BIOSTRATIGRAPHIC CALIBRATION OF HIGH RESOLUTION MAGNETIC SUSCEPTIBILITY STRATIGRAPHY OF THE LATE FRASNIAN (LATE DEVONIAN) LIME CREEK FORMATION IN THE SUBTROPICAL IOWA BASIN OF WESTERN LAURUSSIA

De Vleeschouwer et al. calculated the duration of the Frasnian stage of the Upper Devonian (6.5 m.y + 0.4 m.y) based on recognition of sixteen long eccentricity (E1) astronomical signals recorded by high resolution magnetic susceptibility (MS) stratigraphic records from off-reef basinal sections in the Western Canadian Sedimentary Basin in the Canadian Rocky Mountains. Frasnian long eccentricity signals recorded in our Canadian data sets are identified here as 1 to 16. To test results of our study of the Canadian Frasnian MS stratigraphy, a single subtropical Late Frasnian depositional sequence package from the Iowa Basin of central North America was selected for investigation to compare with coeval Alberta Frasnian depositional sequence 7 of Whalen and Day. Both sequence packages span Late Frasnian Montagne Noire Zones 11 and 12, with Alberta depositional sequence 7 recording five Frasnian Long Eccentricity (405 ky) signals 10 to 14 (2,025,000 years duration).

The Iowa Basin section sampled for this investigation is complete split (35.5 m, 117.5 ft.) of the Late Frasnian Lime Creek Formation in the Iowa Geological Survey (IGS) core from the Cerro Gordo Project Hole # 1 (CG-1). Core chips were taken every 10 cm for bulk MS measurements (346 samples), permitting comparison with the much thicker Alberta Late Frasnian sections (> 120 m) with 50 or 100 cm sample spacing. Conodonts were sampled continuously from 30 cm intervals and brachiopods were removed and identified from 80 sample positions permitting high resolution correlations of the MS stratigraphic record. Bulk Magnetic susceptibility measurements of samples were made on the MFK1 Kappabridge at the AMS laboratory at the University of Wisconsin-Milwaukee. The Lime Creek middle shelf samples record very low MS values ranging from a minimum of 1.49 × 10^-9 to a maximum of 1.02 × 10^-7.

The preliminary plot of the raw MS data from Lime Creek show the presence of E1 signals with smaller scale signals well developed in the MS stratigraphy, likely representing short eccentricity (E2) signals, and possibly Obliquity (O1) signals. These new data have yet to be subjected to time-series analysis but still show that the magnetic susceptibility stratigraphy of the Late Frasnian interval in both central and western North America were significantly influenced by astronomical forcing.