CALCAREOUS TUFA DEPOSITS OF THE ANIENE VALLEY BETWEEN VALLEPIETRA AND MANDELA-VICOVARO (LATIUM, CENTRAL ITALY)

Abstract

Carrara et al. – The calcareous tufa deposits of the Aniene Valley between Vallepietra and Mandela-Vicovaro (Latium,
Central Italy). (IT ISSN 0394-3356, 2006).
Calcareous tufa deposits occurring in the Aniene and Simbrivio Valleys (Latium, Central Italy) between Vallepietra and MandelaVicovaro areas have been studied by means of morpho-geological mapping, mineralogical and facies analysis, malacofauna investigation, geo-archeological research and U/Th dating. The deposits of Subiaco and Mandela-Vicovaro have been studied in detail,
because their formation has modified not only the geomorphic features of the two areas, but has strongly controlled and directed the
human activity. A 15 m thick calcareous tufa body occurring in a bore drilled near Marano Equo, intercalated in a Paleoaniene alluvial
deposits, has also been studied.
After having examined the geological and structural pattern of the valley, controlled by apenninic (NW-SE) and antiapenninic (NE-SW)
faults and by a complex hydrological system, that makes it one of the richest valleys in spring-waters of Latium (most of Roman aqueducts came from the Aniene Valley), the deposits of the different sectors are described. Calcareous tufa deposits, occurring in the
Simbrivio Valley, connected to several cool springs issuing from the right, steep slope are described. The tufa deposits, prograding
vertically and towards the valley bottom, interdigitate and often form highly vegetated terraces, on the front of which still active waterfalls, covered by moss and algae already partially calcified, occur. In the Aniene Valley, upstream of Subiaco very interesting examples of still active tufa deposits, forming fan-shaped bodies, prograding towards the river bed, occur. They are made mainly of phytomicrohermal and stromatolitic laminites, rich in leaf moulds, covered by a thick mat of algae, mosses, liverworts and other hygrophytes. Small barrage tufa deposits can also be observed. In the Subiaco area a tufa deposit, covering an area of about 1 km2
, 70-80 m thick, occurs; it represents a barrage deposit, formed in the gorge at the outlet of the Subiaco basin, probably built up in correspondence to a morphological step of tectonic origin. Its basal portion is mainly made of palustrine, thinly laminated facies, rich in leaf moulds and vegetal frustules, grading upward to massive or grossly stratified, phyto- microhermal framework, containing well-developed large pools, filled with sandy-silty phytoclastic sediments of fluvio-lacustrine environment. The outer part of the deposit is partially covered by vertical hanging festoons and drapes of moss cascade lithofacies. The deposit has been terraced in three orders of terraces by the river, at present flowing in a narrow gorge in the Miocene substratum. The Ist order terrace surface is covered by a thick, pedogenized soil colluvium, rich in volcanic minerals, while the terraces of the IInd and IIIrd orders are covered by reddish-brown or brown soils, poor or devoid of volcanic minerals. In the Mandela-Vicovaro area a continuous calcareous tufa terrace extends along the Aniene gorge for about 3 km, up to 60-70 m thick. It formed as a barrage deposit near Vicovaro, where the emplacement of Middle Pleistocene volcanics, superimposed by younger, coarse alluvial sediments has produced a discontinuity in the river bed. As in the Subiaco deposit, the lower portion of the Mandela-Vicovaro one is composed of thinly stratified, laminitic and microhermal lithofacies, rich in leaf molds and vegetal frustules, of palustrine environment. They grade upward to massive phyto- microhermal litofacies, associated with large sandy-silty pools, rich in malacofauna. The surface of the terrace is covered by brown, calcareous soil. The terrace has been cut deeply as far as the Miocene substratum by the Aniene river and Licenza stream, its main right tributary. The sections of the terrace along the river are partially covered by waterfall moss lithofacies, forming vertical hanging drapes. The study of malacofauna contained in pool filling sands and silts referred the sediments to shallow freshwater, well vegetated environment, under temperate climate. Several Marano Equo, Subiaco and Mandela-Vicovaro samples have been dated by means of U/Th method which allowed the reconstruction of the chronostratigraphic evolution of the three calcareous bodies, referring the first to Late Pleistocene, the second to Upper Pleistocene-Early Holocene and the third to Late Glacial-Middle Holocene. Finally, on the basis of direct and bibliographic researches, the presence of Roman dams built up in the Subiaco area, forming upstream lacustrine basins, at present no longer existent, are discussed.