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Sediments and Sedimentary
Rocks
Chapter 8
Sediments
•Cover most of the land surface and
seafloor
•Your physical geography determines the
dominant process at work in the
sedimentary rock cycle
•e.g. (1) Mountains => weathering and/or
erosion
•e.g., (2) Southern Louisiana =>
•e.g., (3) offshore Louisiana =>
Sediments
•Cover most of the land surface and
seafloor
•Your physical geography determines the
dominant process at work in the
sedimentary rock cycle
•e.g. (1) Mountains => weathering and/or
erosion
•e.g., (2) Southern Louisiana =>
transportation or deposition or erosion
•e.g., (3) offshore Louisiana =>
Sediments
•Cover most of the land surface and
seafloor
•Your physical geography determines the
dominant process at work in the
sedimentary rock cycle
•e.g. (1) Mountains => weathering and/or
erosion
•e.g., (2) Southern Louisiana =>
transportation or deposition or erosion
•e.g., (3) offshore Louisiana => erosion or
sedimentation
erosion
weathering
transportation
Erosion includes BOTH weathering and transportation
Sedimentary rocks are typically
layered, (although layering is not
diagnostic of only sedimentary rocks)
Where do you live?
•What dominant
sedimentary process is at
work where you live?
If deposition is the dominant process, e.g.,
offshore Lousiana then rocks are in the
process of being formed:
Diagnesis includes
(1) compaction = volume loss (mechanical
squeezing)
and is accompanied by dewatering (= water
loss) (by chemical or physical means)
(2) changes in mineral composition (chemical
process with heat and or fluids)
(3) cementation (physical)
If a sediment eventually becomes a rock we
say it is lithified.
DIAGENESIS
compaction
DIAGENESIS
compaction
dewatering
DIAGENESIS
compaction
dewatering
Cementation &
mineral
changes
Geological FUZZY rules
for determining degree of
weathering and transport a
rock or sediment has
experienced respectively
(1) Product composition
(2) Degree of sorting
•Sorting is a measure of how similar grain sizes
are within a sediment or rock and tells us about
the relative strength of the current before it
dropped (deposited) it cargo.
•In a current of water or air, the larger and
denser grains fall faster than the smaller
grains. That is, for the same density, size
determines settling velocity.
(3) Angularity or roundness
(antonym) is a measure of the
distance of transportation
Cross-bedding
Cross-bedding: sets of bedded material
within rock layers that are inclined at
angles as large as 35 degrees from the
horizontal. These latter indicate windblown conditions in either a desert or a
beach.
2 directions of
fluid movement
Fossil example of the past activity of organisms
mixing sediment --- an example of fossil
BIOTURBATION
A sedimentary environment is a
geographic location that has a
peculiar combination of geological
processes
Walther’s Rule (1894)
“The different (sedimentary) rocks
(types) were formed beside each
other in space, but in a crustal profile
we see them lying on top of each
other….”
Bedding sequences--are successions of rock ( in
a vertical profile) that help
geologists work out the
past environment
Three types of sedimentary
Rocks
•Clastic
•Biochemical
•Chemical
Major Classes of sediments
and sedimentary rocks
For clastic sedimentary rocks there is a
classification scheme based on the SIZE of their
clasts, (or rock fragments) that comprise them.
Clst size indicates ancient relative current velocity
weak moderate
>=1.8 km/hr (strong currents)
The End of Chapter 8
Frank Press • Raymond Siever • John Grotzinger • Tom Jordan
Understanding Earth
Fourth Edition
Chapter 8:
Sediments and Sedimentary Rocks
Copyright © 2004 by W. H. Freeman & Company