Sierra Gomez, approximately 20 km east of Aldama, Mexico, consists of approximately 500 m of folded and thrusted middle Cretaceous massive to thin-bedded mudstones and wackestones. Thin-bedded units are generally laminated and show local soft-sediment deformation. The laminated carbonates are dark-gray, with some bedded, black chert and contain gastropods, oysters, and the bivalve Corbula. Massive units are generally dark-gray to tan, with chert nodules and stylolites, and contain rudists, oysters, corals, and gastropods.

The silicified megafauna includes abundant bivalves such as radiolitid and toucasid rudists, Ostrea sp., Corbula sp., Granocardium sp., and several other genera. At least five species of gastropods have been found that include at least two species of Turritella and Heliocryptus planorbis. Abundant corals as well as worm encrustations and sponge borings (Clinos) are also recorded.

These limestones have been described as basinal facies of the Aurora Limestone. A significant portion of the megafauna has apparently not been transported. Shallow-water deposition is also indicated by the presence of rudists in apparent growth position. Normal marine conditions are established by the presence of the stenohaline coelenterates.

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