In a subsurface area of 140 sq km along the La Salle anticline, southeastern Illinois, a 5 to 12-m sequence of shallow-marine, subtidal carbonate mudstone and wackestone in the upper Ste. Genevieve Limestone (Meramecian) has been altered in patches to porous (25 to 40%), oil-productive, microcrystalline dolomite. Information from about 150 wells (46 cored) shows the dolomite to occur in lens-shaped bodies up to 12 m (38 ft) thick, 0.5 to 2.5 km across, by 1 to >5 km long, in places probably interconnected, and markedly oriented east-west to northeast-southwest. The dolomite interfingers with carbonate mudstone/wackestone, which it closely resembles in primary and bioturbate structures, preserved and inferred megafossils, and clay and detrital silt content (< 7%). Mu dy calcareous sediments were the precursors. There is no direct evidence of either deposition or dolomitization under supratidal conditions.

Typical dolomite consists of sharply terminated, clear, 5 to 20-µm rhombs. In cathodoluminescence these have roundish, dully luminescing cores with successive dark and bright, rhomb-shaped overgrowth zones which clearly indicate fabric evolution and progressive reduction of porosity. Bulk isotopic compositions are relatively heavy (mean ^dgrC¹³ and ^dgrO¹⁸ are +2.3 and +2.4 parts per thousand versus PDB); Sr⁺⁺ is 100 to 350 ppm. A later generation of coarse, iron-rich dolomite has similar ^dgrC¹³ but much lower ^dgrO¹⁸ (mean -6.0 parts per thousand) and locally filled molds, fractures, and veins.

Dolomite and carbonate mudstone/wackestone underlie a swarm of elongate lenses of ooid grainstone and sandy ooid-pellet packstone/grainstone that have similar orientations and areal dimensions. Many dolomite lenses directly underlie carbonate-sand bodies. Muddy parts of carbonate sands are commonly altered to microcrystalline dolomite with similar fabrics, isotopic and Sr⁺⁺ compositions, and cathodoluminescence.

These observations suggest that dolomitization may have begun in marine pore water, but continued in a hydrologic system of partly meteoric origin. Dolomitization

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began early in the burial history, after some compaction, ending before stylolites could develop. Hydrology may have been influenced by recharge areas in the northeast, but groundwater flow was mainly through carbonate-sand bodies; these bodies mainly controlled dolomitization patterns. Later dolomite was precipitated from warmer waters expressed through many of the same conduits. Ste. Genevieve dolomites may be early-stage, high-porosity analogs of many sequences of coarser saccharoidal dolomites.

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