Discontinuous outcrops west of Xilitla, San Luis Potosi, Mexico, preserve a sequence of middle Cretaceous (Albian-Cenomanian) carbonate rocks more than 1,500 m thick consisting of the following. (1) (Base) Well-bedded, cherty lime mudstones and wackestones with calcispheres and globular foraminifera. (2) Fine-grained, partly silicified peloidal and bioclastic lime wackestones and thin intraclast layers. (3) Massive graded beds of peloidal-bioclastic lime packstone with abundant echinoid fragments and spines, coral and mollusk fragments, but no rudists. (4) Lime breccias in massive beds with a variety of bioclasts including stromatoporoids and rudistids that are increasingly common upward. Breccias are interbedded with finely laminated, ripple-laminated, or micrograded bed alternating with burrowed mudstones. Dolomitized intervals are present at the base of unit 3 and within unit 4. (5) Massive beds of coarse rudist-fragment lime packstone. (6) (Top) Massive beds of rudist boundstone. Unit 1 is typical of basinal limestones of the upper Tamaulipas Formation. Units 2-4 represent basin-margin facies, the Tamabra Formation. Units 5 and 6 are characteristic of the reefal platform-margin Taminul facies of the El Abra Limestone.

This succession from pelagic basinal limestone to true reefs represents progradation of the eastern margin of the large (200 by 300 km) Valles-San Luis Potosi platform. Such progradational sequences are rare in the middle Cretaceous of east-central Mexico because the platform margins were steep (to 45°, locally near vertical) and relief was great (to 1,000 m). Although some faults may be present, the apparent thickness of the section (>1,500 m) is comparable to the total thickness of platform sections elsewhere.

Porosity is nil in the transitional sequence except for vugs and intercrystalline pores in the dolomite and small vugs in some of the debris beds. Clasts and particles within the basin-margin debris indicate diagenetic stages in the source area ranging from unconsolidated through lightly cemented (both submarine and subaerial) to leached and secondarily cemented.

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