Late Pleistocene stratigraphy and implications for deglaciation and subglacial processes of the Flathead Lobe of the Cordilleran Ice Sheet, Flathead Valley, Montana, USA

Abstract

The Flathead lobe of the Cordilleran ice sheet advanced and retreated across the Flathead Valley, a 90-km-long by 15–25-km-wide basin. The glacier terminated at the southern end of the Flathead Valley during its 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 retreat rates. Units in the 30–200-m-thick stratigraphic sequence include deep sand and gravel (proglacial alluvium), diamicton (till), locally gravelly silt and clay (coarse-grained subaqueous deposits at recessional moraines and glaciolacustrine deposits), and surficial sand and gravel (outwash and Holocene stream deposits). A nearly continuous till overlies alluvium and is in turn directly overlain by glaciolacustrine deposits. Glaciolacustrine shoreline indicators at altitudes of 940–958 m show that the Flathead lobe receded as a proglacial lake expanded in that range of altitudes. This lake, the ancestral Flathead Lake, was impounded behind bedrock that 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 and bedrock. The troughs are mostly 1–5 km in width, 6–25 km in length, sinuous in plan view, and undulatory in longitudinal profile. The troughs extend below the elevation of Flathead Lake and have been below local 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 silts indicates that the Flathead lobe retreated, and the ancestral Flathead Lake expanded, by 87 km before the lake-outlet stream could downcut its dam by <5–30 m. Latest Pleistocene to Holocene dam downcutting rates of ∼1 m 100 year −1 are suggested by the distribution of Glacier Peak tephra over desiccated glaciolacustrine silts in the valley. These downcutting rates suggest minimum glacier retreat rates of 26–150 m year −1 for the Flathead lobe across the ancestral Flathead Lake from the last glacial maximal extent. The maximum estimate of retreat is >500 m year −1.