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Silurian rocks in northern Indiana early inspired
understanding of fossil reefs, ideas on genesis of the Cincinnati arch, and demonstration of the great stratigraphic-paleontologic principles. We have learned only recently, however, that the formations, thickening northward and all exposed, are, in ascending order, the Brassfield limestone, Osgood-Laurel section (so-called), Waldron shale, Louisville limestone, Missisinewa shale, Liston Creek limestone, unnamed rocks, and equivalents of lower Salina rocks of Michigan. Rocks below the Waldron (Llandovery in age) are coextensive with most of the undifferentiated Niagaran and lower rocks in southernmost Michigan and with the Cedarville dolomite and lower rocks in western Ohio. Waldron and higher Niagaran rocks (Wenlock and Ludlow) mostly terminate northward in a reef bank crossing the n rthern quarter of Indiana. The type Huntington dolomite is a reefy facies of Mississinewa and higher Niagaran rocks, although "Huntington" has been applied throughout the Niagaran. The Salina equivalents partly abut against the bank and partly overlie and extend in two tongues south of the bank, partly complement in thickness the reef-bearing formations, and lie from north to south on lower to upper Niagaran rocks.
We postulate southward Silurian expansion of the Michigan basin prototype and consider that lower and middle Silurian sediments were deposited in the subsiding basin in southernmost Michigan and northernmost Indiana and on a relatively stable shallow-water shelf at the south. During late Niagaran and then early Cayugan time, the basin margin became better defined by southward shelf-edge sharpening that resulted from continued relatively greater subsidence northward and extensive fringing bank growth. Bank growth resulted in near-restriction of sea-to-basin circulation to two inlets corresponding to present structural sags in Cass and Jasper Counties. Post-Silurian events helping to define the present basin and the flanking Cincinnati arch include interruption and renewal of basin subs dence; probable inlet closing and deposition of lower-middle Devonian evaporite-bearing sediments, whose southern boundary marks approximately the southern margin of the Michigan basin; and differential subsidence of the Illinois basin, which imparted the southwesterly dip to part of the Silurian shelf area.
We conclude, not entirely facetiously, with the question: Is part of the Cayugan in Michigan and Indiana Niagaran in age, or is part of the Niagaran Cayugan?
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