GEOLOGY OF THE HELENA VALLEY AQUIFER, SOUTHWESTERN MONTANA

The Helena Valley Aquifer, located in an intermontane basin in Southwest Montana, represents the primary drinking water source for over 20,000 residents in the area. The unconfined aquifer provides relatively high yields from high permeability, generally unconsolidated sediments. The Helena Valley is located within the Lewis & Clark fault zone, a major regional east-west strike-slip structure with significant left-lateral displacement. After emplacement of the Boulder Batholith and continental compression during the late Cretaceous, The Helena Valley (Helena-Townsend Valley Structural Basin) was formed by late Paleogene extension on existing faults, with displacement changing to right-lateral motion. Late Paleogene valley-fill deposits comprise thick sequences of volcaniclastic sediments, deposited into lakes in closed basins. These deposits include thick bentonite-rich sequences hundreds of feet thick, with thin coarser beds, similar to Renova Formation facies identified across southwestern Montana. During the Miocene, the original basin was partitioned into separate blocks to account for additional extension, resulting in development of a regional mid-Miocene unconformity. Blocks with granite generally maintain higher elevations, while blocks without granite show basin subsidence. Overlying deposits comprise coarse grained sand and gravel deposits of Six-Mile Creek Formation equivalent.

Water monitoring studies show geologic controls on local aquifer recharge and yield. A major boundary fault on the east side of the valley seals and limits ground water flow, with over 250 feet of ground water depletion from pumping from private potable wells immediately east of the major valley fault. The aquifer exhibits a strong vertical upward gradient in the central part of the valley, attributed to the structural control of the major bounding faults. A thick Oligocene bentonite sequence is used as a marker bed for regional stratigraphic correlations. Field examination of excavations identified the higher energy deposits of Six-Mile Creek equivalent facies in the southeastern part of the valley. Based on this assessment, the Helena Valley Aquifer is comprised of Six-Mile Creek aged deposits combined with younger and similar Pleistocene and Holocene deposits.