Unusual mass-occurrence of small, uncoiled ammonites in a black shale of the Maiolica Formation in the Umbria-Marche Basin (Central Italy)

The unique mass-occurrence of tiny heteromorph ammonites found in a single layer in the Mt. Cipollara locality of Cerreto d'Esi (Maiolica Formation, Umbro-Marchean Basin, Italy) provides critical insights into the depositional environments of the Cretaceous upper Maiolica Formation. The mechanisms behind the formation and preservation of these ammonite assemblages within black shales remain poorly understood. To solve this knowledge gap, we constrained by means of biostratigraphy and stable Carbon isotopes the whole sections hosting the ammonites-rich layer. The latter was then subjected a high-resolution paleoecological analyses. Samples were collected systematically across multiple stratigraphic levels to ensure comprehensive coverage. The ammonite assemblages were documented, focusing on their morphology, abundance, and associated sedimentary structures. Additionally, sedimentological petrographic examinations were conducted to elucidate depositional processes. Our results reveal a rich assemblage dominated by the family Leptoceratoididae, exhibiting relatively good preservation within a predominantly dysoxic low-energy environment at the bottom. Calcareous nannofossils data suggest the presence of a well-stratified water column, with a low salinity water cap. The multidisciplinary analyses indicates that these black shales served not only as a repository for ammonite remains but also reflected localized paleoecological conditions characterized by reduced turbulence and increased organic deposition. This unique sedimentary context suggests that the deposition of these assemblages could have been influenced by both regional sea-level fluctuations and local hydrographic conditions. In conclusion, the study of the Mt. Cipollara heteromorph ammonites underscores the complexity of Cretaceous paleoenvironments and provides an enhanced understanding of the occurrence of black shales within the Maiolica Formation.