Chp 6: Sedimentary Rocks
1. Rocks initially form from solidification of magma or lava
2. These rocks subjected to weathering and erosion processes (wind,
running water, glaciers, etc) which break down those rocks into
particles.
3. These particles are then transported and settle out to become
‘sediments.’
4. These sediments are compacted to form sedimentary rocks.
5. If these sedimentary rocks are subjected to enough pressure and
heat they can then transform into Metamorphic Rocks….later Chp
…this process is known as The Rock Cycle……
Derivation of
Sediments from
Pre-existing rocks-
Fig. 6-7, p.155
II. Rock Types:
Igneous rocks make up 90% by volume of the earth's crust.
-Igneous rocks formed directly from molten material.
-As this molten material cools it solidifies and hardens to become rock.
- Intrusive igneous rock forms below the surface of the earth.
-Extrusive igneous rocks form from molten material that has been
forced out onto the surface of the earth (i.e. volcanoes).
Sedimentary rocks form from the accumulation of eroded debris
of other rocks or chemically from elements in seawater.
-Sedimentary rocks make up 75% of all of the rocks exposed at the
earth's surface.
-most fossilized remains are found in sedimentary rocks. This makes
sedimentary rocks useful in interpreting the earth's geologic history.
Metamorphic rocks are formed from pre-existing rocks that have
been altered as the result of intense heat and pressure.
-Metamorphism increases the “crystallinity“ and hardness of the rock;
sandstone changes to quartzite; shale changes to slate, limestone to marble
Sedimentary rocks in The Valley
of the Gods, Utah
Some characteristics to note:
a. Layering in rocks is horizontal
b. Erosion has exposed layers
differentially by their lithology.
c. Red color results from iron oxide
(Fe02) cement.
d. These rocks are mostly sandstone
Fig. 6-CO, p.148
III. Types of Sedimentary Rocks:
The type of sedimentary rock formed in an area reflects the environment
in which it was deposited, this is referred to by geologists as the facies
of the rock.
Since the facies of sedimentary beds tells the geologists so much
information about the geologic past (paleoenvironments, paleoclimates,
and past life forms), sedimentary rocks are emphasized in Historical
Geology. There are 2 basic groups of sedimentary rocks:
1. Chemical Precipitates from the evaporation of seawater, or
from the concentration of ions in water. These include rocks such as
limestone and various salts such as Halite (NaCl), Sylvite (KCl), Gypsum
(CaSO4), etc. The salts usually indicate periods of massive evaporation
of aqueous environments.
2. Clastic Sedimentary Rocks are formed from the accumulation
of debris from the weathering and erosion of other rocks. The 4 stages
of the formation of clastic sedimentary rocks (“clastic” means "broken")
are described on the following pages.
IV. The Four Steps for Formation of Sedimentary Rocks:
1.
Physical and Chemical Weathering of the “Parent Rock. This results in the
Parent rock being broken into fragments.
2. Transportation is the stage where the clastics are "moved“ (“transported”) from
the source area by water, wind, gravity, or ice.
-size of particle and distance moved very important. Larger particle requires more
energy to move that particle.
-High- energy environments would include white water mountain streams that
are capable of moving almost all sizes of particles. Low-energy environments
include lagoons, lakes, deltas, swamps, etc., that are capable of moving only the
smaller particles.
3.
Deposition is the stage where the sediment is deposited in a particular geographic
environment, which constitutes the sedimentary environment.
-For example, large rocks formed on a mountain range would be carried down the
steep gradient and deposited at the base of the mountain if the energy of the stream
carrying them decreased when it reached the base of the mountain. Since the stream no
longer has the high energy from the gradient, the large rocks are deposited in a manner
indicative of a mountain stream environment. Sedimentary rocks can be interpreted to
find out the environment in which they formed.
