La foresta fossile di Dunarobba (Terni, Umbria, Italia centrale): Contesto litostratigrafico, sedimentologico, palinologico, dendrocronologico e paleomalacologico

Abstract

The Dunarobba Fossil Forest (DFF) is a singular paiaeontological case: namely, it is a perfectly preserved Pliocene forest formed of in situ fossil trees, more than 50 in number and up to 8 m high. The DFF is located near the village of Dunarobba (near Terni, Umbria, in central Italy), and is enclosed in the deposits of the Tiber Basin, which is an extensional basin crossing Umbria from north to south. Continental deposits, from Pliocene to Holocene in age, fill this basin. Four lithostratigraphic units have been recognized in the studied area between the towns of Marsciano and Terni: -"Fosso Bianco" formation (FBf, Middle-Late Pliocene), which was deposited in a complex lacustrine system; - "Ponte Naja" formation (PNf, Late Pliocene), formed of alluvial fan deposits; - "Santa Maria di Ciciliano" formation (SMCf, Early Pleistocene), formed of fluvial alluvial deposits; - "Acquasparta" formation (Af, Early Pleistocene), which is a deposit settled into small isolated lacustrine carbonate basins. The DFF is enclosed in the "Fosso Bianco" formation, in deposits attributed to a coastal lacustrine wetland. Tree trunks are buried by muds and subordinate lignite and sands (facies C1 subassociation, lithofacies a-e). The base of the trunks rests on dark buish grey clayey silts (facies c), interpreted as little evolved and hydromorphic paleosols. Clayey silts with siltysandy non-continuous undulated laminae, sometimes displaying cross-laminations (facies b) and clayey silts with planar, parallel and continuous laminae (facies a) make up deposits yielded by weak wave motion (or distal deposits of a delta system) and small ponds on a wetland lacustrine coastline, respectively; lignite (facies d) evidences swamp organic deposits, whereas uncommon sand lenses (facies e) can be interpreted as due to wave motion (or as deposits of a delta body). The DFF depositional environment was an area subjected to floodings, where the groundwater level was near, or above, the depositional plane for many months in the year. Slow and continuous sedimentation of clastic materials and a high subsidence caused trunks to be buried still during their life. Near the DFF other lacustrine coastal deposits outcrop, which however do not contain in situ fossil trunks (facies C2 subassociation). This depositional succession is characterized by lenticular sandy strata showing planar parallel or undulated laminations (facies f), alternating with muds (facies g); these deposits correspond to sequences formed during stormy wave motion and fair weather, respectively. A clayey sandy silty hydromorphic paleosol, with lignite beds on top (facies h) overlies these deposits. Laminated calcareous sediments, containing freshwater fauna and flora, (facies i) close the outcropping succession; these sediments formed in a small lacustrine basin isolated from clastic inputs. A different wave activity on the lake shores probably controlled the sedimentation of the two facies subassociations. Where morphological barriers or wetland-vegetation hindered the wave action, facies C1 subassociation could be deposited, whereas facies C2 subassociation was deposited where shoreline was directly subjected to wave motion. Palynological aspects - A palynostratigraphic sequence representing the upper 350 cm of the sediments surrounding one of the fossil trunks (trunk no. 49) was studied. The trunk is still in growing position and leans - like all the other trunks - some 10° towards the NE. According to Biondi & Brugiapaglia (1991), all the trunks so far examined appear to belong to one species only, identified as Taxodioxylon gypsaceum (Goppert) Krausel, which became extinct during the Pliocene and had anatomical features similar to the present- day Sequoia sempervirens (Lamb.) Endl. The pollen analyses show (Fig. 14) a qualitatively rich flora with a good proportion of tertiary species that are no longer present in indigenous Italian flora, with a predominance of trees and shrubs (AP) with respect to herbaceous plants (NAP). Forest species most frequently represented in the pollen diagram are: Sequoia-type, Taxodium-type and Pinus subgenus Haploxylon, followed, in decreasing order of frequency, by: Alnus, Larix, Abies, Picea, Pinus subgenus Diploxylon, Zelkova, Tsuga, Cycadaceae (Stangeria-type), Carya, Sciadopitys, Betula, Salix, Quercus, and Castanea; there are also sporadic findings of pollen from Carpinus, Cedrus, Celtis, Ephedra, Eucommia, Fagus, Ilex, Juniperus, Ligustrum, Madura, Nyssa, Ostrya, Pterocarya, Sambucus, and Tilia. We believe that the pollen classified as Sequoia-type should be identified as Taxodioxylon gypsaceum. Although the studies so far carried out suggest that the Dunarobba Forest would consist of a single species, on the contrary pollen analyses show that the forest in that area was mixed. The arboreal taxa