The Importance of Dolomite Cement in the Evolution of Pore Systems in Sucrosic Dolostones

Philip W. Choquette¹ and Eric E. Hiatt^2
1 University of Colorado, Littleton, CO
² University of Wisconsin, Oshkosh, Oshkosh, WI

Dolomite cement is a major, widespread, and under-appreciated, constituent of sucrosic and microsucrosic dolostones. This cement is limpid, with sharp terminations, planar faces (non-saddle forms), and distinct zonation in CL. It forms syntaxial overgrowths on replacive crystals, often lining molds and small vugs, but more typically occurs as sectoral overgrowths pervading entire intercrystal-dominated pore systems in these dolostones. Petrographic, stratigraphic and isotopic evidence suggests that most of these cements form at <0.5 km burial and relatively low temperatures; few are deep-burial or thermobaric cements.

Quantitative analysis shows that dolomite cements comprise 25-50% by volume of peritidal dolostones in samples from the upper Edwards Formation (mid-Cretaceous, outcrop, central Texas); 30-60% in dolostones of the Hawthorn Group and Avon Park Formation (Miocene & Eocene, south Florida); 45-65% of microsucrosic shallow-subtidal dolostones of the Ste. Genevieve Limestone (Mississippian, Illinois Basin); 22-42% of the subtidal dolostones in the Burlington-Keokuk Formation (Mississippian outcrop belt, Iowa); and 25-86% of medium-to-coarsely sucrosic dolostones in the Gambier Limestone (Oligo-Miocene, Gambier Range, south Australia).

Such dolostones exhibit a consistent course of textural maturation that involves lime-mud-replacive dolomitization and carbonate-grain dissolution, followed by increasing induration, coarsening and pore-system occlusion all due largely to cementation by dolomite. Cementation, driven by throughput of large volumes of Mg-rich waters, if unimpeded by hydrocarbons, converts fine, friable, high-porosity replacement dolostones (dolomitization) to substantially less porous or permeable rocks composed of both replacive and cementing dolomite. Recognizing the role and extent of cementation is critical to interpreting these dolostones.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005