DRAINAGE DISRUPTION BY BASALTIC LAVA FIELDS ALONG THE SNAKE RIVER PLAIN HOTSPOT TRACK: DETRITAL ZIRCON EVIDENCE FROM DRILLCORE CONSTRAINS THE COURSE OF THE PLIOCENE BIG LOST RIVER

Analysis of radiometric U/Pb ages of detrital zircons in ten Late Miocene to Pleistocene samples from six drill cores on the Snake River Plain (SRP), Idaho suggests that an ancestral Big Lost River system drained west along the northern side of the SRP prior to late Pliocene and early Pleistocene construction of basaltic volcanoes and rhyolite domes of the Axial Volcanic Zone.

Neoproterozoic (700 to 650 Ma) detrital zircon grains from the Wildhorse Creek drainage of the Pioneer Mountains core complex, and characteristic of the Big Lost system, are found in Pliocene sand from cores drilled in the central SRP (Glenns Ferry Formation and Banbury Basalt near Wendell) and western SRP (Idaho Group at Mountain Home). Ediacaran grains (580-545 Ma were likely derived from alkalic plutons in central Idaho (Lund et al., in press GSA Bulletin).

One sample from Wendell contains a Late Miocene 6 to 3 Ma grain population from the Magic volcanic center.

After the Axial Volcanic Zone was constructed, sediment from the Big and Little Lost Rivers, formerly transported southwestward to the western SRP, was trapped in the Big Lost Trough, a volcanically-silled, closed basin.

Holocene to latest Pliocene samples from drill core from Big Lost Trough wells on the Idaho National Laboratory (INL) reveal interplay between the voluminous Big Lost River system and the relatively minor Little Lost River system in the Big Lost Trough. A mixed provenance signature is recognized in fine-grained sands deposited in a Pleistocene pluvial lake system.