Recognition of primary and secondary Miocene lucinid deposits in the Apennine chain.

The examination of about eighty exposures of lucinid deposits of the Apennine chain based on new field analyses and previous data, and the compositional study of one hundred samples representative of the different geological settings in which lucinid assemblages are found, enable us to distinguish four types of deposits. Type 1 and type 2 are primary (autochthonous) deposits whereas type 3 and type 4 are secondary (allochthonous) deposits. Type 1 deposits consist of marly-calcareous and calcarenitic lenses or columnar bodies, jammed with articulated lucinid-like shells, usually belonging to oligotypic communities and grading into the surrounding pelitic sediments. Most of the samples are made of homogeneous micrites, biomiocrites and fossiliferous micrites, containing variable amounts of biogenic particles, chiefly planktonic foraminifera. Type 2 deposits are made up of lucinid shells, either isolated or associated with a more diverse fauna, in mudstones, marls, arenitic and calcareous marls. Type 3 deposits are constituted of carbonate olistoliths enclosed as blocks within chaotic horizons in turbiditic and hemipelagic formations of the Miocene foredeep. Type 4 deposits are represented by coquina debris, isolated articulated or disarticulated shells in resedimented arenites, calcarenites, carbonate breccias with biogenic debris. Samples of secondary deposits show a noticeable variety of lithologies and textures and are very heterogeneous even at the microscopic scale. The most common lithotypes include fine to medium grained arenites characterized by a loosely detrital fabric and by a pervasive micrite cement. Fossiliferous micrites and biomicrites with brecciated textures are common. The detrital framework show striking similarities to the composition of the host turbidites. Carbonate intraclasts and clasts deriving from lithotypes of the ligurian domain are present. Most of the type 1 and 3 deposits are strongly 13C depleted and are interpreted as chemosynthetic communities (chemoherms), with authigenic carbonate deposition related to methane-rich fluid venting. Type 1 deposits occur only in thick pelitic sediments belonging to the satellite (Termina Formation of the epiligurian sequence) and to the foredeep basins (Vicchio and Letto Marls). Type 2 deposits are instead related to normal marine conditions, whereas the type 4 could have their source from all the other types of deposits. Secondary lucinid deposits occur in a wide range of different tectonic setting, from backland to foreland, with a concentration in the middle Miocene foredeep. The secondary lucinid deposits of the foredeep turbidites evidenced several different grades of allochthony and multiple provenances. Lucinid deposits in the Marnoso-arenacea turbidites show evidence of a moderate reworking, probably from the enclosed pelitic horizons or from adjacent slope pelites. The source of the oldest chemoherms seems to be represented by the Vicchio Marls, whereas isolated shells and coquina debris in resedimented beds of the minor basins and Cervarola foredeep probably originate from satellite basins or from methane-derived biological communities located in fine-grained horizons no longer preserved. The peculiar brecciated structures observed in many lucinid chemoherms are probably the result of a number of superimposed processes, involving fluidization of sediments and reworking for gravity mass transport. Mud diapirism and/or injection, related to overpressuring of sediments due to methanogenic fluid venting, could account for the formation of the brecciated structures resembling those related to gravity flow processes. Fluidization of sediments may also represent an important mechanism able to increase the instability of pelitic marly sediments, thus favouring gravity slumping processes.