Sedimentary Environments can be divided into several categories:
a. Shoreline and Coastal Environments
b.“Fluvial” or Stream, River, and Delta Environments
c. Alluvial Fans or deposits at the bases of mountains
d. “Aeolian” or “wind-borne” deposits
There are numerous other sedimentary environments we will consider later….
4. Compaction is the final stage in the formation of a sedimentary rock. At this stage
the sediments are compacted due to the weight of the overburden (overlying sediments)
and can be eventually “lithified” (turned to stone) as the particles are cemented together
with substances such as Calcite (CaCO3), Silica (SiO2), or forms of Iron Oxide
(i.e. Fe2O3), among other compounds..
Depositional environments
Fig. 6-4, p.153
Red sandstone in New Mexico-note parallel, horizontal bedding
Fig. 6-6, p.155
V. Properties of Clastic Sediments:
These include certain characteristics of the sedimentary rock that give specific
information about the environment of deposition. These include particle size,
degree of roundness, degree of sorting, and color.
1.
Particle Size: Clastic sediments are found in various sizes ranging from <1/256 mm
to >256 mm. Refer to Figure 1. The Wentworth Scale of Particle Sizes.
- The name of a particular sediment size is based on its particle size rather than its
Chemical composition. For example, "sand" refers to particles having a size range
Between 0.125mm – 0.5mm. There can be quartz sand such as that found along the
Gulf Coast or there may be feldspar sands, gypsum sands, etc.
-sediment size indicates the amount of energy operating in the depositional
environment and is therefore a useful clue in determining what the sedimentary
environment was. Boulders represent a high- energy environment such as a river
channel while clays represent a low energy environment such as a floodplain or swamp
2. Roundness: This is simply how “round” (or smooth) the particles in the rock are.
-poorly rounded: angular, irregular shape, sharp edges.
-well rounded: Particles are smooth and have no edges.
The degree of roundness indicates either the amount of agitation the particles were
subjected to before deposition, or the length of time it took to transport the particle.
The Wentworth Scale of Particle Sizes that is a list of sediment particle sizes and the names used to describe them:
The Wentworth Scale of Particle Sizes
Particle Name
Approximate Particle Diameter in millimeters
Boulders
greater than 256mm
Cobbles
Pebbles
128
64
32
16
8
4
Granules
2
Very Coarse Sand
1.0
Course Sand
0.5
Fractional Equivalents
1/2
Medium Sand
0.25
1/4
0.125
0.0625
0.0313
0.0156
1/8
1/16
1/32
1/64
0.0078
0.0039
1/128
1/256
Fine Sand
Very Fine Sand
Silt
Clay
less than 1/256
Lithification and classification of sedimentary particles
Fig. 6-5, p.154
a. Shale in Tenn: note how breaks
along planes….called fissile…
b. Mudstone in Glacier National
Park
Fig. 6-11, p.157
Microscopic view of sandstone-not individual grains, about 0.5mm
what type of rounding is this-well rounded or poorly rounded?
Fig. 6-8a, p.156
Microscopic view of limestone- calcite crystals approx 1mm across
Fig. 6-8b, p.156
Arkose: sandstone that contains pebbles and sand size grains.
Typically this rock has not been transported very far….
Fig. 6-10b, p.157
Quartz sandstone
Fig. 6-10a, p.157
Sedimentary breccia
Fig. 6-9b, p.156
Gravels deposited by swiftly flowing stream (a,b). PreCambrian (1
Billion year old conglomerate in Michigan (c) deposited same way today.
Fig. 6-22, p.172
Conglomerate: note the mixture of rounded grains and fine
matrix
Fig. 6-9a, p.156
“Well rounded” particles: subjected to a high amount of saltation (bouncing along as
they were transported) or being transported for a very long distance-such as from the
center of a continent to its shoreline. Contact with other grains doing same thing…
‘Poorly rounded’ sediments: indicate either a low amount of agitation, or a short distanc
of transportation from the time the particle weathered away from their parent rocks.
