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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 62 (1992)No. 2. (March), Pages 167-189

Characteristics, Distribution and Origin of Porosity in Shelf Dolostones: Burlington-Keokuk Formation (Mississippian), U.S. Mid-Continent

Philip W. Choquette, Ann Cox, William J. Meyers

ABSTRACT

Finely crystalline, massive, porous dolomites comprise 10-70% of the 50-70 m thick Burlington-Keokuk Formation (Osagean) in much of a 120,000-km2 outcrop belt crossing the Mississippian paleoshelf in parts of Iowa, Illinois and Missouri. The dolomites are products of regionally widespread, selective dolomitization of lime-mud-rich subtidal skeletal mudstone to skeletal packstone. Dolomitization was most intense and lime-mud facies are most abundant in the northwestern, inner part of the paleoshelf (SE Iowa) where dolomites comprise 50-70% of the formation.

Pore systems in Burlington-Keokuk dolomites originated during the first of two main regional dolomitization events, a Middle Mississippian event responsible for the conversion of shallow-buried (30-100 m), partly lithified lime-mud-rich sediments to dolomite I. Resulting dolomites are characterized by 1) non-mimetic and sucrosic, planar-e and -s fabrics composed of 30-200 µm rhombs that have intricate concentric CL zoning, 2) abundant intercrystal and moldic pores, and 3) dissolved rather than replaced CaCO3 grains. Study with SEM shows that intercrystal and micro-intercrystal pores resulted from progressive dissolution of micrite during dolomite-I dolomitization. The second major regional dolomitization event, replacement of dolomite I by uniformly dull-red luminesci g dolomite II, had essentially no effect on the dolomite-I pore systems.

A widespread, volumetrically significant (10-27%), terminal CL zone of dolomite I (TD-I zone) partly occludes intercrystal pores and lines most grain molds. The TD-I zone constrains the timing of all such porosity to the later stages of dolomite-I dolomitization rather than a later event and implies a flow system of regional extent. The source of carbonate ions for this pore-occluding dolomite appears to have been CaCO3 grains dissolved at precipitation sites, supplemented from external sources.

Three pervasively dolomitized microfacies comprise up to 70% of the formation and contain > 80% of the formation's pore volume; partly to undolomitized skeletal limestones, although volumetrically important, contain < 20% of the total pore volume. In the most widespread dolomite microfacies, sucrosic lime-mud dolomite, intercrystal pores make up about 70% of the > 5-µm pore-size fraction, skeletal-moldic pores 17%, grain-sized vugs 11%, and intracrystal pores 2%. Micropores (< 5 µm) are chiefly micro-intercrystal and micro-moldic types and comprise 19% of total porosity on average with wide variations. They account for most measured differences between effective (core-analysis) and total (thin-section) porosity.


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