Lucinid deposits from different geological settings of the northern Apennines.

Sixty exposures of lucinid deposits ("calcari a Lucina" of the literature) in different depositional and tectonic settings of the northern Apeninnes, from backland (satellite and minor basins) to foredeep (turbidites and closure deposits) and foreland (Sicily), were studied and, in many cases, reinterpreted in the field. Field and compositional data show that lucinid deposits include not only peculiar carbonate blocks, but a variety of lithologies and fossil assemblages: micritic limestones, marly limestones, mudstones, marls and arenitic marls, fine to coarse-grained arenites, fine to coarse-grained calcarenites, carbonate monomictic and polymictic breccias and microconglomerates. The fossil content is not only adscribable to the genus Lucina, but more generally to lucinids or other bivalves. Lithological and compositional characters are mainly controlled by the depositional and tectonic setting. Lucinid deposits from satellite basins and foredeep slope sediments show evidences of primary deposition while those included in foredeep turbidite deposits are clearly reworked. In some cases only isolated shells are reworked. Primary lucinid deposits occur as: - type 1) marly-calcareous and calcarenitic lenses or column-like bodies, crowded with articulated specimens usually representing the dominant fauna of an oligotypic community; these lenses are often clustered but not related to a precise stratigraphic level and they gradually pass to the host sediments (marly mudstones); - type 2) isolated specimens or associated with a dense high diversity fauna in mudstones, marls, arenitic and calcareous marls. Secondary lucinid deposits occur as: - type 3) carbonate olistoliths crowded with articulated or disarticulated specimens associated with an oligotypic fauna; they are usually enclosed within chaotic horizons made up of intraformational slumps, ligurian-subligurian olistostromes, ligurian-epiligurian olistoliths, resedimented arenitic beds and polymictic breccias, intercalated in turbiditic formations; - type 4) coquina debris, isolated articulated or disarticulated shells, (in few cases associated with abundant fauna from shelf areas) in resedimented arenites, calcarenites, carbonate breccias with biogenic debris (debrites). Most of the type-1 and type-3 deposits are strongly 13C depleted, and are interpreted as chemosynthetic communities (pseudobioherms) with authigenic carbonate deposition related to methane-rich fluid vents.Petrographic data show that the composition of the siliciclastic fraction present in lucinid deposits is consistent with the composition of the host deposits. Lucinid deposits included in the foredeep turbidites of the Marnoso-arenacea and Cervarola Formations display a wide variety of textures and lithologies. Most of the samples are characterized by a sort of "chaotic" texture with an abundant terrigenous fraction associated with micritic intraclasts and planctonic foraminifera. The interstitial fraction is made of micrite, but microspatic to spatitic calcite cements and patchy poikilotopic calcite are also present. The composition of the terrigenous fraction shows a great similarity with the composition of the including turbidites, thus indicating only an intrabasinal reworking of lucinid deposits. In primary, non reworked lucinid deposits of the epiligurian (Termina Formation) and slope sediments (Vicchio and Letto Marls) the terrigenous fraction is absent or very scarse and fine-grained. Samples show more homogeneous textures and lithologies, mainly represented by micritic to biomicritic limestones; planctonic foraminifera are abundant. Detrital carbonate clasts of ligurian affinity are sometimes present in epiligurian lucinid outcrops.According to this study, primary pseudobioherms occur only in epiligurian and foredeep closure pelitic sediments, rich in organic matter. Lucinid deposits in the Marnoso-arenacea and Cervarola foredeep turbidites show evidence of a moderate reworking probably from adjacent slope pelites. Reworking processes are then related to the advancement of the deformational front, as indicated by slumps and olistostromes. Only lucinid deposits located at the top of the thrust belt front (satellite basins) or in mudstones marking the closure stage of the foredeep could have the possibility to be preserved as primary deposits, whereas most of the slope sediments, which represent the main source of secondary lucinid deposits, is reworked and resedimented in the turbiditic units. This is due to foredeep instability phenomena, probably heralding the tectonic events. On the other hand the cold seepage could have lowered the sediment shear strenght and opened the way to foredeep instability which in turn could let the gases out more easily.We hypothesize that the source of the oldest pseudobioherms is represented by the Vicchio Marls, whereas isolated shells and coquina debris in resedimented beds of the minor basins and of the problematic Cervarola-Falterona Formation of the Modena-Bologna Apennine, are probably originated from satellite basins or from faunal communities (methane related ?) located in pelitic horizons no more preserved. Confirming this hypothesis, two pelitic horizons bearing methanogenic lucinid blocks are still preserved in the inner Marnoso-arenacea.A comparison between lucinid concentration peaks (Serravallian-Tortonian and Tortonian-Messinian boundaries) and the neogenic tectonic events of the Apenninic chain do not show any particular relation. No direct connection between fracture planes and the irregular disposition of the non reworked lucinid deposits seems to be evident. Thus further studies are necessary in order to better understand the relations between the advancement of the thrust belt front and fluid escape. Isotopic, sedimentological, stratigraphycal and structural data seem to support that biogenic methane is likely to have been the main source for primary pseudobioherms and related carbonate deposits. The gas could have a shallow origin in local pockets from biodegradation of trapped organic matter. Lucinid pseudiobioherms grew in different open marine environment, from outer shelf to deep basins and do not characterize a precise paleogeographic or paleotectonic setting.Petrographic data also suggest to correlate the Castel Guerrino unit to the Marnoso-arenacea Formation and to interpret the middle Miocene Cervarola Formation of the Modena-Bologna Apennine as a minor foredeep basin (internal part of the Marnoso-arenacea Formation ?).