Geology Reference
In-Depth Information
of dripstone (p. 213). It sometimes precipitates around
springs, where it encrusts plants to produce tufa or
travertine (p. 207). Evaporites form by soluble-salt pre-
cipitation in low-lying land areas and inland seas. They
include halite or rock salt (sodium chloride), gypsum
(hydrated calcium sulphate), anhydrite (calcium sul-
phate), carnallite (hydrated chloride of potassium and
magnesium), and sylvite (potassium chloride). Evaporite
deposits occur where clastic additions are low and evap-
oration high. At present, evaporites are forming in the
Arabian Gulf, in salt flats or sabkhas , and around the mar-
gins of inland lakes, such as Salt Lake, Utah. Salt flat
deposits are known in the geological record, but the mas-
sive evaporite accumulations, which include the Permian
Zechstein Basin of northern Europe and the North Sea,
may be deep-water deposits, at least in part.
Chemicals precipitated in soils and sediments often
form hard layers called duricrusts . These occur as hard
nodules or crusts, or simply as hard layers. The chief types
are mentioned on p. 155.
in humic lakes and pools by the flocculation and precip-
itation of dissolved humic materials. Gyttja comprises
several biologically produced sedimentary oozes. It is
commonly subdivided into organic, calcareous, and
siliceous types. Sedentary organic materials are peats,
of which there are many types.
Sedimentary environments
The three main sedimentary environments are terres-
trial , shallow marine , and deep marine . A single
sedimentary process dominates each of these: gravity-
driven flows (dry and wet) in terrestrial environments;
fluid flows (tidal movements and wave-induced currents)
in shallow marine environments; and suspension set-
tling and unidirectional flow created by density currents
in deep marine environments (Fraser 1989). Transition
zones separate the three main sedimentary environments.
The coastal transition zone separates the terrestrial and
shallow marine environments; the shelf-edge-upper-
slope transition zone separates the shallow and the deep
marine environments.
Sediments accumulate in all terrestrial and marine
environments to produce depositional landforms. As a
rule, the land is a sediment source and the ocean is
a sediment sink . Nonetheless, there are extensive bod-
ies of sediments on land and many erosional features
on the ocean floor. Sedimentary deposits are usually
named after the processes responsible for creating them.
Wind produces aeolian deposits , rain and rivers produce
fluvial deposits , lakes produce lacustrine deposits , ice
produces glacial deposits , and the sea produces marine
deposits . Some deposits have mixed provenance, as in
glaciofluvial deposits and glaciomarine deposits .
On land, the most pervasive 'sedimentary body' is
the weathered mantle or regolith . The thickness of the
regolith depends upon the rate at which the weathering
front advances into fresh bedrock and the net rate of ero-
sional loss (the difference between sediment carried in
and sediment carried out by water and wind). At sites
where thick bodies of terrestrial sediments accumulate,
as in some alluvial plains, the materials would nor-
mally be called sediments rather than regolith. However,
regolith and thick sedimentary bodies are both the prod-
uct of geomorphic processes. They are thus distinct
Biogenic sediments
Ultimately, the chemicals in biogenic sediments and
mineral fuels come from rock, water, and air. They are
incorporated into organic bodies and may accumulate
after the organisms die. Limestone is a common biogenic
rock. The shells of organisms that extract calcium carbon-
ate from seawater form it. Chalk is a fine-grained and
generally friable variety of limestone. Some organisms
extract a little magnesium as well as calcium to con-
struct their shells - these produce magnesian limestones.
Dolomite is a calcium-magnesium carbonate. Other
organisms, including diatoms, radiolarians, and sponges,
utilize silica. These are sources of siliceous deposits such
as chert and flint and siliceous ooze.
The organic parts of dead organisms may accumu-
late to form a variety of biogenic sediments. The chief
varieties are organic muds (consisting of finely divided
plant detritus) and peats (called coal when lithified).
Traditionally, organic materials are divided into sedimen-
tary (transported) and sedentary (residual). Sedimentary
organic materials are called dy, gyttja, and alluvial peat.
Dy and gyttja are Swedish words that have no English
equivalent. Dy is a gelatinous, acidic sediment formed
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