Stratigraphy of the Silurian Rocks of Northern Indiana

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Indiana Geological Survey
The Tenth Indiana Geologic Field Conference treats the stratigraphy of the Silurian rocks that lie between the Cincinnatian rocks and the bedrock surface in the rectangular area whose corners are defined by Cass, Allen, Randolph, and Hancock Counties, Ind. The area forms part of the Tipton Till Plain and is mantled by Wisconsin tills of the Tazewell and Cary Substages. Thus, well logs and cores are essential to an interpretation of the bedrock stratigraphy, but the eight exposures in the itinerary nearly span the Niagaran and Cayugan? Series as known in Indiana. Lowermost Silurian rocks, not exposed in the area, are assigned to the Brassfield Limestone. Three principal post-Brassfield pre-Mississinewa stratigraphic units are here called “lower Niagaran rocks” and are thought to be correlatives of the Osgood Formation and Laurel Limestone, the Waldron Shale, and the Louisville Limestone of southern Indiana. These rocks are present at the bedrock surface in the southeastern and eastern parts of the conference area. Progressively younger rocks are found westward and northward and are assigned to the Mississinewa Shale, the Liston Creek Limestone, the Huntington Dolomite, and the Kokomo Limestone. The Huntington Dolomite of common usage consists of lower Niagaran rocks, generally bedded, in the eastern and southeastern parts of the area and of upper Niagaran rocks, commonly reef facies, in the northern part. The so-called New Corydon Limestone of Huntington County is in the upper part of the Niagaran, but the New Corydon type exposures in Jay County lie stratigraphically well below the Mississinewa Shale. The Kokomo Limestone in its type area is assigned to the Cayugan, but its age and the unconformity that has been described at its base remain questionable. The rocks near Fort Wayne that have been called “Kokomo” are thought to be early-middle Devonian in age. They rest upon rocks of an upper Niagaran reef facies in the one exposure. Southeastward along the Silurian-Devonian contact in the western part of the area Devonian rocks rest upon progressively older Silurian strata, and most of the Mississinewa and younger Niagaran rocks are absent from the southern part. The historical development of stratigraphic terminology and the latest stratigraphic data suggest that the present usages of names for the following rock units in the conference area are less than satisfactory and that revision and redefinition may follow definitive study: Brassfield Limestone, lower Niagaran rocks, Huntington Dolomite, and New Corydon Limestone. Much of the fossil evidence bearing upon early correlations consists unsatisfactorily of species lists that had grown from author to author and through stratigraphic revisions. Many species, from nontypical exposures but presumably characterizing the faunal type, were added after stratigraphic identification had been made by means of lithostratigraphy; nearly half of the classic fossil localities are here assigned new stratigraphic positions. The fauna from the Mississinewa Shale and younger Niagaran strata is thought to be Lockport and Guelph in age.
Indiana Geological Survey Guidebook 10
Stratigraphy, Historical Geology, Paleontology, Limestone, Lithostratigraphy, Silurian, Dolomite, Indiana
Shaver, Robert H. and Others, 1961, Stratigraphy of the Silurian Rocks of Northern Indiana. Indiana Geological Survey Guidebook 10, 62 p., 10 fig.
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