Abstract

The sands presently carried by the Po River differ from those of the Adige River mainly in the content of dolomite and of fragments of acidic volcanic rocks, and to a lesser degree in the content of some heavy mineral species; such differences individuate two petrographic provinces. Compositional data of samples from paleoriver beds and bars reveal that the same differences were present during past times. Complex admixtures of sands from the two petrographic provinces are often observed in sands from beach ridges, whose shape also depends on eolian activity and on the consequent formation of dunes. The beach ridges are the product of an evolution from protruding deltaic bodies to a marked regularity of the coastline. Such modifications are mainly due to marine erosional processes, which are of greatest relative importance during periods of decreased fluvial discharge. Under these conditions the river-supplied sands are redistributed, with resultant admixture of sands from the two petrographic provinces. The sands from deltaic channel-mouth bars have compositions that become increasingly uniprovincial from early to late depositional stages. These bars form at times of increased fluvial deposition, when the rivers build prominent deltas that modify a previously straight coastline. Under such conditions, longshore transport of sands gradually become less important so that mixing of sediments derived from the different provenances gradually becomes negligible. Since pre-Etruscan times, the sands carried by the Adige River were transported longshore towards the south and were mixed with sands from the Po River moving north. The convergence zone between the two provinces was usually controlled by whichever distributary of the Po had the higher discharge among northern Po branches. That distributary would produce a marked deltaic body that modified the coastline until the southward drift of the Adige sands ceased. By comparing compositional data with the historical geomorphic evolution of the system, the most active mouths of the Po in terms of discharge have been identified from the Etruscan age to the present.

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