Formation of dolomite mottling in Middle Triassic ramp carbonates (Southern Hungary)

Research output: Contribution to journalArticle

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Abstract

The Middle Triassic carbonates of the Villany Mountains were deposited on a homoclinal carbonate ramp. Many of the carbonates from the 700 m-thick sequence show partial or complete dolomitization. The present paper describes dolomites that occur in a limestone unit as irregular mottles and as pore- and fracture-filling cements. Replacement-type scattered dolomite rhombs in the mottles having inclusion-rich, very dull luminescent cores and limpid non-luminescent outer zones represent the initial phase of dolomitization. The isotopic composition of these dolomites (δ13C = +2.30‰ VPDB, δ18O = -3.60‰ VPDB) is similar to that of the calcitic micrite (δ13C = +2.6‰ VPDB, δ18O = -4.00‰ VPDB) indicating that no external fluids were introduced during dolomite formation. The elevated Sr content of the micrites implies that sediment was originally aragonite or high-Mg calcite. Dolomitization took place in the burial realm from a 'marine' pore-fluid in a partly closed system. Later fracture-related saddle dolomite reflects elevated formation temperatures and increasing burial. Five calcites were identified. Multiple generations of calcite-filled fractures were formed during burial diagenesis generally having dull or no luminescence (δ13C = +1.80‰ VPDB, δ18O = -6.40‰ VPDB). The latest phase calcites are related to karst formation, having a very negative isotopic composition (δ13C = -5.0 to -7.2‰ VPDB and δ18O ~ -7.4‰ VPDB). The karst-related processes include dissolution, calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite. The timing of dolomitization is uncertain, but the first phase took place in a partly closed system prior to stylolite formation. Late-stage saddle dolomites were precipitated during maximum burial in the Cretaceous. The dissolution of dolomites and karst-related calcite replacement was not earlier than Late Cretaceous. (C) 2000 Elsevier Science B.V. All rights reserved.

Original languageEnglish
Pages (from-to)131-145
Number of pages15
JournalSedimentary Geology
Volume131
Issue number3-4
DOIs
Publication statusPublished - Mar 15 2000

Fingerprint

carbonate ramp
dolomite
Triassic
calcite
dolomitization
karst
micrite
replacement
isotopic composition
stylolite
dissolution
Cretaceous
carbonate
burial diagenesis
fluid
aragonite
luminescence
cement
limestone
mountain

Keywords

  • Dolomitization
  • Middle Triassic
  • Replacement
  • Stable isotopes

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

Cite this

Formation of dolomite mottling in Middle Triassic ramp carbonates (Southern Hungary). / Török, A.

In: Sedimentary Geology, Vol. 131, No. 3-4, 15.03.2000, p. 131-145.

Research output: Contribution to journalArticle

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N2 - The Middle Triassic carbonates of the Villany Mountains were deposited on a homoclinal carbonate ramp. Many of the carbonates from the 700 m-thick sequence show partial or complete dolomitization. The present paper describes dolomites that occur in a limestone unit as irregular mottles and as pore- and fracture-filling cements. Replacement-type scattered dolomite rhombs in the mottles having inclusion-rich, very dull luminescent cores and limpid non-luminescent outer zones represent the initial phase of dolomitization. The isotopic composition of these dolomites (δ13C = +2.30‰ VPDB, δ18O = -3.60‰ VPDB) is similar to that of the calcitic micrite (δ13C = +2.6‰ VPDB, δ18O = -4.00‰ VPDB) indicating that no external fluids were introduced during dolomite formation. The elevated Sr content of the micrites implies that sediment was originally aragonite or high-Mg calcite. Dolomitization took place in the burial realm from a 'marine' pore-fluid in a partly closed system. Later fracture-related saddle dolomite reflects elevated formation temperatures and increasing burial. Five calcites were identified. Multiple generations of calcite-filled fractures were formed during burial diagenesis generally having dull or no luminescence (δ13C = +1.80‰ VPDB, δ18O = -6.40‰ VPDB). The latest phase calcites are related to karst formation, having a very negative isotopic composition (δ13C = -5.0 to -7.2‰ VPDB and δ18O ~ -7.4‰ VPDB). The karst-related processes include dissolution, calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite. The timing of dolomitization is uncertain, but the first phase took place in a partly closed system prior to stylolite formation. Late-stage saddle dolomites were precipitated during maximum burial in the Cretaceous. The dissolution of dolomites and karst-related calcite replacement was not earlier than Late Cretaceous. (C) 2000 Elsevier Science B.V. All rights reserved.

AB - The Middle Triassic carbonates of the Villany Mountains were deposited on a homoclinal carbonate ramp. Many of the carbonates from the 700 m-thick sequence show partial or complete dolomitization. The present paper describes dolomites that occur in a limestone unit as irregular mottles and as pore- and fracture-filling cements. Replacement-type scattered dolomite rhombs in the mottles having inclusion-rich, very dull luminescent cores and limpid non-luminescent outer zones represent the initial phase of dolomitization. The isotopic composition of these dolomites (δ13C = +2.30‰ VPDB, δ18O = -3.60‰ VPDB) is similar to that of the calcitic micrite (δ13C = +2.6‰ VPDB, δ18O = -4.00‰ VPDB) indicating that no external fluids were introduced during dolomite formation. The elevated Sr content of the micrites implies that sediment was originally aragonite or high-Mg calcite. Dolomitization took place in the burial realm from a 'marine' pore-fluid in a partly closed system. Later fracture-related saddle dolomite reflects elevated formation temperatures and increasing burial. Five calcites were identified. Multiple generations of calcite-filled fractures were formed during burial diagenesis generally having dull or no luminescence (δ13C = +1.80‰ VPDB, δ18O = -6.40‰ VPDB). The latest phase calcites are related to karst formation, having a very negative isotopic composition (δ13C = -5.0 to -7.2‰ VPDB and δ18O ~ -7.4‰ VPDB). The karst-related processes include dissolution, calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite. The timing of dolomitization is uncertain, but the first phase took place in a partly closed system prior to stylolite formation. Late-stage saddle dolomites were precipitated during maximum burial in the Cretaceous. The dissolution of dolomites and karst-related calcite replacement was not earlier than Late Cretaceous. (C) 2000 Elsevier Science B.V. All rights reserved.

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