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The Jeffersonville Limestone (lower Middle Devonian) occupies an area south and west of the Kankakee and Cincinnati arches that is about half the size of Indiana. Its spatial and lithologic characters are described conveniently in three facies units: (1) a southern Indiana facies, 0 to more than 200 ft thick, comprising the highly fossiliferous, normal marine biozones, extending from outcrop at the Falls of the Ohio southwestward to far southwest Indiana and northward to cover about 20 counties; (2) a west-central Indiana facies, the Geneva Dolomite Member (new rank), 0 to more than 60 ft thick, consisting of brown, vuggy, pelletoidal, and bioclastic dolomite and mostly restricted in Indiana to 20 counties in the central and central-western parts of the state; and (3) the facies newly named here as the Vernon Fork Member, 0 to more than 80 ft thick, consisting variably of cyclically deposited fine-grained thin-bedded dolomite, brown granular vuggy dolomite, and nearly lithographic laminated limestone and dolomite. This facies extends beyond the Geneva facies both north and south. Facies 2 and 3 together, 3 being superjacent to 2, are northern lateral equivalents of facies 1. Fine-grained quartz sand in a carbonate matrix is present at various levels, including the concentration of sand in the somewhat restricted Dutch Creek Sandstone Member in the basal Jeffersonville. The Pendleton Sandstone Bed (new rank) is a local bed in the base of the Vernon Fork Member.
In southwest Indiana the Jeffersonville lies both on Lower Devonian rocks and a basin facies of Upper Silurian rocks. In the remainder of the study area, the Jeffersonville lies on a large basin-fringing reef bank of Silurian age that has associated pinnaclelike highs both on the bank and basinward of the bank's main expression (called the Terre Haute bank, a new name); also, the formation lies on back-reef and(or) on shelf-deposited interreef Silurian rocks that surround probably hundreds of discrete reefs. The Jeffersonville facies are spatially and genetically related, in both thickness and lithologic expression, to the underlying differentiations among the Silurian rocks.
The southern Jeffersonville facies represents essentially continuous normal-marine deposition in deeper water that in far southwest Indiana was continuous from the time of Clear Creek deposition (Early Devonian), but some fluctuation of this depositional regimen is recorded in tongues of the Amphipora and Paraspirifer acuminatus Zones among rocks of the northern facies. The Geneva Member represents shallow-water, even shoaling, normal-marine deposition over the Silurian reef bank and reef platform. The Vernon Fork represents even shallower, at least more restricted environments that led to carbonate-flat, sabkha, and lagoonal-shelf deposition. During the time of Vernon Fork deposition, central Indiana had mostly hypersaline marine connection with the Michigan basin. The southern Jeffe sonville area was maintained under normal marine conditions and had continuous connection with the Appalachian basin through Kentucky.
The Devonian drape structure (over the Silurian surface) is potentially the result of complex interaction of several factors. Among them are depositional and(or) erosional relief on the Silurian reef bank and pinnaclelike reefs, differential compaction of Silurian reef and interreef rocks (including such differentiation brought about by dolomitization of these rocks), settling of reef masses en masse into their substrate, and deposition of Devonian rocks in differing thicknesses over reef highs and interreef lows. Dolomitization and porosity development in the Geneva facies of the Jeffersonville, in contrast to nondolomitization of most of the southern facies, resulted from hypersaline Vernon Fork waters acting upon porous bioclastic sediments that had been deposited over the shallow ilurian reef platform.
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