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Initiation and Growth of a Giant Paleogene Slope Fan Over Stacked Mass Transport Complexes, Offshore Newfoundland, Canada


The Ephesus fan is a channel-lobe complex, 46 km wide by 18 km long, that extends across 460 km2 along the lower structured slope within the West Orphan Basin of offshore Newfoundland. Based on biostratigraphic dating and long-range correlation from offset wells, the fan appears to have been deposited during the Oligocene, coincident with a 100m+ eustatic sea level fall, thus representing a slope fan within a lowstand systems tract. High resolution 3D seismic data reveal a series of seismic facies that are interpreted to represent high density turbidites and mass transport complexes. Bright parallel continuous seismic facies are interpreted as lobes, bright confined seismic facies are interpreted as channels, and bright to moderate amplitude inclined confined facies are interpreted as lateral accretion packages, together making up a series of stacked channel lobe complexes, with underlying lobes an order of magnitude smaller than overlying lobes. Underneath the channel lobe complexes are dim, inclined, discontinuous seismic facies occurring either as bedded units or imbricate stacked deposits forming mounds. The dim, inclined, discontinuous seismic facies is interpreted as slumps and folds that built up to form mass transport complexes that occur as either extensive bedded intervals or discrete debrite ridges. The interpreted succession is: 1) mass transport complexes deposited across the area, creating a flatter topographic surface within the structured slope that instigated sediments from subsequent turbidity currents and mass wasting events to settle out there. 2) Later slumping events built elongate ridges, creating additional structure in the local area. 3) Small lobes fed by distributary channels were deposited between and on top of the ridges. 4) Sand sediment supply then increased to deposit larger lobes on the south side of the fan. 5) As accommodation space between the ridges filled in, the rugose topography was smoothed and some distributary channels were then able to avulse north, building lobes. Throughout the fan succession, distributary channels also extend down-dip from the fan indicating some sediment bypass beyond the ponding from the mass transport deposits. This unique case study illustrates a fan circumferentially ponded by debrite ridges, which forced the fan to assume its unique wide shape and internally created substrate rugosity that set up compartments within the fan.