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Bahamian Dolomites
A Short Course VU March, 2009 Peter Swart University of Miami
Occurrences in the Bahamas
• Platform Dolomites– San Salvador
– Little Bahama Bank
B h D illi P j t• Bahamas Drilling Project– Unda
– Clino
• Cretaceous Dolomite
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Supko (1977)
Massive
Stratal
Supko P. R. (1977) Subsurface dolomites, San Salvador, Bahamas. Journal of Sedimentary Petrology 47, 1063-77.
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Dawans J. and Swart P. K. (1988) Textural and geochemical alternations in late Cenozoic Bahamian dolomites. Sedimentology 35, 385-403.
Dawans and Swart (1988)
Dawans and Swart (1988)
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CM Dolomite
Distinct differences between CM and MS dolomites.
The CM dolomites preserve there original high U concentrations while the MS dolomites show evidence of a more open system
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Bahamas Drilling Project, 1990
Ocean Drilling Program Leg 166, 1996
Bahamas Transect Drilling Campaign
Joides Resolution Southern Cross II
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100
200
300
m)
1 3 5 7 9
Age (Ma)
bulk
shell
dolomite
400
700
Dep
th (
m
600
500
100
200
bulk
shell
dolomite
)
1 3 5 7 9
Age (Ma)
300
400
500
De
pth
(m
Unda
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0.709200.709100.709000.70890
0
2
87Sr
86
Sr
4
6
8
Ag
e (
Ma
)
86Sr
87Sr
Dep
th (
m)
100
200
300
400
0.70890 0.70900 0.70910
shelldolomite
bulk
Dep
th (
m)
100
200
300
86Sr
87Sr
0.70890 0.70900 0.70910
shelldolomite
bulk
500
600
700
D 300
400
500
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100
200
1 3 5 7 9
Age (Ma)
bulk
shell
dolomite
De
pth
(m)
300
400
500
500
200100
Sr (M)
UNDA 1UNDA 4CLINO 2
300
100
200
mbmp fbmp
1000
1500
2000
Dep
th 300
400
500
600
Evidence for Diagenetic Reactions
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100
200
86Sr
87Sr
0.70890 0.70900 0.70910 0.70920
86Sr
87Sr
0.70890 0.70900 0.70910
100
Dep
th (
m)
300
400
500
600
700
Dep
th (
m) 200
300
400
500
02
000 20 40 60 80 10
0
Aragonite
Carbon
Oxygen
Mineral Percent
500
550
600
650
700
1 2 3 4
3
18
Oo
/oo
DolomiteCarbonate
36 C/1000mo
Dep
th (
m)
40
06
00
Dolomite
g
Clino
-4 +6Isotopic Composition
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Clino 646
3
4
mp
osi
tio
n
Starting Composition
Leach 1
Leach 2
0 20 40 60 80 1000
1
2
% Dolomite
C &
O Is
oto
pic
Co
m
Carbon Oxygen
02
000 20 40 60 80 10
0
Aragonite
Carbon
Oxygen
Mineral Percent
500
550
600
650
700
1 2 3 4
3
18
Oo
/oo
DolomiteCarbonate
36 C/1000mo
Dep
th (
m)
40
06
00
Dolomite
g
Clino
-4 +6Isotopic Composition
m
150
200
CO300
350
OC
NDS
02
0 20 40 60 80 100
Carbon
Oxygen
Mineral Percent
Dep
th m
250
3001 2 3
18O
o/oo
13C
o/oo
400
450
500
550
0 1 2 3 4
Dep
th m
13C
o/oo
18O
o/oo
Clino
UndaNDS
NDS
004
00
600
Dolomite
Aragonite
Clino
-4 +6Isotopic Composition
Oxygen
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02
000 20 40 60 80 10
0
Aragonite
Carbon
Oxygen
Mineral Percent
40
06
00
Dolomite Clino
-4 +6Isotopic Composition
02
000 20 40 60 80 10
0
Aragonite
Carbon
Oxygen
Mineral Percent
500
550
600
1 2 3 4
3
18O
o/oo
pth
(m
)
Strontium (ppm)
200 600
40
06
00
Dolomite Clino
-4 +6Isotopic Composition
650
700
DolomiteCarbonate
36 C/1000mo
De
p
02
000 20 40 60 80 10
0
Aragonite
Carbon
Oxygen
Mineral Percent
350
450
De
pth
m
367
Clino
40
06
00
Dolomite Clino
-4 +6Isotopic Composition
550
1000 2000
D
536.3
Strontium (ppm)
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Swart and Melim, 2000
3.5
4
4.5
5
omite
Co
/oo
1 1.5 2 2.5 3 3.5 42
2.5
3
Dol
o
Coexisiting Carbonate
13C
o/oo
13
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4
4.5
5
olom
ite
Oo
/oo
-0.5 0 0.5 1 1.5 2 2.5 32.5
3
3.5
Coexisiting Carbonate
18O
o/oo
Do
18
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U989.58
U924.5
C1204.19
C1204.17
Three locations of dolomitization have been determined in the sediments from Great Bahama Bank using a combination of stable isotopes and trace elements.
