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.
| Original language | English |
|---|---|
| Pages (from-to) | 111-131 |
| Number of pages | 21 |
| Journal | Sedimentary Geology |
| Volume | 184 |
| Issue number | 1-2 |
| DOIs | |
| State | Published - Feb 1 2006 |
Funding
We are especially grateful to the Confederated Salish and Kootenai tribes for permission to conduct this study. We also would like to thank the Limnological Research Center at the University of Minnesota for providing core equipment and lab space for the analysis of our sediment cores. We are especially thankful to Mark Shapley and Chad Wittkop for their logistical assistance and guidance during the coring process. Comments by Nicolas Eyles and an anonymous reviewer on an earlier version of this paper significantly improved its quality and are greatly appreciated. Financial support for this work was provided by the Geological Society of America student grant program, the American Association of Petroleum Geologists grant-in-aide program, the German Academic Exchange Service (DAAD) and National Science Foundation (grant # ATM-0214273).
| Funders | Funder number |
|---|---|
| ATM-0214273 | |
| Geological Society of America | |
Keywords
- Cordilleran ice sheet
- Flathead Lake
- Glaciolacustrine sediment
- Lake level fluctuations
- Seismic stratigraphy