SESS 2_16022011_TPR2.pub Il Quaternario Italian Journal of Quaternary Sciences 24, (Abstract AIQUA, Roma 02/2011), Congresso AIQUA Il Quaternario Italiano: conoscenze e prospettive Roma 24 e 25 febbraio 2011 PLIO-QUATERNARY PALEOENVIRONMENTAL EVOLUTION ACROSS WESTERN UMBRIA AND TUSCANY Roberto Bizzarri1, Angela Baldanza2 & Patrizia Argenti3 1Via A. Capitini, 8-06055 Marsciano (PG) 2 Dipartimento Scienze della Terra-Università di Perugia, Perugia 3 Lab. Paleontologia dei Vertebrati- Dip. Scienze Geologiche - Università di Roma Tre, Roma Corresponding author: R. Bizzarri ABSTRACT: Bizzarri R., Baldanza A. & Argenti P., Plio-Quaternary paleoenvironmental evolution across Western Umbria and Tuscany. (IT ISSN 0394-3356, 2011) The area across western Umbria and Tuscany records a complex evolution, from Early Pliocene to Late Pleistocene, due to the interaction among coastal marine and continental environments, and among tectonics and sedimentation. Throughout Pliocene, the extensional Valdichiana Basin underwent marine conditions, but marine deposits encom- pass different ages across the area. From Late Pliocene to Early Pleistocene, a continental, river-dominated environ- ment developed; the Gelasian-Santernian Città della Pieve deltaic deposits belong to the “Chiani-Tevere” cycle and are related to this continental evolution. The Trasimeno Basin development and the NE tilt of both Città della Pieve deltaic deposits and Valdichiana continental deposits are due to Early - Middle Pleistocene regional tectonics. RIASSUNTO: Bizzarri R., Baldanza A. & Argenti P., Evoluzione paleoambientale plio-quaternaria tra l’Umbria occi- dentale e la Toscana. (IT ISSN 0394-3356, 2011) L’area compresa tra l’Umbria occidentale e la Toscana ha avuto una complessa evoluzione durante il Pliocene e il Pleistocene, legata agli articolati rapporti esistenti tra ambienti marino - costieri e continentali, nonché alla tettonica. Nel Pliocene, la Valdichiana meridionale ospita una sedimentazione marina, ma tali depositi non hanno ovunque la stessa età. Un ambiente fluviale si sviluppa tra il Pliocene superiore e la fine del Pleistocene inferiore; i depositi deltizi di Città della Pieve appartengono tutti allo stesso ciclo (“Chiani-Tevere”) e possono essere facilmente correlati, su base sedimentologica e stratigrafica, ad almeno parte dei depositi fluviali della Valdichiana. L’impostazione del Bacino del Trasimeno e la generale inclinazione verso NE dei depositi deltizi di Città della Pieve e continentali della Valdichi- ana, tra il Pleistocene inferiore e medio, dipendono dalla tettonica regionale. Key words: Sedimentology, Paleoenvironmental restoration, Pliocene, Pleistocene, central Italy. Parole chiave: Sedimentologia, Ricostruzione paleoambientale, Pliocene, Pleistocene, Italia centrale. The present-day area across western Umbria and Tuscany, among the southern Valdichiana Basin, the Trasimeno Lake and the Tavernelle-Pietrafitta Basin, underwent a complex evolution from Messinian-Early Pliocene to present day, not yet completely defined. Its earliest evolution phases reflect the interaction between marine and conti- nental environments, both referable to Valdichiana Basin, whereas the successive phases have been strongly influenced by tectonics, such as for the development of Trasimeno Basin. The aim of this work is to provide data collected during the last years (BIZZARRI, 2006, 2007; BIZZARRI & BALDANZA, 2009; ARGENTI et al., in press) and to compare it with previous interpretations (VERRI, 1885, 1918; PASSERINI, 1965; JACOBACCI et al., 1967, 1969; AMBROSETTI et al., 1977, 1978, 1987; CATTUTO et al., 1983; COSTANTINI & DRINGOLI, 2002; ARUTA et al., 2004; BARCHI et al., 2007), in order to propose some reasoned paleoenvironmental and strati- graphic schemes for the study area, mainly for the Late Pliocene - Late Pleistocene interval (Fig.1). The better stratigraphic definition for both Città della Pieve deltaic deposits and Valdichiana - Tra- simeno continental deposits, as well as the sedi- mentological and paleoenvironmental reconstruc- tion, allow to fix some points. Marine deposits show a different age across the area, the older being confined on eastern Cetona Mt. side and buried under continental units in the Trasimeno area. They represent the first depositional cycle (Pliocene cycle: AMBROSETTI et al., 1977, 1987). The Città della Pieve coastal marine deposits, re- lated to a deltaic environment, are younger and belong all to the same successive depositional cy- cle (“Chiani-Tevere” cycle: AMBROSETTI et al., 1977, 1987); moreover, they are sedimentologi- cally and stratigraphically related to the Valdichi- ana fluvial deposits (BIZZARRI 2006, 2007; BIZZARRI & BALDANZA 2009). Deposits belong to a braided river dominated envi- ronment, where thick gravel and sand channels alternate to sandy-silty plain massive sediments (BIZZARRI, 2006; ARGENTI et al., in press). This high -energy environment was associated to the Città della Pieve deltaic environment evolution (BIZZARRI, 2006, 2007; BIZZARRI & BALDANZA, 2009). The local recovery of Anancus arvernensis remains into river channel gravels (Vaiano: AR- GENTI, 2004; Cetona: BOSCATO et al., 2008) docu- 81 - 83 82 ZARRI et al., 2008; ARGENTI et al., in press). In the T. Tresa area, alluvial fan to piedmont deposits outcrop, and their