Genesi, età, sollevamento ed erosione dei terrazzi marini di Crosia-Calopezzati (Costa Ionica della Calabria-Italia)

483 download

Collocazione:
Il Quaternario Italian Journal of Quaternary Sciences, 16(1), 2003, 43-90
Autore/i:
Luigi CAROBENE
Abstract:

Sono stati studiati i terrazzi marini del settore costiero ad est del Fiume Trionto, nel territorio di Crosia e di Calopezzati, ai piedi del versante nordorientale del Massiccio della Sila. Il substrato tagliato dai terrazzi è costituito da una successione marina pleistocenica, prevalentementesabbiosa alla base e argillosa nella parte alta; a monte, le sabbie basali pleistoceniche poggiano in discordanza su argille e arenarie mioceniche. Nell’area (di circa 17 km2) sono presenti superfici terrazzate raggruppabili in 4 ordini distinti: il I ordine è rappresentato da relitti di superfici, molto degradate, estese tra 131 e 210 m di quota lungo stretti crinali disposti N-S; gli originari depositi terrazzati sono documentati da piccoli lembi di sabbie, ghiaie e calcari detritici algali ben cementati.Il II ordine è rappresentato da relitti di superfici terrazzate estesi tra 80 e 120 m di quota; i corpi sedimentari terrazzati sono costituiti dadepositi sia marini che continentali aventi uno spessore complessivo massimo di 45 m.Il III ordine è rappresentato da superfici abbastanza ben conservate, estese tra 55 e 75 m di quota; il deposito terrazzato, sia marino che continentale, raggiunge lo spessore massimo di 25 m.Il IV ordine è presente con superfici terrazzate poco estese, aventi margine interno a 25-30 m, dove sono raramente riscontrabili esiguispessori di sabbie e ghiaie.I corpi terrazzati del II e del III ordine hanno fornito, grazie al loro spessore, nuovi elementi per comprendere genesi e significato dei terrazzimarini. I lembi del II ordine iniziano con un debole spessore di ciottoli arrotondati trasgressivi, per svilupparsi poi verso l’alto e lateralmente con corpi progradanti di diversa litologia ed ambiente, in eteropia di facies tra loro. Da mare verso terra sono riconoscibili: pelitisabbiose di spiaggia sottomarina di transizione; sabbie clinostratificate di spiaggia sottomarina; ghiaie sabbiose di battigia e di spiaggiaemersa. Nella parte più interna del terrazzo è sempre presente un corpo lenticolare da dulcicolo a salato, chiuso verso mare dalletre unità litologiche marine sopra descritte e avente spessori massimi di 15 m. Nella parte alta sono presenti infine sabbie di battigia odi spiaggia emersa migrate verso terra fino a ricoprire le peliti dulcicolo-salmastre. La successione è chiusa da 5-6 m di conglomerati continentali pedogenizzati. I corpi terrazzati del III ordine, pur avendo uno spessore minore, mostrano sorprendenti analogie con lasituazione stratigrafica descritta per il II ordine.Le osservazioni geometriche e stratigrafiche effettuate sui terrazzi marini ad est del Fiume Trionto hanno permesso: di riconoscere neidepositi terrazzati 9 litofacies principali; di applicare la stratigrafia sequenziale ai depositi del II e del III ordine di terrazzi; di riconoscere nei depositi pelitici di retrobarriera fasi di emersione e di sommersione; di determinare la corrispondenza tra terrazzi e stadi isotopici marini (MIS), e quindi la loro età; di comprendere il ruolo giocato dal substrato argilloso nel processo trasgressivo e l’importanza degliapporti detritici nello sviluppo del sistema barriera-laguna. L’analisi dei caratteri geometrici delle superfici inferiori di erosione marina hapermesso inoltre di caratterizzare con nuovi criteri descrittivi i terrazzi, ad esempio con il “rapporto sollevamento/distanza” ed il “rapportodistanza/età” delle antiche linee di riva; considerazioni sulle curve climatiche ed eustatiche hanno consentito di calcolare la “velocità di arretramento delle falesie argillose” e di riconoscere la “corrispondenza terrazzo marino-alto picco eustatico interglaciale”.La tettonica ha controllato l’evoluzione dei terrazzi mediante il sollevamento, risultato pari a 0,5 mm/a circa, e con gruppi ben precisi dilineamenti tettonici. Dal punto di vista metodologico si propongono infine nuovi schemi per la descrizione e la nomenclatura dei terrazzied un quadro completo dei processi che intervengono nel rimodellamento degli antichi terrazzi sollevati.