A high-energy environment, allows for a long period of exposure to weathering,
such as a beach or in a stream, is conducive to the formation to the formation of
“well-rounded” sediments. On the other hand, a high-energy depositional environment
that does not allow a long period of exposure to agitation, such as an alluvial fan,
prevents the sediments from becoming “well-rounded”.
3. Sorting: refers to rock fragments separated according to particle size.
-“poorly sorted” sediment would contain particles of varying size. This usually
represents a rapid deposition as the result of a rapid decrease in the energy of an
environment. found in alluvial fans at the base of a mountain. This results in a
"dumping effect" of sediments at the base of the mountain (high- energy to low- energy
Poorly sorted and poorly rounded sediment is said to be "Immature".
-“Well Sorted” sediment contains material that is made up primarily of all the
same sized particles. This indicates that the rate of deposition is slow enough to
allow the materials to be separated. This also means relatively high energy=beach.
Sediment is said to be "Mature" if it is well rounded and well sorted.
Gravel on a
Beach-loose
aggregate of
solids eroded
from existing
rocks.
Fig. 6-1a, p.150
Well Rounded rocks: edges are smooth and worn-NO sharp edges
Fig. 6-3a, p.152
Well rounded, well sorted deposit-smooth edges; approx same size
Fig. 6-3b, p.152
Angular, poorly sorted gravel: sharp edges, particles of many sizes
Fig. 6-3c, p.152
Gravel in outcrophere is a sedimentary
rock-bound together
by chemical cement
filling pore spaces.
Geologists call this
conglomerate
Fig. 6-6b, p.150
4.
Color: The color of sediment can provide useful information about a sedimentary
environment. In general, colors of sedimentary rocks can be interpreted in the
following manner:
a.) Red, yellow, brown - oxidation conditions, probably marine in origin (RUST).
b.) Black, gray, greenish-gray - reducing conditions, probably marine except for
floodplains and swamps.
Glauconite= marine deposition (green mineral)
c.) Light gray or white - little iron present, either marine or non-marine; other
characteristics of the rock must be considered such as the presence of fossils, the
type of fossils, whether or not there is cross-bedding, etc.
VI. Chemical Precipitates:
Chemically formed sediments are produced under various conditions, but generally
speaking, when seawater becomes saturated with chemicals, they will precipitate out
of solution. This is similar to when a lot of sugar is added to hot tea and then it is
allowed to cool. Some of the sugar will "crystallize" or settle out of solution because
the tea was "saturated" with sugar and it could not stay dissolved. Precipitates usually
form only in low energy environments such as lagoons or deep-sea environments.
Chemical Precipitates would not be found in high- energy environments.
Limestone and Dolostone – These “carbonate rocks result from the concentration
and precipitation of Ca+, Mg+, and CO3- ions in the sea.
A. Limestone - Ca CO3 (primarily calcite)- forms offshore from the precipitation
of calcium and carbonate ions that have been dissolved off of the continents.
Limestones may also be formed from the accumulation of microscopic calcareous
tests (shells) of planktonic (or other aquatic level) micro-organisms.
B. Dolostone - Ca,Mg (CO3)2 (primarily dolomite)- forms in a similar manner, but
contains magnesium as well as calcium. Dolostone may start off as limestone and
later is subjected to groundwater replacing Ca+ with Mg+. Or, some dolostones
indicate having formed the calcium/magnesium carbonate all at once.
Table 6-2, p.158
Core of rock salt from
Michigan
Fig. 6-13a, p.160
Gypsum: evaporite
Fig. 6-13b, p.160
Thin layer of bedded chert
Fig. 6-14b, p.160
C. “Bioclastic sediments” are formed by living organisms. Many aquatic marine
organisms produce shells or other protective coverings by secreting calcium carbonate
(limestone) or calcium magnesium carbonate (dolomite). When these organisms die,
their shells accumulate along the sea floor forming layers of broken shell fragments.
