SELECTIVE DOLOMITIZATION ACROSS FACIES OF THE EARLY SILURIAN BRASSFIELD FORMATION (SOUTHWESTERN OHIO, USA)

The Oakes Quarry Park near Fairborn, Ohio exposes Silurian limestones and dolostones of the Brassfield Formation. The depositional environment has been described as a sequence of reef-associated facies. A burrowed dolostone (0.5m thick) comprises the basal unit of the Brassfield at this locality. Five meters of crinoidal grainstone overlies the dolostone. A transition occurs with the appearance of a distinct mollusk-rich marker bed, bounded above and below by shales. Two meters of fossiliferous grainstones and packstones containing patch reefs and reef-flank deposits are present at the top of the section.

The Brassfield Formation in the quarry has undergone varying degrees of dolomitization. Petrographic examination reveals that the dolomitization may be facies specific. Extent and selectivity of dolomitization, texture, and clarity vary by facies. Dolomite occurs in greater quantities in the lower facies in the quarry (below the marker bed). The basal burrowed dolostone contains 63-87% finely crystalline dolomite. The dolomite in the crinoidal grainstones is coarsely crystalline and varies more widely in quantity, ranging from 10-68%. Subhedral, cloudy crystals of dolomite are found in both of these lower facies. In the mollusk-rich bed, dolomite commonly replaces micrite within gastropods and cephalopods although bryozoans and crinoids are also sometimes partially replaced. Fine to medium crystalline dolomite occurs as clear, zoned euhedral rhombs often with cloudy centers. The overlying fossiliferous grainstones and packstones contain the least amount of dolomite (ranging from 0-17%). The dolomite present is commonly finely crystalline and euhedral, with clear to cloudy crystals that often exhibit zonation.

The variation in dolomitization among the facies present in the quarry suggests multiple dolomitization events or multiple sources of dolomitization. Microbial fabrics have been described at this locality and some of the dolomite (inside the mollusk shells) may be of microbial origin.