Imp.Biserni& SHAPE RECONSTRUCTION OF THE PLEISTOCENE/HOLOCENE UNCONFORMITY IN THE GROSSETO ALLUVIAL PLAIN (TUSCANY, ITALY) Giacomo Biserni1, Henk J.A. Berendsen2 & Fabio Sandrelli1 1Earth Science Department, Faculty of Geology, University of Siena, Via Laterina 8, 53100 Siena, Italy E-mail: biserni@unisi.it; sandrelli@unisi.it; 2Department of Physical Geography, Utrecht University, Heidelberglaan 2, 3508 TC Utrecht, The Netherlands ABSTRACT: Biserni G., Berendsen J.A.H. and Sandrelli F., Shape reconstruction of the Pleistocene/Holocene unconformity in the Grosseto alluvial plain (Tuscany, Italy) (IT ISSN 0394-3356, 2004) One hundred two boreholes were drilled in Grosseto alluvial plain (Tuscany, Italy). Coring evidence shows a sharp lithological change that is interpreted as a Pleistocene/Holocene unconformity. This suggests the presence of a paleovalley that was eroded by the Ombrone river during the last glacial interval. Radiocarbon samples and surface’s geometry of lithological changes corroborate this explanation. RIASSUNTO : Biserni G., Berendsen J.A.H. and Sandrelli F., Ricostruzione dell’andamento della unconformity pleistocenica-olocenica all’interno della pianura alluvionale grossetana (Toscana, Italia) (IT ISSN 0394-3356, 2004) La pianura di Grosseto, all’interno della quale scorrono il Fiume Bruna e Ombrone, si estende per circa 370 Km2 ed è delimitata dai rilievi di Castiglione della Pescaia a Nord-Est, dalle colline di Grosseto ad Est e dai Monti dell’Uccellina a Sud (Fig.1). Bravetti & Pranzini (1987), Innocenti & Pranzini (1993), Stea (1995), Bellotti et al., (1999), Bellotti et al., (2001) e Carboni et al., (2002) ricostruisco- no la lenta evoluzione di questa pianura durante il Quaternario. La caduta del livello marino, venutosi a verificare durante l’ultimo glacia- le (Stadio 5e), determinò un abbassamento del livello di base di circa 120 m. I Fiumi Ombrone e Bruna scavarono due paleovalli nelle precedenti alluvioni presso i lati della pianura attuale, lasciando in rilievo la parte mediana. La successiva trasgressione versiliana tra- sformò le due valli in insenature marine in seguito colmate, con velocità diverse dai due fiumi. In questo lavoro, mediante una serie (circa 100) di microperforazioni manuali e alcune perforazioni profonde, viene ricostruito l’anda- mento di una unconformity all’interno della pianura alluvionale grossetana, interpretata, grazie a datazioni radiometriche, come pleisto- cenica-olocenica. La sua forma suggerisce la presenza di una paleovalle erosa dal Fiume Ombrone durante l’ultima glaciazione. Keywords : Pleistocene/Holocene unconformity; paleovalley, radiocarbon dating. Parole chiave : unconformity pleistocenica-olocenica,; paleovalle, datazioni radiometriche. Il Quaternario Italian Journal of Quaternary Sciences 17(2/2), 2004, 443-451 INTRODUCTION The Grosseto plain is delimited by the Castiglione della Pescaia Hills in the northwest, Grosseto Hills in the east, and Uccellina Mounts in the south (Fig. 1). The sedimentary evolution of the Grosseto plain during the Quaternary was previously described based on strati- graphies derived from wells drilled randomly by farmers and with subsurface data collected for a hydrogeologic study (Bravetti & Pranzini, 1987; Bellotti et al., 1999). During the Late Pleistocene, fluvial and aeolian sediments were deposited, which now crop out in low terraces along the borders of the alluvial plain (Mazzanti, 1983; Bravetti & Pranzini, 1987). During the last glacial interval, when sea level dropped approxima- tely 100 m below the present level, the Ombrone and Bruna Rivers presumably cut two valleys into Pleistocene deposits, on opposite sides of the present alluvial plain. The valleys became sedimentary areas for marine and river deposits during the Holocene tran- sgression (Versilian transgression). The southern valley was filled more rapidly because the Ombrone River car- ries more clastic material than the Bruna River. So far, the thickness of the Holocene sediments has not been determined in a reliable way, and eviden- ce for the existence of these paleovalleys was not pre- sented so far. Data from Stea (1995) and Bellotti et al. (2001, 2004) suggest a Holocene thickness of up to 50 m. However, the dates of Bellotti et