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Submarine Canyon Formation During Sea-Level Rise


Submarine canyons are important conduits for terrigenous sediment delivery to the deep sea and the development of coarse-grained deep-water petroleum reservoirs. Sequence-stratigraphic models predict that canyon formation and deep-water sedimentation occur during falling and/or low sea level, as a fluvial system incises across a subaerially exposed continental shelf and delivers terrigenous sediment directly to the head of a submarine canyon. However, recent studies have shown terrigenous sediment delivered during all stands of sea level, with a fundamental control on the delivery of coarse-grained sediment to deep water being the connection between fluvial to shallow-marine environments and submarine canyons. During the past decades researchers have developed models to illustrate the sequence of submarine canyon development including phases of mass wasting of an oversteepened slope, promoted by voluminous fluvio-deltaic sedimentation, followed by headward erosion and canyon expansion by downslope-eroding sediment-gravity flows. Moreover, the terrestrial geomorphologic community has studied headward eroding canyons by knickpoint retreat across faults; the retreat rate is correlated with the throw rate across the fault. Here, we use 3D seismic-reflection data from the Taranaki basin, offshore New Zealand, with biostratigraphic ages and paleoenvironmental classifications to interpret the seismic stratigraphy and timing of canyon formation in the Plio-Pleistocene Giant Foresets Formation. Similar to previous studies, we interpreted north-south progradation of the basin margin coincident with Plio-Pleistocene rifting and the development of the north-south-trending Parihaka normal fault. However, we mapped an extensive northeast-southwest-trending submarine canyon-channel system ~2.4 Ma reflecting a tens-of-km backstep of the basin-margin depositional system; the canyon-channel system incised ~100 m into shallow-marine shelf deposits. This backstep occurred during a period of rising and high eustatic sea-level and the trend of the canyon-channel system is approximately perpendicular to the previous (~2.6 Ma) shelf edge, as well as the Parihaka normal fault. We propose that as the Plio-Pleistocene depositional systems prograded into the Taranaki basin, Plio-Pleistocene offset of the Parihaka fault promoted an unstable shelf margin and, as the shoreline transgressed ~2.4 Ma, fault offset promoted the development of a retreating knickpoint that allowed the submarine canyon to erode headward and maintain a connection to a terrigenous sediment source. Slightly younger canyons in the dataset do not cross the Parihaka fault and they are firmly positioned at the outer shelf. Our interpretations of submarine canyon formation during rising sea level inform predictions of coarse-grained sediment delivery to deep water in frontier basins.