Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example
Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example
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DOI：
10.1111/j.1365-3091.2011.01295.x
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发表时间：
2012-04-01
期刊：
影响因子：
3.5
通讯作者：
Spiske, Michaela
中科院分区：
文献类型：
--
作者：
Bahlburg, Heinrich;Spiske, Michaela
陆上海啸沉积物可由流入和回流沉积物组成。颗粒大小从粘土到直径几米的巨石不等。粒度分布反映了沉积方式，并可用于探索水动力条件的运输。由于海啸沉积物缺乏独特的沉积特征，因此在某些情况下很难区分流入沉积物和回流沉积物。2010年2月27日的海啸在智利中部的Mocha岛留下了特征多变的流入和回流沉积物。海啸流入将沙子、砾石和巨石带到了上海岸、海滩和沿海阶地。重达12吨的巨石被运到内陆300米、海拔13米的地方。在内陆600米和海平面以上19米的地方，发现了粗砂的薄层。回流将流入时沉积的大部分砂砾重新动员起来。侵蚀构造的方向表明，在回流过程中也带走了大量泥沙。反流沉积的一个主要特征是在阶地阶的下游发育了广泛的粗粒沉积物推进扇。扇状沉积多为无结构沉积，但包括交错层理、瓦叠和波纹沉积，表明是由层载牵引流沉积的。沉积物分选差，粒度介于中粗砂到砾石和卵石之间。对这种混合物质回流输运条件的评估表明，劳斯数>2.5处的床质输运是由超临界流实现的，而沉积则发生在低梯度低海岸平原上的水流充分减速时。2010年2月Mocha岛海啸的沉积记录中，回流沉积物占90%以上。由于缺乏沉积构造，前人对现代海啸沉积物的研究发现，大部分碎屑是在入流过程中沉积的。这项研究表明，如果不加批判地使用这一假设，如果不知情地将其应用于回流沉积，可能会导致对古海啸震级和沉积过程的错误解释。
Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain-size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea-level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea-level. Backflow re-mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross-bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2.5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low-gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits.