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2nd year Geology (‫)تخلفات‬
Date: 5/6/2011
Benha University
Faculty of Science
Geology Department
‫ سيد محفوظ أحمد‬/.‫د‬.‫ أ‬:‫اسم الممتحن‬
‫كلية علوم بنها‬-‫أستاذ الصخور الرسوبية قسم الجيولوجيا‬
)‫)ثلث الورقة االمتحانية‬
Sedimentology
Write on the following:1- Weathering in sedimentary rocks
2- The different alluvial subenvironments
3- Eolian sand deposits in the desert environments
4- Turbidities
…………………………………………………………………….
Answer Key
1- Weathering in sedimentary rocks: weathering includes the
processes which break down rocks at the earth's surface to produce
discrete sediment particles. Weathering may be classified into:A) Physical weathering that cause mechanical fracture of the rock.
Three main types of physical weathering; a- freeze- thaw
weathering occurs where water percolates along fissures. When
water freeze, it expands with sufficient force to fracture the rock.,
b- insolation weathering, c- stress release.
B) Chemical weathering that lead to destruction of rock by oxidation
and hydration.
C) Biological weathering and soil formation, that is due to organic
processes.
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2- The different alluvial subenvironments: studies of modern
alluvial deposits show that they can be classified into a number of
subfacies. Each subfacies can be defined by its geometry, by the
type of sediment, type of biota and orientation of its sedimentary
structure. It can be seen that each of these subfacies is formed in
different physiographic subenvironments of the alluvial system.
Channel
Active
Channel floor
Channel bar
Channel bank
Abandoned
channel
Overbank
Levee
Crevasse splay
Flood basin
Pond
Swamp
This table classifies these different alluvial subenvironments.
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3- Eolian sand deposits in the desert environments: sand- sized
sediments are the most important sediments in the desert
environments. Sand forms produced in the wind regime:
A) sand drifts and sand shadows; sand drift are sand accumulation
caused by some fixed obstruction in the path of a sand- laden
wind, such obstructions may bushes rocks, small cliff, sand drifts
are tongue- shaped bodies.
B) Sand sheets; are very large areas of desert country more or less
flat.
C) Sand dunes; the most common characteristic feature of sand dunes
is the presence of a steep face and gentle dipping windward side.
Identification of the desert environments in ancient sediments:1) The presence of a sequence of red beds.
2) Sand grains are well rounded and frosted
3) Sorting is good
4) Better sorted
5) Porosity and permeability is high
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4- Turbidities: the essential elements of this model are a basin
margin dissected by submarine channel systems. Sediment
transported down these by sliding, slumping and grain flow
emerges from the channel mouth as true turbidity flows. At the top
produce the facies of sand, tending to be well sorted, often
glauconitic and with a fraction of skeletal sand. These beds are
cross- bedding. This facies is deposited by traction currents on the
shelf. It passes down into the second facies which consist of two
distinct subfacies. One consists of two distinct subfacies. One
consists of laminated clay and silt which were deposited out of
suspension in the quieter and deeper water of the slope zone. The
slope shales are cut into by the second slope subfacies. These are
the submarine valley- fill deposits. The submarine channel deposits
consist of sand and conglomerate and trace of shale. This facies
passes down wards into the true turbidite deposits of the submarine
fans.
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