Transcript CEE437Lec3Fall06

CEE 437
Rocks!
Thomas Doe
Silica Tetrahedron
Feldspar Structure
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All tetrahedra corners
occupied
Both Si and Al tetrahedra
Cations in voids of
structure to balance
charge
Feldspar Compositions
Clay Minerals
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Extremely Important Mineral Group
Seals
 Stability
 Pore pressure
 Chemical interaction
 Swelling
 Slaking
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Confusion as both “Size” and “Mineral”
Classification
Clay Sources
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Weathering
Hydrothermal Alteration
Deposition
Clay Transformations
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Feldspar  Illite, Kaolinite
Ferro-Magnesian  Chlorite
Volcanics (alkaline conditions)  Smectite
Volcanics (acidic conditions)  Kaolinite
Bentonite: plastic, highly swelling
Clay Units
From West, Geology Applied to Engineering, Prentice Hall, 1995)
Two and Three-Layer Clay Structure
From West, Geology Applied to Engineering, Prentice Hall, 1995)
Mixed Layer Clays
From West, Geology Applied to Engineering, Prentice Hall, 1995)
Clay Viewed from Electron
Microscope
Northwest Igneous and
Metamorphic Rocks
Cascade Volcanoes
(recent)
Cascade Batholiths
(Felsic, CretMiocene)
Columbia River
Basalts
(miocene)
Recent
Basaltic
Volcanism
(Newberry
Crater)
Snake River
Basalts
(pliocene)
Yellowstone Region
Acidic Volcanics
(Pleistocene to
recent)
Geologic Settings for Igneous
Rocks
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Oceanic
Hi Fe, Mg, Ca, low Si
 basalt, gabbro
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Continental
Hi Si, Na, K
 granite, rhyolite, andesite
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Igneous Origins
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Intrusive
Batholithic or plutonic: phaneritic
 Dikes or sills that chill rapidly: aphanitic
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Extrusive
deposition as melt (lava)
 pyroclastic
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tuff
 tephra
 pyroclastic flows
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Identifying Igneous Rocks
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Chemistry
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Acidic: Basic (more Si, less Si)
Texture
Aphanitic: crystals not visible
 Phaneritic: made of visible crystal components
 Porphyritic: Larger crustals in aphanitic or phaneritic
ground mass
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Igneous Rock Classification
Acidic, Felsic
Basic, Mafic
Ultramafic
SERPENTINITE
Magma Generation on Continental
Margins
Magma Generation in Convergent
Continental Plate Margins
Extrusives
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Viscosity varies with Si and water content
Basalt — low viscosity
 Rhyolite — high viscosity
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Rhyolite flows relatively unusual as rhyolite does
not flow well
Explosive
 Tuffs, pyroclastics
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Volcano Types
Basaltic: low
viscosity — Hawaii,
Columbia Plateau
Andesitic/Rhyolitic
Structures of Basalt Flows
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Lava Tubes
Flow Stratigraphy
collonade
 entablature
 flow top breccia/scoria
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Hawaii Basalt Flows
Basalt Flow Structures
Eruptions of Acid-Rock Volcanoes
Rhyolite Dome
Caldera
Mt. St. Helen’s Blast Zone
Mt. Mazama Ash Distribution
Basic Metamorphic Types
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Quartz Sandstone  Quartzite
Limestone, Dolomite  Marble
Shale 
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Slate — cleavage, no visible xl’s
Phyllite — foliation, mica sheen but xl’s not visible
Schist — clear foliation, visible mica
Gneiss — like granite but with foliation/gneissosity
Basalt  greenschist, amphibolite
Non-foliated Metamorphic Rocks
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Sandstone —> Quartzite
Limestone —> Marble
Dolomite —> Dolomitic Marble
Foliated Metamorphic Rocks
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Shale/Mudstone
Slate
Phyllite (Greek for leaves e.g. phyllo dough)
Schist
Gneiss
Origin of Foliation (gneissosity,
schistosity)
Engineering Properties
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Anisotropy of strength and elastic properties
Preferred failure on foliation
Slate
Phyllite
Schist
Chlorite Schist
Gneiss
Banded Gneiss
Metamorphic Grade
Subduction-Zone Metamorphism
Metamorphism at Continental
Collisions
Contact Metamorphism
Sedimentary Rocks
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Clastics, Siliciclastics, and Evaporites
Clastic rocks, depositional medium, and energy
Diagenesis — chemical changes after deposition
Rock Cycle
Crystallization at depth or
extrusion at surface
Igneous Rocks
Sediments
Lithification
Magma
Burial, metamorphism,
recrystallization
Weathering,
Erosion
Melting
Metamorphic
Rocks
