From deepwater sediments to coastal salina deposits: a ten million year journey in the Santerno Valley (N-Apennine)

This is one of the six excursions that will be used as a paradigm of the EDU-GEO project sponsored by the Italian Federation of Earth Sciences (Cantelli & Venturini, this volume). The area is located in the Tuscan-Emilian Apennines, between Firenzuola (FI) and Tossignano (BO), near the town of Imola. The excursion route follows of the Santerno river, which cuts transversally the Apennine chain, forming in places steep, narrow gorges. The well exposed stratigraphic succession is an excellent field opportunity for observing the evolution from deep water sediment to coastal salina deposits, and for becoming aware of the evaporation of the Mediterranean Sea during the late Messinian period. The itinerary along the Santerno Valley reveals striking views of the deformation setting of the Apennine belt and morphological sights of deep fluvial incisions with remnants of orographic terraces. The educational objective of this excursion is to guide the students in the field to recognize and distinguish three different data sets: stratigraphy, tectonics and morphology. The Camaggiore outcrop in the river bed of the Santerno is a well exposed and accessible outcrop of the Marnoso-Arenacea Fm that develops along the river. The favourable exposition enables the excursion leaders to introduce the lithological and sedimentological features of the succession and to stimulate the student to seek similar evidence in the surrounding exposures. Moreover, the site is large enough for educational strategies: e.g. an in situ simulation of turbidite currents using local sand and a prismatic box which approaches the original shape of the turbidite basin. The Coniale outcrop offers a close view of a thrust in the Marnoso-Arenacea Fm (Langhian-Tortonian) with a vertical limb characterized by outstanding large-scale zig-zag folds. The tectonics data set is remarkable all along the excursion route and is essentially related to bed dipping observations. The geological structure forms a wide open anticlinal fold developing at the scale of the route excursion. The third stop in the Tossignano village offers a spectacular panoramic view of the gypsum multilayered thick sequence of the Vena del Gesso Fm. The outcrop exhibits evident faults and block slides. A short trip to an abandoned quarry adds close up views and useful opportunities to study the internal anatomy of the thick gypsum beds. The students will be strongly encouraged by the excursion leaders to look for field data and formulate hypotheses following the sequence effects and causes. All together the three stops allow the students to became aware that: 1) the strata sedimented horizontally and are marked by geological signs (structures) of their depositional environment; 2) inside the mountain belt, the stratigraphic succession shows tilting, folds and faults, documenting strain effects due to orogenetic movements; 3) the relief, exposed to alteration and deep fluvial erosion, generates clasts of different sizes; the fluvial drainage distributes this material over the plains, deltas and, finally, to the sea forming the future stratigraphic successions.