Today: Origin and Interpretation of Sedimentary Rocks
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Transcript of Today: Origin and Interpretation of Sedimentary Rocks
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Today: Origin and Interpretation of Sedimentary Rocks
Wed: Origin and Interpretation of Metamorphic Rocks
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Sedimentary Rocks — Why Care?
• They are the repository of Earth’s history, especially the
history of how the surface environment (including life)
changed over time. To extract this history, we need to be
able to “read” this rock record.
• They hold all of the oil, gas,coal, and water — locating
these materials requires an understanding of the
physical characteristics of different sedimentary rocks
and how those characteristics change in 3-D.
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Coconino Sandstone
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Coconino Sandstone up close
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thin slice of Coconino sandstone seen through microscope
un-cemented quartz sand grains from a sand dune
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So how are sedimentary rocks made?
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Physical & Chemical Weathering → Sediment & Dissolved Ions
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hincks146-147
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Sedimentary Structures in the Coconino
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This is the modern environment where large cross-beds form
Wind Direction
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Fig. 5.21abc
W. W. NortonSo cross beds are tilted in the direction of flow (downstream)
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Fig. 5.21d
Stephen Marshak
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ripples
Stephen Marshak
Uniformitarianism — depositional environments of today are physically the same as ancient environments, so clues from the modern can be used to infer ancient environments from sedimentary rocks, or “the present is the key to the past”
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Ancient ripples
Stephen Marshak
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Mud cracks
Stephen Marhsak
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Ancient mudcracks
Stephen Marhsak
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Fig. 5.24c
© Martin Miller
Modern Alluvial Fan, Death Valley
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Ancient Alluvial Fan Deposits
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OOIDS (made of calcite — CaCO₃)imply shallow (<3m), tropical marine depositional environment
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Ooids with cement, seen in a thin section, through a microscope
800 meters of oolitic limestones
Monte Bove in Italy
2 mm
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So, the question is — how can we explain such a thick pile of shallow-water sediments?
We need to think about isostasy and sea level change in our search for an explanation of how we can get such thick piles of sediment, all of which appear to be deposited in fairly shallow water.
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Isostasy
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Isostasy
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Equilibrium level for wooden block
time 1 time 4
time 1 time 4
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So, the question remains — how to get such a thick pile of shallow-water sediments?
Add a WeightIncrease DensityThin CrustRaise Sea Level
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As sea level goes down, previously deposited sediment will be eroded
In our case, crustal thinning makes sense because the
sediments in question were deposited during and after
continental rifting.
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• Make sediment
• Transport & deposit sediment
• Create space to allow sediment to accumulate
• Compact and cement sediment
Depositional environments are interpreted based on studies of modern environments where sediment is deposited.
Summary on Making and Interpreting Sedimentary Rocks
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Metamorphic rocks
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