Facies and sediment dynamics in Charlie-Gibbs fracture zone during the Late Quaternary

Abstract

Analysis of five cores from the Charlie-Gibbs fracture zone enables us to define the sedimentary characteristics in four different types of environments: a sill disrupting the southern valley, open channels in the northern and southern valleys, a flanking plateau of the northern side wall in the northern valley, and an enclosed depression in the northern valley. Five lithofacies have been recognized: clayey-silty mud (40%), clayey-silty calcareous mud (25%), calcareous sands (20%), sedimentary breccia (10%) and silty-sandy calcareous mud (5%). The predominance of terrigenous clayey material is evidence for a mainly continental origin of the fracture-zone sediments. Yet another striking feature is the abundance of foraminiferal sands rich in pelagic calcareous and siliceous components associated with coarser ice-rafted elements. Despite the pelagic origin of components, a uniform draping of sediments — which kind of sedimentation we call “ubiquiste” (Faugères et al., 1979) — has not been noted in the survey area, due to the reworking processes of already deposited materials by bottom geostrophic currents or gravity flows. The influence of the bottom-current processes on sedimentation is confirmed by the following characteristics: (1) the presence of sharp or erosional contacts between two facies; (2) low clay fraction in certain levels, caused by winnowing of sedimentary materials; (3) high clay fraction in other levels, considered to be the result of dilution of pelagic material by clay particles transported in the nepheloid layer and trapped in a depression downstream of a sill; and (4) Icelandic smectites transported by Iceland—Scotland Overflow Water (I.S.O.W.). The influence of gravity flow dynamic processes is indicated by sandy or silty-sandy centimetric turbidites, very thick mega-sequences (8 m) transported by a single mass flow, and important sedimentary gaps, whence gravity flows originated.