buckhorn samples have age-probability distributions similar, although not identical, to a Buckhorn sample from east of the San Rafael Swell (McGraw et al., 2006, GSA Abstracts, no. 6, p. 232). Our data indicate that the basal Cedar Mountain Formation in Salina Canyon and the type Buckhorn Conglomerate were deposited by two different river systems, separated either geographically or temporally, or both. Buckhorn rivers of the San Rafael region appear to have carried similar detritus and thus drained similar source regions. We postulate that clasts in the Buckhorn Conglomerate were derived from Paleozoic quartzarenites, the Triassic Chinle Formation and Permian and Jurassic eolianites. The San Pitch Formation clasts, which lack Paleozoic grains, contain a detrital-zircon age probability distribution remarkably similar to the Devonian Cove Fort Quartzite of the Canyon Mountains in the thrust belt of west-central Utah, suggesting general unroofing into the Paleozoic section by the end of the Early Cretaceous.

Our data are consistent with progressive unroofing of siliciclastic units from the Sevier thrust belt. They corroborate and refine exposure gates for inverted clast stratigraphies documented in carbonate clast conglomerates of the foreland basin. Future work on these geochronologically distinctive quartzite-clast conglomerates promises to further elucidate correlations, which remain ambiguous, among conglomeratic strata of the foreland.