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Note: Hongbo's poster won an "Outstanding Student Paper Award" at AGU

H. Lu, C.S. Fulthorpe and P. Mann

Upper Miocene to Recent sequences beneath the continental shelf and slope of the offshore Canterbury Basin, New Zealand, are mapped using closely spaced, high-resolution multichannel seismic data collected in January 2000. Sequence geometries and morphologies of unconformities reflect competing influences of eustasy, contour-currents, and rate of sediment supply.

Eighteen regional sequence-bounding unconformities form three seismic stratigraphic units: 1) Miocene unconformities U1-8 (>16-5 Ma; all ages are from ODP Site 1119 and the Clipper exploration well) have smooth shelves. Onlap can occur on both shelf and slope. Most sequences are progradational and of relatively long duration (>1.4 m.y.). Six additional localized unconformities also occur; each onlaps the underlying regional unconformity and is either truncated basinward by sediment drifts or downlaps onto the underlying unconformity. 2) Pliocene to early Pleistocene unconformities U8-13 (5-1.25 Ma) define aggradational sequences with durations ranging from 0.11 to 1.4 m.y. Frequency of onlap decreases up section. Each sequence contains a downlap surface, which onlaps the underlying unconformity and is truncated basinward by the overlying unconformity. 3) Early Pleistocene (~1.25 Ma) to Recent unconformities U13-18 display shelf channel incision and erosional truncation by currents on the slope. Sequences are progradational and of short duration (<0.4 m.y.). They are downlapped on the shelf and slope failure truncates reflections near their well-defined shelf edges. Five additional local unconformities are truncated to landward by the overlying regional unconformity; most are only preserved on the outer shelf and upper slope.

The most recent unconformities (17 and 18, plus three local unconformities) correlate well with 100 k.y. cycles in the oxygen isotope record (stages 6-14) over the last 500 k.y., suggesting that they are of eustatic origin. Older unconformities are of longer duration and encompass multiple oxygen isotope cycles. This could imply local control of older sequences. Alternatively, the sequence stratigraphic record may selectively respond to a subset of the global cycles, or bundling of Milankovitch-scale cycles.

Currents redistribute deposits, control depocenter locations, generate diachronous unconformities, and influence seismic stacking patterns and morphologies. Multistage sediment drifts contain several sequence boundaries. Drift moats may serve as conduits for downslope sediment transport during relative sea-level falls, reactivating as moats when sea level rises. Large sediment drifts in the north transition southward to conventional progradational geometries, in parallel with a north-to-south increase in slope inclination (from <1° to >5°). Geometries transitional between drifts and clinoform sequences suggest criteria by which current deposition can be recognized on margins without well-defined drifts.