Imp.Paiero-Monegato TTHHEE PPLLEEIISSTTOOCCEENNEE EEVVOOLLUUTTIIOONN OOFF AARRZZIINNOO AALLLLUUVVIIAALL FFAANN AANNDD WWEESSTTEERRNN PPAARRTT OOFF TTAAGGLLIIAAMMEENNTTOO MMOORRAAIINNIICC AAMMPPHHIITTHHEEAATTRREE ((FFRRIIUULLII,, NNEE IITTAALLYY)) GGiioovvaannnnii PPaaiieerroo11 && GGiioovvaannnnii MMoonneeggaattoo11 1Dipartimento di Georisorse e Territorio, Università di Udine, via Cotonificio 114, I-33100 Udine. ABSTRACT The Pleistocene evolution of Arzino alluvial fan and western part of Tagliamento morainic amphitheatre (Friuli, NE Italy) - The geologic survey of the sheet 065 “Maniago” (CARG Project) allowed to gain founded data for attempting a reconstruction of Pleistocene evolu- tion of this portion of Friuli piedmont plain, located in left hydrographic slope of Tagliamento River. Units recognised by bounding discontinuity are Unconformity-bounded Stratigraphic Units in the meaning used in CARG Project. Eight units have been distinguished: four related to Arzino alluvial fan, three to Tagliamento Morainic Amphitheatre and one to Ragogna Hill southern slope evolution. These units outcrop along Tagliamento left slope and inside Ponte brook valley. Arzino alluvial fan is made by stratified gravel and sandy deposits, Tagliamento basin units are made by glacial (lodgement till and melt-out till) and fluvioglacial sediments (mainly sandy gravel), while the units of Ragogna Hill southern slope are colluvial (muddy diamicton) and alluvial (sandy gravel and sand). Relationships between Arzino alluvial fan and Tagliamento endmorain system, happened during Quaternary age, let to understand the relative chronology of events, erosive and sedimentary, that has interested Pinzano narrow area during this time span. During early and middle Pleistocene through it Arzino Stream flew down and built an alluvial fan; then from middle Pleistocene glacial advances in Tagliamento basin began to interfere with alluvial fan till it was completely covered during last glacial maximum in Tagliamento basin, for which a find provided the age 19.075±160 14C yr BP. As regards older units, probably many events are juxtaposed without recognisable and classifiable bounding surfaces, whereas chrono- logical reconstruction becomes more reliable for late Pleistocene units. RIASSUNTO L’evoluzione pleistocenica del conoide alluvionale dell’Arzino e della parte occidentale dell’anfiteatro morenico del Tagliamento (Friuli, Italia NE) - La presente nota riguarda la porzione di pianura pedemontana friulana posta in sinistra idrografica del Fiume Tagliamento presso Ragogna, dove affiora una successione di unità di età quaternaria, caratterizzabili in base alle discontinuità che le delimitano ed alla composizione litologica dei clasti. Grazie ai nuovi dati acquisiti durante il rilevamento geologico del Foglio 065 “Maniago” (Progetto CARG) i depositi di età quaternaria sono stati distinti utilizzando come unità di riferimento le Unconformity-bounded Stratigraphic Units con il significato utilizzato per la cartografia del Progetto CARG. Vengono qui descritte otto unità: quattro relative al conoide alluvionale del Torrente Arzino, tre dell’anfiteatro morenico del Tagliamento ed una legata all’evoluzione del versante meridionale del Monte di Ragogna. Tali unità affiorano con buona continuità nella scarpata sinistra del Fiume Tagliamento tra San Pietro e Aonedis e all’interno della valle del Rio Ponte. I depositi relativi al conoide del Torrente Arzino sono caratterizzati da ghiaie e sabbie variamente stratificate e classate; quelli relativi al bacino del Tagliamento sono di origine glaciale (till di alloggiamento e di ablazione s.l.) e fluvioglaciale (in pre- valenza ghiaie sabbiose); mentre quelli del versante meridionale del Monte di Ragogna sono di origine colluviale (diamicton limoso- argilloso) ed alluvionale (ghiaie sabbiose e sabbie). Le relazioni tra il conoide alluvionale dell’Arzino e i depositi legati alle avanzate glaciali nel bacino del Tagliamento hanno favorito la comprensione della cronologia relativa degli eventi, erosivi e sedimentari, che hanno interessato l’area presso la stretta di Pinzano durante il Quaternario. Durante il Pleistocene inferiore ed in parte del medio per essa transitavano le acque del Torrente Arzino, che ha costruito un conoide alluvionale che aveva qui il proprio apice. A partire dal Pleistocene medio i depositi legati alle avanzate del ghiac- ciaio del Tagliamento hanno cominciato ad interferire con l’aggradazione del conoide il quale è stato sepolto interamente durante l’ulti- mo episodio glaciale, per il quale un reperto ha fornito la data 19.075±160 14C a BP. Nel caso delle unità più antiche probabilmente si sovrappongono più episodi sedimentari senza superfici limite riconoscibili e classifica- bili; mentre per le unità del Pleistocene superiore la ricostruzione cronologica si fa più affidabile. Keywords: Alluvial fan, Tagliamento Morainic Amphitheatre, Pleistocene, Arzino, Friuli. Parole chiave: Conoide alluvionale, anfiteatro morenico del Tagliamento, Pleistocene, Arzino, Friuli. Il