The Tansill Formation and the Lamar Member of the Bell Canyon Formation are the uppermost carbonate units equivalent to the Capitan Formation of West Texas and southeastern New Mexico. The Lamar is restricted to the Delaware basin and is equivalent to the lower and middle parts of the Tansill. Surface and subsurface stratigraphic studies by many workers show the Tansill-Capitan-Lamar to represent classic examples of shelf, shelf-margin, and basin deposits.

Because these units will continue to serve as a model for environmental studies of ancient rock, this paper reviews some of the criteria useful in distinguishing the various environments.

In the Guadalupe Mountains area, carbonate units of the Tansill Formation are predominantly light colored, well-bedded dolostone which grades shelfward into evaporite. Along the margin of the Northwestern shelf, the Tansill quadruples in thickness and becomes less dolomitic before grading into the Capitan Formation. Well-preserved depositional textures and sedimentary structures in Tansill carbonates suggest shallow-water environments ranging from supratidal flats and evaporite lagoons to shoal-water areas.

The Capitan Formation of Tansill-Lamar age consists of light-colored, massive to thick-bedded carbonate along the shelf margin and steeply dipping, massive beds of carbonate detritus along the basin margin. Texture and skeletal components of the Capitan are different from those of the Tansill. The Capitan generally is interpreted to be a shelf-edge and slope deposit.

The Lamar Member is a basinward-thinning tongue of limestone debris derived largely from shelf (Tansill) and shelf-edge (Capitan) deposits. It tends to be dark-colored, cherty, and evenly bedded but contains some slump structures. Units of micritic skeletal-intraclast calcarenite grading upward to micrite are common near its transition with the Capitan Formation. Farther basinward, the Lamar is characterized by evenly laminated micrite. Relative to the Tansill the Lamar is interpreted to have been deposited in deeper water (partly by turbidity currents) on a smooth basin floor.

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