ABSTRACT

Except for the Smackover Formation carbonate rocks make up only a small part of the Upper Jurassic sequence, but they are widespread and are sensitive indicators of environments of deposition. Consequently, carbonate studies have yielded data vital for stratigraphic correlation and interpretation of environment. Rocks of Late Jurassic age include, in ascending order, the Smackover and Buckner Formations and the Bossier and Schuler Formations of the Cotton Valley Group. These rocks are in the subsurface at depths ranging from about 3,000 to 12,000 feet.

The Smackover Formation contains three informal members. The lower member, one of the most widespread and easily recognized units in the rocks of Late Jurassic age, consists of dark-gray silty to argillaceous, commonly laminated limestone, that was deposited throughout a deep, possibly stagnant basin. The middle member, generally restricted to basin margins, consists of medium-brown pelletoid or structureless limestone that was deposited in the shallower parts of a basin that supported a relatively abundant fauna. The upper member, also limited to basin margins, consists mostly of light-brown to black oolitic to pisolitic limestone that represents a shallow-water high-energy environment. This member includes the petroleum- producing zone, the Reynolds oolite.

The Buckner Formation contains two members. The lower member consists mostly of laminated micrograined anhydrite and anhydritic mudstone, but in restricted areas it is fine-grained dolomite. It represents deposition in an evaporitic basin and associated mudflats. The upper member consists mostly of nodular anhydritic mudstone that represents deposition in evaporitic mudflats. It contains a bed limestone, known locally as the Azone, that represents a temporary advancement of the sea across the mudflat.

The Bossier Formation represents the offshore equivalent of the Buckner and parts of the Smackover and Schuler Formations. It consists mostly of dark-gray splintery calcareous shale, but contains shell material in various amounts. A limestone at the base of the Q tongue consists mostly of silty micrite containing a fossil assortment that is characterized by algal-encrusted grains but also includes foraminifers, gastropods, ostracods, and echinoid fragments.

The Schuler Formation, which includes a marine and a nonmarine facies, consists mostly of mudstone, shale, and sandstone but contains some limestone in the marine facies. Algal micrite is present in the upper part, and some argillaceous coquina and phosphatic clastic limestone that apparently represents beach environments is present near the base.