Recurring global sea-level changes recorded in shelf deposits near the G/M polarity transition, Wanganui Basin, New Zealand: Implications for redefining the Pliocene-Pleistocene boundary

Abstract

A well exposed 1 km-thick association of late Pliocene and early Pleistocene (ca. 2.6-1.7 Ma) strata in Wanganui Basin, New Zealand, contains a progradational stack of twenty, 5th order (41 ka duration) depositional sequences that accumulated in shelf to shoreface paleoenvironments in response to global glacioeustatic sea level fluctuations. The sequences are correlated with δ 18 O Isotope Stages 100-58, and each 41 ka glacial/interglacial stage couplet is represented by an individual sequence comprising transgressive (TST), highstand (HST) and regressive (RST) systems tracts that can be readily related to generic sequence stratigraphic models. In general, deposition of each of the sequences occurred during odd numbered (interglacial) isotope stages and the sequence boundaries developed during the maxima of even numbered (glacial) isotope stages. The key palaeomagnetic tiepoints used for pinning the isotope record to the section are the Gauss/Matuyama boundary (2.60 Ma in Stage 104) and the top of Olduvai Subchron (1.77 Ma in Stage 63). A deeply incised sequence boundary, with up to 30 m of relief, at the base of Te Rimu Sand, represents the first widespread subaerial exposure in Wanganui Basin during the Matuyama Chron; it is correlated with the high amplitude positive (glacial) δ 18 O shift in Isotope Stage 100, which is considered to mark the initiation of major northern hemisphere continental glaciations at ca. 2.52 Ma. The overlying Hautawa Shellbed, containing the subantarctic bivalve Chlamys delicatula , defines the base of the Nukumaruan Stage, and represents the earliest faunal evidence of climatic cooling in New Zealand Pliocene-Pleistocene marine sequences. The sequence boundary at the base of Hautawa Shellbed is correlated with Isotope Stage 98 (ca. 2.48 Ma). Traditionally, the Pliocene-Pleistocene boundary in New Zealand was located at the base of Hautawa Shellbed, but Beu and Edwards (1984) showed that the boundary, as defined in the Vrica stratotype (astronomically calibrated age ca. 1.81 Ma), actually lies in the upper part of the Nukumaruan Stage. The Hautawa Shellbed, which lies closer to the Gauss/Matuyama palaeomagnetic transition (2.60 Ma), is therefore much older than the currently defined Pliocene-Pleistocene boundary. From a New Zealand perspective, a more logical position for the Pliocene-Pleistocene boundary would be close to or just above the Gauss/Matuyama polarity transition. Such a position would take advantage of the boundaries proximity to a readily identifiable polarity reversal, and lie close to the time of initiation and development of major northern hemisphere ice sheets.