Mixed-layer illite-smectite in low-temperature diagenesis: data from the Miocene of the Carpathian Foredeep

Bulk mineralogy, clay fraction mineralogy and layer ratio in interstratified illite-smectite were studied in three Upper Miocene profiles, up to 200 m deep and representing very low sedimentation rates (∼80 mm/103 yr) with environments of sedimentation ranging from hypersaline (gypsum deposits) to brackish. No correlation was found between mineralogy and depth, or between mineralogy and chemical environment of deposition, despite the use of precise identification techniques. It is concluded that smectites and interstratified illite-smectites are stable in marine sediments until the elevated temperatures of deep diagenesis are reached.