A paleontological assessment of stream sections near Matata, North Island, New Zealand

Abstract

In 2005 the stream sections to the south west of Matata, Bay of Plenty, New Zealand, were affected by large debris flows after a significant storm. These debris flows caused the exposure of excellent cliff outcrops of marine and non-marine units of Pleistocene age in the Ohinekoao and Awatarariki streams. These siliciclastic, volcaniclastic and volcanic deposits constitute part of the Castlecliffian age Matata Formation. These sections lie on the western edge of the Whakatane Graben, the most northerly portion of the Taupo Rift. Due to its position this region is spreading at the fastest rate (~15cm per year, Rowland et al., 2010) of anywhere in the rift, making it a prime location for studying paleoenvironments. The stratigraphy and lithology of the sequences were recorded in detail to aid both paleoenvironmental reconstruction and stratigraphic correlation between stream valleys. Overall, 18 sedimentary units, three tephras and three paleosols were identified. Moreover, X-ray diffraction, transmitted light microscopy, binocular microscopy and Malvern grain size analysis were undertaken on the lithologic units to refine interpretations. Some of the strata contain macrofossils (50 taxa) microfossils (15 taxa) and trace fossils (8 taxa) that were used in this study to assess the depositional environment of formation, which ranged from marine to brackish. A carbonate concretion horizon in mudstone from the Ohinekoao Stream was correlated to a Psilonichnus firmground (Glossifungites ichnofacies) in the Awatarariki Stream, which represents a depositional hiatus, possibly a sequence boundary. The two horizons are located in the same stratigraphic position and occur in mudstones rich in organic matter. The concretions were analysed using paleontology, mineralogy and stable isotope geochemistry to assess their formation conditions, and they were found to have formed by organic matter degradation. By combining the paleontological, sedimentological and mineralogical assessments it was possible to reconstruct paleoenvironmental conditions during the formation of each individual unit within the stream sections. Using these data it was then possible to correlate between them, which allowed for a local assessment in variability of depositional environments (e.g., shallowing vs. deepening, marine vs. nonmarine, etc.), as well as sediment sources. These could then be tied to volcanic and tectonic events taking place during the time of deposition to evaluate the relative roles of sea level change and tectonics in the uplift and subsidence cycles recorded by these strata.