al. (2001, 2004) do not agree with the sea level curve for the Tyrrhenian Sea (Biserni et al., submitted). If the dates of Bellotti et al. (2001) are plotted on the Tyrrhenian sea level curve (Biserni, 2004), it can be seen that their data deviate considerably from the curve, suggesting that their data are not coherent with the Tyrrhenian environmental evolution during the Holocene. Cause of that these data cannot be used to calculate reliable sedimentation rates. A possible reason for this deviation is that the borehole of Bellotti et al. (2001) was drilled in the chan- nel belt of the river. This inevitably leads to an overesti- mation of sedimentation rates, because channels incise into the Pleistocene substratum. BERENDSEN (1982), TÖRNQVIST (1993) and MAKASKE (1998) suggested to col- lect samples within the alluvial plain to reliably measure sedimentation rates. 444 G. Biserni, H.J.A. Berendsen & F. Sandrelli The purpose of this study was to approximately determine the thickness of the Holocene sediments and to reconstruct the shape of the Late-Pleistocene Ombrone paleovalley that was incised during sea level low stand. METHODS We performed a geomorphological and lithostrati- graphical study and radiocarbon dated organic depo- sits in the Ombrone plain. This has been done by dril- ling a series of boreholes and by analyzing deep mechanical wells drilled by farmers. Three radiocarbon samples were obtained for time control. Coring and alluvial architecture The lithological information was obtained by coring, as there were no outcrops available. Boreholes were drilled in a ~ 6 km long section in a NW-SE direc- tion. Whenever possible, boreholes were drilled at 50 m intervals. The cores reached an average depth of 8-9 m, the deepest one is 15 m and the shallowest one is 4 m. The transect is situated approximately 6 km from the coast and crosses the Ombrone River in Punta dello Spolverino (Fig. 1). The cores reached an average depth of 8-9 m. We used a gouge and piston corer to study the stratigraphy and to collect samples for Accelerator Mass Spectometry (AMS) radiocarbon dating. All sediment cores were described in the field at 10 cm intervals with regard to texture, organic content, color, median grain size of sand (using a sand ruler), gravel content, oxydized iron content, calcium carbona- te content (using a 5 % HCl solution), occurrence of groundwater, shells and other characteristics, following Berendsen & Stouthamer (2001). The lithologic units were subsequently grouped into ‘lithogenic units’ of ‘facies units’, an approach that closely follows Berendsen (1982) and Miall (1985). The structure of facies units in a delta complex is generally called allu- vial architecture. Each unit is characterized by its grain size, composition, internal sequence and external geo- metry. Loss On Ignition (LOI) analyses were carried out on samples from borehole 58. Three typical architectural elements occurring in a Fig. 1 - Geological sketch map of the Ombrone River basin (after Geological Map of Italy, simplified). Carta geologica schematica della parte terminale del bacino del Fiume Ombrone (Carta Geologica dell’Italia, semplificata). 445Shape reconstruction of the ... Fi g. 2 a - Li th ol og y of t ra ns ec t A -A R ic os tr uz io ne li to lo gi ca d el t ra ns et to A -A Fi g. 2 b - A rc hi te ct ur al e le m en ts o f t ra ns ec t A -A E le m en ti ar ch ite tt ur al i d el t ra ns et to A -A Fi gu re 2 - C ro ss s ec tio n th ro ug h A -A i n th e O m b ro ne R iv er . T he li th ol og y is in te rp re te d in t er m s of a rc hi te ct ur al e le m en ts . F or lo ca tio n se e Fi g. 1 S ez io ne li to lo gi ca d el b ac in o d el F iu m e O m b ro ne e in te rp re ta zi on e ge ol og ic a in t er m in i d i e le m en ti d el l’a rc hi te tt ur a flu vi al e 446 meandering alluvial plain were recognized: • Small natural levee deposits, consisting of sandy clay. The wedge-shaped natural levees occur near the Ombrone River. • Channel deposits, consisting of sand, deposited by the Ombrone River. The channel deposits are ribbon- shaped sandbodies. • Floodplain deposits, consisting of gray-brown silty- clay that overlies dark gray blue lacustric clay. Thin layers of organic material (peat) occur intercalated