Late Pliocene (2.8 ‐2.4 Ma) cyclothemic shelf deposits, Parikino, Wanganui Basin, New Zealand: Lithostratigraphy and correlation of cycles

Abstract

The Wanganui River valley in the vicinity of Parikino in western Wanganui Basin, New Zealand, contains a late Pliocene, c. 400 m thick, southward dipping (c. 5°) cyclothemic marine succession. Thirteen sedimentary cycles or cyclothems occur within the succession. The lower two sandstone‐dominated cycles are assigned to the Paparangi Group, which includes a renamed formation (Pitangi Sandstone) and a newly defined formation (Moukuku). The overlying Okiwa Group is emended, its base being relocated to the base of the Wilkies Shellbed. This group contains seven new formations, in ascending order: Whauteihi, Whakaihuwaka, Tirotiro, Parikino, Whariki, Shaw, and Wickham Formations. Most formations contain one or more unconformity‐bounded cyclothems, which comprise a lower thin (0.2–1.5 m thick) coquina (shellbed) member, an overlying siltstone member, and an uppermost sandstone member. The distribution of each formation is presented on a new geological map for the area, and their lithologies are shown on stratigraphic columns. Grainsize analysis on a suite of (200) closely spaced sample sites highlights the sedimentary cyclicity and the gradational change from siltstone into sandstone in most cycles. Combined with the depth paleoecology implied by the macrofauna and benthic foraminifers, regular changes in water depth from mid‐shelf to shoreface paleoenvironments are inferred for each cycle. The cyclothemic succession is correlated with the late Pliocene part of the oxygen isotope stratigraphy for ODP site 846 (eastern equatorial Pacific), based on correlation of the Hautawa Shellbed with Stage 98. The lowermost cycle (1) in the Parikino section accumulated between the peaks of Stages Gil and G10, and the uppermost cycle (13) accumulated during Stages 92 and 93. The one‐to‐one match between cyclothems and the isotope stage couplets means that the cyclothems have the same frequency as the isotope curve, which, for the late Pliocene, corresponds to the dominant 41 000 yr Milankovitch orbital rhythm. It follows that the cyclicity results chiefly from repetitive glacio‐eustatic sea‐level changes. Significantly, the first cyclothem is aged c. 2.8 Ma, which precedes what is commonly regarded as the first major 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.

Keywords:

• cyclothems
• Wanganui Basin
• lithostratigraphy
• new stratigraphic names