Sedimentary Rocks

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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
Why???

• Only rocks that contain fossils
• Indicate ancient depositional environments

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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 Earths 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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Dune Cross-Beds

• Large-scale cross-beds in a Permian-aged
  wind-blown dune deposit in Arizona

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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/everest
7.htm
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Moraines and Till

• Origin of glacial drift

• Moraines and poorly sorted till

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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

• Stream/river-dominated deltas
• long distributary channels extending far seaward
• Mississippi River delta

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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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Sorting, Rounding

• 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

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Rounding and Sorting

• A deposit of well rounded and well sorted gravel

• Angular, poorly sorted gravel

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Cross-Bedding

• Tabular cross-bedding forms by deposition on sand
  waves

• Tabular cross-bedding in the Upper Cretaceous Two
  Medicine Formation in Montana

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Current Ripple Marks

• form in response to water or wind currents
  flowing in one direction
• asymmetric profiles allowing geologists to
  determine paleocurrent directions

http//www3.interscience.wiley.com8100/legacy/col
lege/levin/0470000201/chap_tutorial/ch03/chapter03
-5sedstr.html
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Wave-Formed Ripple Marks

• As the waves wash back and forth, symmetrical
  ripples form
• Wave-formed ripple marks in shallow seawater

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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.ht
m
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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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Barrier Islands

• 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

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Barrier Island Complex

• Subenvironments of a 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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Environmental Interpretations and Historical
Geology

• Present-day gravel deposits by a swiftly-flowing
  stream
• (Most transport and deposition takes place when
  the stream is higher)

• Nearby gravel deposit probably less than a few
  thousand years old

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Environmental Interpretations and Historical
Geology

• Conglomerate more than 1 billion years old
• shows similar features

• 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