Easc116 Ch. 6 - Sedimentary Rocks
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Transcript of Easc116 Ch. 6 - Sedimentary Rocks
CH. 6 – SEDIMENTARY RX
Sediment:
- weathering products (gravel, sand, clay minerals)
- chemical precipitates
- organic remains
Sedimentary Rocks
“sedimentum” = “settling”
Sediment settles out of wind or water
- forms layers at the surface
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
Importance of Sedimentary Rx
3) Fossil record
4) Economic value
- coal, petroleum, construction
Sedimentary Rx Classification
1) Detrital = weathering products
(mechanical or chemical)
Named based on particle size
Detrital Sedimentary Rocks
Particle Size Name Gravel Breccia
(angular) Conglomerate (rounded)
Detrital Sedimentary Rocks
Particle Size Name Sand Quartz Sandstone
(quartz sand)
Arkose (K-spar)
Detrital Sedimentary Rocks
Particle Size Name
Sand Graywacke
(rock frags,
mafic minerals)
Detrital Sedimentary Rocks
Particle Size Name
Clay-size *Shale
(clay minerals)
*Shale is the most abundant detrital sedimentary rock
Detrital Rock Process
1) Weathering
2) Erosion (transportation)
3) Deposition (sediment settles)
4) Lithification – turning sediment into rock (“lithos” = rock)
(after sediment is buried)
Types of Lithification Processes
a) Compaction
- most effective on fine-grained rx (ex: shale)
Exception: St. Peters Sandstone
Starved Rock State Park, IL
b) Cementation
Minerals dissolved in groundwater precipitate around sediment
Ex: silica, calcite, hematite
Lithification Processes
Sedimentary Rx Classification
2) Chemical – named based on composition
Ex: calcite = limestone
halite = rock salt
plant remains = coal
Chemical Sedimentary Rocks
a) Inorganic – formed by chemical reactions in environment
Ex: evaporites – rock gypsum, rock salt
Ex: Travertine (limestone)
Inorganic Chemical Sed. Rx
Calcite:*Limestone = mostly marine origin Travertine = speleothems Oolitic Limestone = tidal flats* Most abundant chemical
sedimentary rock
Inorganic Chemical Sed. Rx
Silica (microcrystalline quartz):
Varieties: chert
agate
Silica (microcrystalline quartz):
Varieties:
Petrified Wood
Jasper
Chemical Sedimentary Rx
b) Organic (“biochemical”)Plant remains = coalMicroscopic sea shells = chalkBroken sea shells = coquina
limestone
Detective Analogy
Geologists are solving a mystery
Scene of Clues Mystery
the Crime Solved
Mystery Solved (Sed. Rx)
Depositional environment
- any area on surface where sediment accumulates
Ex:
Principle of Uniformitarianism
“Present is the key to the past”
Study modern depositional environments for clues
Ex: coquina limestone
Ex: ripple marks
Sedimentary Facies
Cumulative physical characteristics of a sedimentary rock which indicates its depositional environment
“Clues” in detective analogy
Principle of Lateral Continuity
Nicolaus Steno (1669)
Sediment is accumulated to edge of its depositional environment
Sedimentary Facies
Note: At the same time, each environment (facies) is accumulating its own sediment and characteristics!
Sedimentary Facies
Clues
1) Sediment sizeDistance deposited from source
areaLarge = (closer to, further from)Small = (closer to, further from)
Clues (sediment size)
High vs. low energy environment
Large particles = (high, low)
Ex: ___________
Small particles = (high, low)
Ex: ___________
Clues
2) Sediment shape
Distance deposited from source area: close to or far from?
angular = ______
rounded = ______
Clues
3) Composition – minerals present
Two most common minerals found in sedimentary rocks are _____ and ______ .
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
One mineral composition:
Well-sorted
- sediment traveled far from source area
Ex: Quartz sandstone, shale
Clues
4) Sedimentary Structures
- features produced in sediment BEFORE it’s lithified
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
Bedding
i) Laminar bedding – horizontal layers
- usually deposited in calm energy environment
Ex: shale, some sandstones
Bedding
ii) Graded bedding – sorted by size
- occurs w/sudden decrease in velocity
Ex: turbidity currents, alluvial fans
Bedding
iii) Cross-bedding (Figure 6.22)
- layers are angled in sets
- occurs with change of current direction (either wind or water)
Sedimentary Structures
b) Ripple marks
- wave action moves sediment
- shallow water
Types of Ripple Marks
i) Asymmetrical - also called “current ripple marks”
- used to determine paleocurrent directions
Types of Ripple Marks
ii) Oscillation (symmetrical)- wave action is back and forth- usually indicates shallow lagoon environment
Sedimentary Structures
c) Mudcracks
Environmental conditions:
a)
b)
Ex:
Sedimentary Structures
d) Trace Fossils
Any evidence an organism was in the environment but no physical remains of the organism exist
Trace Fossils
Examples:
i) Footprints
ii) Coprolites
iii) Bioturbation
Clues
5) Fossils – evidence of prehistoric life
Fossil record is incomplete- shows remarkable pattern of
change from simple to complex life forms
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)
Fossils
Fossil record biased towards marine environment
- abundant life in oceans
- many critters have hard parts
- lots of sediment for quick burial
Fossils
Good evidence for reconstructing past environments
Ex: Beach sandstone vs. desert sandstone
Clues
6) Color
Black or green = unoxidized iron
- lack of oxygen in environment
Ex: deep ocean, deep lake, swamp
Color
Red = oxidized iron
Ex: river floodplain, tidal flats, desert deposits
Green/purple = volcanic ash mixed in with sediment
Clues
7) Geometry of rock unit
- shape & thickness of rock unit
Ex: beach sandstone vs. desert sandstone
Depositional Environments
Handout
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
Lithofacies map
2) Look at the lithologies (rock types) & compare them to:
- nearby rx- geometry of rock unit3) Determine the original
depositional environment
Cyclothems
Cyclical alternations of lithologies deposited as shorelines shift
Sedimentary Facies
Stratigraphic Sequences
1) Transgressions - sea level risesStrat column changes vertically
from coarser sediment to finer sediment
“Fining-upward” sequence
Stratigraphic Sequences
2) Regressions – sea level drops
Strat column changes vertically from finer sediment to coarser sediment
“Coarsening-upward” sequence
Color
Red = oxidized iron
Ex: river floodplain, tidal flats, desert deposits
Green/purple = volcanic ash mixed in with sediment