(S12) Shallow Marine Carbonate and Evaporite Environments Flashcards
1
Q
What is a Carbonate Platform?
A
Shallow marine areas where carbonate sedimentation occur
2
Q
What are the two main conditions for a Carbonate Platform to occur?
A
Isolation from clastic supply, Shallow marine
3
Q
What are the two controls on the type of carbonate grains deposited on a carbonate platform?
A
Climatic conditions, Evolution of groups of organisms
4
Q
What are the two main controls of creation of shallow marine environments?
A
Tectonic subsidence, Sea level rise
5
Q
What is characteristic of sediment supply of clastic material to the shelf?
A
Few large rivers supplying most of sediment to relatively small part of coastline
6
Q
Why is climate ajacent to the continental shelf important?
A
In deserts there is little runoff into shallow marine
7
Q
What is the main control over the amount of carbonate produced?
A
The productivity in the food chain
8
Q
Productivity within the food chain is directly dependent on ___ which in turn is dependant on ___.
A
Availability of light, Amount of suspended material
9
Q
Where is algal productivity likely to be at its highest?
A
Bright, tropical zones
10
Q
What is the name for the oceanic zone which light can penetrate?
A
Photic zone
11
Q
What is the maximum extent light can penetrate to in the ocean and where do most organisms flourish?
A
Maximum up to 100m, usually 10-20m is where most organisms flourish
12
Q
What is the name for the shallow marine region with high biogenic productivity?
A
Carbonate factory
13
Q
What is the optimum temperature for shallow marine productivity?
A
20 - 25 degrees
14
Q
Where are hermatypic corals most productive?
A
Shallow clear water with strong currents
15
Q
What is different about the location of hermatypic corals and most other marine organisms?
A
Other marine organisms tend to prefer a quieter location
16
Q
Where does the precipitation of evaporite minerals usually occur?
A
where ocean bodies become wholly or partially isolated under arid conditions
17
Q
When does sea water become sufficiently concentrated for precipitation of minerals to occur?
A
When evaporation exceeds input of water
18
Q
In the past, where has larger areas of mineral precipitation occurred from seawater?
A
Epiconinental seas and small ocean basins
19
Q
Why were carbonate facies different in the Pre-Cambrian compared to the Phanerozoic?
A
Absence of shelly organisms, lacked bioclastic components
20
Q
Where are corals and calcareous green algae most common?
A
Low latitudes where shallow sea is always over 15 degrees
21
Q
What is the name for a geological association formed from calcareous green algae and corals?
A
Chlorozoan
22
Q
What is the name for a geological association formed from only calcareous green algae?
A
Chloralgal
23
Q
What is the name for the geological assemblage of carbonates formed by the remains of benthic forams and molluscs in cooler waters?
A
Foramol
24
Q
What assemblages are ooids most commonly associated with?
A
Chlorozoan and Chloralgal
25
What two common features found on beaches can form from the reworking of bioclastic debris and ooids by wave action?
Strand Plains, Barrier Islands
26
What is the texture of carbonate sediments deposited on barrier islands and strand plains?
Well sorted, Low mud matrix (Grainstone and Packstone)
27
What local zone is most of the carbonate detritus reworked from in shallow marine settings?
Shoreface
28
What is the typical angle and velocity of sedimentary structures in carbonate detritus in the foreshore and backshore areas?
3-13 degrees, Dips seawards in Foreshore, landwards in Backshore
29
In what tidal regime do laterally continuous barrier systems form, and when in geological time?
Micro-tidal, Slow rise in sea level
30
What is a "Beachrock"?
Carbonate in solution precipitates and acts as a cement for sand/gravel - in fully lithified rock
31
How is it possible to recognise early cementation in a beachrock?
On the foreshore it may act as a host for organisms which bore into the hard substrate
32
What would the succession for a prograding carbonate strandplain or beach barrier look like?
Coarsening upwards, well sorted, stratified grainstone and packstone
33
What is "Aeolianite"?
Carbonate aeolian dunes which have become wet and thus the surface of the rock becomes lithified from cement
34
What is a common occurance around roots of plants in shallow marine carbonate environments?
Precipitation to form Nodular Rhizocretions
35
What does sedimentation in carbonate lagoons look like?
Carbonate mudstone and wackestone with some grainstone and packstone beds deposited as washovers
36
What does sedimentation on a flood-tidal delta in a carbonate lagoon look like?
Cross-bedded oolithic and bioclastic packstone and grainstone formed by sub-aqueous dunes
37
What are the two types of sediment most commonly found on flood-tidal deltas in carbonate shallow marine settings?
Oolithic sediment or Bioclastic debris
38
What is the main source of fine-grained material in carbonate lagoons?
