Cold vent processes in the Miocene Foredeep – thrust belt system of the Northern Apennines

Micritic limestones locally called "calcari a Lucina" packed with dense large bivalves and characterized by autoclastic breccias and fluid-flow conduits are present in middle-late Miocene sediments of the northern Apennines. After many years of geologic controversies, these limestones have been reinterpreted as cold vent deposits, thus becoming special indicators of clathrate dissociation and revealing important relationships between methane seepages, sediment instability, and tectonics.Chemoherms are found from internal tectonic zones (satellite Epiligurian basins) to external ones (foredeep turbidites) of the Apenninic chain. Dominant rock types are calcilutitic limestones, marly limestones and calcarenites; they are composed of authigenic micrite associated with minor neoformed pyrite, biogenic particles and siliciclastic detritus. Host sediments are mudstones, muddy sandstones and marlstones; characterised by low permeability but commonly fractured. In the foredeep chemoherms occur within thick pelitic intervals, deposited in structural highs, which are entirely or partly involved in slumps, frequently associated with extraformational slides. In this contest, chemoherms seem to be indicators of the advancing deformational front of the chain, thus heralding main tectonic phases. Otherwise, in the satellite basins, the distribution of chemoherms is not structurally controlled; a relationship between periods of sea level drops and clathrate destabilization processes in these relatively shallow successions should be taken into account.Two main types of chemoherms, respectively related to focussed and diffuse methane-rich flux venting, can be distinguished in the field. The first type consists of huge isolated lenticular carbonate to calcarenitic levels ranging in extension from 15 to 200 m and a maximum thickness up to 30 m. The second type of chemoherm consists of numerous horizontally and vertically scattered marly-calcareous and calcarenitic lenses with a diameter from decimetric to 5-7 m and a thickness from some dm to 3-5 m. Chemoherm autobrecciation marks the sites where ponded methane at shallow depth is episodically released explosively, as also testified by the offscraping of exotic sediments during the rapid fluid rise along diapiric conduits or fractures. As a whole, widespread discharge of pore fluids lowers sediment shear strength and permits the movement of slumps or debris flows, which are frequently associated with chemoherms.