Tectonosomes and olistostromes in the argille scagliose of the Northern Apennines, Italy

Rocks characterized by an apparently chaotic block in matrix fabric and known as argille scagliose are widespread in the Apennines of Italy. Most of the argille scagliose can be differentiated into two kinds of units, tectonosomes and olistostromes, which have different sedimentological and structural characteristics and implications. The tectonosomes derived from polyphase deformation of lithostratigraphic units of an oceanic cover succession. Stratal disruption resulted from (1) folding and boudinage of nonconsolidated sediments, compatible with shallow tectonic deformation or gravitational spreading and slumping, and (2) prevalent brittle, simple-shear deformation, related to boudinage, transposition of bedding, stacking of blocks, and thrusting. A pervasive scaly fabric enabled the tectonic mobility of the matrix that accompanied the simple shear deformation. Olistostromes are chaotic sedimentary bodies. They have been interpreted as syntectonic deposits related to episodes of slope failure at the front of thrust systems in thrust-bordered, piggyback basins. The inferred depositional mechanisms are debris avalanches that become debris flows downslope. All of these bodies, whether tectonosomes or olistostromes, have, at first observation, common characteristics, such as block-in-matrix fabric and the seemingly chaotic disposition of blocks. The criteria for differentiating tectonosomes from olistostromes may be useful to distinguish tectonically disrupted lithostratigraphic units from submarine debris flow and slide deposits in melanges in other orogenic belts. The basic criteria for field distinction are composition and age of component rocks, different order-disorder ratio in the distribution of blocks, and the presence of scaly fabric or of a matrix hosting millimeter to submillimeter clasts (brecciated matrix).