found in pollen analyses can be divided into three groups, according to the time of their disappearance from Italy. The 1st group comprises the tertiary species which disappeared in the late Pliocene and/or early Pleistocene times, /. e., Sequoia-type, Taxodium-type, Sciadopitys, Stangeria-type, Eucommia, Nyssa, Celtis; and, among the NAP, Tillandsia-type; the 2nd group includes Cedrus, Tsuga, Pinus subgenus Haploxylon, Carya, and Pterocarya which became extinct during the Pleistocene; the 3rd group includes all the other species still growing in Italy today. The fact that Zelkova, Castanea, and Juglans have to be regarded as indigenous species, is discussed. On the basis of the pollen diagram, the time climate had to be warmer and more humid than at present, if the in situ occurrence of plants like the ancient Sequoia and the abundance of spores of Pteridophyta together with Tillandsia-type pollens, an epiphyte living in subtropical environments, are considered. Edaphic conditions in which Sequoia grew were those of an environment subjected to continual alluvial phases, as confirmed by the costant and abundant occurrence of Cyperaceae. Finally, the finding of pollen from Alnus and Salix, together with Potamogetonaceae, and Alismataceae, is evidence of slow-flowing waters. Moreover, the constant finding of Taxodium-type pollen is further evidence for a coastal marshland close to the Dunarobba area. The Taxodioxyion forest had a luxuriant undergrowth, with abundance of Pteridophyta, mainly Lycopodium-type which is replaced by Osmunda-type in the uppermost layers. This change is probably related to the occurrence of different edaphic conditions. Finally, the Sciadopitys curve is discussed, in order to outline possible climatic conclusions from it. The dendrochronological research carried out on Taxodioxyion gypsaceum is a further support to palynological results. On the basis of the pollen results the sediments of the Dunarobba Fossil Forest showing an evident Taxodiaceae fades are attributed to the Pliocene. A more precise attribution is at this time impossible, because the sediment thickness so far studied is very limited. Dendrochronological aspects - A dendrochronological analysis was carried out on a trunk of Taxodioxyion gypsaceum (Gippert) Krausel, a homoxil wood sample put at disposal by the Umbria Region Archaeological Suprintendence. The wood was in a good state of conservation, owing to the impermeability of the embedding clay. Ring widths were measured with Dendroware Aniol; mean value, standard deviation, autocorrelation coefficient and mean sensitivity were calculated. Abrupt growth changes were also calculated and a spectral analysis was performed to obtain indications on past climate lengths of period. An image analysis system was used to investigate anatomical features. Preliminary tracheidograms were elaborated, and a floating curve of 565 years was built. The first period of the tree life is characterized by narrow rings, then by large ring widths for about 60 years. This pattern may be explained by the social status of the tree within the forest. Subsequently, a light biological trend is shown. In regard to the dendrochronological statistical parameters, the mean sensitivity value was calculated at 100 year intervals (see Table 1): the value is in fact high for the total period of 565 rings (0.332) but it is not constant over the whole period. The recorded increase might indicate a worsening in the external ecological conditions; however it is difficult to date environmental modifications in a particular tree life period. As a matter of fact, if abrupt growth changes are take into account, we see that these are slight, recorded for short periods and present over the whole length of the curve. Thus we think that any environmental modification must have occurred step by step. The autocorrelation coefficient values support the observations made on mean sensitivity, important results arise also from the analysis of the lengths of period (Table 2; Figg. 16-18). These were calculated for the total 565 years of the curve and at 100 year intervals such as the dendrochronological statistical parameters. Second and 3rd order frequences are the most common. The distribution in the real curve is at the right of the theoretical distribution indicating a climate tending to the oceanic one. From the interval analysis a decrease of the 3rd order frequencies versus an increase in the 2nd order ones, can be seen. In particular, uniannual variations reach the 62% frequency in the last 65 years, suggesting a notable degree of environmental variability and a trend towards a continental climate. Beside frequency, other high and medium fluctuations seem to be distinguishable. Spectral analysis allows to isolate 5-6 and 11 year fluctuations, whereas 22 year fluctuation does not appear (Figg. 19). A study by Attolini et. al. (1988) on the same area should be used for comparison. The anatomical analysis provides information on short period...