History of sedimentation in the northwestern Lake Superior basin and its relation to Lake Agassiz overflow

Abstract

Following the Marquette glacial advance, which blocked the eastern outlets of Lake Agassiz and reached northern Michigan about 10 000 years BP, the ice margin wasted back toward the northeast, eventually allowing Lake Agassiz to overflow into the Lake Superior basin through a series of channels. Sediments deposited east of Thunder Bay near the mouth of the Wolf, Wolfpup, Shillabeer, and Black Sturgeon channels reflect three phases in the history of Lake Superior and provide the basis for reconstructing the early postglacial history of the region.The lower part of the sedimentary sequence in the northwestern Superior Basin consists of a distinctive red, stoney, sandy till deposited during the Marquette glacial advance and is overlain by pink rhythmites deposited in Lake Superior when it was a deep proglacial lake at the Minong level. The nearly 300 rhythmites deposited at this time typically consist of 4 cm thick silt + clay couplets, which are punctuated by silt laminae and sandy turbidites that probably represent major thaw periods or storms. These are seasonal rhythmites, deposited prior to the reopening of the Lake Agassiz outlets into the Superior Basin, and they display a decrease in dropstones, grain size, and thickness upsection that reflects a receding ice margin.The first eastern outlets of Lake Agassiz were uncovered around 9500 years BP, and water began overflowing into the Superior Basin in a series of catastrophic floods. Subaqueous fans developed at the Wolf, Wolfpup, and Shillabeer confluence and at the mouth of the Black Sturgeon channel. Large sandy turbidites, 45–65 cm thick, were deposited in the proximal part of these fans, with scouring and large (1 m) trough cross-beds resulting from the largest Lake Agassiz floods. These sediments are transitional to distal, clayey silt rhythmites, 10–22 cm thick. A gradual decrease in flooding from Lake Agassiz is reflected in the upward decrease in rhythmite thickness to 1–3 cm by about 8200 years BP. The final sequence of sediments shows a transition to sandy units as water levels dropped in the Superior Basin and the influence of nearshore processes increased.