Download CHAPTER 6 - SEDIMENTS AND SEDIMENTARY ROCKS

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 interpreted 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 and recrystallization.
Sedimentary rocks are classified as clastic or chemical (inorganic or
biochemical) (Tables 9.1 and 9.2). Clastic rocks include breccia, conglomerate,
quartz sandstone, arkose, graywacke, shale, siltstone, and mudstone.
Chemical sedimentary 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. Dolomite (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 interpretation of sedimentary
rocks that includes determination of source area and environment of
deposition. Source area is important as it affects the character of the resulting
sediment. Sedimentary structures can be used to infer the direction of past
currents. Environments of deposition are summarized in Figure 9.38 and
include glacial environments, alluvial fans, river channels, 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.
The 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 often 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 associated 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 regard 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 occurs during transportation, usually by streams. Grain size
generally decreases downstream 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
rocks have a crystalline texture. A crystalline texture can also result from the
destruction of a clastic texture by recrystallization of clastic grains.
5. A section on types of sedimentary rocks discusses clastic, chemical and
organic sedimentary rocks. Clastic sedimentary rocks are classified by grain
size and composition. Coarse-grained clastic rocks form breccia (angular
grains) and conglomerate (rounded grains). Sandstones 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
shale contains both silt and clay sized grains. Claystone 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
recrystallized limestone (original texture lost). Dolomite is a mineral
(CaMg(CO3)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 entirely 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 have 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
materials preserved in marine mud change 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 reflects original
deposition of the 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
traces 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 recognizable 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 different rock
types are called contacts.
10. The source 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 and
grains become smaller and more rounded. Sedimentary structures may also
help determine direction of current flow.
11. Environments in which sediment deposition occurs include continental
environments such as glacial environments, alluvial fans, river channels, flood
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
tectonics. 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 preserved, 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; phosphates 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 between 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 form. 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
upstream. 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 sedimentary 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 depositional history of sedimentary rocks?
3. Why are gypsum and dolomite considered 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 rocks 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 following 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, 1989. 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):134148
Nichols, G. 1999. Sedimentology 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
Publishers.
Reading, H.G., 1986. Sedimentary Environments and Facies (2nd 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 level
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