Integrated delta 13Ccarb and PXRF elemental chemostratigraphy of the Moccasin Springs-Bailey interval (Ludlow-Lochkovian?) in the Illinois Basin, western Laurussia

In this study we outline the results of chemostratigraphic investigation of the upper Moccasin Springs and Bailey formations in southern Illinois in an effort to delineate the Silurian-Devonian boundary. Two Illinois State Geological Survey (ISGS) drill cores penetrated the Bailey down through Girardeau Limestone in southern Illinois in 2014 and 2017 with overlap around the basal Bailey. McAdams and others investigated the Ordovician to Early Devonian (?) at two surface localities and the subsurface section penetrated in the Schlamer #1 core. Results of their high-resolution integrated biochemostratigraphic analysis (2019) of those sections led to the revision of the regional litho- and biostratigraphy and correlations of the Silurian Bainbridge Group. The Schlamer #1 core section provided the first detailed integrated biochemostratigraphic record of all three major Wenlock-Ludfordian delta 13C excursions in the Illinois Basin, but did not penetrate the interval of the Silurian-Devonian (S-D) Boundary. The ISGS McFarland #2 core was drilled S-SE of the Schlamer #1 drill site through the S-D boundary interval and recovered 207 feet of the lower Bailey Formation and 98'. of the Silurian Moccasin Springs Formation (Shepard Point, Lithium and Randol mbs.). The McFarland #2 core was sampled every 10 cm for delta 13C analysis, 654 samples were drilled the full length of the core by Devera and the same positions sampled by Grosch for PXRF analysis at ISU. Elemental data were collected using a hand-held PXRF at the Indiana Geological Survey by Grosch & McLaughlin. Conodonts samples were taken from the upper Randol Member and lower Bailey Formation and processed at ISU (Grosch & Day). Whole rock elemental data indicate a relatively continuous lithologic transition from the Randol Mb. into the lower Bailey Fm. With significant increases in Si in the Bailey reflecting the distinctive silica and chert content of that unit. The delta 13C carb data identify well developed excursions with peak values of +3.27/ml in the Randol (upper part of the Ludfordian Lau Excursion), and another with peak values of +3.69/ml in the lower Bailey (Lower Lochkovian Klonk Event?). ISGS McFarland core may provide an unbroken Ludfordian-Lockhovian record of the Lau and Klonk isotopic excursions, but additional biostratigraphic data is needed to test this hypothesis.