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Analysis of the Geo-Environmental Conditions as Morphological Evolution Factors of the Sand-Clay Series of the Tiberina Valley and the Dunarobba Fossil Forest Preservation


Based on the large presence of fossil trunks in the fluvio-lacustrine deposits of the Tiber basin outcropping near Todi, Umbertide and Dunarobba (Umbria region, Central Italy), the proponents of this research believed extremely interesting to study such geological environment that may be considered as a "natural analogue". It has been considered worth of interest to study the boundary conditions of fluvio-lacustrine deposits and the factors which have made possible the preservation of vegetable substances because of the maintenance of a reducent environment. Other important factors to be investigated appear to be those ones capable to induce modifications and alterations of the original conditions. Based on such analyses, the following topics have been considered of basic importance:
1) the acquisition of the geological, tectonic and hydrogeological boundary conditions;
2) the definitions, in the area of Dunarobba forest, of the whole stratigraphic sequence along with the structural, hydrogeological and geochemical conditions, the mineralogical and petrographical features, the permeability as regards to endogenetic gases, density and water natural content of rock bodies and finally the tensional and consolidation conditions;
3) the reconstruction of the past geomorphological conditions underwent by the continental deposits and the present geomorphological framework.
Stratigraphic and sedimentological analyses of the Tiber basin outcropping near Tode, Umbertide and Dunarobba have been performed in order to define the several lithostratigraphic units. A preliminary photogeologic analysis (scale 1:70000), to define the structural lineaments of the area, has been completed. Gravimetric prospecting along 2 profiles crossing the basin has been conducted. It provided first significant data on the sandy clay sequence thickness. Preliminary laboratory investigations have been carried out to evaluate the geotechnical properties of outcropping fluviolacustrine deposits.

Five main lineament trends were identified: N10W (Eastern ridge); N40W (Northern sector, Todi); N60W (Central sector, Dunarobba); N20W (Western ridge); W50E (Southern sector). Anomalous patterns have been found northeast of Montecastrilli (radial drainage) and south of Avigliano (Trellis drainage). No clear trends occur at Dunarobba but some lineaments are visible around it. A northwest-southeast trend from Montecastrilli to Dunarobba and southwest-northeast trend northwest of Dunarobba occur. The lineaments at the boundary of the basin coincide with the known tectonic features of the area (faults in the western and eastern ridges). The main trends suggest the presence of large tectonic discontinuities from Todi to Montecastrilli cut (and perhaps shifted) by a lineament from the southwestern corner of the basin toward Acquasparta (crossing Avigliano and Dunarobba). The radial drainage near Montecastrilli suggests an uplift of that sector.

On the basis of the gravimetric map 2 preliminary bidimensional models were built. These models, corresponding to the profiles, were drawn along the west-east direction, which is normal to isoanomalous contours; this makes the model meaningful. The first best fit model showed that the sandy clayey formation has its maximum thickness (800 m) in the central part of the investigated area. Moreover, this model suggested a thickness of t he clayey sequence of about 400 m in the Dunarobba area. The second best fit model showed a greater thickness of the clayey formations, which reach a maximum depth of 1300 m. Samples are mainly constituted of silty clay of medium to high plasticity, with a specific unit weight ranging from 25.5 to 27.0 kN/per cubic metre.
Work programme:

The present research has been subdivided into 8 tasks:
Task 1 Geological, structural and morpho-structural surveys on the outcropping rock formations and drawing of geological cartography.
Task 2 Geophysical prospecting (gravimetric and geoelectrical surveys).
Task 3 Geochemical investigations (chemical and isotopic features of groundwater and endogenetic gases).
Task 4 Assessment of rock bodies permeability (water and gas permeability tests, consolidation state analyses, geotechnical property indexes).
Task 5 Petrographical and mineralogical analyses (diffractometry, microscope, microprobe).
Task 6 Paleontological, palinological and organic matter analyses of the clays surrounding the fossil trunks.
Task 7 Evaluation of interaction between organic (wood) and inorganic (clay) matter (uranium/thorium disequilibrium patterns analysis by alpha-spectrometry).
Task 8 Geomorphological investigation aimed at modelling the evolution of the area (morphological evolution rate, alteration of geotechnical properties of the rocks, secondary permeability development, hydrogeochemical evolution).

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5,Piazzale Aldo Moro 5
00185 ROMA

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