Stratigraphic and sedimentological evidence for late Wisconsinan sub-glacial outburst floods to Laurentian Fan

Abstract

Sub-glacial meltwater produces a distinctive stratigraphic and sedimentological response on the continental margin. Seismo-stratigraphy of Laurentian Channel reveals thick till deposits at its seaward end that pass laterally into stratified sediment in deeper basins, that may record periods of water build up beneath the ice. Two scales of meltwater discharge are recognised: large scale that caused catastrophic erosion and transported large volumes of coarse sediment to the abyssal plain and smaller scale, yielding principally muddy sediment. Sub-glacial outburst floods from the Laurentian Channel ice stream delivered distinctive red sediment derived from Permian–Carboniferous strata of the Gulf of St. Lawrence directly to Laurentian Fan between ca. 17 and 14 14C ka, separate from North Atlantic Heinrich events. On levees of Laurentian Fan, three major pulses of meltwater plume muds are separated by intervals dominated by hemipelagic sediments. These meltwater intervals are recognised distally as periods of plume sedimentation on the Scotian Slope and ice-rafting of hematite-stained quartz to the North Atlantic Ocean. In channels of Laurentian Fan, at least one major sediment transport event is recognised that eroded the upper slope and the major fan valleys, depositing a bed of gravel at least 3 m thick in the characteristically wide fan valleys and thick sand on the Sohm Abyssal Plain. The same event was probably responsible for giant flute-like scours. The age of the gravel bed is directly constrained only by the presence of local overlying Holocene sediment. Much of the surface of the gravel bed was re-worked by the 1929 “Grand Banks” turbidity current. An erosional event on the upper slope, likely correlative with the flood-generated gravel bed, has been dated at 16.5 14C ka. Such large scale erosional flood events can be recognised back through several glacial cycles and have played an important role in the architectural evolution of Laurentian Fan.