ICE ADVANCES IN THE WESTERN LAKE SUPERIOR REGION: A REEVALUATION OF THE ST. LOUIS SUBLOBE AND THE MARQUETTE PHASE OF THE SUPERIOR LOBE

Recent investigations in northeastern Minnesota shed new light on long-standing glacial problems. The advance of the St. Louis Sublobe into eastern Minnesota occurred much earlier than previously suggested, and the limits of the Marquette phase of the Superior lobe have been well defined. Most regional chronologies place the Alborn Phase of the St. Louis Sublobe after the Vermilion Phase around 12 kyr BP. However, the presence of Keewatin till at the crest of the Giant's Range but not to the north, combined with evidence for abundant stagnant Rainy Lobe ice south of the Giant's Range, suggests the St. Louis Sublobe advanced while active Rainy Lobe ice was present immediately north of the Giant's Range. A newly identified Rainy Lobe ice margin confirms this relationship; this margin is probably correlative with the previously described Allen moraine and has been named Northofnashwauk Phase. The St. Louis Sublobe then stagnated and the next prominent recessional moraine, the Rabideau Moraine, is correlative with the post-Northofnashwauk Big Rice Moraine of the Rainy lobe. The Rainy Lobe and St. Louis Sublobe retreated from the Big Rice and Rabideau moraines to the Vermilion and Big Stone moraines, respectively.

A long-standing problem in Minnesota glacial geology has been identifying the limit of the Marquette Phase of the Superior lobe. The Marquette Phase is well defined in Michigan's Upper Peninsula by the Grand Marais Moraines, and has been dated at ca. 9.9 kyr BP. Sediment and landforms interpreted to be Marquette Phase have also been described and dated in northwestern Wisconsin. Recently, ice-marginal landforms and till of the Marquette Phase have been identified in eastern Minnesota. A large underflow fan within the Glacial Lake Duluth basin is associated with drainage from glacial lakes Koochiching, Aitkin II, and Upham II. Deposition of the fan post-dates the Nickerson Phase of the Superior Lobe, and the fan is overlain by bouldery clay till of the Marquette Phase. The till is, in turn, overlain by a veneer of lacustrine sand associated with a final phase of Lake Duluth. The ice margin of the Marquette phase is delineated by a narrow belt of hummocky topography along the inner margin of the Nickerson and Thomson Moraines.