Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor
Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor
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美国大西洋中部陆架障壁岛演化第二部分：来自陆架底板的证据
DOI：
10.1016/0025-3227(85)90090-8
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发表时间：
1985
期刊：
影响因子：
--
通讯作者：
G. Freeland
中科院分区：
文献类型：
--
作者：
A. Niedoroda;D. Swift;A. Figueiredo;G. Freeland
在大西洋沿岸坡度较低且基材未固结的地方，岸面过程导致平衡的岸面表面，其特征是平面视图拉直，轮廓凹陷。在冰期后海平面上升期间，这一表面的维持可能会导致大陆海滩分离形成屏障，其中外洋的海岸面是由波浪和洋流作用维持的。下涌的风暴流从大西洋中部的海岸表面扫过沙子，将其沿海岸和向海方向移动，并将其沉积在邻近的内大陆架上。在晴朗的天气里，这些沙子中的一些是由不对称波轨道流的作用返回的。然而，大多数地区都有净损失。河沙的流入并不能弥补这一损失，因为在大西洋沿岸，所有的河沙都被河口截留了。因此，这些地区的海岸正在经历侵蚀性退缩。平均水平退缩率约为1-3米/年，但大多数移动发生在短暂的事件中，复发率约为每世纪几次。在海平面上升的全新世时期，侵蚀性的海岸退缩，加上风沙冲积、风暴冲刷和沙子通过入口进入泻湖的运动，导致大西洋屏障向陆地迁移。整个屏障向陆地的后退既取决于侵蚀性的海岸后退，也取决于入口的形成。反复的进水口破口和进水口的下漂迁移在泻湖内形成了汇合的洪水潮汐三角洲。由此产生的表面形成了一个平台，在风暴冲刷和风积作用的推动下，陆上屏障沉积物可以在其上前进。后屏障沉积物最终在海岸表面重新出现，在那里它们的上层被侵蚀和循环。因此，屏障在其自身的洪水潮汐三角洲和冲积扇的路面上迁移；沙的加入部分是由于上游海岬的侵蚀，但主要是由于入口的冲刷，这种冲刷一直延伸到更新世下部。现代陆架沙层是退缩过程中形成的碎屑层。前缘在海岸表面形成一层薄薄的裂口流沉降物；它周期性地被冬季风暴剥离，露出下面的后屏障地层。进一步向海，陆架砂从海岸面被侵蚀，不整合地沉积在屏障沉积物上；海岸、海滩和沙丘地层总是缺失的。在最近的过去，这个陆架沙板上的每一粒沙子都在海滩或上海岸上占据了一席之地。然而，这张表作为一个整体已经重组；它的主要结构是内部的架子层。以沟槽和向陆地倾斜的反射物为特征的后屏障砂和泥沉积，可以在现代沙层下方的大西洋大陆架中部向海追踪100多公里。后屏障层与上覆的陆架砂层共同构成了全新世大西洋陆架表面屏障退缩的记录。该记录的薄片状性质表明，在适当的时间尺度上观察，屏障迁移是一个连续的过程。
Where the Atlantic coastal gradient is low and the substrate is unconsolidated, shoreface processes lead to an equilibrium shoreface surface, characterized by a straightened plan view and a concave-up profile. Maintenance of this surface during the post-glacial sea-level rise may lead to barrier formation by mainland beach detachment, where the outer oceanic shoreface is maintained by wave and current processes.Downwelling storm currents sweep sand from the shoreface of the middle Atlantic bight, transport it downcoast and seaward and deposit it on the adjacent inner shelf. Some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather. However, there is a net loss in most areas. The loss is not made good by river sand input, since on the Atlantic Coast all river sand is trapped by estuaries. As a result the shoreface in these areas is undergoing erosional retreat. Mean horizontal retreat rates are on the order of 1–3 m yr−1but most movement occurs in brief episodes with recurrence rates on the order of several events per century. Erosional shoreface retreat, coupled with aeolian overshoot, storm washover and the movement of sand through inlets into the lagoons, has lead to landward migration of the Atlantic barriers through the Holocene period of sea-level rise.The landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat. Repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon. The resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action. The backbarrier deposits eventually re-emerge at the shoreface, where their upper beds are eroded and recycled. The barrier thus migrates over a pavement of its own flood tidal deltas and washover fans; sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets, which reaches down to the underlying Pleistocene.The modern shelf sand sheet is the debris blanket resulting from the retreat process. The leading edge occurs as a thin veneer of rip-current fallout on the shoreface; this is periodically stripped off by winter storms to expose the underlying backbarrier strata. Further seaward, shelf sands eroded from the shoreface rest disconformably on backbarrier deposits; shoreface, beach and dune strata are always missing. Every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past. However, the sheet as a whole has been reconstituted; its primary structures are those of the inner shelf floor.Backbarrier sand and mud deposits, characterized by channeling and landward-dipping reflectors, can be traced for over 100 km seaward across the middle Atlantic shelf, beneath the modern sand sheet. The backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the Atlantic shelf surface during Holocene time. The sheet-like nature of this record indicates that, viewed at the appropriate time scale, barrier migration is a continuous process.