Page 41 - Sedimentology and Stratigraphy

3

Biogenic, Chemical and

Volcanogenic Sediments

In areas where there is not a large supply of clastic detritus other processes are impor-
tant in the accumulation of sediments. The hard parts of plants and animals ranging from
microscopic algae to vertebrate bones make up deposits in many different environments.
Of greatest significance are the many organisms that build shells and structures of
calcium carbonate in life, and leave behind these hard parts when they die as calcareous
sediments that form limestone. Chemical processes also play a part in the formation of
limestone but are most important in the generation of evaporites, which are precipitated
out of waters concentrated in salts. Volcaniclastic sediments are largely the products of
primary volcanic processes of generation of ashes and deposition of them subaerially or
under water. In areas of active volcanism these deposits can swamp all other sediment
types. Of the miscellaneous deposits also considered in this chapter, most are primarily
of biogenic origin (siliceous sediments, phosphates and carbonaceous deposits) while
ironstones are chemical deposits.

3.1 LIMESTONE and dams. Limestone strata are common through
much of the stratigraphic record and include some
Limestones are familiar and widespread rocks that very characteristic rock units, such as the Late Cre-
form the peaks of mountains in the Himalayas, form taceous Chalk, a relatively soft limestone that is found
characteristic karst landscapes and many spectacular in many parts of the world. The origins of these rocks
gorges throughout the world. Limestone is also impor- lie in a range of sedimentary environments: some
tant in the built environment, being the construction form in continental settings, but the vast majority
material for structures ranging from the Pyramids of are the products of processes in shallow marine envi-
Egypt to many palaces and churches. As well as being ronments, where organisms play an important role in
a good building stone in many places, limestone is creating the sediment that ultimately forms limestone
also important as a source of lime to make cement, rocks.
and is hence a component of all concrete, brick and Calcium carbonate (CaCO 3 ) is the principal com-
stone buildings and other structures, such as bridges pound in limestones, which are, by definition, rocks