PARAGENESIS OF THE UPPER ORDOVICIAN SALUDA FORMATION: INSIGHTS FROM PETROGRAPHIC ANALYSES

The Saluda Fm. of southeast, Indiana, is Upper Ordovician (Katian) in age and regionally correlated to the Whitewater Fm. in Ohio and the Preachesville Fm. in Kentucky. At Madison, Indiana, the variably dolomitized limestones and dolostones of the Saluda Fm. (North Madison and Hanging Rock Members) are interpreted to reflect deposition in a shallow water marine setting that experienced occasional subaerial exposure based on facies, water depth proxies, and sedimentary structures. Despite numerous field studies conducted at the Madison, Indiana, outcrop location (38°46'45.4"N, 85°21'57.8"W), the timing, mechanism, and fluids of dolomitization remain unclear. Two competing models have been proposed, including early, syndepositional dolomitization in a hypersaline lagoon and later dolomitization via mixed meteoric-marine fluids. The objective of the current investigation is to constrain the timing and conditions of dolomitization and other diagenetic processes in the Saluda Fm using high-resolution petrographic analyses.

A total of 32 thin sections were analyzed from the North Madison (4.2 m) and Hanging Rock (6.4 m) Members. Petrographic analyses show that there are three principal carbonate phases, including (i) microcrystalline calcite, (ii) coarse (30- 120 µm) planar-e fabric destructive dolomite, and (iii) blocky calcite cement. The microcrystalline calcite is observed as fossil grains and matrix. The planar-e dolomite occurs within the muddy matrix and bryozoan fenestrae. The blocky calcite cement is observed to encroach upon dolomite and the crystal boundaries between calcite and dolomite are irregular, suggesting that the calcite postdates the dolomite. Later stage silica, which replaces brachiopods, and pore-filling gypsum cements are also observed within the wackestone-packstone facies of the North Madison Member. Collectively, the petrographic observations are consistent with a paragenetic sequence whereby dolomite predates all other diagenetic phases, thus suggesting early, perhaps syndepositional dolomitization.