SEDIMENTS AND SEDIMENTARY ROCKS
Sediment is a collective term for loose, solid particles that originate from
either weathering and erosion or as chemical (organic or inorganic)
precipitates. Sediment is classified by size, summarized
in Table 9
2mm diameter, sand 2mm<> 1/16mm,"mud"< 1/16 mm. Clay is a
and a group of sheet silicates; most grains< 1/16 mm are clay minerals.
Transportation effects roundness and sorting. Environments of deposition in
nterpreted from the features preserved within the rock record
Sediments deposited in marine environments are most easily preserved in the
Lithification converts sediments to rock and involves compaction
and cementation, or crystallization an
Sedimentary rocks are classified as clastic or chemical (ino
biochemical) (Tables 9.1 and 9
.2). Clastic rocks include breccia, conglomerate,
quartz sandstone, arkose, graywacke, shale, siltstone, and mudstone.
ntary rocks include carbonates, chert and evaporites.
Carbonate rocks form by either organic or inorganic activity. Varieties
discussed include bioclastic
travertine, and recrystallized limestone. Dolomit
calcium is replaced by magnesium in
limestone. Other sedimentary rocks
described include chert, evaporites (including rock gypsum and rock salt), and
coal (not included in Table 9.2). This section concludes with a brief discussion
on the origin of oil and gas.
The origin and character of primary sedimentary structures are described and
shown in several figures
. These include horizontal bedding, cross
graded bedding, mudcracks, ripple marks, and fossils. The
formation and its
as a geologic unit for mapping and interpretation is
introduced. A discussion follows dealing with the inte
rpretation of sedimentary
that includes determination of
source area and environment of
deposition. Source area is
tant as it affects the
sediment. Sedimentary structures can be used to infer the direction of past
Environments of depos
ition are summarized in Figure 9
include glacial environments,
alluvial fans, river channe
ls, flood plains,
Shallow marine environments include deltas, beaches, barrier islands,
. Sediment can also be deposited in deep marine
e chapter concludes with an examination of the influence of
on the distribution of sedimentary rocks. Convergent plate boundaries are
rapid erosion and accumulation of
in marine basins adjacent to
belts themselves o
ften contain uplifted marine deposits.
are also characterized by high erosion and deposition rates and the rapid
makes them good sites for petroleum
exploration. Divergent plate
boundaries are associat
evaporites and volcanic sediments
unconsolidated particles of either preexisting rocks or
chemical precipitates. It is classified by size: gravel> 2 mm< sand>
mm<"mud", without r
egard to composition, although most grains of
. Rounding (grinding away sharp edges) and sorting (separation by size)
during transportation, usually by streams.
ream in a river. Deposition occurs when agents of
transportation lose energy
and can no longer transport their load
of sediments requires their burial and is favored in subsiding basins.
. Lithification converts loose sediment to sedimentary
rock, usually by
compaction (reduces pore space) and cementation (fills remaining pore
Sedimentary rocks consisting of loose grains bound by cement
. Not all sedimentary rocks form from
form through crystallization of
minerals from solution (e.g.
cks have a crystalline texture. A crystalline text
ure can also result from
destruction of a clastic texture by
of clastic grains.
. A section on types
of sedimentary rocks discuss
es clastic, chemical
rocks. Clastic sedimentary rocks are classified by grain
size and composition.
grained clastic rocks form b
) and conglomerate (rounded
es contain sand
and may be differentiated
as quartz sandstone (>90% quartz), arkose
(>25% feldspar), and graywacke (>15% matrix = silt and clay). Graywackes
result from deposition by turbidity currents. Lithified silt forms siltstone, while
ale contains both silt and clay sized grains
laystone and mudstone
formed by predominately clay
. Limestone is composed mostly of calcite
through the action of
organisms or as an inorganic precipitate. Varieties include
shells), bioclastic limestone (
containing fossils or coralline algae
formed from microscopic organisms),
oolitic limestone (small spheres of
calcite), tufa and travertine (crystalline precipitates
fro fresh water
zed limestone (original texture lost). Dolomite is a mineral
and a rock (sometimes called dolostone) that occurs as a
replacement of limestone and destroys its original texture.
. Chert is a fine
grained, sedimentary rock composed almost entir
ely of silica.
form as nodules within other rocks (such as dolomite) or as a layered
deposit resulting from the accumulation of microscopic marine organisms
the sea floor.
Evaporites are sedimentary rocks formed from evaporation of
water. They h
ave crystalline textures and include rock gypsum and rock salt.
Coal forms from consolidation of plant material, originally as peat. Compaction
transforms peat to coal and several varieties
s preserved in marine mud
e to oil and natural gas through
burial and the effects of
increased heat and pressure
. Sedimentary structures form
during or shortly after deposition of sediment
sedimentary rocks and reflect
deposition of th
e sediment as horizontal layers.
layers within a bed
forms as sand is deposited on the steep face
of migrating bedforms such as ripples or dunes. Ripple marks are either
symmetric (waves) or asymmetric (currents) and
can be preserved in
conglomerates, sandstones or siltstones
Graded bedding exhibits a vertical
change in grain size and
is typically produced by
turbidity current deposition.
