Pleistocene Laurentide Ice Sheet drainage into the Labrador Sea: Sedimentary facies, depositional mechanisms, stratigraphy and significance of Heinrich events.

摘要

On the basis of sedimentary structures and textures, six depositional facies have been identified in Labrador Slope, Rise and Basin cores. They include: (1) turbid-surface plume sediments (facies P; plumites) comprising 4% of the total length of the slope cores; (2) hemipelagic sediments (facies HI and H) with and without ice-rafted debris (IRD) (48% and 20% on the slope/rise and basin centre, respectively); (3) nepheloid-layer deposits (facies N; 9% on the slope); (4) contourites (facies C; 4% on the slope); (5) turbidite facies (30% on the slope and {dollar}>{dollar}40% on the levees of the Northwest Atlantic Mid-Ocean Channel, NAMOC) with three subfacies: thin-bedded silt and mud turbidites (T); turbidites with laminae of IRD (TI), and sand turbidites (MS); and (6) debris-flow facies (10% on the slope) with four subfacies: gravelly (D1), sandy silt (D2), thin bedded (D3) and sandy gravelly debris-flow deposits (D4).; Facies P occurs on high-relief slope sections, deposited by buoyantly rising meltwater plumes entrained by the south-flowing Labrador Current. The high relief was caused by retrograde canyon erosion after deposition. Facies N is best developed and thickest on the slope and upper rise. It was deposited when sediment-laden meltwater from the Laurentide Ice Sheet with high concentrations of suspended sediment spread out in mid-water or along the bottom. Facies T occurs on the levees of the NAMOC and its tributaries. It originated from the remobilization of detrital carbonate-rich sediments on the slope south of the Hudson Strait. Extensive sand turbidites occur on a braided floodplain east of NAMOC. Deposition of sand turbidites by high-density turbidity currents, probably of sheet-flow type, resulted from bedload-rich meltwater discharges on the low-relief slope sector off the Hudson Strait. They may have been caused by subglacial-lake outburst flooding, which might be linked to Heinrich events. Facies C occurs on the lower slope to upper rise. Facies H is present in all parts of the basin but most abundant on the slope; together with facies T, it is the major facies in the intercanyon regions. Facies D is found mainly on low-relief slope sectors, in front and north of major glacier outlets. Debris-flow tongues in the slope canyons merge downslope forming an extensive stacked megadebris-flow deposit on the floodplain west of NAMOC. Facies D makes up {dollar}>{dollar}75% of the sediment thickness in the western floodplain cores.; Four types of Heinrich layers (HL) were identified. (Abstract shortened by UMI.)