Hemipelagic and turbiditic deposits constrain lower Bengal Fan depositional history through Pleistocene climate, monsoon, and sea level transitions
Hemipelagic and turbiditic deposits constrain lower Bengal Fan depositional history through Pleistocene climate, monsoon, and sea level transitions
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DOI：
10.1016/j.quascirev.2018.09.027
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发表时间：
2018-11
影响因子：
4
通讯作者：
M. Weber;B. Reilly
中科院分区：
文献类型：
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作者：
M. Weber;B. Reilly
孟加拉扇包含了喜马拉雅气候和构造的最完整的记录。作为世界上最大的扇系统，它是由恒河-雅鲁藏布江系统提供的。国际海洋发现计划（IODP）第354次考察队在下孟加拉扇沿8°N东西向岩心样带沿着采集了7个岩心。我们研究了这些网站的更新世部分，以提供过去1.25马的沉积历史的限制。低分辨率的生物和磁性地层的限制确定了两个区域广泛的半远洋单位和夹层浊积岩沉积，提供了一个年代地层框架的320公里的断面。利用岩石地层学和沉积物物理性质在轨道时间尺度上的变化，我们将高分辨率数据集调整到LR 04海底δ 18 O同位素叠加，以获得沉积变化时间的新约束。在底层单元1（由早更新世至中更新世浊流沉积物组成）之上，我们将过去1.25 Ma的另外四个单元划分为：单元2，在海洋同位素阶段（MIS）37至1917的所有地点沉积的中更新世半远洋层（0.124 -0.68 Ma）在整个中更新世过渡时期，孟加拉扇沉积中心一定远离我们的样带;第3单元，主要为浊积岩，夹有一些半盆状沉积物，沉积时间为19916 - 19913年（0.68-0.48 Ma），仅在九洲岭西侧最东端发现桑迪岩性;第4单元，块状浊流沉积物，从中布鲁赫过渡期（MMIS 12）开始主导沉积，直到MIS 7/8（0.48-0.25 Ma），河道-堤防体系集中在85°E基底脊以东8°N处;第5单元为晚更新世半远洋层，其沉积时间跨度为MIS 7/8-1（距今0.25Ma），浊流沉积集中在85°E基底脊以西的8°N处。我们发现的证据表明，在这些时间尺度上，整个下孟加拉扇的沉积随着更新世气候，季风和海平面历史的演变而变化，因此响应于沉积物积累和风扇结构的外部控制，而不仅仅是风扇内部，自循环机制。
The Bengal Fan contains the most complete record of Himalayan climate and tectonics. As the largest fan system of the world it is fed by the Ganges-Brahmaputra river systems. International Ocean Discovery Program (IODP) Expedition 354 cored seven sites along an east-west core transect at 8°N in the lower Bengal Fan. We studied the Pleistocene sections of these sites to provide constraints on the depositional history of the last 1.25 Ma. Low-resolution bio- and magnetostratigraphic constraints identified two regionally-extensive hemipelagic units and intercalated turbidite deposits that provide a chronostratigraphic framework for the 320 km transect. Using a combination of lithostratigraphy and sediment physical properties that vary on orbital timescales, we tuned high-resolution data sets to the LR04 benthic δ18O isotope stack to obtain new constraints on the timing of depositional changes.Above the underlying Unit 1, which consists of Early to Middle Pleistocene turbiditic deposits, we divide four more units for the last 1.25 Ma: Unit 2, a Middle Pleistocene hemipelagic layer deposited at all sites from Marine Isotopic Stages (MIS) 37 to ∼17 (∼1.24–0.68 Ma) during the entire time of the Mid-Pleistocene Transition, when the Bengal Fan depocenter must have been distal to our transect; Unit 3, mostly turbidites with some intercalated hemipelgic sediments deposited from ∼MIS 16–∼13 (∼0.68–0.48 Ma), with sandy lithologies only found at the easternmost site on the west flank of the Ninetyeast Ridge; Unit 4, massive turbiditic sediments that started to dominate deposition with the Mid-Brunhes Transition (∼MIS 12) until MIS 7/8 (∼0.48–0.25 Ma), when the channel-levee system was focused east of the 85°E basement ridge at 8°N; and Unit 5, a Late Pleistocene hemipelagic layer at the top that spans MIS 7/8–1 (∼0.25 Ma to present) while the turbidite deposition was focused west of the 85°E basement ridge at 8°N. We find evidence that, on these timescales, deposition across the lower Bengal Fan changed with the evolution of Pleistocene climate, monsoon and sea-level history and hence responded to external controls on sediment accumulation and fan architecture rather than only fan-internal, autocyclic mechanisms.