Morfoneotettonica dell'Altopiano delle Mànie e zone circostanti (Liguria occidentale)

Morphoneotectonics of the High Plaine of Mànie and surroundings (Westem Liguria, ltaly)

Abstract

The High Plain of Mànie (in the Finale area, Western Liguria) is mainly composed of carbonate lithotypes, subject to considerable karst processes. While the karst microforms can be referred to the dynamics of the present slope, the karst macroforms are similar to those typical of tropical karst landscapes. Thanks to a quantitative geomorphological survey, it has been found that the morphogenesis of the top surface of the High Plain dates back to a hotter and wetter period than the present one, responsible for the cockpit shape of the largest karst depressions, which is still recognisable in spite of remodelling at later times. This morphogenetic stage has been framed in the reconstruction of the post-Serravallian evolution of the whole area. Such reconstruction has been achieved through the examination of the hypogeous karst phenomena, the analysis of the 73 karst pits (most of which have been surveyed for the first time), the discovery in some places of Quaternary flora and fauna, the study of the hydrographic grid (thanks to which the apparent contradictions between the indicative parameters of the maturity degree of the basins and the hydrography have been explained), the study of a «surface of peaks» extended to most of the surveyed area and of the morphological steps at the edge of the high plains. The results can be summarised as follows: after the settling of the «Pietra di Finale» (Serravallian) the zone definitely emerges, forming a structural high, also emergent during the Pliocene transgression. As in many circum-Mediterranean regions, also in this zone, probably during the Plio-Pleistocene, there was the formation of an erosion glacis, proof of which is the existence of a «surface of peaks». Following the uplift movements in that area, the glacis is eroded and a karst high plain forms in the zone of carbonate rocks. Later, once or more than once, the zone becomes hotter and wetter than it is at present, with the ensuing formation of cockpits, which are later partly dismantled by an erosive phase. This erosive phase transforms the high plain into smaller units such as the Mànie High Plain. During a period of standstill in the relative uplift movements of the region, the formation of large valley bottoms (the one of the Rio Ponci is still well preserved) takes place together with a series of morphological steps, 100-145 metres above the present sea level, such as the alluvial deposits of the Ponci. The recurrence of relative uplift movements leads to the present situation and to the dismantling of most of the old valley bottoms through a new phase of erosion which has resulted in a rejuvenation of the whole region. At present, this erosive phase is in its final stage. The current erosion process, mostly in areas composed of metamorphic or quartzitic rocks, is mainly due to anthropic causes. The uplift of the region has been accompanied by Quaternary eustatic oscillations. Evidence of two of them, which may be referred to the Upper Pleistocene, is the existence of fossil deposits at Capo Noli.