Observations on depositional environments and benthos of the continental slope and rise, east of Newfoundland

Abstract

Sedimentologic, biologic, and morphologic criteria permit recognition of four depositional environments on the continental slope and rise, east of Newfoundland. The 'upper slope' (300–700 m) has a hummocky substrate with a mantle of terrigenous, gravelly muddy sand which is a mixture of ice-rafted detritus and sediment reworked from underlying glacial drift deposits. Reworking presumably took place during the last major lowering of sea level and it is continuing today under the influence of the Labrador Current and other oceanographic and biologically-related forces. The featureless bottom of the 'middle slope' (700–2000 m) is the principal depositional site of Recent mud. Fines, reworked from shelf and upper slope sediments, settle out together with fines transported to the area by the southeast-flowing Western Boundary Under-current (WBU). Compared to the upper slope this deeper environment receives less ice-rafted clasts, supports a richer macrofauna, and has a higher total species diversity of foraminifera. The 'lower slope' (2000–2500 m) is characterized by higher amounts of gravel and sand mixed with the mud, increasing numbers of current bedforms, and a more diverse foraminiferal assemblage, all of which correlate with the increasing power of the WBU with depth. The gravel was ice rafted probably at the end of the late Wisconsin to early Holocene and its presence on the seabed reflects the power of the WBU to inhibit deposition of Recent mud. The 'rise' (2500 to > 3000 m) is heralded by a subtle break in slope at about 2500 m. A high speed core of the undercurrent is situated in this area as indicated by the coarseness of the sediments (gravelly muddy sand) and the observed current bedforms. A marked increase in the numbers of benthonic and planktonic foraminifera is related primarily to the winnowing capacity of WBU. Numerous arenaceous deep sea forms first occur between 2500 and 3000 m and appear to reflect the reduced salinity, low temperature, high dissolved oxygen characteristics of the watermass that is associated with this depth interval.