L’affioramento del Caio (Lago di Corbara, Orvieto, Italia centrale) nell’ambito dell’evoluzione paleogeografica Plio – Pleistocenica della Valle del Tevere: evidenze sedimentologiche e stratigrafiche
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L’affioramento del Caio (Lago di Corbara, Orvieto, Italia Centrale), per i suoi peculiari caratteri sedimentologici e stratigrafici, suggerisceun riesame del contesto paleoambientale e paleogeografico dell’area durante il Pliocene superiore – Pleistocene inferiore.L’affioramento, situato a circa 6 km dall’abitato di Corbara, mostra una successione di depositi marini e continentali del Pliocene medio– superiore e Pleistocene inferiore, passanti da sabbie, a sabbie argillose, ad argille e infine a ghiaie, riferibili alle Unità delle “Sabbie aFlabellipecten” e delle “Argille e Sabbie del Chiani – Tevere” (Ambrosetti et al., 1987). La parte finale della successione, affiorante all’internodi una piccola cava, è costituita da diversi corpi ghiaiosi, che sono stati interpretati come depositi della parte medio - prossimale diun conoide alluvionale, probabilmente deposti in un breve intervallo temporale. Sia nelle argille che nelle ghiaie sono state inoltre rinvenute piroclastiti. E’ stata effettuata sia un’analisi di facies che uno studio biostratigrafico; inoltre, i campioni contenenti materiale vulcanoclasticosono stati oggetto di indagini mineralogico - cristallochimiche, eseguite in diffrattometria a raggi-X da polveri e in microscopiaelettronica a scansione (SEM-EDS). Il rinvenimento di Hyalinea balthica e nannoplancton calcareo del Pleistocene inferiore basaleall’interno di depositi riferibili alle “Argille e Sabbie del Chiani – Tevere”, il ritrovamento di piroclastiti a probabile affinità Toscana, ed infinela presenza di depositi continentali di età confrontabile con quella delle argille forniscono nuovi dati utilizzabili per la ricostruzione dell’evoluzione paleogeografica dell’area durante il Pliocene superiore – Pleistocene inferiore.
The Caio outcrop (Corbara Lake, Orvieto, Central Italy) suggests a re-evaluation of the palaeoenvironmental and palaeogeographicalscenery during the Late Pliocene – Early Pleistocene induced by its peculiar sedimentological and stratigraphical feature. The outcropis located about 6 km from the Corbara village, and shows a succession of marine and continental deposits, varying from sands, toclayey sands, to clay and finally to gravels. The latest part of Caio succession, made of several gravel bodies, outcrops into a smallabandoned quarry. Both clay and gravel bodies contain pyroclastic material. A facies analisys, based on sedimentological and palaeontological features, and a biostrathigraphic study have been made; moreover , the samples with volcanoclastic material were object ofSEM – EDS analisys and X-ray diffrattometry for mineralogical and petrological determination. On the basis of paleontological content,the sand sediments (from 0 m to 15 m) are referred to the “Sabbie a Flabellipecten” Unit (Ambrosetti et al., 1987) for the presence ofPecten (Flabellipecten) flabelliformis (Brocchi) in assemblage with common Amphistegina sp. and benthic microforaminifers typical ofdetritical environments (Conti et al., 1983). The overlaying clayey sand sediments (from 15 m to 17.5 m), in spite of the lack of Flabellipecten, for their macropaleontological content, with no particular variation in respect to underlying sands, are here related to“Sabbie a Flabellipecten” Unit; however, some problems still remain about their stratigraphical attribution. The following (from 17.5 m to23 m) clay sediments are instead referred to the “Argille e Sabbie del Chiani – Tevere” Unit (Ambrosetti et al., 1987). The presence of Amusium sp., Globorotalia inflata (d’Orbigny), common Bulimina marginata and B. elegans, Gephyrocapsa oceanica, Helicosphaerasellii and Calcidiscus macintyrei allows the identification of Early Pleistocene age; this age attribution is supported by the finding of Hyalinea balthica from 19.50 m. The Pliocene – Pleistocene boundary is hypothetically placed at 18.50 m, in the lower part of clay sediments.The gravel bodies (from 23 m to 29 m) are interpreted as middle – proximal alluvial fan deposits, probably deposited over ashort time period. Their age attribution is still an open problem, but some considerations could be made: 1) the reworked microfossil assemblages contain only Middle Pliocene – Early Pleistocene specimens, accompained by H. balthica; 2) the younger clastic componentsof gravel bodies come from Middle – Upper Pliocene biocalcarenites (Amphistegina level sensu Conti et al., 1983); 3) the pyroclastites,found from 17.5 m to 29 m, contain the same pyroxenes not altered with typical habitus, showing no or only little abrasion; 4) onthe basis of preliminary mineralogical and petrological data, it is possible to make some generalizations on the affinity of the volcanic products with the neighbouring magmatic provinces (HKS-Roman Comagmatic Province; KS- Monti Vulsini, Roccamonfina, Ventotene;Tuscan Magmatic Province; ULUD-Umbria-Latium ultra-alkaline district). The presence of ortopyroxenes suggest, for at least part of thepyroclastites, a Tuscan affinity and consequently an older age. On the other hand, our geological study suggests the probable existenceof small eruptive centres in the vicinity; 5) a reworked mollusc assemblage, recorded in the middle-upper part of the Caio outcrop, is composed of poor littoral marine and freshwater species, the last ones referable to the Early Pleistocene. These considerations, althougha Middle Pliocene age is not totally rejectable, allow the hypothesis of an Early Pleistocene age (Upper Emiliano - Siciliano) for alluvial– fan deposits, with the consequence of a minimal age gap in respect to the underliyng marine clay sediments. Thus, Caio’s gravelbodies could be considered as heteropical to upper part of “Argille e Sabbie del Chiani – Tevere” Unit. In this way, the upper portion of Caio outcrop, still marking a local paleoenvironmental situation, testifies the great lateral variability of Early Pleistocene deposits, referableto “Argille e Sabbie del Chiani – Tevere” Unit. Could be ipotized a coastal morphology articulation, somehow inherited but surely exasperated by strong tectonics at Pliocene – Pleistocene boundary: this tectonic phase, with structural axes striking NE – SW to E -W, displaced Pliocene deposits, that were partially reworked, causing a lateral environmental variability, testified by facies heteropy.In a similar scenery, with clay testifying marine environments of increasing depth from east to west, alluvial – fan complexes, as Caioalluvial - fan, took place, draining SW or NW and partially reworking different deposits of the same marine basin in which they came,during a regressive phase of marine level.