Ciottoli e blocchi di origine recifale nei conglomerati basali della Formazione a Colombacci (Messiniano, bassa Val Secchia, Appennino settentrionale)

Recent erosion in the lower valley of the Secchia River has uncovered part of the late Messinian and Pliocene succession, which consists of the Colombacci and Argille Azzurre formations, respectively. A conglomerate unit, about twenty meters thick and discordant on the underlying epi-Ligurian substratum, occurs at the base of the Colombacci Formation. This conglomerate holds carbonate cobbles and boulders of reefal origin. A detailed analysis of these carbonate clasts allowed to recognize some typical microfacies (bioclastic rudstone, coral packstone, algal crust wackestone, mudstone-wackestone rich in bryozoans and serpulids), which suggest a correlation with the lower Messinian reefal deposits outcropping in several circum-Mediterranean localities. At present, lower Messinian deposits characterized by similar reefal facies do not occur in the Apennines of Modena and Reggio provinces; the only outcrops of Messinian reefal carbonates located on the Po plain side of the Apenninic chain occur in the Parma-Piacenza foothill area (Vigoleno, Scipione). The large size and the sedimentological characters (alluvial and fan-delta environments) of the conglomerate suggest that the provenace of the carbonate clasts was the present-day sector of the Modenese Apennines and that the original reefal deposits were completely eroded during a relative short time, i.e. between the late Messinian and the Early Pliocene. These reefal sediments, likewise those of northwestern Emilia, most probably constituted the basal unit of the Messinian depositional sequence which includes also the pre-evaporite and evaporite deposits. The reefal carbonates of the Modena Apennines, together with those of northwestern Emilia and Monferrato, represent the northern boundary of the Messinian coral reefs distribution. Their occurrence at these latitudes may correspond to a particular warm period which, according to isotope and radiometric data from oceanic sediments, should be placed between 6.4 and 6.2 Ma ago.