Temporal and spatial complexity in post-glacial sedimentation on the tectonically active, Poverty Bay continental margin of New Zealand

摘要

On the eastern Raukumara Ranges of the New Zealand East Coast, active tectonics, vigorous weather systems, and human colonisation have combined to cause widespread erosion of the mudstone- and sandstone-dominated hinterland. The Waipaoa River sedimentary dispersal system is an example that has responded to environmental change, and is now New Zealand's second largest river in terms of suspended sediment discharge. This paper presents new sediment accumulation rates for the continental shelf and slope that span century to post-glacial time scales. These data are derived from radiochemical tracer, palynological, tephrostratigraphic, and seismic methods. We hypothesise on the temporal and spatial complexity of post-glacial sedimentation across the margin and identify the broad extent of sediment dispersal from the Waipaoa system. The similar to 15 km(3) Poverty Bay mid-shelf basin lies adjacent to the mouth of the Waipaoa River, reaching a maximum thickness of similar to 45 m. A post-glacial mud lobe of an additional similar to 3 km(3) extends through the Poverty Gap and out onto the uppermost slope, attaining 40 m thickness in a structurally controlled sub-basin. Here, an offset in the last-glacial erosion surface indicates that deposition was sympathetic with fault activity and the creation of accommodation space, implying that sedimentation was not supply limited. Contrary to classical shelf sedimentation models, the highest modern accumulation rate of 1 cm y(-1) occurs on the outer-shelf sediment lobe, approximately similar to 2 times the rate recorded at the mid-shelf basin depocentre, and similar to 10 times faster than the excess Pb-210 rates estimated from the slope. Pollen records from slope cores fingerprint Polynesian then European settlement, and broaden the spatial extent of post-settlement sedimentation initially documented from the Poverty Bay mid-shelf. Changes in sub-millennial sedimentation infer a 2-3-times increase in post-settlement accumulation on the shelf but a smaller 1-2 times increase on the slope. Over longer time scales, seismic evidence infers slower but steady sedimentation since the last transgression, and that significant cross-shelf sediment pathways pre-date the increase in sedimentation resulting from colonisation and deforestation. From a summation of coastal bedload, shelf and slope sediment mass accumulation, the total sediment budget for the Holocene is similar to 1 Mt y(-1). Under modern conditions a larger proportion of the Waipaoa sediment dispersal system likely extends onto the slope and beyond. (c) 2006 Elsevier Ltd. All rights reserved.