midterm Flashcards
(65 cards)
1
Q
Abundance of rocks
Carbonates -
Sandstone -
Shale -
A
Abundance of rocks
Carbonates - 21%
Sandstone - 37%
Shale - 42%
2
Q
Production by rock type
A
Miscellaneous 2.5%
Sandstone 37%
Carbonates 61.5%
3
Q
Carbonate mineralogy
A
Calcite (caco3), dolomite CaMg(Co3)2
4
Q
Constituents of carbonate rock grains or particles in carbonate rocks
A
○ Non-skeletal grains
○ Skeletal grains
○ Matrix, cements, and pore space
5
Q
In modern environment aragonite occurs in
A
warm shallow waters
6
Q
Calcite sea =
A
high sea level
7
Q
dolomite:
A
CaMg(co3)2 diagenetic, dolostone
8
Q
Less than ———of dolomite formed in a modern environment formed in a modern environment
even though there is more dolomite in modern seas.
A
1%
9
Q
Dolomites are important hydrocarbon reservoirs with ______ and ______
A
t porosity and permeability
10
Q
Dolomite is more resistant to porosity less with depth than limestone
A
3-5km depth oil companies look for dolomite
11
Q
Constituents of carbonate rocks
A
1) Grains or particles
2) Matrix: the grains and matrix are primary sedimentary products that accumulate in the
depositional environments
12
Q
Non - skeletal grains:
A
- Coated grains: ooids and oncoids
- Pelids
- Aggregates
- Intraclasts
13
Q
Skeletal grains
A
- Ooids: coated grains with a calcareous cortex and nucleaus the cortex is smoothly and evently
laminated - Oncoids: coated grains with a calcareous cortex of irregular, partially overlapping laminae.
14
Q
Ooids
Definition of ooid:
Size:
Shape:
Cortex:
Oolite:
A
Coated grains with a calcareous cortex and nucleus
Size: < 2mm
Shape: spherical to ellipsoidal
Cortex: smooth, even laminae (concentric)
Oolite: rock largely made of ooids
15
Q
Three types of ooid cortical microfabrics
A
tangential microfacris
radial microfabrics
random microfabrics
16
Q
Controls on microfabrics
A
o Energy of the depositional environment
o Salinity of the water
17
Q
Low energy environment with hypersaline water
A
favor radial aragonite ooids with a loose structure. E.g.
protected Bimini lagoon of Bahamas
18
Q
High energy environments with normal salinity
A
favor tangential microfabrics with tightly packed
crystals: e.g Bahamian ooids
19
Q
Suerficial ooid
A
only few thin coatings
20
Q
collapsed ooid
A
undergone partical dissolution and internal collapse of remaining undissolved
materials to the bottom of the mold
21
Q
composite ooids
A
represent the coalescence of two or more ooids, with concentric layers covering all
the incorporated particles
22
Q
deformed ooids
A
reflect compaction or tectonic compression or shearing, commonly with some
separation of cortical layers from their nuclei
23
Q
Preservation of ancient ooid microstructures
A
- Dissolution and refilling with spary calcite, dolomite, ect
- Calcitised (recrystallized) aragonite with relic structures
- Micritization with relic structure
- Preservation of the original fabric (e.g, radial microstructure)
24
Q
Mineralogy of recent ooids
aragonite
high mg calcite
low mg calcite
A
Aragonite: most common marine ooids, some in salie lakes with high Mg/Ca ratio
- High – mg calcite (mg > 4%): less common but resent in marginal marine hypersaline environments.
- Low- Mg calcite (Mg<4%): present day lakes, streams, caves and calcareous soils.
25
origin of ooids
mechanical accretion
chemical processes
biochemical processes
26
The vast majority of ooids in the geological record were formed in
o Very shallow (<2m deep)
o Warm tropical and
o Agitated marine settings, e.g. ooids shoals of the Bahaman banks
o However, pisoids and ooids can form in a variety of environments: from shallow marine, to
lagoons, lakes, rivers, caves and calcareous soils.
27
Oncoids
```
Size
cortex
shape
orgin
environments
```
Size > 2mm
Cortex: irregular, overlapping laminae (non – concentric)
Shape: irregular
Origin: microbial – biogenically coated grains, formed by coatings of encrusting organisms such as algae carbonates Page 2
Origin: microbial – biogenically coated grains, formed by coatings of encrusting organisms such as algae
or cyanobacteria, due to trapping and binding of sediments
Environments: each type of biogenic oncoid will be controlled by the tolerances of the organisms
involved.
