INSIGHTS ON HIGH-LEVEL GRAVELS OF THE GLENNS FERRY FORMATION IN THE WESTERN SNAKE RIVER PLAIN OF WESTERN IDAHO

New mapping in the western Snake River Plain of western Idaho shows that high-level gravels associated with the Glenns Ferry Formation may represent the initial draining of Ancient Lake Idaho. These Glenns Ferry gravels are a significant source of aggregate materials for the region and in places require a substantial removal of overburden. Gravel terraces in the region associated with modern river systems in the area (Snake River, Boise River, Payette River) have been well studied and mapped; however, the well buried Glenns Ferry gravels don’t appear to easily align with older modern terraces. The Tenmile Gravel is the oldest identified gravel associated with the modern river systems in the Boise Valley and it appears to lie atop the Glenns Ferry Formation. Aggregate pits, water-well logs, and rare outcrops show these gravels appear anywhere from the surface to ~24 m (80 ft) in depth and vary in thickness from less than a meter (3 ft) with poor lateral continuity to 12 m (40 ft) with wide lateral distribution. Pebble and cobble clast compositions include porphyritic felsic clasts, Idaho batholith granite and granodiorites, quartzites, Miocene fine grained volcanic rocks and rare vesiculate volcanic rocks. In several places the gravel appears to be coated with a black manganese stain. The Glenns Ferry gravels are covered by up to 2 m (6 ft) of Bonneville slack water deposits, which are underlain by 6 to 18 m (20 to 60 ft) of massive silt, clay, fine sands, and thin beds of gravel, and up to 3 m (10 ft) of fluvial sand. The thick middle zone of fine massive material above the gravels may correlate to the Quaternary Froman’s Ferry section of the Glenns Ferry Formation located 30 km (15 mi) south. By analyzing numerous water-well logs as well as active and abandoned aggregate pits in the area it is possible to spatially delineate the maximum extent of these economically significant gravels. Further work is necessary to try to constrain the age of the deposits.