STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE BARNETT FORMATION AND EQUIVALENT MISSISSIPPIAN ROCKS IN THE FT. WORTH BASIN

The recent emergence of the Barnett Formation as a target for unconventional gas production has sparked renewed interest in the geological character of the Mississippian succession in the subsurface of Texas. Examination of more than 25 cores from the Ft. Worth Basin area, most previously undescribed, provides new insights into the sedimentology, and depositional setting of the Barnett and equivalent rocks.

Along the western basin margin, Ordovician carbonates are overlain by the Early Mississippian Chappel, a thin succession of crinoid-rich, shallow water shelf buildups and flank deposits (packstones and grainstones). These rocks are overlain by transported, mud-rich, deeper water, crinoidal packstones and wackestones assigned to the White's Crossing. The Barnett “shale” succession, which variably overlies the White's Crossing or in some areas rests directly on the Chappel or older rocks, comprises organic-rich, parallel-laminated, calcitic and siliceous mudrocks and very little shale. Sedimentary structures suggest these rocks were deposited by distal turbidity flows in a sediment-starved, below storm wave-base setting under restricted, anoxic or dysoxic conditions. This stratigraphic succession chronicles the downwarping and progressive flooding of the previously emergent southern margin of the Laurussian paleocontinent

The Forestburg, which is commonly distinguished from the Barnett of the central Ft. Worth Basin by its low gamma ray/high resistivity wireline log response, is dominantly a lime mudstone containing sedimentary features like those of the Barnett. Sedimentary structures, organic matter content, the lack of in situ fauna, and regional paleooceanography imply that the Barnett/Forestburg accumulated in a deeper water, intercratonic seaway developed between approaching North American and Gondwanan plates.