LATE PLEISTOCENE STRATIGRAPHY AND IMPLICATIONS FOR DEGLACIATION AND SUBGLACIAL PROCESSES IN THE FLATHEAD VALLEY, NW MONTANA

The Flathead lobe of the Cordilleran ice sheet retreated across the Flathead Valley, a 90 km-long by 15–25 km-wide basin, at the last glacial maximum. I used geologic mapping, sedimentologic study, and mapping from 3300 descriptive water-well logs to define the stratigraphy of glacial and post-glacial deposits in the Flathead Valley and to infer subglacial processes and glacier retreat rates. Units in the 30–200-m-thick stratigraphic sequence include basal sand and gravel (proglacial alluvium) overlain by diamicton (till), locally gravelly silt and clay (coarse-grained subaqueous fans at recessional moraines and glaciolacustrine deposits), and capping sand and gravel (outwash and Holocene stream deposits).

Shorelines at altitudes of 940–958 m show that as the glacier receded a proglacial lake expanded in that range of altitudes. This ancestral Flathead Lake was impounded by a terminal moraine and an underlying bedrock dam; the bedrock was downcut 60–78 m since the late Pleistocene. Glaciolacustrine deposits are as much as 180 m thick in discontinuous troughs that locally cut through till to alluvium or bedrock. The troughs are mostly 1–5 km wide, 6–25 km long, sinuous in plan view, and undulatory in longitudinal profile. The troughs extend below the elevation of Flathead Lake and have been below base level since glaciation. These characteristics point to incision by meltwater hydrostatically pressurized beneath the Flathead lobe in subglacial tunnel channels. Sand and gravel at the trough bases and part of the glaciolacustrine fill in the troughs are subglacial deposits. Tunnel channels that terminate at recessional moraines suggest excavation occurred sporadically during deglaciation.

The distribution of glaciolacustrine silt indicates that the Flathead lobe retreated, and the ancestral Flathead Lake expanded, by at least 87 km before the lake-outlet stream could downcut the bedrock dam by <5–30 m. I calculated a downcutting rate of at most 1 m 100 yr-1 assuming an initial recession date and the date of Glacier Peak tephra deposited on desiccated silt at 920 m (15,000 and 11,200 14C yr BP, respectively). The downcutting rate suggests minimum glacier retreat rates of 30–150 m yr-1 for the Flathead lobe across the ancestral Flathead Lake from the last glacial maximal extent. A maximum estimate of retreat is >500 m yr-1.