PETROGRAPHIC AND ISOTOPIC INVESTIGATION OF DEPOSITIONAL ENVIRONMENTS IN THE EARLY CRETACEOUS (APTIAN-ALBIAN) TLAYÚA QUARRY, MEXICO

The Tlayúa Quarry in central Mexico has yielded a tremendous diversity of Early Cretaceous fossils that include well preserved marine, freshwater, and terrestrial fossils including vertebrates and plants remains (Applegate et al. 2006). Detailed petrographic and isotopic studies of these units contribute to our understanding of the paleoenvironments of these deposits. Our results suggest that these deposits, consistent with the fossil assemblages, ranged from open marine to coastal freshwater dominated lagoons.

Tlayúa Quarry deposits include foraminifera and gastropod packstones, wackestones, fenestrate wackestones and mudstones. Bird's-eye fabric and locally abundant rip-up clasts of laminated material (e.g. cyanobacterial mats) suggest deposition in tidal flat environments. Limestone layers are commonly separated by a thin residue of red clay and rarely by centimeter-scale red silty claystone, on which evidence for exposure is preserved in the form of raindrop impressions and mud cracks. Hypersaline conditions are suggested by calcite-filled gypsum molds that grew displacively within the sediments

Four trends can be distinguished in the isotopic data. Meteoric calcite lines defined by variable δ¹³C and invariable δ¹⁸O trends are present in numerous horizons and indicate freshwater deposition or early meteoric diagenesis. Enriched δ¹⁸O values associated with petrographic evidence of supratidal deposition indicate evaporative conditions. Many horizons show positive covariance between δ¹³C and δ¹⁸O indicating mixing between fresh and marine water. Calcite precipitation from tectonic fluids resulted in highly negative δ¹³C and δ¹⁸O values of late calcite spars, and are pervasive in some horizons.