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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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Dawans and Swart (1988)

Dawans and Swart (1988)

Microsucrosic

Dawans and Swart (1988)

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Sucrosic

Dawans and Swart (1988)

Dawans and Swart (1988)

Dawans and Swart (1988)

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Swart 1989

235U(n,f)f Fission Tracks

10B(n,)7Li Alpha Tracks

CNM Dolomite SS

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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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Swart, unpublished

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Dolomitization by marine fluids

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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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Unda 1133Unda 1133

Unda 961

Unda 1061

Clino 1769

Unda 863Unda 863

Unda 961

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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

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Unda 1133Unda 1133

Unda 961

Unda 1061

Clino 1769

Unda 863

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