| dc.description.abstract | El Abra Formation was cyclically deposited on an isolated, steep-sided, high-relief carbonate platform in the Tampico-Misantla basin during the mid-Cretaceous. Sea-level oscillations recorded by shallowing-upward cycles, truncated cycles, and rhythms were of low amplitude (<10 m) and high frequency, possibly modulated by sub-Milankovitch (< 12 ka) cycles. The El Abra platform missed some sea level oscillations above the platform resulting in relatively thick subtidal units. Paleosols, karst surfaces, laminated caliche crusts, voluminous solution porosity, blackened clasts, rhizoliths, desiccation and wetting cracks, and solution-collapse breccia record subaerial disconformities in the inner platform margin. Meteoric diagenesis associated with intraformational disconformities (D-1) produced molds, vugs, and solution-enlarged fenestrae, followed by the deposition of crystalline internal sediment (CIS) and precipitation of early equant cement (EEC). Crosscutting relationships and cement stratigraphy document intraformational origin of EEC, with cyclic cementation beneath most successive disconformities. Fluids varying in salinity from freshwater, hypersaline water, normal seawater and mixed seawaters precipitated EEC in the inner margin. In the outer margin contemporaneous marine-phreatic diagenesis precipitated meniscate micritic cements and isopachous radiaxial fibrous cements (RFC-1). 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. A major dissolution event (D-2) in a regional meteoric aquifer, perhaps in the late Cenomanian, created voluminous megaporosity, reduced by a second generation of marine cement (RFC-2) in the inner margin and by pelagic internal sediments (PIS) and RFC-2 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. An extensive diagenetic history, most of it near-surface, resulted in 19.6% extant porosity and permeability as much as 553 md in the outer-platform-margin rocks of El Abra Formation. Inner-margin rocks were not so blessed, but retain 9.3% porosity. Microporosity of inner-margin rocks averages 3.3% with permeability of 0.04 md. Outer-margin rocks likewise retain 3.6% microporosity and permeability of 0.03 md. Mesoporosity, visible in thin section, is negligible for both settings. This range of values underscores the need to measure porosity and permeability in vuggy carbonate rocks over the widest range of scales feasible, more than six orders of magnitude in this study. It also clarifies that, although matrix porosity is very low, large vugs are interconnected, at least locally. Various diagenetic stages affected porosity evolution on different scales and different magnitudes. In order of increasing volume in the inner margin: radiaxial fibrous cement-2, early equant cement, crystalline internal sediment, and late equant cement reduced the reservoir quality. Late, near-surface dissolution produced considerably more pore space than intraformational dissolution episodes beneath successive disconformities. Intraformational diagenesis beneath disconformities compartmentalized the inner margin by reducing porosity to half of its surface values in the first 50 cm, followed by a dramatic increase to a depth of 2 m. In the outer margin, in order of increasing volume: crystalline internal sediment, radiaxial fibrous cement-2, radiaxial fibrous cement-1, and late equant cement reduced reservoir quality. The late dissolution event, which pervaded the entire platform margin, produced virtually all of the extant porosity in the outer margin, much of it large scale and locally interconnected. The outer margin is thus the optimum zone for reservoir quality in a heavily cemented shallow-water peritidal carbonate system. | |
