Petrology of the Buckner Formation in Adjacent Parts of Texas, Louisiana, and Arkansas

Abstract

ABSTRACT The Buckner Formation is an evaporitic mudstone unit of Late Jurassic age that is present only in the subsurface around the margin of the Gulf Coast embayment. The formation has been informally divided into two members, lower and upper. The most common rock types in the Buckner Formation are nodular anhydritic mudstone and nodular anhydrite. The nodules are lathy anhydrite occurring as swirls in a groundmass of blocky to anhedral grains. The matrix consists of a red or gray nonlaminated or poorly laminated mudstone. The clay minerals in the mudstone are mostly illite and chlorite. The next most common rock type in the formation is light-gray crypto- to mirograined laminated anhydrite that is largely confined to the lower member. This anhydrite contains scattered minute rounded dolomite grains, clay minerals, and fine-grained quartz sand and silt. Some less common rock types in the Buckner Formation are shale, oolitic and detrital limestone, oolitic-micritic and detrital-micritic limestone, rock salt, micrograined dol mite, and medium-grained dolomite. The Buckner was deposited in elongate basins that paralleled the margin of the Gulf Coast embayment. Circulation of water in the basins with that of the sea apparently was restricted by contemporaneously growing salt-cored anticlines along the seaward margin. The lower member was deposited for the most part in standing bodies of water, and the upper member was deposited on a tidal mudflat that was, from time to time, flooded both by marine and by terrestrial water. In general, the sea was regressing during deposition of the Buckner Formation. Nodular anhydrite is associated with brackish-water fossils and was probably deposited as gypsum. Micrograined anhydrite is associated with rock salt and considered here to be primary.