ITEM METADATA RECORD
Title: Spheroidal dolomites in a Visean karst system - Bacterial induced origin?
Authors: Nielsen, P
Swennen, Rudy
Dickson, JAD
Fallick, AE
Keppens, E #
Issue Date: Feb-1997
Publisher: Blackwell science ltd
Series Title: Sedimentology vol:44 issue:1 pages:177-195
Abstract: 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 S-34 (delta(34)S = - 22.4 to - 25.5 parts per thousand CDT), testifies to a period of bacterial sulphate reduction. The isotopic composition of the spheroidal dolomites (delta(13)C = - 2.4 to - 3.2 parts per thousand PDB and delta(18)O = - 3.8 to - 3.4 parts per thousand PDB) suggest a contribution from oxidized organic carbon produced during bacterial sulphate reduction. Sulphate reduction may also result in a concomitant O-18 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 O-18 depletion of the spheroidal dolomite crystals (delta(18)O = - 3.8 to - 3.4 parts per thousand PDB) 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 delta(18)O values in the range of 0 to + 5.3 parts per thousand PDB, which reflect precipitation from normal to slightly evaporated contemporaneous seawater.
ISSN: 0037-0746
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Division of Geology
# (joint) last author

Files in This Item:

There are no files associated with this item.

Request a copy

 




All items in Lirias are protected by copyright, with all rights reserved.

© Web of science