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第五章 三角洲与河口湾. 一、三角洲研究历史及世界主要三角洲 二、三角洲的分类与沉积过程 三、三角洲的沉积特征 四、河口湾. 大陆边缘的海岸带主要沉积环境. 研究历史和定义. Delta: firstly used by the Greek philosopher Herodotus ( 希罗多德 ) about 490 B.C. to describe the triangular-shaped alluvial plain formed at the mouth of the Nile River.
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第五章 三角洲与河口湾 一、三角洲研究历史及世界主要三角洲 二、三角洲的分类与沉积过程 三、三角洲的沉积特征 四、河口湾
研究历史和定义 • Delta: firstly used by the Greek philosopher Herodotus (希罗多德) about 490 B.C. to describe the triangular-shaped alluvial plain formed at the mouth of the Nile River. • The term (alluvial) Delta is applied to any deposit, subaerial or subaqueous, formed by fluvial sediments that build into a standing body of water. • Delta are “discrete shoreline protuberances formed where rivers enter oceans, semi-enclosed seas, lakes or lagoons and supply sediment more rapidly than it can be redistributed by basinal processes” (Elliott, 1986) • Deltaic sediments are known to be important hosts from petroleum and natural gas, coal, and some minerals such as uranium.
Although ancient deltaic sediments are common in the rock record, much of what we know about delta systems comes from study of modern deltas. • Pioneer work by Johnston (1921,1922): 弗雷塞河三角洲，密西西比河三角洲 • Study on ancient delta triggered by Gilbert (1885, 1890): Bonneville湖更新世三角洲相，使用了前积层、顶积层和底积层的术语，并描述了每一部分的层理、结构、颜色和生物群，奠定了三角洲沉积的相分析方法。“Gilbert”型三角洲
20世纪40年代末期，美国开始将露头和地下资料按古环境进行解释，逐渐认识到在古代三角洲体系中产有大量的矿产。同时，欧美认识到三角洲组成的向上变粗的沉积旋回（韵律层），反映了从海相到陆相的过渡。20世纪40年代末期，美国开始将露头和地下资料按古环境进行解释，逐渐认识到在古代三角洲体系中产有大量的矿产。同时，欧美认识到三角洲组成的向上变粗的沉积旋回（韵律层），反映了从海相到陆相的过渡。 • Main research interests: Mississippi Delta, Nile River Delta, Niger River Delta, Rhone River Delta.. • 我国在80年代初兴起三角洲研究的热潮：长江三角洲、黄河三角洲、珠江三角洲、滦河三角洲等
Deltas are particularly common in the modern ocean owing to post-Pleistocene sea-level rise coupled with high sediment loads carried by many rivers. • Modern deltas occur on all continents, with the possible exception of Antarctica. • Deltas form where large, active drainage systems with heavy sediment loads exist. They are particularly developed in east coasts of Asia and the Americas where tectonic activity is low. Fewer than 10% of major modern deltas occur on collision coasts, where tectonic activity is high and drainage divides are close to the sea.
The distribution and characteristics of alluvial deltas are controlled by a complex set of interrelated fluvial and marine/lacustrine processes and environmental conditions. • These factors include climate, water and sediment discharge, river-mouth processes, nearshore waver power, tides, nearshore currents, and winds (Coleman, 1981). • Other factors that influence the formation of deltas are slope of the shelf, rate of subsidence and other tectonic activity at the depositional site, and geometry of the depositional basin. • River/sediment input, wave-energy flux, and tidal flux are the most important processes that control the geometry, trend, and internal features of the progradational framework sand bodies of deltas.
Classification of deltas on the basis of dominant processF: fluvialW: waveT: tidal
Behaviors of sediment-laden river water into basin water • Homopycnal flow等密度流: river water entering basin water of almost equal density, leading to rapid, thorough mixing and abrupt deposition of much of the sediment load. • This type of jet outflow presumably causes the formation of Gilbert-type deltas that displays a topset, foreset, and bottomset arrangement of beds.
Hyperpycnal flow高密度流: river water that has higher density than basin water flows beneath the basin water, generating a vertically oriented, plane-jet flow. • This type of jet flow moves along the bottom as a density current that may be erosive in its initial stages but eventually deposits its load along the more gentle slopes of the delta front to form turbidites. • It may also occur in lakes and under marine conditions.
Hypopycnal flow低密度流: river outflow is less dense than basin water, as when rivers flow into denser seawater, it flows outward on top of the basin water as a horizontally oriented plane jet. • Fine sediment may thus be carried in suspension some distance outward from the river mouth before it flocculates and settles from suspension. • This type of flow tends to generate a large, active delta-front area, typically dipping at 1º or less, as contrasted with the 10 º -20 º dip of most Gilbert-type deltas (Miall, 1984). • It is probably the most important type of river outflow in marine basins.
the Mississippi River Delta • The Mississippi River Delta is the a classic example of a birdfoot-type, river-dominated delta. It consists of seven distinct sedimentary lobes that have been active during the past 5000 to 6000 years, indicating that periodic channel or distributary abandonment is a common process. • Bar-finger sands developed at the mouth of the distributaries. • Common sediment facies on the Mississippi delta include marsh and natural-levee deposits, delta-front silts and sands, and prodelta clays. • Other fluvial-dominated deltas include the Danube (Black Sea) and the Po (Adriatic Sea) river deltas.
Tide-dominated Delta • If tidal currents are stronger than river outflow, these bidirectional currents can redistribute river-mouth sediments, producing sand-filled, funnel-shaped distributaries. • The distributary mouth bar may be reworked into a series of linear tidal ridges that replace the bar and extend from within the channel mouth out onto the subaqueous delta-front platform. • Tide-dominated deltas include the G-B delta, Colorado Delta, Fly (Gulf of Papua, New Guinea), and the Yalujiang (Korea Bay).
The area size of the delta is more than three times that of the Mississippi delta. Mean tidal range is large, about 4m, and wave energy is relatively low. • The delta is characterized by tidal-flat environments, natural levees, and flood basins in which fine sediment is deposited from suspension. The strong tidal influence is manifested by the presence of a network of tidal sand bars and channels oriented roughly parallel to the direction of tidal current flow. • A variety of sediment types include tidal-bar or tidal-ridge sands, braided, channel-fill sands, and natural-levee, tidal-flat, and floodbasin muds.
Wave-dominated Delta • Strong wave cause rapid diffusion and deceleration of river outflow and produce constricted or deflected river mouths. Distributary-mouth deposits are reworked by waves and are redistributed along the delta front by long-shore currents to form wave-built shoreline features such as beaches, barrier bars, and spits. • A smooth delta front, consisting of well-developed, coalescent beach ridges, may eventually be generated. • The Sao Francisco delta of Brail is well known as typical wave-dominated delta. Others include the Brazos (Gulf of Mexico), the Rhone, the Tiber (Italy).
The San Francisco Delta • It is smaller in areal dimensions than the Mississippi and has lower discharge. Tidal range is about 2m; however, wave power is reported to be about 100 times that of the Mississippi delta (Cileman, 1976). • Owing to the extreme wave energy, this delta is dominated by high-energy environments in which sand deposition take place. Muds accumulate locally in marshes and floodplains, but the interdistributary bay mud deposits characteristic of the Mississippi delta are absent. • It is dominated by beach-ridge barrier sands that cover much of the delta surface. A belt of eolian dune sands is distributed along the outer margin of the delta. • Deltaic sediments are not supplied dominantly by the river, but transported from the inner shelf (relatively fall in sea level of about 5m during the last 5000 years) by longshore drift.
Characteristics of fluvial-, tide-, and wave-dominated deltas
