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 Tlayúa 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 volcaniclastic, 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 Tlayúa 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 feldspars. 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 Tlayúa 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.

少数矿床中化石的特殊保存很难解释。必须逐案检查在其死亡、埋葬和成岩过程中保存软体动物或脆弱的无脊椎动物和脊椎动物结构的确切条件。我们研究了墨西哥普埃布拉的 Tlayúa 组的中段，这是一个石灰岩岩石地层单元，其中包含保存完好的动物群，主要是鱼类。其成岩历史已通过粘土矿物学和地球化学、火山和自生碱长石表征以及粘土和长石的 K-Ar 测年进行了研究。我们的研究结果表明，化石生物的死亡和最初的腐烂是由薄层的反复沉积控制的，可能是火山碎屑，中断与藻垫相关的浅海环境中石灰石的沉淀。这些层表现出高浓度的重金属（V、Cr、Ni、Cu、As、Mo），表明缺氧。Tlayúa 组经历了低程度的热成岩作用，这取决于早期的蒙脱石伊利石化和锂辉石的存在，但观察到化学成岩作用是粘土形成和长石的自生作用。鉴定了两代长石并确定了日期。第一个是原生火山sanidine，其矿物精矿中的K-Ar年龄和单晶上的原位K-Ar年龄，给出了Tlayúa组中段上部的沉积年龄在~104至~100 Ma（晚期Albian-Lower Cenomanian）和一系列高达 118 Ma 的继承火山晶体。第二长石，98 至 82 Ma 之间形成的自生 adularia，表明埋葬的主要阶段（Cenomanian 到 Campanian）。后来的初期非法化发生在 70-60 Ma (Maastrichtian-Danian)，恰逢墨西哥造山带的发育和后来的挖掘。