Paul Brenckle, H. Richard Lane, Charles Collinson; Progress toward Reconciliation of Lower Mississippian Conodont and Foraminiferal Zonations. Geology ; 2 (9): 433–436. doi: https://doi.org/10.1130/0091-7613(1974)2<433:PTROLM>2.0.CO;2
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Calcareous foraminifers and algae and conodonts indicate that the late Osagean Keokuk Limestone near its type section correlates with Visean strata in Belgium. This discovery necessitates shifting the Mamet foraminiferal zonal scheme so that the Keokuk is no older than Zone 10 and possibly as young as Zone 13, rather than corresponding to Zones 8 and 9. The occurrence of the conodonts Eotaphrus burlingtonensis and Polygnathus mehli near the top of the Burlington Limestone in its type area suggests that the Tournaisian-Visean boundary probably corresponds with the Burlington-Keokuk contact.