Petrography and Depositional Environments of the Upper Buckner Formation, Smith County, Mississippi

Core logging and petrographic examination of about 240 feet (73 m) of the upper Buckner section in the Pan American, R.M. Thomas #1 well in northern Smith County, Mississippi, revealed interbedded anhydrite, limestone, and sandstone. This unsuccessful wildcat was drilled in 1964 as a Smackover test, and penetrated 1,958 feet (597 m) of Buckner and 682 feet (208 m) of Smackover before encountering Louann Salt at 18,310 feet (5581 m) subsea, north of Burns Dome. Excluding incomplete units, there are 60 beds ranging in thickness from 0.4 to 18.0 feet (0.12 - 5.49 m). Anhydrite beds (26) are most common, and the thickest is 18.0 feet (5.49 m). The anhydrite is light to dark gray, and is mostly nodular with chickenwire structure. Limestone beds (19) are also common, and range in color from light brownish gray to black. The thickest is 17.0 feet (5.18 m), and it includes intramicrite, intrasparite, pelmicrite, oomicrite, and oosparite. Sandstone beds (15) are least common, and the thickest is only 4.0 feet (1.22 m). The sandstone beds are light to dark gray, fine- to medium-grained quartz arenites, cemented either by calcite, anhydrite, or clay, and lack discernible bedding or lamination. The lithologic units are interpreted to be sabkha anhydrites that were periodically covered by shallow marine limestones or eolian sandstones. Based on the frequency of the nature of the lithologic change, limestone deposition was then most commonly terminated by a return to evaporitic conditions, or occasionally by influx of a quartz sand blanket. The sand units provided a substrate for either a return to sabkha evaporite deposition or deposition of marine limestone. However, because all of the sand units are underlain, overlain, or both underlain and overlain by anhydrite, and because limestones overlain by sandstone are rare, significant eolian sand deposition seaward of associated sabkhas seldom occurred