Sedimentary architecture of detached deep-marine canyons: Examples from the East Coast Basin of New Zealand

Submarine canyons are conduits for the distribution of sediment across continental margins. Although many canyons connect directly with fluvial or marine littoral system feeders, canyons detached from direct hinterland supply are also recognized. The fill of detached canyons remains enigmatic, because their deep-water setting restricts analysis of their evolution and stratigraphic architecture. Therefore, this study aims to investigate the sedimentary processes that infilled deep-water canyons and the resulting architecture. Miocene outcrops of an exhumed deep-water system from the East Coast Basin, New Zealand, are documented and compared with the morphology and seismic scale architecture of a modern detached canyon system on the same convergent margin. The outcropping system preserves the downstream margin of a sub-basin deposited at palaeo-water depths >700 m. A 6 km wide by 430 m deep incision is filled by heterogeneous siliciclastic sediments, 50% of which comprise graded thin-beds with traction structures, interpreted to result from oscillatory flows. These are intercalated with concave-up lenses, up to 15 m thick, of sigmoidally-bedded, amalgamated sandstones, which preserve ripple casts on bed bases, interpreted as deposits at the head of a deep-marine canyon. Palaeo-flow was eastward, into the sub-basin margin. On the adjacent margin of the sub-basin down-dip, stacked and amalgamated sandstones and conglomerates represent the fill of a submarine channel complex, at least 3 km wide. The channels are inferred to have been fed by the up-dip canyon, which traversed the intervening structural high; similar relationships are seen in the bathymetry data. Seismic studies on this margin demonstrate that multiple phases of canyon cut and fill may occur, with downstream architectural evolution comparable to that seen at outcrop, demonstrating that detached canyons may act as sediment conduits. Breaching of developing sea-floor structures by detached canyons can modify tortuous sediment pathways, supplying sediment to otherwise starved areas of the slope.