Aragonite Relic Preservation in Jurassic Calcitereplaced Bivalves

  1. Maurice E. Tucker3 and
  2. Robin G. C. Bathurst4
  1. Philip A. Sandberg1 and
  2. John D. Hudson2

Published Online: 29 APR 2009

DOI: 10.1002/9781444304510.ch23

Carbonate Diagenesis

Carbonate Diagenesis

How to Cite

Sandberg, P. A. and Hudson, J. D. (1990) Aragonite Relic Preservation in Jurassic Calcitereplaced Bivalves, in Carbonate Diagenesis (eds M. E. Tucker and R. G. C. Bathurst), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304510.ch23

Editor Information

  1. 3

    Department of Geological Sciences, University of Durham, UK

  2. 4

    Derwen Deg Fawr, Llanfair DC, Ruthin, Clwyd, North Wales, UK

Author Information

  1. 1

    Department of Geology, University of Illinois, 245 NHB, 1301 W. Green Street, Urbana, IL 61801, USA

  2. 2

    Department of Geology, University of Leicester, Leicester LEI 7RH, UK

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 21 AUG 1990

ISBN Information

Print ISBN: 9780632029389

Online ISBN: 9781444304510

SEARCH

Keywords:

  • aragonite relic preservation in Jurassic calcite-replaced bivalves;
  • coarse neomorphic calcite, replacing aragonite bivalve Neomiodon;
  • TEM micrographs, revealing criss-cross laths;
  • aragonite relics, occurring in calcite-replaced Mesozoic aragonite shells;
  • aragonite relics, occur in calcite-replaced Mesozoic aragonite shells

Summary

Shells of the aragonite bivalve Neomiodon (Great Estuarine Group, Jurassic, Scotland) replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure. The presence of these relics in such old altered shells, as well as the high Sr content of the replacement calcite, indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events, but rather a rapid, one-step process. Aragonite relics, once encased in neomorphic spar, will survive as unequivocal evidence of original aragonite mineralogy, barring total remobilization of the enclosing stable calcite, a generally unlikely event. The retention of this residual aragonite and high-Sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration (‘recrystallization’) invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones. Retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or, as in the case of our Neomiodon material, deeper burial environments. Pseudopleochroism of the replaced Neomiodon shells appears to be due to organic, largely graphitic, relics, not to the aragonite relics.