LINKING DETRITAL ZIRCON DOUBLE DATING WITH CHRONOSTRATIGRAPHY IN THE EOCENE ELKO BASIN

In the North American Cordilleran hinterland, significant drainage ponding occurred in eastern Nevada during the Eocene. Lakes of the Eocene Elko Basin were situated on a 3-3.5 km-high Cordilleran orogenic plateau prior to extensional collapse during the Neogene. Despite recognition of lacustrine deposits in the Cordilleran hinterland for over a century, widespread extensional segmentation and an absence of geochronologic constraints has hindered previous attempts to unravel hinterland basin evolution. New single-crystal sanidine ⁴⁰Ar/³⁹Ar ages for 21 Elko Basin ash beds permit detailed stratigraphic correlations and expand the previously estimated duration of lacustrine deposition to 50-39 Ma. Enhanced chronostratigraphy also provides a robust framework for interpreting detrital zircon U-Pb and (U-Th)/He ages from double dating of basin sediments.

Decimeter-scale stratigraphy and sedimentologic observations from across northeastern Nevada provide the basis for interpreting the timing and style of lacustrine sedimentation. These observations have revealed regionally correlative lithofacies that show a lake type progression from overfilled to balanced-fill conditions followed by volcanic infilling from proximal late Eocene calderas. Detrital heavy mineral analysis and U-Pb geochronology indicate that a range of proximal and distal (≥ 400 km) sources contributed to basin sedimentation. U-Pb dating of Eocene sandstones indicates that the Roberts Mountain Allochthon (RMA) and conglomeratic detritus derived from the RMA during the Antler orogeny contributed the majority of clastic sediment to the Elko Basin. U-Pb-He double dating of detrital zircons from these sediments shows predominantly muted exhumation (average of 0.2 km/Myr) across the hinterland, and a lack of significant sediment burial throughout the Phanerozoic.

Lag time is the difference between the cooling age and depositional age of detrital grains and can therefore be used to elucidate the exhumation history of eroded orogens. Elko Basin sediments show a significant decrease in mean lag time from 458 Myr in the early-middle Eocene to 39 Myr in the late Eocene-early Oligocene, possibly reflecting progressive exposure and capture of deeper crustal sources in the hinterland.