Glaciations, interglaciations and reworked microfossils in Poukawa Basin, New Zealand

The Poukawa Basin on the eastern North Island of New Zealand was cored to a maximum depth of 234 m during investigations from 1976 to 1984, and again in 1997 as part of the New Zealand contribution to Pole Equator Pole Transect 2. The nonmarine sediments recovered were fossiliferous Quaternary silts and sands with some peat and marl layers. The common and diverse marine fossils present (molluscan fragments plus diverse microfossils) resemble assemblages in the Raukawa Mudstone, indicating that fossils in the cores have been reworked from this Upper Pliocene mudstone outcropping in nearby ranges. Variations in microfossil abundances and preservation indicate changes in erosion patterns, which in turn can be linked to climatic change. In core PEP 97-2, dominantly reworked siliceous microfossils with few calcareous specimens indicate that the interval 198–185 m (Marine Isotope Stage 6) was deposited slowly. Freshwater microfossils dominant from 185 to 117 m indicate a stable vegetated catchment in late MIS 6 and in MIS 5; high concentrations of diatoms indicate some wet periods in MIS 5. Microfossils in samples from 117 to 98 m (MIS 4 and beginning of MIS 3) indicate catchment erosion followed by stabilization. Reworked siliceous microfossils are again dominant from 98 to 9 m (MIS 3 and MIS 2). Good preservation of coccoliths from 98 to 50 m indicates rapid erosion and deposition. This may indicate rapid tectonic subsidence in MIS 4 and MIS 3, possibly associated with anticline growth. The top 9 m deposited during MIS 1 contains dominantly freshwater diatoms, which indicate a vegetated catchment and formation of a lake in the basin.