The timing and extent of illite formation in Ordovician K-bentonites at the Cincinnati Arch, the Nashville Dome and north-eastern Illinois basin |
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| Authors: | W CRAWFORD Elliott JAMES L Aronson |
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| Institution: | Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA |
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| Abstract: | The K/Ar ages of illite/smectite (I/S) were measured from Middle Ordovician K-bentonites both west and east of the present crest of the Cincinnati Arch and the Nashville dome to test a previous hypothesis that I/S formed by reaction with migrated saline solutions during the Alleghanian Orogeny. The K/Ar ages of I/S at the distal margin of the southern Appalachian basin and from central Indiana range from 251 to 277 Ma. However, the ages of I/S from west of the crest of the Cincinnati Arch are slightly older (286–301 Ma) and the ages of I/S from north-eastern Indiana, on the northern edge of the Kankakee Arch and in effect in the Michigan basin, are the oldest measured in this study (315–325 Ma). The westward decrease in the K/Ar ages of I/S from Late Pennsylvanian ages in the proximal basin (286–303 Ma) to Permian (251–277 Ma) at the distal margin suggest that I/S was formed by the westward migration of fluids during the Alleghanian Orogeny as opposed to being formed by projected deep burial by Permian sediments. Moreover, the available thermal maturation data suggest the Cincinnati Arch was not buried deeply. The ages of I/S west of the Cincinnati Arch are an enigma as they are older than the ages in the distal Appalachian basin. The ages of I/S from central Indiana within the Illinois basin suggest the possibility that I/S was formed by reaction with fluids that migrated from the Ouachita orogenic belt in Mississippi. The oldest ages of I/S from north-eastern Indiana suggest the formation of I/S might have been influenced by the presence of potassic brines from the Michigan basin. |
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