Foraminiferal biostratigraphical and paleobathymetric constraints in geohistory analysis: the example of the Epiligurian Succession of the Secchia Valley (Northern Apennines, Mid Eocene-Late Miocene)

ABSTRACT A geohistory analysis has been proposed for the Mid Eocene- Late Miocene Epiliguran succession cropping out in the Enza, Secchia end Reno Valleys (Northern Apennines) in order to quantify the evolution of the studied areas in terms of both vertical motions (subsidence/ uplift) and sedimentary infill. The three curves integrate and complete the geohistory scenario proposed by DI GIULIO et alii (2002) in order also to discriminate local events from large-scale episodes related to the Northern Apennine orogenesis. New biostratigraphic and paleobathymetric data were collected for the late Bartonian-early Burdigalian Epiligurian succession from the Secchia Valley, conversely, literature data have been used for the youngest investigated units (Pantano, Cigarello and Termina Formations; Burdigalian-Tortonian), which were not sampled for this work. Both chronostratigraphic and bathymetric data have been wholly derived from the literature for the Enza and Reno Valleys (MANCIN & PIRINI, 2001; 2002). From a paleobathymetric point of view, the Secchia Valley Epiligurian deposition began in the earliest Bartonian in a very deep marine environment (average depth 1900 m). After a deepening phase during late Bartonian-Priabonian, with a shift of the bottom (average depth 2200 m) to probably close to the calcite compensation depth, an overall shallowing trend was recorded culminating in Burdigalian shelf deposits (Pantano Fm., average depth 50 m). Finally, during Mid-Late Miocene, a second, lesser, deepening phase occurred, ending with the Tortonian sediments of the marly Termina Fm. (average depth 200 m). This paleobathymetric evolution has been related to the vertical motion and sedimentary infill undergone by the Secchia Valley area during the time interval considered. The results described highlight the occurrence of large-scale episodes involving the whole analysed area (from the Enza to the Reno Valleys), alternating with periods of quite different growth for each sector recording the existence of independently evolving domains of the Ligurian orogenic wedge in the geodynamic context of a mobile belt