PLIO-PLEISTOCENE DELTAIC DEPOSITS IN THE CITTÀ DELLA PIEVE AREA (WESTERN UMBRIA, CENTRAL ITALY): FACIES ANALYSIS AND INFERRED RELATIONS WITH THE SOUTH CHIANA VALLEY FLUVIAL DEPOSITS Roberto Bizzarri & Angela Baldanza Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Perugia (Italy) e-mail: roberto.bizzarri@libero.it ABSTRACT: Bizzarri R. & Baldanza A., Plio-Pleistocene deltaic deposits in the Città della Pieve area (western Umbria, central Italy): facies analysis and inferred relations with the South Chiana Valley fluvial deposits. (IT ISSN 0394-3356, 2009). The Plio-Pleistocene coastal marine deposits in the Città della Pieve area (western Umbria, central Italy) can be considered as a bor- derline among the Paglia Valley marine basin southward, and the South Chiana Valley and Tavernelle-Pietrafitta continental basins, north- and eastward respectively. Deposits, particularly on the Città della Pieve hill, have been traditionally referred to a deltaic envi- ronment and put in relation with river systems (Paleo-Tiber, Paleo-Nestore) draining the Tavernelle-Pietrafitta Basin. Moreover, pre- vious authors considered the Città della Pieve lithostratigraphic succession as a reference scheme to reconstruct the evolution of western Umbria marine environments during Pliocene and Pleistocene. New data allow a revaluation of the Città della Pieve strati- graphic succession, whereas sedimentological and stratigraphic analyses lead to both a detailed description and a deltaic depositio- nal context attribution. At the same time, studies on coeval South Chiana Valley continental deposits suggest a depositional continuity with coastal marine deposits and, in spite of previous authors’ interpretations, a main northern (Chiana Valley) alimentation for the Città della Pieve deltaic system. RIASSUNTO: Bizzarri R. & Baldanza A., Depositi deltizi plio-pleistocenici nell’area di Città della Pieve (Umbria Occidentale, Italia Centrale): analisi di facies e ricostruzione dei rapporti con i depositi fluviali della Val di Chiana. (IT ISSN 0394-3356, 2009). I depositi marino costieri plio-pleistocenici, affioranti nell'area di Città della Pieve (Umbria occidentale, Italia centrale), fanno idealmente da confine tra il Bacino marino della Valle del Paglia, a sud, e i bacini continentali della Val di Chiana meridionale e di Tavernelle- Pietrafitta, rispettivamente a nord e a est. Tali depositi, in particolare nella collina di Città della Pieve, sono stati tradizionalmente riferiti a un ambiente deltizio e messi in relazione con i sistemi fluviali (Paleo-Tevere, Paleo-Nestore) che drenavano il Bacino di Tavernelle- Pietrafitta. Inoltre, gli autori precedenti hanno considerato la successione litostratigrafica di Città della Pieve come uno schema di riferi- mento per la ricostruzione dell’evoluzione paleoambientale dell’Umbria Occidentale durante il Pliocene e il Pleistocene. I nuovi dati stratigrafici hanno permesso una riconsiderazione della successione pievese. Inoltre, l'analisi sedimentologica e stratigrafica ne ha con- sentito sia una descrizione dettagliata che l'attribuzione a un contesto deposizionale deltizio. Gli studi parallelamente condotti su coevi depositi continentali della Val di Chiana meridionale hanno permesso di verificare la contiguità di ambienti deposizionali con l'area pie- vese e di suggerire, contrariamente a quanto proposto in letteratura, una prevalente alimentazione dalla Val di Chiana per l'apparato deltizio di Città della Pieve. Keywords: River deposits; Deltaic deposits; Pliocene; Pleistocene; Umbria; Tuscany. Parole chiave: Depositi fluviali; Depositi deltizi; Pliocene; Pleistocene; Umbria; Toscana. Il Quaternario Italian Journal of Quaternary Sciences 22(2), 2009 - 127-138 1. INTRODUCTION The main features of the Plio-Pleistocene C i t t à della Pieve deltaic deposits and South C h i a n a V a l l e y gravel braided river deposits are here illustrated. The aim is to mark their sedimentological and petrographic affinity and the paleodepositional contiguity, as well as to propose a reliable depositional model. Some resul- ting considerations on origin and provenance of the gravel are also proposed. Since the end of 19t h Century, the Città della Pieve area (western U m b r i a, central Italy) has been considered as a key-zone to understanding Plio-Pleistocene paleoenvironmental evolution across the South C h i a n a Valley (Fig. 1A), particularly in the T r a s i m e n o Lake area, and P a g l i a Valley. Deposits cropping out on the C i t t à della Pieve hill have been traditionally referred to a del- taic environment and linked to the Paleo-Tiber (VE R R I, 1877; 1885; 1889; 1892; 1918; LO T T I, 1926; ME R L A, 1944) or to the Paleo-N e s t o r e river systems (AM B R O S E T T I et al., 1977; 1987; 1989; CA T T U T O et al., 1983; 1992) drai- ning the T a v e r n e l l e - P i e t r a f i t t a Basin (Fig. 1). Eastern ali- mentation, therefore, has been classically considered as prevailing, although some authors also hypothesized a subordinate northern provenance (CA T T U T O et al., 1992). North of Città della Pieve, the South Chiana Valley Basin evolved as a river environment from Middle-Late Pliocene to Early Pleistocene (BA R B E R I et al., 1994; AR U T A et al., 2004; 2006; BI Z Z A R R I, 2006; BA R C H I et al. , 2007; BIZZARRI et al., 2008), becoming a fluvial-lacustri- ne environment, with the early development of “Paleo- T r a s i m e n o”, from Middle Pleistocene onward. On the other hand, AMBROSETTI et al. (1977; 1987; 1989) discus- sed the implications of the Città della Pieve lithostrati- graphic succession and proposed a reference scheme for western Umbria marine deposits (Fig. 2), underlining the occurrence of two main depositional cycles: an Early-Middle Pliocene cycle, made up of three superim- posed Units (“Argille di Fabro”, “Sabbie a Flabelli- p e c t e n” and “Conglomerati di Città della Pieve”), cha- 128 R. Bizzarri & A. Baldanza racterized by an average north-east