Part 1. Physical Stratigraphy and Petrology of the Cretaceous Sierra Madre Limestone, West-Central Chiapas

Abstract

A 2,575 m thick composite stratigraphic section of the Sierra Madre Limestone was measured southwest of Ocozocuautla, Chiapas. The formation is believed to be Neocomian to Santonian in age, although biostratigraphic control is poor. Eight major lithofacies were identified, representing four periods of carbonate platform deposition interrupted by three major marine inundations. The lithofacies occur in predictable vertical stratigraphic sequences, interpretable as facies tracts of analogous modern environments. The lithofacies represent deposition in hypersaline evaporite platform interior, carbonate platform interior, open interior lagoon, restricted interior lagoon, tidal mudflat, ooid sand shoal, open marine, and basinal open marine environments.

The dolomite and collapse breccia lithofacies represents deposition in a hypersaline platform interior environment. Collapse breccias indicate the former presence of evaporites. Petrographic evidence and field relationships indicate two primary generations of dolomite. Finely crystalline dolomite associated with the former evaporites is interpreted as penecontemporaneous. Coarsely crystalline sucrosic dolomite, exhibiting anhedral crystalline mosaics and zones of euhedral rhombs, is interpreted as the result of pervasive deep burial dolomitization. The unit is believed to conformably overlie the San Ricardo Formation, although stratigraphic and faunal evidence is inconclusive.

The lime mudstone, pellet, miliolid lime wackestone, and requieniid lime wackestone lithofacies were deposited in a quiet, shallow, low-energy platform interior environment. The environment was laterally equivalent to the hypersaline platform interior environment and was also reestablished following periods of rapid platform progradation after marine inundations.

The ooid and abraded skeletal fragment grainstone lithofacies was deposited in an ooid sand shoal environment, marginal to the prograding platform edge. The ooids are small (less than 0.2 mm mean diameter) and exhibit radial rather than concentric layering. The structure implies a relatively low-energy depositional environment for ooid formation, possibly as a result of diminished wave and current energy on the shallow open shelf. The shelf slope between the platform and the open marine environment was very gentle. A platform margin was established southwest of Ocozocuautla only during periods of tectonic instability or maximum global eustatic rises in sea level when the precipitous platform margin in the Reforma area was submerged.

Lime mudstones, requieniid wackestones, and thinly laminated dolomitic lime mudstones were deposited in a low-energy sublittoral and tidal flat environment directly leeward of the ooid sand shoal complex. Desiccation features and birdseye structures are common. The rapid lithologic change reflects a rapid energy change resulting from early lithification in the ooid sand shoal complex, which formed an effective, although discontinuous, barrier to open marine circulation.

The whole to fragmented radiolitid lime packstone to wackestone and pellet-intraclast lime packstone to wackestone lithofacies were deposited in a broad, gently circulated interior lagoon environment seaward of the platform interior environment and leeward of the ooid sand shoal complex and tidal mudflat environments. Planktonic foraminifers and echinoids are characteristic of the lithofacies in areas of the lagoon with more direct contact with open marine circulation and normal marine salinity. Better circulation and salinity probably resulted from lateral discontinuities in the ooid sand shoal barrier. The restricted lagoon environment was characterized by the absence of planktonic foraminifers and echinoids. Thinly laminated lime mudstones and stromatolites were deposited on supratidal exposures which restricted circulation in parts of the lagoon. Requieniid rudist lime wackestones are common in the restricted areas.

The fossiliferous marl and nodular mollusk lime wackestone lithofacies was deposited in the open marine shelf environment seaward of the ooid sand shoal environment. Planktonic foraminifers, echinoids, calcispheres, and sponge spicules are common. An open marine shelf environment developed when the carbonate platform submerged as a result of rapid rises in global sea level or rapid subsidence associated with tectonism and the influx of terrigenous clay. The carbonate platform was inundated during the mid-Cenomanian, late Cenomanian, and Coniacian. The maximum inundation occurred during the mid-Cenomanian when the planktonic foraminifer lime mudstone to wackestone lithofacies was deposited in a basinal open marine environment.