Late Pleistocene and Holocene depositional history of sediments in Flathead Lake, Montana: Evidence from high-resolution seismic reflection interpretation

Abstract

This paper presents an interpretation of 3.5 kHz reflection seismic data together with information from lower frequency seismic data and sediment cores from Flathead Lake in northwestern Montana. Flathead Lake is located at the margin of the Cordilleran Ice Sheet during the last glacial maximum and is the repository for about 160 m of syn- and post-glacial sediment. Six seismic stratigraphic units (A–F) are identified in Flathead Lake above the acoustic basement reflector. The deepest unit is interpreted to be late Pleistocene glacial till (unit A) overlain by well-layered glaciolacustrine sediments (unit B). Overlying seismic reflectors (unit C) include slump deposits, turbidites fed by meltwater surges associated with retreat of the Flathead glacial lobe, and homogeneous silty mud. Preliminary age models suggest that the transition from glacial rhythmites (unit B) to more homogenous lake sediments (unit C) occurred approximately between 14,500 and 13,180 cal.y.b.p. Parallel high amplitude reflectors of the oldest Holocene unit (unit D) drape Pleistocene stratigraphy. Sediment core analyses suggest that these fine sediments were deposited below wave base by suspension settle out in a lake similar to today's. The Mt. Mazama tephra (7630 ± 80 cal.y.b.p.) is interpreted as the top reflector of unit D. Onlap geometries between units D and E imply a significant drop in lake level before deposition of unit E. Sedimentation of unit E was confined to several sub-basins that were separated by bedrock highs. Lake level rose gradually during deposition of unit E to form the well-developed basal onlap geometry. Parallel high amplitude reflectors of the youngest Holocene sediments (unit F) drape all of the older stratigraphy and bedrock highs across the lake basin, suggesting a late Holocene lake highstand.