Bahamian Dolomites. Occurrences in the Bahamas 2/25/2009. Platform Dolomites. Cretaceous Dolomite. San Salvador Little Bahama Bank.

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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 Bahamas Drilling Project Unda Clino Cretaceous Dolomite 1

2 2

3 Supko (1977) Massive Stratal Supko P. R. (1977) Subsurface dolomites, San Salvador, Bahamas. Journal of Sedimentary Petrology 47,

4 Dawans J. and Swart P. K. (1988) Textural and geochemical alternations in late Cenozoic Bahamian dolomites. Sedimentology 35, Dawans and Swart (1988) Dawans and Swart (1988) 4

5 Dawans and Swart (1988) Dawans and Swart (1988) Microsucrosic Dawans and Swart (1988) 5

6 Sucrosic Dawans and Swart (1988) Dawans and Swart (1988) Dawans and Swart (1988) 6

7 235 U(n,f)f Fission Tracks 10 B(n, ) 7 Li Alpha Tracks Swart 1989 CNM Dolomite SS 7

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 8

9 Swart, unpublished 9

10 Dolomitization by marine fluids 10

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13 Bahamas Transect Drilling Campaign Bahamas Drilling Project, 1990 Ocean Drilling Program Leg 166, 1996 Joides Resolution Southern Cross II 13

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15 Depth (m ) Depth (m m) 2/25/2009 Age (Ma) dolomite bulk shell Age (Ma) dolomite bulk shell Unda 15

16 Depth (m) Depth (m) 2/25/ Sr Sr Age (Ma) Sr 86 Sr 87 Sr 86 Sr bulk shell dolomite 200 bulk shell dolomite

17 Depth (m) 2/25/2009 Age (Ma) dolomite bulk shell mbmp fbmp Sr ( M) 200 UNDA 1 UNDA 4 CLINO Depth Evidence for Diagenetic Reactions 17

18 Depth (m) 0 Depth (m) 18 2/25/ Sr 86 Sr 87 Sr 86 Sr o O /oo Mineral Percent Oxygen 60 Carbon 80 Depth (m) Carbonate Dolomite o 36 C/0m Aragonite Dolomite Clino Isotopic Composition 18

19 /25/2009 C & O Isotopic Com mposition Starting Composition Clino 646 Leach 1 Carbon Oxygen Leach % Dolomite o O /oo Mineral Percent Oxygen 60 Carbon 80 Depth (m) Carbonate Dolomite o 36 C/0m Aragonite Dolomite Clino Isotopic Composition Mineral Percent Carbon Oxygen Dolomite Aragonite Clino Depth m O C o 18 C /oo O Clino o /oo NDS Depth m NDS O Unda o 13 O /oo C C o /oo NDS Isotopic Composition 19

20 /25/2009 Mineral Percent Oxygen Carbon Aragonite Dolomite Clino Isotopic Composition Mineral Percent Oxygen Carbon o O /oo Strontium (ppm) Aragonite Dolomite Clino Dep pth (m) Carbonate Dolomite o 36 C/0m Isotopic Composition Mineral Percent Clino Oxygen Carbon Aragonite Depth m D Dolomite Clino Isotopic Composition Strontium (ppm) 20

21 Swart and Melim, Dolo omite 13 C o /oo o C /oo Coexisiting Carbonate 21

22 2/25/ Do olomite 18 O o /oo o O /oo Coexisiting Carbonate 22

23 Unda 1133 Unda 961 Unda 1061 Clino 1769 Unda 863 Unda

24 0 2/25/2009 U U924.5 C C Mineral Percent Three locations of dolomitization have been determined in the sediments from Great Bahama Bank using a combination of stable isotopes and trace elements Oxygen Carbon Aragonite These are Hardground Dolomites Background Dolomites Massive Dolomites Dolomite Clino Isotopic Composition 24

25 C O /25/2009 Mineral Percent Dolomite is found throughout the core, but is particularly abundant immediately below non-depositional surfaces. These surfaces represent hiatuses of between several K to several Myrs Oxygen Dolomite Carbon Aragonite Clino Isotopic Composition C & O Isotopic Com mposition Starting Composition Clino 646 Leach 1 Carbon Oxygen Leach % Dolomite Dolomite o /oo o C /oo Coexisiting Carbonate 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. 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. Dolomite o /oo o O /oo Coexisiting Carbonate 25

26 Origin of Dolomite Depth (m) O o /oo Carbonate 3 Dolomite o 36 C/0m Dolomites show a gradient in their O isotopic composition indicating that they formed in the presence of a geothermal gradient of 36oC/km. Mg diffused from overlying seawater during the time represented by the hiatus 700 Isotopes and Strontium The concentration of Sr also increases with depth away from the non-depositional surface, similar to porewater Srgradients. This supports the timing of dolomitization Depth (m) O o /oo Carbonate 3 Dolomite o 36 C/0m Strontium (ppm) Clino 367 Background 0 Unda Depth m Hardground Marine Strontium (ppm) Strontium (ppm) 26

27 0 2/25/2009 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 formation 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 % 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. 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 Mineral Percent Carbon Oxygen Dolomite Aragonite Clino Isotopic Composition 27

28 Strontium (ppm) Increase in Sr/Ca ratio Change in stoichiometry Mol% MgCO3 Strontium (ppm) Sr/Ca (x 0) Mol% MgCO O C th m Dep Clino o 18 o C /oo O /oo 28

29 Hardgrounds with increased dolomite content below Bahamas Transect: 7 Drill Sites across Prograding Margin of Great Bahama Bank Santaren Channel Site 4 WSW Site 6 Site 7 Site 3 Site 5 0 Clino Unda Great Bahama Bank ENE ms (twt) Progradation Bimini Bank Line 106 Western Line Pleistocene/Holocene a b c d e f h i k l g m n o p 1.0 Pliocene e d f g hi k l m q 1.5 Miocene ms (twt) Base of Neogene n o p p2 q Platform Slope Drifts 10 km Mineralogy along the Bahamas Transect 10 km 29

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 down Sulfate reduction - Microsucrosic dolomite recrystallisation of sediment and precipitation into pore space Conclusions High sea level: Thermal convection dominant force for fluid flow Low sea level: Reflux is dominant, if a constant recharge is maintained, not likely on an exposed platform Lithology: plays minor role for fluid flow Anisotropy: important, high anisotropy inhibits vertical movement Compaction: less important than thermal convection Flow (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 30

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 31

32 Unda (292.8 m) 1 mm 150 Dolomite at Sequence Boundary C f/g O Above: 200 Sucrosic Dolomite Fabric-destructive Depth m Below: 250 Fabric-preserving Dolomite Unda o O /oo 13 o C /oo NDS Mineralogy in Unda and Clino within Sequence Stratigraphic Framework Plio-Pleistocene Sea Level Changes 32

33 STRATIGRAPHY OF A STEEP SLOPE Erosional escarpment LST/TST deposits *unconformity surface HST wedge (derived from platform top) 0 m m 200 m Inherited topography Transgressive fringing reefs source coarse debris Pervasive marine cementation preserves steep slope Offbank transport of sand and mud form onlapping wedge Grammer & Ginsburg 1992 SB f/g 5 cm Graded bed with lithoclasts and erosive base Uncemented periplatform ooze Coarse-grained slope deposit with lithoclasts Hardground in upper slope deposits Fine-grained slope deposit dolomitized Pliocene SB f/g Miocene Karstified Reef dolomitized Eberli et al., 2001 Clino

34 Unda 1133 Unda 961 Unda 1061 Clino 1769 Unda

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