The Brassfield Formation (Lower Silurian), which crops out in southwestern Ohio, through parts of Indiana and Kentucky, to western Tennessee, is a transgressive sequence consisting of interbedded shales, limestones, and dolostones. Shales dominate the eastern edge of the outcrop, whereas carbonate sediments are characteristic of the western and southern exposures. A detailed study of outcrops in southwestern Ohio shows that two separate dolomitizing processes have altered the rocks. Elsewhere, the Brassfield Formation appears to be dolomitized by a single process. In southwestern Ohio, initial dolomitization was restricted to the basal Belfast Member and probably occurred penecontemporaneously in a supratidal environment similar to a modern sabkha. Dolomite in the Belfast member occurs as small (10 to 40 µm), cloudy, anhedral crystals. Thinly laminated sediments with a few gypsym casts are characteristic of this unit.

Regional dolomitization was a later diagenetic event related to the formation of a freshwater and seawater mixing zone beneath a landmass created by upwarping of the Cincinnati arch and Nashville and Ozark domes. Intensity of dolomitization in the outcrop belt is controlled by the proximity of the original carbonate sediments to the source of dolomitizing fluids in the mixing zone. The Brassfield is undolomitized away from these structural highs. Dolomite from this portion of the formation is typically large (> 10µm), limpid, euhedral rhombs replacing bioclasts and sparry calcite cement.

This "Dorag" type of dolomitization suggests the presence of landmasses in the areas of the Cincinnati arch and the Ozark and Nashville domes during the time of dolomitization of the Brassfield Formation.

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