Geology Reference
In-Depth Information
drift is impeded, coastal landforms develop. Longshore
currents and beach drifting may act in the same or
opposite directions.
a fluid medium (air, water, or ice) they become clastic
sediments.
Size is the normal criterion for grouping clastic sedi-
ments. Loose sediments and their cemented or com-
pacted equivalents have different names (Table 3.1). The
coarsest loose fragments (2 mm or more in diameter)
are
rudaceous deposits
. They comprise gravels of vari-
ous kinds - boulders, pebbles, cobbles, granules - and
sometimes form distinct deposits such as glacial till.
When indurated, these coarse deposits form rudaceous
sedimentary rocks. Examples are conglomerate, which
consists largely of rounded fragments held together by a
cement, breccia, which consists largely of angular frag-
ments cemented together, and gritstone. Loose fragments
in the size range 2-0.0625 mm (the lower size limit
varies a little between different systems) are sands or
arenaceous deposits
. Indurated sands are known as
arenaceous sedimentary rocks. They include sandstone,
arkose, greywacke, and flags. Loose fragments smaller
than 0.0625 mm are silts and clays and form
argilla-
ceous deposits
. Silt is loose particles with a diameter in
the range 0.0625-0.002 mm. Clay is loose and colloidal
material smaller than 0.002 mm in diameter. Indurated
equivalents are termed argillaceous rocks (which embrace
silts and clays). Examples are claystone, siltstone, mud-
stone, shale, and marl. Clay-sized particles are often
made of clay minerals, but they may also be made of
other mineral fragments.
Biological activity
Some marine organisms build, and some help to build,
particular coastal landforms. Corals and other carbonate-
secreting organisms make coral reefs, which can be
spectacularly large. The Great Barrier Reef extends along
much of the north-east coast of Australia. Corals grow in
the tropics, extratropical regions being too cold for them.
Coral reefs cover about 2 million km
2
of tropical oceans
and are the largest biologically built formation on Earth.
Calcareous algae produce carbonate encrustations along
many tropical shores.
Salt-tolerant plants colonize salt marshes. Mangroves
are a big component of coastal tropical vegetation. With
other salt-tolerant plants, they help to trap sediment in
their root systems. Plants stabilize coastal dunes.
SEDIMENT DEPOSITION
Sediments are material temporarily resting at or near
the Earth's surface. Sedimentary material comes from
weathering, from volcanic activity, from the impact of
cosmic bodies, and from biological processes. Nearly
all sediments accumulate in neat layers that obligingly
record their own history of deposition. In the fullness
of Earth history, deposition has produced the geologi-
cal or stratigraphic column (see Appendix). The sum-
ming of the maximum known sedimentary thickness for
each Phanerozoic period produces about 140,000 m of
sediment (Holmes 1965, 157).
Chemical sediments
The materials in chemical sediments derive mainly from
weathering, which releases mineral matter in solution
and in solid form. Under suitable conditions, the solu-
ble material is precipitated chemically. The precipitation
usually takes place
in situ
within soils, sediments, or
water bodies (oceans, seas, lakes, and, less commonly,
rivers).
Iron oxides
and
hydroxides
precipitate on the
sea-floor as chamosite, a green iron silicate. On land,
iron released by weathering goes into solution and,
under suitable conditions, precipitates to form vari-
ous minerals, including siderite, limonite (bog iron),
and vivianite.
Calcium carbonate
carried in ground-
water precipitates in caves and grottoes as sheets of
flowstone or as stalagmites, stalactites, and columns
Clastic sediments
Clastic
or
detrital sediments
form through rock weath-
ering. Weathering attacks rocks chemically and physically
and so softens, weakens, and breaks them. The pro-
cess releases fragments or particles of rock, which range
from clay to large boulders. These particles may accu-
mulate
in situ
to form a regolith. Once transported by
