ABSTRACT: Anatomy of North America's Largest Quaternary Lake

TELLER, JAMES T., University of Manitoba, Winnipeg, Manitoba

Lake Agassiz was the largest lake in North America at the end of the last glaciation, as runoff and meltwater from a 2 million sq. kilometers area of central Canada and the northern United States were impounded by glacial ice. Although time transgressive toward the retreating ice barrier and regressive in the south because of crustal rebound and lowering outlet levels, its deposits cover 950,000 sq. kilometers and include large (>6000 sq. kilometers) and small sandy to gravelly fan deltas, multiple beach and lagoon facies, and low-organic (<4%, largely humic) offshore muds that contain irregular fluvial sand bodies, soil desiccation zones, and ice-rafted dropstones and till masses. Much of the offshore silty clay is poorly laminated or massive.

In the <4000 years of the lake's history, up to 75 m of coarse-grained fan delta sediments (commonly capped by eolian dunes) and 50 m of offshore muds were deposited; however, there are large areas in the center of the basin where no lacustrine sediment was deposited. The sedimentary sequence has all been isostatically deformed, and beaches in the north lie 170 m higher than those formed contemporaneously 600 km to the south. Isotopic, paleontological, and sedimentological data provide evidence for rapid fluctuations in the level of Lake Agassiz; this resulted from the opening and closing of lake outlets by the fluctuating glacier margin and by outlet erosion. Baseline outflow from the lake, which occurred south through the Mississippi River and east through the Great Lakes and St. Lawrence valley, typically ranged from 14,000 to 50,000 cubic meters s-1 but was punctuated by catastrophic flood bursts that were up to 200,000 cubic meters s-1 (>10 times the flow of the modern Mississippi).

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)