ASTROCHRONOLOGICAL CALIBRATION OF LATE FRASNIAN SEA LEVEL AND BIOEVENTS: IOWA BASIN, USA

Identification of Milankovitch periodicities within Magnetic Susceptibility data permitted documentation of 16.5 long 405-kyr eccentricity cycles (Fr-LECs) within the Frasnian Stage of the Upper Devonian of western Alberta and Poland. To further test the cyclostratigraphic model for the Frasnian, we conducted an integrated biostratigraphic, chemostratigraphic, and paleomagnetic investigation to develop a comparative record for the Late Frasnian of the subtropical Iowa Basin. The Late Frasnian Lime Creek Formation in Iowa was sampled in the Cerro Gordo Project Core #1 (CG-1) from northern Iowa and the H-32 core from southeastern Iowa. Conodont biostratigraphy (CG-1 and H-32) and δ¹³C_carb provide Late Frasnian time control. We carried out bulk magnetic susceptibility (MS) measurementsat high sampling resolution on both cores (10 cm resolution in CG-1; 4 cm resolution in H-32). The MS series of both cores display the imprint of 100-kyr and 405 -kyr eccentricity, and suggests that the studied interval covers the last ~2.6 million years of the Frasnian. These new data indicate the base of the Lime Creek Formation corresponds to the base of Frasnian Long Eccentricity Cycle 11 (Fr-LEC 11). The shift in δ¹³C_carb values from -1‰ to +2‰ in the upper Lime Creek is an expanded record of the LKE δ¹³C_carb excursion that spans two short eccentricity cycles within Fr-LEC 15 (200 ky duration). The onset of sea level rise that initiated Late Frasnian Sequence 7 in Alberta begins within Fr-LEC 10. In Iowa the transgression of Devonian T-R cycle IId-1 initiated deposition across the deeply incised karst surface capping the emergent Middle Frasnian Shell Rock Platform during the earliest part of Fr-LEC 11 indicating a duration of the basal hiatus estimated at 300-400 ky. The estimated duration of the high-stand interval of T-R cycle IId-1 in Iowa spans all of Fr-LEC 11 to 14 and lower short eccentricity cycle 1 of Fr-LEC 15 . All data sets from Iowa, Canada and Poland, indicate he Lower Kellwasser δ¹³C_carb excursion occurs within Fr-LEC 15, approximately 800 thousand years before the Frasnian-Famennian boundary. These results confirm that global cyclostratigraphic correlations are possible with a clear astronomical imprint, and represent the first steps towards a high-resolution chronology for the Frasnian - Famennian boundary interval.