Progressive Recrystallization and Stabilization of Early-Stage Dolomite: Lower Ordovician Ellenburger Group, West Texas

  1. Bruce Purser,
  2. Maurice Tucker and
  3. Donald Zenger
  1. J. A. Kupecz1 and
  2. L. S. Land2

Published Online: 14 APR 2009

DOI: 10.1002/9781444304077.ch15

Dolomites: A Volume in Honour of Dolomieu

Dolomites: A Volume in Honour of Dolomieu

How to Cite

Kupecz, J. A. and Land, L. S. (1994) Progressive Recrystallization and Stabilization of Early-Stage Dolomite: Lower Ordovician Ellenburger Group, West Texas, 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.ch15

Author Information

  1. 1

    ARCO Exploration and Production Technology, 2300 W. Plano Parkway, Plano, Texas 75075, USA

  2. 2

    Department of Geological Sciences, University of Texas, Austin, Texas 78713-7909, 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:

  • progressive recrystallization and stabilization of early-stage dolomite - Lower Ordovician Ellenburger Group, west Texas;
  • stratigraphy, diagenesis and regional tectonics of Ellenburger;
  • depositional environments and facies;
  • cyclicity, karstification and associated diagenesis;
  • dolomites from mud-supported facies;
  • regional tectonics - Pennsylvanian to earliest Permian Ouachita Orogeny northward thrusting and regional metamorphism;
  • cathodoluminescence photomicrograph of Breccia clast of bioturbated facies;
  • textural and geochemical modification of dolomite;
  • diagenetic fluids and relative timing of superimposed recrystallization events

Summary

The Lower Ordovician Ellenburger Group of west Texas is composed predominantly of dolomitized mud-supported (bioturbated, mm-laminated, and cryptalgal-laminated) facies. These facies constitute approximately 90% of the Ellenburger section. Dolomites from these facies are very fine to coarse-crystalline, with an increase in non-planar crystal boundaries corresponding to larger crystal sizes. Cathodoluminescence indicates several generations of dolomite. Dark-brown-luminescent dolomite rhombs (E1), are overgrown by non-luminescent dolomite (E2) and dark-brown-luminescent dolomite (E3). These early generations are cemented and replaced by orange-luminescent dolomite (L2). The abundance of E2 and E3 dolomite overgrowths, and the degree of replacement by dolomite-L2, increases with crystal size and the number of non-planar crystal boundaries.

Dolomites from the mud-supported facies are characterized by low concentrations of iron (180–7160 ppm; generally <2000 ppm), strontium (20–210 ppm) and manganese (20–420 ppm), by a wide range in δ18O values (–2.4 to –8.8‰ PDB), variable stoichiometry (49.53–56.00 mole % CaCO3), and variable 87Sr/86Sr ratios (0.70812–0.71238). Although most geochemical parameters are variable, there is a covariance between increasing textural modification, increasing stoichiometry, decreasing δ18O, increasing 87Sr/86Sr and decreasing trace element concentrations.

Dolomites of the mud-supported facies probably originated as sabkha-like sequences. However, the covariance between textural changes and geochemistry suggests significant postdepositional modification. Based on petrographic and geochemical data, we conclude that two major diagenetic events have been superimposed following initial dolomite formation: meteoric modification, synchronous with early Middle Ordovician regional exposure and karstification; and late modification via basinal-derived Pennsylvanian pore fluids expelled upon thrusting during the Ouachita Orogeny. These major diagenetic events have resulted in the progressive recrystallization and stabilization of early Ellenburger dolomites. Evidence of extensive modification suggests that it is critical that the textural and geochemical evolution be considered when interpreting the origin of the Ellenburger and other dolomites on the basis of their present geochemical signatures.