HIGH RESOLUTION UPPER DEVONIAN BIOSTRATIGRAPHY OF THE SWEETLAND CREEK AND GRASSY CREEK SHALES, KELLWASSER EXTINCTION SIGNATURES, AND THE FRASNIAN-FAMENNIAN STAGE BOUNDARY IN THE IOWA BASIN-CENTRAL NORTH AMERICA

In southeastern Iowa, the Sweetland Creek and Grassy Creek shales comprise a highly condensed deep ramp succession of hemipelagic shale and mudstone facies of Late Frasnian and Early Famennian age. Intensive sampling in 2016-2020 of the type section of Sweetland Creek Shale and overlying Grassy Creek yield diverse and abundant Upper Frasnian assemblages of Frasnian Zone 11 to Subzone 13c, and the Lower Famennian Palmatolepis subperlobata to Pa. d. platys Zones. Species First and Last Appearance Datums (FADs & LADs) clearly identify the Lower and Upper Kellwasser Extinction (LKE & UKE) intervals.

A diverse Frasnian Zone 11 fauna reported from the basal 50 cm shale by Johnson and Klapper, and new samples, in the lower 60 cm of the overlying “siltstone”. Important elements of Zone 11 assemblages include abundant Palmatolepis plana, with Pa. proversa, Pa. kireevae, Pa. ljaschenkoae, Pa. playfordi, Pa. amplificata, Ancryognathus? deformis and Pa. semichatovae. The lowest occurrence of Pa. foliacea with Pa. rehnana identifies the base of Zone 12 in the upper 5 cm of the “siltstone” above a prominent prytitic hardground. Above the “siltstone”-capping hardground Pa. winchelli and Ag. asymmetricus have their FADS in gray shales of the Zone 12 interval. The LKE interval is marked by the LADs of Pa. kireevae, Pa. amplificata, Pa. foliacea and Pa. muelleri. The base of Subzone 13a is 185 cm above the base of the Sweetland Creek, marked by the FAD of Pa. bogartensis, with FADS of Pa. hassi s .s., Ag. calvini, P. brevicarina, and Ozarkodina dissimilis low in S.z. 13a . The FAD of Pa. linguiformis defines the base of Subzone 13b. The onset of the UKE is marked by LADs of Pa. bogartensis and Pa. boogardi 5 cm below the first K-bentonite bed in the lower Grassy Creek Shale. Extinctions of most typical Frasnian conodont taxa characterize the 20 cm thick UKE crisis interval with species LADs within the second K-bentonite bed in the Grassy Creek. Frasnian survivors and FADS of Pa subperlobata and Ancyrognathus cryptus denote the position the lowermost Famennian and F-F boundary immediately above the second K-bentonite.

Integrated whole rock magnetostratigraphic, C13carb and O18 apatite isotope and REE chemostratigraphic work is in still in progress and will provide valuable insights as to the environmental perturbations behind these extinction events.