Abstract

Silurian and Devonian strata of Oklahoma crop out in three areas: (1) Arbuckle Mountains and Criner Hills of southcentral Oklahoma (including a small inlier on Turkey Creek in northern Marshall County), (2) northeastern Oklahoma, and (3) the Ouachita Mountains of southeastern Oklahoma and western Arkansas. The Ouachita Mountains, which constitute a distinct structural-stratigraphic province with poorly understood Silurian-Devonian relationships, is discussed separately. Exclusive of the Ouachitas, Silurian-Devonian strata in Oklahoma are represented by two distinct lithofacies: an upper dark shale sequence of Late Devonian to Early Mississippian age, and an underlying carbonate section ranging in age from Early Silurian (Llandoverian) to Early Devonian (Esopusian, Emsian; no Middle Devonian rocks are known at the surface or in the subsurface of Oklahoma). The dark shales are called Woodford Shale in the Arbuckle Mountains - Criner Hills and throughout most of the subsurface, and Chattanooga Shale in the outcrop area of northeastern Oklahoma. An unconformity separates the Woodford-Chattanooga from the underlying rocks, and this shale may rest on Lower Devonian, or Silurian, or on pre-Silurian strata. The Silurian and Lower Devonian strata constitute a thin sequence of discrete lithostratigraphic and biostratigraphic units separated by unconformities. The combined maximum thickness of Silurian and Lower Devonian beds at the surface is about 400 feet.