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The Cape Freels Fan: 3-D Seismic Observations That Challenge the Turbidite Fan Paradigm

Abstract

The Eocene-Oligocene Cape Freels Fan (CFF), Offshore Newfoundland, is a large submarine fan deposited at the toe-of-slope in the West Orphan Basin. The proximal portion of the CFF contains the Ephesus prospect. At first glance, the CFF appears simply to be an example of a classic “basin floor fan”, but further investigation reveals otherwise. The CFF is a hybrid submarine depositional system recording the interaction of simultaneous southward-oriented contour currents and easterly-oriented sandy turbidity currents. The CFF was deposited at the base of contourite drift slope, within a sub-basin created by antecedent topography associated with drift slope systems and basement structures. Sandy sediment was delivered to the CFF via a fixed a slope feeder channel to the NW. The CFF is subdivided into a Lower and Upper Fan, and an inboard Moat, marking distinct phases of interactions between turbidity and contourite currents. The Lower and Upper Fan contain evidence for turbidity current origins, including distributary channel networks and depositional lobes, but also evidence of reworking by contourite currents. The Moat is a partially filled, large, curvilinear channel-form feature located at the toe-of-slope, between a thick drift slope complex and the CFF. The Moat was formed by bottom currents active before, during and after CFF deposition. Significant reworking is observed near the top of the CFF, in the form of depositional ridges that appear to have been “streamlined” by bottom currents focused across the surface of the CFF near the Moat. The ridges are an en echelon set of depositional features oriented at a high angle to the Moat and have the appearance of large sediment waves cross-cutting and modifying the distributaries of the CFF. Locally, erosional scouring associated with bottom currents modified the depositional margins of the fan and also created a significant erosional escarpment across the front of the CFF. Elsewhere, isolated, linear seismic-scale features represent material removed from the CFF and re-deposited as sand ribbons around its periphery. Clearly, standard turbidite concepts and paradigms for submarine fan deposition do not fully explain many of the important seismic stratigraphic features of the CFF. This interpretation provides a new example to a growing list of hybrid, submarine depositional systems being recognized by industry and academia.