Such material is biochemically produced and is ultimately broken by water action.
They are then referred to as "bioclastic sediments". The sedimentary rock coquina
is a good example of a bioclastic deposit. The availability of nutrients decreases the
further from the shore therefore most marine organisms live in the coastal, shallow
water areas. As the distance from shore increases, generally the number of marine
organisms decreases. The facies of bioclastic sediments such as coquina usually indicates
a lagoon to beachfront.
D. “Organic Rocks” form as the result of organics (such as vegetative matter) accumulating in low energy, reducing, anaerobic environments such as swamps. The material
does not rot quickly and the volatiles are driven off leaving behind the carbon. A good
example of an organic rock is coal. The first stage is called peat. As the peat gets
compressed over time, it becomes lignite coal. As lignite becomes compressed, it
becomes bituminous coal. As bituminous coal becomes compressed, it forms the
metamorphic rock anthracite, the final stage of coal. Other types of organic rocks
may form from accumulations of dead organisms (such as fish) in low energy lagoons.
Coquina-pile of shells….
Fig. 6-12b, p.159
Chalk cliff in Denmark-made up of skeletal remains of microscopic’
organisms
Fig. 6-12c, p.159
Limestone with shells
Fig. 6-12d, p.159
Peat: plant remains,
buried and compressed
Fig. 6-15a, p.161
Lignite: peat that has been
buried deeper, compressed
and heated.
Fig. 6-15b, p.161
Bituminous coal-about 80% carbon; dense, black, burns
efficiently
Anthracite=highest grade coal, 95% carbon; yields more heat than
Fig. 6-15c, p.161
other types of coal
VII. Bedding or Layering of Sedimentary Materials:
Sedimentary rocks are deposited in layers known as "beds". The type of bedding will
vary depending on the environment of deposition. Under normal conditions, beds are
deposited in horizontal layers with the bedding planes (the line of contact between the
beds) parallel to one another. "Cross-bedding" occurs when the surface of deposition is
inclined (i.e. a delta) or a current is present (i.e. a stream). This type of bedding is
called "cross-bedding" and is indicative of these environments.
The types of currents that form cross-bedding strata are:
Aeolian - wind action
Fluvial - river and stream action
Marine in Origin - current action
Types of cross-bedding include planar - the bedding planes separating the cross-bedded
units are parallel, wedge - the bedding planes are at an angle to one another and form a
wedge; and trough - the bedding planes separating the cross-bedded units are curved.
Thick planar or wedge cross-bedding always indicates an aeolian (wind) deposit such
as a sand dune in the desert.
Thin planar or wedged units may be aeolian, fluvial, or marine. Because of this, other
characteristics such as color must be used to determine the environment of deposition.
Principle of Horizontality
• Illustration of the principles of superposition
– and original horizontality
• Horizontality: These sediments
were originally
– deposited horizontally
– in a marine environment
– This outcrop is Chattanooga Shale, Tennessee
Principle of Superposition
• Illustration of the principles of superposition
– and original horizontality
• Superposition: The youngest
– rocks are at the top
– of the outcrop
– and the oldest rocks are at the bottom
Jurassic Navajo Sandstone, Zion National Park, UT
wind blown dune deposit. Note the thickness!! the
cross bedding….
Fig. 6-23a, p.173
a. Bedding in sandshale layers.
b. Fluvial cross beddin
originally deposited
on sloping surface.
c. Ancient cross beddi
in Montana: hammer i
30 cm long.
Fig. 6-17, p.164
Ripple Marks: form in response to flow in one direction
Cross bedding within a ripple
To and fro motion of waves in shallow water
Current ripples in shallow stream
Wave formed ripple marks in shallow seawater
Fig. 6-19, p.165
Lion monument in Lucerne, Switz
Carved in sandstones which
illustrate cross bedding.
