RAINY LOBE RETREAT AND KOOCHICHING LOBE ADVANCE DOCUMENTED BY GLACIAL VARVES FROM PELICAN AND VERMILION LAKES (MINNESOTA)

Glacial retreat rates are being determined by analyzing varve thickness records from sediment cores in northern Minnesota and northwestern Ontario. Here we report on varves from Pelican and Vermilion Lakes, located just north of the Vermilion moraine of the Rainy Lobe. The varves were deposited in Glacial Lake Norwood. Lake Norwood dropped in elevation through time, presumably by a combination of sill incision of a southern outlet (the Embarrass Gap) and differential post-glacial rebound. Lake Norwood also coalesced with Glacial Lake Agassiz when the western boundary of the lake became ice free after retreat of the Koochiching Lobe.

There are 626 glacial varves in the Pelican Lake cores. The oldest, beginning at varve year 1, are thick with sandy summer layers deposited when the glacier was initially close to the core site. Up section the varves get thinner and finer-grained, due to the retreat of the Rainy Lobe. By varve year 277, the varves are very thin (<5 mm) and have occasional laminae of reddish-gray clay, which could be sourced from the Lake Superior basin. At around varve year 417 the varves increase in thickness for around 100 years before thinning again. These thicker varves include ice-rafted pebbles of limestone, presumably sourced from the Koochiching Lobe.

The Vermilion Lake cores have 331 varves and their thickness patterns closely match those from Pelican Lake, which is 50-km to the northwest. The Lake Vermilion record begins 86 years (varves) before the Pelican Lake record, however, the Pelican Lake core site is 3.7-km farther from the Vermilion moraine. This suggests the Rainy Lobe retreated from the Vermilion moraine at a rate of ~43 m/yr.

We are attempting to correlate varve thickness records from rotosonic drill cores southwest of Pelican Lake that preserve varved clays overridden by a Koochiching Lobe advance, which we suspect resulted in the ~140 thin varves in Pelican Lake during a period of cooler climate. The thicker varves, with ice-rafted limestone pebbles, are explained by the subsequent retreat of the Koochiching Lobe. Scanning XRF elemental analyses and total inorganic carbonate data are expected to document a shifting provenance from dominantly Rainy Lobe (Canadian Shield) to Koochiching Lobe (Paleozoic carbonates and shale).