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SEDIMENTARY ROCKS
Sedimentary rocks are rocks that have formed from (1) lithification of any type of sediment, (2) precipitation
from solution, or (3) consolidation of the remains of plants and animals.
A.
: The collective name for loose, solid particles that originate from:
•
•
and
of preexisting rocks.
Chemical precipitation from solution, including secretion by organisms in water.
Relationship of Transported Particle Size
to Water Velocity
Rock Cycle in Earth’s Crust
This generalized graph shows the water velocity needed to
maintain, but not start movement. Variations occcur due to
d i f f e r e n c e s i n p a r t i c l e d e n s i ty a n d s h a p e .
1.
Types of Sediment are classified by their size (Wentworth Classes).
a)
: A rounded particles coarser than 2 mm in diameter.
i.
: Range from 2 to 64 mm
ii.
:
iii.
_______________:
Range from 64 to 256 mm
Coarser than 256 mm
Angular fragments larger than 2 mm in diameter are called
b)
.
grains:
i.
ii.
ESC 101: Professor Vorwald
1/16 mm to 2 mm
Feels gritty between the fingers.
Sedimentary Rock Notes
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c)
i.
ii.
iii.
grains:
1/256 mm to 1/16 mm
Too small to see without magnification
Doesn’t feel gritty between fingers but feels gritty between teeth.
d)
i.
ii.
L
2.
The finest sediment
Less than 1/256 mm
is the term used for wet silt or clay.
Transportation of Sediment
a)
:
b)
The grinding away of sharp edges and corners of sand and
gravel as rivers, glaciers, or waves cause particles to hit and
scrape against one another
: The process by which sediment grains are selected and separated
according to: (1) size, (2) density, or (3) shape.
• Rounding and Sorting
# Well sorted and well rounded gravel
# Angular and poorly sorted gravel
• Degrees of Sorting - Well sorted to poorly sorted.
• Sorting by a stream - Horizontal sorting
3.
a)
:
When transported material settles.
b)
Also includes organic and biological extraction of sediment from solution
c)
Factors that affect rate of deposition: (
i.
Sediment Size:
ii.
Sediment Density:
iii.
Sediment Shape:
4.
)
: The general term for a group of process that convert loose
sediment into sedimentary rock.
a)
Process of lithification,
i.
ESC 101: Professor Vorwald
:
Weight of overlying sediment (
packs loose sediment grains tightly together
Sedimentary Rock Notes
)
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ii.
:
Precipitation of cement around sediment grains
binds them into a firm, coherent rock.
a)
Pore space is
.
b)
Common cements include:
1.
Calcium carbonate (
2.
Silica:
)
3.
(less common)
iii.
B
: Minerals may precipitate from solution, without
passing through the loose sediment stage.
Types of Sedimentary Rocks
1.
Sedimentary Rocks (from land-derived or terrigenous sediments)
a)
b)
A sedimentary rock is said to have a detrital texture when it consists of sediment
grains bound by cement into a rigid framework.
i.
Pore spaces are not completely filled.
ii
Rock fragments can be identifiable pieces of rock, or individual mineral
grains.
iii
Clay minerals are also considered fragments.
Detrital sedimentary rocks are classified by the
i.
ii.
iii.
:
of the particles they contain.
Cemented rounded gravel
______________________: Distinguished from conglomerate by angular fragments
and is not common.
_____________________:
a)
Medium grained
b)
_____________ : The fine-grained silt and clay found in spaces between the
sand grains and makes up a substantial volume of the rock.
c)
Types of sandstone
•
Sandstone:
More than 90% of grains are quartz . Because Sand grains have been
transported great distances the are well rounded and feldspar and other less
resistant minerals have weathered away.
•
: More than 25% of sand grains are feldspar. Most
grains are coarse and angular.
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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•
(“gray-whacky”
More than 15% of volume is a fine-grained matrix and consist of quartz,
feldspars of sedimentary, volcanic, and metamorphic rocks. Most probably
were transported by dense masses of sediment-laden water flowing downslope
along the sea floor (called turbidity currents)
iv
: Composed mostly of silt grains and does not
split into thin layers or have laminations.
v
a
b
c)
:
Fine-grained.
