Three-dimensional architecture of shelf-building sediment drifts in the offshore Canterbury Basin, New Zealand

Abstract

A grid of high-resolution, multichannel seismic profiles from the offshore Canterbury Basin, New Zealand, reveals that at least 11 large (up to 1000 m thick, >50 km long, along-strike, and 20 km wide, down-dip) elongate sediment drifts developed within the lower Miocene to Recent shelf-slope sediment prism. The drifts overlie a condensed section of late Eocene to late Oligocene limestone and cover an area of ∼5000 km 2. The drifts were deposited in water depths of 300–750 m, probably by a northward-flowing contour current, and aggraded to shelf depths. The drifts exhibit mounded morphologies with channel-like moats along their landward flanks. Erosion of the landward flanks creates prominent unconformities; these unconformities are diachronous and, therefore, not sequence boundaries. The internal architecture of the drifts defines two end members of elongate drift, which we describe as simple and complex. Early (early to middle Miocene) simple drifts are small (<400 m thick, 10–28 km long and several km wide) and concentrated in the southern part of the survey area. Drift thickness increased as the shelf aggraded and the locus of drift development migrated northeastward through time. Late (late Miocene to Recent) simple drifts are, therefore, larger (up to 1000 m thick, >50 km long and up to 20 km wide) and occur in the northeastern part of the survey area. These late, simple elongate drifts are subdivided into three parts (base, core, and crest) based on seismic facies. These facies form in response to progressive confinement of current flow within the moat. Complex drifts may be multi-crested or multistage. Multi-crested drifts form in response to rapid lateral shifts in position of the moat, perhaps associated with relative sea-level change, modulated by paleoslope inclination and orientation. Such drifts, together with the observation that several drifts were often active simultaneously, indicate that flow patterns were complex and involved multiple pathways. Multistage drifts comprise superimposed subdrifts whose retrogradational and progradational stacking patterns indicate fluctuations in the rate of sediment supply. Complex drift formation may require intermediate shelf relief: high enough to sustain drift development during changes in sea level and rate of sediment supply, but low enough so that such changes are still able to influence moat position. In addition, coeval climatic cooling may amplify changes in sea level, current intensity and sediment supply, thereby contributing to complex drift formation.