Page 107 - Fundamentals of Geomorphology

90 INTRODUCING LANDFORMS AND LANDSCAPES

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