The stratigraphic architecture of Late Pliocene (2.8-2.4 Ma) asymmetrical shelf sequences, western Wanganui Basin, New Zealand

The western Wanganui Basin in the vicinity of Parikino contains a 410 m thi ck succession of thirteen sequences that accumulated between 2.8 and 2.4 Ma . Traditional facies analysis of the succession indicates accumulation of t he sequences in shoreface to mid-shelf depositional palaeoenvironments. The Late Pliocene sequences have been correlated with oxygen isotope stages G1 0 to G92, and each of 41 ka glacial-interglacial stage couplets is represen ted by an individual depositional sequence comprising transgressive (TST), highstand (HST) and regressive (RST) systems tracts. Most sequence boundari es are unconformable, and coincide with transgressive surfaces of erosion ( TSE = ravinement surface), with Ophiomorpha burrows extending down into und erlying sandstone. One sequence boundary at the base of the Wilkies Shellbe d is a correlative conformity. Two types of shellbed occur within TSTs and are associated with key sequence stratigraphic surfaces: onlap and backlap shellbed. Onlap shellbeds onlap the TSE and sequence boundary; backlap shel lbeds result from stratigraphic condensation at the top of the TST and are overlain by the downlap surface (DLS). In most of the sequences the onlap a nd backlap shellbeds are superposed, forming compound shellbeds. Rarely a d ownlap shellbed overlies the DLS within the base of HSTs. HSTs are composed typically of massive, aggradational, mid-shelf siltstone that grades up in to sandstone, which forms thick progradational inner-shelf to shoreface lit hofacies attributed to regressive systems tracts (RST). Lensoidal toplap sh ellbeds comprised of reworked, nearshore storm emplaced fauna occur within the upper part of RSTs. All of the Parikino sequences have asymmetrical arc hitectures, with thin TSTs. This arises because of the very low rates of no n-tropical carbonate accumulation during the transgressions, that is about half of the 41 ka period of each cycle, amounting to a few metres at most o f shellbed, followed by very high rates of terrigenous sediment accumulatio n during the latter half of cycles. An implication of this work is that mar ked (40-60 m) eustatic sea level changes occurred repeatedly during the int erval 2.8 to 2.4 Ma. This implies substantial and successive continental gl aciations from at least 2.8 Ma, probably in the Northern Hemisphere. (C) 19 98 Elsevier Science B.V. All rights reserved.