Sedimentation in a large lake: A reinterpretation of the late Pleistocene stratigraphy at Scarborough Bluffs, Ontario, Canada

A lithofacies code developed for diamicts emphasizes facies variability in the classic upper Pleistocene glaciogenic sequence at Scarborough Bluffs, Ontario, Canada. Lithological breaks along the bluffs are traditionally assigned to incursions of grounded ice margins and to interstadial lakes formed during ice retreat, during the Early and Middle Wisconsin. Detailed sedimentological logging through three diamict units (previously formalized as Sunnybrook, Seminary, and Meadowcliffe Tills) and intervening sandy lithofacies shows absence of glaciotectonic structures and diamicts associated with grounded glacier ice, traction-current activity during diamict accumulation, postdepositional resedimentation of diamicts into topographic lows accompanied by turbidite activity, a subaqueous deltaic origin for intervening sandy lithofacies, and loaded, transitional, and interbedded contacts between sand and diamict. The Scarborough Bluffs sequence may be the preserved bottom stratigraphy of a large lake. The bottom stratigraphy results from repeated basinward progradation of deltaic sandy lithofacies over glaciolacustrine diamicts deposited below floating ice, whether ice shelf, ramp, bergs, or lake ice. A facies model is presented for glaciolacustrine diamict deposition on the floors of enlarged Pleistocene lakes trapping substantial volumes of fine-grained suspended sediment.