Q 1: Write ( √ ) or ( × ) on the
following statements, and correct the "wrong" word(s):
(10 Marks)
1)
Satin spar
gypsum vein is formed during uplift diagenetic stage ( )
2)
Packstone lacks
mud and is grain supported. ( )
3)
Ooids form in
shallow, agitated water have concentric dolomite structure ( )
4)
The decrease in
PCO2 in warm water favor carbonate precipitation ( )
5)
Polymodal,
planar-e dolomite consists of one size of euhedral dolomite crystals ( )
6)
Alabastrine
gypsum is a primary gypsum that formed in a sabkha setting ( )
7)
Meniscus cement
indicates vadose diagenetic environment ( )
8)
Columnar
stromatolites are formed in intertidal environment ( )
9)
Micrite envelope
means the filling of algal pores by micrite ( )
10) Halite
chevron is formed during shallow burial diagenesis ( )
Model Answer
of Question No. 1:
1)
Satin spar
gypsum vein is formed during uplift diagenetic stage
(√)
2)
Grainstone
lacks mud and is grain supported. (×)
3)
Ooids form in
shallow, agitated water have concentric calcite structure (×)
4)
The decrease in
PCO2 in warm water favor carbonate precipitation (√)
5)
Unimodal,
planar-e dolomite consists of one size of euhedral dolomite crystals (×)
6)
Alabastrine
gypsum is a secondary gypsum that formed during uplift. (×)
7)
Meniscus cement
indicates vadose diagenetic environment (√)
8)
Columnar
stromatolites are formed in intertidal environment (√)
9)
Micrite envelope
means the filling of algal pores by micrite (√)
10) Halite
chevron is a primary texture of halite (×)
Q 2: Complete the following statements: (5
Marks)
1)
Wackestone
contains …………… lime mud, and is mud Supported.
(<
90 %, > 90 %, 100 %)
2)
……………………….. is
composed of low-Mg calcite that preserve the internal microstructure during
diagenesis.
(Gastropodes, Corals, Cephalopodes)
3)
Planar
stromatolites are formed in …………………….….
(Protected tidal flats, subtidal zone,
supratidal zone)
4)
Enterolithic fold and chickenwire anhydrite textures
are formed by the coalescence of ………………………….
(nodules, rosettes, rafts)
5)
…………………….. are
several carbonate particles that are bound with organic matter.
(grapestones, intraclasts, peloids)
Model Answer of Question No. 2:
1)
Wackestone
contains < 90 % lime mud, and is mud Supported.
2)
Corals
are composed of low-Mg calcite that preserve the internal microstructure during
diagenesis.
3)
Planar
stromatolites are formed in Protected
tidal flats
4)
Enterolithic fold and chickenwire anhydrite textures
are formed by the coalescence of nodules
5)
Grapestones
are several carbonate particles that are bound with organic matter.
Q
3: Write short note on ONE ONLY from the following: (5 Marks)
1)
Diagenesis of
carbonate sediments in marine environment.
2)
The
discrepancies between the theoretical and observed sequences of
salts.
Model Answer
of Question No. 3:
Q. 3. 1 - Diagenesis of carbonate sediments in marine environment.
1. Intertidal and supratidal diagenesis
n Diagenesis
takes place on and just below the sea-floor in both shallow and deep water, and
intertidal-supratidal zone.
n Cementation
in the intertidal zone produces cemented beach sands known as beachrock.
n The
cements in modern beachrocks are aragonite and/or high Mg calcite.
n Aragonite
typically occurs as fringes 10 to 200 µm thick of acicular crystals, oriented
normal to the grain surfaces.
1.A. Isopachus Cement
n
In many cases the
cements fringes are isopachus, i.e. of equal thickness, indicating marine
phreatic (below the water table) precipitation where pores were constantly
water-filled.
1.B. Meniscus
cements
n Asymmetric
cement fringes, thicker on the underside of grains, and meniscus cements,
concentrated at grain contacts are indicating precipitation in the marine
vadose zone.
2. Shallow-subtidal cementation
In shallow marine areas, it involves cementation and
microbial micritization.
The cements are mainly acicular aragonite and high
Mg-calcite, that form isopachus fringes.
3. Deep water cementation
Carbonate sediments are cemented by micritic low
Mg-calcite.
Q. 3. 2. The discrepancies
between the theoretical and observed sequences of salts.
1) Incomplete
cycles of evaporation
Reflux of
dense brine through porous sediments
Surface inflow /
deep outflow ratio will determine long-term salinity balance
Full evaporation
rare (K, Mg salts rare): replenishment with low-salinity waters
2)
Disequilibrium
High rate
of precipitation, Inhibition by Sr, Mg, Organic complexing, Hydration of ions
3) Diagenesis
a) Evaporites very soluble -- readily altered:Concentrated solutions react with earlier precipitates
e.g. anhydrite + K, Mg polyhalite
K minerals react
sylvite
Aragonite
/ dolomite magnesite
b) Bacterial reduction of sulphates
c) Dehydration of gypsum to anhydrite during burial begins at 30-60 m, complete by ~ 1300 m Gypsum Anhydrite + H2O 38% reduction in solid volume
d) Rehydration of anhydrite to gypsum during uplift volume increase
4) Dissolution
Bulk loss of evaporites by solution
e.g. collapse
breccias, with jumbled limestone blocks where evaporites were dissolved away.
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