CONTROLS ON THE PLEISTOCENE EVOLUTION OF THE UPPER SNAKE RIVER BETWEEN ST ANTHONY AND BLACKFOOT, IDAHO

Geologic mapping by the Idaho Geological Survey has clarified tectonic, volcanic, and climatic controls on the Pleistocene evolution of the upper Snake River. Since 2005, fifteen 7.5-minute quadrangles have been mapped and released on the IGS website at www.idahogeology.org. The mapping has used optically stimulated luminescence to date terrace deposits and loess, geochemical and paleomagnetic analyses to correlate volcanic rocks, and hundreds of domestic water well logs to trace units in the shallow subsurface. Tectonic control is most evident at the junction of the South Fork and Henrys Fork where the Grand Valley fault zone enters the Snake River Plain and splays into multiple northeast-stepping normal faults. Structural accommodation space and subsidence in this area created a major Pleistocene depocenter. The depocenter contains over 210 m of unconsolidated, saturated sediments interbedded with basalt lava flows. Basaltic magma intruded into the saturated sediments, forming the tuff cone and tuff ring complexes of Menan Buttes. Shield volcanoes form a topographic high west of the Snake River, pinning the course of the river between the basalts and uplands underlain by late Miocene-early Pleistocene rhyolites and basalts. Several basalt flows temporarily dammed the Snake River, forming reaches underlain by shallow basalt and knick zones at Idaho Falls and St Anthony. Fine-grained deposits associated with lava dams influence the region of high water tables at the confluence of the South Fork and Henrys Fork. Deposition of the Huckleberry Ridge Tuff at 2.1 Ma disrupted the drainage of the Teton River. Subsequent incision driven by subsidence of the Snake River Plain resulted in bedrock canyons that largely confine the Teton River. Climatic controls include intermittent glaciation of the Snake River headwaters. Gravel-dominated outwash was deposited in the late Pleistocene within a wide braid plain. A huge gravel fan was deposited at the confluence of the South Fork and Henrys Fork. Large floods streamlined basalt bedrock at Idaho Falls. Loess derived from glacial meltwaters was also deposited throughout the region. With cessation of glaciation, the Snake River metamorphosed into a dominantly single-thread meandering stream incised into the late Pleistocene outwash.