0200
40 20 40 60 80 100
Aragonite
Carbon
Oxygen
Mineral Percent
These areHardground DolomitesBackground DolomitesMassive Dolomites
006
00
Dolomite Clino
-4 +6Isotopic Composition
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Dolomite is found throughout the core, but is particularly abundant immediately below non-depositional surfaces. These surfaces represent
0200
40 20 40 60 80 100
Aragonite
Carbon
Oxygen
Mineral Percent
phiatuses of between several 100K to several Myrs.
006
00
Dolomite Clino
-4 +6Isotopic Composition
Clino 646
3
4
mp
osi
tio
n
Starting Composition
Leach 1
Leach 2
0 20 40 60 80 1000
1
2
% Dolomite
C &
O Is
oto
pic
Co
m
Carbon Oxygen
1 1.5 2 2.5 3 3.5 42
2.5
3
3.5
4
4.5
5
Dol
omite
Coexisiting Carbonate
13
Co
/oo
13
C
o/o
o
There is a positive correlation between the C in the calcites and the dolomites with an offset of about 1 per mille, suggesting equilibrium between the calcite and dolomite.
-0.5 0 0.5 1 1.5 2 2.5 32.5
3
3.5
4
4.5
5
Coexisiting Carbonate
18
Oo
/oo
Dol
omite
18
O
o/o
o
There are two relationships between the O of the dolomites and the precursors. One +ve and one with no apparent relationship. No relationship is what I would expect. The +ve relationship I will discuss in a moment.
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Origin of Dolomite
• Dolomites show a gradient in their O isotopic composition indicating that they formed in the presence of
500
550
1 2 3 4
3
18
Oo
/oo
pa geothermal gradient of 36oC/km.
• Mg diffused from overlying seawater during the time represented by the hiatus
600
650
700
DolomiteCarbonate
36 C/1000mo
De
pth
(m
)
Isotopes and Strontium
• The concentration of Sr also increases with depth away from the non-depositional
500
550
1 2 3 4
3
18
Oo
/ooStrontium (ppm)
200 600
surface, similar to porewater Sr-gradients. This supports the timing of dolomitization
600
650
700
DolomiteCarbonate
36 C/1000mo
Dep
th (
m)
350367
Clino0
200
100 108.1
Unda
Background
450
550
1000 2000
Dep
th m
536.3
Strontium (ppm) Strontium (ppm)
300
400
270
1000 2000
Hardground Marine
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Summary
• Background Dolomites: Are microsucrosic and contain Sr concentrations in excess of 2000 ppm and are formed from pore waters saturated with respect to calcium carbonate. No unusla O or C isotopic composition.
• Hardground Dolomites: Dolomites are formed below hardgrounds or firmgrounds. Dolomitization is mediated by the decomposition of organic material and concentrations are highest nearest to the surface and decrease with depth.
Summary
• Gradients in Sr indicate formation from fluids close to the composition of seawater to a composition similar to that which formed the background dolomites.
• Gradients in the O isotopic composition indicate p pformation in the presence of a geothermal gradient.
• Massive dolomites: Theses dolomites can be sucrosic or fabric replacing and are distinguished from the other dolomites by being composed of 100% dolomite and having a uniformly low Sr concentration.
Origin of Dolomites
• Dolomites formed below hardgrounds with the thermodynamic drive being supplied by the decay of organic material and Mg by diffusion from the overlying seawater
0200
0 20 40 60 80 100
Carbon
Oxygen
Mineral Percent
seawater.• Background dolomites formed
in the the porewater with Mg being supplied from local pore waters and by diffusion.
