DEPOSITION IN GLACIO-ISOSTATIC MOATS IN COASTAL ENVIRONMENTS

Thick and extensive bodies of Pleistocene proglacial outwash accumulated in isostatically depressed crustal ‘moats’ located outboard of advancing and retreating ice sheets in British Columbia (B.C.) and southern Argentina. Quadra Sand was deposited in a subsiding trough in front of large lobes of the Cordilleran ice sheet in the Salish Sea and on Haida Gwaii during the early part of the Fraser Glaciation (MIS 2). The presence of brackish and marine diatoms in Quadra Sand near Vancouver indicates that relative sea level was at least 18 m above present 28,000 cal yr ago. Eustatic sea level at that time was at least 85 m below its present position, thus there was more than 100 m of glacio-isostatic depression near the margin of the ice shee early during the Fraser Glaciation. Similarly, on Haida Gwaii, Quadra Sand dating to about 25,000-28,000 years ago was deposited in a moat situated inboard of a glacial forebulge. In both instances, low mantle viscosity associated with high heat flow inboard of plate boundaries off the B.C. coast led to strong gradients in isobases and short glacio-isostatic response times. With isobase slopes of at least 1-2 m/km in the direction of glacier flow at the southern and western margins of the Cordilleran ice sheet, migrating moats with up to 100 m of depositional accommodation space were created within 50-100 km of advancing ice margins. This accommodation space was filled with outwash graded to sea level, which was rising relative to the land. Similarly, a sequence of ice-proximal subaqueous sediments that are exposed over distances of tens of kilometers along the Atlantic coast of Argentina north of Tierra del Fuego filled a glacio-isostatically depressed trough when the Patagonian ice sheet advanced to near its maximum limit during the Cabo Virgenes glaciation (Middle Pleistocene). In this case, however, eustatic sea-level lowering appears to have exceeded the amount of glacio-isostatic depression, thus the depositional environment was glaciolacustrine rather than glaciomarine.