GEOLOGIC MAPPING IN THE CENTRAL MOUNT BENNETT HILLS: VOLCANISM ASSOCIATED WITH THE NORTH-CENTRAL BOUNDARY OF THE SNAKE RIVER PLAIN

Geologic mapping of rocks along the margins of the Snake River Plain can provide useful constraints on models for the development of the Snake River Plain. Mount Bennett Hills area is of considerable interest because its in an area where the northeast trending plume related Eastern Snake River Plain and the northwestern trending Western Snake River Plain converge. The area for this EDMAP project was Dempsey Meadows Quadrangle within the central Mount Bennett Hills, NW of Twin Falls, in the Mountain Home 1/100k sheet. The Mount Bennett Hills area is an east-west trending horst along the northern margin of the west central Snake River Plain. The area is approximately 100 x 20 km and bounded on the north by normal faults and the Camas Prairie Rift and to the south by the Snake River Plain. The area is comprised of thick sequences of Miocene and Pliocene silicic volcanic rocks, Pliocene and Pleistocene mafic volcanic rocks, and interbedded sedimentary deposits which generally dip and thicken toward the Snake River Plain to the south. This area represents the northern juncture of the eastern SRP trend with the western SRP graben. Understanding the volcanic history of this area, and its geochemical development, are central to our understanding of how the western and eastern parts of the SRP relate to one another, and to the effects of different lithosphere on the history of basaltic volcanism. Mapping within the central Mount Bennett Hills will complement our current studies of basaltic volcanism in the western SRP graben (near Mountain Home, Idaho) and along the Yellowstone plume track (SW of Twin Falls).