A MODEL FOR THE ORIGIN OF DEVONO-MISSISSIPPIAN MUDROCKS IN GREATER MIDCONTINENT NORTH AMERICA

The Woodford Shale, Sycamore Formation, and lower Caney Shale of southern Oklahoma accumulated during the late Devonian to the middle Mississippian, when southern Laurentia was largely covered by low-latitude epeiric seas, at paleolatitude of ~30˚S-15˚S. The Woodford is dominated by organic-rich siliceous mudstone and chert beds. The Sycamore is mostly calcareous quartz siltstone, while the Caney is carbonaceous, silty mudstone with thin limestone and siltstone beds. While the silty, silica-rich nature of these strata is well known, the origin and delivery of the detrital silicate fraction remains enigmatic and little studied. To address this gap, we present new grain size analyses, whole-rock geochemistry, and U-Pb detrital zircon geochronology framed within a reanalysis of regional paleogeographic reconstructions. Grain sizes of the detrital silicate mineral fractions (N=23) range from fine silt to very fine sand. The Woodford and Sycamore have modes of 10 – 31 µm and 14 – 84 µm, respectively. Geochemical data (N=21) show an enrichment in SiO₂ (74 – 97%) and depletion in Al₂O₃ (1 – 15%). U-Pb detrital zircon data from the Sycamore (N=1; n=215) and Caney (N=1; n=218) units include Archean – Paleozoic age grains. The Sycamore has primary peaks at 1047 Ma (Grenville Province), 1382 and 1463 Ma (Granite-Rhyolite Province), and multiple peaks between 363 – 617 Ma (Appalachian source areas). The Caney Shale has primary peaks at 1062 Ma (Grenville Province), 2715 Ma (Archean Provinces), and a secondary peak at 1356 Ma (Granite-Rhyolite Province). Zircon ages from these strata are interpreted to have primary detrital contributions from recycled midcontinent Cambro-Ordovician sandstone and Appalachian source areas including recycled foreland-basin strata and exhumed Appalachian basement. Paleogeographic reconstructions show no major local feeder systems for the Woodford, Sycamore, or lower Caney. These observations coupled with grain size statistics and relatively low geochemical weathering indices suggest that eolian processes likely played a major role in delivering the silicate fraction to these midcontinent systems.