DETRITAL ZIRCON GEOCHRONOLOGY OF BULL RUN AND COPPER BASINS, NV

Sediment provenance data can be used to reconstruct paleodrainage patterns, relative paleotopography, and regions of active erosion and volcanism at the time of sediment deposition. Provenance data also provide constraints on the timing of basin formation, expansion, and periods of basin connectivity. Here we investigate the relationship between active deposition in the Bull Run and Copper Basins in northeastern Nevada and early extension in the Cordilleran hinterland as part of an orogen-scale study. U-Pb ages on detrital zircon grains were obtained from several stratigraphic levels. The youngest population of three or more grains is interpreted as the maximum depositional age of the sample, which range from 35.1-37.2 Ma in Copper Basin strata and 34.0 Ma in Bull Run Basin strata. These ages are consistent with ⁴⁰Ar/³⁹Ar sanidine ages from Henry (2008), but are 2-5 Ma younger than ⁴⁰Ar/³⁹Ar biotite ages of McGrew and Foland (unpublished). A sample collected just below the Copper Basin flora described by Axelrod (1966) provides a new maximum depositional age of 35.5 Ma for the flora.

Major grain age populations range from 34-40 Ma in Bull Run strata and 35-45 Ma in Copper Basin strata, and are dominated by volcanic material primarily sourced from the northeastern Nevada volcanic field. A secondary 101-111 Ma grain age population found in upper Copper Basin strata was likely sourced from intrusions related to the Cordilleran arc system. Bull Run strata also contain minor grain age populations of 120-360 Ma, 1080-1240 Ma, 1450-2390 Ma. Dominant grain age populations in Copper Basin sediments are similar to those in middle Eocene strata in basins to the south, while Bull Run Basin grain age populations are more similar to those in late Eocene southern basin strata. Although strata in both basins are dominated by northeastern Nevada volcanic grain ages, the diversity of older grain age populations between the two basins and lack of 40-45 Ma grain ages in Bull Run strata shows that the two basins were not part of one continuous paleodrainage. Conglomerate clast counts and paleocurrent analysis support this interpretation. This provenance data suggests a localized, small-scale basin formation mechanism, such as minor upper crustal extension or geomorphic damming of paleodrainages.