Sedimentary Environment

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Sedimentary Environment

Terrestrial environments

Coastal environments

Marine environments

Continental shelf environments

Deep oceanic environments

Interpreting the sedimentary record

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A sedimentary, or depositional, environment is an area on Earth’s surface, such as a lake or stream, where large volumes of sediment accumulate. All environments of deposition belong to one of three settings: terrestrial, coastal (or marginal marine), and marine. Subenvironments, each with their own characteristic environmental factors and sedimentary deposits, make up a sedimentary environment. For example, streams consist of channel, sand bar, levee, and floodplain subenvironments, among others.

Sedimentary environments display great complexity and almost infinite variety. Variations in environmental factors such as climate, latitude, surface topography, subsurface geology, and sediment supply help determine the characteristics of a particular sedimentary environment, and the resulting sedimentary deposits. This entry deals only with typical examples of common environments, with greatly simplified descriptions.

Terrestrial environments

Water, wind, and ice erode, transport, and deposit terrigenous sediments on land. Geologists recognize five common terrestrial sedimentary environments: stream, lake, desert, glacial, and volcanic.

Streams are the most widespread terrestrial sedimentary environment. Because they dominate landscapes in both humid and arid climates, stream valleys are the most common landform on Earth. Streams naturally meander and coarse-grained sediments accumulate along the inside of meanders where water velocity decreases, forming sand and gravel bars. When flood-water overflows a stream’s banks, fine-grained sediment accumulates on the land surface, or floodplain, adjacent to the channel. Coarser sediment collects on the channel banks during floods, forming a narrow deposit called a levee. Sorting, rounding, and sediment load generally increase downstream.

Where a stream rapidly changes from a high to low slope on land, for example at the base of a mountain, gravel, sand, silt, and clay form a sediment pile called an alluvial fan. Where a stream flows into standing water its sediments produce a deposit called a delta. Deltas are usually finer grained than alluvial fans. In both alluvial fans and deltas, grain size rapidly decreases downslope.

Most lakes form from water contributed by one or more streams as well as precipitation directly into the lake. As it arrives at a lake, stream velocity drops very rapidly, depositing the coarsest sediment at the lake-shore and forming a delta. Farther from shore, as the water continues to lose velocity, finer and finer grained sediment falls to the lake bottom. Only in the deepest part of the lake is water movement slow enough to permit the finest grained sediment to accumulate. This produces thin layers of clay. Hence, grain size generally decreases from the lakeshore to its center.

Deserts develop where rainfall is too sparse to support abundant plants. Contrary to popular belief, most deserts are not vast seas of sand. Instead, they consist mostly of a mixture of gravel and sand. However, the sand may be eroded away, or deflated, by the wind leaving behind a layer of gravel called a desert pavement, or reg. The deflated sand is later heaped into piles downwind, producing dunes. Despite the prevalence of regs and dunes in deserts, water is nonetheless the most important agent of erosion. Alluvial fans are common at the base of mountains. Dry lake beds, or playas, and salt deposits, or sabkhas, resulting from lake evaporation, commonly occupy the adjacent valley floor.

Where snowfall exceeds snowmelt and snow persists from year to year, ice accumulation can eventually form a glacier. Alpine glaciers occur throughout the world on mountains at high elevations. Modern continental glaciers now cover Antarctica and Greenland.

From around two million years ago to about ten thousand years ago—the Pleistocene epoch, or Ice Age— glaciers deposited sediments over large areas at mid- to high-latitudes. These glacial ice deposits, called till, are characterized by a wide range of sediment sizes (geologists refer to sediment comprised of a wide range of particle sizes as being poorly sorted). They generally are thick, widespread sheets or narrow, sinuous ridges. Ice meltwater forms thick, well-sorted, and widespread layers of sediment called stratified drift.

Though volcanism involves igneous processes, many terrestrial volcanic deposits are sedimentary in origin. These volcaniclastic, or pyroclastic, sediments form when ash, cinders, and larger volcanic materials fall to the ground during eruptions. Running water often modifies volcaniclastic sediments after deposition. They also may move downhill as a mudflow, or lahar, when saturated with water. Generally, volcani-clastic sediments form thin lobe-shaped deposits and widespread sheets, which thicken toward the volcanic source.

Coastal environments

Where the land meets the sea, interplay between terrestrial and marine processes causes sedimentary environments to be complicated. In areas where wave energy is low and the tidal range (the difference between high tide and low tide) is also low, terrestrial processes usually dominate. For example, sediments flowing into the sea from a river will form a well-developed triangular shaped deposit known as a delta. If wave energy is high and tidal range low, the river’s sediments will be reworked into a beach or barrier island. However, if tidal range is high, tidal currents flood the river mouth daily, forming a drowned river mouth, or estuary, with scattered sand bars.

In coastal areas far from rivers, the nature of the coast changes rapidly. The balance between tidal and wave processes influences coastal character. The higher the tidal range, the more important tidal processes become. In wave-dominated areas, currents flowing parallel to the shoreline move sand along the coast, producing barrier islands and beaches for long stretches. If a barrier island protects the coast, channels, or tidal inlets, pass between the islands and allow tidal currents to flow from the open ocean into the bay behind the island. Landward from the bay, a tidal marsh will occur. When high tide approaches and tidal currents flow landward, the marsh will be flooded. As the water level drops toward low tide, tidal currents flow seaward, exposing the marsh to the elements. If no barrier island is present, coasts are simple with only a few river mouths and coastal marshes to break the monotony of long stretches of beach.

