T. C. Buschbach
The Croixan Series (Winchell, 1873, p. 70; 1900) is named for the St. Croix River in eastern Minnesota.
The type section consists of all strata of Cambrian age (about 600 feet) exposed in the St. Croix Valley.
The Croixan Series has also been called the Upper Cambrian, the St. Croix, and the St. Croixian Series.
Extent and Thickness
All Cambrian strata in Illinois are assigned to the Croixan Series. Therefore, the thickness of the Croixan is the same as that for the Cambrian System (1000-3500 feet) (fig. Є-2).
In the St. Croix Valley, Cambrian strata overlie Precambrian rocks, and the uppermost Cambrian formation, the Jordan Sandstone, is overlain by the Oneota Dolomite, the oldest Ordovician formation in that area.
The Croixan Series is subdivided into the Dresbachian (oldest), Franconian, and Trempealeauan Stages (fig. Є-3). Information about Cambrian rocks in extreme southern Illinois is scarce and middle, or even lower, Cambrian strata may be present in that part of the Illinois Basin.
The Croixan Series is defined on the basis of its trilobite faunas; it extends from the Cedaria Zone at the base to the Plethopeltis Zone (Howell et al., 1944) or the Saukia Zone (Raasch, 1951) at the top.
HOWELL, B. F., et al., 1944, Correlation of the Cambrian formations of North America: Geological Society of America Bulletin, v. 55, p. 993-1003.
RAASCH, G. O., 1951, Revision of Croixan dikelocephalids: Illinois Academy of Science Transactions, v. 44, p. 137-151 ; Illinois State Geological Survey Circular 179.
WINCHELL, N. H., 1873, Potsdam Sandstone: Minnesota Geological and Natural History Survey 1st Annual Report (1872), p. 68-80.
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