Mud cracks require air
drying of very fine
grained sediments. Fossils are
s of plants or animals buried by sediment and
unaltered original material, replacements, molds or carbon films. Fossils may
occur in any sedimentary rock type, but are most common in limestones.
. Formations are bodies of rock
ognizable characteristics that can be
to map, describe and interpret the geology of a region. The first name
a geographic location where it is well exposed, and the
second name is its rock type. The bounding surfaces
are called contacts.
ource area of
determined by the
composition of its grains (for example feldspar, quartz and mica indicate a
granitic source). Sedimentary deposits thin away from their source
grains become smaller and more rounded. S
edimentary structures may
help determine direction of current flow.
Environments in which sediment deposition occurs include c
glacial environments, alluvial fans, river ch
plains and lakes. Shallow marine environments include deltas (usually with
thick siltstone and shale, cut by sandstone channels), beaches and barrier
islands (well sorted, quartz sandstone), dunes (high angle cross bedding),
shallow marine shelves (widespread sandstone, siltstone and
shale), and reefs (massive limestone cores). Deep marine environments
receive deposition from turbidity currents.
. The distribution of sedimentary rocks
s. Convergent boundaries accumulate thick clastic deposits in
adjacent to rising mountain belts
the mountains at these boundaries
organic rich deposits
to be pre
, while diverging
boundaries form rift valleys
gravels, lake deposits and evaporites.
VALUABLE SEDIMENTARY ROCKS
Many sedimentary rocks have value. Among these are: limestone for cement,
chalk and soil conditioning; coal for fuel; gypsum for plaster
and soil conditioning; rock
salt for hydrochloric acid, seasoning and ice
melting; diatomite for filtering; shale for ceramic clays; sulfur for matches,
fungicides, and sulfuric acid; phosphat
es and nitrates extracted from
sedimentary rocks for fertilizers; evaporites for potassium (soap), boron
(cookware and fiberglass), sodium (baking and washing soda and soap);
quartz sandstone for glass; sedimentary iron ores for steel; and reservoir
for ground water, oil and gas.
HOW FAST DID THE CURRENT FLOW?
approximate velocity of a flowing current can be determined from the grain
size of the sediment and any
n grain size, current velocity and bedform type is shown on
a bedform stability diagram. Sand sized sediment will not move on the bed of
a stream if current velocities are very low
(< 20 cm/sec)
but as velocities
increase small bedforms such as ripples fo
rm. As velocities continue to
(> 50 cm/sec)
ripples are replaced by larger dunes
‘washed out’ at higher velocities (> 60 cm/sec) to form a flat bed.
At current velocities of over 100 cm/sec antidunes develop and migrate
change from one bedform type to another as current velocity
increases varies according to grain size of the sediment. Using the bedform
stability diagram geoscientists can reconstruct past flow conditions through
analysis of grain size and sedi
mentary structures within a deposit.
How does sediment originate?
2. What is the significance of changes in grain size, rounding and sorting to
of sedimentary rocks?
gypsum and dolomite
both rocks and minerals?
How does coal form?
Explain the process of lithification.
be used to help understand
depositional origin of
How can sedimentary roc
ks formed in continental depositional environments
be distinguished from those formed in marine environments?
What factors determine the mineralogy of a sedimentary rock?
How do fossils form?
Describe the types of environment in which the follow
ing sedimentary rocks
may form: breccia, arkose, chert, oolitic limestone, conglomerate, gypsum,
There are numerous textbooks dealing with sedimentary rocks. Most treat
either petrology/petrography or sedimentation/stratigraphy
. A few more
recent editions are given here.
Blatt, H. 1992. Sedimentary Petrology. 2nd edition. New York:
W.H. Freeman and Co.
Boggs, S. Jr., 1987
. Principles of Sedimentology and Stratigraphy, Merrill,
Collinson, J.D. and Thompson, D.B, 1
Sedimentary Structures. Allen
and Unwin, 194p.
An essential ‘handbook’ of all sedimentary structures and
Fichter, L.S. 1996."Tectonic Rock Cycles," Journal of Geological
gy and Stratigraphy
. London: Blackwell
Science Ltd. 355pp
. A good introductory sedimentology text.
Prothero, D.R. and Schwab, Fred. 1996. Sedimentary Geology.
New York: W.H. Freeman
Raymond, L.A. 1995. Petrology. Dubuque, IA: Wm. C. Brown
Reading, H.G., 1986
. Sedimentary Environments and Facies (2
Blackwell Scientific, Oxford, 615pp.
Thorough coverage of sedimentary
processes, environments and deposits.
Walker, R.G. and James, N.P., 1992
. Facies Models
response to sea lev
change. Geological Association of Canada, 454pp
. An excellent, advanced
level coverage of sedimentary environments and the deposits they produce.
An interesting article on classification:
Brown, V.M. and Harrell, J.A. 1991."Megascopic classification
Rocks," Journal of
Geological Education 39:379