Modelling tidal current-induced bed shear stress and palaeocirculation in an epicontinental seaway: the Bohemian Cretaceous Basin, Central Europe
Modelling tidal current-induced bed shear stress and palaeocirculation in an epicontinental seaway: the Bohemian Cretaceous Basin, Central Europe
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DOI：
10.1111/j.1365-3091.2009.01082.x
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发表时间：
2010-02-01
期刊：
影响因子：
3.5
通讯作者：
Pain, Christopher C.
中科院分区：
文献类型：
--
作者：
Mitchell, Andrew J.;Ulicny, David;Pain, Christopher C.
波希米亚白垩纪盆地（中欧）内的中土兰阶沉积物由粗粒三角洲砂岩组成，这些砂岩向远端延伸到细粒近海沉积物中。叠加在三角洲前缘斜形上的沙丘级交错层表明，盆地古循环旺盛，能够将粗粒沙输送到斜形的整个深度范围（约 35 m）。双向、沿岸导向、槽交叉轴、淤泥覆盖层和再激活表面表明潮汐活动。然而，此时的波希米亚白垩纪盆地距离陆架断裂处有一千多公里，并被一系列小岛屿与公海隔开。在同振荡潮汐可能被海底摩擦抑制并被新兴陆地块阻挡的环境中，受潮汐影响的沉积物的存在是有问题的。帝国理工学院海洋模型是一种完全流体动力学、非结构化网格有限元模型，用于检验这一假设：该孤立区域的潮汐环流能够生成观测到的颗粒尺寸分布、河床形态类型和古水流方向。该模型首先针对当今不列颠群岛周围的北欧陆架海的床剪应力大小和沉积物输送路径的预测进行了验证。该模型通过一系列敏感性测试（当地海湾潮差升高）预测了波西米亚白垩纪盆地的微潮至中潮态。与海峡相关的漏斗会增加潮汐流速度，产生能够形成现场观察到的沉积结构的床剪应力。该模型还预测瞬时双向电流，其方向与现场测量的方向相当。总体而言，帝国理工学院海洋模型预测波希米亚白垩纪盆地内存在着强烈的潮汐驱动的古环流，这无疑会影响沉积物的扩散和相分布。然而，古水流载体和沉积物迁移途径在不同的敏感性测试中存在显着差异。在这种情况下，对这些参数进行精确建模需要比从现有岩石记录中提取的古地理确定性更大的确定性。
Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse-grained deltaic sandstones passing distally into fine-grained offshore sediments. Dune-scale cross-beds superimposed on delta-front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse-grained sand across the entire depth range of the clinoforms (ca 35 m). Bi-directional, alongshore-oriented, trough cross-set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally-influenced deposits in a setting where co-oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain-size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present-day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi-directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide-driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.