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

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.