ENGINEERING GEOLOGY (101)
CHAPTER .4 SEDIMENTARY ROCK
 Sedimentary rocks are types of rock that are formed by
the deposition of material at the Earth's surface and within bodies of
water.
 Particles that form a sedimentary rock by accumulating are
called sediment. Before being deposited, sediment was formed
by weathering and erosion in a source area, and then transported to
the place of deposition by water, wind, mass movement or
glaciers which are called agents of denudation.
 The study of sedimentary rocks and rock strata provides
information about the subsurface that is useful for civil engineering,
for example in the construction of roads, houses, tunnels, canals or
other constructions. Sedimentary rocks are also important sources
of natural resources like coal, fossil fuels, drinking water or ores.
 The study of the sequence of sedimentary rock strata is the main
source for scientific knowledge about the Earth's history
Genetic classification
Based on the processes responsible for their formation, sedimentary
rocks can be subdivided into four groups:
a) clastic sedimentary rocks,
b) biochemical (or biogenic) sedimentary rocks,
c) chemical sedimentary rocks and
d) "other" sedimentary rocks formed by impacts,
volcanism, and other minor processes.
a)
Clastic sedimentary rocks
Clastic sedimentary rocks are composed of silicate minerals
and rock fragments that were transported by moving fluids (as
bed load, suspended load, or by sediment gravity flows) and
were deposited when these fluids came to rest. Clastic rocks
are composed largely of quartz, feldspar, rock (lithic)
fragments, clay minerals, and mica; numerous other minerals
may be present as accessories and may be important locally.
 gravel (>2 mm diameter), conglomerates and breccias
 sand (1/16 to 2 mm diameter), and sandstones
 mud (clay is <1/256 mm and silt is between 1/16 and
1/256 mm). mud rocks.
Boulder of
conglomerate
sandstone
mud rock
Biochemical sedimentary rocks
are created when organisms use materials dissolved in air or
water to build their tissue. Examples include:
b)
 Most types of limestone are formed from the calcareous
skeletons of organisms such as corals, mollusks,
and foraminifera.
 Coal which forms as plants remove carbon from the
atmosphere and combine with other elements to build their
tissue.
 Deposits of chert formed from the accumulation of siliceous
skeletons
from
microscopic
organisms
such
as radiolaria and diatoms.
limestone
Anthracite coal
Chert
Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents
in solution become supersaturated and inorganically precipitate.
Common
chemical
sedimentary
rocks
include oolitic
limestone and rocks composed of evaporite minerals such
as halite (rock salt), sylvite, barite and gypsum.
c)
Modern ooids from
a beach
halite
gypsum
"Other" sedimentary rocks
This fourth miscellaneous category includes rocks formed
by Pyroclastic flows, impact breccias, volcanic breccias, and
other relatively uncommon processes.
d)
Pyroclastic flows
sweep down the flanks
breccia
breccia
3-3-2 Mineralogy
Most
sedimentary
rocks
contain
either quartz (especially siliciclastic rocks)
or calcite (especially carbonate rocks). In contrast with igneous and
metamorphic rocks, a sedimentary rocks usually contains very few
different major minerals. However, the origin of the minerals in a
sedimentary rock is often more complex than those in an igneous rock.
3-3-3 Fossils
Sedimentary rocks are the only type of rock that can contain fossils, the
remains or imprints of dead organisms.
Fossil-rich layers in a sedimentary rock
3-3-4 Primary sedimentary structures
Structures in sedimentary rocks can be divided into 'primary' structures
(formed during deposition) and 'secondary' structures (formed after
deposition). Unlike textures, structures are always large-scale features
that can easily be studied in the field. Sedimentary structures can tell
something about the sedimentary environment or can serve to tell which
side originally faced up where tectonics have tilted or overturned
sedimentary layers.
Sedimentary rocks are laid down in layers called beds or strata. A bed is
defined as a layer of rock that has a uniform lithology and texture.
 The sequence of beds that characterizes sedimentary rocks is called
bedding
 Cross-bedding forms when small-scale erosion occurs during
deposition, cutting off part of the beds. Newer beds then form at an
angle to older ones.
bedding
Cross bedding
3-3-5 Secondary sedimentary structures
Secondary sedimentary structures are structures in sedimentary rocks
which formed after deposition. Such structures form by chemical,
physical and biological processes inside the sediment. They can be
indicators for circumstances after deposition.
3-3-6 Stratigraphy
That new rock layers are above older rock layers is stated in
the principle of superposition. There are usually some gaps in the
sequence called unconformities. These represent periods where no new
sediments were laid down, or when earlier sedimentary layers raised
above sea level and eroded away.
Sedimentary rocks contain important information about the history of
the Earth. They contain fossils, the preserved remains of
ancient plants and animals. Coal is considered a type of sedimentary
rock. The composition of sediments provides us with clues as to the
original rock. Differences between successive layers indicate changes to
the environment over time. Sedimentary rocks can contain fossils
because, unlike most igneous and metamorphic rocks, they form at
temperatures and pressures that do not destroy fossil remains.