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SEDIMENTARY ROCK: Made from sediments consolidated at the
earth's surface. The sediments are deposited at the earth's
surface by water, wind, glacial ice, or bio-chemical processes.
Typically the sediments are deposited in layers under COOL
conditions [thereby distinguishing it from a layered volcanic
rock (TUFF)].
Clastic - made up of CLASTS (also termed grains or detritus)
& CEMENT (calcite, quartz, or hematite)
- Cementation is due to DIAGENESIS (p. 44)
Chemical - made of chemical or evaporite sediments (see below); rock usually made up
of 1 mineral
- Examples: Limestone, Dolomite, Evaporites (p. 50).
WEATHERING: Changes that take place in a rock exposed at the
Earth's surface.
- Mechanical - Big to little particles - NO CHEMICAL
CHANGE!
- Chemical - refers to 1) dissolution of minerals or 2)
formation of new minerals that are more stable at the
lower temperature, lower pressure, and higher moisture at
the Earth's surface (Example: feldspar --> clay).
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For Clastic rocks: Clasts (sediments) are physically
transported by wind, water, glaciers, and/or gravity.
- Clasts are deposited when the transportation energy is
insufficient to move the particle. As transportation
energy decreases, the larger (heavier) particles are
deposited first.
- HIGH ENERGY environments can transport large and small
particles
- LOW ENERGY environments can transport only small
particles
Examples of high and low energy environments?
See p. 46, Fig. 3-1
| Low Energy Environment |
High Energy Environment |
Lagoon
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Storm-Dominated Beach
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Deep Lake
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Steep Mountainside (Landslide)
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Deep Ocean
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Tornado
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Tsunami
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NOTE: The longer the transportation distance, the more
exposure the sediment has to chemical and physical weathering!
For Chemical rocks: Chemical
sediments are generated by the precipitation of minerals out of
solution by biological activity, chemical change, or evaporation.
These minerals (chemical sediments) are precipitated when
the dissolved ion concentration becomes too large to remain in
solution.
Where do these dissolved ions come from?
- This is an important question since the dissolved ions
are the "building blocks" for the chemical
sediments. It is the chemical weathering of sediments
(dissolution of minerals) that releases these ions into
solution. Once in solution, the ions can travel in
groundwater, rivers, etc. to their final destination
(such as an ocean, lake, cave, etc.) where they are
precipitated as minerals.
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1) Particle Size:
- Clay (Very Fine Grained): < 1/256 mm
- Silt (Fine Grained) : 1/256 to 1/16 mm
- Sand (Medium Grained) : 1/16 to 2 mm
- Pebbles (Coarse Grained) : > 2 mm
Particle size indicates the energy of the transporting medium.
The larger the size of grains in a clastic rock, the more energy
it took to move that particle to the place of deposition!
2) Angularity: Degree to which the individual
sedimentary particles are rounded.
- WELL ROUNDED - all corners of a grain are rounded off
- ANGULAR - all corners of a grain are still sharp
(pointed)
The more the corners of an individual grain in a clastic rock
are rounded, the longer the distance the grain has been
transported.
3) Sorting: Degree to which the sedimentary particles
are the same size.
- POORLY SORTED - large and small grains jumbled together
- WELL SORTED - all grains are the same size!
Poor sorting suggests that the particles have not been
transported very far.
What would be some general trends seen if you were to look at
the sediments being weathered, eroded, and transported from a
granite mountain to the plains to a deep lake or ocean?
General trends:
Distance from sediment source vs. texture and
mineralogy
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At source
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Near source
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Intermediate
from source
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Far from
source
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Farthest
from source
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| Largest grain size seen |
boulder-sized |
pebble to sand |
sand |
silt |
clay-sized |
| Angularity |
angular |
sub-rounded |
rounded |
well rounded |
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| Sorting |
poor |
poor to moderate |
moderate to well |
well |
well |
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| Mineralogy |
Q,F,M |
Q,F,(M) |
Q,(F) |
Q |
clay |
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| Typical clastic rock |
breccia |
conglomerate, arkose |
quartz sandstone |
siltstone |
shale |
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Sedimentary Structures and
Fossils--things found in sedimentary rocks
- Stratification - horizontal layering at time of
deposition
- Ripple Marks - undulatory structures due to wind/water
current
- Cross Bedding - internal angular layering within a
horizontal bed (due to ripple mark remnants)
- Mud Cracks (Desiccation Cracks) - when mud (basically
clay) dries, it shrinks and pulls apart from
itself--forming polygonal columns.
Fossils - evidence of past life (either a remnant of the
organism itself or its activity); can indicate where the
sediments were deposited!
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Sedimentary rocks listed in black are clastic
sedimentary rocks. Those listed in blue
are chemical sedimentary rocks.
- Breccia (consists of coarse, angular grains w/poor
sorting)
- Conglomerate (coarse, rounded grains w/poor
sorting)
- Sandstone (rock consisting of sand sized particles
of any composition (quartz is most common)
- Arkose (sandstone rich in orthoclase feldspar)
- Siltstone (Grain size in between sandstone and
shale)
- Shale, mudstone (Very fine grained clastic rock
made of clay minerals derived by chemical weathering of
feldspar)
- Coquina -
Limestone made of cemented shell fragments
- Limestone - made
mostly of calcite precipitated chiefly through the agency
of living organisms -- "fizzes" vigorously
- Dolomite - an altered
limestone that "fizzes" sluggishly
- Rock salt - NaCl
(halite); an evaporite mineral
- Rock gypsum - CaSO4 *
2 H2O (gypsum); an evaporite mineral
- Chert - dense, hard
sediment consisting of micro-crystals of quartz (looks
like unglazed porcelain)
- Coal - carbon
rich rock consisting of altered plant material
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