Geology Exam #2- Sedimentary

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Sedimentary Rock

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rock that forms at or near the surface of the earth by the precipitation of minerals from water solutions, by the growth of skeletal materials in organisms or by the cementing together of shell fragments or loose grains derived from pre-existing rock

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Sediments/sedimentary rocks occur in the . . .

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cover, or upper part of the crust, not in the basement

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Types of weathering

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physical, chemical

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clastic sedimentary rocks

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rock created from solid grains

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big grain size, medium, tiny (give specific rock examples)

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conglomerate, sandstone, shale

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chert

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made up of animals who have shells of silica, not clastic

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biochemical sedimentary rocks

give examples

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rocks made from deceased organisms (particularly shells), many are carbonates

examples: fossiliferous limestone, micrite, chalk, biochemical chert, coal and oil shale (<-- organic rocks)

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chemical sedimentary rocks

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rock formed primarily by the precipitation of minerals out of water solutions

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evaporites form . . .

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1. closed basin
2. salt becomes concentrated
3. they reach saturation
4. precipitates right out of the water

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evaporites examples

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CaCO3- evaporitic limestone or travertine
CaSO4H4O- gypsum
NaCl- halite

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salt deposits consists of . . ./if the sea evaporated entirely . . .

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inner rung- halite (80%)
middle run- gypsum (13%)
outer rung- carbonates

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Aral Sea

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inland sea in Central Asia from which Stalin diverted water for agriculture, it is now mostly dried up

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bedding plane

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boundary between two beds in a strata

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Patterns of different rock types in sedimentary environments offer us . . .

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A record of geologic change

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Sed. structure:
Ripples/ripple marks

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-relatively small elongated ridges that form on a bed surface at right angles to the direction of the current flow
-asymmetric if: current flow is always in the same direction
-symmetric if: water is flowing back and forth, like waves along the shore

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Sed structure:
Cross-bedding

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internal laminations that are inclined at an angle to the boundary of the main sedimentary layer

cross beds form in aquatic environments

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Sed structure:
Graded bedding

How are they formed?

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a layer of sediment in which grain size varies from coarse at the bottom to fine at the top. formed by turbidity currents.

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turbidity current

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underwater landslide

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Mud cracks

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mud layer dried up after deposition, cracks in to roughly hexagonal plates that typically curl at their edges.

they later fill with sediment and are preserved.

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Mountain stream environments

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Fast moving water can carry large clasts. When water flow slows, larger clasts settle out to form gravel and boulder beds, while smaller sediments are carried on.

Deposits would include:
breccia (coarse, angular)
sandy channels in a broad, muddy floodplain

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Alluvial Fan enviros

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Found in arid regions, where a fast moving stream empties on to a plain. Deposits right at a mountain from.

Depositions would include:
gravel & sand

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Delta enviro

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where a river build a delta of sediment out in to the sea

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Deep sea
and Reef/lagoon
Carbonate deposition

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Deep sea- massive limestone and chert depositions
Reef/lagoon-carbonate clastics

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dunes are . . .

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giant ripple marks or can occur from wind

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zone of leaching

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where downward transport occurs

towards the top

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zone of accumulation

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where minerals pile up

towards the bottom

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dome

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fold that is shaped like an overturned bowl