Turbidites in foreland and on-thrust basins of the northern Apennines

Abstract

Turbiditic sediments are associated with all stages of development of the Apennines orogen: not only the classical “flysch” of Oligo-Miocene age (Macigno, Cervarola, Marnoso-arenacea) but also the coeval, smaller bodies based on thrust sheets (“Loiano” of “Ranzano” Sequence) and the younger “molasse” of the Plio-Pleistocene Foredeep. Except for more recent ones (Quaternary), turbidite units are affected to some degree by thrusting and later block-faulting The equivalent marginal deposits were also deformed, uplifted and eroded to a great extent. However, identification and correlation of sequences and paleoenvironments across the orogenic belt can be helped by (a) deposits with rather strong eustatic imprint (high stands), and (b) preservation of marginal sediments by early remobilization (cannibalism) into deeper depocentres. The following categories of basins can be recognized: • — main foreland basin, or foredeep, in front of thrust sheets; based on undeformed or previously rifted continental crust; • — thrust-top basins (meso-autochthonous auct.) in absence of a well-defined, major foredeep; • — satellite basins: thrust-based but inwardly associated with a foredeep, and of much smaller size (linear dimensions in the order of kilometers and ten of kilometers, volume of fill in the order of tens to hundreds cubic kilometers). The foredeep persisted for the whole orogenic cycle but changed its character and was more or less segmented; two main stages can be recognized, conventionally labelled as the flysch stage (up to the late Miocene, Appenines mostly submerged) and the molasse stage (late Miocene to Recent, present relief already in existence). Satellite basins “floating” on the Ligurian Sheet (Epi-Ligurian) persisted up to the mid-Pliocene but received turbidites only in the Oligocene and Miocene. Others were created later on thrusts of different style involving the foreland sedimentary cover (piggyback basins of Ori and Friend, 1984). Turbidite lithosomes of the flysch group are the largest ones (volumes up to 30,000 km 3), with maximum size and lateral continuity of individual beds (megaturbidites). They are not seen as overlapping units filling coexisting and adjacent basins but as distinct, superposed isochronous units, within which only, time-transgressive sedimentation could occur. Macigno thus stands for all Oligocene turbidites of the northern Apennines foredeep, Cervarola for early Miocene ones, and Marnoso-arenacea for mid-late Miocene ones. It is assumed that these units reflect sedimentary cycles of some order, correlatable with “global” charts and splittable into depositional sequences sensu Vail and collaborators.