Burial,
metamorphism,
recrystallization
Sedimentary Rocks
Sediment Sources
Clastic Sedimentary Rocks
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Clastic — broken like iconoclast)
Often referred to as Siliciclastics as having Si
based rock forming minerals
Based on grain size and to a lesser extent
composition
Grain size related to energy of depositional
environment
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Relationship of medium velocity to maximum grain
size)
Clastic Sedimentary Rocks
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Clay, muds  shales, mudstones, claystones
(difference based on fissility)
Silts  siltstones
Sands  sandstones
Gravels  Conglomerates (Breccia if angular,
breccia may also be a term for tectonically
fragmented rock)
Weathering Cycle
Clastic Sediments
Clay Minerals
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Sheets of linked silica tetrahedra sandwiching
octahedral layers of gibbsite composition, Al2(OH)6, or
brucite Mg3(OH)6
Major Clay Groups
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kaolinite: single gibbsite layer
montmorillonite:weak water bonding between layers,
moderated by Ca, Na, or K (near-shore environments)
illite: K bonds between layers (off-shore environments)
bentonite: highly expansive, volcanic-derived, Na-rich
montmorillonite
Clay Structure
Clay Structure Cont’d.
Kaolinite
Illite
Montmorillonite
Lithification
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Cementation
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Crystallization
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deposition of a material different from clasts
crystal growth on clasts to fill pore space
Compaction
Diagenesis
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Early post-depositional chemical transformation of
sediments, e.g. calcite to dolomite
Carbonates
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Generally like siliciclastics — carbonate muds,
sands, etc.
Often deposited in reefs
Major portion of world oil deposits
Properties depend strongly on post-depositional
pore chemistry
Cementation
 Dissolution
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Karst topography, cave formation
Carbonate Environments
Evaporites
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Rock salt (NaCl), Gypsum-Anhydrite (CaSO4),
Sylvite (KCl)
Deposition in regions where evaporation
exceeds recharge
desert lakes
 restricted seas (Mediterranean)
 lagoons, back-reef areas
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Subject to flow and diapirism
Other Sedimentary Rocks
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Chert: finely crystalline silica
as replacement/diagenetic nodules
 as bedded material from silica-shelled biota
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Coal
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Derived from vegetation
Banded Iron Formation
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Likely bacteria derived, mainly Pre-Cambrian
Sedimentary Rocks and Rock
Properties
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Properties for a given geologic description vary
wildly based on cementation, porosity and other
diagenetic factors.
Properties can be strongly anisotropic and
heterogeneous based on bedding
Depositional Environments
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Synchroneity of deposition of different rock types
Sedimentary facies
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Related to energy of environment
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A rock unit is not everywhere the same age: Bright Angel
Shale
(example channels and banks in fluvial systems)
Energy related to topography, climate, and tectonic
activity
Sediment Sorting
Sedimentary Structure — Cross
Bedding
Fluvial and Lacustrine Environments
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Fluvial
Channelization
 Complex and close interrelationship of fine and
course sediments
 Challenge for characterization due to high
variability
 Special examples: glacial environments
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Lacustrine
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Deltaic deposits at margins, finer materials in lake
beds
Deltaic Environments
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Variability based on proximity to source
Stratigraphy effected by progradation
Deltaic Development and
Sedimentary Facies
Continental Slope Environments
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Turbidites and turbidity currents
Graded bedding
poor sorting
 vertical zonation with fining upwards
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Turbidites and Turbidity
Currents
Metamorphic Classification
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Original Material
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sandstone, limestone, shale, basalt)
Metamorphic Grade (Temperature, Pressure)
Source of Metamorphism (Regional, Contact)
Glacial Environments
K. Troost