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29 Cards in this Set

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Where do sedimentary rocks form? What are the 3 main categories?
Form at or near the surface from 1 of 3 types of sediment:

1. Clastic
2. Biochemical
3. Chemical

Form on land or sea floor.
Clastic sediment def...
Made of a variety of clasts (solid rock & mineral grains derived from preexisting rocks)
Biochemical sediments def..
made of shells, shell debris, plant debris, or other organic materials
Chemical sediments def...
made of minerals precipitated directly from a water solution
What is weathering and what are the 2 types?
def: the chemical or physical disintegration of a rock exposed at or near the Earth's surface.

Chemical & Physical
Chemical weathering def...
minerals chemically dissolve/partially dissolve. Dissolved solids are released and carried away by groundwater, streams, and rivers.

Feldspar weathering gives us soils.
Physical weathering def...
when rocks exposed at/near surface are physically broken apart.

EX: whack w/ hammer, frost wedging, root wedging, etc...
Where do the salts in sea & groundwater come from?

Where do salts in your tissues come from? How do soils form?
Chemical weathering is main source of salts in sea and our bodies.

Partial dissolution causes soils. Only certain minerals are removed from feldspars creating clay minerals.
What is erosion?
removal of material from the site of weathering.
What is the grain size cutoff that separates sandstones from shale?Sandstone from conglomerates?
1/16 to 2mm

2 to 64mm
Why are silt and clay-sized sediments not eroded at velocities significantly less than those for sand?
Fine-grained sediments have electrical attractions that help them stick together.

Plus there is a thin layer at the base of a flowing fluid that does not move. This insulates very small particles from a moving fluid.
Relationship between the slope of a stream and current speed.
Steep slopes = fast currents, only coarse deposition.

Low slopes = slow currents, finer deposition.
Limestone formation...
if outer shelf is warm and shallow enough for sunlight to reach bottom, biological activity can create thick deposits of limestone that built a carbonate platform.
What are limestones made of?
dominated by the mineral calcite. Reflects the accumulation of lots of bioclastic materials.
In what depositional environment are coals deposited?
Swamps - where pure plant materials accumulate.

Nearby river channels are on the other side of a vast tangle of trees and plants. tangle keeps contaminating sand and mud away.
What conditions favor the deposition of thick deposits of chemical sediments in the oceans?
1. Sea water flows into a restricted subtropical basin from the open ocean.
2. Evaporation concentrates salts until they precipitate.
3. Results: moving from the inlet, 1st limestone, then gypsum, & halite precipitate.
4. Sea water recharge allows vast accumulation.
Why are thick deposits of chemicals rarely exposed outside of deserts?
Every time it rains, chemical weathering occurs. Rainfall flows down steams and fills temporary lakes (playa lakes). Water evaporates and salts are left behind.

currents are too fast to deposit sediment.
What is a sedimentary environment? What is a sedimentary facies?
sed. environ - place where sediment is deposited

sed. facies - collection of sediment types that distinguish each environment
Would you expect Pitt to have a sedimentary record preserving the 21st century in 100mil. yr? Where you find this?
No because our land is eroded away by water and deposited into hills far away.
What are the deposits left behind by a melting glacier? What are their distinguishing features?
Glacial till

sediments of all sizes in one unlayered jumble
Why do fast mountain stream channels tend to be filled w/ lots of boulders but little sand or mud?

Why don't we expect to see ancient mountain stream deposits in the geologic record?
finer sediment is washed down streams because it can't settle in fast channels.

They are constantly moving downstream.
What is an alluvial fan? What distinguishes alluvial fan deposits from those of a glacier?
Mountain stream reaches a valley floor, flow rapidly slows and spreads out over floor. Instant deposition of gravel, boulders, and sand. Sand/gravel deposited between boulders.

Clast-supported conglomerate vs matrix-supported conglomerate

layered vs. jumbled
What sediment features characterize a wind-blown sand dune deposit?

What are cross beds?
Series of complex dunes made of well-sorted, well-rounded sand.

Cross-beds record the direction in which ripples and dunes migrate by preserving the down-wind slip-face.
River Facies...
lens-shaped sandstone deposits represent the channel, shale represents the flood plain muds.
Pittsburgh Coal represents which part of a delta environment?
Vast swamps on the delta plain.
What sediments characterize the beach facies? Shelf facies? Why do clastic sediments get finer moving out to sea?
Pure sand (sandstone)

gray shale in nice layers. some limestone.

Currents slow
On a carbonate platform, where do you expect to find the coarsest sediments? Coral fossils in growth position? Finer sediments?
Off the reef face

Reefs

Lagoons
Do deep sea sediments tend to be coarse or fine grained?
biological particles, fine clay, and silt

"marine snow"
How do sedimentary facies shift in response to a rise and fall in sea level?
Rise - pushes shore landward

A rise followed by a fall leaves a "V" of sedimentary layers, each representing a depositional environment.