CHAPTER 9 - SEDIMENTS AND SEDIMENTARY ROCKSOverview

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Feb 22, 2014 (3 years and 1 month ago)

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CHAPTER 9

-

SEDIMENTS AND SEDIMENTARY ROCKS


Overview

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
.1: gravel>
2mm diameter, sand 2mm<> 1/16mm,"mud"< 1/16 mm. Clay is a
grain
size
and a group of sheet silicates; most grains< 1/16 mm are clay minerals.
Transportation effects roundness and sorting. Environments of deposition in
the past
can

be i
nterpreted from the features preserved within the rock record
.
Sediments deposited in marine environments are most easily preserved in the
rock record.
Lithification converts sediments to rock and involves compaction
and cementation, or crystallization an
d recrystallizat
ion.

Sedimentary rocks are classified as clastic or chemical (ino
rganic or
biochemical) (Tables 9.1 and 9
.2). Clastic rocks include breccia, conglomerate,
quartz sandstone, arkose, graywacke, shale, siltstone, and mudstone.
Chemical sedime
ntary rocks include carbonates, chert and evaporites.
Carbonate rocks form by either organic or inorganic activity. Varieties
discussed include bioclastic

limestone
,
coquina,
chalk, oolitic

limestone
, tufa,
travertine, and recrystallized limestone. Dolomit
e (dolostone)
forms when
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
-
bedding,
graded bedding, mudcracks, ripple marks, and fossils. The
definition
of a

formation and its
use

as a geologic unit for mapping and interpretation is
introduced. A discussion follows dealing with the inte
rpretation of sedimentary
rocks

that includes determination of

source area and environment of
deposition. Source area is
impor
tant as it affects the
character of
the resulting
sediment. Sedimentary structures can be used to infer the direction of past
currents.
Environments of depos
ition are summarized in Figure 9
.38

and
include glacial environments,
alluvial fans, river channe
ls, flood plains,
and
lakes
.


Shallow marine environments include deltas, beaches, barrier islands,
lagoons, shelves,
and
reefs
. Sediment can also be deposited in deep marine
environments.

Th
e chapter concludes with an examination of the influence of
plate tectonics
on the distribution of sedimentary rocks. Convergent plate boundaries are
marked by
rapid erosion and accumulation of
thick clastic,
(
frequently
turbiditic
)
, sediments
in marine basins adjacent to
mountain belts.
Mountain
belts themselves o
ften contain uplifted marine deposits.
Transform boundaries
are also characterized by high erosion and deposition rates and the rapid
burial of
organic
-
rich deposits

makes them good sites for petroleum
exploration. Divergent plate
boundaries are associat
ed with
coarse clastics,
evaporites and volcanic sediments
.


Learning Objectives

1
. Sediment
consists of
unconsolidated particles of either preexisting rocks or
chemical precipitates. It is classified by size: gravel> 2 mm< sand>
0.0625

mm<"mud", without r
egard to composition, although most grains of

clay

size
are
made of
clay minerals.

2
. Rounding (grinding away sharp edges) and sorting (separation by size)
of
sediment
occur
s

during transportation, usually by streams.

Grain s
ize
generally
decreases downst
ream in a river. Deposition occurs when agents of
transportation lose energy

and can no longer transport their load
. Preservation
of sediments requires their burial and is favored in subsiding basins.

3
. Lithification converts loose sediment to sedimentary

rock, usually by
compaction (reduces pore space) and cementation (fills remaining pore
space).

Sedimentary rocks consisting of loose grains bound by cement
have a
clastic texture.

4
. Not all sedimentary rocks form from
pre
-
existing
sediment

grains
. Some
form through crystallization of

minerals from solution (e.g.
,
rock salt
). These
ro
cks have a crystalline texture. A crystalline text
ure can also result from
the
destruction of a clastic texture by
recrystallization
of clastic grains.

5
. A section on types

of sedimentary rocks discuss
es clastic, chemical
and
organic
sedimentary
rocks. Clastic sedimentary rocks are classified by grain
size and composition.

Coarse
-
grained clastic rocks form b
reccia (angular

grains
) and conglomerate (rounded

grains
). Sandston
es contain sand
-
size
grains,
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
sh
ale contains both silt and clay sized grains
.
C
laystone and mudstone

are
formed by predominately clay
-
size particles
.

6
. Limestone is composed mostly of calcite
precipitated
through the action of
organisms or as an inorganic precipitate. Varieties include
coquina (cemented
shells), bioclastic limestone (
containing fossils or coralline algae
), chalk
(
formed from microscopic organisms),

oolitic limestone (small spheres of
calcite), tufa and travertine (crystalline precipitates

fro fresh water
) and
recrystalli
zed limestone (original texture lost). Dolomite is a mineral
(
CaMg(CO
3
)
2
)
and a rock (sometimes called dolostone) that occurs as a
replacement of limestone and destroys its original texture.

