Late Eocene to Oligocene Calcareous Biostratigraphy in the Northern Apennines Satellite Basins: The Ranzano Sandstone.

The abundance distribution patterns of selected calcareous nannofossils were investigated by means of quantitative methods in the late Eocene to early Oligocene Ranzano Sandstone (RS, Epiligurian Succession) outcropping in the Northern Apennines. Our goals were 1) to test the reliability, in the terrigenous sediments of the RS, of the biohorizons used in the standard zonations of Martini (1971) and Okada and Bukry (1980) and of others recently proposed in the literature, 2) to establish biostratigraphic correlation within the RS on a regional scale, 3) to establish a chronology of the depositional history of the RS on the basis of calcareous nannofossil biostratigraphy, focusing on the main changes in facies and petrofacies described by Cibin (1993), Martelli et al. (1993), Cibin et al. (in press) and Mutti et al. (1995), and 4) to evaluate the regional and global significante of major stratigraphic esenta identified by previous studies. The standard zonations of Martini (1971) and Okada and Bukrv (1980) provide low resolution in this time interval. Wc suggest that sine calcareous nannofossil biohorizons are fairly reliable for classifying and correlating the stratigraphic record of the Northern Apennines from the uppermost Eocene to the topmost lower Oligocene. On the basis of these biohorizons, we have proposed a set of six biozones wihich have been tentatively correlated to the global standard chronostratigraphic scale. The provided time resolution is high in the latest Eocene and basal Oligocene (less than 1.0 m.y.) and low in the advanced early Oligocene. On the basis of the calcareous nannofossil biochronology developed, it has been possible to establish a chronology of the depositional history of the Ranzano Sandstone which was unavailable so far. The RS outcropping between the eastern Tertiary Piedmont Basin and the Enza Valley (Reggio Emilia Province) is terminal Priabonian (upper Eocene) to the terminal Rupelian (lower Oligocene) in age and covers a time interval of 4-6 m.y. The calcareous nannofossil biostratigraphy provides a time grid which allows the recognition, correlation and timing of the main stratigraphic events indicated by field stratigraphic analysis, and the evaluation of their degree of synchroneity on a regional scale. This is important because the RS shows a high degree of stratigraphic variability which prevents the tracing of physical stratigraphic correlation among the various outcrop areas. Finally, the availability of a global time frame allows comparison of the local stratigraphic history with the global glacioeustatic record suggested by oxygen isotope stratigraphy, thus contributing to the ongoing harsh debate on the tuie of tectonics versus eustasy in stratigraphy. Tettonics is the dominant factor in determining the stratigraphic architecture of the RS. Some of the events which characterize the RS (the base of the unit, the beginning of the supply of ophiolitic detritus, a widespread hiatus at the Eocene-Oligocene boundary) are reasonably synchronous in the investigated area. This finding suggests that, if tectonics were the dominant causative mechanism in determining these features, tectonics must have been operating at a regional scale, and, most probably, at a very fast rate. However, there is good matching between the timing of the above mentioned widespread basal Oligocene erosional event (Valle dei Cavalieri event) and a major oxygen isotope shift (Oi1 of Miller et al., 1991a, 1996) which may support the hypothesis of eustatic control over the development of this stratigraphic feature. The other major events recognized in the RS Succession are not sufficiently constrained in time to allow, a confident correlation to the oxygen isotope stratigraphy. However, two regional chaotic mass wasting events (Specchio and Zermagnone events) and a confined thick bedded coarse grained turbiditic body (Lagrimone Sandstone) could be tentatively, linked to eustatic lowering suggested by oxygen isotope stratigraphy. This would suggest that, even in an active tectonic setting like that of the RS, eustasy is a major factor in controlling sedimentation.