in the floodbasin deposits. The alluvial architecture of the Ombrone area is shown in Figure 2. Two organic samples were selected for Accelerator Mass Spectrometry (AMS) radiocarbon dating using terrestrial macrofossils. Laser diffraction analysis Soil samples were collected randomly all over the alluvial plain, between 50 and 80 cm below the surface. In the low lying part of the alluvial plain, the upper 40- 50 cm are thought to be a result of reclamations (La bonifica grossetana, 1956), hence no samples were taken from the upper 50 cm. All samples were taken using an Edelman hand auger. Grain size of these sam- ples was studied using laser diffraction analysis. Results of the laser diffraction analyses are shown in a soil map of the alluvial plain presented in Figure 4. Geomorphological-geological mapping In mapping the Ombrone fluvial system, borehole descriptions were interpreted following the approach of Berendsen (1982), Miall (1985) and Berendsen & Stouthamer (2001). The main features of the Ombrone River and the Grosseto alluvial plain have been analyzed from a morphological and sedimentological viewpoint, using maps, aerial photographs and field evi- dence. Four morphological elements are shown in Figure 3: ➢ a beach-ridge delta plain parallel to the coastline, characterized by a wide dune-belt close to the present river mouth, that becomes narrow near Castiglione della Pescaia, where high foredunes occur (Mori, 1935; Bird & Jones, 1988); ➢ a wide, fairly f lat and low-lying floodplain located behind the beach-ridge. The floodplain is higher near the channel belt, reaching an elevation of 3-4 m above sealevel; ➢ an approximately 1 km wide channel belt of the Ombrone River. ➢ a higher elevated area can be distin- guished in the middle-Eastern part of the alluvial plain. A scarp of 3-4 m occurs near to “Il Poggiale”, “Le Gorarelle” and “La Rugginosa”, SW, S and ENE of Grosseto. Elevation increases from 5-6 meters to 15-19 m close to the Istia Hills. RESULTS Lithostratigraphy Borehole Ombrone 58 (see Fig. 2), can be regar- ded as representative for the Ombrone alluvial plain, and will be described here in more detail. The lithology has been studied through accurate field and laboratory inspection. The core was divided in 8 lithozones (Fig. 5) reflecting varying environmental conditions. The sediments of lithozone a indicate a sporadic high-energy environment in a generally low energy set- ting, dominated by clay sedimentation. The presence of sand layers and broken and complete marine shells (like Cerastoderma sp., Hydrobia sp., Tellinea pulchella) in a clayey matrix suggests marine flooding. From this layer, at a depth of 8.76 m below the surface, a sample was taken for radiocarbon analyses, which yielded a radiocarbon age of 7724-7651 cal yr BP. The smaller grain size of the overlying lithozone (b) indicates a general decrease of energy. The fine grain size and the presence of a thin peaty horizon in lithozone c indicates a low-energy environment no lon- ger affected by floods and storms. Lithozones d and e are characterized by slightly organic gray-blue clay; the organic content in lithozone e decreases upward. Lithozones (f-h) are characterized by lithological change, from blue clay to gray-brown silty clay, to gray- brown clayey silt characterized by an upwards decrea- sing organic content and a gradual increase of oxida- tion coatings. Those characteristics indicate an alluvial environment connected with the Ombrone River. A change of grain size, the occurrence of reworked fossils G. Biserni, H.J.A. Berendsen & F. Sandrelli Fig. 3 - Morphological and sedimentological sketch map of the Grosseto alluvial plain Carta morfologica e sedimentologica della pianura alluvionale grossetana 447Shape reconstruction of the ... Fig. 4 - Soil map of the Ombrone alluvial plain (after Sevink et al., 1982; modified) Carta dei suoli della pianura alluvionale del Fiume Ombrone (Sevink et al., 1982; modificata) 448 and the presence of Glomus indicates a supply of sedi- ments from the river. The uppermost part of the litholo- gical log is affected by pedogenesis and agriculture. Evidence for a stratigraphic unconformity Approximately 15 new wells mechanically-drilled were accurately examined, in addition to one