North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 5
Presentation Time: 8:00 AM-12:00 PM


RAMIREZ, Diane L., Department of Geoscience, The Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242 and HECKEL, Philip H., Department of Geoscience, The Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242-1379,

Pennsylvanian cyclothems on the Northern Midcontinent Shelf are laterally continuous marine transgressive-regressive units that resulted from glacial-eustatic fluctuations of the North American Midcontinent Sea. The most laterally extensive (major) cyclothems consist of a nearshore shale–transgressive limestone–offshore shale–regressive limestone–nearshore shale sequence, in which the typically black, phosphatic, and conodont-rich offshore shale represents sea-level highstand. Major cyclothems exhibit distinct vertical sequences of conodont genera that reflect changes in depositional environments. Nearshore shales are dominated by sparse Adetognathus, while the condensed gray and black facies of the offshore shales are characterized by high abundances of Idiognathodus and Streptognathodus. At the species level, enough change took place among the latter two genera between successive cyclothems to allow the identification of individual cyclothems on the basis vertical species distribution.

The Eudora Shale Member, which can be traced from southwest Iowa to northern Oklahoma, represents the offshore condensed interval of the Stanton cyclothem. This dark gray to black shale unit displays a change in Streptognathodus species from base to top. The lower gray facies contains several species that are common in older cyclothems, including S. elegantulus. Although Idiognathodus simulator dominates the deepest-water middle black facies, the upper gray shale is dominated by a similar, but distinct of species Streptognathodus, S. firmus.

My purpose is to determine whether or not forms intermediate between S. elegantulus in the lower gray shale and S. firmus in the upper gray shale exist in the middle black facies in several sections across the shelf. If intermediate forms are present, they would indicate that S. firmus is a descendant of S. elegantulus. Their absence would suggest extinction of the earlier Streptognathodus species and immigration of S. firmus from Eurasia, where it was first described. Results so far indicate a lack of intermediate forms, suggesting that S. firmus migrated from Eurasia. Studies of this type will aid in evaluating worldwide correlation of glacial-eustatic cyclothems.