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North-Central Section (44th Annual) and South-Central Section (44th Annual) Joint Meeting (11–13 April 2010)

Paper No. 24
Presentation Time: 8:30 AM-12:00 PM

GEOCHEMICAL ANALYSIS OF A GLACIAL LAKE BENSON VARVE SEQUENCE, WEST-CENTRAL MINNESOTA


MOHR, Audrey R.1, JOHNSON, Grace C.2, COTTER, James F.P.3, MAHONEY, J. Brian1, SYVERSON, Kent M.4 and FERGUSON, Jill W.5, (1)Dept. of Geology, University of Wisconsin, Eau Claire, WI 54702, (2)Dept. of Geological Sciences, University of Minnesota, Duluth, MN 55812, (3)Geology Discipline, University of Minnesota, Morris, Morris, MN 56267, (4)Dept. of Geology, Univ. of Wisconsin-Eau Claire, 105 Garfield Ave, Eau Claire, WI 54701, (5)Materials Science Center, University of Wisconsin, Eau Claire, WI 54702, mohrar@uwec.edu

Glacial Lake Benson formed in western Minnesota as meltwater from the retreating Des Moines Lobe filled low areas of the Minnesota River valley during the Late Wisconsin Glaciation (Rittenour et al., 1998). The lake had a maximum depth of 19 m and covered ~3900 sq. km in west-central Minnesota. Evidence for Glacial Lake Benson includes relict deltas and exposures of offshore rhythmites. In this study, we analyzed the internal geochemistry of a 15-year varve sequence to investigate how geochemistry may change in a single location during a short time interval. In addition, we compared lake sediment geochemistry to the geochemistry of the surficial till in western Minnesota (Thorleifson et al., 2007, MN Geological Survey OFR-07-01).

Fifteen samples were obtained from an outcrop of consecutive varves near the center of former Glacial Lake Benson (21 km southwest of Benson on Cty Rd 11; UTM 287,900 mE, 5,003,100 mN, Zone 15). A bulk sample of one summer and winter layer was collected for each varve. Varve couplets were 7 to 13 cm thick. Sediment was dry sieved to obtain the silt and clay fraction for major and trace element analysis by x-ray fluorescence (XRF).

Bulk geochemistry within the varved succession is relatively homogeneous, although there is a distinct geochemical partitioning between the bottom and the top of the succession. In particular, large ion lithophile (LIL) elements such as Ba, La and Rb are depleted at the base of the section. Depletion of these mobile LIL elements may indicate selective removal of elements by hydrologic scavenging associated with groundwater movement. XRF results were also compared to regional till geochemistry in the Glacial Lake Benson area, as analyzed by the Minnesota Geological Survey using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Thorleifson et al., 2007). Lake Benson sediments are significantly enriched in Cu, Zn, Nb, Zr and Y relative to the till. This geochemical enrichment, particularly in nonmobile elements such as Zr, suggests that Lake Benson sediments might have been derived from a different source than the surficial Des Moines Lobe till.

Research for this study was funded by a grant to Cotter from the NSF-R.E.U. Program (NSF-EAR 0640575).

Session No. 18--Booth# 24

T33. Undergraduate Research (Posters)
Monday, 12 April 2010: 8:30 AM-12:00 PM
Taneycomo A (Branson Convention Center)
Geological Society of America Abstracts with Programs. Vol. 42, No. 2, p.62
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