A base-level stratigraphic approach to determining Holocene subsidence of the Ganges–Meghna–Brahmaputra Delta plain
A base-level stratigraphic approach to determining Holocene subsidence of the Ganges–Meghna–Brahmaputra Delta plain
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DOI：
10.1016/j.epsl.2018.07.008
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发表时间：
2018-10
影响因子：
5.3
通讯作者：
C. Grall;M. Steckler;J. Pickering;S. Goodbred;R. Sincavage;C. Paola;S. H. Akhter;V. Spiess
中科院分区：
文献类型：
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作者：
C. Grall;M. Steckler;J. Pickering;S. Goodbred;R. Sincavage;C. Paola;S. H. Akhter;V. Spiess
相对海平面历史是地面沉降、泥沙供给和绝对海平面历史共同作用的结果，可以由保留的沉积物厚度恢复。然而，不同沉积环境之间保存的沉积物厚度的差异极大地限制了这种地质方法的准确性，特别是在河流沟道化体系中，如三角洲平原。为了解决这一问题，我们对恒河-雅鲁藏布江-梅格纳三角洲(GBMD)采用了三种不同和独立的地层学方法。我们的方法是通过我们收集的广泛的地质档案数据集实现的，其中包括400多口手工钻探的地层井、198个放射性碳测年和河流地震反射数据(255公里高分辨率多道地震图像)。假定三角洲接近或处于基准面，即相对海平面，估算了下沉三角洲平原的向海可容梯度和极限。首先，对保存下来的沉积物厚度的变异性进行了统计分析，得出了全新世等厚地层的平均可容纳模式。其次，根据地貌构造域对保存的沉积物厚度进行分析，以估计平均可容纳模式。第三，重建了地震成像的雅鲁藏布江末次冰川切割的埋藏史。结果表明，在同一相对海平面历史上，位于河道和泛滥平原之间的三角洲平原，保存的沉积物厚度变异性可达35%。考虑到这种变异性，确定了GBMD的全新世相对海平面历史和最可能的下沉模式。研究结果提供了三角洲全新世适度下沉的证据，从可被认为是下沉三角洲平原北缘的铰链带向陆地的0.2 mm/年，向海逐渐增加，中游河流三角洲为2±0.7 mm/年，低潮三角洲为4±1.4 mm/年。这使我们能够在GBMD上构建第一张千年尺度的下沉模式地图，其中提供了下沉速度的不确定性。这张地图可能有助于评估人为改变可能对大堡礁的下沉和相对海平面产生的负面影响，从而有助于确定大堡礁和其他大型三角洲平原更好的可持续海岸管理做法。
Relative sea level history, which is the result of the combined effects of land subsidence, sediment supply and absolute sea level history may be reconstructed from preserved sediment thicknesses. However, variations in the preserved sediment thicknesses between different sedimentary environments strongly limit the accuracy of this type of geological approach, particularly in fluvial channelized systems, such as delta plains. To address this, we apply three different and independent stratigraphic approaches to the case of the Ganges–Brahmaputra–Meghna Delta (GBMD). Our approach has been made possible by a broad dataset of geological archives we have collected, which includes more than 400 hand-drilled stratigraphic wells, 198 radiocarbon ages, and river seismic reflection data (255 km of high-resolution multichannel seismic images). The seaward gradient of accommodation and the limit of the subsiding delta plain are estimated, assuming that the delta is near or at the base-level, which is considered to be the relative sea-level. First, a statistical analysis of the variability of preserved sediment thicknesses is used to derive the average pattern of accommodation from the Holocene isopach. Secondly, the preserved sediment thicknesses are analyzed by geomorphotectonic domains to estimate an average pattern of accommodation. Thirdly, the burial history of the seismically imaged last glacial incision of the Brahmaputra River is reconstructed. Results suggest that the variability of preserved sediment thicknesses can be up to 35% in a delta plain between river channel and flood plain deposits for the same relative sea-level history. Taking this variability into consideration, the Holocene relative sea-level history of the GBMD and the most likely pattern of subsidence are determined. Results provide evidence of moderate Holocene subsidence over the delta, gently increasing seaward from <0.2 mm/yr landward of the Hinge Zone, which can be considered as the northern limit of the subsiding delta plain, to 2 ± 0.7 mm/yr in the middle fluvial delta to 4 ± 1.4 mm/yr in the lower tidal delta. This enables us to construct the first millennial-scale map of subsidence pattern on the GBMD in which uncertainties on subsidence rates are provided. This map may aid in evaluating the negative impact that human modification may have on subsidence and relative sea level in the GBMD, and thereby help to determine better sustainable coastal management practices for the GBMD and other large delta plains.