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

Abstract

The Caio outcrop (Corbara Lake, Orvieto, Central Italy) in the Plio – Pleistocene palaeogeographic evolution of Tiber Valley: sedimentological and stratigraphical evidences.
The Caio outcrop (Corbara Lake, Orvieto, Central Italy) suggests a re-evaluation of the palaeoenvironmental and palaeogeographical
scenery during the Late Pliocene – Early Pleistocene induced by its peculiar sedimentological and stratigraphical feature. The outcrop
is located about 6 km from the Corbara village, and shows a succession of marine and continental deposits, varying from sands, to
clayey sands, to clay and finally to gravels. The latest part of Caio succession, made of several gravel bodies, outcrops into a small
abandoned 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 of SEM – 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 of Pecten (Flabellipecten) flabelliformis (Brocchi) in assemblage with common Amphistegina sp. and benthic microforaminifers typical of detritical 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 to 23 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, Helicosphaera sellii 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 a short 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 components of 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) on the 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 the pyroclastites, a Tuscan affinity and consequently an older age. On the other hand, our geological study suggests the probable existence of 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, although a 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 gravel bodies 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, referable to “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 Caio
alluvial - 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.