LATE DEVONIAN CONODONT BIOSTRATIGRAPHY KEY TO UNDERSTANDING THE TIMING AND CAUSE OF FAUNAL EXTINCTIONS AND CARBONATE PLATFORM EVOLUTION IN THE IOWA AND WESTERN ILLINOIS BASINS

During the Late Frasnian (Late Devonian), a profound sea level rise of a minimum estimated magnitude of 90-125 m began during Montagne Noire (M.N.) Zone 11 initiated Lime Creek carbonate ramp deposition in western, central and northern Iowa. Progressive deepening and highstand during the upper part of M.N. Zones 11-12 are reflected by the incoming and progressive diversification of the platform-dwelling platform shelly faunas with shelly taxa reaching peak diversity during M.N. Zone 12. Distal ramp clinoform shale wedges downlapped onto the Middle Frasnian erosional surface across central and eastern Iowa and filled paleokarst cavities hosted in older Devonian and Silurian carbonates (Independence Shale). During peak highstand, inner shelf facies of the Owen Member prograded over middle shelf facies in the Lime Creek Formation platform. Distal ramp muds prograded into southeastern Iowa as recorded by Sweetland Creek Shale deposition. The Lower Kellwasser Event is recorded by extinction of the diverse middle shelf shelly biota high in the interval of M.N. Zone 12 and lower M.N. Zone 13. Resumed deepening in the very late Frasnian resulted in Lime Creek platform backstepping indicated by abrupt juxtaposition of middle shelf facies with offshore conodonts and brachiopods over shallow platform carbonates in the middle part of the Owen Member. Lowstand emergence eroded platform deposits spanning the Frasnian-Famennian boundary interval in northern Iowa, although a comformable F-F boundary is recognized in basinal sections of the lower part of the Grassy Creek Shale (type locality of the Sweetland Creek Shale) and newly documented subsurface well sections in southeastern Iowa. Subtropical sea surface temperatures (SST) reconstructed from oxygen isotope ratios of conodont apatite from the and Lime Creek Formation and coeval units in Germany indicate warming during the Late Frasnian (SST > 30° C) and with two significant SST cooling events coinciding with the Lower and Upper Kellwasser bioevents and stepped Kellwasser extinctions of late Frasnian platform shelly faunas in central North America.