Petrographic analysis in regional geology interpretation: case history of Macigno (northern Apennines)

The Macigno is a widely outcropping terrigenous turbidite succession, up to 3000 m thick, in the northern Apennines. It was deposited in an Upper Oligocene– Lower Miocene perisutural basin that flanked the Apennine chain during uplift. In the Abetone study area, the Macigno lithofacies are characterized downward by prevailing thick-bedded, frequently amalgamated coarse sandstones (lower Macigno) and upward by recurrent fine and thin-bedded turbidites and siltstones (upper Macigno). The paleocurrent indicators are generally oriented toward the southeast and east. In the upper Macigno, a 250-m-thick intercalation known as the “Monte Modino Olistostrome” is present, represented by Cretaceous to Oligocene, locally chaotic, varicolored shale, marl, and limestone. Some authors hold that the latter is really an olistostrome that briefly interrupted the turbidite sedimentation. Others interpret the intercalation as the stratigraphic base of a tectonic unit that was thrusted onto the lower Macigno. Detailed modal petrographic analyses (Gazzi-Dickinson) performed on medium- to coarse-grained sandstones of the turbidite succession, along with plots of the data following the stratigraphic order of the samples, help to solve the geologic debate on stratigraphic continuity or tectonic discontinuity of the succession. The new data show that: (1) the main (QFL + C) and secondary components (Lv, Lm, Ls + C) are substantially similar along the succession; (2) from the stratigraphic base to the top of the turbidite succession, the petrographic parameters are characterized by appreciable trends and variations; and (3) the shaly-marly-calcareous intercalation (i.e., the Monte Modino Olistostrome) does not interrupt these trends and fluctuations. Therefore, we suggest that: (1) there is an overall common source area for the turbidite beds, even if minor compositional variations occur; and (2) the Macigno is a thick, stratigraphically continuous succession, and the sedimentary emplacement of the Monte Modino Olistostrome briefly interrupted the turbidite sedimentation. The new results contribute to geological mapping, to local- and regional-scale correlations, and to a better definition of the paleogeographic and tectonic setting of the Oligocene-Miocene siliciclastic turbidite successions of the northern Apennines.