Page 238 - Sedimentology and Stratigraphy

15

Shallow Marine Carbonate and

Evaporite Environments

Limestones are common and widespread sedimentary rocks that are mainly formed
in shallow marine depositional environments. Most of the calcium carbonate that
makes up limestone comes from biological sources, ranging from the hard, shelly parts
of invertebrates such as molluscs to very fine particles of calcite and aragonite formed by
algae. The accumulation of sediment in carbonate-forming environments is largely con-
trolled by factors that influence the types and abundances of organisms that live in them.
Water depth, temperature, salinity, nutrient availability and the supply of terrigenous
clastic material all influence carbonate deposition and the build up of successions of
limestones. Some depositional environments are created by organisms, for example,
reefs built up by sedentary colonial organisms such as corals. Changes in biota through
geological time have also played an important role in determining the characteristics of
shallow-marine sediments through the stratigraphic record. In arid settings carbonate
sedimentation may be associated with evaporite successions formed by the chemical
precipitation of gypsum, anhydrite and halite from the evaporation of seawater. Shallow
marine environments can be sites for the formation of exceptionally thick evaporite
successions, so-called ‘saline giants’, that have no modern equivalents.

15.1 CARBONATE AND EVAPORITE processes influencing the supply of detritus, except
DEPOSITIONAL ENVIRONMENTS where increased terrigenous clastic supply reduces
carbonate productivity, i.e. the rate of formation of
There are a number of features of shallow marine calcium carbonate by biological processes. Second,
carbonate environments that are distinctive when the grain size of the material deposited is largely
compared with the terrigenous clastic depositional determined by the biological processes that generate
settings considered in Chapter 14. First, they are lar- the material, not by the strength of wave or current
gely composed of sedimentary material that has action, although these processes may result in break-
formed in situ (in place), mainly by biological pro- up of clasts during reworking. Third, the biological
cesses: they are therefore not affected by external processes can determine the characteristics of the