Studio geomorfologico della pianura di Pisa

Geomorphology of the Pisa plain (Tuscany)

Abstract

In this note we present a geomorphologic study of the Pisa plain which was prepared on the basis of field studies and the study of historic documents, aerial photographs, and satellite images. We have defined the Pisa plain as a unit in terms of its physiography, administration, history, geology, and geomorphology. We began by studying the substratum of the plain, which we divided into three parts: inferior (underlying the upper Miocene sediments, which in Tuscany represent the beginning of the neoautocthonous sedimentary complex), intermediate (which is made up of the sediments of the neoautocthonous complex that date through the lower Pleistocene), and upper (includes middle and upper Pleistocene and Holocene sediments, all of which were strongly affected by glacio-eustatic variations in sea level). The exanimation of the substratum was followed by a study of the surface of the plain, for which the data supplied by historic sources as well as that derived from the interpretation of areal photographs and satelite images proved very important. Most of the Pisa plain lies within the Pisa-Versilia basin, a graben bounded by the Alpi Apuane and Monte Pisano to the East, and the Meloria-Maestra submarine crest to the West. The Livorno and Casciana Terme mountains close off the basin to the South. During the middle-upper Pleistocene this region emerged and was crossed by rivers from the Apennines that drained into the Tyrrhenian Sea. Unfortunately, the stratigraphic, documentation of this phase is scanty. The development of the Pisa-Versilia basin reached its maximum extent in the beginning of the lower-middle Pliocene, after which the basin became associated with a large emerged region that seems to have extended as far as Corsica and Sardegna during the upper Pliocene (as is indicated by the vertebrate faunas of the islands), and then subsided considerably in the lower Pleistocene. The upper-middle Pleistocene (Mindel-Riss Interglacial) marks the beginning of a period of intense fluvial activity on the part of the Arno and the branch of the Serchio that passed to the East of Monte Pisano. During the Würm II there was a second phase of fluvial activity on the part of the Arno and the Serchio, whose hed has been mapped at depths between 40 and 60 meters belov the surface of the plain to the East of Monte Pisano. In the latter phases of Würm II aeolian sediments were also deposited, which have undergone pedogenesis and contain Musterian remains. These sediments line the southern border of the plain (Sabbie di Vicarello), and form a transverse barrier (Sabbie dell’isola di Coltano) behind the Holocene littoral sediments produced by the Versilian transgression. The first overflow of the Serchio through the Ripafratta gorge to the sea seems to have occurred during Würm I. The gorge had been dug much earlier, before the upper Miocene in fact, by a stream flowing in the uppusite direction. It had then become inactive when the Pisa-Versilia basin subsided. We have also been able to make a satisfactory reconstruction of the evolution of the coastline with the help of historic documents, photointerpretation, and satellite imagery. The shore reached ist most inland position during the Versilian transgression between the end of the Atlantic Dak Phase and the first and second centuries B.C. Its present position in the result of the intense erosion of the cusp of the Arno delta since the 18th century, which in probably due to human activities. Photointerpretation and the study of satellite imagery has also allowed us to retrace many sections of the paleostream channel of both the Arno and the Serchio, the locations of which are confirmed by historic documents. We were also able to trace the histories of the areas that were covered by swamps during the Middle Ages through the time they were drained in the 1930s. On the other hand, we were only able to propose hypothesis as to the locations of rivers and marshes in antiquity.