ABSTRACT

The Zuloaga Formation (Upper Jurassic) is well-exposed in the mountains of northeastern Mexico. It is stratigraphically equivalent to the Smackover Formation of the Northern Gulf Coast. Sixteen Zuloaga outcrops were studied, and a petrographic analysis of samples was made. Twelve 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 and 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 unknown, possibly intertidal to supratidal
11. evaporate facies--environment unknown, probably shallow, restricted lagoon
12. pelagic fossil lime mud facies--low-energy shelf, deeper than facies 1 through 9.

The general range of paleoenvironments suggests 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 numerous 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 relationships in the Smackover, which is more difficult to examine because it does not crop out.