inclination of depo- sits, and an Early Pleistocene cycle, coinciding with the “Argille e Sabbie del Chiani-Tevere” Unit, whose depo- sits are prevailing sub-horizontal. On the basis of geo- morphologic data (orientation of main valleys, asymme- try on valley profiles, different altitude of river terraces), chiefly documented east and north-east of Città della Pieve, a strong structural and tectonic control on land- scape evolution has been inferred (AM B R O S E T T I et al. , 1989; CA T T U T O et al., 1983; 1992). According to strati- graphic data, previous authors individuate a main tecto- nic event corresponding to “Acquatraversa” phase (Late Pliocene-Early Pleistocene: AM B R O S E T T I et al., 1977; 1987; 1989; CATTUTO et al., 1983; 1992). Most of the data collected during the last few years (BI Z Z A R R I, 2006; BI Z Z A R R I & BA L D A N Z A, 2007) can hardly be included into the former scenario; thus, a revaluation of supply origin, as well as of paleodeposi- tional and stratigraphic implications for the Città della Pieve succession, seems to be appropriate. Figure 1 - A) Geographic location of study area and main geological and depositional domains. B) Simplified geological map for study area (detail). a) Alluvial deposits (Holocene); b) "Paleotrasimeno" fluvial and lacustrine deposits (Middle-Late Pleistocene); c) Ancient Chiana Valley piedmont deposits (Early Pleistocene); d ) T a v e r n e l l e - P i e t r a f i t t a Basin piedmont and fluvial-lacustrine deposits (Early Pleistocene); e ) C h i a n a Valley sandy braided river deposits (Early Pleistocene); f ) C h i a n a Valley gravel braided river deposits (Late Pliocene-Early Pleistocene); g) Città della Pieve deltaic deposits (Late Pliocene-Early Pleistocene); h ) Brackish-transitional deposits (Late Pliocene-Early Pleistocene); i ) Macigno s.l. (Oligocene-Miocene); l) Scaglia Toscana s.l. (Cretaceous-Eocene); m ) L i g u r i d s (Cretaceous-Palaeocene); n) Main extension faults; o) Faults (presumed); p) Main thrusts; q) Main rivers; r) Geological profile tract; s) Sub-horizontal strata; t) 5° to 10° dipping strata; u) Main outcrops: 1) Frazzi Brickyard, 2) CDP1, 3) CDP2, 4) CDP3, 5) CDP4, 6) CDP5, 7) CDP6, 8) CDP7, 9) CDP8, 10) P.gio al Piano, 11) S. Biagio, 12) Casaltoldo, 13) P.gio Vaccaio, 14) Le Coste, 15) La Trincea. A) Collocazione geografica dell’area di studio e principali domini geologici e deposizionali. B) Carta geologica semplificata (ingrandi- mento). a) Depositi fluviali recenti (Olocene); b) Depositi fluvio-lacustri del "Paleotrasimeno" (Pleistocene medio-superiore); c) Depositi pedemontani antichi della Val di Chiana (Pleistocene inferiore); d) Depositi pedemontani e fluvio-lacustri del Bacino di Tavernelle- Pietrafitta (Pleistocene inferiore); e) Depositi di fiumi a canali intrecciati sabbiosi della Val di Chiana (Pleistocene inferiore); f) Depositi di fiumi a canali intrecciati ghiaiosi della Val di Chiana (Pliocene superiore-Pleistocene inferiore); g) Depositi deltizi di Città della Pieve (Pliocene superiore-Pleistocene inferiore); h) Depositi di ambiente salmastro-transizionale (Pliocene superiore-Pleistocene inferiore); i) Macigno s.l. (Oligocene-Miocene); l) Scaglia toscana (Cretaceo-Eocene); m) Liguridi (Cretaceo-Paleocene); n) Faglie dirette; o) Faglie presunte; p) Sovrascorrimenti; q) Fiumi principali; r) Traccia di sezione geologica; s) Strati a giacitura suborizzontale; t) Strati con incli- nazione tra 5° e 10°; u) affioramenti principali: 1) Fornace Frazzi, 2) CDP1, 3) CDP2, 4) CDP3, 5) CDP4, 6) CDP5, 7) CDP6, 8) CDP7, 9) CDP8, 10) P.gio al Piano, 11) S. Biagio, 12) Casaltoldo, 13) P.gio Vaccaio, 14) Le Coste, 15) La Trincea. 2. GEOLOGICAL SETTING The study area (Fig. 1A) is located in central Italy, across the Umbria-Tuscany boundary; it covers a bran- ch of the South Chiana Valley Basin, a NW-SE oriented extensional basin bounded to the east by the Mt. Peglia range, the Paciano-Panicale lineage and the Trasimeno Lake and to the west by the R a p o l a n o - C e t o n a r a n g e . On western areas, the pre-Pliocene substratum is refe- rable to the Scaglia Toscana Auct. and Ligurids A u c t. Units. Whereas on the eastern ones it is ascribable to the Macigno s.l. Auct. Unit, with a subordinate occur- rence of Scaglia Toscana and Ligurids, mainly south of 129Plio-Pleistocene deltaic deposits in the Città Della Pieve area ... Città della Pieve (Fig. 1A). East of Città della Pieve, on both the Paciano-Panicale lineage and the northern ter- mination of Mt. P e g l i a range the substratum is repre- sented by prevailing Oligocene to Miocene “Turbiditic” sandstone Units (Macigno s.l. Auct.); limestone occur- rence is very scarce in the area. Ligurids locally crop out west of Trasimeno Lake (near Castiglione del Lago, S a n f a t u c c h i o and P a c i a n o), whereas they are more widely documented on the eastern side of R a p o l a n o - Cetona range, respectively north and west of Città della P i e v e. The South C h i a n a Valley Basin developed as a marine environment from Late Miocene?-Early Pliocene to Middle Pliocene (PA S S E R I N I, 1965; JA C O B A C C I et al. , 1967; 1969; BARBERI et al., 1994; CO S T A N T I N I & DR I N G O L I, 2002; AR U T A et al. , 2004; 2006); between Middle Pliocene and Early Pleistocene, a continental and coastal marine sedimenta- tion took place. Coastal marine deposits crop out in the C i t t à della Pieve and C h i u s i a r e a s (deltaic deposits), as well as in the eastern side of the R a p o- l a n o - C e t o n a range (beach to offshore deposits). Previous authors attributed the same Early-Middle Pliocene age to deposits on both sides of the valley (Fig. 2), with local occur- rence of Lower Ple istocene deposits both north and south of Città della Pieve ( PA S S E R I N I, 1965; JA C O B A C C I et al., 1967; 1969; AM B R O S E T T I et al., 1977; 1987). Particularly, AM B R O S E T T I et al. (1977; 1987) individuate two main depositional cycles