POSSIBLE HIGH-ORDER MILANKOVITCH CYCLES PRESERVED IN THE HIGH-STAND SYSTEMS TRACT (HOLT SHALE MEMBER) OF THE TOPEKA FORMATION (VIRGILIAN, UPPER PENNSYLVANIAN) OF NORTHERN MIDCONTINENT NORTH AMERICA

Lithologic evidence, seen as interbedded gray and black shales, and conodont morphotypes of Streptognathodus sp., Hindeodus sp., and Adetognathus sp. in the Holt Shale Member of the Topeka Formation, suggest that high-order (possibly of 20,000-40,000 year duration) glacio-eustatic sea-level changes (T-R cycles) with probable maximum fluctuations on the order of a few meters, may be preserved in the high-stand systems tract (HST) of this Upper Pennsylvanian (Virgilian) cyclothem. The Holt Shale (HST) lies between the underlying DuBois Limestone (TST) and the overlying Coal Creek Limestone (RST) in the upper part of the Topeka Formation. At Forest City, MO, the lower part consists of about 20 cm of non-sandy, black, fissile, phosphatic shale. This is interpreted to represent deposition in deeper, more offshore, anoxic waters during maximum transgression. The upper part consists of about 96 cm of light gray shale with three thin (each ~10 cm thick) interbedded dark gray to black zones. We interpret the upper light gray part to represent early regression under more oxic conditions, while the interbedded dark gray to black zones are inferred to represent short duration transgressive events that placed the sea bottom into more dysoxic conditions. The Holt Shale was deposited during a major sea level high-stand, and in the northern shelf region of the Midcontinent Basin, exhibits a complex architecture with several probable minor sea level fluctuations being superimposed upon the early regressive phase of the cyclothem.