PLIOCENE-PLEISTOCENE MARINE CYCLOTHEMS, WANGANUI BASIN, NEW-ZEALAND -A LITHOSTRATIGRAPHIC FRAMEWORK

The Rangitikei River valley between Mangaweka and Vinegar Hill and the surrounding Ohingaiti region in eastern Wanganui Basin contains a lat e Pliocene to early Pleistocene (c. 2.6-1.7 Ma), c. 1100 m thick, sout hward-dipping (4-9 degrees), marine cyclothemic succession. Twenty sed imentary cycles occur within the succession, each of which contains co arse-grained (siliciclastic sandstone and coquina) and fine-grained (s iliciclastic siltstone) units. Nineteen of the cycles are assigned to the Rangitikei Group (new). Six new formations are defined within the Rangitikei Group, and their distribution in the Ohingaiti region is re presented in a new geologic map. The new formations are named: Mangare re, Tikapu, Makohine, Orangipongo, Mangaonoho, and Vinegar Hill. Each formation comprises one or more cyclothems and includes a previously d escribed and named distinctive basal horizon. Discrete sandstones, sil tstones, and coquinas within formations are assigned member status and correspond to systems tracts in sequence stratigraphic nomenclature. The members provide the link between the new formational lithostratigr aphy and the sequence stratigraphy of the Rangitikei Group. Base of cy cle coquina members accumulated during episodes of sediment starvation associated with stratigraphic condensation on an open marine shelf du ring sea-level transgressions. Siltstone members accumulated in mid-sh elf environments (50-100 m water depth) during sea-level highstands, w hereas the overlying sandstone members are ascribed to inner shelf and shoreface environments (0-50 m water depth) and accumulated during fa lling eustatic sea-level conditions. Repetitive changes in water depth of 50-100 m magnitude are consistent with a glacio-eustatic origin fo r the cyclothems, which correspond to an interval of Earth history whe n successive glaciations in the Northern Hemisphere are known to have occurred. Moreover, the chronology of the Rangitikei River section ind icates that Rangitikei Group cyclothems accumulated during short durat ion, 41 ka cycles in continental ice volume attributed to the dominanc e of the Milankovitch obliquity orbital parameter. The Ohingaiti regio n has simple postdepositional structure. The late Pliocene formations dip generally to the SSW between 4 degrees and 9 degrees. Discernible discordances of c. 1 degrees between successively younger formations a re attributed to synsedimentary tilting of the shelf concomitant with migration of the tectonic hingeline southward into the basin. The outc rop distribution of the Rangitikei Group is strongly influenced by thi s regional tilt and also by three major northeast-southwest oriented, high-angle reverse faults (Rauoterangi, Pakihikura, and Rangitikei Fau lts).