Sedimentation and diagenesis along open and island-protected windward carbonate platform margins of the Cretaceous El Abra Formation, Mexico

Abstract

The windward margin of the mid-Cretaceous Valles-San Luis Potosi carbonate platform in northeastern Mexico included open and island-protected segments. Depositional environments and diagenesis vary markedly with margin type. Sand shoals near the windward open margin are composed of oncoid-bioclastic and cross-laminated carbonates. Increasingly restricted and finer lagoonal and tidal-flat environments occurred bankwards, recording gradually decreasing wave and current energy. Lithofacies include peloid-miliolid, cryptalgal laminite, lime mudstone, and molluscan carbonates. Islands along the windward margin are composed of rudistid-skeletal debris from adjacent reefs. Lagoonal to tidal-flat sediments were deposited bankwards. Similar lithofacies occur as in these environments along the open margin but they are muddier and contain less diverse fauna. The different energy regimes along the margin influenced the distribution and packaging of banktop sediments. The bankward transition to low-energy, restricted environments was gradual across the open margin. In contrast, muddy sediments with restricted fauna accumulated in close proximity to the island-protected margin. Non-cyclic vertical lithofacies successions characterized the open platform margin, whereas asymmetric shoaling-upward sequences characterized the island-protected margin. Early diagenesis along the open margin was minor; burial diagenesis was of major importance. Thin rinds of marine cement are widespread but meteoric diagenesis was minor. Burial promoted extensive compaction. Mg-rich connate brines expressed from Guaxcama gypsum resulted in dolomitization and lithification, thereby precluding further compaction. Pore fluids resulting from dehydration of Guaxcama gypsum to anhydrite yielded pore-filling and replacement anhydrite in the El Abra Formation. Burial and Laramide deformation (Maastrichtian-Paleocene) resulted in stylolitization and extensive fracturing. Uplift produced widespread meteoric diagenesis, for the first time resulting in dissolution of anhydrite, calcitization of dolomite, and equant calcite cementation. Early diagenesis along the windward protected margin was extensive. Prolonged subaerial exposure resulted in calcrete cementation and secondary porosity. Subsequent marine cementation and internal sedimentation reduced porosity. Equant calcite cement precipitated during local Upper Cretaceous exposure. Burial diagenesis was limited to traces of dolomite, anhydrite and late-stage, equant calcite cement, suggesting similar pore fluids as in the open margin. Depositional environment determined the type and degree of early diagenesis, which strongly influenced subsequent diagenetic events. Subtidal sediments of the open margin underwent minimal early diagenesis and entered the burial realm much as they were deposited. They were subjected to intensive compaction and alteration by brines. In contrast, sediments exposed to meteoric diagenesis and marine cementation on near-backreef islands and tidal flats, entered the mesogenetic realm as stabilized, lithified rocks; much as they are at present.