borehole obtained by Bellotti et al. (2001). The stratigraphies of the wells in the lower part of the alluvial plain (Table 1) are characterized by an alternation of aquatic gray-blue G. Biserni, H.J.A. Berendsen & F. Sandrelli Fig. 5 - Lithological log of Borehole 58 (depth in cm below ground level) Ricostruzione litologica dei sedimenti rinvenuti dal Borehole 58 (la profondità è espressa in cm) clay and dark-blue or gray clay with a varying micro- and macrofossil content in the upper 11-12 m (Biserni et al., 2001), very similar to the sediments of borehole B-58 described above. A sudden and sharp change of sedimentary facies from gray-blue clay and dark-blue or gray clay to brown-yellow-beige clay-silt characteri- zed by oxydation coatings occurs in the Ponte Nuovo and Casotto dei Pescatori wells at a depth of approxi- mately 9-10 m and 11-12 m below the surface respecti- vely. 449Shape reconstruction of the ... Wells drilled in the ele- vated part of the alluvial plain (e.g. Fattoria la Principina, Rugginosa, via Canada, via Madagascar and via Oberdan, see Table 1) show the same brown-yellow-beige clay-silt from the surface on downwards. Samples collec- ted from this depth for paleo- botanical and paleontological analyses are extremely poor in macrofossils, and pollen grains, if present, are not well preserved (e.g. in samples from a depth of 4.0, 4.5, 5.0, 5.5 ,7.0 and 7.5 m below the surface in well Casotto dei Pescatori). These sediments seem to have formed under subaerial conditions. The same lithological and paleobotanical transition occurs at a depth of approxi- mately 16-17 m below the surface in boreholes P. Casole and P. Isonzo that are located close to the river. In Figure 6 the subsurfa- ce elevation of the transition is plotted in section 1,2 and 3. The geometry of the transition suggests the presence of a paleovalley below the present Ombrone river, that may have formed during the last seale- vel lowstand (Bravetti & Pranzini, 1987). New AMS data, here presented, give a new contri- bution to reconstruct the development of this area during the transition between Pleistocene and Holocene. A sample for AMS radiocarbon dating was collected from well Casotto dei Pescatori at a depth of 22 m below the surface. This sample yielded an age of 36400 ± 2800 yr BP. In borehole 58 a radiocar- bon sample from a depth of 8.76 m was dated at 6860 +/- 50 yr BP. These AMS date, lithological observation and the surface’s geometry of lithological changes descri- bed above and reconstructed in Fig. 6 point out that the lithological transition may represent an unconformity, which represents the Pleisto- cene/Holocene boundary. 450 Fig. 6 - Schematic sections through the Grosseto alluvial plain Sezioni schematiche attraverso la pianura alluvionale grossetana G. Biserni, H.J.A. Berendsen & F. Sandrelli CONCLUSIONS The higher part of the Grosseto alluvial plain is lithologically and morphologically different from the low alluvial plain. It is delimited by a scarp of 3-4 m. The higher part is interpreted as a terrace, originally separa- ting two palaeovalleys incised during the last glacial (MIS 2-4) (Mazzanti, 1983; Bravetti & Pranzini, 1987; Bellotti et al., 2001). Based on lithological and micropa- leontological similarities, the surface of the terrace is correlated with the sediments that occur in boreholes below the sharp lithological change. Therefore the litho- logical transition in the boreholes is interpreted to respresent an unconformity at the Pleistocene/ Holocene transition. Radiocarbon ages corroborate this interpretation. The paleovalley most likely formed by incision during the lowstand of the Tyrrhenian Sea. ACKNOWLEDGMENTS We thak Prof. Maria Pareschi and “Acquedotto del Fiora S.p.A.”. REFERENCES Bellotti P., 2000 - Il modello morfo-sedimentario dei maggiori delta tirrenici italiani. Boll. Soc. Geol. It., 119, 777-792. Bellotti P., Belluomini G., Bergamin L., Carboni M.G., Di Bella L., Improta S., Letuov P.P., Mandra L., Potyomkina T.G., Valeri P., and Vesica P., 2001 - Nuovi dati cronostratigrafici sul sottosuolo della piana deltizia del Fiume Ombrone (Toscana Meridionale): Studi costieri, v. 4, p. 33-42. Bellotti P., Caputo C., Davoli L. 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