28
Peliods (non – skeletal grains)
size
environment
origin
-A sand sized grains (0.1-0.5mm) composed of microcrystalline calcite. Internally structureless; rounded,
subrounded; spherical, ellipsoidal, to irregular in shape. The term is purely descriptive
- Environmnets: shallow, low-energy, restricted marine settings
- Origin: peloids are a polygenetic group of grains including faecal pellets, calcareous algae, micritized
grain, and mud clasts
29
Aggregates
Several carbonate particles become bound and cemented together (0.5-3mm), and have irregular
shapes
30
Grapestone
aggregates of spherical grains ( commonly micritized ooids), which resemble small clusters
of grapes
31
Lumps
aggregates with smoother outline, which commonly have hallow interiors
32
Botryoidal lumps
grapestone or lumps with a thin oolitic coating
33
Aggregates
stage 1:
stage 2:
stage 3:
stage 4:
Stage-1: sediment grains are bound together by microbial ilaments and/ or forminifers
Stage-2: calcification of microbial braces and progressive micritization occur to from grapestone
Stage 3: increased cementation at grain contacts by microbially – induced precipation, fills depressions
to create a smoother relief of surface (lump stage)
Stage 4: filling of any central cavity to form a dense heavy micritized and matrix – rich aggregate
34
Clasts (last of non-skeletal grains)
Rewoked fragments of at least partially consolidated carbonate sediments
35
Intraclasts
fragments of weakly consolidate sediments reworked from within the area of deposition,
e.g. mud breccias formed by desiccation and cementation on supratidal zones
36
Extraclasts
consists of lithologies not represented in the immediate deposition area, e.g. a trilobite – bearing bioclastic grainstone in a tertiary limestone
37
Skeletal grains/ bioclast
Hard (or body) parts of organisms, which change with time and environments, as the organism change.
depends on the mineralogy of skeletal grains, their microstructure can be modified during diagenesis.
38
Six kingdoms
Prokaryokes
Eukaryotes
Cyanobateria
Eubacteria
39
Characteristics of carbonate sediments
Carbonate sediments (calcite and aragonite) are autochthonous ( formed or produced within the basin)
Produced largely biochemically, as skeletal fragments
o Also form through direct precipation from sea water with the help of cyanobacteria (e.g. ooids). - Carbonate sediments (calcite and aragonite) are autochthonous ( formed or produced within the basin)
- In other words, carbonate sediments are “born”
40
Crinoids
Are an echinoderm related to starfish, sea urchains
41
Trilobites
- Trilobite shells (carapaces) were composed of chitin with large amounts of calcium carbonate and
variable amounts of calcium phosphate ( up to 30% in some species).
42
Bryzones
Are a group of small animals; some that superficially resemble corals
43
Organism biology and evolution
Most carbonate sediments are produced biologically or by biochemical mediatio
44
Matrix
fine - grained (<62 micrometers) carbonate, deposited at the same time as much larger and particles
45
Cements
diagenetic products added to the sediments after their deposition
46
Pore spaces
pore spaces are generally present and filled by air, water and/ or petroleum products
47
The origin of lime mud
the origin of matrix is polygenetic and after diagenesis it is usually impossible to ascertain its origin.
- Origins of micrite: abrasion of carbonate sedimetns
- Orgaic breakdown of calcareous green algae
48
Major controls on carbonate sedimentation (carbonate factory)
