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第三章 海岸带. 第三节 滨海沉积相及其特征. 波动底流速的不对称和海岸砂体的特征. 沉积相 Sedimentary facies.
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第三章 海岸带 第三节 滨海沉积相及其特征
沉积相 Sedimentary facies • Each sedimentary environment is characterized by a particular suite of physical, chemical, and biological parameters that operate to produce a body of sediment characterized by specific textural, structural, and compositional properties. Such distinctive bodies of sediments or sedimentary rock are defined as sedimentary facies: 一定沉积环境中形成的沉积岩石与古生物的组合. • Lithofacies 岩相are distinguished by physical characteristics such as color, lithology, texture, and sedimentary structures. • Biofacies生物相are defined on the basis of paleontologic characteristics.
滨海沉积相特征 • 海岸带是波浪和水流强烈作用的地带；水动力变化决定了沉积物的分布和地貌特征，而沉积物的分布和地貌特征反过来又影响水动力条件。 • 海岸带生物组合受温度、盐度控制显著，在海岸带不同地区具有不同的组合类型。 • 滨海沉积相划分主要决定于水动力条件、沉积物性质、生物组合和地貌特征。
几个术语 • Beach海滩– A gently sloping accumulation of coarse sediment along a lake or ocean shoreline. The beach extends from the low-water line landward to where there is a change in morphology, such as a cliff, dunes, or permanent vegetation. • Mainland Beach – long, narrow accumulations of sand aligned parallel to the shoreline and attached to land. • Barrier island障壁岛– An elongate body of coastal sand separated from the mainland by a body of water. • Barrier-island beaches – similar to mainland beaches but are separated from land by a shallow lagoon, estuary, or marsh, and also commonly dissected by tidal channels or inlets.
海滩-近岸带的地貌类型 • The beach is divided into the backshore, foreshore and shoreface. • Backshore 后滨extends landward from the beach berm 后滨阶地 above high-tide level and commonly includes back-beach dune deposits. • Foreshore前滨mainly encompasses the intertidal (littoral) zone between low-tide and high-tide levels. • Shoreface临滨extends from about low-tide level to the transition zone between beach and shelf sediments, that is, to fair-weather wave base at a depth of about 10-15m. • Nearshore 近滨：including foreshore and shoreface • Offshore远滨/远滨 covers the area from the fair-weather wave base to the continental shelf break.
近滨 远滨
障壁岛的形成机制 • Shoal and longshore-bar aggradation, that is, upward building and eventually emergence of offshore bars 沙坝加积 • Spit segmentation by breaching and detachment of spits oriented parallel to the coast 沙嘴分割 • Mainland ridge engulfment owing to submergence and drowning of shoreline-attached beaches 大陆脊-海滩沉陷 • Welding or veneering of Holocene dune, beach, and foreshore sand into and over pre-Holocene topographic highs 全新世沉积汇聚 • Lateral shifting of coastal sands during transgression to form the barrier island 海侵中海岸沙侧向迁移 -Reineck and Singh(1980)
障壁岛的组成单元 • The sandy barrier-island chain itself (the subtidal to subaerial barrier-beach complex) • The enclosed lagoon, estuary, or marsh behind it (the back-barrier, subtidal-intertidal region) • The channels that cut through the barrier and connect the back-barrier lagoon to the open sea (the subtidal-intertidal delta and inlet-channel complex)
封闭泻湖 海湾 半封闭泻湖 半封闭泻湖
沙坝-泻湖的各沉积单元 半封闭泻湖
沙嘴拦截出的泻湖 半封闭泻湖 海湾-泻湖
沉积背景 • Beach and barrier-island complexes are best developed on wave-dominated coasts where tidal range is small to moderate (<4m). 波控，小到中潮的海岸 • Beach is the most dynamic of all depositional environments • Microtidal: 0-2m tidal range 小潮 mesotidal 中潮: 2-4 m tidal range, macrotidal 大潮: >4m tidal range • Barriers are generally absent on macrotidal coasts; extremely tidal range causes wave energy to be dispersed and dissipated over too great a width of shore zone to effectively form barrier. • 同三角洲沉积环境的差别：动力环境—海洋作用为主，风
现代海滩和障壁岛沉积相特征 • Overall geometry and lithofacies: a narrow body of sediments elongated parallel to the depositional strike, or strike of the shoreline; mainly composed of sand of hundreds of meters broad, up to hundreds of kilometers long, and 10-20m thick (Reineck and Singh, 1980). • Beach sediments may be interrupted along its length by deltaic, estuarine, bay, and other deposits where these features cut across the beach. • Sands of the barrier beach grade landward into back-barrier sediments that may include washover sands, tidal-delta sands and muds, lagoonal silts and muds, and sandy, muddy tidal-flat and marsh deposits.
海滩沉积相特征 • 后滨：中-细砂；受间歇性的风暴潮和风的作用；向陆缓倾；水平纹层；甲壳类 动物掘穴构造发育；风成槽状交错层理。 • 前滨：中细沙，往复水流形成平行纹层, 向海缓倾 (2-3º);沉积层薄,重矿物富集层常见，和石英质砂层交替。 • 临滨：可分上、中、下三部分，对应于涌浪带、破浪带和碎浪带。
Upper-shoreface/surf zone: dominated by strong bidirectional translation waves and longshore currents. • Sediments range from fine sand to gravel. • Structures consist primarily of multidirectional trough cross-bed sets, but also include low-angle, bidirectional cross-beds and subhorizontal plane beds.
Middle-shoreface/breaker-zone deposits also form under high-energy conditions owing to breaking waves and associated longshore and rip currents. • Sediments consist mainly of fine- to medium-grained sand with minor amount of silt and shell material. • Structures can be highly complex, depending on the presence or absence of longshore bars, and can include landward-dipping ripple cross-laminations; and seaward- and landward-dipping low-angle planar bedding; subhorizontal plane laminations, and seaward- and landward-dipping trough cross-beds. Trace fossils consisting of vertical burrows are common.
Lower-shoreface/shoaling-zone deposits form under relatively low-energy conditions and grade seaward into open-shelf deposits. • They are composed dominantly of fine to very fine sand but may also contain thin layers of silt and mud. • Small-scale cross-stratification and planar, nearly horizontal laminatd bedding are the main sedimentary structures present. • Such deposits may contain abundant plant materials, mica flakes, and other hydraulically light particles. Laminae in lower shoreface deposits tend to be obliterated by bioturbation, and suspension-feeder and deposit-feeder traces.
理想的海滩沉积层序 进积的全新世低能海滩
