Features of Authigenic Mineralization and Geochemical Processes of Fossilization of Marine Reptile Vertebrae (Oxford-Kimmeridge, Republic of Tatarstan)

Studies were conducted on the sequence and conditions of authigenic mineral formation in fossil bone tissue, as well as the chemical composition and geochemical characteristics of fossilization processes in marine reptile vertebrae discovered in undivided Upper Jurassic clay deposits on the right bank of the Volga River (Tetyushsky District, Republic of Tatarstan). Using a combination of optical microscopy, electron microscopy, and spectral methods, the following set of authigenic minerals composing the fossilized remains was identified: fluor-hydroxyapatite, pyrite, barite, gypsum, anhydrite, siderite, calcite, hematite, goethite, lepidocrocite, quartz, and albite. The morphology, structural-anatomical features, mineral composition, and trace element content of the fossilized vertebrae indicate that sedimentation occurred under anoxic conditions on the floor of the Middle Russian Sea. Mineralization mechanisms included both biogenic (activity of sulfate-reducing bacteria) and chemogenic processes (seep-related activity), with the latter predominating. The sequence of mineral formation stages encompassed early and late diagenesis, catagenesis, and epigenesis (hypergenesis zone). Positive Ce anomalies (1.25–1.42) suggest a coastal-marine sedimentary environment in the Middle Russian Sea. The determined concentrations of rare earth elements and uranium are highly promising for further evaluation studies of bone-bearing deposits in the Oxfordian-Kimmeridgian clay formation.