Calcareous algae living within the lagoon
39
What is the main control over diversity and abundance of life in carbonate lagoons?
Salinity
40
Give two examples of life which are found in hypersaline carbonate lagoons
Stromatolites, Marine Grasses (Thalassia)
41
What is the name for an area of hypersaline shallow water which precipitates evaporite minerals?
Saltern
42
How thick may saltern deposits become?
Meters to tens of meters thick
43
What type of gypsum growth may be found in saltern deposits and why?
Selenitic gypsum, restricted circulation in a lagoon - grows upwards from lagoon bed
44
What succession would be indicative of a fluctuating Arid Lagoon?
Alteration between laminated gypsum (sub-aqueous) and nodular gypsum in a supratidal sabkha
45
What is the name for the zone which is above the mean high water mark?
Supratidal Zone
46
When does the supratidal zone become inundated?
Extreme high tides or storms
47
What does the supratidal zone usually look like where the gradient to the shoreline is very low?
Marshy area
48
What usually forms in the marshy supratidal carbonate zone?
Microbial mats
49
What role does aoelian blown material play in supratidal carbonate zones?
Dust and sand can be bound by the microbes forming carbonate pavement
50
What may occur after the formation of a carbonate pavement?
Desiccation, but broken pieces are re-incorporated into sediment and re-cementation
51
What would the fabric of a succession of carbonate pavement look like?
Primary lamination with in-situ breccia
52
Give and example of a modern arid sabkha flat
Arabian Gulf
53
What is the definition of a "Sabkha"?
Sites of evaporite formation within coastal sediments
54
What are two common general features of a sabkha?
Not always a well defined beach, Usually very low relief
55
What is the most important supply of water to a sabkha?
Through groundwater transportation from the sea
56
How does the process of a sabkha form?
Evaporation through the sediment, water is drawn up and becomes increasingly concentrated in salts, dense, highly concentrated brine forms
57
Where do gypsum, halite and anhydrite form within a sabkha?
Gypsum forms within the sediment near a body of water, Anhydrite forms within the sediment further away from the water, Halite forms a crust at the surface
58
What forms do gypsum and anhydrite take when forming in a sabkha?
Gypsum = clusters, Anhydrite = Nodules
59
What is the name for the layers of anhydrite which have remnants of sediments?
Chicken-wire strucutre
60
What colour are terrigenous sediments in sabkhas usually?
Red
61
List the succession formed by sedimentation along an arid coast. (5 Main Stages)
Bottom: Wave reworked shallow subtidal setting, Overlain by: Intertidal microbial limestone beds, Gypsum forms in upper intertidal/lower supratidal, Anhydrite with chicken-wire structure, then coalesced beds of anhydrite in uppermost - may be contourted due to mineral growth
62
What is the name for the contourted anhydrite structure found in arid coastal environments?
Enterolithic bedding structure
63
What would cause a repeat in the cycle of an arid coastal succession?
Continued subsidence
64
What is the most distinctive feature found in arid coastal successions?
Displacive growth of gypsum within the sediment
65
What would the succession of carbonate sands deposited on migrating bars in tidal channels look like?
Cross-bedded grainstone/packstone beds
66
What is prominent in intertidal carbonate settings and why?
Desiccation, carbonate mud in warm climates dries out forming a crust by syndepositional cementation
67
What is the name of the feature which forms in carbonate muds where the surface layer expands from repeated precipitation? (2 names)
Teepee structures (Pseudoanticlines)
68
How big are psuedoanticlines usually?
A few 10's cm across
69
What secondary process occurs from the formation of psuedoanticlines? What is the name of the fabric formed?
Growth of sparry calcite cements within the cavities - Fenestrae (Fenestral cavities)
70
What is the name for lime mudstones with small cavities filled with calcite?
Birds-eye limestone
71
What forms from the trapping of fine grained sediment on algal mats?
Stromatolite formation
72
What is stromatolite formation and abundance controlled by?
Activity of organisms which feed on the algae
73
Where in the system do stromatolites tend to be better developed and why?
Better developed in the higher parts of the intertidal area which are less favourable for other organisms which may graze on the mats
74
What size grain usually accumulates on a carbonate shoal?
Sand to granule-sized
75
What environment do carbonate shoals usually form in?
Shallow, high energy areas
76
What are carbonate shoals usually made up of?
Broken shelly debris and accumulations of benthic foraminifers
77
What texture of material is usually found on carbonate shoals?
Well sorted/well rounded
78
When lithified, what type of rock forms from a carbonate shoal?
Grainstone or packstone
79
What type of sedimentary structures would be expected from a carbonate shoal?
Planar and trough cross-bedding from subaqueous dune migration
80
What form would a carbonate shoal in a wave dominated environment take?
Banks parallel to coastline
81
What form would a carbonate shoal in a tidal dominated environment take?
