RECOGNITION AND SIGNIFICANCE OF MINOR SEA LEVEL CHANGES IN PENNSYLVANIAN CYCLOTHEMS OF THE NORTHERN MIDCONTINENT IOWA SHELF REGION

Minor transgressive events have been seen in the regressive systems tract of several Middle and Upper Pennsylvanian cyclothems in the northern Midcontinent and Illinois basins. Some examples are the Ardmore Limestone of the Verdigris cyclothem, Worland Limestone of the Altamont cyclothem, Bethany Falls Limestone of the Swope cyclothem, Winterset Limestone of the Dennis cyclothem, Flat Creek Limestone (Illinois) and correlative Cement City Limestone (Midcontinent) of the Dewey cyclothem, Plattsmouth Limestone of Oread cyclothem.

At least one minor regressive event has been recognized for some time in a high stand systems tract, Haynies Limestone in the Larsh-Burroak Shale (Deer Creek cyclothem). Recently three other minor high-order cycles have been recognized in the early regressive deposits of the Larsh-Burroak Shale and two in the overlying Ervine Creek Limestone in the southwest Iowa region. More recently, several minor high-order cycles have been recognized in the high-stand and early regressive deposits of the Holt Shale (upper Topeka cyclothem) and the mid-Hartford Limestone. These deposits include a limestone in the Holt Shale and mid-Hartford shale that are analogous to the Haynies Limestone of the Larsh-Burroak interval, and dark gray to black conodont-rich shales separated by lighter gray shales.

For the most part, these minor cycles can only be recognized lithologically, in the shallower water of the northern Midcontinent Shelf region of northeastern Kansas, northwestern Missouri, southeastern Nebraska and southwestern Iowa. This is where relatively small changes in sea level could affect bottom sedimentation, by changing where dysoxic/anoxic water and sunlight impinged on the ocean bottom. Farther to the south, in deeper water, these smaller sea level changes may not be recognized lithologically, because bottom sedimentation was not significantly affected.