Late Quaternary stratigraphy of Laurentian Fan: a record of events off the eastern Canadian continental margin during the last deglacial period

Abstract

Examination and correlation of 10 piston cores from Laurentian Fan, off the eastern Canadian continental margin revealed a high-resolution stratigraphic record of events since the Last Glacial Maximum (LGM). The five lithofacies identified in the cores are: (1) bioturbated, olive-grey, silty mud with varying amounts of biogenic carbonate; (2) olive-grey and reddish-brown silt-laminated mud with rare fine to very fine sand beds; (3) poorly sorted, light grey and brick-red gravelly, sandy mud; (4) colour-banded silty mud with colours alternating at a centimetre-scale between olive-grey and reddish-brown; and (5) brown silty mud, commonly massive but occasionally silt-laminated. The dominant depositional processes include hemipelagic sedimentation, turbidity currents, and ice rafting. In these sediments, colour is diagnostic of the sediment source area, with reddish sediment being delivered to the fan from the Gulf of St. Lawrence and Laurentian Channel whereas greenish sediment arrives on the fan from St. Pierre Bank through Grand Banks Valley. Turbidites are the most volumetrically significant facies. Conversely, ice-rafted beds are volumetrically minor, but they are the most chronostratigraphically significant facies, and include Heinrich layers H0, H1, and H2. The chronological control provided by radiocarbon dating and stable oxygen isotopes suggests that sedimentation rates for the reddish-brown turbidites deposited between the LGM and 14 ka exceeded 10 m kyr −1. Mapping of the units, together with a simple sediment deposition model, suggest that these beds resulted from deposition by turbidity currents over 1 km thick that lasted on the order of weeks. Less information can be ascertained from the olive-grey turbidites that interbed with H2 and directly overlie H1 because of a lack of sampling, except that they had a lower sedimentation rate and include sediment derived from Newfoundland and the Grand Banks. Comparison of the stratigraphy on Laurentian Fan with that of other deep-water areas further north (southwest Grand Banks Margin, Flemish Pass, and the Labrador Sea) shows striking similarities over the late Quaternary, with the deglacial period characterized by rapid sediment accumulation. However, only on Laurentian Fan does sediment gravity flow deposition occur during the LGM. Reddish-brown turbidites deposited on Laurentian Fan during the deglacial period have no equivalent facies further north. These unique features suggest important glaciological differences between the ice stream occupying Laurentian Channel and ice streams farther north. The onset of widespread hemipelagic deposition occurred after ca. 12 ka on Laurentian Fan and in Flemish Pass, but not until ca. 8.2 ka in the Labrador Sea, consistent with the earlier retreat of glacial ice out of Laurentian Channel as compared to Hudson Strait.