Disrupted and chaotic rock units

Many important geological concepts and features were first recognized and described in the Apennines and Sicily. In these areas, among the most distinctive features are rock units characterized in outcrop by variably disrupted strata or blocks of diverse sizes disposed in a clay-rich matrix. The stratal disruption and the intense weathering give an overall chaotic or non-bedded appearance to these units, and various names have been used for them, including argille scagliose, argille brecciate, caotico eterogeneo, Chaotic Complex, Undifferentiated Complex, Ligurian mélange, and olistostrome. Some of these terms, such as argille scagliose and olistostrome, have been applied to mélanges in other mountain chains, even while Apennine geologists continued to debate whether local examples resulted from tectonic or gravitational processes. The purpose of this chapter is to provide a brief review of past and current research on disrupted rock units in the Apennines. First, we briefly review how the nomenclature originated and evolved. Second, we summarize the criteria that are currently being used in the Northern Apennines to subdivide these units into two major types, tectonosomes and olistostromes, which originated by in situ tectonic deformation and by gravitational mass movements, respectively.