in western Umbria marine depo- sits, Early-Middle Pliocene and Early Pleistocene in age, respectively, separated by a hiatus coincident with “A c q u a- traversa” erosion phase (Fig. 2). COSTANTINI & DRINGOLI (2002) still confirm the Early-Middle Plio- cene age attr ibution for the western C e t o n a coastal marine deposits, whereas a Late Pliocene-Early Pleistocene age has been recently inferred for the Città della Pieve d e l t a i c deposits (Fig. 2: BI Z Z A R R I, 2006; BIZZARRI & BALDANZA, 2007). As a result of the average NE dip of both deltaic and coastal marine sediments (Fig. 1), older depo- sits crop out on the western side of the Chiana Valley Basin, at least across the Cetona–Città della Pieve transect. Lower Pleistocene continental deposits cover a wide area north of Città della Pieve, west of T r a s i m e n o Lake, at least from Tresa Valley Figure 2 - Summarized stratigraphic scheme for Plio-Pleistocene deposits between Città della Pieve (BIZZARRI, 2006; BIZZARRI & BALDANZA, 2007) and Trasimeno Lake areas (CARG project “F. 310 - Passignano sul Trasimeno”). A comparison with Units proposed by AM B R O S E T T I et al. (1977; 1987) for the Città della Pieve area is also provided. CCP=”Conglomerato di Città della Pieve”; ASCT=”Argille e Sabbie del Chiani-Tevere”. Schema stratigrafico riassuntivo per i depositi plio-pleistocenici dell’area tra Città della Pieve ( BI Z Z A R R I, 2006; BI Z Z A R R I & BA L D A N Z A, 2007) e il Lago Trasimeno (Progetto CARG “F. 310 - Passignano sul Trasimeno”). Viene proposto il confronto con le Unità proposte per l’area Pievese da AM B R O S E T T I et al. (1977; 1987). CCP=”Conglomerato di Città della Pieve”; ASCT=”Argille e Sabbie del Chiani-Tevere”. 130 to Farneta village; Upper Pliocene continental deposits are documented only in south-western areas, south from Vaiano-Villastrada alignment (Fig. 1B). The deve- lopment of Trasimeno Basin (Figs. 1A, 2), from Middle Pleistocene onward, reflects the definitive emersion of western Umbria and the sea retreat from T r a s i m e n o hills to Orvieto area (GIROTTI & MANCINI, 2003; MANCINI et a l., 2004; BI Z Z A R R I, 2006; BA R C H I et al., 2007). Sedi- mentological analysis, as well as geological mapping for the CARG “F. 310-Passignano sul Trasimeno” project (BI Z Z A R R I, 2006; BA R C H I et al., 2007; PA Z Z A G L I A, 2007; BI Z Z A R R I et al., 2008), allow us to describe four main Unconformity-Bounded Stratigraphic Units (UBSU), in the area amongst Città della Pieve, the T r a s i m e n o Lake and the T a v e r n e l l e - P i e t r a f i t t a B a s i n , with a t ime span from Late Pliocene to Early Pleistocene (Figs. 1B, 2): i) Città della Pieve d e l t a i c deposits; ii) South Chiana Valley gravel braided river, sandy braided river and piedmont deposits; i i i) Tavernelle-Pietrafitta piedmont and lacustrine deposits; iv) Tavernelle-Pietrafitta lacustrine, swamp and alluvial plain deposits. This paper deals with f a c i e s a s s o c i a- tions, depositional setting and stratigraphic implications of Città della Pieve deltaic deposits and South Chiana Valley gravel braided river deposits (Figs. 1B, 2), hence designated simply as Unit A and Unit B, respectively. 3. FACIES ANALYSIS Some outcrops (Fig. 1, Tab. 1), roughly aligned along a SW-NE transect, have been chosen as repre- sentative of the facies variation documented in the area (BIZZARRI, 2006; 2007). Furthermore, they allow both the collection of paleontological and stratigraphic samples and statistic sedimentological analyses. facies associations are described (Fig. 3): A1 facies association – Mixed marine and brackish deposits – It is made up of heterogeneous deposits represented by alternations of three prevailing facies: i) 10÷20 cm thick alternations of silty sand, silt and clay, with occurrence of wave ripple and clay chips on coarser horizons. ii) Ripple-laminated, moderately well sorted fine to medium sand, with <2 cm thick clay horizons. iii) 0,5 to 1 m thick, weakly cemented lenticu- lar-shaped conglomerate, made up of sub-rounded to rounded, plate to equi-dimensional cobbles (φmax12-15 cm, MPS-Mean Particle Size, s e n s u NE M E C & ST E E L, 1984, ranging from 4 to 8 cm). Parallel orientation of flattened clasts is common, as well as surface oxidation and borings. Samples collected on fine-grained depo- sits reveal a brackish mollusc assemblage, associated to rare benthic foraminifera and Late Pliocene-Early Pleistocene nannofossil assemblages (Tab. 2). A2 facies association - Proximal delta front d e p o s i t s – Association is represented by about 2 m thick, wedge-shaped, roughly cross-stratified gravel, laterally grading to sand, alternated to up to 5 m thick, dune- to ripple-laminated sand (Figs. 3A-C). Gravel bodies locally show a channelled basal surface, chiefly on E-W oriented cuts, and are mainly represented by rounded to well rounded, diffusely bored, limestone medium pebbles to coarse cobbles (Fig. 5A). Gravel lithology implications will be discussed further (see § 3.3, 5). Sandy bodies, made of well sorted, medium to fine sand, show an upward transition of sedimentary structures from 2D-dunes to wave ripples (Fig. 3C). Some less than 5 cm thick, leaf-bearing silty horizons locally occur (Fig. 3B). An oligotypic association with oysters and pectinids, as well as Late Pliocene-Early Pleistocene nannofossil assemblages, have been docu- mented on sandy horizons (Tab. 2). A3 facies association - Outer delta front deposits A3a - Sand bars – They are organized in sets of about 4 m thick, parallel cross-laminated sand bodies, made up of medium to fine, locally cemented and diffu- sely bioturbated, well sorted sand (Fig. 3D). Each body shows a similar organization, with a mollusc horizon (mainly Flabellipecten sp.) at the base, a tangential and concave-upward cross-lamination and a diffuse Thalassinoides isp. and Ophiomorpha isp. bioturbation. Scattered pectinids, oysters and rare gastropods are also present, whereas