Transcript Sedimentary rocks

Sedimentary Rocks —
The Archives of Earth History
?
Disconformity
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
Historical geology focuses on
sedimentary rocks
•Why???
• Only rocks that contain fossils
• Indicate ancient depositional
environments
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?
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?
What is a sedimentary rock?
• Rock that forms at or near Earth’s surface
• 3 types
– Clastic
– Chemically-precipitated
– Biogenic
How do clastic sedimentary rocks
form?
• Weathering
• Transport
• Deposition
• Lithification
How do other sedimentary rocks
form?
• Chemical – precipitation of dissolved
materials
• Biogenic (organic) – accumulations of
organic material
Environments of Deposition
• At or near surface of Earth
• Marine
• Continental
• Transitional (deltas, barrier islands,
beaches)
Marine
• Coastal
• Shelf
• Deep water
www.ngdc.noaa.gov/mgg/image/sedthick.jpg
Continental
• Fluvial
– Meandering
– Braided
• Desert
• Lacustrine
www.uoregon.edu/~millerm/depenv.html
• Glacial
Continental
• Fluvial
– Meandering
– Braided
• Desert
• Lacustrine
www.uoregon.edu/~millerm/depenv.html
• Glacial
Continental
• Fluvial
– Meandering
– Braided
• Desert
• Lacustrine
www.uoregon.edu/~millerm/depenv.html
• Glacial
Dune Cross-Beds
• Large-scale crossbeds in a Permianaged wind-blown
dune deposit in
Arizona
Continental
• Fluvial
– Meandering
– Braided
• Desert
• Lacustrine
www.mikelevin.com/DLBlissParkTahoe.jpg
• Glacial
Continental
• Fluvial
– Meandering
– Braided
• Desert
• Lacustrine
www.peakware.com/encyclopedia/peaks/photos/everest7.htm
• Glacial
Moraines and Till
• Origin of glacial drift
• Moraines and poorly sorted
till
Delta
• Form in oceans or
lakes (marine and
non-marine)
www.uoregon.edu/~millerm/depenv.html
Stream/River-Dominated Deltas
• Stream/riverdominated
deltas
– long
distributary
channels
extending far
seaward
– Mississippi
River delta
Where would you find different
sedimentary rocks?
Where would you find different
sedimentary rocks?
Where would you find different
sedimentary rocks?
• Grain size is controlled by energy
• High energy
– River
– Beach
Large grains
• Low energy
– Lake
– Deep ocean
Small grains
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
Rounding and Sorting
• A deposit of well
rounded and well
sorted gravel
• Angular,
poorly sorted
gravel
Cross-Bedding
• Tabular crossbedding forms by
deposition on
sand waves
• Tabular crossbedding in the
Upper Cretaceous
Two Medicine
Formation in
Montana
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.com:8100/legacy/college/levin/047000
0201/chap_tutorial/ch03/chapter03-5sedstr.html
Wave-Formed Ripple Marks
• As the waves
wash back
and forth,
symmetrical
ripples form
• Wave-formed
ripple marks in
shallow
seawater
Modern Deposition near Houston
• Fluvial
– Brazos, Colorado, Trinity, San Jacinto Rivers
• Transitional
– Deltas, barrier islands
• Marine
– Gulf of Mexico
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?
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
Brazos River
• Longest river in Texas – 1450 km
• Highest sediment supply of any Texas
river
• Originates in New Mexico
Where does deposition occur?
www.uwsp.edu/geo/courses/geog391/toriv/Diagrams.htm
Point Bars
• Sediment deposited within the inside bank
of a meander loop
• Fining upward sequence (grain size
decreases)
• Coarsest sediment deposited by highest
energy
Brazos River – Point Bar
Brazos River – Cut Bank
Coastal – Galveston Island
Barrier Islands
• Formed during sea level rise
• Rate of SL rise and rate of sediment
deposition approximately equal
• Wave-dominated environment
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
Barrier Island Complex
• Subenvironments of a barrier island complex
Texas Coast
• Most Texas beaches relatively fine-grained
• Low gradient of rivers like Brazos
• Why is this a problem?
Brazos Delta
gulf.rice.edu
Environmental Interpretations and
Historical Geology
• Present-day gravel
deposits by a swiftlyflowing stream
(Most transport and
deposition takes place
when the stream is
higher)
• Nearby gravel deposit
probably less than a
few thousand years old
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