AGE AND CORRELATION OF THE WOODFORD SHALE, UPPER DEVONIAN-LOWER CARBONIFEROUS, IN THE SUBSURFACE OF EASTERN NEW MEXICO AND WESTERN TEXAS

The Woodford Shale in the type area of southern Oklahoma consists of approximately 75 m of predominately dark organic-rich shales, as well as light shales, bedded chert, and interbedded phosphate-rich strata that lie unconformably on Middle Devonian or older units. The Woodford includes strata of Upper Frasnian, Famennian, and lower Carboniferous. In the subsurface of west Texas and eastern New Mexico the Devonian light and black shales assigned to the Woodford rest unconformably on Silurian or Lower Devonian carbonates. Although Middle Devonian and lowest Frasnian deposition is indicated by conodonts recovered from cavity fills in the underlying carbonates and a thin basal green-gray shale, typical Woodford rocks in seven cores in the study area, from Chaves County NM to Howard County TX, consist of 4 to 20 m of Famennian strata that starts in the Upper crepida Zone, coincident with a global rise in sea level and deposition of other black shale units in the central and eastern United States, and ranges into the Upper marginifera Zone. The Lower marginifera Zone strata interval correlates across 350 km of the Upper Devonian basins. A well in Glasscock County TX did not have lower Famennian Woodford, but preserves high Famennian strata of the Middle expansa Zone or higher, as well as lower Carboniferous strata of the Upper duplicata Zone or higher. The Middle expansa Zone is characterized by deposition of another wide spread black shale interval in the eastern United States.