LATE PLIOCENE (2.8-2.4 MA) CYCLOTHEMIC SHELF DEPOSITS, PARIKINO, WANGANUI BASIN, NEW-ZEALAND - LITHOSTRATIGRAPHY AND CORRELATION OF CYCLES

The Wanganui River valley in the vicinity of Parikino in western Wanga nui Basin, New Zealand, contains a late Pliocene, c. 400 m thick, sout hward dipping (c. 5 degrees) cyclothemic marine succession. Thirteen s edimentary cycles or cyclothems occur within the succession. The lower two sandstone-dominated cycles are assigned to the Paparangi Group, w hich includes a renamed formation (Pitangi Sandstone) and a newly defi ned formation (Moukuku). The overlying Okiwa Group is emended, its bas e being relocated to the base of the Wilkies Shellbed. This group cont ains seven new formations, in ascending order: Whauteihi, Whakaihuwaka , Tirotiro, Parikino, Whariki, Shaw, and Wickham Formations. Most form ations contain one or more unconformity-bounded cyclothems, which comp rise a lower thin (0.2-1.5 m thick) coquina (shellbed) member, an over lying siltstone member, and an uppermost sandstone member. The distrib ution of each formation is presented on a new geological map for the a rea, and their lithologies are shown on stratigraphic columns. Grainsi ze analysis on a suite of (200) closely spaced sample sites highlights the sedimentary cyclicity and the gradational change from siltstone i nto sandstone in most cycles. Combined with the depth paleoecology imp lied by the macrofauna and benthic foraminifers, regular changes in wa ter depth from mid-shelf to shoreface paleoenvironments are inferred f or each cycle. The cyclothemic succession is correlated with the late Pliocene part of the oxygen isotope stratigraphy for ODP site 846 (eas tern equatorial Pacific), based on correlation of the Hautawa Shellbed with Stage 98. The lowermost cycle (1) in the Parikino section accumu lated between the peaks of Stages G11 and G10, and the uppermost cycle (13) accumulated during Stages 92 and 93. The one-to-one match betwee n cyclothems and the isotope stage couplets means that the cyclothems have the same frequency as the isotope curve, which, for the late Plio cene, corresponds to the dominant 41 000 yr Milankovitch orbital rhyth m. It follows that the cyclicity results chiefly from repetitive glaci oeustatic sea-level changes. Significantly, the first cyclothem is age d c. 2.8 Ma, which precedes what is commonly regarded as the first maj or Northern Hemisphere continental glaciation (Stage 100, 2.54 Ma) by 0.26 m.y., but coincides with the first occurrence of significant ice- rafted debris in core from the Arctic Barents Sea.