Decline and recovery of the Aptian carbonate factory in the Southern Apennine Carbonate Shelves (southern Italy): Climatic/oceanographic vs. local tectonic controls

The Cretaceous peri-Tethyan carbonate systems record alternating phases of vigourous development and rapid decay of the shallow-water carbonate factories. After reaching their maximum palaeogeographic extent during the early Bedoulian time, the carbonate factories underwent unhealt episodes in relation to well-known crisis events (OAE1). These events resulted in drowning episodes, as experienced in the northern Tethyan carbonate platforms, or in signals of water stress in the carbonate factories, evidenced by drastic changes in the floro-faunistic assemblages, as experienced in the carbonate platforms of the central-southern Tethyan belt. In interpreting the inception and decay of different carbonate depositional systems, climatic-oceanographic variations together with sea-level oscillations are commonly considered key elements. However, local tectonic controls cannot be ruled out. In attempts to discriminate among the main factors controlling the evolution of the southern Apennine mid-Cretaceous carbonate system, detailed analyses have been performed on Aptian-Albiano carbonate successions in the Matese Group (southern Apennines). Since the middle Aptian, the analysed successions suggest a scenario characterised by a complex, tectonically driven topography, replacing the previous large tropical shallow-water domain. From the Bedulian-Gargasian transition onwards, the analysed area evolved into subdomains characterised by diversified sedimentological trends. A tectonic control influenced the Gargasian evolution of the considered area with local evolution into small intraplatform basins bordered by channelised margins. Nevertheless, since the latest Bedulian onward, the studied successions register palaeoecological signals that cannot be linked exclusively with tectonic disturbance episodes because of the coeval appearance of analogous signals at a global scale, including a significant shift in the biological assemblages and an outbreak of organisms indicative of stress conditions in the water mass. The analysed Gargasian strata show impoverished biota: caprinids totally disappear, both as in situ and storm-related layer components, and hermatypic corals are drastically reduced. Muddy lithofacies prevail in intertidal metric cycles in which cyanobacterial consortia, both in the form of dense laminae and coalescent oncoids, orbitolinids and small gastropods (cerithiids) suggest restricted, nutrient-rich water. Large oncoids of Bacinella irregularis/Lithocodium aggregatus and mollusk (mostly oyster and gastropod) shell fragments significantly contribute to storm-related coarse skeletal intercalations, in which oligotrophic condition-adapted forms are reduced or absent. This pattern suggests generalised conditions of stress in the water mass and in more marginal open areas. Following the mid-Aptian unproductive episode, characterised by the flourishing of assemblages adapted to mesotrophic-eutrophic conditions, the studied southern Apennines shallow-water domain changed again, with a progressive reduction of the previous mainly aragonite-dominated chlorozoan assemblages and an increase of calcite-dominated skeletal components, including rudists with calcitic outer shell layers. Pioneer biota (e.g., cyanobacteria and polychetes) characteristically marked the first phases of recovery in the still-deteriorated shallow-water domains, rapidly evolving into more complex and differentiated assemblages. The outbreak of nerineid and acteonid gastropods seems to be related to a flourishing of cyanobacterial mats and related microphytae and also to the presence of polychetes. Moreover, the grazing activity of the nerineids favoured the flourishing of oysters and oyster-like condrodonta. Among the rudists, the persistence/radiation of rudist species adapted to a wide range of temperatures and that adopted functional strategies that allowed them to thrive in unstable, open, unsuitable sea bottoms such as Requieniidae and Monopleuridae as well as the first elevator Radiolitidae; this suggests some kind of oceanographic change (e.g., seawater chemistry and/or temperature changes). To date, the rudist ecological constraints have yet to be defined. Nevertheless, on the basis of the associated benthic floro-faunistic assemblage, we can infer the complex environmental parameters in which mesotrophic/eutrophic conditions, presumably coupled with cooler and locally oligophotic conditions, cooperatively modified carbonate factory characteristics.