Storm layer deposition on the Mississippi-Atchafalaya subaqueous delta generated by Hurricane Lili in 2002
Storm layer deposition on the Mississippi-Atchafalaya subaqueous delta generated by Hurricane Lili in 2002
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DOI：
10.1016/j.csr.2005.08.023
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发表时间：
2005-11-01
影响因子：
2.3
通讯作者：
Stone, GW
中科院分区：
文献类型：
--
作者：
Allison, MA;Sheremet, A;Stone, GW
阿查法拉亚内大陆架位于路易斯安那州近海的墨西哥湾中北部，是一个泥浆快速堆积的区域，与源自密西西比河支流的水下三角洲的进积有关。 2002年9月至10月，该地区受到相隔仅7天的两次热带气旋（热带风暴伊西多尔和飓风莉莉）的影响。沿海观测平台（WAVCIS 网络）的水柱和水动力记录与丽丽通过后 4-7 天的海底采样相结合，以检查这些事件对阿查法拉亚内陆架的影响。三角洲 CSI-3 平台（位于 4.5 m 水深）的风速在伊西多尔期间达到峰值 20 m/s，在距离更近、强度更大的 Lili 事件期间风速超过 30 m/s。丽丽期间，CSI-3 平台的显着波高峰值超过 2m，与约 2m 的风暴潮同时发生。尽管风向随风暴通过而变化，两次风暴期间的水柱流结构与风暴潮（海岸设置-沉降）周期密切相关。在渐盈阶段（莉莉为 1.5 天，伊西多尔为 4 天），整个水柱的水流都在岸上，在风暴过后（类似于 12 小时渐弱阶段），水流迅速（1-2 小时）逆转为离岸水流。在丽丽事件期间，ADCP 测量的整个水柱（距底部 > 65 厘米）的流速保持在 1 m/s 以上超过 2 天。沉积物岩心揭示了基底侵蚀面的存在，推测其代表了两次风暴联合再悬浮造成的海底紧缩，其上覆盖着 2-19 厘米厚的粉质粘土风暴沉积物。与风暴前后在两个三角洲站（水深 5 m）拍摄的 7 Be 海底剖面和 X 射线照片相比，这些站发生了 3-13 厘米和 7-17 厘米的侵蚀性收缩。上覆的物理分层风暴沉积物含有放射性同位素库存（Be-7、Th-234、(CS)-C-131、Pb-210），这些放射性同位素库存与主要来自风暴上蜡阶段重新悬浮的颗粒的再沉积的来源一致。 X射线照相术和粒度分析表明存在两相再沉积：初始的、正常分级的基底沉积物厚度为1-2厘米，含有可能是由于正常沉降而沉积的沙子，以及一个稍微正常分级的、贫沙的单元，假设是由于流体泥浆（> 10克/1）的固结而沉积的，在衰弱阶段后期阻碍了沉降悬浮液。风暴沉积物中的大型动物洞穴表明幸存动物群快速（数天）定居，可能是由于每年这个时候沉积物丰度较高，而埋藏率（来自阿查法拉亚河沉积物供应）和足以底部再悬浮的能量通常较低。 (c) 2005 Elsevier Ltd. 保留所有权利。
The Atchafalaya inner continental shelf, located along the north-central Gulf of Mexico offshore of Louisiana, is an area of rapid mud accumulation associated with the progradation of a subaqueous delta originating from this Mississippi River distributary. In September-October 2002, this region was impacted by two tropical cyclones (Tropical Storm Isidore and Hurricane Lili) separated by only 7 days. Water-column and hydrodynamic records from coastal observation platforms (WAVCIS network) are combined with seabed sampling 4-7 days after passage of Lili, to examine the impact of these events on the Atchafalaya inner shelf. Wind speeds at the CSI-3 platform on the delta (located in 4.5 m of water) peaked at 20 m/s during Isidore, and more than 30 m/s during the closer, and stronger, Lili event. Significant wave heights during Lili peaked at more than 2m at the CSI-3 platform, coincident with a storm surge of about 2m. Water-column flow structure during both storms was closely tied to the storm surge (coastal setup-setdown) cycle despite variations in wind direction with storm passage. Flow was onshore throughout the water column during the waxing phase (1.5 days in Lili, 4 days in Isidore), with a rapid (1-2 h) reversal to offshore flow after storm passage (similar to 12 h waning phase). Flow velocities remained above I m/s throughout the ADCP-measured water column (> 65 cm above the bottom) for more than 2 days during the Lili event. Sediment cores reveal the presence of a basal erosional surface, hypothesized to represent seabed deflation from the combined resuspension attributable to both storms, overlain by a silty clay storm deposit 2-19 cm thick. Comparison with 7 Be seabed profiles and X-radiographs taken at two delta stations (5 m water depth) prior to and following the storm suggests erosional deflation of 3-13 and 7-17cm occurred at these stations. The overlying, physically stratified storm deposit contains radioisotopic inventories (Be-7,Th-234, (CS)-C-131, Pb-210) that are consistent with an origin primarily from redeposition of particles resuspended in the waxing phase of the storm. X-radiography and granulometry suggest two-phase re-deposition: an initial, normally graded basal deposit 1-2 cm thick containing sand that likely was deposited from normal settling, and a slightly normally graded, sand-poor unit hypothesized to be deposited from consolidation of a fluid mud (> 10 g/1), hindered settling suspension later in the waning phase. Macrofaunal burrows in the storm deposit suggest rapid (days) settlement of surviving fauna, likely due to high abundance in the sediments at this time of year when burial rates (from Atchafalaya River sediment supply) and energies sufficient for bottom resuspension are normally low. (c) 2005 Elsevier Ltd. All rights reserved.