2/25/2009 1 Bahamian Dolomites A Short Course VU March, 2009 Peter Swart University of Miami Occurrences in the Bahamas • Platform Dolomites – San Salvador – Little Bahama Bank Bh D illi P j t • Bahamas Drilling Project – Unda – Clino • Cretaceous Dolomite
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
2/25/2009
1
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
2/25/2009
2
2/25/2009
3
Supko (1977)
Massive
Stratal
Supko P. R. (1977) Subsurface dolomites, San Salvador, Bahamas. Journal of Sedimentary Petrology 47, 1063-77.
2/25/2009
4
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)
2/25/2009
5
Dawans and Swart (1988)
Dawans and Swart (1988)
Microsucrosic
Dawans and Swart (1988)
2/25/2009
6
Sucrosic
Dawans and Swart (1988)
Dawans and Swart (1988)
Dawans and Swart (1988)
2/25/2009
7
Swart 1989
235U(n,f)f Fission Tracks
10B(n,)7Li Alpha Tracks
CNM Dolomite SS
2/25/2009
8
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
2/25/2009
9
Swart, unpublished
2/25/2009
10
Dolomitization by marine fluids
2/25/2009
11
2/25/2009
12
2/25/2009
13
Bahamas Drilling Project, 1990
Ocean Drilling Program Leg 166, 1996
Bahamas Transect Drilling Campaign
Joides Resolution Southern Cross II
2/25/2009
14
2/25/2009
15
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
2/25/2009
16
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
2/25/2009
17
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
2/25/2009
18
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
2/25/2009
19
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
2/25/2009
20
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)
2/25/2009
21
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
2/25/2009
22
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
2/25/2009
23
Unda 1133Unda 1133
Unda 961
Unda 1061
Clino 1769
Unda 863Unda 863
Unda 961
2/25/2009
24
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
2/25/2009
25
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.
2/25/2009
26
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
2/25/2009
27
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
2/25/2009
28
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
2/25/2009
29
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
2/25/2009
30
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
2/25/2009
31
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
2/25/2009
32
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