The marine terraces of the coastal sector at the foot of the north-eastern slope of the Massiccio della Sila, east of the Trionto River,between Crosia and Calopezzati (Ionian coast of Calabria) were the subject of this study. The terraces cut through a substratum consistingof a Pleistocene marine succession, the base of which is sandy-pebbly and the rest clayey-silty; the layers dip towards the NNE atan angle of 10°-20°; uphill the basal Pleistocene sands lay discordantly on Miocene clays and sandstones.The Miocene-Pleistocene sequence that crops out along the Ionian coast of Calabria today constitutes the proximal part of the sedimentary filling of a basin caused by the subduction of the African lithospheric plate under the Calabrian Arch, a complex chain consistingof a thrust-sheet of continental origin and ophiolite-bearing units. The dramatic uplift that Sila (like Serre and Aspromonte furthersouth) is undergoing in the Quaternary is contributing to the partial emergence of this Miocene-Pleistocene sedimentary wedge, is causingthe terracing along the Tyrrhenian and Ionian coasts and is provoking the strong seismicity of Calabria today. In the area studied,in particular, 4 orders of marine terraces are identifiable and have been the subject of morphological and sedimentological research(Fig. 1) The 1st order is represented by very remodelled relics lying between altitudes of 131 and 210 m; the 2nd by the remnants of terraced surfaces lying between altitudes of 80 and 120 m; the 3rd by fairly-well preserved surfaces lying between altitudes of 55 and 75 m,and the 4th by surfaces lying between altitudes of 20 and 30 m.The conclusions drawn from data collected are synthesised as follows:1)A detailed nomenclature has been proposed to describe the genetic, geometric and morphological characteristics of the terraces and those derived from their remodelling, above all in the presence of a terraced body of a certain thickness (Scheme in Fig. 31).2)The age of each terrace has been established, considering that it is not the result of a single sea level highstand, but of “an interglacialeustatic peak”. The chronology of the four orders of terraces in the area (Fig. 1) is as follows: 1st order (+210 m) = stages 9+11; 2ndorder (+105÷120 m) = stage 7e; 3rd order (+64÷71 m) = stage 5e, 4th order (+25÷30 m) = substages 5a-5c. The uplift rate is 0.5 mm/yr(Tab.1).3)The substratum, which consists of early-middle Pleistocene silty clays in the area studied, played a determining role in the transgressive process. The lithology of the substratum influenced the generation and rapid retreat of the seacliffs (see Point 10), and also favoured the development of an “erosion marine platform” rather than an abrasion one. 4)The terraced deposits have diversified lithologies; and so it is possible to distinguish 9 lithofacies, correlatable to various beach environments.These are (Figs. 6, 14, 20, 25): (Cb) transgressive basal conglomerate; (Pm) marine pelites; (Si) lower grey sands; (Ags)alternating gravel and sand; (Ss) upper yellow sands; (Pds) fresh-water, brackish and lagoon pelites; (C.a.-A.-C.c.) arenaceous limestones-bioturbate sandstones-calcareous shellstones; (Ca) algal limestones; (Cc) continental conglomerates.5)The great thickness that characterises the deposits of the 2nd order of terraces (up to 45 m) and the 3rd (up to 25 m), and their divisioninto distinct lithofacies that distinguish the sedimentary bodies between them in a heteropy of facies, makes it possible to easily identify two depositional sequences (for both orders) indicative of a coastal barrier-lagoon system. In fact, a transgressive systems tract(TST) and a highstand systems tract (HST), divided by a “maximum flooding surface” (MFS) are identifiable (Fig. 33). The regressive depositional system was probably destroyed by the genesis of the most recent terraces. The HST deposit was formed by the strong progradation of the beach ridge, accompanied by aggradation; this would have determined the development of wide back-barrierlagoon or fresh-water basins.6)The stratigraphic analysis of the pelitic successions (Pds) characteristic of the back-barrier basins mentioned above highlight periodsof emersion (paleosoil) and submersion (transgressive beach deposits) (Fig. 22) These testify to eustatic oscillations during the periodof the “interglacial eustatic peak” and the consequent polyphase genesis of the 2nd and 3rd orders.7)The generation of the marine terraces