DISCONFORMITY-CONTROLLED DIAGENESIS IN EL ABRA CARBONATE PLATFORM (MID-CRETACEOUS), MEXICO

Paleosols, micro-karst surfaces, laminated caliche crusts, secondary porosity, blackened clasts, rhizoliths, desiccation and wetting cracks, and solution-collapse breccias record subaerial disconformities in the mid-Cretaceous El Abra Formation. Surficial diagenesis associated with intraformational disconformities produced solution-enlarged primary porosity, molds, and vugs, with deposition of crystalline internal sediment followed by precipitation of early equant cement (EEC) in the inner margin. Contemporaneous marine-phreatic diagenesis resulted in micritization and precipitation of isopachous radiaxial fibrous cements (RFC-1) in the outer margin. Crosscutting relationships and cement stratigraphy document informational origin of EEC, with cyclic cementation beneath successive disconformities. Salinities of the precipitating fluids ranged from freshwater to evaporated seawater, including seawater and various mixtures. A pervasive dissolution event, probably in a regional meteoric aquifer preceding the Turonian transgression, created voluminous porosity, reduced by a second generation of radiaxial fibrous cement (RFC-2) in the inner margin and by RFC-2 and pelagic internal sediment (PIS) in the outer margin. Major pore occlusion in both the inner- and outer-margin rocks was by late equant cement (LEC) during shallow burial. Four pervasive, distinctive cathodoluminescence zones in LEC indicate approximate synchronicity of cementation throughout the platform margin. Fluid-inclusion data from LEC indicate precipitation from freshwater at temperature less than 50°C. Freshwater may have infiltrated through clastic wedges developed at the front of the rising Sierra Madre Oriental during the Late Cretaceous and passed through fractured (?) basinal rocks and porous platform limestones, to exit through the elevated rim of the platform. Deeper burial and the Laramide orogeny produced only minor fractures, fracture-filling cement, and stylolites. Disconformities are more numerous and better developed within inner-margin skeletal rudstone and tidal deposits than among the outer-margin reefs and sands, indicating that the inner margin had the highest elevation on the platform, probably as rubble islands and tidal flats.