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Investigating sediment transport on the Waipaoa margin: Linking in situ observations with preserved deposits.

机译：研究Waipaoa边缘的泥沙运移：将原位观测与保存的沉积物联系起来。

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Connecting the processes that control sediment transport to the deposits they leave behind is critical for understanding sedimentary system dynamics on modern timescales. Small mountainous river systems (SMRs) have been the focus of intense investigation for several decades, as they provide an ideal setting in which to study the conditions responsible for generating and preserving individual event deposits. In this study, bottom-boundary-layer tripods were deployed in three locations on the continental shelf seaward of the Waipaoa River (New Zealand), to monitor sediment and water movement from January 2010 to February 2011. Four research cruises were conducted at approximately four-month intervals during this time period to service and repair instrumentation, as well as to collect short (<50 cm) sediment cores at repeat locations across the shelf. Cores were examined for their textural and radiochemical properties as evidence of recent sediment deposition.;Typical of SMRs, water and sediment discharge from the Waipaoa River is episodic, with the potential for relatively major (>2000 m3 s-1) events to occur in any season. Sediment transport on the Waipaoa margin is similarly event driven, with the majority of near-bed sediment transport occurring in 36 discreet events (defined in this study by U *wc >0.03 m s-1) of <60 h duration. Events which result in a net aggradation of the seabed tend to have longer duration than erosive events (~100 h), though on an annual timescale this is still rapid. In general, short-duration events with relatively low-height, long-period waves result in seabed erosion on the mid-shelf. Longer-duration events with tall-height, short-period (i.e., steep) waves typically result in sediment deposition. Event duration is particularly important in the case of large floods of the river, as the associated sediment requires a minimum of ~120 hours of event-level shear velocity before being deposited on the mid-shelf.;When events (both discharge and U*wc) occur coincidentally, and are of sufficiently long duration, wave-supported fluid mud (WSFM) can form at the mid-shelf. Near-bed suspended-sediment concentrations (SSC) >50 g l-1 are observed in the near-bed layers (<26 cm above bed), which is well above the theoretical threshold for downslope travel under the influence of gravity. Gravity-driven velocity is slow at the mid-shelf due to the shallow slope, however material travelling from the mid-shelf under the influence of gravity alone can reach the shelf break in ~50 h during periods of highest SSC.;The deposit from a WSFM that occurred in early July, 2010, was plainly visible in x-radiographs of sediment cores collected at 17 locations in September 2010, and again at the same locations in February 2011. The deposit is identified as relatively low-bulk-density material, 1-12 cm thick, which tends to be physically stratified, especially at greater thicknesses. Using a newly developed method for estimating the degree of physical lamination, 10 locations show more surface variability in September than in February, while the opposite is true in 5 locations. Visual inspection of the cores agrees with these results, which demonstrate how biological mixing of event layers destroys strata, and prevents long-term event-bed preservation. This new method for using down-core bulk-density to analyze quantitatively sediment x-radiographs over a decadal-to-century scale faithfully reproduces the results of qualitative analyses performed in this location by previous studies.
机译：将控制沉积物传输的过程与它们留下的沉积物联系起来对于理解现代时间尺度上的沉积系统动力学至关重要。几十年来，山区山区河流系统（SMR）一直是深入研究的重点，因为它们为研究负责产生和保存单个事件沉积物的条件提供了理想的环境。在这项研究中，底部边界层三脚架被部署在怀帕阿河（新西兰）的大陆架海域的三个位置，以监测2010年1月至2011年2月的沉积物和水运动。在大约4个研究巡航中进行了4次在此时间间隔内每月间隔一次，以维修和修理仪器，并在整个架子的重复位置收集短（<50厘米）的沉积物芯。检查岩心的质地和放射化学特性，作为近期沉积物沉积的证据。；典型的SMR，Waipaoa河中的水和沉积物是偶发性的，可能发生较大的（> 2000 m3 s-1）事件。任何季节。 Waipaoa边缘的泥沙运移是类似的事件驱动，大部分近床沉积物运移发生在持续时间少于60 h的36个离散事件（在本研究中，U * wc> 0.03 m s-1定义）中。与侵蚀性事件相比，导致海床净沉积的事件持续时间长（〜100小时），尽管在每年的时间尺度上仍较快。通常，高度相对较低，较长周期的短时间事件会导致中层海床侵蚀。高高，短周期（即陡峭）波的持续时间较长的事件通常会导致沉积物沉积。在河水泛滥的情况下，事件持续时间尤为重要，因为相关的沉积物在沉积到中层之前至少需要约120小时的事件水平剪切速度; wc）巧合地发生，并且持续时间足够长，因此波状流体泥浆（WSFM）会在中层架上形成。在近床层（距床上方<26 cm）观察到近床悬浮沉积物浓度（SSC）> 50 g l-1，该浓度远高于重力影响下坡行驶的理论阈值。重力驱动的速度由于斜度较浅而在中层架上较慢，但是在最高SSC期间，仅在重力作用下从中层架运来的物料可以在约50小时内到达层架破裂处。于2010年7月上旬发生的WSFM，在2010年9月在17个地点以及2011年2月在相同地点收集的沉积物芯的X射线照相中清晰可见。该矿床被确定为相对低密度的物质厚度为1-12厘米，往往会在物理上分层，特别是在较大的厚度下。使用一种新开发的方法来估算物理层压程度，9月的10个位置比2月的表面变化更大，而5个位置则相反。岩心的目视检查与这些结果一致，表明了事件层的生物混合如何破坏地层，并防止长期的事件床保存。这种使用下堆堆积密度在十年到世纪规模上定量分析沉积X射线照相仪的新方法，可以忠实地再现以前研究在此位置进行的定性分析结果。

著录项

  • 作者

    Hale, Richard Prescott.;

  • 作者单位

    University of Washington.;

  • 授予单位 University of Washington.;
  • 学科 Marine Geology.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 132 p.
  • 总页数 132
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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