CONODONT BIOSTRATIGRAPHY OF MISSISSIPPIAN CARBONATES AND SHALES ON THE BEND ARCH, NORTH TEXAS

During the early Mississippian, a carbonate shelf comprising offshore marine carbonates with "Waulsortian" mounds developed across on the Bend Arch, along the western margin of the Fort Worth Basin. The mound-associated deposits are commonly called the "Chappel Limestone," and are overlain by shallow-water, carbonate sands (Mississippian Lime). Above the carbonates lie the organic-rich shales of the late Mississippian Barnett Shale. Analysis of conodont faunas from a 270 ft. core in Shackelford County, Texas, permits better resolution of ages for each of these Mississippian units on the Bend Arch.

The "Chappel Limestone" comprises a basal unit (20 ft.) of gray argillaceous wackstone that is late Kinderhookian in age (Siphonodella isosticha-upper S. crenulata zone) that is overlain by reddish argillaceous wackstone/packstone (25 ft.) of early Osagean age (Gnathodus typicus Zone). The overlying gray argillaceous wackstone/packstone (20 ft.) yielded typicus Zone conodonts near the base and species of the middle Osagean Scaliognathus anchoralis-Doliognathus latus Zone toward the top. Crinoid-bryozoan grainstones (35 ft.) appear above this, but produced mostly sparse faunas dominated by Hindeodus. Rare Bactrognathus, including B. distortus, place this grainstone unit in the upper part of the anchoralis-latus Zone. A 5-ft. interval of vuggy grainstone is followed by 50 ft. of crinoid-bryozoan grainstone with shale stringers (Mississippian Lime). Forms of the Gnathodus texanus group dominate and poorly constrain the age to the late Osagean to Meramecian. Thin beds of black shale are interbedded with a variety of partially silicified carbonates through the next 40 ft. of the core, but produced few undiagnostic conodonts. An overlying dark gray wackstone with articulated brachiopods (10 ft.) yielded the lowest occurrences of Gnathodus girtyi and G. bilineatus, characteristic Barnett species that appear at the base of the Chesterian. The overlying grainstone unit (10 ft.) produced just a few Cauvusgnathus. The uppermost 60 ft. of the core is dominantly dark gray to black shale, typical Barnett Shale, with a few thin beds of partially silicified carbonates. Lochreia commutatus, another Chesterian species, and G. girtyi and G. bilineatus occur through this entire shale interval.