What type or size sediment would you expect to find in each of the following env
ID: 235541 • Letter: W
Question
What type or size sediment would you expect to find in each of the following environments (there may be more than one type or size). What type of sedimentary rock would form if each of the depositional environments you chose were buried and the sediment was lithified?
Depositional Environment
Type/size (e.g. boulders, sand, dissolved) Sediment Found in this environment.
Sedimentary Rock(s) Produced (if sediment becomes lithified)
A. Alluvial Fan
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B. Rivers and Streams
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C. Beach
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D. Dune
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E. Delta
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F. Inner Continental Shelf
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G. Outer Continental Shelf
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H. Marine Evaporite Basin
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Depositional Environment
Type/size (e.g. boulders, sand, dissolved) Sediment Found in this environment.
Sedimentary Rock(s) Produced (if sediment becomes lithified)
A. Alluvial Fan
_____________________
_____________________
B. Rivers and Streams
_____________________
_____________________
C. Beach
_____________________
_____________________
D. Dune
_____________________
_____________________
E. Delta
_____________________
_____________________
F. Inner Continental Shelf
_____________________
_____________________
G. Outer Continental Shelf
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_____________________
H. Marine Evaporite Basin
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_____________________
Explanation / Answer
A. Alluvial fan: An alluvial fan is a cone-shaped deposit of coarse stream sediments, sheet flood deposits and debris flows that forms where a canyon stream suddenly disgorges into a flat valley. The sudden change of a stream from a narrow, confined channel with a steep gradient to the broad flats of the valley causes a sudden drop in the hydraulic power of the stream. The decrease in the competence of the stream allows the coarser material to drop and accumulate.
Coarsening upward sequence of cross bedded sandstone, channel lag conglomerates and unsorted debris flow deposits. Conglomerate and cross bedded sandstone are most common. Debris flow deposits are unsorted can show reverse bedding and contain boulders. Sieve deposits form conglomerates with no matrix of fine material as well as lenticular bodies composed of cross bedded channel sand and pebble conglomerates . Ripples and convulate lamination can occur in fine grained sheet flow deposits. Sediments can be very immature and angular with abundant coarse rock fragments and feldspars
B. RIvers and streams:
Meandering rivers:
Channel sequences typically form long ribbonlike bodies of sand within a thick sequence of shales . There is fining upward from a basal channel lag gravel to the sandy point bar sequence of plane beds, trough cross beds and ripple drift. Grain size range from channel lag gravels to floodplain muds. Laterally point bar sands show decreasing flow velocity sedimentary structures( plane beds, trough cross beds, ripple cross lamination). Floodplain muds are finely laminated and vertically accreted.
Braided rivers:
There is a fining upward sequence of channel lag gravels, abundant sandy trough cross beds filling channels and occasional tabular cross beds migrating across channels topped by vertically accreted, laminated sand and mud.
Gravel is more common in longitudinal bars of the upper reaches of the system but sand is dominant throughout. So sandstone as well as conglomerate can occur but sandstone more common
C. Beach: sediments are well sorted due to action of swash and backwash of sea water . Here sandsone is most commonly found.
D. Dune: extremely well sorted sand, well rounded with no fine matrix and fewer coarse gravel lags found. Large scale cross beds composed of smaller scale low amplitude wind ripples form cross lamination that moves up or down the lee face of the dune. Here the dominant mineralogy is sand so sandstone is most common.
E. Delta: Deltas are roughly traingular in plan view and wedge shaped in cross section, tens to thousands of square kilometers in area and tens to thousands of metre thick. A coarsening upward sequence of prodelta muds and clays is followed by distributary bar finger sands and then muds and coals of interdistributary marshes and levees.
There is a wind range of grain size from coarse sand to fine mud, generally becoming finer away from land. Distributary sands show small cross beds and ripple marks, levee muds are mudcracked, prodelta muds are finely laminated with occasional sandy layers, laminated muds of interdistributary. So from shale to sandstone can form in this environment.
H. Marine evaporite basin: It consists of bedded evaporite minerals epecially halite and gypsum. These rocks formed from dissolved components that precipitate to form these rocks decomposed from preexising rocks and minerals hence also known as chemical sedimentary rocks.
G,H. Inner and outer continental shelves: Typical sequences of shale with long sheetlike or lenticular sandstone bodies are found. Fining or coarsening upward sequences occur. Burrowed glauconitic muds folllowed by storm graded beds or large scale cross beds or gravel lags.
Quartz and clay minerals are predominant although carbonate minerals and shell fragments can also occur. Glauconite is characteristic of shalllow marine muds and sands. Hummocky cross stratification, wave generated cross beds, large scale cross stratification, storm graded beds characteristic of this environment. So the rock most commonly found can be sandstone, shale and limestone
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