Controls on sequence stratigraphy of a middle Miocene-Holocene, current-swept, passive margin: Offshore Canterbury Basin, New Zealand

Abstract

The offshore Canterbury Basin exemplifies sequence development on a prograding passive margin influenced strongly by submarine currents. Nineteen middle Miocene–Holocene, regional, sequence-bounding unconformities are interpreted by using high-resolution multichannel seismic data. The sequences can be grouped into larger units based on seismic geometry and facies that reflect different combinations of controls on sequence architecture. Correlation with oxygen isotope records suggests that eustasy controls the timing of sequence boundaries. The number of sequences is similar to that of coeval cycles on a temperature-adjusted, Miocene and early Pliocene δ 18 O record. The late Pliocene–Pleistocene sequence record is of lower frequency than the isotopic record of this period, either because of the limitations of seismic resolution or because of removal of sequence boundaries by erosion associated with high-amplitude eustasy. However, the last two sequence boundaries correlate well with the last two 100 k.y. isotopic cycles. In contrast, sequence architecture is influenced strongly by local processes. Along-strike currents create large, elongate sediment drifts that control sequence thickness; current erosion in drift moats forms diachronous unconformities. Drifts focus deposition on the slope, reducing the rate of basinward advance of the shelf edge, but increasing that of the slope toe, thereby reducing slope inclination. Replacement of along-strike processes by downslope processes increases rates of shelf-edge progradation, and the slope steepens as the reduced accommodation space over the expanded slope is filled. Clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.