Dorrik A. V. Stow

Institute of Petroleun Engineering, Heriot Watt University

The morphology and early growth history of the Laurentian Fan have been investigated from seismic reflection profiles. The Fan has acted as an important depocentre since the early Mesozoic. Major input of material during the Plio/Pleistocene glaciations resulted in deep (up to 850 cm) channels being carved through the existing sediment pile, and the simultaneous construction of very thick interchannel deposits. The present Fan morphology is most likely inherited from the Illinoian glacial stage.

Twelve facies and subfacies have been characterized in late Quaternary sediments of more than 60 cores from the outer margin off Nova Scotia. Olive gray, biogenic-rich, bioturbated sediments were deposited through much of the Holocene, and represent "hemipelagic" sedimentation with bottom current reworking. Red-brown, silt-laminated muds were deposited over most of the Rise and Fan during the Wisconsin glacial period from large-scale turbidity currents. The change from red to gray sediments was time transgressive across the Holocene/Pleistocene boundary. Three, inferred interstadial periods are recognized in the Wisconsin when sedimentation was more similar to that of the Holocene, but with increased ice rafting. Thick (up to 3 m) turbidite sands and gravels are a minor facies restricted to the channels.

Detailed sedimentological analyses were carried out on the red-brown muds in particular. A standard sequence of sedimentary structures is proposed for fine-grained turbidite units, analogous to the Bouma sequence for sandy turbidites. A physical model has been developed for sorting in muddy turbidity currents. Hydraulic sorting within the flow, and downslope lateral sorting explain the textural and compositional grading through individual units (2 to 10 cm thick), and over hundreds of km away from the source. Depositional sorting by increased shear in the bottom boundary layer separates clay flocs from silt grains and results in the regular silt/mud lamination. Single units take between 2 and 6 days to deposit from very thick flows (up to 1500 m) which are moving at current velocities of 9 to 16 cm/s.

The interbedded, olive gray muds and associated coarse layers have been more affected by bottom current activity. The most important criteria for distinguishing between these fine-grained turbidite and contourite facies have included: textural trends, regional compositional patterns, and grain fabric indicating either downslope or along-slope transport. It is apparent that many of the currently used criteria are not applicable to fine-grained turbidites and contourites.

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Supervisors: D. J. W. Piper