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.