Historical Geology Notes

Sedimentary Environments

Page and figure numbers refer to the textbook: Stanley, Earth System History, 2nd ed.

Understanding modern environments of deposition allows geologists to understand the environments in which ancient sedimentary rocks were deposited and thereby help us recreate past conditions on the Earth.

glacial deposits (p. 108-109)
Glaciers are flowing streams of ice. They may be huge continental ice sheets or small alpine (mountain) glaciers. All glaciers scrape up sediments and incorporate them into the base of the ice sheet. Sand, gravel, and large boulders polish and gouge the surface of the bedrock that they are dragged over leaving glacial striations (Fig 5-4). Glaciers do not sort sediments as flowing water and wind do. Poorly sorted glacial sediments are known as till. Large boulders often lie in a matrix of sand and silt (matrix-supported conglomerate) (Fig 5-5). At the end of a glacier, where ice is melting as fast as it is being supplied from upstream, the sediments are deposited in a terminal moraine, a ridge of poorly-sorted glacial till. Thinner depostits of glacial sediments called a ground moraine or till plain are found behind the terminal moraine. Sorted sediments carried by networks of braided streams out from the terminal moraine form an outwash plain.

deserts and aeolian (wind) deposition (p. 109-112)
In the desert belts centered around 10 to 20 degrees north and south of the equator there is very little rainfall. Because of this there is only sparse vegetation. The soil is exposed. The soil is also dry, due to the lack of rain, so the particles have no cohesion as it does when moist. Here, wind is an important means of sedimentary transport and deposition. Because the soil and sediments are not protected by a covering of vegetation or held together by roots and cohesion, the wind is free to pick up and carry sediments.

Fine particles, clay and silt are picked up as windblown dust (analogous to the suspended load in stream transport). The dust will eventually settle in an area adjacent to the desert in a more humid area with sufficient vegetation to protect sediments from further wind transport. The resulting deposits are called loess.

sand dunes (Fig. 5-9)
Sand is transported by a means called saltation. The sand grains tumble and bounce along the desert floor close to the ground or perhaps as high is several feet in a strong wind. Where they encounter an obstacle they may settle behind it, protected from the wind. Sand may build up here eventually forming a dune. The sand is blown up a gentle slope facing the wind and is deposited on a steep slope opposite to the wind. Over time layers of sand dune deposits may be preserved as large scale cross-bedded sandstone (Fig 5-10).

Desert sands are typically well sorted and rounded. The sand grains appear frosted under a micoscope because of constant collisions with other grains during wind transport.

alluvial fans (p. 110-112, Fig. 5-11)
Where a steep mountain stream flows out into a valley the reduction in gradient and stream velocity causes the stream to deposit its coarser sediments. A pile of coarse sediments (sand and gravel) builds up at the base of the mountains. The sediments are piled in a semicircular, fan-shaped body that is tallest at the base of the mountains. These coarse sediments when lithified are preserved as conglomerates. Stream deposited conglomerates are typically clast-supported (the large clasts lie against one another, and finer sediments fill in the voids between them) in contrast to typically matrix-supported glacial deposits. An apron of overlapping alluvial fans deposited adjacent to a tectonic escarpment or uplift is often called a "fanglomerate."

stream deposition (p. 113-114; Figs. 5-15 & 5-16)
In humid climates on the continents, streams are the primary medium for transport and deposition of sediments. Streams carry sediments from uplifted (mountains) source areas, eventually to the sea. Lowlands are often sites of stream deposition. Meandering streams deposit point bar sands that may be preserved as a sheet of sandstone as a result of the migration of the point bar across the stream valley. Occasional flooding carries suspended silt and clay out of the stream channel and onto the flood plain. As the flood waters recede, the fine sediments are deposited in sheets in the backswamp area. These so-called overbank deposits may be preserved as layers of shale. The sandstones and shales formed in a stream valley will contain terrestrial and fresh water fossils, not marine fossils. The overbank shales often contain mudcracks that form when the floods recede and the clay dries out (Fig. 5-12) and raindrop impressions. Assymetric or current ripple marks also indicate deposition in a stream. Lakes normally have muddy bottoms and perhaps a narrow shoreline of sand and gravel. Shales with fossils of fresh-water organisms are commonly formed in lakes.

coastal deposition
Sediments that reach the ocean may be deposited in a delta (Figs. 5-17, 5-18, 5-19), which is in many ways like an underwater alluvial fan. Sediments are distributed in a fan-shaped body that grows outward (seaward) with time. Longshore currents will transport sediments along the coast.

All along a coast, sediments derived from longshore drift and sediments formed in place from wave action are distributed by wave energy. Wave action is strongest at the ocean surface and decreases with depth in the water down to a depth of half the wavelength (L/2). Because of this, in shallow water near the shore the fine sediments are washed away as suspended load. Only coarse sediments are deposited in shallow water. As the depth to the bottom increases, the bottom is stirred less and less by wave action; progressively finer sediments can be deposited in increasingly deeper water. Deposited sediments progress from sands near the shore to silts and clays farther offshore. In cool or turbid (murky) water, fine sediments will dominate to the edge of the continental shelf.

In warm tropical waters, if most of the fine sediments have already been deposited, coral reefs will grow in shallow water on the continental shelf (p. 119-123). Modern coral reefs do not form in the deep ocean abyss where there is no light because symbiotic algae that lives in the coral needs light to grow. Coral reefs also do not grow in turbid nearshore waters where terrestrial sediments have not yet been deposited.

In a sedimentary sequence, alternating sandstone, shale, and limestone generally indicates a marine environment. Almost all limestone is deposited in the ocean. The sandstones and shale would contain fossils of marine organisms. The shales would almost certainly have no mudcracks.

off the continental shelf (p. 124-125; Figs. 5-33, 5-34)
The continental shelf is the shallow ocean surrounding the continent. The depth at the edge of the shelf is usually not more than 100 to 150 meters (the length of one to one-and-a-half football fields). Some sand and mud are carried to the edge of the continental shelf via submarine canyons which are like undersea river valleys. Sediments build up at the edge of the shelf and when too much has accumulated these flow down the continental slope and rise as turbidity currents (like underwater mud flows). The resulting deposits, called turbidites, contain some chaotic, poorly sorted coarse layers at their base and then finer layers on top. Repeated sequences of turbidites indicate deposition on the continental slope and continental rise.

deep abyssal plains (p. 125-125; Fig. 5-35)
In the deep sea, out on the abyssal plains, the depth to the seafloor varies from about 2.5 to 6 km (2500 to 6000 meters) or more below sea level. The abyssal plains receive very little sediment from the continents. Pelagic clays from windblown dust from the continents and oceanic volcanoes form finely laminated (layered) shales. Biogenic oozes: Calcareous oozes from deposits of single-cell, microscopic organisms with calcite shells result in finely laminated limestone. Siliceous oozes from single-cell, microscopic organisms with silica shells form finely laminated chert (silica) layers. Furthermore, the limestones indicate warm water; limestone dissolves in cold water. Chert indicates high biological productivity and cool water.