OLISTOSTROMES, MASS-TRANSPORT DEPOSITS AND THE ORIGIN OF MéLANGES

The terms olistostrome and olistoliths have been introduced by G. Flores (4th World Petroleum Congress, 1955, Rome) to respectively indicate 1) sedimentary bodies with a chaotic block-in-matrix fabric intercalated between layered sequences in the Tertiary succession of Sicily and 2) the individual masses included in the matrix. 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. Olistostromes are, therefore, part of the more general category of mass-transport deposits (MTD). In their extended meaning, they are complex bodies, often involving the entire spectra of mass-transport processes.

Mass transport processes lead to stratal disruption up to block-in-matrix fabric and the complete chaoticization of sediments. They are also a powerful tool for mixing of rocks of different ages, state of consolidation, diagenetic and metamorphic degrees, and provenance. Because of this, en-mass transport is theoretically considered as the sedimentary end-member of melange-forming processes.

Mass transport deposits (MTD), often named to as olistostromes, have been recursively proposed as consistent part of mélanges worldwide, even if strongly deformed by post-depositional, tectonic and/or mud-diapiric processes. These interpretations have been, however, often sustained by general considerations on regional geology and tectonic setting and not by studies on the mélange structures and fabric. Moreover, the mechanisms of internal deformation and stratal disruption, and the origin of the matrix in MTD are not completely defined so far. The mechanism and structures related to the post-sedimentary reactivation of MTD are still not completely clear, as well as their rheological behaviour and the relationship with fluid overpressure.

Moreover, in the sedimentary record of collisional chains, the majority of fossil MTD, including olistostromes, originated during the stages of intracontinental deformation, having been deposited in foreland and wedge-top basins. In some cases, collisional orogeny has allowed MTD related to extensional tectonics and passive margin to become exposed. This contrasts with the observed abundance of present-day MTD, which prevail in passive and divergent margins and along the flanks of volcanic islands. The present-day submerged contractional margins, however, do not show a significantly high concentration of MTD, apart from the erosional margins off the coasts of Peru. Basin-wide MTD are only present when catastrophic events occur, as in the case of the subduction of seamounts and volcanoes. This also contrasts with the suggested possibility that several mélanges in fossil accretionary wedge originated as MTD.

There are more, possibly concomitant, explanations to these discrepancies: 1) the method of investigation might prevent the imaging of geometrically complicated MTD in present-day submerged active margins; 2) the large scale MTD originated in the passive margins completely lack of preservation because of the intense tectonic reworking during the stages of subduction, collision and intracontinental deformation, and, 3) the MTD originated in passive margins and accretionary wedges are still present in the geological record of collisional chains, but their presence has been disregarded so far.

A complete and integrated sedimentological study of the “olistostromes” can: 1) add a wider spectra of internal structures and flow mechanisms to the sedimentological knowledge of mass-transport processes, 2) give a benchmark for sedimentary-related, melange-forming processes, 3) might help in explaining the discrepancies about the present-day vs. fossil MTD distribution.