FIELD EVIDENCE FOR SUBGLACIAL DEFORMATION PROCESSES IN LATE QUATERNARY SEDIMENTS EXPOSED AT VINELAND, ONTARIO

Subglacial sediment deformation process have been theoretically modeled and described from modern ice margins but there are few field descriptions of Late Quaternary sediments created by these processes. This poster will present field evidence for subglacial deformation processes in fine-grained sediments exposed at the Vineland Quarry in Vineland, Ontario. The quarry is located close to the crest of the Niagara Escarpment in an area affected by the Ontario lobe of the Laurentide ice sheet. The studied section is 7m high and directly overlies an eroded and striated limestone bedrock surface.

Three distinct facies types can be identified within the exposed Quaternary section. A laterally discontinuous massive silty clay unit up to 1m thick, and containing 1-3 centimeter subrounded limestone clasts, directly overlies bedrock. This silty clay unit is capped by an irregular boulder horizon consisting of subrounded to subangular limestone clasts between 15 and 40cm diameter. These facies are overlain by 4-5 m of laminated clays and silts that show progressive upward increase of laminae thickness (from mm scale at the base to 1 cm at the top). Fine silt and clay laminations are flat-lying at the base of the unit but become increasingly folded and contorted upsection. Silt clasts, ranging in size from mms to cms also become more abundant upwards; some silt laminae consist entirely of silt clasts. Scattered clasts within this unit are subrounded to rounded. The uppermost facies consist of approximately 2m of massive to crudely stratified clayey sand diamict containing pockets of deformed medium to coarse-grained sands. Clasts are subrounded, range in size from 1 to 30cm and include local bedrock lithologies (limestone, dolomite) as well as far-travelled granites and gneisses derived from the Canadian Shield. Clast long axes have no preferred orientation.

Sediments exposed at the Vineland Quarry are interpreted to record initial glaciolacustrine deposition in a deep lake ponded in front of the Laurentide ice margin (massive silty clays). Rapid advance of the ice margin created sediment instability in the basin and allowed deposition of upward-coarsening laminated silts and clays. Overriding of the site by ice caused deformation of laminated silts and clays and allowed deposition of clayey diamict facies as deformation till.