DETRITAL ZIRCONS FROM THE KIMAMA COREHOLE, CENTRAL SNAKE RIVER PLAIN: PROVENANCE CHANGE BETWEEN 6.1 AND 4.9 MA AS THE PALEO-WOOD RIVER SYSTEM MEETS THE AXIAL VOLCANIC ZONE

New detrital zircon U-Pb ages of cored sand from the Kimama corehole, on the central Snake River Plain, suggest episodic Late Miocene, south-draining fluvial incursions into the Axial Volcanic Zone of the subsiding Yellowstone hotspot track. No evidence of a main Snake River nor north-flowing streams are recorded, thus the Axial Volcanic Zone acted as a drainage divide since Late Miocene time.

The 1912 m Kimama core contains basaltic lava flows, eolian loess, and fluvial sand. Ar/Ar and paleomagnetic dating establish a basalt accumulation rate of ~305 m/m.y and a projected bottom hole age of 6.5 Ma. Averaging of ages of the youngest zircon groupings suggest maximum ages for the two upward fining fluvial intervals. The lower interval, 18.5 m thick, and sampled at 1842 and 1844 m is younger than 6.2 Ma. The upper interval, 50 m thick and sampled at 1708, 1733, and 1749 m is less than 4.9 Ma. At the base, both intervals contain mainly Miocene detrital zircons of the Yellowstone-Snake River Plain magmatic system (mainly 10 to 4.9 Ma). Stratigraphically higher sands contain successively older zircon groupings including Challis magmatic event (50 to 45 Ma), Idaho batholith (100 to 90 Ma), Silurian and Ordovician magmatic grains (415 to 435 Ma) recycled from Paleozoic rocks west of the Pioneer thrust fault, and, variably, Grenvillean and Meso- and Paleoproterozoic grains, or mainly Archean grains.

Detrital zircon age groupings (barcodes) within the five samples indicate two incursions of the Wood River system. Each fluvial interval systematically changes upward from dominantly young zircons to mainly older zircons, but variable Archean vs. Proterozoic populations in the upper interval suggest a 4.9 Ma paleo-Wood River tributary that tapped the structurally uplifted lower and middle plates of the Pioneer core complex. This observation suggests unroofing of the core complex and breaching of the Wildhorse gneiss by about 5.0 Ma.

The top of the upper interval records the only population of Paleozoic grains present within the Kimama core, indicating a source west of the Pioneer thrust fault. We suggest that after 4.9 Ma, the source of the paleo-Wood River system shifted westward, and that basaltic volcanism along the Axial Volcanic Zone diverted the paleo-Wood River system southwestward through the Hagerman area between 3.8 and 3 Ma.