HIGH-RESOLUTION CONODONT BIOSTRATIGRAPHY AND EVENT STRATIGRAPHY OF A KEY NORTH AMERICAN REFERENCE SECTION FOR THE UPPER FRASNIAN-LOWER FAMENNIAN (UPPER DEVONIAN) FROM THE IOWA BASIN, CENTRAL NORTH AMERICA

Upper Devonian strata from eastern Iowa in the Iowa Basin (central United States) provide a complete highly resolved Upper Frasnian to Lower Famennian conodont succession in condensed (540 cm) thermally pristine hemipelagic facies of the Sweetland Creek and lower Grassy Creek shales. New conodont sample series (128 total) collected in 2016-2020 document a continuous conodont sequence of 60 species spanning Frasnian Zones 11 and 12, Subzones 13a to 13c, and the Lower Famennian Palmatolepis subperlobata to Pa. platys Zones. Associated benthic and pelagic taxa include low diversity assemblages of polychaete scolecodonts, phosphatic linguiform brachiopods (Barriosella sp.), pyritized rhynchonelliform brachiopods Eostrophalosia sp. and Nervostrophia canace, and dacryonarid tentaculites. The Lower Kellwasser Extinction (LKE) interval is highly condensed within a 10-15 cm interval within the upper Part of Frasnian Zone 12. The Upper Kellwasser Extinction (UKE) occurs within a 10 cm interval immediately below a distinctive K-bentonite bed in the Grassy Creek Shale. The onset of the Upper Kellwasser Extinction (UKE) is marked by the LADs of a majority of Frasnian taxa in the uppermost part of Subzone 13b, followed by LADs of the remaining typical Frasnian taxa in a 5 cm thick interval representing Subzone 13c. This is immediately overlain by a 3 cm thick Grassy Creek K-bentonite with FADS of earliest Famennian conodont taxa and small moldic post-larval brachiopods, gastropods and ammonoids within the Palmatolepis subperlobata Zone. The FAD of Pa triangularis within the second Grassy Creek K-bentonite bed marks the base of that zone in the lower Grassy Creek and is 4 cm thick. Dark brown shale immediately above the second K-bentonite yield conodonts of the Palmatolepis platys Zone. This high resolution sequence provides chronostratigraphic control for ongoing integrated chitinozoan biostratigraphic, chemostratigraphic, and magnetostratigraphic investigations. The latter apply cyclostratigraphic analysis of bulk MS data to astronomically calibrate the succession.