NUCLEATION PROCESSES IN HIGH-RELIEF CARBONATE PLATFORMS: SEDIMENTOLOGIC CHARACTERIZATION, FACIES COMPOSITION AND DEPOSITIONAL EVOLUTION OF THE ESINO LIMESTONE (LADINIAN-LOWER CARNIAN) IN LOMBARDY

High-relief carbonate platforms evolve from a poorly known nucleation stage that has different environmental conditions and depositional processes with respect to the mature stage of the platform. To understand the processes that lead to the development of a high-relief carbonate platform, it is crucial to analyze facies type and distribution during the early phase of their development. The Esino Limestone (up to 800m thick, with a final platform-to-basin relief of 550 m) preserves exposed the inception stage deposits. In its mature configuration the carbonate system was characterized by a wide inner platform (bedded peritidal limestones with stromatolites, oncoids, fenestrae and dasycladaceans), bordered by a narrow reef (Tubiphytes and microbial mounds associated with coral framestones with calcisponges and intrabioclastic packstone) that sourced the breccias of the steep slopes. 20 stratigraphic sections have been measured and about 600 samples for microfacies analysis have been collected in the 30-130 meters-thick succession of the nucleation stage. Facies type and distribution led to the stratigraphic reconstruction and to the identification of major phases in the early platform evolution, which are linked with relative sea-level changes. A first transgression, following the drowning of the antecedent peritidal platform (Camorelli Limestone), is characterized by the deposition of dark bioturbated bio-intraclastic packstone and wackestone passing to bedded black marly limestones and marls (Prezzo Limestone). A general regressive trend is characterized by intraclastic packstones and decreased terrigenous input. Sparse reefal bafflestones (porifera, algae, corals, Tubiphytes) and peritidal facies (stromatolitic bindstone with fenestrae, early marine cements, intrabioclastic packstone) record the final onset of the core of the future high relief carbonate platform. The end of the inception stage is thus controlled by the progradation of sparse patch reefs that fill up the depositional space, leading to the coalescence of isolated small nuclei of carbonate production. This study documented the key role played by antecedent topography and sea-level changes in the localization and growth of the first platform nuclei. Furthermore, environmental conditions (e.g. terrigenous input, water circulation) possibly exerted a control on the organisms’ associations that characterized the first stages of platform evolution.