1. Characteristics of carbonate sediments
2. Carbonate factory model
3. Factors that controls sediment production and distribution in the carbonate factory.
49
2. Carbonate factory model
- Carbonate factory is the core of carbonate deposition throughout the world
Carbonate factory is defined as the shallow illuminated seafloor, where sediment articles are
"born" through crystallization or precipitation out of sea water
- Most carbonate sediments are produced in warm, shallow marine (< 15m) environment in low
latitudes (within 30 degrees of equator
- Temperate carbonates ( 30-60 degree latitudes) make up less than 10% of total modern carbonate
production
50
Factors that controls sediment production and distribution in the carbonate factory
- Depth of water
- Temperature and salinity of water
- Cleanness of water (clastic input)
51
Factors related to water depth T and S of water, cleanness of water
```
○ Antecedent topography
○ Climate
○ Carbonate growth potential
○ Tectonic subsidence
○ Eustatic sea level changes
○ Late movement
```
52
Water depths (light and T)
The rate carbonate production is depth dependent
- Phototrophic organisms are the source of most carbonate sediments in warm water factories
Coral growth on barrier reefs goes down to more than 50m depth, but most skeletal
carbonate is produced in water depths < 5m
○
Green algae can exist into deep water (>50m), the highest production rates are in water
depths < 15m
○
The depth of photic zone is usually 80m. Most organic productivity however, takes place in
< 10 -15m of water
- Since formation of carbonate sediments in most intense depths < 15m, this shallow subtidal
environment is often referred to as the carbonate factory - On modern cool water platforms, the photic zone is of lesser importance because carbonates are
generated almost entirely by nonphototrophic organism
53
Temperature and salinity of water
A warm water, low - latitude realm (30 degrees), and
○ A cool to cold - water mid to high- latitude realm
- A water T about 20 degrees c divide carbonate in modern shallow seas into - Carbonates are produced mostly in warm water environments
54
Warm water systems
typically in low latitudes, the photic zone extends to about 70m; carbonate
production is highest in the upper 10-20m; produces chlorozoan assemblages of organisms
55
Water salinity
Increasing salinity reduce biotic diversity and above 4.0% most invertebrates disappear;
calcareous algae, however, continue to be sediment producers for a time
56
Water temperature and salinity
Less and Buller (1972) recognized 3 principal skeletal associations related to changes of T and
salinity
1. Chlorozoan association; characteristic skeletal carbonate produces are corals and calcareous green
algae, along with other organisms; T > 15oC; salinity 3.2-4.0%
2. Chloralgal association; characteristic skeletal carbonate produces are calcareous green algae;
along with other organisms ( e.g. benthic foram, mollusks, brachiopods); T > 15oC; salinity > 4.0%
3. Foramol association: sediments are dominated by benthic foram and mollusc, with carbonate also
coming from echinoderms, bryozoans, calcareous red algae, and ostacodes T < 15oC salinity
3.2-4.0%
57
Cleanness of water (clastic input)
Siliciclastic influx related to tectonics as it controls hinterland topography and river drainage.
Clastic sediments can shut off carbonate sediment production and kill the carbonate factory in
following three ways
1. Reduction of water transparency
2. Cloggs the feeding and/or respiration of sessile benthic organisms
3. Increasing nutrient and particulate organic concentration of the water. This causes algae to
replace the carbonate factory. When nutrient levels increase corals and other organisms lose
their advantage and can die off quickly
58
Antecedent topography:
The antencedent topography features will influence the growth patterns and position of modern
reef complexes; and is the blueprint for carbonate sedimentation, (e.g. ooid shoals, barrier and
patch reefs on high; but muds in lows). - Four types of antecedent topography have been recognized 1) older reefs 2) erosinal teraces 3)
siliclastic / volcanic topoographic features 4) karstic topography
59
Climate
Climate can influence the carbonate factory in a number of ways, e.g. by changing temperature,
salinity, weathering rate
60
The rate of evaporation depends upon
- wind speed
- Temperate
- Humidity
61
Carbonates growth potential
Organism production rates vary depending on specific individuals, provided ambient conditions are ideal
The average growth potential of modern reefs ad carbonate platforms is about 100 cm/ky, which is: - Higher than the rate of relative eustatic sea level change;
But could be lower than sea level changes caused by faulting or quick rises associated with glacial
melting
- The rate carbonate production is also depth dependent
62
Organism biology and evolution
Biologically evolution: Most paleozoic bioclasts were calcite whereas the majority of Mesozoic and
Cenozoic skeletal particles were aragonite.
63
Porosity
rock volume that is not filled with solid material; is generally present and filled by air, water
and/or petroleum products
64
Total or absolute porosity
percentage of the total volume of the rock that is pore space, whether the
pores are connected or not.
65
Effective porosity
considers only the interconnected pore space and is a measure of the void space that
is filled by recoverable oil and gas. Usually 40% to 755 of total porosity
