Global and local controls on depositional cyclicity: Canterbury basin, New Zealand
The eastern margin of the South Island of New Zealand is part of a continental fragment that rifted from Marie Byrd Land in Antarctica beginning at about 80 Ma. High rates of sediment supply since the middle Miocene have resulted in the preservation of a high-frequency, seismically resolvable record of depositional cyclicity in the offshore Canterbury basin. In addition, along-strike currents have influenced deposition, modifying sequence architecture while forming large sediment drifts. High-resolution multichannel seismic (MCS) data provide insights into the interplay of global (eustastic) and local controls (tectonics and current activity) responsible for continental margin depositional cyclicity and sequence architecture. At least eleven large, elongate sediment drifts are identified within the Neogene shelf-slope sediment prism. The drifts were deposited in water depths of 300-750 m and developed in response to a northward flowing current. Their subsequent evolution is influenced by sediment supply, relative sea-level change, and seafloor morphology. Unconformities created by paleoslope erosion at the landward edges of drift moats differ from sequence boundaries by being strongly diachronous. Nineteen regional sequence-bounding unconformities are interpreted. Correlation with oxygen isotopic records suggests that eustasy controls the timing of sequence boundaries. In contrast, sequence thickness, shelf edge stacking patterns, internal seismic facies, and slope inclination, are strongly influenced by local processes, principally the along-strike currents responsible for drift development. Sequences with conventional clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking. Reconstruction of Australian-Pacific relative plate motion reveals divergence in the central Southern Alps prior to ~20.1 Ma, followed by increasing average rates of convergence, with a marked increase after ~6 Ma. Sedimentation rates, calculated from sequence volumes, correlate well with convergence rates since ~11.5 Ma, indicating that tectonism has been the dominant control on sediment supply. In particular, high rates of sediment supply since 6 Ma, may reflect previously recognized increased plate convergence and Southern Alps uplift. In contrast, high sedimentation rate from ~15-11.5 Ma correlates with low convergence rates and is therefore probably a response to global climatic forcing and eustasy.