Sedimentary Environment
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.
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.