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Abstract:

Miocene sequences composed of bioclastic limestones rich in red algal concretions (rhodoliths) and bryozoans (Bryozoan and Lithothamnium Limestones- BLL) crop out in the central-southern Apennines (Italy). In general, these limestones document the evolution of an open carbonate shelf, the upper portion of which shows evidence of a drowning event. The facies consist of rhodalgal-type grain associations which typically are indicative of carbonate platforms developing in temperate seas or in subtropical to tropical areas that are characterized by anomalous (e.g., cooler, eutrophic, upwelling) water conditions. Most of the facies constituents were derived from organisms adapted to low-light intensities (sciaphile assemblages) in the cooler deep euphotic zone, whereas components derived from organisms living in the photic zone (photophile assemblages) and warmer waters are subordinate. These facies appear to be analogous to Modern bioclastic sediments that cover large sectors of the middle to outer shelf in the Mediterranean Sea, described by Pérès and Picard (1964) as Détritique Côtier biocenosis sediments. Within the Détritique Côtier deposits, the Faciès à Pralines (cf. Pérès and Picard, 1964) which is characterized by red algae concretions, match well with the rhodolith-rich Apennine Miocene facies. BLL skeletal-rich deposits were locally (and partially) stabilized to form complex agglomerates. These are analogous to present-day concretionary hard grounds dominated by encrusting red algae (Coralligène bottoms in Pérès and Picard, 1964).

The facies identified in this study were likely formed as a result of the upwelling of cold, nutrient-rich water on the outermost-shelf sectors which produced anomalous water conditions on the Apennine Miocene subtropical shelves. As a consequence, rhodalgal-type associations were favored and a "temperate-type" platform developed, where growth potential was relatively low. A significant Burdigalian trasgressive event resulted in increased water depths and exposure of the uppermost bioclastic level of the BLL to the sediment-water interface thereby creating a sheet of relict sediment. A further increase in water depth and enlargement or lateral shifting of the anoxic levels connected with an upwelling maximum resulted in exposure of the shelves to nutrient-rich and oxygen-poor waters and phosphatization of the sediments. The drowning event culminated in deposition of planktonic-rich sediments that were mixed with the relict neritic bioclasts resulting in a complex basal depositional interval (palimpsest interval sensu Swift et al., 1971) that pass upwards into hemipelagic globigerinid-rich wackestones).

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