Groundwater Dolocretes from the Upper Triassic of the Paris Basin, France: A Case Study of an Arid, Continental Diagenetic Facies

  1. Stuart D. Burley and
  2. Richard H. Worden
  1. Christoph Spötl1 and
  2. V. P. Wright2,†

Published Online: 30 MAR 2009

DOI: 10.1002/9781444304459.ch17

Sandstone Diagenesis: Recent and Ancient

Sandstone Diagenesis: Recent and Ancient

How to Cite

Spötl, C. and Wright, V. P. (2003) Groundwater Dolocretes from the Upper Triassic of the Paris Basin, France: A Case Study of an Arid, Continental Diagenetic Facies, in Sandstone Diagenesis: Recent and Ancient (eds S. D. Burley and R. H. Worden), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304459.ch17

Author Information

  1. 1

    Geologisches Institut, Baltzerstrasse 1, CH-3012, Berne, Switzerland

  2. 2

    Postgraduate Research Institute for Sedimentology, The University, PO Box 227, Whiteknights, Reading RG6 2AB, UK

  1. US Geological Survey, Office of Energy and Marine Geology, National Center, MS. 915, 12201 Sunrise Valley Drive, Reston, VA 22092, USA

Publication History

  1. Published Online: 30 MAR 2009
  2. Published Print: 21 MAR 2003

ISBN Information

Print ISBN: 9781405108973

Online ISBN: 9781444304459

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Keywords:

  • petrography of quartz-cemented Penrith sandstone;
  • unicrystalline grains;
  • polycrystalline grains;
  • electron microscopy;
  • experimental considerations of crystal growth

Summary

Thick dolomite-cemented horizons (dolocretes) occur within a fluvial sandstone–mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and(b)massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.

The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar-filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar–septal fabrics were observed in the thin, nodular dolomite beds.

The dolomite is near stoichiometric, well ordered and non-ferroan. δ13C values range from −7.7 to −0.4‰ PDB and δ18O values range from −5.1 to +1.8‰ PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0.7101 to 0.7126 and are invariably higher than the contemporary Triassic seawater.

A vadose–pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.

Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non-marine, continental origin for the groundwaters. The poorly ordered and non-stoichiometric, protodolomite probably underwent stabilization upon further burial resulting in a near-stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.