Quaternario Italian Journal of Quaternary Sciences 1166(1), 2003, 185-193 11.. IINNTTRROODDUUCCTTIIOONN The study area (Fig. 1) includes a part of piedmont plain between Tagliamento river, Ragogna hill and western ridges of the morainic amphitheatre. Tagliamento river flows in the last stretch of the valley northern-side of Ragogna Hill, where Arzino stream flows into Tagliamento, that, after crossing Pinzano nar- row, flows out in the plain where it shaped the late Pleistocene plain into a slope 60÷70 m high. Here a complex of Quaternary units outcrops; this is the main object of this paper. Continuity of Tagliamento's left slope is cut off by three tributary brooks, namely Mordaro, Ponte and Fos (Fig. 1). In particular in Ponte brook's slopes, about 2 km long, and in Fos brook's ones, about 1.1 km long, many outcrops allow to observe in the third dimension units outcropping in Tagliamento's left slope and to cor- relate them to Tagliamento amphitheatre ones. This complex morainic amphitheatre witnessed that, during Pleistocene times, Tagliamento valley was the site of a glacier that many times flew out southward in the Friuli piedmont plain. The portion of the glacier intere- sting this area corresponded to "San Daniele" lobe, that flew inside the sag between Ragogna Hill and Susans Hill. Morainic ridges, in the southern side of Ragogna Hill, are set against the versant, then they are continuos eastward as far as San Daniele village, where are inter- rupted by the valley of Corno stream, originate by one of the main and lasting meltwater streams of last glacier 186 G. Paiero & G. Monegato Fig. 1 - Geologic sketch map of the study area and its location (modified after M.U.R.S.T., 1997). Carta geologica schematica dell’area studiata e sua ubicazione (modificato da M.U.R.S.T., 1997). (Feruglio, 1920; Comel, 1955; Croce & Vaia, 1985). Pre-Quaternary substratum outcropping in this area is given by Montello Conglomerate (Massari et alii, 1986), an upper Tortonian - lower Messinian terrigenous sequence and by the Osoppo Conglomerate (Dalla Vecchia & Rustioni, 1996; Zanferrari et alii, in prep.), an upper Messinian – Pliocene continental conglomeratic sequence . Study area lies on the SSE-verging Arba-Ragogna thrust (Fig. 1), the more external thrust of the Eastern Southalpine Chain front (Zanferrari et alii, 2002; Zanferrari et alii, in prep.). Such structure is characteri- sed by tectonic activity from Upper Miocene to Present (Zanferrari et alii, in prep.). This paper is aimed at illustrating the Quaternary succession of a part of Friuli piedmont plain (NE Italy). In particular the complex sequence of existing sedimen- tary units, so far undervalued by detail stratigraphic stu- dies, was sorted out, in order to reconstruct, downflow of Pinzano narrow, the evolution of Arzino alluvial fan and its relationships with glacial and fluvioglacial units of Tagliamento. This study uses new data gained during CARG- FVG Project's geological survey for the new 065 "Maniago" geologic sheet at the scale 1:50.000. 22.. PPRREEVVIIOOUUSS SSTTUUDDIIEESS Early authors surveyed Quaternary deposits of this area since late ‘800 (Taramelli, 1875; Tellini, 1892; Penck & Brückner, 1909). First official geological map was "Udine" sheet of Carta Geologica delle Tre Venezie at the scale 1:100.000 (Feruglio, 1925; 1929b). In this map the glacial deposits, located in the study area, have a Würm age; older deposits given to Riss glacial age are marked in the Borgo di Mezzo terrace slopes (Tellini, 1892; Feruglio, 1929a) and recognised by the presence of a strongly weathered level of gravel, called ferretto, between these glacial units. Later authors accepted Feruglio’s assumptions for amphitheatre gla- cial units, except of Sacco (1937). The latter gave a Mindel? age to Feruglio's Riss one, for the strong reworking of outer morainic ridges; a Riss age to higher morainic ridge, that Feruglio gave to Würm acme, since Riss glaciation were considered the longest; and a Würm age to inner ridges because less developed. Whole Tagliamento morainic amphitheatre was studied also by Gortani (1959), Carraro & Petrucci (1977; 1982) 187The Pleistocene evolution of Arzino ... and Croce & Vaia (1985). Whereas the presence of a Pleistocene glacier inside Arzino valley was witnessed by glacial deposits only in the head of valley; this glacier was probably a lobe of Tagliamento glacier, overflowing through Chiampón and Chianzután saddles (Tellini, 1892; Penck & Brückner, 1909; Zenari, 1927; Sacco, 1937; Comel, 1956; Gortani, 1959). The deposits of Tagliamento left slope were gene- rally studied joined with amphitheatre ones. The depo- sits of amphitheatre are directly related to glacial advan- ces evidenced by morainic ridges; only Di Bernardo et alii (1998) gave a short stratigraphy of the fluvial suc- cession, ascribing all them to Tagliamento basin. 