DETRITAL ZIRCONS IN NEOGENE AND HOLOCENE FLUVIAL SYSTEMS OF THE SNAKE RIVER PLAIN-YELLOWSTONE PROVINCE, U.S.A

Few studies have documented densely spaced sampling in areas of known source-area geology, to validate the detrital zircon provenance technique and demonstrate repeatability of the method. On the eastern Snake River Plain we have accumulated one of the most dense sample networks currently available. Significant findings include: 1. Pliocene beds at Hagerman Fossil Beds National Monument contain Idaho batholith (85 to 70 Ma) and Challis volcanic (50 to 40 Ma) grains, and were sourced from streams that flowed southward across the Snake River Plain, though today these streams flow westward into the Snake River. 2. In the subsurface of the Idaho National Engineering and Environmental Laboratory (INEEL), modern drainage patterns were established about 1.8 Ma ago, when basaltic volcanism blocked off southwestward drainage of Medicine Lodge Creek, which contains Grenville (1.2 to 1.0 Ga), Idaho batholith, and Miocene to Recent (16 to 0.6 Ma) Yellowstone-Snake River Plain rhyolite grains. Pleistocene to Recent drainage has been controlled by the Big Lost River, which has a distinctive detrital zircon signature of abundant Challis volcanic grains, lesser 1.8 to 1.4 Ga grains recycled through the 1.47 to 1.4 Ga Belt Supergroup, and enigmatic 750 to 600 Ma grains with no known source 3. In surface trenches on the INEEL, Pleistocene and Holocene fluvial sands contain the Big Lost River signature, whereas Holocene eolian loess contains a mixed provenance, with significant contribution from the Medicine Lodge Creek system to the northeast. This suggests that during loess deposition, fine sand and silt were transported to the southwest, opposite to Modern wind patterns. 4. The distinctive zircon population derived from erosion of Belt Supergroup sediments can be detected in Cretaceous to Recent stream deposits, indicating that it persists through 3 or more episodes of recycling. This suggests that major orogenic periods (as at 1.7 Ga, and the 1.1 Ga Grenville orogeny) generate zircons that remain as a recognizable population, through multiple episodes of recycling, for 2 billion years. The detrital zircon method is thus surprisingly robust and repeatable as a provenance indicator in 2nd and 3rd order drainage basins like those of southern Idaho. These distinctive populations are eventually mixed into a less specific continental signature.