Post-Last Glacial Maximum fluvial incision and sediment generation in the unglaciated Waipaoa catchment, North Island, New Zealand

摘要

Small river systems contribute a significant component of sediment delivered to oceans, but the temporal evolution of fluvially eroded landscapes is needed. A sequence of postglacial terraces in the unglaciated Waipaoa River catchment provides the opportunity to document fluvial incision and sediment flux on an -2000-year timescale since the Last Glacial Maximum (LGM), which has previously only been undertaken for the entire post-LGM period. This study also calculates sediment mass, where previously sediment volume was calculated. Using a 15-m DEM, field mapping and surveying, and tephrochronology, we calculate rates of fluvial incision and sediment volumes excavated during successive age-constrained, postglacial, incision events and correlate these with a framework of inferred climatic events established for New Zealand. We identify seven periods of terrace formation each succeeded by a period of fluvial incision, six in total. Although the magnitude of the response during each incision event and thus the sediment volumes generated varied through time and across subcatchments draining two contrasting lithological terrains, we conclude that incision events were essentially synchronous, at least within the timeframe constrained by the ca. 2000 year interval between successive eruptive airfall events. Slope relaxation processes were simultaneous with incision thereby indicating that both processes were likely climate drivea We identify a period of accelerated fluvial incision -7 mm y~(-1) commencing before ca. 14.0 cal. ka BP (during the early postglacial period) and ceasing ca. 7.9 cal. ka BP toward the end of the Early Holocene Warming period. The magnitude of this incision response was significantly higher in subcatchments draining highly erodible lithologies in the higher uplifting parts of the catchment when river bedload was at over capacity. In contrast, within the remainder of subcatchments draining the more resistant lithologies and in areas of lower uplift (and in parts subsiding), incision and sediment generation was moderated by the presence of knickpoints. Overall, since abandonment of the LGM to present day, fluvial incision in the Waipaoa and the adjacent Waimata catchments generated ～16.7 km~3 of sediment of which -10 km~3 (～90% of the estimated 35 Mt of glacial-postglacial slope and shelf sediment mass) was potentially available for transport offshore. Of this, 14.08 km~3 (7.4 km~3 derived from 'upper' and 6.7 km~3 from 'remainder' of subcatchments) was excavated from Waipaoa catchment at an average of-0.6 km~3 ka~(-1) of which ～80% was generated by ca. 7.9 cal. ka BP. This potentially validates previous accounts of high rates of offshore sediment flux before 8000 ~(14)C YBP (ca. 8877 cal. YBP). Thereafter, for the period mid-Holocene cooling and variability (MHCV) (ca. 6.5 cal. ka BP) until the present day, the rate of incision across all subcatchments slowed to ～2 mm y~(-1) and generated just ～20% of the total sediment volume. In part, this reflected a depletion of available sediment as rivers in the upper subcatchments returned to a steady state and, coincidental with an increase in accommodation space in the rapidly growing coastal floodplain, sediment flux to the marine depocentres was thereby limited.