Olistostromes, mélanges and mass-transport deposits

The terms olistostrome and olistolith have been introduced by G. Flores (4th World Petroleum Congress, 1955, Rome) to indicate either sedimentary bodies with a chaotic block-in-Matrix fabric or single slide blocks, intercalated between layered sequences in the Tertiary succession of Sicily. Both terms soon became extensively used by the international geological community worldwide. With the extended usage, they evolved to generally indicate stratally disrupted to chaotic complexes and “exotic” bed packages, which originated by mass-transport events. In this extended meaning, the concept of olistostrome played a significant role in other, important scientific debates, such as the origin of mélanges. Mélanges are considered the products of either tangential tectonics, sedimentary processes, mud diapirism, or the interplay of these processes (FESTA et alii, 2010 and references therein). Among the sedimentary processes, mass gravitational transport is appointed to create chaotic, blocks-in-matrix rocks (olistostromes) that is a possible sedimentary end-member for mélanges. Most studies in literature have been aimed at distinguishing olistostromes from chaotic bodies deriving from other processes (broken formation, tectonic mélange, mud diapirs), to outline their paleogeographic distribution according to certain geodynamic and structural conditions (i.e., subduction vs.obduction, accretion vs. erosion, subduction of sea mounts), or to define more or less cyclic tectonic-stratigraphic events. As a result, olistostromes have been poorly studied from a sedimentological point of view, with some notably exceptions (see, e.g., ABBATE et alii, 1981).