Lake Agassiz Superflood


Late Quaternary deposits in the Moose River Basin (James Bay Lowlands) area of northern Ontario consist of a succession of glacial tills with interceding interglacial deposits, which culminate in the sequence shown above, corresponding to the last glacial cycle (Skinner, 1973). Two distinct tills (Adam Till, Kipling Till) were deposited after the last interglacial (Missinaibi Fm.) and are separated by a relatively thin gravel/sand lag of fluvial origin, indicating a period of melt-back that reached to just south of the Hudson Bay either during the Younger Dryas or during Marine Isotope Stage 3 (Friday Creek Sediments). Overlying the final till is a very interesting ‘deglacial succession’ (shown below) that records the demise of a vast pro-glacial lake (roughly the size of England) that had pooled against the dwindling Laurentide ice-sheet.

AC-02_LCS2It is generally thought that the sudden drainage of the vast pro-glacial Lake Agassiz significantly altered the ocean circulation in the North Atlantic and its associated heat transport, thus causing an abrupt and intense regional cooling called the ‘8.2 kyr event’. This event, which is most clearly recorded in temperature reconstructions from the Greenland ice-cores, appears to have had impacts as far away as the monsoon region of Asia.

Despite the ‘8.2 kyr event’ receiving a great deal of attention in recent years, relatively little direct field-evidence has emerged of the timing and hydraulics of the flood event that accompanied the demise of the Laurentide ice-sheet, and that is presumed to have triggered this major climate event.

New work (funded by the Royal Society and aided by the Geological Survey of Canada) is providing new sedimentological and AMS radiocarbon evidence from uplifted flood deposits in the James Bay Lowlands. The flood deposits overlie a set of remnant Lake Agassiz blue-grey rhythmites (which in turn overlie late-glacial Kipling Till), and are overlain in turn by grey marine clay and intercalated silt layers that represent the off-lap deposits of the Tyrell Sea.

The flood deposits themselves consist of crudely stratified clay- and silt pebble ‘breccias’, with variable inclusion of dispersed sand- to cobble sized lithic clasts (drop-stones). The flood sequence culminates in a striking couplet of blue- and (overlying) red- ‘pebble breccias’ (shown below).


Reb clay pebble breccia overlying blue clay pebble breccia, at the top of the final Lake Agassiz flood sequence.

The main focus of our current work is on reconstructions of the hydraulic and sedimentary conditions that led to the deposition of the ‘flood deposits’ (especially the clay breccias), as well as the occurrence of sudden changes in the provenance of siliciclastics incorporated into these deposits.


R.G. Skinner, A late glacial short-lived saline underflow in southern James Bay, Canada, Northeastern Section 7th Annual Meeting 4, The Geological Society of America, Buffalo, New York, 1972, p. 45.