Presentation on theme: "第三节 滨海沉积相及其特征 第三章 海岸带. 波动底流速的不对称和海岸砂体的特征 沉积相 Sedimentary facies Each sedimentary environment is characterized by a particular suite of physical, chemical,"— Presentation transcript:
第三节 滨海沉积相及其特征 第三章 海岸带
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.
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.
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) -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 半封闭泻湖 海湾 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 海湾 - 泻湖 半封闭泻湖
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 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 m 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.
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.