Shelf-to-Basin Transect of Middle Paleozoic Organic-Rich Shales of the North American Midcontinent (Chattanooga, Woodford, Arkansas Novaculite)

Middle Paleozoic (Frasnian-earliest Tournaisian) black shales mapped as Chattanooga Shale in NE Oklahoma, southern Missouri and western Arkansas, along with the Woodford Shale of the Arbuckle Mountains and the Arkansas Novaculite of SE Oklahoma and Arkansas form a continuum of coeval shelfal-basinal deposits. The high shelfal expression consists of the Chattanooga Shale and is typified by black fissile shale with minor silt but generally lacking in non-skeletal phosphate and bedded cherts. Distal shelf and slope equivalents mapped as the Woodford Shale include abundant interbeds of radiolarian cherts and black fissile shales with locally abundant non-skeletal phosphate. The phosphate-rich interval is generally restricted to a relative short stratigraphic interval in the upper Famennian part of the Woodford Shale or those near the Devonian-Mississippian (Famennian-Tournaisian) boundary. Locally, the Woodford Shale has large early diagenetic carbonate concretions that contain abundant conodonts and radiolarians (Ardmore Basin area). The basinal area deep Ouachita trough contains interbeds of radiolarian-cherts (mapped as Arkansas Novaculite) and thin black shale beds. Phosphate is apparently lacking from this facies. In addition to the interbeds, red and green shales with minor novaculite are present near the top of the member. Their origin is enigmatic. Conodonts contained in the Arkansas Novaculite indicate that only the upper part of the Novaculite is stratigraphically equivalent to the Woodford Shale with a considerable thickness in the middle part that is Middle Devonian with no shelfal equivalents.