Late Quaternary (marine isotope stages 6-1) seismic sequence stratigraphic evolution of the Otago continental shelf, New Zealand

A proposed chronostratigraphic model for the Late Quaternary evolution of the Otago continental shelf, New Zealand, includes fluvially incised sequence boundaries eroded during marine isotope stages 2, 4 and 6, bounding three sequences subdivided into lowstand, transgressive and highstand/regressive systems tracts. Datable material is limited to the uppermost 1 m of the sequence, and consequently the model is based on correlation of new high-resolution seismic reflection data (Geopulse™) to a compilation of independent global sea-level curves. A second chronostratigraphic model that lacks a stage 4 sequence boundary and distinct stage 3 highstand/forced regressive deposit is discounted because its subsidence-corrected paleoshoreline depths do not match lowstands in the eustatic curves or temporally constrained sequences at other far-field locations. In the preferred chronostratigraphic model, sea-level lowstands during stages 2, 4 and 6 reached maximum depths of 126–131 m, 101–111 m and 116–135 m, respectively, producing sequence boundaries, landward-pinching deltaic wedges on the outer Otago shelf, and submarine canyon incision. Lowstand paleoshoreline depths have been corrected for subsidence from thermal cooling, sediment compaction and loading, but may be erroneously deep owing to erosion of paleoshoreline indicators. Clastic, shallow marine wedges and back-barrier valley-fill deposits accumulated during the stage 6–5, 4–3 and 2–1 marine transgressions. Contrary to the traditional sequence stratigraphic model, deltaic and strandline units deposited during highstand through falling sea level (stages 5–4, 3–2, 1) volumetrically dominate Otago Late Quaternary sequences. Discrete units and minor marine erosion surfaces within the transgressive and regressive systems tract deposits are interpreted as strong evidence for seventh- (∼ 20 ka) or higher-order sea-level fluctuations influencing sedimentation on the Otago shelf.