The Cabot Head Archipelago: evidence of glacial Lake Algonquin on the Northern Bruce Peninsula, Ontario

Abstract

The occurrence of abandoned shoreline bedrock erosional features at the edge of the Niagara Escarpment at Cabot Head indicates the existence of a group of islands in glacial Lake Algonquin during early postglacial time, referred to herein as the Cabot Head Archipelago. The abandoned shoreline features are situated as much as 80 m above the level of contemporary Georgian Bay. The range of abandoned shoreline bedrock erosional features, including shoreline cliffs, adjacent wave-cut platforms, wave-cut notches, shore stacks, shore caves, and other features, are described. The occurrence of these features is thought to be the result of the interaction between wave action in glacial Lake Algonquin and two distinct lithological facies representing the Wiarton -Colpoy Bay and Lions Head members of the Amabel Formation. The exceptional development of the abandoned shoreline bedrock erosional features in massive reefal dolostone between elevations of ∼250 and 255 m above sea level (asl) is interpreted as representing the relatively long-lived and stable Main stage of glacial Lake Algonquin (∼11 000-10 200 years BP). Shoreline erosional forms at elevations between ∼240 and 250 m asl may be indicative of declining lake levels partially controlled by bedrock structural factors. The final abandonment of the glacial Lake Algonquin shoreline in this area occurred when the eastern outlets of the lake became ice-free and its level dropped rapidly some 10 200 years BP. The Cabot Head Archipelago and the associated suite of raised and abandoned shoreline bedrock erosional features represent a rare assemblage of landforms within the Great Lakes basin, and possibly within Canada.Résumé : La présence de lignes de plages abandonnées et les caractéristiques d'érosion du socle à la bordure de l'Escarpement du Niagara à Cabot Head indiquent l'existence, au début de l'ère postglaciaire, d'un groupe d'îles dans le lac glaciaire Algonquin; ce groupe sera appelé ici l'archipel de Cabot Head. Les caractéristiques des lignes de plages abandonnées sont situées à des élévations pouvant atteindre 80 m au-dessus du niveau contemporain de la baie Georgienne. L'étendue de lignes de plages abandonnées, les caractéristiques d'érosion du socle, incluant les falaises littorales, les plateformes adjacentes érodées par les vagues, les encoches, les piliers sur les plages, les grottes et d'autres caractéristiques sont décrites. La pré-sence de ces caractéristiques serait le résultat de l'interaction entre l'activité des vagues dans le lac glaciaire Algonquin et deux faciès lithologiques distincts représentant les membres Wiarton -Colpoy Bay et Lions Head de la Formation d'Amabel. Le développement exceptionnel des caractéristiques d'érosion des plages abandonnées dans la dolomie récifale entre les élévations d'environ 250 et 255 m au-dessus du niveau de la mer suggère qu'elles aient été formées durant l'étage Main, stable et de longue durée, du lac glaciaire Algonquin (environ 11 000 à 10 200 ans avant le présent). Les formes d'érosion littorale à des élévat...

Postglacial lake shoreline surveys and lacustrine paleobiotic records in northern Bruce and Grey counties, Ontario, Canada

“…The Algonquin cluster of "upper" shorelines is not separately distinguished as Deane (1950) was able to do in the Lake Simcoe area, but instead records the water level regression brought about by glacioisostatic crustal tilting during ice retreat and operation of the southern and eastern outlets. This history embraces as well the bedrock shoreline features recently described by Kor et al (2012) near Cabot Head, to the north of the study area.…”

“…The spectacular bedrock-shoreline erosional features at Cabot Head (Kor, 1994;Kor et al, 2012), at the northeast end of the Bruce Peninsula, are isolated at a considerable distance from the Lake Algonquin mainland. Their elevations appeared too low for common Algonquin shoreline projections and their elevations near Cape Croker were sought to aid interpretations of Algonquin paleogeography.…”