Criteria Useful in Interpreting Environments of Unlike But Time-Equivalent Carbonate Units (Tansill-Capitan-Lamar), Capitan Reef Complex, West Texas and New Mexico


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Depositional Environments in Carbonate Rocks

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The Tansill Formation and the Lamar Member of the Bell Canyon Formation are the uppermost carbonate units equivalent to the Capitan Reef Complex of West Texas and Southeast New Mexico. Fusuli-nid occurrences establish the equivalency of the Lamar and the pre-Ocotillo Member part of the Tansill. Surface and subsurface studies by numerous workers show the Tansill-Capitan-Lamar to represent classic examples of shelf, shelf edge and slope, and basin deposits, respectively. Tansill carbonates are restricted to the shelf areas surrounding the Delaware Basin. The biota, strati-graphic relationships, depositional textures, and sedimentary structures suggest these carbonates were deposited in shallow water environments which include (1) evaporite, (2) “mixed shelf carbonate,” (3) unwinnowed shelf carbonate sand and (4) winnowed shelf carbonate sand environments. Facies characteristic of these environments can be further subdivided. Tansill carbonates are mostly light colored dolostones that thin sheifward by interfingering with evaporites. The biota is “restricted marine” consisting mostly of algae, molluscs, and Foraminifera. The “mixed shelf carbonate” environment is characterized by irregularly laminated mudstone and textures interpreted to have been superimposed on original shallow water deposits during exposure and (or) under intertidal and supratidal conditions. Carbonate sands in the winnowed shelf environment are better sorted, contain less fine matrix, more skeletal grains, and osagid-type coated grains than the unwinnowed shelf carbonate deposits. The portion of the Capitan Formation time-equivalent to the Tansill contains a diverse “normal marine biota.” Captain deposits of the shelf edge environment are massive lime packstone, wackestone, and locali’ boundstone which are commonly altered by recrystallization. Capitan deposits of the basin slope environment are generally light colored, partly dolomitized, and poorly bedded packstone with primary basinward dips. Characteristic grain types are sand- and coarser-sized skeletal and intraclast debris. The Lamar Member is restricted to the Delaware Basin, thins basinward by wedging, and then grades into a thin unit of dark shaly siltstone and silty shale. Lamar environments include (1) basin margin, (2) basinal, and (3) starved basin. In contrast to the Tansill and Capitan carbonates, the Lamar consists predominantly of cherty, dark-colored lime packstone, wackestone, and mudstone with well preserved depositional texture. Graded beds are common in the basin margin limestones. Grain types are mostly redeposited skeletal and intraclast debris derived from the shelf edge and basin slope environments. Textural evidence and stratigraphie framework suggest these grains were periodically transported into the Delaware Basin by turbidity currents. The basinal carbonate environment is characterized by evenly laminated mudstone and fine skeletal-intraclast wackestone. The starved basin environment is represented by a thin, widespread unit of black, shaly siltstone and silty shale.

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