only scarce Late Pliocene-Early Pleistocene benthic foraminifera and nannofossil assemblages have been documented (Tab. 2). A3 b - Interdistributary bay deposits – The sub association is represented by up to 2 m thick, locally ripple-laminated, fining-upward moderately sorted medium to very fine sand, alternated to 10 cm thick pebbly sand (Fig. 3E). Erosion surfaces and bioclastic lags are common, as well as large pectinid horizons. Irregular cementation also occurs. Some levels also grade upward to parallel-laminated silty clay. Scarce Late Pliocene-Early Pleistocene benthic foraminifera and nannofossil assemblages (Tab. 2) have been col- lected. A4 facies association - Prodelta deposits – Association A4 is represented by massive to thinly lami- nated silty clay (Fig. 4), passing upward to sandy silt and very fine sand; cemented horizons also occur. Marine macrofossils, vegetal remains, rich foraminifer Table 1 - Geographic data (Latitude, Longitude, Elevation a.s.l.) of sampled sites and sedimentological logs. Dati geografici (Latitudine, Longitudine, Quota s.l.m.) dei siti campionati e dei log sedimentologici. 3.1 Unit A - Città della Pieve deltaic deposits The Unit is characterized by about 250 m thick superposition of coastal marine deposits, related to a deltaic environment, with an average 5°÷8° NE dip. Five R. Bizzarri & A. Baldanza 131 and Early Pleistocene nannofossil assemblages have been documented (Tab. 2). 3.1.1 Depositional architecture A transition from distal to proximal environments can be documented in the SW-NE distribution of facies associations (Figs. 4, 6). Concerning the attribution of facies associations to sub-environments (Fig. 4), mixed marine and brackish deposits of the A1 facies associa- tion represent the distributary channel zone, at the tran- sition between the external delta plain and the delta front. In turn, the latter seems to be articulated, both along- and across-shore (NW-SE, NE-SW respectively), with documentation of proximal and distal mouth bars (A2- A3 a), as well as of interdistributary bays among channel mouths (A3 b). On proximal zones, mouth bars show alternations of cross-stratified prograding beach- face gravel and dune- to ripple-laminated shoreface sand (A2). Outer delta front is represented by tangential cross-laminated sandy dunes (A3a), related to high flow velocity and shallow water conditions. Finally, silty clay deposits correspond to the prodelta (A4). Facies asso- ciations suggest a broad and long-lasting river-fed coa- stal environment. Both river and marine sedimentation Table 2 - Paleontological assemblages collected on main sampled sites. Associazioni paleontologiche documentate nei principali siti campionati. Plio-Pleistocene deltaic deposits in the Città Della Pieve area ... 132 processes are well represented (Fig. 5C), with important effects on deposit organization. This leads to a hypothesise of shallow water, wave dominated delta, in which river supply and wave reworking alternated (Fig. 6). The large amount of gravel implies high-competence river flows, at least during build-up phases, and sugge- sts alimentation from one or more gravel braided rivers. Due to a total 250-300 m thick- ness of the Città della Pieve del- taic deposits, a long-lasting river system may be supposed, although a discontinuous sup- ply, related to the complex interplay of climatic, tectonic and eustatic changes, may be expected. 3.2 Unit B – South Chiana Valley coarse-grained river deposits The Unit is constituted of at least 200 m thick continental deposits, showing a common 5÷10° NE inclination. Two main f a c i e s associations have been documented: B1 facies association - Channelled gravel deposits - It is represented by about 1-2 m thick and 10 m wide lenticular- shaped bodies, with a bi-parted gravel and sand fill. The lower- most part is filled with 0,5-1,5 m thick clast-supported gravel (Fig. 3F); clasts are variable in shape but always from sub- rounded to well rounded, and range from coarse cobbles to fine pebbles, with MPS~10-15 cm. Limestone lithology domi- nates (Fig. 5B). The a and b axes tend to dispose horizontal- ly or with a ( t ) b ( i ) current imbri- cations (CO L L I N S O N & TH O M P S O N, 1982). A subordinated poorly to moderately sorted, very coarse sand/granules matrix also occurs. A weak cementation has been locally documented, as well as occurrence of surface oxidation, due to subaerial exposition. The uppermost part of channel fill consists of trough cross-laminated, well-sorted medium to very coarse sand; locally cross-lamination is not clearly recognizable. B2 facies association - Roughly organized overbank d e p o s i t s - Association B2 i s mainly represented by finer deposits, compared to associa- tion B1, with prevalence of sand and only a local sensible amount of clay and silt (Fig. 4). Sandy deposits are represented by tabular to lenticular bodies, 2÷5 m thick and up to 20 m long, made up of through cross-laminated, moderately sorted to well sor- ted, fining-upward coarse to fine sand. Clay/silt depo- sits are subordinate and represented by alternations of weakly cemented, massive sandy silt, massive clay bearing freshwater molluscs, mainly Gyraulus albus Figure 3 - Described facies associations. A) Gravel-sand alternations of A2 facies association (NE-SW cut). B) Channelled gravel of A2 f a c i e s association (E-W cut); Sy=leaf-bearing silty horizons. C) Particular of dunes and ripple in the sand from A2 facies association. D) Cross- laminated and bioturbated sand of A3a facies association. E) Ripple-laminated sand and silt- clay draping of A3b facies association F) Gravel-sand alternations of Unit B. G, H) Thin section features of calcarenite clasts from A2 gravel. Associazioni di facies descritte. A ) Alternanze ghiaie-sabbie dell’associazione di facies A2 (taglio NE-SW). B) Ghiaie a geometria canalizzata dell’associazione di facies A2 (taglio E-W); Sy=livelli siltosi con materia vegetale (foglie). C) Particolare dell’organizzazione a dune e ripple dell’associazione di facies A2. D) Sabbie a laminazione incrociata, bioturbate, dell’associazione di facies A3a. E) Sabbie con ripple e drappeggio di silt-argilla dell’associazione di facies A3b. F) Alternanze ghiaie-sabbie dell’Unità B. G, H) Sezione sottile di clasti di calcarenite appartenenti ai ciottoli dell’associazione A2. R. Bizzarri & A. Baldanza (MULLER) and Bithynia leachi (SHEPPARD), and lami- nated silty clay with root traces and vegetal remains (paleosoils). 