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Spheroidal dolomites in a Visean karst system – bacterial Induced origin?
Author(s) -
NIELSEN P.,
SWENNEN R.,
DICKSON J. A. D.,
FALLICK A. E.,
KEPPENS E.
Publication year - 1997
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1997.tb00431.x
Subject(s) - dolomite , pyrite , geology , karst , calcite , geochemistry , pseudomorph , mineralogy , carbonate , mineral , sedimentary rock , paleontology , chemistry , quartz , organic chemistry
Spheroidal dolomite crystals occur in the karstified top of a Dinantian dolomite sequence in eastern Belgium. The spheroidal dolomite crystals are best developed at the base of the karst system. The dolomite crystals are characterized by a spherulitic or dumb‐bell inclusion pattern, and are overgrown by dolomite cements with a rhombohedral outline. They are considered to be bacterially related precipitates based on, (1) textural similarities with documented bacteriogenic precipitates, (2) the presence of ‘bacterial’microspheres and framboidal pyrite embedded within the dolomite, and (3) their general geological setting. The geochemical characteristics of the dolomites and associated minerals support a bacterial origin. The ubiquity of framboidal pyrite, depleted in 34 S (δ 34 S=— 22.4 to — 25.5%oCDT), testifies to a period of bacterial sulphate reduction. The isotopic composition of the spheroidal dolomites (δ 13 C=— 2.4 to ‐ 3.2%oPDB and δ 18 O=— 3.8 to ‐ 3.4%oPDB) suggest a contribution from oxidized organic carbon produced during bacterial sulphate reduction. Sulphate reduction may also result in a concomitant 18 O depletion if the system is nearly closed. It is however, evident from the sulphur isotopic composition of associated framboidal pyrite that the system was fairly open. The 18 O depletion of the spheroidal dolomite crystals (δ 18 O=— 3.8 to — 3.4%oPDB) and their occurrence adjacent to, and within karst cavities suggests a mixing zone origin, with a significant proportion of freshwater in it. The rhombohedral cement‐overgrowths have calculated δ 18 O values in the range of 0 to +5.3%oPDB, which reflect precipitation from normal to slightly evaporated contemporaneous seawater.