7
. Chert is a fine
-
grained, sedimentary rock composed almost entir
ely of silica.
It

can
form as nodules within other rocks (such as dolomite) or as a layered
deposit resulting from the accumulation of microscopic marine organisms

on
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
can be

recognized. Organic
material
s preserved in marine mud

chang
e to oil and natural gas through
burial and the effects of
increased heat and pressure
.


8
. Sedimentary structures form
during or shortly after deposition of sediment
.
Horizontal bedding
is
common
in
sedimentary rocks and reflect
s original
deposition of th
e sediment as horizontal layers.

Cross bedding
refers to
inclined
layers within a bed
and
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
trace
s of plants or animals buried by sediment and
may be
preserved as
unaltered original material, replacements, molds or carbon films. Fossils may
occur in any sedimentary rock type, but are most common in limestones.

9
. Formations are bodies of rock
with rec
ognizable characteristics that can be
used
to map, describe and interpret the geology of a region. The first name
of
a formation
is
often
a geographic location where it is well exposed, and the
second name is its rock type. The bounding surfaces
between di
fferent rock
types
are called contacts.

10
.
The s
ource area of
a

sedimentary rock
can be
determined by the
composition of its grains (for example feldspar, quartz and mica indicate a
granitic source). Sedimentary deposits thin away from their source

area a
nd

grains become smaller and more rounded. S
edimentary structures may
also
help determine direction of current flow.

11
.
Environments in which sediment deposition occurs include c
ontinental
environments
such as
glacial environments, alluvial fans, river ch
annels,
f
lood
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),
lagoons (shales),

shallow marine shelves (widespread sandstone, siltstone and
shale), and reefs (massive limestone cores). Deep marine environments
receive deposition from turbidity currents.

12
. The distribution of sedimentary rocks
is

controlled
in part
by plate
tectonic
s. Convergent boundaries accumulate thick clastic deposits in
sedimentary basins

adjacent to rising mountain belts
.

Uplifted marine

deposits
can

also be

found in
the mountains at these boundaries
. Transform
boundaries
allow
organic rich deposits

to be pre
served
, while diverging
boundaries form rift valleys
containing

gravels, lake deposits and evaporites.

Boxes

9
.1
-

ENVIRONMENTAL GEOLOGY
-
VALUABLE SEDIMENTARY ROCKS

-

Many sedimentary rocks have value. Among these are: limestone for cement,
building stone,

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
rocks
for ground water, oil and gas.

9
.2


IN

GREATER DEPTH


HOW FAST DID THE CURRENT FLOW?



The
approximate velocity of a flowing current can be determined from the grain
size of the sediment and any
preserved
sedimentary structures
.

This
relationship betwee
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
increase
(> 50 cm/sec)
ripples are replaced by larger dunes
which are
eventually
‘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
upst
ream.
The

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.

Shorter Discussion/Essay

1.
How does sediment originate?

2. What is the significance of changes in grain size, rounding and sorting to
the
deposition
al history
of sedimentary rocks?

3.
Why are
gypsum and dolomite
consid
ered as

both rocks and minerals?

4.
How does coal form?

5.
Explain the process of lithification.

Longer Discussion/Essay

1.
How can
sedimentary structures
be used to help understand

the
depositional origin of
sedimentary rocks?

2.
How can sedimentary roc
ks formed in continental depositional environments
be distinguished from those formed in marine environments?

3.
What factors determine the mineralogy of a sedimentary rock?

4.
How do fossils form?

5.
Describe the types of environment in which the follow
ing sedimentary rocks
may form: breccia, arkose, chert, oolitic limestone, conglomerate, gypsum,
coquina.

Selected Readings

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,
Ohio, 784pp.



Collinson, J.D. and Thompson, D.B, 1
989.
Sedimentary Structures. Allen
and Unwin, 194p.

An essential ‘handbook’ of all sedimentary structures and
their interpretation.

Fichter, L.S. 1996."Tectonic Rock Cycles," Journal of Geological
Education 44(2):134
-

148

Nichols, G.

1999.
Sedimentolo
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

and Co.

Raymond, L.A. 1995. Petrology. Dubuque, IA: Wm. C. Brown
Publis
hers.

Reading, H.G., 1986
. Sedimentary Environments and Facies (2
nd

ed.),
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
el
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
of

Rocks," Journal of

Geological Education 39:379
-
387