occurrence seems to be confined to Tasso F.U.. In the Tavernelle-Pietrafitta area, a pro- gressing transition from lacustrine to swampy deposits took place during the Tasso - Farneta interval. Finally, sandy and silty-clayey deposits are widely diffused in the area from the Farneta Abbey to the Città della Pieve hills. De- posits are still related to a fluvial envi- ronment: the prevailing sandy sedimen- tation, with common St and Sp cross- laminations, testifies of a less energetic and more organized river pattern. Sandy channel deposits are intermin- gled to clay, the latter bearing pa- leosoils and sheetflood deposits and indicative of a wider and more stable alluvial plain. As at the Tasso F.U. in- terval occasional communications oc- curred among the Tavernelle Basin and the sea nearby, the Pietrafitta swamp environment can be imagined as a lat- eral and temporary abandoned river branch, now clearly related to Valdichi- ana continental environment. Difference in age among deltaic depos- its related to Valdichiana rivers suggest a progressive shift southward of basin depocenter (Fig. 1), presumely related to a tectonic activity of eastern Cetona master fault (BARCHI et al., 2007). The on-set of the Trasimeno Basin is related to NE rotation of the area be- tween Early and Middle Pleistocene, that is also responsible for the north- eastward inclination of both Città della Pieve deltaic deposits and Valdichiana continental deposits (Fig. 1). The evolu- tion model underlines a progressive sea-retreat south-westward during Early Pleisto- cene, probably representing the first indication of the regional extensional tectonics, which deeply modified the area from Middle Pleistocene on- wards. This main event is about 1 My younger than what thought by AMBROSETTI et al., (1977, 1987) and CATTUTO et al. (1983): in fact, it is not due to the so called “Acquatraversa” phase (Piacenzian: AMBROSETTI et al., 1977, 1987) but represents the consequences of a successive regional tectonics, as inferred by BARCHI et al. (2007). In our recon- struction, the main unconformity separating, on seismic sections, Valdichiana marine and conti- nental deposits (U1 unconformity of BARCHI et al., 2007) can be referred to “Acquatraversa”: in that case, this phase represents the moment of diffuse ments the occurrence of Middle Villafranchian fau- nas. As the southern Valdichiana was character- ized by high energy rivers, draining toward the near marine coast and building a large and steady fan-delta in the Città della Pieve area, the Tav- ernelle-Pietrafitta area represented an intermoun- tain closed basin. In the Valdichiana continental basin, a stable allu- vial plain environment persists until the end of Early Pleistocene (Fig. 1). The majority of fossil record regards Tasso and, principally, Farneta F.Us (ARGENTI, 2004 cum biblio). Three main de- positional continental environments are docu- mented in the area through the end of Middle Villa- franchian and the Late Villafranchian (BIZZARRI, 2006; PAZZAGLIA, 2007; BARCHI et al., 2007; BIZ- Fig. 1, Evolution of the study area from Late Pliocene to Late Pleisto- cene. Evoluzione dell'area tra il Pliocene superiore e il Pleistocene superiore. R. Bizzarri, A. Baldanza & P. Argenti 83 Ms. received: Testo ricevuto il Soc. Geol. It., Nuova Serie, 5, 54-56. BIZZARRI R. (2006) - Depositi plio-pleistocenici tra la Val di Chiana e il Lago di Corbara: caratterizzazione sedi- mentologica e ricostruzione paleoambientale. PhD Thesis, Perugia University, 408 p. BIZZARRI R. (2007) - Late Pliocene-Early Pleistocene fluvial paleocoasts in western Umbria (central Italy): facies analysis and sedimentation models. Geoitalia Epitome, 2, 393-394. BIZZARRI R. & BALDANZA A. (2009) – Plio-Pleistocene deltaic deposits in the Città della Pieve area (western Umbria, central Italy): facies analysis and inferred rela- tions with the Valdichiana fluvial deposits. Il Quater- nario, 22 (2), 127-138. BIZZARRI R., PAZZAGLIA F., BARCHI M.R., PASSERI L. & ARGENTI P. (2008) - Inquadramento stratigrafico dei depositi continentali quaternari nel F. 310 “Passignano sul Trasimeno” ed evoluzione dei Bacini plio- pleistocenici del confine Tosco-Umbro. Atti del Con- vegno “Il Quaternario nella Cartografia CARG” (Roma, 21-22 Febbraio 2008), p. 10. BOSCATO P., COLTORTI M. & REGGIANI P. (2008) - Plio- cene Anancus arvernensis (Croizet & Jobert, 1828) remains from Cetona (Siena): stratigraphy, chronology and paleoenvironment. Boll. Soc. Geol. It., 127 (1), 151 -162. CATTUTO C., GREGORI L. & PARISI G. (1983) - Indizi di tettonica pleistocenica nel bacino del T. Tresa (Lago Trasimeno). Geogr. Fis. Dinam. Quat., 1, 16-20. COSTANTINI A. & DRINGOLI L. (2002) – Geologia dell’area fra Chianciano Bagni e Sarteano (Provincia di Siena). Carta geologica, Università di Siena. S.E.L.C.A., Firenze, 2002. JACOBACCI A., MALATESTA A. & MARTELLI G. (1969) - Note illustrative della Carta Geologica d'Italia alla scala 1:100000; foglio 121 "Montepulciano". Serv. Geol. d'I- talia. JACOBACCI A., MARTELLI G. & NAPPI G. (1967) - Note illustrative della Carta Geologica d'Italia alla scala 1:100000; foglio 129 "Santa Fiora". Serv. Geol. d'Italia. PASSERINI F. (1965) – Il Monte Cetona (Provincia di Siena). Boll. Soc. Geol. It., 83 (4), 219-338. PAZZAGLIA F. 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