The Zuloaga Formation (Late Jurassic) is well exposed in the mountains of northeastern Mexico. It is stratigraphically equivalent to the Smackover Formation of the northern Gulf Coast. From 16 Zuloaga outcrops and a petrographic analysis of samples 12 distinct lithofacies are recognized within the formation.

The lithofacies and their inferred depositional environments are as follows (numbers 1 through 9 are shallow-water to supratidal deposits): (1) detrital facies--littoral marine to marginal marine to arid playa; (2) clean oolite facies--high-energy bar, shoal, or shelf; (3) muddy oolite facies--lagoon or shelf, washover; (4) clean pellet-fossil facies--medium to high-energy shoal and storm washover; (5) muddy pellet--fossil facies--intertidal to shallow subtidal; between shoals; (6) burrowed lime-mud facies--low-energy lagoon or low-energy shelf; (7) algal-laminated facies--very shallow subtidal to intertidal, possible supratidal; (8) oncolite facies--medium- to low-energy subtidal; (9) limestone-breccia facies--intertidal to supratidal storm deposit; (10) dolomite facies--environment nknown, possibly intertidal to supratidal; (11) evaporite facies--environment unknown, probably shallow, restricted lagoon; and (12) pelagic fossil lime-mud facies--low-energy shelf, deeper than facies 1 through 9.

The general range of paleoenvironments suggest a very shallow, slowly subsiding, trough-shaped epicontinental sea, the Mexican "geosyncline." A sequence of depositional environments, similar to those represented in the Mexican geosyncline area, may be expected east of the Tamaulipas Peninsula in the Mexican Gulf coastal plain. The Zuloaga Formation was deposited during a major marine transgression with many minor sea-level fluctuations.

The Zuloaga and Smackover Formations are very similar in lithology and depositional environments. Detailed studies of the Zuloaga may aid in defining facies relations in the Smackover, which is more difficult to examine because it does not crop out.

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