GLACIAL AND SEA LEVEL CONSTRAINTS, NORTHEASTERN GRAND BANK

Geoscience surveys on NE Grand Bank provide data to delineate surficial geology, interpret glacial history, and constrain estimates of former sea level.

The Late Quaternary record comprises thin sediments over a regional near-seabed unconformity. The Grand Banks Drift blankets this unconformity from ca. 220 mbsl to 110 mbsl where it pinches out. A thin lag Grand Banks Gravel overlies the upper part of the till and extends to < 60 mbsl. A recessional moraine topography is preserved in deep water, but is absent above 150-160 mbsl. Erosional trimming of the moraines is thought to have occurred during a low sea level prior to the last marine low-stand. It follows that the Grand Banks Drift predates the last glacial maximum. Between ca. 105 and 150 mbsl, the gravel and till are overlain by continuous Adolphus Sand formed by nearshore processes during the last sea level lowstand. Above 100 mbsl, the Grand Banks Gravel is discontinuously overlain by thin sand deposits and ridges thought to have formed during the transgression following the last glacial maximum. More than 60 marine shell samples were dated to estimate the timing and extent of sea level fall during the last glacial lowstand. Twenty radiocarbon ages, ranging from 2-34 ka, were obtained on single shells or fragments. The collections are from different water depths, and assumed to indicate a time when sea level was high enough to provide a marine habitat. 19 ages from the 70-85 m range fall in two groups indicating that sea level was higher than this elevation from 34-25 ka, and after 13 ka, but possibly lower in-between. Shells from vibrocores in the Adolphus Sand at 115 m and 125 mbsl indicate continuous sand accumulation from > 20 ka to < 10 ka, suggesting minimum sea level either did not descend or fell only briefly to these depths.

Did ice exist over NE Grand Bank during the last glacial maximum (LGM)? While widely distributed sub-rounded boulders of diverse lithologies suggest erosion of former glacial deposits, no glacial diamict has been found that can be dated to the LGM. It's possible the boulders are remnants of earlier glaciations such as that which deposited the Grand Banks Till below 110 mbsl. The presence of relatively friable shells in surficial deposits dating from prior to the LGM favours absence of LGM ice, or if present, one that exerted little basal deformation or erosion.