INTERACTION OF TECTONIC, SEDIMENTARY AND DIAPIRIC PROCESSES IN THE ORIGIN OF MÉLANGES: EXAMPLES FROM THE MESSINIAN OF THE TERTIARY PIEDMONT BASIN (NW ITALY)

The origin of chaotic rock bodies (mélanges) is commonly attributed to tectonic disruption and mixing of originally coherent sequences, gravitational submarine downslope movements and shale diapirism, caused by the rising towards the sea floor of overpressured, fluid-permeated fine grained sediments. The recognition of the role played by each of these processes in the geologic record is problematic, due to the strong facies convergence of their products and to the fact that these mechanisms can coexist and interact in a complex way. In northwestern Italy, the Messininan succession is made up of chaotic sediments, consisting of a fine-grained unconsolidated matrix enveloping blocks of different size and composition, including gypsum and a wide range of carbonate facies. The study of these sediments, including geologic mapping and integrated stratigraphic and structural observations, allow to evaluate the relative contribution, the time relationships and causative links among tectonic, sedimentary and diapiric processes in their genesis. It appears that these mechanisms, triggered by regional tectonic deformation, impacted on each other and operated sequentially in a short time span. Faulting was responsible for the disruption of an originally coherent stratigraphic succession, favored the failure of mechanically weakened sediments and encouraged shale diapirism through the creation of mechanical discontinuities working as conduits for the rising of overpressured sediments. Loading provided by deposition of gravitative chaotic sediments could have contributed (together with the circulation of methane-rich fluids in the sedimentary column) to diapir intrusion, which in turn caused a partial reorganisation of the dismembered sediments produced by the other two processes. Large volumes of Messinian chaotic sediments characterize many sectors of the Mediterranean area. In these sedimentary bodies, the imprint left by gravitational movements can usually be seen. However, it must be taken into account that in the highly mobile geodynamic setting where these deposits formed, slope failure was likely to be the prevailing process, able to completely conceal the traces of both tectonic faulting and shale diapirism.