H. B. Willman and Elwood Atherton
The Cayugan Series (Clarke and Schuchert, 1899; Swartz et al., 1942, chart 3) is based on the Cayugan Group of New York and includes all the strata of Silurian age above the contact of the Guelph and Salina Formations in the New York type section.
As the uppermost Racine strata in northern Illinois have been correlated with the Guelph Formation, no Cayugan age strata are recognized in that area. However, the generally accepted interpretation that the reefs isolated the basin in which the Salina evaporites accumulated introduces the possibility that some of the highest reefs may have continued to grow in Cayugan time.
Reefs of Cayugan age could have been eroded from Illinois before they were overlapped by Middle Devonian strata. In the deeper part of the Illinois Basin, the Moccasin Springs Formation appears in places to grade upward into the Bailey Formation, which is largely of Lower Devonian age (Collinson and Schwalb, 1955), but in the outcrop area in Illinois the contact is sharp (Pryor and Ross, 1962). As the conodonts suggest that the lowermost beds of the Bailey are upper Silurian (Collinson, 1967), the Cayugan Series probably includes the upper part of the Moccasin Springs and the lower part of the Bailey Formation.
Late Silurian graptolites have been reported from strata equivalent to the upper part of the Moccasin Springs in Missouri (Berry and Satterfield, 1972).
BERRY, W. B. N., and I. R. SATTERFIELD, 1972, Late Silurian graptolites from the Bainbridge Formation in southeastern Missouri: Journal of Paleontology., v. 46, p. 492-498.
CLARKE, J. M., and CHARLES SCHUCHERT, 1899, Nomenclature of the New York series of geological formations: Science, v. 10, p. 874-878.
COLLINSON, CHARLES, 1967, Devonian of the north-central region, United States, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1 , p. 933-939; Illinois State Geological Survey Reprint 1968-G.
COLLINSON, CHARLES, and H. R. SCHWALB, 1955, North American Paleozoic Chitinozoa: Illinois State Geological Survey Report of Investigations 186, 33 p.
PRYOR, W. A., and C. A. ROSS, 1962, Geology of the Illinois parts of the Cairo, La Center, and Thebes Quadrangles: Illinois State Geological Survey Circular 332, 39 p.
SWARTZ, C. K., et al., 1942, Correlation of the Silurian formations of North America: Geological Survey of America Bulletin, v. 53, p. 533-538.
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