3.2.1 Depositional architecture Deposits of Unit B (Figs. 4, 6) can be referred to a braided river environment. The roughly-organized over- bank, with only local occurrence of paleosoils, suggests a high lateral mobility of interbedded channels in the river system. Concentration of channelled bodies in well defined portions of the succession, with interbedding to overbank sands, leads to the hypothesis of an intermit- tent river activity, with alternation of high transport and long-lasting low activity phases, the latter recorded as erosion surfaces and paleosoils, with interpositions of overbank sands. Gravel imbrications, as well as through cross-lamination of the sands, testify the existence of a channelled river current and refer B1 facies association to channel fill. Although the scarce gravel organization could be partially explained with a mass movement component, inside river channel, and large clast move- ment itself requires high current velocity, a(t)b(i) imbri- cations are related to sub-critic flow conditions (Froude number Fr<1: CO L L I N S O N & TH O M P S O N, 1982; EI N S E L E, 1992). Sand is in vertical continuity with gravel, and appears to be part of the same channel fill; through cross-lamination which is indicative of both high current velocity, still in condition of sub-critic flow, and reduced channel depth. The channel bi-partition, with a lower gravel and an upper sandy fill, partially resembles the Donjek type river model (MI A L L, 1985); it can be inter- preted in terms of different sedimentation phases, with alternation of gravel and sand transport and deposition by bed load streams inside channels. Most of the sedi- mentation took place shallow water channels during floods, with large emersion of deposits during low water phases. Channels of B1 facies association repeat for some tenth of metres, and irregularly alternate to B2 f a c i e s association deposits; the latter represent an unchannelled flow/sheetflood dominated environment, only temporarily involved by very fine sedimentation and soil development, referable to the floodplain. 3.3 Preliminary statistic analyses on gravel On gravels of f a c i e s association A2 and B1, two distinct statistic analyses have been carried out, in 133 Figure 4 - Top: geological profile across Units A and B; bottom: selected sedimentological logs crossed by the geological profile. C.N.Z.=Calcareous Nannofossil Zones. See Figure 1B for tract localization and Table 1 for geographical data (Latitude, Longitude, Elevation a.s.l.) of sedimentological logs. In alto: sezione geologica attraverso le Unità A e B; in basso: log sedimentologici attraversati dalla sezione. C.N.Z.= Zone a Nannofossili Calcarei. La traccia di sezione è riportata in Figura 1B, mentre i dati geografici (Latitudine, Longitudine, Quota s.l.m.) dei log sono riportati in Tabella 1. Plio-Pleistocene deltaic deposits in the Città Della Pieve area ... order to reconstruct paleocurrent patterns and gravel li- thology, and to verify the hypothesis of a common sour- ce. Despite their very preliminary value, the collected data support the hypothesis of relations between the Città della Pieve deltaic deposits and the South Chiana Valley gravel river deposits. Gravel lithology from four outcrops referable to A2 f a c i e s association and from four outcrops referable to B1 f a c i e s association has been considered (Figs. 5A, 5B). For each outcrop, a 1 m high and 0,5 m long transect was considered, with determination of lithology of every clast inside. Two lithologies prevail: i) Cretaceous-Palaeogene Tuscan and Ligurid calcarenite and micrite, ii) Macigno s.l. sandstones. The percentages of other lithologies, parti- cularly Pliocene calcarenite, are negligible. Limestone prevails on sandstone, with calcarenite dominance (Figs. 5A, 5B); the bioclastic component of the calcare- nites document a supply of various benthic and plank- tonic microfossils, Cretaceous to Oligocene in age (Figs. 3G-H). The micropaleontological assemblages (D i s c o c y c l i n a spp., N u m m u l i t e s spp., A l v e o l i n a s p p . , G l o b o t r u n c a n a spp., Globigerinoids, Globorotalids, Bryozoans, Crinoids, Echinoids, red Algae), characteri- zed by marked reworking phenomena, show affinities with the “Calcareniti di Dudda” A u c t. (Tuscan succes- sion) and “Monte Morello” Auct. (Ligurids) Units. On the whole of coastal marine deposits, paleocurrents con- centrate along two main directions, SW-NE and NW- SE, respectively orthogonal and parallel to the paleo- coast. Within the first group, SW directed currents pre- vail on NE ones, while NW and SE directed currents have equi- valent distribution (Fig. 5C, left). The paleocurrent pattern can be related both to a SW coastal system progradation, linked to river supply, and to a redistribu- tion of deposits alongshore. On the other hand, paleocurrents measured on a single outcrop referable to Unit B (La Trincea: Fig. 5C, right), although docu- menting only local flow direc- tions, seem to be consistent with a W-NW provenance of river deposits. 