Also angular unconformity
with rocks above….
p.167
1.Many times paleocurrents of water (and sometimes wind) can be traced by the ripple
marks left in some sedimentary rocks indicating ancient river channels or beachfronts.
2. Mud cracks can also be preserved indicating ancient low energy mud flats.
3. Another type of bedding is known as graded bedding. This is where there is a
gradation in the size of particles within a unit of deposition. Larger particles are found
on bottom with successively smaller sediments on top. This type of bedding is formed by
"turbidity currents", which are the sudden flows of material down the continental
slopes. This causes the finer particles to be suspended in the water while the larger
particles fall out and are deposited on the bottom with smaller and finer sediment on top.
This results in a "gradation" in particle size. Graded bedding is deep water marine facies
VIII. The Marine Lithofacies:
This refers to the depositional sequence found in a cross section of a shore to deep-water
environment. The usual sequences of rock types are:
1. Sandstone formed on beach areas
2. Siltstone formed near-shore
3. Claystone/Shale formed further out
4. Limestone formed even further out in deeper waters
Mudcracks in ancient rocks in Montana-note cracks filled with
sediment
Fig. 6-20b, p.166
Mudcracks form in clay rich rocks-due to drying and shrinking
Fig. 6-20a, p.166
Formation of graded bedding-typically found in turbidites (deep sea)
Fig. 6-18, p.165
Bedding
Some features form long after rock is deposited: note the red-brown agate
that filled cavity, later filled by calcite crystals.
Fig. 6-21, p.166
Transgression: - the advancement of the sea onto the land because of a worldwide
increase in sea level or a subsidence of the landmass.
Regression: - the retreat of the sea from the land due to a worldwide drop in sea level
or the uplift of the land.
Transgressional and Regressional sequences of strata can be used to interpret and retrace
ancient coastlines.
Transgressional Sequence - reflects RISE in Sea Level
Regressional Sequence – reflects a FALL in Sea Level
Marine Transgression
Marine Regression
Note vertical succession of facies in each case-they are very different
Fig. 6-16, p.162
Marine transgression, evidence in Grand Canyon:
c. Muav Limestone
b. Bright Angel shale
a. Tapeats sandstone
Fig. 6-23b, p.173
Glacial Environments
• All sediments deposited in
– glacial environments are collectively called drift
• Till is poorly sorted, nonstratified drift
– deposited directly by glacial ice
– mostly in ridge-like deposits called moraines
• Outwash is sand and gravel deposited
– by braided streams issuing from melting glaciers
• The association of these deposits along with
– scratched (striated) and polished bedrock
– is generally sufficient to conclude
– that glaciers were involved
Moraines and Till
• Origin of glacial drift
• Moraines and poorly sorted till
Carbonate Environments
• Carbonate rocks are
– limestone, which is composed of calcite
– dolostone, which is composed of dolomite
• most dolostone is altered limestone
• Limestone is similar to detrital rock in some
ways
– Many limestones are made up of
• gravel-sized grains
• sand-sized grains
• microcrystalline carbonate mud called micrite
– but the grains are all calcite
– and are formed in the environment of deposition,
– not transported there
Limestone Environments
• Some limestone form in lakes,
–
–
–
–
but most limestone by is deposited
in warm shallow seas
on carbonate shelves and
on carbonate platforms rising from oceanic depths
• Deposition occurs where
– little detrital sediment, especially mud, is present
• Carbonate barriers form in high-energy areas
and may be
– reefs
– banks of skeletal particles
– accumulations of spherical carbonate grains known
as oolites
• which make up the grains in oolitic limestone
Ooids: carbonate grains deposited in high energy environment-rounded!!
