A Sedimentological Investigation of the Sunnybrook Diamict , Scarborough Bluffs Region, Ontario, Canada

Abstract

<p>Exposures of Quaternary-aged sediments along the Scarborough Bluffs, near Toronto, provide considerable information concerning environmental conditions which existed during the last interglacial-glacial cycle along the southern margin of the Laurentide Ice Sheet. The present exposures dissect a thick interglacial deltaic body (Scarborough Formation), overlain by a set of interfingered fine-grained diamicts (Sunnybrook, Seminary, Meadowcliffe, Halton) and interstadial sands (Thomcliffe Formation). The depositional origin of the diamicts have been vigorously debated in the literature, particularly that of the Sunnybrook.</p> <p>Two facies associations for the Sunnybrook are introduced in this study, Channel (CFA) and High Facies Association (HFA), based upon: facies and contact descriptions, and contact geometries. The CFA consists of a thick package (10-18m) of massive and stratified diamicts, conformably overlain by a succession (11-25m thick) of finely and coarsely bedded turbidites, and infills topographic lows within the underlying strata. Silt clast breccia, flow noses, debris flow deposits, and turbidites, within the CFA, are proof of a subaqueous resedimentation origin. The HFA is located on the relative highs, away from channels, forming horizontally-planar contacts with the underlying Scarborough Formation. The HFA consists of an association of: massive and stratified diamicts, and deformed laminated silts and clays. In addition, the HFA has several unique characteristics which include: basal clast horizons, an interbedded contact zone (ICZ), and reverse density loading along the upper contact of the Sunnybrook with the overlying Lower Thorncliffe Formation.</p> <p>The proposed depositional model for the Sunnybrook's two facies associations suggests combined rain-out from suspended sediment plumes and from ablating ice masses, and resedimentation in a glaciolacustrine setting. As well, a model of formation for basal clast horizons in the HFA was developed in this study. A subaqueous interpretation of the Sunnybrook implies the existence of a high level lake, in the Metro Toronto Region, for the duration of the early and middle Wisconsin period. Previous interpretations, which suggest subglacial conditions during deposition of the Sunnybrook and related diamicts, are not substantiated by the sedimentological and glaciotectonic data of the present study.</p>