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In a 140 km2 subsurface area along the La Salle anticline, southeastern Illinois, a 5–12 m sequence of shaUow-marine, subtidal lime mudstone and wackestone in the upper Ste. Genevieve Limestone (Meramecian) has been altered in pa?chy fashion to porous (25–40%), 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, 0.5–2.5 km (0.3–1.5 mi) across by 1–5 km (0.6–3 mi) long, in places probably interconnected, and strongly oriented E-W to NE-SW. The dolomite interfingers with lime mudstone/wackestone, which it closely resembles in primary and bioturbate structures, preserved and inferred megafossils, and clay and detrital silt content (<7%). Muddy lime sediments were the precursors. There is no evidence of either deposition or dolomitization under supratidal conditions.

Typical dolomite consists of sharply terminated, clear, 5–20 μm rhombs, each composed in cathodoluminescence of a very dully luminescing core, a younger bright overgrowth, and in some sequences a still younger dull overgrowth. Bulk isotopie compositions are relatively heavy (mean δ13C and δ18O + 2.4 and +2.0% VS PDB); Sr+2 is substantially lower (mean 166 ppm) and Na+ somewhat lower (mean 810 ppm) than those of Holocene supratidal or penesaline-lagoon dolomite. A later generation of coarse, equant-blocky, iron-rich dolomite cement has similar δ13C but much lower δ18O (mean −6.0‰) and locally fills molds, fractures, and veins (Choquette, 1971).

Dolomite and lime mudstone/wackestone underlie a swarm of elongate lenses of ooid grainstone and sandy ooid-pellet packstone/grainstone that have orientations and areal dimensions like those of the dolomite lenses. Oohte bodies have in places small amounts of freshwater-vadose and beachrock cements, suggesting short-lived exposure, but show few or no signs of dissolution. subaerial exposure seems to have been brief but sufficient to create freshwater lenses. Many dolomite lenses he directly under carbonate sand bodies. Muddy parts of carbonate sands are commonly altered to microcrystalline dolomite with similar fabrics, isotopie compositions, and cathodoluminescence.

These considerations suggest that dolomitization took place in a hydrologie system of mixed meteoric and marine origin. Dolomitization began early in the burial history, perhaps in mixing zones of local freshwater lenses after some compaction but while porosities were high. subsequently, the hydrology was influenced by recharge areas to the northeast and groundwater flow was mainly through carbonate sand bodies, which strongly influenced dolomitization patterns.

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