DIAGENETIC EVOLUTION OF THE CENOZOIC FORMATIONS ASSOCIATED WITH THE CHICXULUB IMPACT CRATER, NORTHWESTERN YUCATAN PENINSULA, MEXICO

The northwestern Yucatan Peninsula, Mexico is a unique place for studying the control exerted by the large end-of-Cretaceous Chicxulub Impact Crater on sediment deposition and diagenesis during the Cenozoic. Together with glacio-eustatic sea-level fluctuations this crater has affected the distribution of facies and the type and intensity of diagenetic processes.

The facies succession inside the Chicxulub Sedimentary Basin, which roughly corresponds to the crater, shows an upward-shallowing trend, more gradual toward the center. Pelagic limestones with planktonic forams occur only within the basin, are in some places laminated and may contain marly intervals with pyrite. They may also contain chert bands or nodules. Dolomitized intervals separated by limestones are present inside the basin where the stable isotope trends in diagenetic calcites are also well defined statistically. Outside, the Cenozoic sequence is much thinner, has karstified intervals, and is represented mainly by dolomitized shallow-water facies. Dolomite (51 to 56 mole % CaCO3) is in general matrix and/or grain replacive, the texture changing from planar-e to planar-s and non-planar as matrix dolomitization progresses. d18O in dolomite varies between -5.54 and 0.87 ‰ PDB and d13C between -4.62 and 3.38 ‰ PDB; many of the most negative values come from samples that show features of subaerial exposure (subaerial crusts, vugs) whose frequency increases upward and outward from the center of the basin. In general, dolomite formation was rock-buffered with respect to d13C. The largest dolomite crystals line biomolds and vugs. Dedolomitization is suggested by the presence of hollow dolomite crystals and dolomite rhombs partially replaced by calcite. Prior to dolomitization, most of the initial aragonite and high-magnesium calcite was replaced by low-magnesium calcite in vadose and phreatic environments; meniscus and pendular cements and syntaxial overgrowths are present.

The petrographic, microprobe, XRD, and stable isotope data suggest that diagenetic processes including conversion to low-magnesium calcite and dolomitization were more extensive outside the Chicxulub Sedimentary Basin where more subaerial exposure also took place during the Cenozoic. The facies and stable isotope trends are much better defined inside the basin.