A revised hydrostratigraphic framework model of Halton Till in the Greater Toronto area, Ontario

Abstract

Halton Till, mapped south of Oak Ridges Moraine from Niagara Escarpment eastward to Pontypool, has traditionally been inferred to represent a climatically induced readvance of late-glacial ice from Lake Ontario basin that deposited sandy drumlinized till. Basin analysis techniques, seismic profiling, outcrop and core logging, and downhole geophysics, are integrated with detailed mapping to revamp the sedimentary and stratigraphic framework for Halton Till. Integration of glaciological process permits a reinterpretation of Halton Till depositional models. Field and core data reveal Halton Till as a muddy, clast-poor diamicton with interbeds of graded sediment. It is transitional upward from Oak Ridges Moraine sand and gravel to units of graded sand, silt, and clay rhythmites, with interbeds of sand, gravel, and muddy diamicton. It may abruptly overlie stony, sandy Newmarket Till and cross-sections show that Halton Till sediment fills basin lows and thins across Oak Ridges Moraine sediment lobes. Halton Till sedimentary architecture and facies are reinterpreted within an oscillating, ice-marginal, glaciolacustrine depositional model. The transitional lower contact with Oak Ridges Moraine sediment and upward-fining into Halton Till sediment records waning flow and deposition within lower-energy, ice-marginal, glaciolacustrine environments. Glaciolacustrine sedimentation is indicated by diminishing (mud-rich) grain sizes and gradational sediment style. Rhythmically laminated mud, diamicton interbeds, and intraclasts indicate ponded, ice-marginal debris flow and semibuoyant ice loading. Paleoflow data indicate westward transport of fine sand and mud from an eastern channellized, high-energy, sandy Oak Ridges Moraine depositional setting. The Halton Till depositional model may be applicable to other muddy diamicton units in the Great Lakes basin.