Aspetti dell’evoluzione quaternaria della costa tra Cogoleto e Varazze (Liguria occidentale)

Le indagini condotte nel settore costiero compreso tra Cogoleto e Varazze (Liguria occidentale) hanno permesso di riconoscere in dettaglioforme di origine marina presenti lungo il percorso turistico che corrisponde al tracciato della Ferrovia litoranea costruita alla finedell’800. La ricerca sul terreno, oltre al rilevamento geomorfologico, ha compreso la misura dei sistemi di fratture che, successivamente,sono stati confrontati con quelli ottenuti dall’analisi delle carte topografiche e delle foto aeree. I rilevamenti effettuati hannoportato al riconoscimento di numerosi relitti di piattaforme marine sollevate che testimoniano 2 alti livelli di stazionamento compresitra 8 e 12 m s.l.m. Lungo la costa è stata, inoltre, scoperta e studiata una grotta scavata dal mare nel substrato peridotitico tra 9 e 12m s.l.m. Uno studio a carattere generale è stato effettuato sull’intero versante costiero fino al crinale Pian della Noia (378 m), Santuariola Guardia (416 m) e Bric Piano Donne (320 m), tra i torrenti Teiro e Arenon, e anche nell’area a Est oltre il T. Arrestra; le indagini hannoconsentito il riconoscimento di un “paleopaesaggio” degradato; la mappatura dettagliata di due ordini di terrazzi sollevati delPleistocene inferiore e medio (tra 60 e 125 m di quota). Con i dati raccolti è stata redatta la Carta delle “Forme plio-quaternarie dellacosta tra Cogoleto e Varazze” (chiamata nel testo Tav. I) che permette una lettura cronologica delle forme e quindi una miglior comprensionedell’evoluzione del paesaggio dal Pliocene ad oggi.

With the aim of recognizing marine and continental landforms in the coastal stretch between Cogoleto and Varazze, different kinds ofinvestigations have been carried out. The collected data were used to draw a map (see Plate); they provide a descriptive frame thatallows to clarify some phases of the Pliocene-Quaternary evolution of the area.1) The relic of a Pliocene paleo-landscape. It’s partially shown in Plate and in Fig. 1 by a series of topographic summits, mostlybetween 300 and 400 m in elevation, which originated from the progressive emersion of the area since the middle Pliocene.2) The Quaternary landscape. From a morphological point of view it appears well distinguishable from the Pliocene landscape (Fig. 3)because of the presence of several marine terraces (see Plate). Considering the average Quaternary uplift rate (equal or less than 0.1mm/y – Cap. 5), the relicts of the Quaternary surface cannot occurr over the elevation of 180 m. Between 180 m and the altitude of the1st order terrace’s inner margin (95-100 m, see Plate, central stretch), marine terraces are not visible because of the slope’s intenseregularization. Distinct and well-represented is the first order terrace, which still preserves marine deposits (inner margin’s altitude90÷115 m); its age has been attributed to the late lower Pleistocene (1.2-0.9 M.y.). Less obvious but still represented is the secondorder’s terrace, whose inner margin is located at 65 meters a.s.l..3) The rocky band located between 0 and 12 meters. Along the coast, between the sea level and a tourist path located in correspondenceto the route of the railway built in the late 1800s (Europa Promenade), a rocky band (gabbros and peridotite) preserving remainsof terraced marine areas appears; this indicates two paleo shore-lines located at 8 and 12 m elevation, respectively (see Plate, profilesS1 to S4). Marine deposits constituted by sands and rounded pebbles and, locally, by rounded boulders (often with diameters largerthan 1 meter), are kept isolated in small flaps. The level at 7.5÷8 m has been attributed to the Tyrrhenian (MIS 5.5) (Fig. 7).4) The coastal slope. The slope has been divided into altitudinal sections with distinct morphological aspects and attributed to differentages. The oldest and most settled side (VC. in Plate) stretches between 250 meters and 95÷105 meters elevation with an average inclinationof 25° (see Tab. 1); between 70÷80 meters (external edge of the 1st order terrace) and the sea level is a band (VC.T1 in Plate)with an angle between 22° and 32°. Between 50÷55 meters (outer edge of the 2nd order terrace) and the sea level is a steeper and irregularsector (VC.T2 in Plate) with an inclination between 27° and 37°.5) The coastal plains. The Arrestra and Teiro plains (partially visible in Plate) present: the oldest side (Medium and Late Pleistocene)between 10 meters and 25 meters in height, characterized by a thick toe of slope deposits (mainly colluvium) with evidence of strongrubefaction; a lower part (Holocene) between 5 and 10 meters; a beach between 5 meters and the sea level.6) The Mizar cave. It is a marine cave, between 9 and 12 meters in height, entirely excavated in peridotite rocks. The interior walls havevery regular profiles (similar to parables); its formation is referable to the Tyrrhenian eustatic highstand. The dominant erosive process isrelated to the waves abrasion during storm surges (Figs. 16 e 13).7) The Plio-Quaternary vertical dislocations. The present day coastal area is the result of a moderate lifting that began in middlePliocene; in contrast, the marine platform area has been lowered. The two areas are divided by tectonic lines stretching 56°-57° (ca.ENE-WSW) e 21°-22° (ca. NNE-SSW) (Fig. 1 e Plate). It was possible to calculate an average rate of Plio-Quaternary uplift not exceeding0.2 mm/year and an average rate of Quaternary lifting less than 0.1 mm/year.8) The disjunctive tectonics. All the landforms in this area, and particularly the hydrographic network, have been controlled by activePlio-Quaternary faults and fractures. The “Teiro Fault” (partially visible in Fig. 1), futhermore, shows greater evidence and significance;its effect determined the rise of the eastern sector over the western one. In this area the dislocations are testified by changes in theheight of first order marine terraces, in which the inner margin increases from 80 to 95 metres.. The tectonic features, derived from thehydrographic network analysis are more fragmented and discontinuous to the East of the “Teiro fault”. However it has been possible torecognize three fault/fracture sets characterized by different directions: 55-60°; 21-23°; 125-136°. These directions are confirmed byfield surveys carried out along the rock band situated down the “Europe Promenade” (Fig. 17).9) The valley cut. The main rivers of this area (Rio Mola, Rio Partigliolo, T. Arrestra – see Plate) have a uniform and very high inclinationin the lower part, which differs significantly from that of the higher part (VV. in Tab.1). This is the consequence of the valley deepeningduring the Quaternary.The obtained results, which were briefly listed above, provide new and sufficiently detailed evidences for clarifying the evolution of thearea from a morphological, genetic and chronological point of view. In conclusion, the landscape we see today began its formation inthe middle Pliocene and evolved over time. It changed its shape under the actions of tectonic events and both continental and marineexogenous processes.