• Massive dolomites formed by the circulation of seawater
400
600
Dolomite
Aragonite
Clino
-4 +6Isotopic Composition
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100
200
300
400
500
600
Str
ontiu
m (
ppm
) Increase in Sr/Ca ratio
Change in stoichiometry40 42 44 46 48 50
0
Mol% MgCO3
Change in stoichiometry
38
4244
4648
40
Mol% MgCO 3
100300
500700
900
Sr/Ca (x 1000)
400
800
1200
1600
2000S
tron
tium
(pp
m)
CO300
350
400
450
th m
367
500
550
0 1 2 3 4
Dep
13C
o /oo18
Oo/oo
Clino
536
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Hardgrounds with increased dolomite content below
0
0.5
1.0
1.5
ms(twt)
WSWGreat Bahama Bank
Bimini BankProgradation
Clino UndaSite 1006 Site 1007Site 1004
Site 1003 Site 1005ENE
Santaren Channel
Bahamas Transect: 7 Drill Sites across Prograding Margin of Great Bahama Bank
0
0.5
1.0
1.5
ms(twt)
10 km
qpn
mk
he il
o
gfd
p2
b
cd
ef
g
h i k l
mn
op
q
a
Line 106 Western Line
Drifts
PlatformSlope
Base of Neogene
Miocene
Pliocene
Pleistocene/Holocene
Mineralogy along the Bahamas Transect
10 km
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Dolomitization Model based on geochemistry in Great Bahama Bank
- Dolomitization early and episodic
- Massive dolomite = normal marine sea water after diagenesis from aragonite to LMC
Mechanism unclear (Reflux, Kohout)
- Hardground dolomite = cold marine waters at top into partly altered sediment
Mg diffusing from top downSulfate reduction
- Microsucrosic dolomiterecrystallisation of sediment and precipitation into pore space
ConclusionsHigh sea level: Thermal convection dominant force for fluid flowLow sea level: Reflux is dominant, if a constant recharge is maintained, not likely on an exposed platform
Lithology: plays minor role for fluid flowLithology: plays minor role for fluid flowAnisotropy: important, high anisotropy inhibits vertical movementCompaction: less important than thermal convectionFlow (1 m/y) over 1 Ma sufficient for pervasive dolomitization of reef at Unda by Kohout convection
Dolomitization on Great Bahama Bank:
• Normal Marine Sea water
• Episodic
• Diffusive below submarine hardgrounds
• Kohout convection possible flow
mechanism for dolomitized buried margin
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Meteoric
Mixing Zone
Marine Burial Realm
Moldic Porosity created in the marine diagenetic environment
Marine cements
Mineralogy in Unda and Clino within Sequence Stratigraphic Framework
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1 mm
150
Unda (292.8 m)
Dolomite at Sequence Boundary f/g
Above: Sucrosic DolomiteFabric-destructive
Below: Fabric-preservingDolomite
Dep
th m
150
200
250
3001 2 3
18O
o/oo
13C
o/oo
OC
UndaNDS
Mineralogy in Unda and Clino within Sequence Stratigraphic Framework
Plio-Pleistocene Sea Level Changes
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Erosional escarpment
STRATIGRAPHY OF A STEEP SLOPE
LST/TST depositsLST/TST deposits*unconformity surface*unconformity surface
HST wedgeHST wedge(derived from platform top)(derived from platform top)
0 m
100 m
(derived from platform top)(derived from platform top)
Grammer & Ginsburg 1992
200 m
Inherited topographyInherited topography
Transgressive fringing reefs Transgressive fringing reefs source coarse debrissource coarse debris
Pervasive marine cementationPervasive marine cementationpreserves steep slopepreserves steep slope
Offbank transport of sand Offbank transport of sand and mud form onlapping wedgeand mud form onlapping wedge
Fine-grainedslope depositdolomitized
Graded bed
Coarse-grainedslope depositwith lithoclasts
SB f/g
Pliocene
5 cm
SB f/g
KarstifiedReefdolomitized
Hardgroundin upper slopedeposits
with lithoclasts and erosive base
Uncementedperiplatformooze
Pliocene
Miocene
Eberli et al., 2001
Clino 1777