HR: 1330h
AN: PP32B-0288    [PDF]
TI: Sedimentary Indicators of Significant Late Pleistocene - Early Holocene Lake-Level Fluctuation: Preliminary Results From Flathead Lake, Montana
AU: * Hofmann, M H
EM: michael.hofmann@umontana.edu
AF: University of Montana, Department of Geology, Missoula, MT 59812 United States
AU: Hendrix, M S
EM: marc@selway.umt.edu
AF: University of Montana, Department of Geology, Missoula, MT 59812 United States
AU: Moore, J N
EM: gl_jnm@selway.umt.edu
AF: University of Montana, Department of Geology, Missoula, MT 59812 United States
AU: Sperazza, M
EM: sperazza@mso.umt.edu
AF: University of Montana, Department of Geology, Missoula, MT 59812 United States
AU: Shapley, M
EM: shap0029@umn.edu
AF: University of Minnesota, Limnological Research Center, Minneapolis, MN 55455 United States
AU: Wittkop, C
EM: wittk004@umn.edu
AF: University of Minnesota, Limnological Research Center, Minneapolis, MN 55455 United States
AU: Stone, J
EM: jstone@unlserve.unl.edu
AF: University of Nebraska, Department of Geosciences, Lincoln, NE 68588 United States
AB: Flathead Lake is a large (550km$^{2}$) open lake system, locally $>$100m deep, occupying an active half-graben in NW-Montana. During the summer of 2003, we recovered eleven piston cores from the central and southern portions of the lake. Core locations were selected on the basis of available sub-bottom 3.5 kHz seismic reflection data and designed to yield the most information about Pleistocene and Holocene lake-level history. Particularly elucidating are seismic data and three cores (FL-03-14K, FL-03-15K, FL-03-16K) obtained along a north-south transect within Big Arm Bay in the western part of the lake in water depths of 17m. Seismic data along this transect indicate the presence of a fundamental unconformity that likely relates to a significant fluctuation in lake level. Sub-unconformity reflectors in the shallowest part of the transect are clearly truncated. The unconformity itself is onlapped by overlying reflectors. Down depositional dip, the degree of sub-unconformity erosion and post-unconformity onlap diminishes to a point at which the surface appears to be conformable. Core FL-03-14K recovered from the furthest updip position along the line of transect, contains a condensed section of gray-brown fine sand and silt in the uppermost part of the core overlying tan Pleistocene(?) mud ($\sim$5m). Core FL-03-15K, recovered midway along the line of transect, consists mostly of dark brown-gray mud (0cm-613.5cm) with a tan clay and silt section towards the bottom (613.5cm-777cm). Core FL-03-16K, recovered furthest down-dip, contains very gassy dark gray-brown mud (0cm-740cm) with a very well pronounced, $\sim$1 cm thick ash layer (Glacier Peak ash, 13,375 cal.y.b.p.) at a depth of 650cm below the sediment-water interface. We interpret the transition from gray-brown mud to tan silt and clay to represent the unconformity imaged on our seismic data and to record the change from a glacially influenced lake system to the post-glacial open lake system present today. This unconformity, along with the condensed section in core FL-03-14K, provides information about an early post-glacial lake-level lowstand, during which the surface of the lake was about 15m below the modern lake level. Additionally we recovered cores from the center of Flathead Lake (cores FL-03-19 and FL-03-26K) and the east side of the lake (core FL-03-22K) respectively. Core FL-03-26K was collected in water depths of about 50m and provides an undisturbed record of Pleistocene and Holocene Lake sedimentation. A color change from brown-gray mud to tan clay and silt at a depth of 570cm below the sediment-water interface records the transition from glacial to interglacial conditions. Well-displayed rhythmic sedimentation containing centimeter-thick laminae with darker, coarser grained (very fine sand) layers and lighter colored silt and clay layers occupy the lower 460cm of this core (570cm-1030cm). A similar record has been recovered in core FL-03-22K in the deepest part of the lake (93m). The uppermost 150cm of this core consists of heavily degassing gray-brown mud with sulfide rich dark (black) layers. At a depth of 635cm below the sediment-water interface, we observed another sharp contact between the interglacial brown-gray mud and the tan glacial clay and silt sedimentation. The lower part (635cm-868cm) of core FL-03-22K contained nicely developed rhythmites, similar to those observed in core FL-03-26K.
DE: 1620 Climate dynamics (3309)
DE: 1827 Glaciology (1863)
DE: 1845 Limnology
DE: 9350 North America
DE: 9604 Cenozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting