CONSTRAINING SEDIMENT SOURCES ACROSS THE CORDILLERAN HINTERLAND IN THE EOCENE FROM DETRITAL ZIRCON U/PB DATING

Cordilleran hinterland deformation and Farallon slab rollback influenced fluvial systems across the central North American Cordillera during the Eocene. The extent and evolution of drainages across and off of the Cordilleran hinterland during the Eocene may be traced through detrital zircon U/Pb dating. Fluvial samples from across the Northwestern U.S. were collected and analyzed to track Eocene sediment provenance and paleoflow directions. Source regions are reflected in multiple prominent grain age populations, which may aid in defining the evolution of the Cordilleran drainage divide and rivers crossing the Sevier thrust front.

This study focuses on two sampling regions: 1) Cordilleran hinterland basins and 2) basins in the northern Sevier-Laramide foreland. Detrital zircon samples from the southern part of the Elko Basin in NE Nevada have two dominant grain age populations: early Eocene (42-59 Ma) and Jurassic (146-206 Ma). Local Mississippian basement conglomerates have two dominant grain age populations (1800-2100 Ma and 2500-2800 Ma). Previous detrital zircon analyses from the Elko Basin show similar dominant grain age populations. Latest Eocene to Oligocene sediments from the uppermost part of Greater Green River Basin section show early Eocene dominant grain age populations, with the addition of Precambrian grain age peaks: (48-56 Ma and 1100-1700 Ma). Renova Formation sediments in southwestern Montana contains a dominant grain age population of Late Cretaceous (65-80 Ma) and a smaller Precambrian grain age population of 1400-1700 Ma.

Renova Formation grain age populations correspond with ages from the Idaho batholith, and ages from zircon grain cores also indicate Precambrian grain recycling. Detritus from the Challis volcanic province in Idaho reached the Elko Basin in northeastern Nevada. Local Mississippian basement rocks also acted as a major sediment source to the Elko basin. Oligocene sediments capping the Eocene Green River basin show a greater diversity of Precambrian grain age populations than in the hinterland, likely sourced from the Cordilleran thrust front.