THE BIRTH OF THE PLEISTOCENE BIG LOST TROUGH: DETRITAL ZIRCONS CONSTRAIN LATE PLIOCENE DRAINAGE DIVERSION BY A BASALTIC RIFT ZONE, EASTERN SNAKE RIVER PLAIN, IDAHO

Detrital zircon populations from Pleistocene sands in coreholes at the Idaho National Engineering and Environmental Laboratory have distinctive age populations which match those of tributary streams. Modern Medicine Lodge Creek drains the Beaverhead and Tendoy Mountains along the Idaho-Montana continental divide and contains distinctive 105 to 98 Ma Idaho batholith grains, likely recycled through Cretaceous synorogenic fluvial strata, as well as diverse Proterozoic grains (2160 to 1700 Ma) recycled through Mesoproterozoic Belt Supergroup strata. The modern Big Lost River is dominated by 50 to 45 Ma grains from the Challis Volcanic Group. Corehole samples from 1840 ft in Hole 2-2A (2.5 to 3 Ma, Pliocene) contain the Medicine Lodge Creek signature. At around 2.3 Ma the basaltic Circular Butte-Kettle Butte and Lava Ridge-Hells Half Acre volcanic rift zones began to produce lava which formed a northwest-trending topographic high. This effectively prevented Medicine Lodge Creek from entering what was to become the Big Lost Trough, an underfilled basalt-silled sediment and lava basin south of the Lemhi Range. Sediments from 1697 feet and higher (1.8 Ma to 600 ka) in corehole WO-2 have the Big Lost River detrital zircon signature. This suggests that the Big Lost River, diverted northward by growth of the Craters of the Moon volcanic field, or by more rapid subsidence of the Big Lost Trough, has been the primary sediment source for the INEEL subsurface for the last 1.8 Ma. Our work suggests that SHRIMP U-Pb ages on random detrital zircon populations provide unique and repeatable signatures for 2nd order intermontane drainage basins. Detrital zircons provide fast and repeatable signatures to distinguish Pleistocene and Neogene stream systems.