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.