Continental slope sedimentation adjacent to an ice-margin. II. Glaciomarine depositional facies on Labrador Slope and glacial cycles

Abstract

Eight depositional facies have been identified in Labrador Slope and Basin cores on the basis of sedimentary structures and textures. These are combined into three genetic groups: (1) hemipelagic facies HI with ice- rafted debris (IRD) and H without IRD (with a combined thickness of about 53% of the total core length); (2) debris-flow facies D (7%) and three turbidite facies (34%) T (thin, fine-grained bodyspill turbidites), TH (headspill turbidites) and TI (turbidites interlayered with laminae of IRD); and (3) contourite (< 3%) and nepheloid-layer deposits (3%) of facies C and N. The latter and facies TI are characteristic of proximal Heinrich layers. The intercanyon regions of the slope are mainly covered by hemipelagic sediment with intercalated spillover turbidites. The canyons, if filled, contain debris-flow deposits and turbidites. The dominant sediment-delivery and transport mechanisms change from vertical settling and bedload transport by glacial stream-discharge on the upper slope to down-slope mass-flow processes, which predominate over along-slope processes on the lower slope and rise. Up to 15 lithostratigraphic units have been differentiated in the slope cores, based on the recognition of various combinations of the 8 facies types and short-distance correlations of AMS 14C-dated intervals. Very dark hemipelagic muds with low detrital carbonate content, little ice-rafted debris, a low degree of bioturbation and abundant sinistral-coiling, cold- water foraminifera (Facies H of units 3, 5, 7, 9, 11 and 13) are tentatively ascribed to times of the Mid-to Late Wisconsinan glaciation with extensive seasonal sea-ice cover in the Labrador Sea. Turbidites and debris-flow deposits cannot be related to specific phases of the glacial cycle and result largely from redeposition of upper and mid-slope sediments. Ice-rafted debris dispersed in nepheloid-flow deposits or interlayered as separate laminae between thin mud-turbidites are related to short-lived Heinrich events during times of partial ice-cap collapse, but are not restricted to these events. The youngest slope sediments, which were deposited since the early part of marine oxygen-isotope stage 2 (<32 ka) correlate chronologically and lithologically with adjacent shelf, rise and basin sediments. The oldest AMS 14C date of 54, 530±2090 yr B.P. was obtained for the unit 11/12 boundary at 1008 cm subsurface depth in core 90-26.