HIGH-FREQUENCY SEQUENCES DEPOSITED DURING QUATERNARY SEA-LEVEL CYCLES ON A DEFORMING CONTINENTAL-SHELF, NORTH CANTERBURY, NEW-ZEALAND

A closely spaced grid of high-resolution seismic reflection profiles reveals twelve unconformity-bounded sedimentary units of Late Pliocene to Recent age beneath the inner to middle north Canterbury continental shelf, New Zealand. This part of the shelf lies in < 80 m deep water and is undergoing tectonic deformation at the edge of the Australian-Pacific plate boundary zone. Coastal uplift, folding and wave planation have exposed the units at the seabed. Nannofloral biostratigraphy of sediment cores from at least six units constrain the age of the succession. The oldest unit (unit 12) of Late Pliocene-Early Pleistocene age is exposed in a submerged coastal platform, 3-8 km wide. Units 11 to 1 of middle Pleistocene to Recent age each thin shoreward, wedge out against the outer edge of the platform, and onlap the underlying erosion surface. The units are interpreted to be predominantly highstand systems tract (HST) silty muds separated by thin (< 1 m), transgressive systems tract (TST) gravel layers that mantle the underlying ravinement surface. The HSTs, which were deposited at rates ranging from 0 to > 3 m/ka, have prograded along shelf, shorewards and offshore, so that they onlap the TSTs beneath the inner shelf and downlap the TSTs beneath the outer shelf. Isolated pockets of sediment that infill channels beneath the ravinement surfaces may represent the preserved remnants of lowstand systems tract (LST) fluvial sediments and Type 1 sequence boundaries, respectively. Seismic units 11 to 1 and intervening unconformities A to J are tentatively interpreted to represent nine sea-level cycle sequences, deposited during high-amplitude (> 100 m) glacio-eustatic fluctuations corresponding to 100 ka (eccentricity) cycles in Earth's orbit. The sequences are tentatively correlated with marine oxygen isotope stages 19 to 1 (most represent odd-numbered, interglacial stages of ca. 0.75 Ma to Recent age) and with a succession of alternating glacial and interglacial deposits recovered in an onland testbore at Bexley, near Christchurch City. The outer edge of the submerged coastal platform, exposing unit 12, is close to the axis of regional tilting. Landward of this axis, unit 12 has been progressively uplifted, deformed and repeatedly eroded, and coastal hills and marine terraces have been uplifted. Seaward of the axis, the middle to outer shelf has been subsiding at about 0.4 m/ka, which created accommodation space for mostly HST deposition. Large-scale growing folds beneath the inner shelf have progressively deformed and modified the geometry of the units as a result of contemporaneous sedimentation and tectonic shortening. Therefore, although the sequences and unconformities owe their origin mainly to glacio-eustatic fluctuations, regional tilting and tectonic deformation have played a role in modifying the geometry of the units.