L’evoluzione quaternaria della pianura di Grosseto: prima interpretazione dei dati di sottosuolo

Quaternary evolution of the Grosseto plain: preliminary interpretation of surface data

Abstract

The sedimentary evolution during the Pleistocene/Holocene is schematically outlined using subsurface data collected for a hydrogeologic study. During the Upper Pleistocene fluvial and aeolian sediments were deposited, which now crop out in low terraces along the borders of the plain (gravel and sand: q), and in the southern-central part of the plain (gravelly alluvium: acg). During last glacial interval, when the sea level dropped about 100 metres below its present level, the Ombrone and Bruna rivers cut two valleys in the Pleistocene deposits, on the opposite sides of the plain. These valleys became inlets during the Versilian transgression and were later filled by the sediments supplied by the rivers. The filling of the inlets took place for the most part in historic times. Because the Ombrone River carries more solid material than the Bruna Rivers does, the southern inlet was filled more rapidly. During Roman times the north-western part of the plain was still a lagoon, and became a marsh during the Middle Ages (favoring the spread of malaria in the process) which would still be there if the land had not been reclaimed. The interpretation we are presenting is confirmed by the faunal analisysi of the samples recovered during the drilling of eight water wells. The data show that the recent sediments underlying the land fill were deposited under predominantly swampy and lagoonal conditions. Fluvial episodes, more frequent in the lower part of the sections, were probably due to cataglacial conditions. The sands of the coastal dunes, whose innermost ridge is related to a tombolo that is probably Etruscan in age, continue inland under the land fill for 2-3 km, and reach the thickness of 30 m. This indicates that the formation of a beach ridge certainly predates the cultivation of large areas in the Bruna and Ombrone drainage basins, that had formerly been woodlands. The deforestation of these areas resulted in increased land erosion and transportation of solid material in streams, mostly the Ombrone, which is responsible for the prograding of the beach ridges and the Ombrone delta into the sea.