Upper Cretaceous-Lower Eocene succession of the Monte Postale and its relationship with the “Pesciara di Bolca” (Lessini Mountains, northern Italy): deposition of a fossil-fish lagerstätte

Abstract

The Lower Eocene (Cuisian) deposits of Pesciara di Bolca in northern Italy represent one of the most important Konservat-Lagerstatte in the world, well known since the 16th century. Over a period of around 500 years, numerous high-quality fossils have been found in the Pesciara and nearby olistoliths. In order to explain the exceptional nature of these finds, the relationship between the Pesciara and the nearby Monte Postale successions are considered in terms of their facies and depositional environments. The sites are approximately 300 m apart, but the presence of volcanics, the local tectonics, and the dense vegetation make it difficult to correlate between the two. The complete M. Postale succession has been studied from the Maastrichtian Scaglia Rossa basinal lime mudstones to the Cuisian bioclastic grainstones. Volcanic activity that began in the Late Paleocene provided topographic highs that allowed the development of Ilerdian (Spilecco Limestone) and Cuisian (Nummulite Limestone) platforms carbonates. The Ilerdian platforms were successively dismantled. The Cuisian platforms included the Pesciara olistolith and the main part of the M. Postale succession. Within the latter, three lithofacies were identified: massive bioclastic packstone (lithofacies 1), bedded bioclastic packstone (lithofacies 2), and laminated lime mudstone with fish and plants (lithofacies 3). Lithofacies 1 is absent in the nearby Pesciara olistolith, whereas lithofacies 2 and 3 are interdigitated in different arrangements in the Pesciara and M. Postale. The palaeoenvironmental model envisaged for the Pesciara-M. Postale system consists of a coastal lagoon in a subtropical zone, which is bordered by an emergent area (most likely represented by a volcanic archipelago surrounded by a rather deep basin), and a carbonate buildup, in which larger benthic foraminifera played a fundamental part. Brief anoxic episodes occurred within the more distal back-reef areas, close to the center of the lagoon, which enabled the preservation of the fossil fish.