Noted for
(splitting into very thin layers.
Actually contains both silt and clay. Because clay is 2/3 the volume, it
is often classified by its smallest sized particles.
d)
is rock composed of predominantly clay-sized
particles.
e)
2.
is often used for all fine-grained detrital rocks.
Sedimentary Rocks:
•
Rocks formed by inorganic chemical processes.
•
Formed from
•
All have a
a)
_________________________
Examples include
evaporating seawater or from a saline lake.
b)
of minerals from water.
texture.
Formed from
_____________________________
Contain ______ as part of their chemical composition. The two main types of carbonates are
_________________ and ____________________.
i.
Most limestones are formed either directly or indirectly by the action of living things.
Limestones formed completely as a result of inorganic chemical process do not make
up a great volume of sedimentary rocks
However, limestone can be precipitated directly as the result of inorganic processes
and are included in the group of chemically formed sedimentary rocks by many
geologists
•
ESC 101: Professor Vorwald
Types of limestone formed by inorganic chemical processes include:
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limestone: Cementation of sand-sized spheres (oolites or
ooids) inorganically precipitated in warm, shallow seawater.
: Forms from fresh water in caves and in hot
springs when droplets of carbonate-rich water lose CO2.
The Formation of Travertine in Caves.
A.
Aqueous solution of water, Ca ions, and bicarbonate ions seeps into
caves. Dripstone forms.
B.
Limestone bedrock is dissolved by acidic rain near Earth’s surface
C.
A stalactite is an aggregate of calcite crystals that have been
chemically precipitated
___________: Precipitated in the fresh water of a continental spring or lake.
ii.
Dolostone (also called dolomite after its main mineral constituent)
a
Forms from limestone as calcium in calcite is partially replaced by magnesium
as water solutions move through the limestone.
Mg++
+
Magnesium
in solution
b.
c.
3.
a)
b)
2 CaCO3
calcite
v
CaMg(CO3)2 + Ca++
dolomite
calcium in
Solution
Mg-rich brines from surface evaporation trickle through existing limestone
layers.
Chemical reactions at the boundary between seawater and fresh underground
water.
(Organically formed) Sedimentary Rocks.
Limestones (Biochemical)
i.
These are carbonate rocks (contain CO3 as part of their chemical composition)
ii.
Precipitated through the actions of organisms (such as algae and corals) on
continental shelves in warm, shallow water.
iii.
These rocks have a crystalline texture and contain fossil remains of the
organisms still in their growth positions.
Limestones (Skeletal)
•
i.
Formed from wave-broken fragments of shells, corals, and algae.
________________: Coarse-grained with recognizable fossils shell fragments
ii.
______________: fine-grained, light colored, and porous from
marine organisms (plankton).
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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c)
:
i
ii
Forms from the compaction of plant material that has not
completely decayed.
Environment of deposition: Shallow swamps in a temperate or tropical climate
that have rapid plant growth and water with a low oxygen content.
Several varieties are recognized on the basis of the type of original plant
material and the degree of compaction (peat, lignite, bituminous)
C
•
•
1.
Features found within sedimentary rock
Usually form during or shortly after deposition
Significance
a)
Important because they provide clues of past environments and methods of sediment
transportation.
b)
May reveal the upward direction of deposition allowing for interpretation of the
geometry of folded and faulted rocks in tectonically active regions.
2
Types of structures:
a)
i
:
A series of visible layers within a rock.
Most are originally deposited as_________________________________
_________________________________________________, and the oldest
layer is usually on the bottom with the layers becoming younger upward
(____________________________________________________).
ii.
: Nearly flat surface of deposition separating
two layers of rock.
•
_________________: Boundary surfaces between two different types
or ages of rocks. These are usually bedding planes.
b)
:
i
ii
A layer with a
change in particle size (coarse to fine at the
top.
Most likely to occur in a turbidity current.
c)
(Figure 6.28 on page 145 of your text)
•
•
•
A series of thin, inclined layers within a larger bed of rock.
