easc116 ch. 6 - sedimentary rocks
TRANSCRIPT
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CH. 6 – SEDIMENTARY RX
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Sediment:
- weathering products (gravel, sand, clay minerals)
- chemical precipitates
- organic remains
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Sedimentary Rocks
“sedimentum” = “settling”
Sediment settles out of wind or water
- forms layers at the surface
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Importance of Sedimentary Rx
1) Form 75% of exposed rx at surface (outcrops)
- compose only 5% of total crust
2) Contain clues to reconstruct past environments on Earth
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Importance of Sedimentary Rx
3) Fossil record
4) Economic value
- coal, petroleum, construction
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Sedimentary Rx Classification
1) Detrital = weathering products
(mechanical or chemical)
Named based on particle size
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Detrital Sedimentary Rocks
Particle Size Name Gravel Breccia
(angular) Conglomerate (rounded)
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Detrital Sedimentary Rocks
Particle Size Name Sand Quartz Sandstone
(quartz sand)
Arkose (K-spar)
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Detrital Sedimentary Rocks
Particle Size Name
Sand Graywacke
(rock frags,
mafic minerals)
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Detrital Sedimentary Rocks
Particle Size Name
Clay-size *Shale
(clay minerals)
*Shale is the most abundant detrital sedimentary rock
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Detrital Rock Process
1) Weathering
2) Erosion (transportation)
3) Deposition (sediment settles)
4) Lithification – turning sediment into rock (“lithos” = rock)
(after sediment is buried)
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Types of Lithification Processes
a) Compaction
- most effective on fine-grained rx (ex: shale)
Exception: St. Peters Sandstone
Starved Rock State Park, IL
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b) Cementation
Minerals dissolved in groundwater precipitate around sediment
Ex: silica, calcite, hematite
Lithification Processes
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Sedimentary Rx Classification
2) Chemical – named based on composition
Ex: calcite = limestone
halite = rock salt
plant remains = coal
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Chemical Sedimentary Rocks
a) Inorganic – formed by chemical reactions in environment
Ex: evaporites – rock gypsum, rock salt
Ex: Travertine (limestone)
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Inorganic Chemical Sed. Rx
Calcite:*Limestone = mostly marine origin Travertine = speleothems Oolitic Limestone = tidal flats* Most abundant chemical
sedimentary rock
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Inorganic Chemical Sed. Rx
Silica (microcrystalline quartz):
Varieties: chert
agate
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Silica (microcrystalline quartz):
Varieties:
Petrified Wood
Jasper
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Chemical Sedimentary Rx
b) Organic (“biochemical”)Plant remains = coalMicroscopic sea shells = chalkBroken sea shells = coquina
limestone
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Detective Analogy
Geologists are solving a mystery
Scene of Clues Mystery
the Crime Solved
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Mystery Solved (Sed. Rx)
Depositional environment
- any area on surface where sediment accumulates
Ex:
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Principle of Uniformitarianism
“Present is the key to the past”
Study modern depositional environments for clues
Ex: coquina limestone
Ex: ripple marks
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Sedimentary Facies
Cumulative physical characteristics of a sedimentary rock which indicates its depositional environment
“Clues” in detective analogy
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Principle of Lateral Continuity
Nicolaus Steno (1669)
Sediment is accumulated to edge of its depositional environment
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Sedimentary Facies
Note: At the same time, each environment (facies) is accumulating its own sediment and characteristics!
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Sedimentary Facies
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Clues
1) Sediment sizeDistance deposited from source
areaLarge = (closer to, further from)Small = (closer to, further from)
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Clues (sediment size)
High vs. low energy environment
Large particles = (high, low)
Ex: ___________
Small particles = (high, low)
Ex: ___________
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Clues
2) Sediment shape
Distance deposited from source area: close to or far from?
angular = ______
rounded = ______
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Clues
3) Composition – minerals present
Two most common minerals found in sedimentary rocks are _____ and ______ .
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Variety of minerals:
Ex: quartz, K-spar, plagioclase feldspar, mafic minerals, rock fragments
- very little chemical weathering- indicates rapid deposition close
to source areaEx: arkose in alluvial fan
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One mineral composition:
Well-sorted
- sediment traveled far from source area
Ex: Quartz sandstone, shale
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Clues
4) Sedimentary Structures
- features produced in sediment BEFORE it’s lithified
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Sedimentary Structures
a) Bedding – layering w/in rock
Bedding plane – flat surfaces along which rx tend to break or separate
- separates different episodes of deposition
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Bedding
i) Laminar bedding – horizontal layers
- usually deposited in calm energy environment
Ex: shale, some sandstones
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Bedding
ii) Graded bedding – sorted by size
- occurs w/sudden decrease in velocity
Ex: turbidity currents, alluvial fans
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Bedding
iii) Cross-bedding (Figure 6.22)
- layers are angled in sets
- occurs with change of current direction (either wind or water)
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Sedimentary Structures
b) Ripple marks
- wave action moves sediment
- shallow water
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Types of Ripple Marks
i) Asymmetrical - also called “current ripple marks”
- used to determine paleocurrent directions
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Types of Ripple Marks
ii) Oscillation (symmetrical)- wave action is back and forth- usually indicates shallow lagoon environment
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Sedimentary Structures
c) Mudcracks
Environmental conditions:
a)
b)
Ex:
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Sedimentary Structures
d) Trace Fossils
Any evidence an organism was in the environment but no physical remains of the organism exist
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Trace Fossils
Examples:
i) Footprints
ii) Coprolites
iii) Bioturbation
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Clues
5) Fossils – evidence of prehistoric life
Fossil record is incomplete- shows remarkable pattern of
change from simple to complex life forms
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Steps to becoming a fossil
Step 1: Death
Step 2: Hard parts – bones, teeth, shells (Who you are!)
Step 3: Rapid burial (Where you die!)
Step 4: Time (prehistoric)
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Fossils
Fossil record biased towards marine environment
- abundant life in oceans
- many critters have hard parts
- lots of sediment for quick burial
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Fossils
Good evidence for reconstructing past environments
Ex: Beach sandstone vs. desert sandstone
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Clues
6) Color
Black or green = unoxidized iron
- lack of oxygen in environment
Ex: deep ocean, deep lake, swamp
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Color
Red = oxidized iron
Ex: river floodplain, tidal flats, desert deposits
Green/purple = volcanic ash mixed in with sediment
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Clues
7) Geometry of rock unit
- shape & thickness of rock unit
Ex: beach sandstone vs. desert sandstone
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Depositional Environments
Handout
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Lithofacies map
For a particular moment in geologic time, rocks indicate the landscape
1) Map out the areas where each rock type is found & draw boundaries
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Lithofacies map
2) Look at the lithologies (rock types) & compare them to:
- nearby rx- geometry of rock unit3) Determine the original
depositional environment
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Cyclothems
Cyclical alternations of lithologies deposited as shorelines shift
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Sedimentary Facies
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Stratigraphic Sequences
1) Transgressions - sea level risesStrat column changes vertically
from coarser sediment to finer sediment
“Fining-upward” sequence
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Stratigraphic Sequences
2) Regressions – sea level drops
Strat column changes vertically from finer sediment to coarser sediment
“Coarsening-upward” sequence
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Color
Red = oxidized iron
Ex: river floodplain, tidal flats, desert deposits
Green/purple = volcanic ash mixed in with sediment