Sedimentary mélanges and fossil mass-transport complexes: a key for better understanding submarine mass movements?

Although the 1978 Penrose Conference stated that “Mélanges are mappable, internally fragmented and mixed rock bodies” formed by “tectonic movements, sedimentary sliding, or any combination of such processes”, an exclusive origin from tectonic processes is often aprioristically assumed. Due to the recent advances in the investigating methods and the increasing work of comparison between mélanges in mountain ranges world-wide, an increasingly larger number of sedimentary mélanges is recognized. Although strongly deformed by post-depositional, tectonic and/or mud-diapiric processes, these mélanges prevailingly originated from sedimentary processes. Moreover, the chaotic sedimentary bodies known as olistostromes in the literature fall inside the more general category of sedimentary mélanges, further increasing their relative abundance and diffusion. Sedimentary mélanges are prevailingly related to mass-transport processes. They often are, therefore, excellent fossil examples of mass-transport complexes (MTC), cropping out in well preserved and continuously exposures. This communication will show the results of the holistic study of both sedimentary mélanges and more classic cases of fossil MTC, with a focus on the Apennines of Italy. Fossil MTC, especially the large-scale, basin-wide ones, are complex units involving the entire spectra of mass-wasting processes and often originated by retrogressive sliding of basin margins up to the shelf. The translation of the bodies is enabled by the relative movement of discrete masses, with progressive stratal disruption of rocks/sediment involved and flow transformation. Fluid overpressure inside either diffuse or concentrated shear zones enable these movement and trigger the stratal disruption. Detailed information on internal geometry and structures and on the mechanisms of deformation can be gathered from fossil MTC, which are less readily achieved by submarine geophysical studies of modern submarine landslides. The synergic study of modern and fossil MTB should result in a better knowledge of mass-transport processes and bodies, in relation with the basin floor geometries.