Where tidal range is high, strong tidal currents dominate coastal processes. Tidal sand flats occur below low tide level. These are generally covered with large ripples to small sand dunes. Between the low tide and high tide marks, ripples are abundant on a mixed sand and mud flat. A mud flat, backed by a tidal marsh, forms above the high tide mark. Landward of the low tide level, tidal creeks cut through the deposits as well.

Marine environments

Sediments may accumulate and be preserved virtually anywhere in the oceans. Consequently, marine sedimentary environments are numerous and widespread. Water depth also plays a major role in shaping these environments. For simplicity, marine environments can be divided into two broad groups: shelf and deep oceanic. Shelf environments range in depth from low tide level to depths of 425 ft (130 m), typical for the outer edge of the continental shelf.

Continental shelf environments

The average continental shelf is about 45 mi (75 km) wide. Shelf sediments generally decrease in grain size with increasing distance from shore. This occurs for two reasons: (1) greater distance from sediment sources and (2) decreasing sediment movement (transport) with increasing water depth.

Shelf sediments vary significantly with latitude. At high latitudes, glacial ice flowing into coastal water generates icebergs, which transport large sediment loads of various sizes out onto the shelf. As icebergs melt, they drop their load. These glaciomarine sediments are generally less sorted and coarser grained than lower latitude deposits. In fact, boulders known as dropstones occur on the sea floor in deep water, hundreds of miles from shore.

Rivers deliver most of the sediments to mid-latitude shelves. Therefore, grain size routinely decreases with distance from shore; sediment sorting also tends to be good. Shallow water, nearshore sediments form thick sand blankets with abundant ripple marks. As depth increases and water movement decreases, average grain size decreases, and sand, silt, and clay occur interbedded. In water depths greater than 150 to 200 ft (45-60 m), even storm waves do not stir the bottom; consequently, silts and clays predominate. Scattered sand deposits are also located on outer shelf margins. During periods of lower sea level, rivers flowing across what is now the inner shelf deposited these so-called relict sediments.

At low latitudes, bottom-dwelling plants and animals secrete large volumes of calcium carbonate, producing thick blankets of carbonate sediment. Perhaps the best known carbonate environment is the coral reef. Corals produce a rigid framework of carbonate rock (limestone), which is also a major source of sediment of various grain sizes. Where stream input is great, terrigenous sediments discourage habitation by carbonate-producing organisms and dilute any carbonate sediment that is produced.

Deep oceanic environments

Seaward of the continental shelves, continental slopes incline more steeply, so relict and modern sediments form deposits called deep-sea fans. These are similar to alluvial fans, but generally consist of sand-to clay-sized particles with little or no gravel. Deep-sea fans form the continental rise, a continuous apron of sediment at the base of the continental slope.

Even farther from land, the monotonous abyssal plains begin. Here mostly clay-sized sediment forms sheets up to 0.6 mi (1 km) thick. These deposits, composed of sediments that settle through the water column from shallow depths, thin to a feather edge at the oceanic ridges where new sea floor forms. Abyssal sediments are generally a mixture of three grain types: carbonate muds and siliceous muds of biogenic (organic) origin, and red clays of terrigenous origin. Carbonate-rich muds generally accumulate in water depths of less than 2 to 2.5 mi (3-4 km); at deeper depths, colder water and higher pressures combine to dissolve the carbonate. Siliceous muds occur where abundant nutrients in surface waters support high rates of biogenic silica (SiO₂) production. Red clays, transported from the land by winds and stream flow, predominate where quantities of carbonate and siliceous muds are insufficient to dilute these finegrained terrigenous deposits.

Interpreting the sedimentary record

Geologists associate sub-environments with specific sediment features by observing modern sedimentary environments and the resulting sediments. These features include sediment composition, sediment texture (size, shape, and sorting), vertical changes in grain size, and various sedimentary structures such as wave and current ripples, desiccation cracks in mud, plant and animal remains, and bedding thickness. The assortment of sediment features that is typical of a particular subenvironment is called a sedimentary facies.

Geologists compile characteristic facies from each sedimentary environment to produce a facies model.

KEY TERMS

Biogenic sediment —Sediment produced by, or from the skeletal remains of, an organism.

Grain size —The size of a sediment particle; for example, gravel (greater than 2mm), sand (2mm-1/16 mm), silt (1/16 mm-1/256 mm) and clay (less than 1/256 mm).

Sediment load —The amount of sediment transported by wind, water, or ice.

Sorting —The range of grain sizes present in a sediment deposit; a sediment with a narrow range of grain sizes is said to be well sorted.

Terrigenous sediment —Sediment eroded from a terrestrial source.

A facies model may be a complicated diagram (sometimes developed using three dimensional computer graphics software), a table of information, or simply a detailed verbal description. It indicates which sedimentary features characterize a particular environment, and the lateral and vertical distribution of facies within sedimentary deposits.

Geologists use facies models for paleoenviron-mental reconstruction, which is the practice of deducing the environment where sediments or sedimentary rocks originate. This is useful for predicting the distribution of economically important earth materials, such as gold, tin, coal, oil, or gas, in a sedimentary deposit. When doing paleoenvironmental reconstructions, geologists look for sources of variation in environmental conditions. For example, rising sea level or a decreasing sediment supply influence the sediment deposit formed, so facies models are altered accordingly. Geologists constantly work on refining facies models to improve the accuracy of paleoenvironmental reconstructions.