Episodic postglacial deltaic pulses in the Gulf of Cadiz: Implications for the development of a transgressive shelf and driving environmental conditions

IF 2 4区 地球科学 Q1 GEOLOGY
Álvaro Carrión-Torrente, F. Lobo, Á. Puga-Bernabéu, I. Mendes, S. Lebreiro, Marga García, D. Van Rooij, M. Luján, M. I. Reguera, L. Antón
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引用次数: 2

Abstract

The postglacial sea-level rise after the Last Glacial Maximum provided ideal conditions to study the transgressive sedimentary response to sudden shelf flooding driven by different rates of sea-level rise. In this study, a high-resolution seismic stratigraphic interpretation and sedimentological analysis were conducted on data from the northern Gulf of Cadiz continental shelf (SW Iberian Peninsula), in order to: 1) understand the succession of sedimentary processes during each shelf flooding episode and 2) explore the significance of variable rates of sea-level rise, sediment fluxes, and climatic conditions on the development of postglacial deposits. Four backstepping seismic postglacial transgressive units (PTUs; 4 to 1 from oldest to youngest) that are linked to the retreating mouth of the Guadiana River were interpreted. Together, these seismic units display a wedge-shape geometry, are located over the inner to middle shelf, and overlie a regional unconformity formed during the Last Glacial Maximum. Each PTU can be divided into several sub-units with distinctive seismic facies that have a similar stratigraphic organization. Each PTU contains lower sub-units that are composed of low-angle tangential-oblique clinoforms. The clinoforms are locally topped by a channelized sub-unit. The distal and/or lateral parts of the clinoforms are occasionally buried by sheet-like semitransparent subunits. The uppermost sub-units are present over the proximal and central parts of each seismic unit and are also sheet-like. PTUs can also be subdivided and described sedimentologically. Fine-grained sands with intercalated silty layers dominate the lower part of each PTU (lower clinoform sub-units). The upper part of each PTU (upper sheet-like sub-units) is characterized by reworked facies, composed of highly fragmented bioclasts within a mixture of silt and coarse to medium sand. Finally, mud deposits occur as a sediment drape over the PTUs. The internal structure of each PTU reveals several phases of development under a general process of transgressive submergence in which both coastal and marine deposits were formed and eventually preserved. The initial phase involved the development of coarse-grained deltas in shallow water, which were locally eroded by a network of distributary channels. In a transitional phase, the infilling of distributary channels and the offshore export of fine-grained sediments is related to a change in sediment sources, possibly triggered by enhanced hydrodynamic processes. The final phase involved the reworking of fluvio-deltaic sediments by shoreface processes to generate a sediment sheet. Age correlation with a suite of postglacial sea-level curves indicates that the formation of the postglacial transgressive deposits is bracketed between 14 ka and 9 ka. The studied deposits are related to a period of reduced sea-level rise, culminating in the Younger Dryas event (two oldest PTUs), and to phases of enhanced sea-level rise, such as Meltwater Pulse (MWP) 1B (two youngest PTUs). In spite of high rates of sea-level rise over MWP-1B, each PTU exhibits progradation and preservation of much of the delta. The preservation of progradational deltaic units is likely caused by increased sediment supply during progradational pulses. We suggest that those pulses of enhanced sediment fluxes during MWP-1B were strongly driven by the overall climatic conditions in the southwest of the Iberian Peninsula, probably resulting from enhanced rainfall runoff during humid periods and scarce land vegetation cover.
加的斯湾的幕式冰后三角洲脉冲:对海侵陆架发展和驱动环境条件的影响
末次盛冰期后的冰后海平面上升为研究不同海平面上升速率下陆架突发性洪水的海侵沉积响应提供了理想条件。本文对加的斯湾北部大陆架(伊比利亚半岛西南部)的数据进行了高分辨率地震地层解释和沉积学分析,以期:1)了解每次大陆架洪水期间沉积过程的演替过程;2)探索海平面上升速率、沉积物通量和气候条件变化对冰后沉积物发育的意义。4个退步地震冰后海侵单元;4比1(从最老到最年轻),与瓜迪亚纳河口的退缩有关。总的来说,这些地震单元呈现楔形几何形状,位于陆架内侧至中部,并位于末次盛冰期形成的区域不整合之上。每个PTU可划分为几个亚单元,具有不同的地震相,具有相似的地层组织。每个PTU包含由低角度切斜斜斜形组成的较低亚单元。斜形岩的局部顶部是一个水道化的亚单元。斜形的远端和/或外侧部分偶尔被片状半透明亚基埋没。最上面的亚单元出现在每个地震单元的近端和中心部分,也是片状的。ptu也可以在沉积学上进行细分和描述。下斜形亚单元下部主要为细粒砂质夹层。每个PTU(上片状亚单元)的上部以改造相为特征,由高度破碎的生物碎屑组成,混合了粉砂和粗砂至中砂。最后,泥浆沉积作为沉积物覆盖在ptu上。每个PTU的内部结构显示了在海侵淹没的一般过程下的几个发展阶段,在这个过程中海岸和海相沉积物形成并最终被保存下来。最初阶段是在浅水区发育粗粒三角洲,这些三角洲局部受到分流河道网络的侵蚀。在过渡阶段,分流河道的充填和细粒沉积物的外泄与沉积物来源的变化有关,可能是由水动力过程的增强引起的。最后一个阶段是河流三角洲沉积物在滨面作用下的再加工,形成沉积板。与一系列冰期后海平面曲线的年龄对比表明,冰期后海侵沉积的形成时间介于14 ~ 9 ka之间。所研究的沉积物与海平面上升减少的时期有关,最终在新仙女木事件(两个最古老的ptu),以及海平面上升增强的阶段,如融水脉冲(MWP) 1B(两个最年轻的ptu)。尽管MWP-1B的海平面上升速度很高,但每个PTU都显示出大部分三角洲的退化和保存。前积三角洲单元的保存可能是由于前积脉冲期间泥沙供应增加所致。我们认为,MWP-1B期间泥沙通量增强的脉冲受到伊比利亚半岛西南部整体气候条件的强烈驱动,可能是由于湿润期降雨径流增加和土地植被覆盖稀缺造成的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
CiteScore
3.80
自引率
5.00%
发文量
50
审稿时长
3 months
期刊介绍: The journal is broad and international in scope and welcomes contributions that further the fundamental understanding of sedimentary processes, the origin of sedimentary deposits, the workings of sedimentary systems, and the records of earth history contained within sedimentary rocks.
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