Sedimentation on a High Input Continental Shelf at the Active Hikurangi Margin, Poverty Bay, New Zealand

Abstract

Only in recent years has it been fully appreciated that small mountainous rivers play a major role in global sediment budgets. The MARGINS Source-to-Sink initiative aims to gain a comprehensive understanding of such systems by detailed, multidisciplinary study of the Waipaoa River sedimentary system, which is among the world's muddiest river systems. The Waipaoa shelf adjacent to Poverty Bay, New Zealand, lies on the tectonically active Hikurangi margin. Geophysical and sedimentary datasets are used to provide a new, comprehensive portrayal of the modern sedimentary framework. Complexity in the pattern of seafloor sediments is attributed to the interplay between a very high, but episodic sediment supply, and the local tectonic and hydraulic regimes. This complexity is clarified by the recognition of multimodal sediments, representing the variable mixing of several simple sedimentary components. Outer-shelf anticlines provide a partial structural wall for subsiding mid-shelf mud depocentres, and also act as a source area for palimpsest mid-shelf sands. Active uplift of these structures has preserved relict gravels, which were originally deposited on the Waipaoa River's lower floodplain during the last lowstand of sealevel. Broad patterns within the sedimentary framework have been consistent in historic times, even in the face of catastrophic inundations of sediment, exemplified by Cyclone Bola. Despite a moderate to high energy wave climate, sediment delivery by the Waipaoa River exceeds the redistributive power of shoaling waves. On the open shelf through the Poverty Gap, which is exposed to prevailing south to southeast ocean swell, a seaward-fining textural trend exists. However, it is not a true equilibrium profile, with muds residing further inshore than would be expected on lower input shelves with comparable wave climates. Fairweather circulation is dominated by the Wairarapa Coastal Current and tides. Net northward flows lead to the dispersal of Waipaoa River hypopycnal plumes beyond the northern Waipaoa shelf, and the introduction of suspended sediment from Hawke Bay. Fairweather wave influence is generally limited to the innermost-shelf. The broad patterns in shelf sedimentary facies suggest that southerly storm-related swell and wind drift-reinforced currents, particularly characteristic of El Nino winters, are the dominant drivers of sedimentation beyond the inner-shelf. Such conditions cause major mud resuspension, and are capable of transporting medium sand on the mid-shelf.