Petrographic, Geochemical and Structural Constraints on the Timing and Distribution of Postlithification Dolomite in the Rhaetian Portoro (‘Calcare Nero’) of the Portovenere Area, La Spezia, Italy

  1. Bruce Purser,
  2. Maurice Tucker and
  3. Donald Zenger
  1. J. K. Miller and
  2. R. L. Folk

Published Online: 14 APR 2009

DOI: 10.1002/9781444304077.ch12

Dolomites: A Volume in Honour of Dolomieu

Dolomites: A Volume in Honour of Dolomieu

How to Cite

Miller, J. K. and Folk, R. L. (1994) Petrographic, Geochemical and Structural Constraints on the Timing and Distribution of Postlithification Dolomite in the Rhaetian Portoro (‘Calcare Nero’) of the Portovenere Area, La Spezia, Italy, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch12

Author Information

  1. University of Texas at Austin, Austin, Texas 78713, USA

  1. Exxon Company USA, Houston, Texas 77210-4698, USA

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 25 MAY 1994

ISBN Information

Print ISBN: 9780632037872

Online ISBN: 9781444304077

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Keywords:

  • petrographic, geochemical and structural constraints on timing and distribution of postlithification dolomite in Rhaetian Portoro;
  • postlithification dolomites;
  • petrography of postlithification dolomites;
  • fracture-controlled dolomitization;
  • fluids migrating along bedding surface - slight hiatus in sedimentation;
  • stylolite-controlled dolomitization;
  • Breccia-controlled dolomitization;
  • postlithification dolomitization in Triassic Portoro limestone and bedding

Summary

The Uppermost Triassic Portoro Limestone of Liguria, Italy, shows a striking structural control of late dolomitization. Large white fronts and irregular pods consisting of coarse, commonly anhedral, dolomite crystals replace jet-black limestone. Dolomitizing fluids were fed along faults, dolomitizing laterally until they contacted joints, fractures or stylolites. The fluids then either migrated elsewhere if the joint or stylolite was open, or were stopped by vein-filling cements of less permeable sparry calcite.

The Portoro underwent three major episodes of dolomitization. The first occurred soon after deposition and preferentially dolomitized certain beds and fabrics. This was followed by two fault- and fracture-related postlithification dolomitization events. The genesis of these two later, petrographically indistinguishable, dolomites was determined using field relations and Sr isotopes. The first of the later events occurred after complete lithification of the rock but prior to a major Oligocene–Miocene thrusting event. The dolomites associated with it can be identified in the field by the shearing of the more ductile microsparry calcite around the dolomite masses, and in the laboratory by their 87Sr/86Sr ratio of 0.7078. This ratio falls within the range of values for late Triassic seawater. Identification of the dolomites associated with the second postlithification event is more difficult. These dolomites can be distinguished only where field evidence shows that the dolomite is associated with a Plio-Pleistocene fault or replaces sheared limestone. These latest fault-related dolomites give 87Sr/86Sr ratios of 0.7080–0.7090, which we interpret as representing various degrees of buffering of relatively recent seawater by Triassic host rock. 87Sr/86Sr ratios thus are useful in determining the timing and genesis of diagenetic events. Radiogenic Sr decreases laterally away from the Plio-Pleistocene faults until it reaches a value equal to that of the earlier dolomites, suggesting that dolomitizing fluids were buffered after only a few metres of transport away from the faults.