sedimentary rocks — the archives of earth history ? disconformity
TRANSCRIPT
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Sedimentary Rocks —The Archives of Earth
History
?
Disconformity
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There are actually 3 types of rocks…
• Igneous – cool from liquid (magma or lava)
• Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure
• Sedimentary – form mainly from deposition of sediments
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Historical geology focuses on sedimentary rocks
• Only rocks that contain fossils
• Indicate ancient depositional environments
•Why???
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What is a sediment?
• Fragment of pre-existing rock (or animal shell)
• Why does water off Galveston look murky, while water off Florida looks clear?
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What kind of rocks do we find around Houston?
• Not many rocks!
• Lots of unlithified sediment
• Why do many houses in Houston have foundation problems?
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What is a sedimentary rock?
• Rock that forms at or near Earth’s surface
• 3 types– Clastic– Chemically-precipitated– Biogenic
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How do clastic sedimentary rocks form?
• Weathering
• Transport
• Deposition
• Lithification
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How do other sedimentary rocks form?
• Chemical – precipitation of dissolved materials
• Biogenic (organic) – accumulations of organic material
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Environments of Deposition
• At or near surface of Earth
• Marine
• Continental
• Transitional (deltas, barrier islands, beaches)
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Marine
• Coastal
• Shelf
• Deep water
www.ngdc.noaa.gov/mgg/image/sedthick.jpg
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Continental
• Fluvial– Meandering– Braided
• Desert
• Lacustrine
• Glacial
www.uoregon.edu/~millerm/depenv.html
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Continental
• Fluvial– Meandering– Braided
• Desert
• Lacustrine
• Glacial
www.uoregon.edu/~millerm/depenv.html
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Continental
• Fluvial– Meandering– Braided
• Desert
• Lacustrine
• Glacial
www.uoregon.edu/~millerm/depenv.html
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• Large-scale cross-beds in a Permian-aged wind-blown dune deposit in Arizona
Dune Cross-Beds
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Continental
• Fluvial– Meandering– Braided
• Desert
• Lacustrine
• Glacial
www.mikelevin.com/DLBlissParkTahoe.jpg
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Continental
• Fluvial– Meandering– Braided
• Desert
• Lacustrine
• Glacial
www.peakware.com/encyclopedia/peaks/photos/everest7.htm
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• Moraines and poorly sorted till
Moraines and Till
• Origin of glacial drift
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Delta
• Form in oceans or lakes (marine and non-marine)
www.uoregon.edu/~millerm/depenv.html
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• Stream/river-dominated deltas – long
distributary channels extending far seaward
– Mississippi River delta
Stream/River-Dominated Deltas
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Where would you find different sedimentary rocks?
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Where would you find different sedimentary rocks?
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Where would you find different sedimentary rocks?
• Grain size is controlled by energy
• High energy– River– Beach
• Low energy– Lake– Deep ocean
Large grains
Small grains
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• If the size range is not very great, – the sediment or rock is well sorted
• If they have a wide range of sizes, – they are poorly sorted
• Wind has a limited ability to transport sediment so dune sand tends to be well sorted
• Glaciers can carry any sized particles because of their transport power, so glacier deposits are poorly sorted
• Grains more rounded with longer transport
Sorting, Rounding
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• A deposit of well rounded and well sorted gravel
Rounding and Sorting
• Angular, poorly sorted gravel
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• Tabular cross-bedding forms by deposition on sand waves
Cross-Bedding
• Tabular cross-bedding in the
Upper Cretaceous Two Medicine Formation in
Montana
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• form in response to water or wind currents flowing in one direction
• asymmetric profiles allowing geologists to determine paleocurrent directions
Current Ripple Marks
http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch03/chapter03-
5sedstr.html
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• As the waves wash back and forth, symmetrical ripples form
• Wave-formed ripple marks in shallow seawater
Wave-Formed Ripple Marks
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Modern Deposition near Houston
• Fluvial– Brazos, Colorado, Trinity, San Jacinto
Rivers
• Transitional– Deltas, barrier islands
• Marine– Gulf of Mexico
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Ancient Environments
• Important for historical geology
• Important for oil companies (need to know where sand was deposited)Why are we looking at
modern depositional environments?
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Present is the key to the past
• Study modern depositional environments to learn about ancient ones
• Knowledge of ancient environments helps oil companies and historical geologists
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Brazos River
• Longest river in Texas – 1450 km
• Highest sediment supply of any Texas river
• Originates in New Mexico
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Where does deposition occur?
www.uwsp.edu/geo/courses/geog391/toriv/Diagrams.htm
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Point Bars
• Sediment deposited within the inside bank of a meander loop
• Fining upward sequence (grain size decreases)
• Coarsest sediment deposited by highest energy
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Brazos River – Point Bar
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Brazos River – Cut Bank
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Coastal – Galveston Island
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Barrier Islands
• Formed during sea level rise
• Rate of SL rise and rate of sediment deposition approximately equal
• Wave-dominated environment
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• On broad continental margins with abundant sand, long barrier islands lie offshore separated from the mainland by a lagoon
• Barrier islands are common along the Gulf and Atlantic Coasts of the United States
• Subenvironments of a barrier island complex: – beach sand grading offshore into finer deposits– dune sands contain shell fragments (not found in
desert dunes)– fine-grained lagoon deposits
Barrier Islands
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• Subenvironments of a barrier island complex
Barrier Island Complex
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Texas Coast
• Most Texas beaches relatively fine-grained
• Low gradient of rivers like Brazos
• Why is this a problem?
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Brazos Delta
gulf.rice.edu
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• Present-day gravel deposits by a swiftly-flowing stream(Most transport and
deposition takes place when the stream is higher)
Environmental Interpretations and Historical Geology
• Nearby gravel deposit probably less than a few thousand years old
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• Conglomerate more than 1 billion years old – shows similar
features
Environmental Interpretations and Historical Geology
• We infer that it too was deposited by a braided stream– Why not deposition by glaciers or along a
seashore?– No evidence for either glacial activity or
transitional environment