occurred at the end of the middle and during the late Pleistocene, in an extended deformational regime characterised by uplift (0.5 mm/yr) and the recommencement of fault activity. This favoured erosion and the production of detritus;the abundant detrital supply of the Trionto River caused the accentuated progradation of the paleobeaches and the formation of thelagoon-barrier systems, as documented by the 2nd and 3rd orders of terraces.8)In the area studied each order of terraces was formed, at least in part, at the expense of the order immediately above it (Fig. 6, 14,20). Thus, each terrace utilised a considerable quantity of the marine and continental sediments from the terraced deposit immediately above it during its formation. The recycling of these clastic sediments influenced the generation of the lower transgressive surfaces ofthe terraces and explains the carpet of large pebbles (Fig. 34) covering them (the surface is cut into the clays of the substratum) andthe presence of rounded blocks of algal limestone coming from the 1st order of terraces (Ca lithofacies) in those of the 2nd, 3rd and 4thorders.9)During the uplifting of the area, the high eustatic levels of the last interglacials created the characteristic stepped terracing of the area(Fig. 36). In this way the ancient coastlines of the 1st, 2nd, 3rd and 4th orders were gradually “distanced” from the actual coastline of today.The ratio of uplift:distance that can be calculated for each coastline gives angular values varying between 2.1° and 3°. Instead, the ratioof distance:age gives values of the “velocity of distancing” for each coastline varying between 1 and 1.5 cm/yr.10)Our detailed knowledge of the width of the lower erosion surfaces of the terraces and the thickness of the terraced marine deposits make it possible to calculate the velocity of the seacliffs retreat of the terraces of the 2nd and 3rd orders. The deposits of these two terracesare transgressive in the lower part and progradant-aggradant in the upper part; thus they can be linked to the “first semiamplitude ofa eustatic and climatic oscillation” (which lasted approximately 7,500-10,000 years) (Fig. 37). A comparison with the eustatic uplift curveof the last interglacial makes it possible to indirectly deduce the times from the paleocurves and to calculate the partial times of the transgressive phases and the highstand. The rates of the retreat of the seacliffs are reported in Tab. 2; the mean values are 11 and 9 cm/yrfor the 2nd and 3rd orders respectively, while the values referring to the transgressive tract of the lower surfaces only are 34 and 38 cm/yrrespectively.11)The polyphase genesis of a “terrace” (3.6.) requires long time periods that are compatible with an entire interglacial period, to whichan interglacial eustatic peak corresponds. The climatic oscillations into which an interglacial period is divided can produce single sea level highstands (of much shorter time spans and different elevations) to which single and well-defined “shorelines” correspond (Tab. 3).12)Tectonics. The generation and evolution of the terraces were tectonically checked using : a) the ratio of eustatic rise:tectonic uplift;b) the tectonic uplift (equal to 0.5 mm/yr) that raised the most ancient terrace (1st order) to an altitude of 210 m; c) the presence offaults or fractures along which erosion has worked during the different phases of the remodelling of the terraces. The tectonic lineaments have the following mean directions: N26°, 40°, 60°, 120° and 140° (Fig. 38).13)After the uplift and emersion the terraces underwent remodelling, the oldest being the most degraded. The processes operating during the remodelling were of various types that can act for hundreds of thousands of years, but were not continuous as they are dependent on periodic climatic variations. Schematising an uplifted marine terrace as in Fig. 41, it is possible to highlight 6 groups of processes that operated on 6 distinct surfaces: (PF) Paleocliff; (SS) Upper surface of the terrace, (FA) Active cliff; (V.V.) Valley slopes;(F.V.) Valley bottom, (F.C.) Coastal area. The final complex effect is to reduce the area of the upper surface of the terrace, which can become one narrow ridge (tract of one drainage divide with subhorizontal trend) or divide itself into more or less isolated culminations of similar height. Over a long period the processes described in Fig. 41 lead to the complete destruction of the terrace.

Abstract (PDF)