Controls on clastic sequence geometries in a shallow‐marine, transtensional basin: the Bohemian Cretaceous Basin, Czech Republic
Controls on clastic sequence geometries in a shallow‐marine, transtensional basin: the Bohemian Cretaceous Basin, Czech Republic
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对浅海海相扭张盆地碎屑层序几何形状的控制：捷克共和国波希米亚白垩纪盆地
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发表时间：
2009
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通讯作者：
S. Čech
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作者：
D. Uličný;J. Laurin;S. Čech
波希米亚白垩纪盆地结合了全球海侵期间形成的浅水（大部分<100 m）陆缘海道和构造活动的张扭背景的特征。 该盆地形成于温室气候条件下，并受到强烈轴向流的影响。密集的测井覆盖面，结合当地高质量的暴露和生物地层控制，使得有可能在三维空间中检查遗传序列的几何形状并解释其控制变量。砂质三角洲在几个空间尺度上形成层序，反映了嵌套的海进-海退旋回，持续时间从数万年到数百万年不等。各层序的剥蚀方向和距离、厚度和内部几何形状主要受盆地内断层、盆地尺度沉降速率变化、海平面波动和盆地连续充填引起的局部水深变化的控制。来自源区不同部分的沉积物输入的沿着走向变化和次级碎屑源的短暂抬升提供了对层序几何形状的额外控制。有效的低密度输运与浅水中细碎屑的再沉积相结合，促进了以砂为主的三角洲陡坡的发育，同时防止了泥质斜坡的下超;大部分悬浮物在近水平或轻微倾斜的底流中沉积下来。长期容纳率在早期至中期Turonian期间较低，有轻微的盆内断层，但在晚期Turonian和早期Coniacian加速。这种加速至少部分是由于沉降速率增加，伴随着沉积中心的结构分区，部分由增加的沉积物输入，表明在西苏台德岛源区的上升速率增加补偿。这一事件可能反映了中欧区域应变率的增加。Turonian早期和早期Coniacian晚期的两个主要洪水事件的连续性，在Turonian中期被低可容纳间隔隔开，显示出与已发表的长期海平面变化曲线的估计非常相似。然而，在波希米亚晚期Turonian和Coniacian的可容纳速率的海平面升降分量是很难分开的加速沉降。在某些情况下，短期（100 kyr尺度）强迫海退的证据，独立于盆地的结构活动，表明小规模的海平面福尔斯下降的速度，如目前所理解的，不能解释的冰川海平面升降机制以外。
The Bohemian Cretaceous Basin combines features of a shallow‐water (mostly < 100 m) epicontinental seaway formed during a global transgression with those of a tectonically active, transtensional setting. The basin formed under a greenhouse climate and was affected by strong axial currents. Dense well‐log coverage, combined with locally high‐quality exposures and biostratigraphic control, make it possible to examine in three dimensions the geometries of genetic sequences and interpret their controlling variables. Sand‐dominated deltas formed sequences at several spatial scales that reflect nested transgressive–regressive cycles with durations ranging from tens of thousands of years to millions of years. Progradation directions and distances, thicknesses and internal geometry of the individual sequences were controlled primarily by intrabasinal faulting, basin‐scale changes in subsidence rate, eustatic fluctuations and localized bathymetric changes due to successive filling of the basin. Along‐strike change in sediment input from different parts of the source area and a short‐lived uplift of a secondary clastic source provided additional controls on the sequence geometry. Efficient hypopycnal transport combined with redeposition of fine clastics in shallow water promoted development of steep slopes of sand‐dominated deltas while preventing downlap of muddy clinoforms; most of the suspended load became deposited downcurrent in subhorizontal or gently dipping bottomsets. Long‐term accommodation rates were low during the Early to Middle Turonian, with minor intrabasinal faulting, but became accelerated in the Late Turonian and Early Coniacian. This acceleration was caused at least partly by increased subsidence rate accompanied by structural partitioning of the depocentre and partly compensated by increased sediment input indicating increased uplift rates in the Western Sudetic Island source area. This event probably reflected an increase in the regional strain rate in Central Europe. The succession of two major flooding events in the Early Turonian and late Early Coniacian, separated by a low‐accommodation interval in the Middle Turonian, shows a close similarity to published estimates of long‐term eustatic curves. However, the eustatic component of accommodation rate in the Bohemian Late Turonian and Coniacian is difficult to separate from accelerated subsidence. In several cases, evidence for short‐term (100 kyr scale) forced regressions, independent of basinal structural activity, suggests small‐scale eustatic falls at rates which, as presently understood, cannot be explained other than by a glacio‐eustatic mechanism.