| Summary: | 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 ...
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