THE EARLY DEPOSITIONAL PHASES OF THE NORTHERN APENNINE FOREDEEP-THRUST BELT SYSTEM: IMPLICATIONS FROM THE “MACIGNO COSTIERO” (LATE OLIGOCENE, ITALY)

Abstract

The late Chattian “Macigno costiero” represents the innermost and the oldest portion of the well developed siliciclastic wedge of the migrating Oligo- Miocene foredeep-thrust-belt system of the Northern Apennine. Its stratigraphic features help to understand the early phases of the sin-collisional sedimentation and the evolution of such a thrust belt. In the turbiditic siliciclastic sequence, carbonates were found at different stratigraphic levels (chaotic-debris flow key-level, calcareous turbidites and Subligurian olistostromes and olistoliths). Stratigraphical studies of the sequence based on facies analysis, petrographical observations and biostratigraphical analysis of calcareous nannofossils allow to suggest some hypotheses of the palaeogeographic setting and the evolution of the foredeep-thrust belt system. The vertical distribution of the sediments and their sedimentological characters show a low efficiency sand-rich turbidite system (sensu Mutti and Normark, 1987), characterised by a prograding fan terminated by the emplacement of slices of the orogenic wedge. Olistostromes and olistoliths found in the turbidite succession were the precursor of the allochthonous nappe. The proposed hypothesis shows that turbidite sediments were probably fed transversally to the basin, from a shallow marine shelf adjacent to a crystalline basement. The “Macigno costiero” was deposited in an internal position of the foredeep system.