First report of a Messinian coralgal facies in a terrigenous setting of Central Apennines (Italy) and its palaeogeographic significance

Abstract

The upper Miocene dominantly terrigenous succession cropping out at Monte San Fabrizio (Simbruini Mts.) shows peculiar, previously unreported bio‐sedimentological features, which bring fresh elements in a regional palaeogeographic/palaeotectonic key. Overlying the regional foredeep succession, an array of different lithofacies either form mappable outcrops or occur as clasts, remarkably not referable to any stratigraphic units already known in the Central Apennines. These lithologies, which include coral‐bearing siliciclastic sediments, mixed carbonate/siliciclastic deposits with benthic macrofaunas, rhodalgal facies, and conglomerates, essentially made of Mesozoic pelagic lithoclasts, are here grouped in a comprehensive informal unit, called Monte San Fabrizio Unit (MSFU). The most striking feature of the MSFU is a coralgal facies characterized by the occurrence of zooxanthellate (z) corals (Porites and Siderastrea crenulata). While the age of these deposits is not directly determinable due to the lack of biostratigraphic markers, their composition and field relationships clearly place their origin within a late‐orogenic setting: only the uplift, subaerial exposure, and erosion of large portions of the pre‐orogenic stratigraphy, also belonging to different geological domains, and subsequent sediment transport by rivers, can account for the heterogeneous composition of the conglomerates. Their sedimentological characters (i.e., shape and size of the clasts) suggest reworking of the pebbles in a shore environment. A shallow marine photic environment is confirmed by the presence of z‐corals, red algae, and abundant macrofauna. The occurrence of Porites and Siderastrea crenulata represents the first occurrence of corals subsequent to the drowning of the bryozoan limestone carbonate factory, being the youngest occurrence of corals in the Central Apennines and the first ever post‐Burdigalian z‐corals described in the region. This also constrains the age of the MSFU as early–middle (pre‐evaporitic) Messinian, due to the almost total demise of z‐corals known in the Mediterranean in response to the Messinian Salinity Crisis. In conclusion, sedimentology, palaeobiology, and field geology all point to a shallow‐water environment, close to a shoreline, for the MSFU. This unit can be interpreted as representing the vestiges of an ephemeral depositional environment, such as a small wedge‐top basin, an evidence that this portion of the Apennine chain was already structured in the early–middle Messinian.