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Two widespread Rocklandian age (Middle Ordovician) K-bentonite beds, the Deicke and Millbrig, can be correlated from southern Minnesota and northwestern Iowa through eastern Missouri across Kentucky and central Tennessee to the valley and Ridge of the southern Appalachian Mountains. They are equivalent to beds previously called T-3 or Pencil Cave and T-4 or Mud Cave metabentonites, and thus constitute Ordovician chronostratigraphic marker horizons throughout much of the eastern Mid-Continent. Previous correlations of these beds in the Mississippi Valley by chemical fingerprinting is extended toward the southern Appalachian basin by tracing in surface exposures and on wireline logs. Both beds thicken toward the southeast indicating a volcanic source area that was probably ast of South Carolina. The interval between the K-bentonite beds also thickens toward the southeast owing to increased rates of carbonate deposition between the two volcanic episodes.
The areal extent of known correlatives of the Deicke and Millbrig is minimally estimated to be 600,000 sq km and at least 1122 cu km of pre-compaction Deicke K-bentonite accumulated as ash in eastern North America. The volume of both beds suggests they rank among the largest air-fall ash deposits documented in the stratigraphic record.
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