Plio-Pleistocenic evolution of Rio Mannu di Mores valley (Logudoro, northern Sardinia)

Abstract

This work is part of the research activity carried out on the territory of Logudoro-Mejlogu and its aim is to study the changes in the drainage pattern induced by Plio-Pleistocene volcanic activity. The general trend of this area shows clear tectonic control by geomorphic processes of the size of the Mores-Chilivani plain, which is overestimated with respect to the watercourse that crosses it. This condition is the result of a vast erosive action of denudation of the pre-Miocene basin, fossilised by marine sediments. On the basis of the acquired data, the evolution of the of the Mannu di Mores river valley should be referred to the early phases of formation of a runoff system, probably in the Upper Messinian-Lower Pliocene. Phases of vertical erosion by Rio Mannu di Mores during the Upper Pliocene-Lower Pleistocene (Villafranchian) are assumed to be a result of uplifting movements, which are clear in the relief inversion of the Pliocenic lava flows dated 2-1.8 M.y.b.p. and in the gravity deformations along the tectonic faults. Alternate phases of linear and lateral erosion have led to the recession of the carbonaceous banks to the west and the exhumation of the volcanocineritic plane. This structural plane should be considered partly degraded by the water system, whose candlestick geometry shows a selective process induced by a pomiceus-cineritic substrate. Nevertheless, effusive events that have caused stretches of the watercourse to fossilise have affected its dynamics significantly. This intense extensional phase seems to end in Middle Pleistocene (0.8 M.y.b.p.) since the lava flows referred to this age show a poor relief inversion (0-40 metres), which is also observed in other parts of Logudoro. From the reconstruction of the evolutionary events, it emerges that in the last few million years the land was affected by a prevalent erosive morphogenesis, which manifested itself in an extended denudation process with the exhumation of Pre-Miocene forms. Through morphochronological correlation (based on absolute dating) with the surfaces buried under the basalt lava flows, a different morphodynamic control of the erosive processes emerged. An important erosive phase due to an uplifting of the territory, which caused important changes from the Upper Pliocene-Villafranchian onwards, is attributable to an initial stage of tectonic control. A prevalent modelling of the slopes into terraced surfaces is attributable to a second stage of palaeoclimatic control. The morphoclimatic action on the modelling of the slopes found its greatest expression during the Upper Pleistocene and the Holocene. This work led to the compilation of a geomorphological map of the area on a 1:25.000 scale, so far lacking, and to the definition of detailed geomorphological features in an area of Logudoro. The most significant results are the recognition of deep gravity deformations and the landslide survey which can be related to one morphoevolutionary event linked with Plio-Pleistocene neo-tectonics.