3.4 Depositional model The two described units delineate a depositional and paleoenvironmental contiguity, during Late Pliocene and the beginning of Early Pleistocene, between the South C h i a n a Valley continental basin and the Città della Pieve delta. The col- lected data lead to the recon- struction of a gently seaward incl ined delta profi le, with subenvironment articulation both along- and across-shore (Fig. 6). Mouth bar deposits, seaward decreasing in grain- size, were redistributed along- shore as gravel beaches and organized as prograding gravel and sand dunes across- shore, with interposition of shoreface-like deeper inter- distributary bays. On a profile parallel to paleocoast, shallower and deeper zone alternate, referable to chan- nel mouths and bays respectively (Fig. 6B). The C i t t à della Pieve delta was fed by one or more braided rivers (Figs. 6A, C). Leaf-bearing silt horizons interposition to gravel (Fig. 3B), as well as occurrence of mixed marine and brackish deposits, document the transitional zone between the two environments (Fig. 6). 4. PALEONTOLOGICAL AND STRATIGRAPHIC DATA Mollusc and foraminifera assemblages of Unit A are scarce and oligotypic on gravelly and sandy depo- sits (A1 to A3 f a c i e s associations), whereas prodelta deposits (A4 f a c i e s association) are characterized by richer and more diversified assemblages (Tab. 2); in any case, they lack biostratigraphic significance. On the other hand, nannofossils provide better stratigraphic data, and two main assemblages have been documen- ted. Samples collected on sands (S. Biagio, CDP1, CDP4 outcrops: Figs. 1B, 4; Tab 1) are characterized by the occurrence of small Gephyrocapsa spp. and can be referred to Late Pliocene-Early Ple istocene (MNN18–19a Nannofossil Zones). Early Pleistocene richer assemblages come from P.gio al Piano and F.ce Frazzi quarry outcrops (Tab. 2): the former is characteri- zed by small G e p h y r o c a p s a spp. and C a l c y d i s c u s 134 Figure 5 - Statistic analyses on gravel. Lithology of A2 (A) and B1 (B) facies associations gra- vels on eight selected outcrops (see Figure 1B and Table 1 for outcrop sites, text for details). C) Paleocurrent patterns (flow directions) derived from A2 gravel on Città della Pieve area (left) and from B1 gravel on La Trincea outcrop (right). Analisi statistiche sui ciottoli. Litologia dei ciottoli provenienti da otto affioramenti selezionati e riferibili alle associazioni di facies A2 (A) e B1 (B) (si veda Figura 1B e Tabella 1 per l’ubicazione degli affioramenti ed il testo per maggiori dettagli). C) Distribuzione delle paleocorrenti (direzio- ne dei flussi) dedotta dai ciottoli dell’associazione di facies A2 nell’area pievese (a sinistra) e da quelli dell’associazione di facies B1 nell’affioramento de La Trincea (a destra). R. Bizzarri & A. Baldanza macyntirei (BUKRY & BRAMLETTE) LOEBLICH & TAPPAN and can be referred to MNN19b Zone, whereas the latter, for the presence of medium Gephyrocapsa spp. (sensu RAFFI, 2002), can be referred to MNN19b-c Zones. Both marine and brackish Early Pleistocene mollusc and foraminifer assemblages (G. inflata - Gl. cariacoensis Zones), reported south and north of Città della Pieve (AMBROSETTI et al., 1987), agree with the collected data. A Late Pliocene-Early Pleistocene age for the Città della P i e v e deltaic deposits can be presumed (Figs. 2, 4). Differently to previous authors (AMBROSETTI et al., 1977, 1987), Early to Middle Pliocene foraminifera assembla- ges, with occurrence of Globorotalia puncticulata (DESHAYES) and G. gr. crassaformis GALLOWAY & WISSLER, as well as the nannofossil assemblages dominated by Discoaster spp. (AMBROSETTI et al., 1977), have not been documented. Unit B continental deposits are very scarce in paleontological content; mollusc assemblages (Tab. 2) col lected during the CARG-P a s s i g n a n o p r o j e c t (BIZZARRI, 2006; BIZZARRI et al., 2008), and the remains of Anancus arvernensis ( CR O I Z E T & JO B E R T) reported from gravel bodies in the Vaiano area (ARGENTI, 2004), allow the attribution of Unit B to Late Villafranchian (Figs. 2, 4). 5. DISCUSSION In order to allow a reliable inference of the new data here proposed, some considerations about depo- sitional and stratigraphic implications of the Città della P i e v e deltaic deposits are needed. The occurrence of two main depositional cycles (Fig. 2) proposed by AMBROSETTI et al. (1977; 1987) based upon two assump- tions: i) deposits have a noteworthy difference in age and ii) they are divided by a main angular unconformity. These statements also lead authors to propose simple superposition contacts among units of lower cycle, and to mark the occurrence of a large non depositional and/or erosive gap, covering a time span from Late Pliocene to Early Pleistocene (“Acquatraversa”: AMBRO- SETTI et al., 1977; 1987). The gravel and sand alternation (A2 facies association), in fact, develops with stable fea- tures along about the 2/3 of Città della Pieve hill thick- ness and not only on its top, as it was stated by pre- vious Authors, showing lateral continuity to sand of A3 f a c i e s associations (Fig. 4). Furthermore, nannofossil assemblages at the top of F.ce Frazzi deposits (A4 f a c i e s association) and on sandy deposits (CDP1, CDP4 outcrops: A3 facies associations) can be referred to Early Pleistocene and to Late Pliocene-Early 135 Figure 6 - Depositional model. A) Paleoenvironments distribution in plan. B) Across-shore section. C) Along-shore section. 1-1’, 2- 2’=section tracts. Modello deposizionale. A) Articolazione in pianta dei paleoambienti. B) Profilo ortogonale alla paleocosta. C) Profilo parallelo alla paleo- costa. 