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

Abstract

The western Wanganui Basin in the vicinity of Parikino contains a 410 m thick succession of thirteen sequences that accumulated between 2.8 and 2.4 Ma. Traditional facies analysis of the succession indicates accumulation of the sequences in shoreface to mid-shelf depositional palaeoenvironments. The Late Pliocene sequences have been correlated with oxygen isotope stages G10 to G92, and each of 41 ka glacial–interglacial stage couplets is represented by an individual depositional sequence comprising transgressive (TST), highstand (HST) and regressive (RST) systems tracts. Most sequence boundaries are unconformable, and coincide with transgressive surfaces of erosion (TSE = ravinement surface), with Ophiomorpha burrows extending down into underlying sandstone. One sequence boundary at the base of the Wilkies Shellbed 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 shellbeds 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 and backlap shellbeds are superposed, forming compound shellbeds. Rarely a downlap shellbed overlies the DLS within the base of HSTs. HSTs are composed typically of massive, aggradational, mid-shelf siltstone that grades up into sandstone, which forms thick progradational inner-shelf to shoreface lithofacies attributed to regressive systems tracts (RST). Lensoidal toplap shellbeds comprised of reworked, nearshore storm emplaced fauna occur within the upper part of RSTs. All of the Parikino sequences have asymmetrical architectures, with thin TSTs. This arises because of the very low rates of non-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 of shellbed, followed by very high rates of terrigenous sediment accumulation during the latter half of cycles. An implication of this work is that marked (40–60 m) eustatic sea level changes occurred repeatedly during the interval 2.8 to 2.4 Ma. This implies substantial and successive continental glaciations from at least 2.8 Ma, probably in the Northern Hemisphere.