Provenance of the Cotignola quarry sands (northern Apennines) as a tool for palaeogeographical studies on the action of Senio and Santerno rivers in the Romagna floodplain

Abstract

Marconi et al.: Provenance of the Cotignola quarry sands (Northern Apennines) as a tool for palaeogeographical studies on
the action of Senio and Santerno rivers in the Romagna floodplain. (IT ISSN 0394-3356, 2008).
Palaeogeographic studies on western Romagna plain focus on the location of the ancient pattern of Santerno river, that nowadays
joins Reno river before reaching the Adriatic sea near Ravenna. Several authors agree on locating this river in an eastern position
during Roman Age and suggest an overlap of Santerno and Senio rivers in the Cotignola floodplain (Fig. 2).
Outcrops of recent fluvial sediments are exposed at Cotignola quarry, near Faenza (Northern Apennines, Italy). According to the the
present hydrography, the top sediments (age: ca 1.000 yrs) were deposited only by the action Senio river. On the contrary, the most
ancient alluvial deposits of Cotignola quarry display different geological features suggesting a combination of Santerno and Senio
contributions.
In the sediment source areas of both rivers there are extensive sandstones and marls outcrops belonging to the “Marnoso-Arenacea”
formation. Though, the Santerno source area stands out because it also contains basic volcanic and metamorphic rocks, coarse-grained carbonates and cherts (deep oceanic and ophiolithic lithotypes belonging to the “Ligurian Units”). These rocks can be considered as petrographic tracers of Santerno drainage basin provenance.
Sand samples were collected at Cotignola quarry and compared by means of petrographical analysis to the present day fluvial sands
of Santerno deposits, which are to be considered as compositional standard for sediments resulting from the contribution of this river.
At Cotignola quarry basic volcanic and metamorphic, coarse-grained carbonate and chert rock fragments occur in the most ancient
sediments, whereas they decrease or disappear upward in the more recent sediments. These results prove that the upper sand top of
Cotignola quarry is due only to sedimentation by Senio river, while the lower layers can be considered as the result of contributions
from both rivers.
Furthermore, a sand sample was collected near Maiano Monti (age: ca 2.200 yrs BP, radiometric dating), which is located in the floodplain included between Senio and Santerno close to the northern part Senio river (Fig. 4). The petrographic analyses detect the presence of the ophiolithic tracers in the composition of sands collected at Maiano-Monti.
The sediments provenance implement the hypothesis of an eastern location of Roman Santerno river in the Cotignola floodplain and
suggest an overlap of its ancient pattern with that of Senio river, according to the archeologic and stratigraphic evidences (Fig. 2 and 3).