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/

A well exposed 1 km-thick association of late Pliocene and early Pleis tocene (ca. 2.6-1.7 Ma) strata in Wanganui Basin, New Zealand, contain s a progradational stack of twenty, 5th order (41 ka duration) deposit ional sequences that accumulated in shelf to shoreface paleoenvironmen ts in response to global glacioeustatic sea level fluctuations. The se quences are correlated with delta(18)O Isotope Stages 100-58, and each 41 ka glacial/interglacial stage couplet is represented by an individ ual sequence comprising transgressive (TST), highstand (HST) and regre ssive (RST) systems tracts that can be readily related to generic sequ ence stratigraphic models. In general, deposition of each of the seque nces occurred during odd numbered (interglacial) isotope stages and th e sequence boundaries developed during the maxima of even numbered (gl acial) isotope stages. The key palaeomagnetic tiepoints used for pinni ng 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 Stag e 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 correlate d with the high amplitude positive (glacial) delta(18)O shift in Isoto pe Stage 100, which is considered to mark the initiation of major nort hern hemisphere continental glaciations at ca. 2.52 Ma. The overlying Hautawa Shellbed containing the subantarctic bivalve Chlamys delicatul a, defines the base of the Nukumaruan Stage, and represents the earlie st faunal evidence of climatic cooling in New Zealand Pliocene-Pleisto cene marine sequences. The sequence boundary at the base of Hautawa Sh ellbed 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 b oundary, as defined in the Vrica stratotype (astronomically calibrated age ca. 1.81 Ma), actually lies in the upper part of the Nukumaruan S tage. The Hautawa Shellbed, which lies closer to the Gauss/Matuyama pa laeomagnetic transition (2.60 Ma), is therefore much older than the cu rrently defined Pliocene-Pleistocene boundary. From a New Zealand pers pective, a more logical position for the Pliocene-Pleistocene boundary would be close to or just above the Gauss/Matuyama polarity transitio n. Such a position would take advantage of the boundaries proximity to a readily identifiable polarity reversal, and lie close to the time o f initiation and development of major northern hemisphere ice sheets. (C) 1997 INQUA/Elsevier Science Ltd.