The origin of dolomites in tertiary sediments from the margin of Great Bahama Bank

Peter K Swart, Leslie A. Melim

Research output: Contribution to journalArticle

80 Citations (Scopus)

Abstract

Based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on Great Bahama Bank, we have determined three different environments of formation for the dolomites that are common throughout the Pliocene and Miocene parts of these cores. The first environment of dolomitization occurs in association with development of nondepositional surfaces. Dolomite typically forms below each of these surfaces, the concentration and extent of which is governed by the length of the period of nondeposition. These dolomites are recognized by their association with the nondepositional surfaces, characteristic positive δ 18O values indicative of formation from cold bottom waters, and δ 18O and Sr profiles with depth that suggest formation in the presence of diffusive temperature and Sr gradients. The second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes. The dolomite forming here is termed background dolomite. This is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg 2+, and consequently the dolomite never constitutes more than between 5 and 10% of the sediment. This type of dolomite is characterized by extremely high Sr concentrations, which reflect concentrations of Sr 2+ in the pore fluids from which it formed. The high concentrations of Sr 2+ in the pore fluids arise through the continued recrystallization of metastable aragonite and high-Mg calcite to dolomite and low-magnesium calcite driven by oxidation of organic material by sulfate. Not only does sulfate reduction provide an additional thermodynamic drive for recrystallization, but because the absolute concentration of Sr 2+ in the pore fluids is governed by the solubility of celestite (SrSO 4), removal of sulfate allows the Sr 2+/Ca 2+ ratio of the interstitial fluid to become much higher than normally encountered. The final type of environment of dolomitization is associated with coarse-grained reefal sediments. The pervasive nature of the dolomitization and the relatively normal Sr concentrations suggest the circulation of normal marine water in a relatively open system.

Original languageEnglish (US)
Pages (from-to)738-748
Number of pages11
JournalJournal of Sedimentary Research
Volume70
Issue number3
StatePublished - May 2000

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dolomite
dolomitization
sediment
fluid
sulfate
calcite
celestine
aragonite
cold water
bottom water
void
anion
magnesium
Pliocene
solubility
cation
thermodynamics
Miocene
oxidation

ASJC Scopus subject areas

  • Geology
  • Stratigraphy
  • Global and Planetary Change

Cite this

The origin of dolomites in tertiary sediments from the margin of Great Bahama Bank. / Swart, Peter K; Melim, Leslie A.

In: Journal of Sedimentary Research, Vol. 70, No. 3, 05.2000, p. 738-748.

Research output: Contribution to journalArticle

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abstract = "Based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on Great Bahama Bank, we have determined three different environments of formation for the dolomites that are common throughout the Pliocene and Miocene parts of these cores. The first environment of dolomitization occurs in association with development of nondepositional surfaces. Dolomite typically forms below each of these surfaces, the concentration and extent of which is governed by the length of the period of nondeposition. These dolomites are recognized by their association with the nondepositional surfaces, characteristic positive δ 18O values indicative of formation from cold bottom waters, and δ 18O and Sr profiles with depth that suggest formation in the presence of diffusive temperature and Sr gradients. The second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes. The dolomite forming here is termed background dolomite. This is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg 2+, and consequently the dolomite never constitutes more than between 5 and 10{\%} of the sediment. This type of dolomite is characterized by extremely high Sr concentrations, which reflect concentrations of Sr 2+ in the pore fluids from which it formed. The high concentrations of Sr 2+ in the pore fluids arise through the continued recrystallization of metastable aragonite and high-Mg calcite to dolomite and low-magnesium calcite driven by oxidation of organic material by sulfate. Not only does sulfate reduction provide an additional thermodynamic drive for recrystallization, but because the absolute concentration of Sr 2+ in the pore fluids is governed by the solubility of celestite (SrSO 4), removal of sulfate allows the Sr 2+/Ca 2+ ratio of the interstitial fluid to become much higher than normally encountered. The final type of environment of dolomitization is associated with coarse-grained reefal sediments. The pervasive nature of the dolomitization and the relatively normal Sr concentrations suggest the circulation of normal marine water in a relatively open system.",
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