Caratteristiche compositive della porzione silicizzata delle 'marne di Antognola' nelle sezioni di ponte delle Lepri e di Cereglio (BO)

The occurrence of coeval silica-rich and volcaniclastic layers within diverse tectonic domains of the northern Apennines provides the opportunity to discuss and redefine the old term "lithozone" in a modern light, showing its interest when used for stratigraphic correlation of genetically related deposits. Silica-rich horizons (bedded cherts and nodules) have been reported in the Lower Miocene strata of circum-Mediterranean countries as occurring at distinct stratigraphic levels. In the northern Apennines at least, they can be included in a single siliceous zone, representing the upper part of an unconformity-bounded depositional sequence. The siliceous zone is characterized by a reduced rate of sedimentation, with the local aspect of a condensed sequence, and the almost ubiquitous presence of lithologic markers: cherts, ash layers, bentonites, and locally glaucony. None of them can be considered distinctive of the zone, but their assemblage is. The local intercalation of turbidite siliciclastics complicates the picture, especially the location of the lower boundary of the zone. These terrigenous units form the lower part of the sequence and dilute the overlying siliceous and volcanogenic horizons, but do not mask them completely. Turbidites cause a thickening of the sequence and of the intervals between the markers, up to two orders of magnitude in the main foredeep area where the Cervarola and Falterona Formations were deposited. Smaller turbidite bodies occur in the semi-allochthonous basins located onto the thrust belt (Antognola Formation), whereas the ramp connecting the foredeep and foreland to the east is virtually turbidite-free (Bisciaro Formation). In semi-allochthonous and foredeep depocentres, both sand input and frequence of resedimentation events decrease upwards within the siliceous zone, concurrently with a concentration of chert and volcaniclastic layers. Carbonate or mixed carbonate-siliciclastic megaturbidites are interbedded in the more shaly, upper sections of the Cervarola and Falterona Formations and are probably correlatable with calcareous tempestites (or relatively shallow-water turbidites) of the Bisciaro area. The siliceous zone has a geochemical signature, too, showing a silica enrichment, which is not accompanied by major or minor elements concentrations. This supports the hypothesis that silica is related more directly to biological activity than to volcanic events. Abrupt facies variations from deep-water turbidite/hemipelagite to shelf deposits, concurrently with a change in the terrigenous supply, occur at the top of the zone, allowing its interpretation as a sequence boundary. This boundary is marked by obvious unconformities both in the semi-allochthonous basins and in the foredeep, laterally passing into correlative conformable surfaces in the foreland ramp. The regional significance of the siliceous zone and its possible use as a stratigraphic marker over a great areal extent suggest using a single name for this unit instead of the chaotic terminology which has grown up in the past. The evidence of a pronounced tectonic (Burdigalian) phase that caused a substantial rearrangement of all orogenic basins at the upper zone boundary, supports the idea that the siliceous zone marks a divide in the tectono-sedimentary evolution of the Apenninic orogenic system