Defining benefit and hazard: Distribution of Upper and Lower Tertiary units on the northeast flank of the Missoula Valley Montana

Two Tertiary units on the northeast flank of the Missoula Valley are distinct from each other; the lower unit bears resemblance to the middle Eocene to late early Miocene Renova Formation, and the upper unit closely resembles the middle Miocene to Pliocene Sixmile Creek Formation. However, previous studies mapped the two units together or partially dififerentiated the units in an inconsistent manner. Exposures are limited, and strata are difficult to trace on the valley flank. But the units can be mapped separately. Outcrops of conglomerate containing mylonite clasts indicate the depositional environment of the lower Tertiary unit included a river flowing north from an ancestral Bitterroot Valley. Deposits near the southern end of the valley flank indicate the river was flanked by series of coalescing alluvial fans Though similar to the Renova Formation, the lower Tertiary unit is distinct because it contains exotic clasts deposited in a fluvial environment. I identified two lithologie facies of the upper Tertiary unit A conglomerate facies is in angular unconformable contact with the northeast-dipping lower Tertiary unit This is a fluvial deposit and may be ancestrally related to the Clark Fork River A boulderdominated conglomerate facies of the upper Tertiary unit, coeval with the fluvial deposit, was generated from a broad debris-flow/alluvial fan system which flowed primarily out of an ancestral Grant Creek. A lack of age evidence within the upper Tertiary unit makes its assignment to the Sixmile Creek Formation premature. The mylonite-bearing, non-lenticular, coarse fluvial deposits of the lower Tertiary unit can transmit significant quantities of water to wells. Based on bedding orientations taken fi-om outcrops, these beds can be targeted in the subsurface. Landslides on the valley flank result fi’om the interplay of the lower and upper Tertiary units. Interstitial water drains through the coarse upper Tertiary unit and travels along the contact between the upper and lower units. Near the topographic surface, where the water-logged contact intersects with poorly lithified, clayey, ash-rich beds of the lower Tertiary unit, landslide failure may occur if the topography along the contact is sufficiently steep. Overburden fi-om the upper Tertiary unit adds to the driving forces necessary for slippage. A reasonable estimate of landslide potential can be made based on proximity to the geologic contact between the upper and lower Tertiary units. Note: Plates 1 and 2 are available below