Download Benha University

Benha University
Faculty of Science
Geology Department
3rd year Geology & chemistry
Date: 7/6/2011
‫ سيد محفوظ أحمد‬/.‫د‬.‫ أ‬:‫اسم الممتحن‬
‫كلية علوم بنها‬-‫أستاذ الصخور الرسوبية قسم الجيولوجيا‬
)‫)نصف الورقة االمتحانية‬
Benha University
Faculty of Science
Geology Dept.
3rd year Geology & Chemistry (‫)تخلفات‬
Date: 7/6/2011
Sedimentary rocks
Write on the following:1- Types of breccias
2- Color, fissility and laminations of shales
3- Cementation and dolomitization of limestones
4- Quartz arenites and arkoses in sandstones
5- Laminated sulphate and secondary gypsum
…………………………………………………………………….
Answers
1- Types of breccias:
A) Cataclastic breccias: in cataclastic rocks, fragmentation was
accomplished by movement of large masses of rock past one
another. The movement that occurs along a fault surface gives rise
to fault breccias.
B) Landslide and slump breccias: some breccias are formed by earth
movements by simple gravitational stress.
C) Collapse and solution breccias: related to simple downward earth
movements as a result of removal of the underlying materials by
solution. They consist of highly angular fragments.
D) Pyroclastic breccias: include volcanic breccia.
2- Color, fissility and laminations of shales:
- Color of shales is a reflection of the mineral composition and the
oxidation state (Eh) of the environment of deposition. Red shales have
their iron in the ferric state as hematite, while in black and green
shales it is present in the ferrous state. Black shales possess a high
content of organic matter. Siderite bearing shales are grey or bluish on
fresh surfaces, but become brown after a short period of exposure to
air. Red shales are indicative of an oxidizing environment of
deposition, while blue, green and black shales indicate a reducing
environment.
- Fissility is a characteristic of shales that result due to compaction
and crystallization. Also, may be due to parallel orientation of the
micaceous minerals parallel to the bedding plane.
- Laminations: they appear due to:1- Alteration of coarse and fine particles (silt and clay).
2- Alteration of light and dark layers.
3- Alteration of calcium carbonate and silt.
3- Cementation and dolomitization of limestones:
- Cementation: is a process by which loose sediments are changed
into consolidated rocks by means of precipitation of the cementing
material in the interstitial pore spaces of the rocks. Marine to fresh
water cementation may affect the sediments. Carbonate cements in
shallow marine environment are aragonite and high Mg calcite, the
aragonite in most cases exhibits acicular, fibrous and needle shape.
High Mg calcite is microcrystalline and cryptocrystalline. Cements
in the subaerial environments are low Mg calcite and appears as
coarse to very coarse sparry calcite crystals.
- Dolomitization: the conversion of CaCO3 minerals into dolomite
(Ca Mg (CO3)2) may take place after the sediments have been
deposited. During early diagenesis referred to as syngenetic
dolomitization, or a long time after deposition (epigenetic
dolomitization). The fine grained dolostone which are composed of
xenotopic dolomite crystals, cloudy and dark in color and unzoned
are formed early diagenetic dolomitization. The late diagenetic
dolomites are large and zoned.
4- Quartz arenites and arkoses in sandstones:
- Quartz arenites: at least 95% quartz grains. So, the sandstones are
mature. They consist of well- rounded and well- sorted grains so, the
textural maturity is also very high. Cements are quartz overgrowths.
Monocrystalline quartz grains are dominated. The majority of the
quartz grains are second cycles, derived from pre-existing sediments.
- Arkoses: contain more than 25% feldspars, much quartz and some
rock fragments. The feldspar is usually fresh although some may be
altered to kaolinite and sericite. Arkoses are derived from granites and
gneisses. Arkoses were deposited in fluviatile environments.
5- Laminated sulphate and secondary gypsum:
- Laminated sulphate: consists of thin sulphate laminations
alternating with laminae of different composition, often calcite,
organic – rich calcite or organic matter. These laminated anhydrites
occur in the lower sections of thick basional evaporite sequences
and the term varved has been applied to the couplets.
- Secondary gypsum: uplift of anhydrite sequences perhaps along
time after their formation and burial, results in the generation of
secondary gypsum. Secondary gypsum consists of two varieties,
porphyroblastic and alabastrine gypsum. Gypsum porphyroblasts
are large crystals which scattered through the anhydrite.
Alabastrine gypsum consists of small to large, often poorlydefined interlocking crystals, many with irregular extinction.