Whanganui Basin, an archive of prehistoric earthquakes? Waitapu Shell Conglomerate (c. 0.9 Ma), North Island, New Zealand

Abstract

ABSTRACTThe Waitapu Shell Conglomerate is an important marker horizon in the eastern Whanganui Basin, occurring within a Pleistocene volcaniclastic record that contains early eruption products from the Taupo Volcanic Zone. The unit comprises a cross-bedded pebbly-shell conglomerate containing the first influx of Kaukatea Pumice (c. 0.9 Ma) within the Rangitikei succession. We document soft sediment deformation structures that occur in close stratigraphic proximity to the Waitapu Shell Conglomerate and other laterally equivalent units within the basins Castlecliffian outcrop belt. Soft sediment deformation structures formed through a combination of liquefaction and fluidisation, triggered by a range of mechanisms, including evidence of high sedimentation rates, loading, slope instability and potential for wave and earthquake-induced seismicity. Lateral changes in depositional style toward the basins eastern margin relate to relative position on the paleo-shelf, reduction of accommodation space, intermittent preservation of low stand deposits and proximity to the uplifting paleo-axial range.