Fig. 6-12a, p.159
Carbonate Shelf
• The
carbonate
shelf is
attached to
a continent
– Examples
occur in
southern
Florida
and the
Persian
Gulf
Carbonate Platform
• Carbonates may be deposited on a platform
– rising from oceanic depths
• This example shows a cross-section
– of the present-day Great Bahama Bank
– in the Atlantic Ocean southeast of Florida
Mineral Resources in Sedimentary Rocks
Oil and natural gas traps: a. stratigraphic traps, b. structural traps, c. salt
dome
Fig. 6-24, p.174
Alaska pipeline taking crude oil from Prudhoe Bay south to terminal.
Fig. 6-25, p.177
Economic Uses:
a. Iron ore mined from sedimentary rocks
b. Banded iron formation, Michigan. Alternating
Layers of red chert and silver colored iron minerals.
c. Iron ore mined and shaped into pellets.
Fig. 6-26, p.178
Chp 6: Sedimentary Rocks- Summary
A. Sedimentary Rocks are derived from weathering of igneous rocks
B. Two types of Sedimentary rocks:
-Chemical precipitates: Calcium carbonate
-Clastic rocks: sandstones, shales, etc.
C. 4 Steps to Formation of Sedimentary Rocks:
-Erosion: various weathering processes
-Transportation: air, glaciers, running water, etc
-Deposition
-Compaction: overburden, burial
D. Characteristics of Clastic Sedimentary Rocks
-Roundness: well rounded vs angular
-Sorting: well sorted vs poorly sorted
-Color: red=exposed to air; glauconite=green=marine
-Particle Size (see following diagram)
Table 6-1, p.152
Fossils commonly occur
In Sedimentary rocks:
a. Dinosaur excavation in
Wyoming.
b. Paleontologists excavating
Rhinoceros (foreground) and
Horse (background).
c. Mural showing animals
found in La Brea Tar Pits
from Los Angeles
p.170-172
Chp 6: Sedimentary Rocks- Summary
II. Chemical Precipitates
-Limestone: calcium carbonate
- Dolostone: calcium and magnesium carbonate
-Bioclastic deposits: form around remains of marine organisms.
e.g. chalk, coquina (shells),
-Evaporites: halite, gypsum, etc. Evaporitic conditions
-Coal: peat, lignite, bituminous coal, anthracite
Bedding/Layering of Sedimentary Rocks
-Aeolian: wind, cross bedding
-Fluvial: cross bedding, laminations, etc.
-Turbidites: graded bedding.
-Marine: currents-ripples
-Marginal marine to terrestrial: mud cracks
Laws of: Superposition and horizontality: oldest on bottom, flat
Chp 6-Sedimentary Rocks: Summary
Vertical Succession of Facies
-Transgressive: sea level rise
-Regressive: sea level fall
Economic Uses of Sedimentary Rocks:
-iron ore from fluvial deposits
-placer (fluvial) deposits: precious minerals-gold and silver
-Drilling for hydrocarbons (oil and gas)
stratigraphic traps, structural traps, salt domes
-gravel pits for road use
Chp 6: Sedimentary Rocks- Aeolian dunes
Chp 6: Sedimentary Rocks- Ripples
Chert: notice the concoidal fracturing along bottom edge; hard rock,
Composed of microscopic particles of quartz.
Fig. 6-14a, p.160
Alluvial Fans and Playa Lakes
• Alluvial fans form best along the margins of
desert basins
–
–
–
–
where streams and debris flows
discharge from mountains onto a valley floor
They form a triangular (fan-shaped) deposit
of sand and gravel
• The more central part of a desert basin
– might be the site of a temporary lake, a playa lake,
– in which laminated mud and evaporites accumulate
Sheep Rock, ORE.:
a. Sedimentary rocks
capped by thin lava
flow at top.
b. Sedimentary rocks
in Dry Fossil Beds Natl
Monument.
c. Mammals that lived in
Area from 37 to 55 million
Years ago:
1.Titanotheres
2. A carnivore
3. Ancient horses
4. Tapirs
5. rhinoceroses
Fig. 6-2, p.151
Ooids: carbonate grains deposited in high energy environment-rounded!!
Fig. 6-12a, p.159