Análisis sedimentológico-facial de clastos formadores de brechas calcáreas cretácicas de una zona transicional plataforma–cuenca en el centro de México
A sedimentological facies analysis of the carbonate breccias deposited in a transitional platform to basin setting was conducted in the central part of San Luis Potosí, Mexico. The goal was to determine the origin and provenance of breccia components, mechanisms of deposition and the final depositional environment in order to better understand the depositional development of the Valles – San Luis Potosí Platform and western slope to the Central Mexico Mesozoic Basin during the mid-Cretaceous.
The sedimentological characteristics of these breccias include planar basal contacts with evidence of erosion, transitional upper contacts, thicknesses from 0.5 to 14 m, lenticular to massive bedding with local normal grading, polymictic and clast sizes up to blocks of 1 m to 2 m. Clast morphology includes tabular clasts ranging from angular to subrounded with low sphericity to rounded with high sphericity. Sorting is poor to moderate. Clast orientation is random with local imbrication indicating W-NW transport. Matrix is micritic or sparitic and locally argillaceous. Textures of these carbonate breccias range from floatstone to rudstone with deep water clasts, bioclasts and resedimented shallow water clasts.
Two types of breccias record different sedimentation processes. The first type is represented by breccias with tabular, angular clasts, which range from spiculite wackestone to mudstone or wackestone with pelagic fauna and millimeter lamination. These indicate pelagic sedimentation in deep water, in a basin, deep shelf and even toe-of- slope. The second is represented by subrounded clasts of resedimented material, transported by gravitational and suspension processes from the margin of the platform and back-reef zones. Noteworthy components are bioclasts, algae, sponges, corals, rudists, brachiopods and foraminifers (orbitolinids and miliolids). Resedimented clasts include. packstone with coated grains, bioclasts and pellets, ooid grainstones and grainstone or packstone with abundant benthic foraminifers.
The lithologic, morphologic and textural characteristics of the breccias indicate that the transitional zone between the Valles-San Luis Potosí Platform and the Central Mexico Mesozoic Basin was an abrupt margin. This is suggested by the coexistence of deep-water and platform components. The breccias contain reef and back-reef material transported through channels or collapse of the unstable margin of the platform, generating debris flows and turbidity currents. Microfacies analysis indicates that the breccias with less developed textural maturity (tabular, angular and poorly sorted clasts) show more transport, than breccias with rounded and moderately sorted clasts. Important variations in the presence of rudist bioclasts suggest a discontinuous reefal rim along the western margin of the platform.