Banks elongated perpendicular to the shoreline
82
What is the geological definition of a reef?
Carbonate bodies made of frame-work building benthic organisms
83
What is a bioherm?
A reef buildup in a moundlike shape
84
What is a biostrome?
A reef buildup which is not moundlike - bedded
85
Why are Scleractinian corals successful?
Hermatypic (symbiotic relationship with algae) and so grow rapidly in nutrient poor waters
86
Aside from corals, what is the other main reef builder?
Calcareous algae
87
What were the earliest reef builders?
Cyanobacteria (Stromatolites)
88
What followed stromatolites as early reef builders in the Palaeozoic?
Rugose and Tabulate corals, Calcareous Sponges, Stromatoporoids
89
What is the name for the unusual reef-forming organism which was a type of bivalve?
Rudists
90
How often in the Phanerozoic have reefs dominated as depositional systems? What dominated when reefs did not?
Four peaks, Mud mounds
91
What do corals which dwell in the reef crest look like?
Robust structures, encrusting and massive forms able to withstand wave force
92
What do corals which dwell on the reef front look like?
More branching and more delicate plate-like forms
93
What do corals which dwell in the reef back look like?
More globular in forms
94
How do encrusting organisms and calcareous algae contribute to a reef?
Stabilise the framework and provide mass
95
What is the name for the process by which organisms remove mass from a reef?
Bioerosion
96
What two common organisms contribute the most to bioerosion?
Fish and Molluscs
97
What are the two options for the filling of voids in a reef?
Filled with remains of organisms and bioerosion, or not filled and crystalline calcite is precipitated
98
What is the forereef?
A talus slope/reefal debris from reef crest
99
What type of rock comes from a forereef?
Carbonate breccia - bioclastic rudstone or grainstone facies
100
What would be the expected incline of a forereef?
Steeply sloping as they are gravity deposits (10 - 30 degrees)
101
What type of rock would be found on a back reef?
Gradation from rudstone to grainstone
102
What type of material could be expected at the fringe of a carbonate lagoon?
Broken reef material, shells, ooid formation
103
What are the three main forms of reef?
Fringing reef, barrier reefs, patch reefs
104
What is the name for a reef which builds out directly from the shoreline?
Fringing reef
105
What is the name for a linear reef which forms parallel to the shoreline, off shore?
Barrier reef
106
How far offshore can barrier reefs form?
Kilometers to tens of kilometers
107
What is the name for a reef which forms on a sea mount in open ocean areas?
Coral atolls
108
What latitude range to present day reefs occur in?
within 35 degrees of the equator
109
Why is it incorrect to assume that reefs only form at low latitudes?
Corals, especially in the past, seemed to have a synbiotic relationship with algae and thus availability of nutrients was the main control over location
110
What is the main cause of cessation of reef development? (2)
Environmental conditions: Flux of terrigenous clastic supply, or availability of nutrients
111
What is the name for successions deposted over long time periods in thin layers?
Condensed sections
112
What is a carbonate mud mound?
Structureless/crudely bedded body of fine crystaline carbonate
113
Where do carbonate mud mounds commonly form?
Deeper deposits than reefs, but within the photic zone
114
What are the two types of mud mounds we know of?
Mounds made up of microbes build in place, mounds made up of detrital material piled
115
What is required for the cementation of mud?
Circulation of large amounts of calcium carbonate rich water
116
What is the name for lithified fine grained carbomnate sediment formed in outer platform settings?
Chalk
117
How does chert nodules form within chalk facies?
Redistribution of silica within beds from siliceous organisms
118
How is bedding picked out in chalk deposits? (2)
Slight variations in proportions of clay minerals or by variations in degree of cementation
119
What are the three ages were chalk is predominatly found?
Mesozoic, Cenozoic, Late Cretaceous
120
What is the name for the unit which forms the White Cliffs of Dover, and when did it form?
The Chalk, Late Cretaceous
121
What does a by-pass margin ook like for a carbonate platform?
Abrupt change in slope, can form a vertical wall with a lower angle at the bottom
122
How is sediment transported to the base of the slope off a carbonate platform? (5)
Blocks fall, Talus slopes, Slumps, Debris flow, Turbidites
123
What type of deposit does the most proximal material of a by-pass margin form?
Rudstones, AKA Megabreccias
124
What is the name for the gentle slope which has a continuous spectrum of sediments from reef boundstones to wackestones and mudstones?
Depositional margins
125
What may result from mudstones and wackestones deposited on steep slopes?
Contourted, redeposited beds - tend to be unstable on steep slopes
126
What is the modern limitation of marine evaporites?
Coastal regions
127
What does the stratigraphic record tell us about ancient evaporite deposition?
