Forme terrazzate relitte di genesi marina lungo la costa ligure tra Genova e Savona (Liguria occidentale)

Vengono esaminati in maniera organica i terrazzi marini del tratto di costa Genova – Savona; essi si presentano ingenerale come relitti, rimodellati da una forte e prolungata erosione.Lo studio ha permesso infatti di collegare la genesi dei terrazzi all’evoluzione plio-quaternaria di questo settore costiero, che presenta sedimenti del Pliocene inferiore oggi affioranti fino alla quota di 100 - 200 m.Le considerazioni dedotte dalla presente ricerca riguardano aspetti morfologici, stratigrafici e tettonici, così sintetizzabili: 1) sono riscontrabili varie tipologie di forme terrazzate relitte; 2) le superfici più basse sono correlabili in 3 ordini di terrazzi, probabilmente quaternari,che occupano una fascia costiera fino alla quota di 145 m; 3) i relitti più alti sono mal correlabili tra loro, si presentano molto erosi, sono probabilmente pliocenici e occupano una fascia costiera più interna fino a circa 400 m di quota; 4) le superfici più basse di 145 m conservano quasi sempre depositi marini costieri pedogenizzati; 5) il sollevamento costiero ha prodotto un basculamento, ben riconoscibile nel terrazzo che si alza progressivamente in quota da 60 m (Savona) a 145 m (Genova); 6) la fascia costiera terrazzata può essere suddivisa in settori trasversali diversi tra loro per caratteri morfologici e geologici; 7) l’età dei terrazzi è ipotizzabile in maniera indiretta in base a considerazioni sullo stato di erosione che essi presentano e in base ai rapporti con i depositi del Pliocene inferiore; 8) il sollevamentosi è manifestato in due fasi principali caratterizzate da differente velocità; 9) la linea di costa ad andamento rettilineo circa ENE–OSO (tratto Savona - Voltri) denuncia il controllo tettonico sulla separazione tra l’area di terraferma sollevatasi nel plio-quaternarioe l’area di piattaforma in subsidenza.

A geological survey of the coast between Genoa and Savona was carried out with the aim of fully describing, correlating and presenting all the ancient terraced landforms and deposits associated with ancient sea levels that rise to more than 300 m above actual mean sea level.The area falls within the eastern sector of the Ligurian Alps, where a number of metamorphic units crop out: the Savonese and Arenzano Crystalline Massifs (Paleozoic continental crust) and the Voltri Group (oceanic crust and serpentinised peridotites) (Fig. 1).The coastal terraces examined had previously been studied by Issel (1883), Rovereto (1939), Limoncelli and Marini (1969), Cortemiglia(1983), none of whom was, unfortunately, able to give a complete, systematic description of the morphology or deposits or adequate correlations between the terraces.The survey obtained the following information:1) The relics associated with ancient marine terraces are of various types: terraced surfaces that stretch along the main ridges (that fall directly to the sea), the outer edge and inner margin of which are identifiable; sub-horizontal parts of ridge delimited by outer edge andinner margins; successions of culminations of similar altitude aligned along the main ridges; isolated remains of summital terraced surface slacking inner margins; terraced surfaces running along the coast and corresponding to a marine platform covered by thick marine and continental deposits. The “inner margin” is some metres higher than the ancient coastline (terrace 25 m above mean sea level inTab. 1) (Fig. 11).2) Three orders of terraces were identified. The first order consists of clearly correlatable terraced surfaces whose inner margins rise toan altitude of from 60 m at Savona to 145 m a.s.l. at Genoa; there is almost always a weathered marine deposit on these surfaces.The second order, only clearly identifiable in Sector 9, consists of small terraced remains with inner margins 60 m a.s.l., on which weathered pebbly deposits are preserved. Terraced surfaces with inner margins 35 m a.s.l., identified in the less uplifted Sector 1,could belong to this order.The third order consists of terraced surfaces with inner margins at 25 m a.s.l., referring to two marine platforms (Fig.11), the more high of the two with an inner margin at 17 m a.s.l. and the lower one with an inner margin at 7 m a.s.l. Both platforms are overlaid by marine deposits and do not seem to show regional tilting.3) Along the stretch of coast surveyed it was also possible to identify terraced relics higher than the order of terraces between 60 m and145 m described above. These relics have an average width of about 450 m and inclinations of about 2°; they are distributed between 100 and 400 m a.s.l. and are difficult to correlate as they are very eroded (Plate I) and without deposits. A better understanding of these relics will require further studies.4) Both marine and continental deposits were identified on the terraced surfaces included in the four orders cited above. The marine deposits have a maximum thickness of 4-5 m and mainly consist of gravel with rounded pebbles, often flattened and imbricated towards the sea; those at Natta (Fig. 5) and Voltri (Plate I) are particularly well preserved. The deposits are always strongly weathered (red, yellowish red, reddish brown; 2.5 YR and 5 YR).5) The uplift of the ancient terraced surface caused the tilting that is attested by the order of terraces that varies progressively in height from 60 m a.s.l. at Savona to 145 m at Genoa (orange coloured terrace in Plate I). It is more difficult to establish the degree of tilting ofthe terraces above this order. The lower orders (+7 m and +17 m) seem to have remained sub-horizontal.6) The coastal area surveyed can be divided into different sectors (Fig. 2) on the basis of its general morphology and the lithology of the substratum. Furthermore, the variation in the height of the terraces suggests that some sectors have been separated by neotectonic transverse faults (e.g. Sector 4, Tab. I).7) The age of the terraces has been approximately calculated on the basis of the reliable data available:- the Early Pliocene is the most recent part of the substratum which is preserved in scattered relics along the coast at less than 100 ma.s.l;- the terraces lying above the order at 60÷145 m a.s.l. are very eroded and without deposits;- the terraced order lying at 60÷145 m a.s.l. formed after the Early Pliocene emersion;-the lower terraces, including that lying at 60÷145 m a.s.l., have better preserved surfaces on which it is possible to observe both marine and continental deposits.The absence of Middle and Late Pliocene sediments and Pleistocene shelf deposits along the stretch of coast surveyed would suggest that the uplifting of the Early Pliocene bathyal clays was very rapid and largely finished during the Pliocene. The very eroded highest terraced remains, visible today as narrow sub-horizontal ridges without deposits, can be dated to this period. Instead, the uplift would have proceeded very slowly in the Quaternary, lifting the early Pliocene layers to their present-day height of about 100 m a.s.l.; the terrace that rises to an altitude of from 60 m at Savona to 145 m at Genoa also probably belongs to this period as it is relatively well preserved.The two lowest coastlines, at +7 m (attributed to OIS 5e) and +17 m (attributed to OIS 9 or 11) document a very slow rate ofuplift, less than or equal to 0.05mm/yr.The terraces considered to be Quaternary occupy a coastal stretch approximately 750 m wide and up to 145 m a.s.l. The oldest terraced relics, probably Pliocene in age, occupy a more inland stretch, up to 400 m a.s.l. and are about 2 km wide. Landward of these, theslopes rise rapidly towards the Ligure-Padano watershed line.