Stratal order in Pennsylvanian cyclothems: Discussion
In summation, the attempt by Wilkinson et al. (2003) to statistically test for the presence of high-frequency, terrestrial to marine sequences in the cyclothem "type section" in western Illinois may have been hindered by the absence of the marine components of the cyclothems. Because of the lack of marine strata, as shown by Wanless's (1957) descriptions, the presence of some bias in the database, and problems of classifying marine strata, the conclusion that glacioeustatic processes had no apparent effect on the western Illinois succession is not persuasive The conclusion of Wilkinson et al. 2003 that the importance of stochastic depositional processes in western Illinois has been underappreciated is probably correct, although Wanless (1964) recognized that some cyclothemic strata demonstrate the presence of stochastic processes. This study has strengthened my conviction that the ten unit "ideal" cyclothem is not very representative of the cyclothems of western Illinois or of any cyclothemic strata of the Pennsylvanian in the Illinois Basin. Perhaps a more representative and less complex succession based on the most commonly occurring cyclothem beds (Wartless, 1957) is more suitable for studies of cyclothems: basal sandstone, underclay units, coal, marine units (fossiliferous gray shale, limestone, black shale, or various combinations thereof), and upper shale. ?? 2004 Geological Society of America.