1-1’, 2-2’=tracce di sezione. Plio-Pleistocene deltaic deposits in the Città Della Pieve area ... Pleistocene, respectively (Tab. 2). This little difference in age suggests f a c i e s eteropy between clays and sands, or a partial superposition of clays top to sands (Fig. 4), but it is in contrast to the hypothesized vertical continuity between “Argille di Fabro” clay unit and “Sabbie a Flabellipecten” sandy unit proposed by pre- vious authors (AMBROSETTI et al., 1977; 1987). Analogous relations of lateral continuity/partial superposition occur between A2 and A1 facies association (Figs. 4, 6, Tab. 2). As a consequence, hypothetic time lines cross facies associations of Unit A (Figs. 4, 6, Tab. 2) rather than corresponding to lithological variations, as pre- viously inferred by AM B R O S E T T I et al. (1977; 1987). The hypothesis of vertical unit superposition, indeed, is not confirmed. All these deposits are referable to a deltaic environment, with a time span from Late Pliocene to Early Pleistocene: a single stratigraphic unit, grouping the whole Città della Pieve deposits and including also mixed marine and brackish deposits of A1 facies asso- ciation, seems to be more reliable. On the other hand, all the Città della Pieve deposits show a common, less than 10° north-eastern dip, and main unconformities, referable to 3r d order cycle boundaries, which have not been documented on outcrop. Both deltaic and mixed deposits (A1 to A4 f a c i e s associations), documented in the Città della Pieve area, belong to a unique strati- graphic unit, Late Pliocene to Early Pleistocene in age, partially referable to the second depositional cycle of the succession proposed by AM B R O S E T T I et al. (1977; 1987) for western Umbria (“Argille e Sabbie del Chiani-Tevere” Unit). A better connection can be found with the Late Pliocene – Early Pleistocene depositional cycle (“C h i a n i - T e v e r e” Unit) recently proposed for Tiber Valley south of Orvieto (GI R O T T I & MA N C I N I, 2003; MA N C I N I et al., 2004). The other topic to debate regards the source and provenance of sediments that build up the Città della P i e v e delta. Taking into account deposit thicknesses, gravel-sand alternations of A2 facies association (Figs. 3A-B) repeat 12 to 15 times in the core of Città della Pieve hill, for a total of 100-150 m, as can be observed on deep ravines west of the town. Comparable values characterize sandy bodies of A3 a f a c i e s a s s o c i a t i o n (Fig. 3D), whereas A4 deposits crop out only for ~20 m and show no variations on sedimentary structures. The whole thickness of Unit A reaches 250-300 m. These values suggest a wide and long-lasting river alimenta- tion for the delta system to accommodate the entire successions, although characterized by a disconti- nuous supply, with alternation of high river discharge and prevailing wave reworking phases. The repetition of the facies associations suggests a cyclic organization in depositional sequences, as the result of interaction among tectonic subsidence, sea-level variations, sedi- ment supply and compaction. An account of the cycli- city is beyond the aims of this work, and study is still in progress. In summary, the facies associations of Unit A can be referred to a wave-dominated delta built in cor- respondence to one or more stable and long-lasting river mouth. This kind of fluvial system, with steady characters along a time span of at least 500 ka, inferred by biostratigraphic data, necessarily implies the hypothesis of a stable, long-lasting and wide catchment area. The present day geological setting underlines the occurrence of two past continental domains (Figs. 1A, 1B), coeval with the Città della Pieve delta, which can supposedly represent source areas for deltaic sedi- ments: the South Chiana Valley Basin to the north, and the Tavernelle-Pietrafitta Basin to the east. Deposits of Unit B indicate the occurrence on the south-western part of Trasimeno hills (Figs. 1A, 1B) of a roughly orga- nized braided river environment, characterized by a distinctive bi-parted gravel/sand fill. Fluvial environment had a discontinuous activity, with the highest gravel transport consequent to main river activity phases. Overbank is represented by prevailing sheetflood depo- sits, alternated with alluvial plain fine sediments and paleosoils. Stratigraphic data (Fig. 2) point out the sta- bility of these environmental conditions for at least 500 ka, between Late Pliocene and Early Pleistocene. Besides stratigraphic data, depositional architecture of that river environment matches with the alternation of river supply and marine reworking described for C i t t à della Pieve deposits. Moreover, the >200 m thickness estimated for Unit B deposits (BI Z Z A R R I et al., 2008) is comparable to values proposed for Unit A. F a c i e s analysis and paleontological determinations mark the occurrence, NE and N of Città della Pieve, of mixed marine and brackish deposits (Fig 1B, Tab. 2), easily referable to delta plain environment and transitional to braided river environment of Unit B. Paleocurrent pat- tern reconstructed for Unit A agree with a NW-SE proximal-distal facies transition. Measures indicate two main orthogonal directions (Fig. 5C); NW-SE values ali- gned on along-shore direction, show a symmetrical distribution and are here related to longshore currents. Across-shore, SW-NE oriented values confirm the dispersion of channel axis about a N-S direction, marking also a clear asymmetry, with a prevalence of SW-directed flows (Fig. 5C), referable to a mean mouth bar progradation in the same direction. The subordinate NE-directed set also testify for across-shore wave reworking. The La Trincea paleocurrent pattern, althou- gh indicative of only local flow conditions, still denotes a coherent trend compared to Città della Pieve ( F i g . 