ISOTOPIC AND ASTRONOMICAL CALIBRATION OF A LATE FRASNIAN (LATE DEVONIAN) CONODONT SEQUENCE IN THE IOWA BASIN-CENTRAL NORTH AMERICA

A calculated the duration of the Frasnian Stage of the Upper Devonian at 6.5 m.y + 0.4 m.y was based on recognition of sixteen long 405-kyr eccentricity (E1) astronomical signals recorded by magnetic susceptibility (MS) signal from stratigraphic records in the Western Canadian Sedimentary Basin. The Late-Latest Frasnian Lime Creek Formation (Fm.) in the Iowa Basin was sampled in the Cerro Gordo Project Hole #1 (CG-1) for conodonts and δ¹³C_carb at 30 cm intervals, and every 10 cm for bulk MS, allowing for a comparison with the Western Canadian Frasnian MS and astrochronology. Previous outcrop and core studies of the Lime Creek conodont sequence indicate correlations with Late Frasnian Montagne Noire Zones 11 and 12, but have not directly identified faunas of Latest Frasnian M.N. zones 13A to 13C in platform facies. The new δ¹³C data record Late Frasnian base line values of -1‰, with a pronounced shift to +2‰ in the upper Cerro Gordo Member interpreted as an expanded record of the Lower Kellwasser δ¹³C Excursion known to begin near the base of Late Frasnian Montagne Noire Zone 13A in European sections. Time series analyses of Iowa MS data from the CG-1 core identify parts of six of seven six Late-Latest Frasnian E1 signals (FR-LES 11 to 16) in the Lime Creek Fm.

Conodont faunas recovered in 109 (71 new) samples through the CG-1 core are typical of the Frasnian Polynathus biofaces. Samples from the lower 23 meters of the Juniper Hill and lower Cerro Gordo members include Palmatolepis semichatovae and Pa. kireevae correlated with Late Frasnian Montange Noire (M.N.) Zone 11. The first appearance of Polygnathus planarius is interpreted as marking the base of M.N. Zone 12 based on its known range in western Canada. The occurrence of Polygnathus samueli high in the Cerro Gordo Member, just below the onset of the Lower Kellwasser δ¹³C Excursion, appears to be high within M.N. Zone 12. The occurrence of Ancyrognathus asymmetricus in the Owen Member interpreted to fall within M.N. Zone 13A, above the interval of peak values of the Lower Kellwasser δ¹³C Excursion. Cross-calibration of the MS, δ¹³C and conodont data sets indicate that the base of the Lime Creek begins within FR-LES 11 (10 is missing), the Lower Kellwasser δ¹³C Excursion in within FR-LES 15, with the upper part of core recording the lower part of FR-LES 16 (part of two E2 signals).