Glacial Isostatic Adjustment of the Laurentian Great Lakes Basin: Using the Empirical Record of Strandline Deformation for Reconstruction of Early Holocene Paleo-Lakes and Discovery of a Hydrologically Closed Phase*

Abstract

In the Great Lakes region, the vertical motion of crustal rebound since the last glaciation has decelerated with time, and is described by exponential decay constrained by observed warping of strandlines of former lakes. A composite isostatic response surface relative to an area southwest of Lake Michigan beyond the limit of the last glacial maximum was prepared for the complete Great Lakes watershed at 10.6 ka BP (12.6 cal ka BP). Uplift of sites computed using values from the response surface facilitated the transformation of a digital elevation model of the present Great Lakes basins to represent the paleogeography of the watershed at selected times. Similarly, the original elevations of radiocarbon-dated geomorphic and stratigraphic indicators of former lake levels were reconstructed and plotted against age to define lake level history. A comparison with the independently computed basin outlet paleo-elevations reveals a phase of severely reduced water levels and hydrologically-closed lakes below overflow outlets between 7.9 and 7.0 ka BP (8.7 and 7.8 cal ka BP) in the Huron-Michigan basin. Severe evaporative draw-down is postulated to result from the early Holocene dry climate when inflows of meltwater from the upstream Agassiz basin began to bypass the upper Great Lakes basin.Dans la région des Grands Lacs, le soulèvement isostatique lié à la dernière glaciation a ralenti selon une courbe de décroissance exponentielle établie à partir du gauchissement observé dans les anciennes lignes de rivage lacustres. Une surface de référence composite de la réponse isostatique datant de 10,6 ka BP (12,6 cal ka BP) a été préparée pour l’ensemble du bassin-versant des Grands Lacs, par rapport à une région au sud-ouest du lac Michigan située au-delà de la limite du dernier maximum glaciaire. Le calcul du soulèvement des sites en fonction des altitudes de la surface de référence a facilité la conversion d’un modèle altimétrique de terrain du bassin actuel des Grands Lacs en des cartes paléo-géographiques pour différents âges choisis. De plus, afin de définir l’évolution des niveaux lacustres, les altitudes initiales des indicateurs géomorphologiques et stratigraphiques des paléo-niveaux lacustres, datés au 14C, ont été reconstituées puis reportées en fonction de l’âge. La comparaison de cette courbe à celles des paléo-altitudes des exutoires lacustres, calculée indépendamment, révèle, entre 7,9 et 7 ka BP (8,7 et 7,8 cal ka BP), une phase d’abaissement majeur des niveaux lacustres au-dessous de ceux des exutoires et la fermeture hydrologique des lacs dans le bassin des lacs Huron-Michigan. La forte évaporation nécessaire à l’abaissement du niveau d’eau est attribuée à un climat sec peu après le début de l’Holocène, dans un contexte de détournement progressif hors des Grands Lacs des eaux de fonte du lac Agassiz

Stratigraphic analysis of Late Wisconsin and Holocene glaciolacustrine deposits exposed along the Nottawasaga River, southern Ontario, Canada

“…This erosional phase likely records a drop in regional base level during the Kirkfield low water phase, following the ice retreat beyond an isostatically depressed outlet near Fenelon Falls (Figs. 1, 3c, 14d, 15;Finamore 1985;Lewis et al 2005). Water levels are previously interpreted to have fallen by as much as 10-30 m during this time ( Fig.…”

“…3c). Isostatic rebound of this lake outlet allowed water levels to rise towards southern outlets and form main Lake Algonquin (11.4-10.5 14 C ka BP (13 300-12 400 cal year BP); Lewis et al 2005;Fig. 3c).…”

“…Ice recession beyond isostatically depressed outlets in the northern parts of Algonquin Park and near North Bay (Fig. 1) after 10.5 14 C ka BP (11 700-12 800 cal year BP) resulted in drainage of glacial Lake Algonquin from the study area and led to the establishment of the Stanley Low Phase which was punctuated by multiple short-lived high stands (Lewis et al 2005;Lewis 2016). Finally, uplift of the North Bay outlet following deglaciation triggered water level rise in Georgian Bay; water levels south and west of the outlet rose towards southern outlets, up to 15 m above modern Lake Huron (192 m asl) within the study area, during the establishment of the Nipissing phase of the upper Great Lakes (6-4 14 C ka BP (6900-4400 cal year BP); Eschman and Karrow 1985).…”