The Quaternary geology of the Labrador Shelf

The regional distribution of glacial and postglacial sediments on the Labrador Shelf was mapped by medium- and high-resolution seismics and shows a succession of tills that extend to the continental shelf edge at approximately 600 m. The uppermost of these tills is less laterally extensive and does not occur on the Labrador Shelf bank tops in water depths less than 160 m. We infer that the last glacial episode was less extensive than the preceding ones. Glaciomarine stratified silts and postglacial muds are found overlying the upper till in the deep basins that separate the banks. The upper till, glaciomarine silts, and postglacial muds are interpreted to constitute one complete deglacial cycle. Radiocarbon age determination suggests this deglacial sequence began at approximately 20 000 years BP. On the northernmost Labrador Shelf and Hudson Strait, seismic evidence indicates a late glacial readvance locally deposited a till that overlies the glaciomarine sediments associated with the last glacial advance on the Labrador Shelf. The stratigraphic position of this Hudson Strait till suggests the local readvance occurred at approximately 8000 years BP. The acoustic stratigraphic framework (data) is ground truthed by a regionally extensive piston core and grab sample network. Multidisciplinary analysis of these sediment samples included X-ray, textural, geotechnical, paleontological, lithological, trace-element, and 14C analyses. The results confirm the acoustic interpretations and show that each of the stratigraphic units defined have unique and regionally consistent properties. Geotechnical analyses from the upper till indicate low shear strengths (± 25 kPa) and low to normal consolidation ratios (0.8–2.8), which we interpret to indicate deposition under low basal loading as a result of hydrostatic support on an ice shelf in a marine setting. Provenance studies on the sands and gravels from each of the stratigraphic units of the last deglacial sequence (i.e., till, glaciomarine silts, postglacial muds) indicate that the till is mostly derived from mainland Labrador and transported by an easterly ice flow. Since limestone outcrops are rare to absent on the Labrador mainland and continental shelf, high limestone concentrations (up to 80%) within the glaciomarine sediments that overlie the upper till indicate a northern and distal source for that unit. We interpret an active ice margin overlying the Paleozoic limestone outcrops in Hudson Strait and Ungava Bay as being the dominant source for the glaciomarine silts. The absence of limestone within the postglacial muds that overlie the glaciomarine silts indicates a significant change in provenance, which we attribute to the disappearance of the active ice margin over the limestone terranes. The depositional style and sedimentary structures within the glaciomarine silts indicate low bottom currents and a uniform rain out of ice-rafted (limestone) debris that extends to the southernmost part of the Labrador Shelf. The depositional style of the postglacial muds indicates a significant increase in bottom currents, with deposition restricted to the deep basins. We suggest these increased currents resulted from the disappearance of the Hudson Strait ice barrier, which allowed the Canadian current to develop and combine with the west Greenland current. This combination of currents occurred at approximately 8000 years BP and marks the inception of the Labrador current, which presently traverses the Labrador Shelf.