Controls on Late Quaternary Sediment Transport and Sequence Architecture on the Otago Continental Margin, New Zealand

The Otago margin on the east coast of the South Island, New Zealand, is a sink for sediment sourced in the outboard portion of the Southern Alps orogen, and periodically a conduit for that sediment to the abyssal Bounty Channel and Fan system. Sequence stratigraphic interpretation of high-resolution seismic reflection data suggests that several frequencies of climate-induced sea-level fluctuation, a variable sediment supply and a high-energy hydraulic regime regulate both the late Quaternary sequence architecture of sediment stored on the shelf, and the transport of sediment to the deep sea via the Otago submarine canyons. Following copious sediment influx to the margin through the shoreface erosion of Tertiary marine sediments, more-cohesive Mesozoic schist basement was exposed along the Otago coast in the early Quaternary. Shoreline erosion consequently decreased, and the principal sediment input shifted to local rivers. Deltaic and strandline units, deposited during the highstand through falling stages of sea-level, volumetrically dominate late Quaternary fourth- ( ~100 ka) and fifth-order ( ~40 ka) shelf sequences. Deltaic deposits are characterized by internal truncation surfaces indicative of deposition during higher-order sea-level fluctuations within the lower-order regression, and are associated with progradation of the inter-canyon shelf. Strandline deposits, however, do not display evidence of deposition during higher-order sea-level oscillations, and taper out at the depth of Otago canyon heads. Both deltaic and strandline deposits are confined to < ~55 m depth by strong barotrophic currents, tides and swell as sea-level rises and falls. High-energy waves and currents also rework older deposits into backstepping, shallow marine barriers and wedges during higher-order stillstands and regressions within fourth-order transgressions. Such transgressive deposits are subsequently drowned and preserved by rapid, higher-order sea-level rises. Sediment transport to the Bounty Channel/Fan system occurs primarily during glacio-eustatic lowstands when sediment bypasses the subaerially exposed shelf. High outer shelf/upper slope sedimentation rates and a lowered storm wave base trigger retrogressive gravity failure within Otago submarine canyons, and incision of canyon heads into the shelf.