Deglaciation of the Central Lake Superior Basin Imaged by High-Resolution Seismic-Reflection Profiles

Abstract:

The Lake Superior basin experienced multiple episodes of glaciation, resulting in a variety of glacial deposits and landforms, most of which date to the final retreat of Laurentide ice from the basin. Prominent among these features are moraines and thick glacial lacustrine varve sequences in the central part of the lake. Because these features are now beneath deep water, they can be well imaged by modern marine seismic-reflection methods, providing a variety of insights into glacial processes and history. Two prominent moraines occur east of Isle Royale, and other morainal deposits exist. The prominent moraines are as much as 75 m high, asymmetric, and locally concave down-ice in plan view. They are steeper up-ice than down, but vary in morphology along strike. Air-gun seismic-reflection data show that the moraines are underlain by thick, acoustically massive deposits (till) over a smooth bedrock surface, and that, in front of the moraines, the till grades laterally into increasingly stratified deposits interpreted as glacial lacustrine outwash. Such lateral relations between till and outwash are rarely displayed so well in natural exposures. The moraines relate to the Marquette advance of the Laurentide ice sheet, but they are difficult to directly correlate with the terrestrial deposits used to define that advance. Overlying the till and moraines is a thick sequence of glacial lacustrine varves, which are well imaged by high-resolution CHIRP seismic-reflection profiles. Although the CHIRP data cannot resolve even the thickest of the individual varves, the section comprises distinct acoustic packages. The CHIRP data show that the base of the varve sequence becomes younger to the northeast, the direction of ice retreat. Throughout the varved sequence are lenses of acoustically massive material and local features interpreted as iceberg plow marks, which are especially concentrated at one horizon. Limited 3-D seismic data show the curvilinear plan view of the plough marks at this horizon, and the horizon can be mapped throughout the central part of the basin. These features indicate that ice retreat from the basin was not instantaneous or simple in detail.