Environmental Engineering Reference
In-Depth Information
Figure 12.8
Principal metamorphic facies and their
environments of formation.
Source: Partly after Skinner and Porter (1995).
boundaries. Sediments of pelagic (surface) and
benthic
(deep-water) marine origin,
formed by biogeochemical processes in the oceans themselves, share a similar fate.
Denudation transforms rocks into disaggregated minerals and lithic fragments or
detritus
, lacking cohesion, and mineral solutions which are readily removed from their
source. At this stage they possess textural (particle size, shape) and chemical properties
diagnostic of their parent rocks and, to some extent, the denudation process involved.
Three principal styles of sediment are recognized.
Clastic sediments
(from the Greek
word for 'broken') are formed by particles broken off parent rocks and initially reflect
fragment size and shape. A distinction is drawn between
clasts
, or fragments larger than
sand size, and finer grains (sand, silt and clay sizes) which form a
matrix
(Figure 12.9).
Chemical sediments
are precipitated primarily from dissolved salts, silicates and
carbonates. They form
biogenic sediments
when their origins are organic, as do clastic
shell and bone beds and carbonifer-ous (peat and coal) sediments (see Colour Plate 5
between pp. 272 and 273). Figure 12.10 shows the relation between sourcing process,
product and eventual sedimentary forms.
SEDIMENT TRANSPORT AND DEPOSITION
Once in transit, the character of the original denudation product is weakened and
sediments begin to acquire the signature of the transport environment. This occurs
through particle attrition and sorting, in response to trans-portational energy, and further
chemical weathering or fractionation as less stable minerals are taken in or out of
solution. Particle edge roundness or sphericity tends to