These layers form a distinct angle to the horizontal.
Most common in sandstone
i.
Development of Cross-Bedding in Wind-Blown Sand
(1)
Sand is deposited in inclined layers on the downwind side of a dune
(2)
The second dune covers first.
(3) Cross-bedding changes direction as the wind direction shifts.
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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ii.
Development of Cross-Bedding by a Water Current
(1)
The current fills in a depression in the river or sea floor bottom with sediment.
(2)
Continued sedimentation often covers the first set of cross-beds with another
set.
(3)
The cross-bedding remains in the same direction since the current doesn’t
change direction.
i
ii
Polygonal cracks formed in very fine-grained sediment as it dries.
Only form in environments where sediment is exposed above water.
d)
e)
Small ridges formed on the surface of sediment by
moving wind or water.
i.
ii.
Symmetrical Ripples:
From oscillating (back and forth) motion of waves.
Asymmetrical Ripples:
•
Formed by water currents
•
Steeper sides in the down-current direction
f)
:
i.
ii.
iii.
iv.
Any trace of plants or animals preserved in rock.
Hard parts of organisms are most likely to be fossilized.
Must be covered by a protective layer (usually sediment) before it decays.
Original bone or shell is rarely preserved.
Body Fossils
(1)
Unaltered remains: Remains of organisms (e.g, bone, shells, teeth,
mummification, freezing, preservation in amber) - They keep their original
composition and structure)
(2)
Altered remains: Change in structure as a result of permineralization,
recrystallization, replacement, carbonization
D
1.
A body of rock of considerable thickness with
characteristics that distinguish it from adjacent
rock units.
2.
Usually composed of one or more sedimentary
rock beds.
3.
Often based on rock type.
4.
Criterion for distinguishing and naming a formation is some visible characteristic that makes it
recognizable. For example:
•
A sequence of limestone beds may have different fossils in the lower half than in the
upper half.
•
It would be divided into two formations based on the fossil content.
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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G
Environments of Deposition
1.
Sedimentary rocks allow geologists to determine ancient environments of deposition. The
type of sedimentary rock, type of sorting and rounding of the grains, sedimentary structures
and fossils present are clues used.
2.
Continental Environments:
a
Glacial Environments are characterized by an unsorted mix of unweathered boulders,
cobbles, pebbles, sand, silt, and clay.
b
Alluvial Fans
i
Broad fan-shaped piles of sediment deposited as rivers flow from mountains
onto broader plains.
ii
Often are characterized by coarse sandstones and cross-bedding
c
River Channel and Floodplains
i
River deposits typically are cross-bedded and have ripple marks
ii
Floodplains have thin-bedded shales characterized by mudcracks and fossil
animal footprints.
d
3.
Lakes (Often referred to as lacustrine deposits)
i
Thin-bedded shales
ii
Often contain fish fossils
Shallow Marine Environments
•
Deltas
i.
A body of sediment deposited when a river flows into a sea or lake.
ii.
Generally made of thick sequences of siltstone and shale
iii.
Characterized by low-angle cross-bedding cut by coarser channel deposits
•
Beach, Barrier Island, Dune
i.
ii.
iii.
Well-sorted quartz sand with rounded grains
Cross-bedding
Tropical regions have carbonate sands
•
Lagoon
i.
Semi-enclosed, quiet bodies of water between a barrier island and a mainland
ii.
Fine-grained, dark shale
iii.
Many fossils of marine organisms
•
Shallow Marine Shelves
i.
Widespread deposits of sandstone, siltstone, and shale deposits as sediments
are transported offshore
ii.
May contain symmetrical ripples and cross-bedding
•
Reefs
i.
Massive limestones
ii.
Fossils of corals, coralline algae, and many other marine organisms
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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4.
Deep Marine Environment
•
Deep sea floor
•
Shales and graywacke sandstone deposited by turbidity currents
•
Characterized by graded bedding and asymmetrical current ripple marks
ESC 101: Professor Vorwald
Sedimentary Rock Notes
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