5C), and underlines a local western or north-western provenance of flows. Gravel lithology analyses on both Unit A and Unit B sampled outcrops (Figs. 3, 5A, 5B) reveal a prevalence, or a significant amount, of limesto- ne coming from Tuscan and Ligurid lithostratigraphic Units, although percentages vary with outcrops. Abundance of Cretaceous to Palaeogene bio-calcareni- tes is a feature of Tuscan and Ligurid lithostratigraphic Units, whereas they lack on Umbria-Marche western successions. Sandstone gravels become important only close to the cropping out Macigno s.l. f o r m a t i o n (Figs. 1B, 5A, 5B); on the other hand, western outcrops show an absolute dominance of limestone (Figs. 1B, 5 A, 5B). The scarce sandstone occurrence can be ascri- bed to its low preservation potential and its rapid disag- gregation during river transport; in fact, both marine and continental sands contain a large amount of quartz presumably derived from sandstone erosion. Gravel lithology agrees with a prevailing northern and western sediment supply, from sectors in which Tuscan and Ligurid lithostratigraphic Units are more represented, whereas an eastern supply, due to prevailing sandstone substratum lithology east of Città della Pieve, seems negligible (Figs. 1A, 1B). Indeed, both gravel lithology and paleocurrent pattern are consistent with a northern alimentation for the Città della Pieve delta. Basing on 136 R. Bizzarri & A. Baldanza depositional environment, age, f a c i e s transition and analogous sediment supply, this feeder system can be found on South C h i a n a Valley braided rivers (Unit B) . Nevertheless, the northern hypothesis has been essen- tially neglected by previous authors (VERRI, 1877; 1885; 1889; 1892; 1918; LOTTI, 1899; 1900; MERLA, 1944; CAT- TUTO et al., 1983; 1992; AMBROSETTI et al., 1987; 1989), and an eastern alimentation, throughout the Tavernelle- Pietrafitta Basin, as well as subordinated relations with Tiber Basin, was chosen. The lack of lithologies refera- ble to the Umbria-Marche stratigraphic Units both among Unit A and B gravel, which should be documen- ted on deposits of a large river, with a hydrographical basin of several hundred square kilometres, sharply contrasts with the hypothesis of a Paleo-Tiber delta ( VE R R I, 1877; 1885; 1889; 1892; 1918; LO T T I, 1899; 1900; MERLA, 1944). A local eastern supply hypothesis (Paleo-Nestore: CATTUTO et al., 1983; 1992; AMBROSETTI et al., 1987; 1989) denotes some incongruence with the previously explained data. First of all, pre-Pliocene sub- stratum all around the Tavernelle-Pietrafitta Basin, both on the Paciano-Panicale lineage and the northern termi- nation of M. Peglia range, is today represented by Oligo-Miocene turbiditic sandstones (Macigno s.l. Auct.), whereas limestone Units are not widely diffused and mainly confined on a far, south-western portion (Fig. 1A). This lithological homogeneity reflects on gra- vel, almost exclusively arenitic and with less than 10% of limestone, on all the outcrops of the basin (PAZZAGLIA, 2007). The depositional context appears even more in contrast: the Tavernelle-Pietrafitta Basin was characte- rized, during Late Villafranchian, by a lacustrine- swampy rather than by a fluvial environment, and was widely bounded, also toward Città della Pieve, by a large thickness of piedmont deposits (Figs. 1B, 2). The Tavernelle-Pietrafitta Basin shows, indeed, a centripetal and prevailing eastward river drainage at least during Early Pleistocene, not consistent with the alimentation of the Città della Pieve delta. Thus, the hypotheses involving eastern supply and temporary communication with the paleo-sea between Late Pliocene and Early Pleistocene, still not rejectable, are not satisfactory for a comprehensive interpretation of the collected data. Another possibility to evaluate is an origin from depo- sits far (both in time and space) from their final sedi- mentation area. From this point of view, the reworking of previous deposits, a sort of “cannibalization”, related to a different environment and/or a previous basin must been considered. The hypothesis of reworked gravel, with the Città della Pieve delta representing only the last depositional environment, still leads to a northern or north-western supply. The limestone-dominated deposits can result from substratum erosion at the Miocene-Pliocene boundary or during Early Pliocene; that substratum, tectonically-dislocated and buried under about 400 m thick Plio-Pleistocene deposits, is now only inferred from seismic sections under T r a s i m e n o Lake (BA R C H I et al., 2007; BI Z Z A R R I et al. , 2008). In fact, the small substratum occurrences in the wide Trasimeno hill area (Fig. 1A) are characterized by Ligurids A u c t . (C. del Lago, Sanfatucchio, Paciano) , Scaglia Toscana s.l. (Isola Minore) and Macigno s.l. (Isola Maggiore, Isola Polvese), and only partially reflect the composition of substratum on the eastern side of present-day Trasimeno Lake. On the other hand, mate- rial origin can be somehow put in relation with thick gravel deposits, with prevai ling bio-calcarenit ic Tuscan/Ligurid clasts, deposited during Early-Middle Pliocene on R a p o l a n o - C e t o n a range western side (Conglomerati de La Foce: PASSERINI, 1965; JACOBACCI et a l., 1967; 1969; CO S T A N T I N I & DR I N G O L I, 2002), interpre- ted as northern river fan delta deposits into the R a d i c o f a n i Basin (CO S T A N T I N I & DR I N G O L I, 2002). At the emersion of R a d i c o f a n i Basin area, from Middle-Late Pliocene onward (BA R B E R I et al., 1994), the hypothesis of reworking, due to South Chiana Valley rivers must be considered: the southward shift of river systems led to deposition on the Città della Pieve area between Late Pliocene and Early Pleistocene. 6. CONCLUSIONS Sedimentological and stratigraphic data here pro- posed, still supporting the deltaic origin for Città della Pieve coastal marine deposits, which shed new light on both age of deposits and river supply source. Depositional and paleoenvironmental contiguity, gravel lithology and stratigraphic position mark a clear lateral SW continuity between these deposits and the South Chiana Valley gravel river systems; indeed, they repre- sent the transition to open marine environment. From this point of view, the Plio-Pleistocene Città della Pieve deltaic deposits belong to the South Chiana Valley Basin and can be related to a 3rd order Late Pliocene- Early Pleistocene depositional cycle, only partially cor- responding to “Argille e Sabbie del Chiani-Tevere” cycle of AMBROSETTI et al. (1977; 1987) and comparable to the Chiani-Tevere Unit described in the Orvieto area (GIROTTI & MANCINI 2003; MANCINI et al., 2004). 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