现代地层学与地史学实习指导书

现代地层学与地史学实习指导书

中国石油大学(北京)

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2007年6月

实习一沉积环境分析

一、实习目的

1、熟悉现代主要沉积环境及各种环境中沉积作用条件及产物的特点。

2、掌握一些常见的沉积相标志

3、学习相分析的基本方法

4、理解沉积相分析结果的表示方法

二、实习内容

1、阅读并翻译下文，总结现代主要沉积环境的基本特征。CONTINENTAL ENVIRONMENTS

Alluvial fans are fan-shaped deposits formed at the base of mountains. Alluvial fans are most common in arid and semi-arid regions where rainfall is infrequent but torrential, and erosion is rapid. Alluvial fan sediment is typically coarse, poorly- sorted gravel and sand

Fluvial environments include braided and meandering river and stream systems. River channels, bars, levees, and floodplains are parts (or subenvironments) of the fluvial environment. Channel deposits consist of coarse, rounded gravel, and sand. Bars are made of sand or gravel. Levees are made of fine sand or silt. Floodplains are covered by silt and clay

Lacustrine environments (or lakes) are diverse; they may be large or small, shallow or deep, and filled with terrigenous, carbonate, or evaporitic sediments. Fine sediment and organic matter settling in some lakes produced laminated oil shales.

Deserts (Aeolian or aolian environments) usually contain vast areas where sand is deposited in dunes. Dune sands are cross-bedded, well sorted, and well rounded, without associated gravel or clay.

Swamps (Paludal environments) Standing water with trees. Coal is deposited.

Transitional environments

Deltas are fan-shaped deposits formed where a river flows into a standing body of water, such as a lake or sea. Coarser sediment (sand) tends to be deposited near the mouth of the river; finer sediment is carried seaward and deposited in deeper water. Some well known deltas include the Mississippi River delta and the Nile River delta.

Beaches and barrier islands are shoreline deposits exposed to wave energy and dominated by sand with a marine fauna. Barrier islands are separated from the mainland by a lagoon. They are commonly associated with tidal flat deposits.

Lagoons are bodies of water on the landward side of barrier islands. They are protected from the pounding of the ocean waves by the barrier islands, and contain finer sediment than the beaches (usually silt and mud). Lagoons are also present behind reefs, or in the center of atolls.

Tidal flats border lagoons. They are periodically flooded and drained by tides (usually twice each day). Tidal flats are areas of low relief, cut by meandering tidal channels. Laminated or rippled clay, silt, and fine sand (either terrigenous or carbonate) may be deposited. Intense burrowing is common. Stromatolites may be present if conditions are appropriate.

Marine environments

Reefs are wave-resistant, mound-like structures made of the calcareous skeletons of organisms such as corals and certain types of algae. Most modern reefs are in warm, clear, shallow, tropical seas, between the latitudes of 30o N and 30o S of the equator. Sunlight is required for reef growth because of the presence of symbiotic algae called zooxanthellae which live in the tissues of corals. Atolls are ring-like reefs surrounding a central lagoon (such as Bikini Atoll in the Pacific Ocean).

The continental shelf is the flooded edge of the continent. The continental shelf is relatively flat (slope < 0.1o), shallow (less than 200 m or 600 ft deep), and may be up to hundreds of miles wide. (The flooding of the edges of the continents occurred when the glaciers melted at the end of the last Ice Age, about 10,000 years ago.) Continental shelves are exposed to waves, tides, and currents, and are covered by sand, silt, and mud.

The continental slope and continental rise are located seaward of the continental shelf. The continental slope is the steep (5- 25o) "dropoff" at the edge of the continent. The continental slope passes seaward into the continental rise, which has a more gradual slope. The continental rise is the site of deposition of thick accumulations of sediment, much of which is in submarine fans, deposited by turbidity currents.

The abyssal plain is the deep ocean floor. It is basically flat, and is covered by very fine-grained sediment, consisting primarily of clay and the shells of microscopic organisms (such as foraminifera, radiolarians, and diatoms).

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