The excavation of clay for bricks production in a quarry close to Dunarobba (Umbria Region, Central Italy) brought to light a significant number of still in life position sequoia tree trunks. Their outstanding state of preservation led to consider the site as a natural analogue; therefore, the undertaken researches were aimed at analyzing the characteristics of the soil masses which have been protecting the trunks of the Dunarobba Fossil Forest until present and at evaluating the effects which have been induced by vulnerability factors acting at the boundaries of the hosting system. Geophysical investigations, soil gas prospecting, geological and structural surveys, as well as logs from mechanical boreholes allowed to reconstruct the evolution and the present boundary conditions of the satellite basin where Dunarobba Forest is located. Such a basin is filled up by Plio-Pleistocene alluvial deposits, about 100 m thick, which unconformably overly the carbonatic Meso-Cenozoic bedrock. On the western bank of this basin the trunks of Dunarobba Forest formed during Upper Pliocene. The trunks, classified as Taxodioxylon gypsaceum, were buried still in life position with a rate of buring of about 4 m/1000 yrs, as suggested by the fades analysis of the surrounding deposits. Results from hydrogeological and geochemical investigations show that the circulating waters concentrate along the main vertical faults and then move upward, because of the high dissolved gas pressure, to meet the low permeability masses represented by the Plio-Pleistocene deposits. The main causes of such a low hydraulic conductivity can be related to the physical properties of these deposits, to their high density conditions, and to their overconsolidation state. Such factors contribute in preventing the migration of circulating waters and the chemical exchange among the different levels and inside each one of them. Therefore each sedimentary horizon behaved as a closed geochemical system as shown by the different values of 87Sr/86Sr isotope ratios in the carbonate fraction of samples collected at various depths along the drilled sequence. All the above mentioned factors, accompanied by a high content of organic matter in the examinated continental soils, contributed in creating a reducing environment that allowed the trunks preservation and guaranteed the low geochemical mobility of some natural radionuclides such as U, Th and Ra.
The stratigraphic, depositional, geomorphological and structural characteristics of the sediments which contain the fossil forest of Dunarobba (FFD) / Ambrosetti, P.; Basilici, G.; DI FILIPPO, M.; DI MAIO, Rosa; Gentili, S.; Lombardi, S.; Patella, D.; Toro, B.. - STAMPA. - EUR 17479 EN:(1997), pp. 81-104.
The stratigraphic, depositional, geomorphological and structural characteristics of the sediments which contain the fossil forest of Dunarobba (FFD)
DI MAIO, ROSA;
1997
Abstract
The excavation of clay for bricks production in a quarry close to Dunarobba (Umbria Region, Central Italy) brought to light a significant number of still in life position sequoia tree trunks. Their outstanding state of preservation led to consider the site as a natural analogue; therefore, the undertaken researches were aimed at analyzing the characteristics of the soil masses which have been protecting the trunks of the Dunarobba Fossil Forest until present and at evaluating the effects which have been induced by vulnerability factors acting at the boundaries of the hosting system. Geophysical investigations, soil gas prospecting, geological and structural surveys, as well as logs from mechanical boreholes allowed to reconstruct the evolution and the present boundary conditions of the satellite basin where Dunarobba Forest is located. Such a basin is filled up by Plio-Pleistocene alluvial deposits, about 100 m thick, which unconformably overly the carbonatic Meso-Cenozoic bedrock. On the western bank of this basin the trunks of Dunarobba Forest formed during Upper Pliocene. The trunks, classified as Taxodioxylon gypsaceum, were buried still in life position with a rate of buring of about 4 m/1000 yrs, as suggested by the fades analysis of the surrounding deposits. Results from hydrogeological and geochemical investigations show that the circulating waters concentrate along the main vertical faults and then move upward, because of the high dissolved gas pressure, to meet the low permeability masses represented by the Plio-Pleistocene deposits. The main causes of such a low hydraulic conductivity can be related to the physical properties of these deposits, to their high density conditions, and to their overconsolidation state. Such factors contribute in preventing the migration of circulating waters and the chemical exchange among the different levels and inside each one of them. Therefore each sedimentary horizon behaved as a closed geochemical system as shown by the different values of 87Sr/86Sr isotope ratios in the carbonate fraction of samples collected at various depths along the drilled sequence. All the above mentioned factors, accompanied by a high content of organic matter in the examinated continental soils, contributed in creating a reducing environment that allowed the trunks preservation and guaranteed the low geochemical mobility of some natural radionuclides such as U, Th and Ra.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
