Mineralogy, geochemistry, and K-Ar dating of feldspars and clays from an exceptional Cretaceous fossil locality (Tlayua, Puebla, Mexico): Insights into the depositional and diagenetic ages and processes

The exceptional preservation of fossils in a small number of deposits is difficult to explain. The precise conditions for preserving soft-bodied animals or delicate invertebrate and vertebrate structures during their death, burial, and diagenesis must be examined case by case. We studied the Middle Member of the Tlayua Formation, Puebla, Mexico, a limestone lithostratigraphic unit that contains exceptionally preserved fauna, mainly fishes. Its diagenetic history has been investigated using clay mineralogy and geochemistry, volcanic and authigenic alkali feldspar characterization, and K-Ar dating of clays and feldspars. Our results indicate that the death and initial decay of the fossilized organisms was controlled by the repeated deposition of thin layers, probably volcani-clastic, interrupting the precipitation of limestones in a shallow marine environment associated with algal mats. These layers exhibit high concentrations of heavy metals (V, Cr, Ni, Cu, As, Mo) indicative of anoxia. The Tlayua Formation experienced a low degree of thermal diagenesis, as determined by the incipient smectite illitization and the presence of liptinite, but chemical diagenesis was observed as clay formation and authigenesis of feld-spars. Two generations of feldspars were identified and dated. The first is primary volcanic sanidine, whose K-Ar ages in mineral concentrates and K-Ar in situ on single crystals, gave the deposition age of the upper part of the Middle Member of the Tlayua Formation at-104 to-100 Ma (Late Albian-Lower Cenomanian) and a range of inherited volcanic crystals up to 118 Ma. The second feldspar, an authigenic adularia formed between 98 and 82 Ma, indicates the main stage of burial (Cenomanian to Campanian). Later incipient illitization was dated at 70-60 Ma (Maastrichtian-Danian), coincidental with the Mexican Orogen development and later exhumation.