Has occurred over huge scales
128
How large can evaporite units be when formed from shallow marine platforms?
Tens of kilometers across
129
What is the name for a basin connected to ocean which becomes blocked?
Barred basin
130
How deep can evaporite deposits accumulate to in barred basins?
Hundreds of meters
131
What depth of seawater is required to produce a meter bed of halite?
75m
132
What is required to generate a particularly thick succession of evaporite minerals?
Seawater being repeatedly evaporated and replenished
133
What are the three ways thick successions of halite can form?
Shallow-water to deep-basin setting, Shallow-water to shallow basin setting, Deep-water to deep-basin setting
134
Explain the process of evaporite formation in shallow-water to deep-basin settings
Basin is below sea level, sea-water is periodically replenished
135
What is characteristic of shallow-water to deep-basin successions?
Evaporites overlain by deep-marine successions such as turbidites from the periodic overflow
136
Explain the process of evaporite formation in shallow-water to shallow-basin settings
Evaporites deposited in salterns, continued subsidence in the basin allows build up of succession
137
What is characteristic of shallow-water to shallow-basin successions?
Succession will show characteristics of shallow-water deposition throughout
138
Explain the process of evaporite formation in deep-water to deep-basin settings
Evaporite sediments formed at shallow margins redeposited by gravity flows - normally graded beds from turbidites
139
What is characteristic of deep-water to deep-basin successions?
Lower part of succession will be deeper water facies and overlying will be characteristic of shallow-water (two patterns)
140
What are the two patterns of deep-basin successions?
Enclosed/barred basin: bulls-eye pattern where most soluble salts in basin center, Equilibrium between inflow/evaporation: Teardrop pattern with salinity increasing with distance from inlet
141
What may depositions cycles in barred basins be related to? (2)
Global sea level fluctuation, Local tectonics
142
Why may organic material brought to a barred basin be preserved when the salinity increases?
Hypersaline basins are anoxic
143
What is the name given to very large barred evaporitic basins?
Saline gians
144
What is a typical thickness and expanse of a saline giant?
1000m thick deposits, hundreds of thousands of km2
145
What is an example of a saline giant?
Partial closure of the Mediterranean Sea caused the Messinian Salinity Crisis
146
What could be the cause of a alternation of limestone and mud beds?
Flux of sediment supply to shelf, relative sea level rise
147
Where may carbonate deposits co-exist with terrigeneous sediments?
Deltas built by ephemeral rivers in arid environments - carbonate deposits on the delta front
148
Where may carbonate deposits co-exist with volcaniclastic sediments?
Between eruption episodes of island arc volcanoes - periods long enough to form carbonate shelves
149
What are the typical lithologies associated with shallow marine carbonate deposits?
Limestone
150
What would be expected of the mineralogies of shallow marine carbonate deposits?
Calcite and Aragonite
151
What would be expected of the texture of shallow marine carbonate deposits?
Variable, biogenic structures in reefs, well sorted in shallow water
152
What would be expected of the bed geometry of shallow marine carbonate deposits?
Massive reef build-ups, well sorted in shallow water
153
What sedimentary structures would be expected in shallow marine carbonate deposits?
Cross-bedding in oolite shoals
154
What paleoflow would be expected in shallow marine carbonate deposits?
Not usually diagnostic, with tide, wave and storm driven currents
155
What fossils would be expected in shallow marine carbonate deposits?
Usually abundant, shallow marine fauna most common
156
What would be expected of the sedimentary colouring of shallow marine carbonate deposits?
Usually pale white, cream or grey
157
What facies would be expected to be associated to shallow marine carbonate deposits?
May occur with evaporites, associations with terrigenous clastic material may occur
158
What are the typical lithologies associated with marine evaporite deposits?
Gypsum, anydrite and halite
159
What would be expected of the mineralogies of marine evaporite deposits?
Evaporite minerals
160
What would be expected of the texture of marine evaporite deposits?
Crystalline or amorphous
161
What would be expected of the bed geometry of marine evaporite deposits?
Sheets in lagoons and barred basins, nodular in sabkhas
162
What sedimentary structures would be expected in marine evaporite deposits?
Intrastratal solution in breccias and deformation
163
What paleoflow would be expected in marine evaporite deposits?
Rare
164
What fossils would be expected in marine evaporite deposits?
Rare
165
What would be expected of the sedimentary colouring of marine evaporite deposits?
Typically white, buy may be coloured by impurities
166
What facies would be expected to be associated to marine evaporite deposits?
Often with shallow marine carbonates
167
What is the importance of limestone beds with hydrocarbons?
Limestones form some of the largest hydrocarbon reservouirs in the world
168
What role does diagensis play on limestone successions in terms of hydrocarbon potential?
The formation of cement inbetween the grains decreases hydrocarbon potential
