Depositional cycles in two turbidite formations of northern Apennines

ABSTRACT Apart from Bouma sequences, recurrent patterns of sedimentation can be recognized at two hierarchic levels in turbidite formations of the northern Apennines. First order cycles (turbidite suites) encompass most of the basin fill, second order cycles (megasequences) occur as organized groups of beds 2-70 m thick within the suites. The Marnoso-arenacea (Lower-Upper Miocene) is an example of a progradational or offlapping suite with thickening and coarsening upward trend. The Laga Formation (Upper Miocene-Lower Pliocene) is characterized by a "transgressive" (retrogradational) or onlapping trend. Vertical facies changes go either way through the following steps: basin plain outer fan midfan inner fan slope deposits ( = prograding, = receding). Both suites are enclosed by hemipelagic pelites and chaotic deposits, the start of turbidite sedimentation being gradual in the progradational case, and abrupt (large scale erosion) in the reverse one. Second order cycles follow each other or are separated by monotonous (uniform or irregularly bedded) sequences of extremely variable thickness (2 to more than 1,000 m) composed either of fan (overbank, fringe) or basin plain deposits (with key beds laterally continuous up to 175 km). The trend of cycles may be asymmetric (termed positive when beds thin upward, negative when they thicken upward), symmetric, composite or not definable. Thick to massive beds with sand/shale ratio >> 1 or > 1 and well developed coarse divisions (products of high density turbidity currents, grain flows, debris flows, fluidized flows) form the thicker bedded portion of cycles and sometimes the whole cycle. One hundred and thirty-two cycles were analyzed and subdivided into simple and complex (= multiple or composite). By splitting multiple cycles into simple components, a total of 170 cycles was reached. Thirty-nine of them were measured in channelized deposits, 131 in non-channelized turbidites, laterally continuous exposures enabling the distinction to be made. Results of sequence analysis are illustrated by columnar profiles and bed thickness diagrams, i.e. CD (coarse division) and L (layer) diagrams. Eighty percent of channelized cycles show a positive trend, whereas a negative trend characterizes 60% of non channelized cycles. This strengthens the assumption that most positive cycles reflect filling of fan channels, while negative cycles are the expression of prograding depositional lobes, i.e. localized areas of preferred sand accumulation in front of distributary channels. Among progradational cycles, a distinction was made between "fast" and "slow" accretion types by means of relative number of beds (L/T ratio), relative number of coarse divisions (L/CD ratio), and frequence of interbedded hemipelagites. The possible non-prograd tional nature of some "slower" cycles is discussed; they could alternatively reflect sporadic phases of sand accumulation in the basin plain environment due to huge flows triggered by tectonic events and bypassing the fan system (or independent from a fan system). Attempts of correlation of negative cycles both downcurrent and across current are shown.