Growth and burial of a submarine canyon off Motunau, North Canterbury, New Zealand

Abstract

An interpretation is provided for the growth and burial of a Pleistocene submarine canyon underlying the continental shelf off Motunau, New Zealand. Active erosion began in early Pleistocene time in a structural trough trending across the continental shelf. Gravitational mass movements, akin to some that have taken place recently in low-gradient (6.8 m/km) shelf muds in the same region are inferred to have been localized by this trough and to have transported surficial sediment out onto the deeper sea floor. Retrogressive slumping, accompanied by debris flows, extended the depression westwards towards a prograding shoreline. Sand and gravel transported from the south by shelf and longshore currents were funnelled into this depression providing the coarse, hard material required to erode a deep canyon. Feeder channels to the canyon system, filled in by debris-flow deposits, are exposed along the adjacent uplifted coast. Erosion is inferred to have ceased when the neighbouring middle Pleistocene to Recent Pegasus Canyon cut across the continental shelf and pirated the supply of coarse bedload sediment transported from the south. Muds carried in suspension began to bury the canyon, relief decreased, and gravitational mass movements were inhibited. Additional influences may have been changes in the tectonic activity and/or river regiment of the western landmass.