33.. MMEETTHHOODDSS Aimed at planning and addressing properly the field work, the study area has been previously studied by interpreting aerial images and analysing the microre- lief on topographic maps (scale 1:10.000 and 1:5.000) with 1 m contour lines. Each outcrop has been mapped and the observed features were recorded for the geolo- gic data base of CARG Project (Zanferrari et alii, 2000), according to a modified version of the guidelines after Baggio et alii (1997). Sedimentary facies and disconti- nuities were examined in detail to distinguish the ele- ments of the sedimentary architecture. The facies classifications, after Miall (1996) for flu- vial deposits and Eyles et alii (1983) for glacial deposits, have been used; for classification of the latter Bini & Orombelli (1988) review has been adopted. The boun- ding surfaces classification after Miall (1996) (Tab. 1), with Boyce & Eyles (2000) review for glacial deposits, has been used. Hierarchization of bounding surfaces was above all obtained using the evolution degree of the soils developed during their shaping. Units obtained are Unconformity-bounded Stratigraphic Units (ISSC, 1994), in the meaning of CARG Project (Zanferrari et alii, in prep.), and they were split up by relationship between bounding surfaces, lito- facies, petrographic nature of clasts and sedimentary body alteration and deformation. Blair & McPherson (1999) grain-size scale was adopted, while the given color for sediments, weathered horizons and soils refers to the Munsell Soil Color Charts. Radiocarbon data were obtained at Radiocarbon Lab of “Dipartimento di Scienze della Terra” of "La Sapienza" University of Rome. Tab. 1 - Classification of bounding surfaces by Miall (1966) / Classificazione delle superfici limite secondo Miall (1966). RRaannkk DDeeffiinniittiioonn TTiimmee ssccaallee ooff pprroocceessss ((aa)) 1st order Set bounding surface; microform, minor erosion 10-5 - 10-4 - 10-3 2nd order Coset bounding surface; mesoform 10-2 – 10-1 3rd order Dipping 5-20° in direction of accretion; macroform growth increment 100 - 101 4th order Minor channel scour, flat surface bounding floodplain elements; immature paleosol 102- 103 5th order Flat to concave-up channel base; mature paleosol 103 - 104 6th order Flat, regionally extensive or base of incised valley 104 - 105 7th order Sequence boundary; flat, regionally extensive, or base of incised valley 105 - 106 8th order Regional disconformity, basin fill complex 106 - 107 188 A critical element for fixing units was clasts' petro- graphic analysis, that let to distinguish deposits on their source basin, because the two watercourses whose sediments characterised the area have a lithologic diffe- rent source basin: - light and monochromatic carbonate deposits, with dolomites and limestones more than 95%, as concer- ning Arzino stream; - polychromatic carbonates, with remarkable siliceous and volcanic clasts portion and presence of clasts of metamorphics, as concerning Tagliamento river. In the investigated area two different main sites can be distinguished: - Rio Ponte section (A-A’ in fig. 1), a glacial sequence heteropic with Ragogna Hill hillslope sediments, whose most complete sections crop out inside Ponte brook valley, where it has an overall thickness of 90 m, even if some isolated boulders were found 50 m higher than morainic ridges. - San Pietro - Aonedis section (B-B’ in fig. 1), where an alluvial sequence of 60-70 m thick is exposed in left fluvial slope of Tagliamento river from Pinzano Bridge to Aonedis. Some stratigraphic units are present in both sec- tions because their deposits are phisically continuous from one to another. 44.. RRIIOO PPOONNTTEE SSEECCTTIIOONN The Rio Ponte section is 500 m long and 150 m high. It is located in the head of Ponte brook valley. Here many units crop out (Zanferrari et alii, in prep.) (Fig. 2): - the complex Rio Ponte Unit, having source from geo- morphologic evolution of southern side of Ragogna hill, - three different glacial units: Plaino, Santa Margherita and Canodusso synthems, related to Tagliamento Glacier expansions. 44..11.. RRiioo PPoonnttee UUnniitt ((UURRPP)): this unit outcrops only inside Ponte brook incision and it is made by different sedimentary bodies with an avera- ge dip of strata of 25-30 degrees southward; unit has been divided in the following facies: - Massive to stratified gravel (Gci-Gh), locally cemen- ted; clasts are subangular and carbonate, resulted from Montello Conglomerate and Osoppo Conglomerate erosion; texture is clast-supported to matrix-supported; matrix is muddy and weathered, having a strong brown (7.5YR4/6) color. Interstratified sandstones are present, laminated to massive, about 50 cm thick. These are deposits related to small fans originated from Ragogna Hill southern side incisions and active before the deposition of glacial units. - Muddy massive diamicton, matrix-supported (Dmm), made of rounded, strongly alterated clasts; carbonate lithologies are absent; muddy matrix is weathered and has a yellowish red (5Y5/6) color. This facies has a variable thickness, from less than 50 cm to some meters, it is interbedded with gravel facies. Its origin is probably colluvial from weathering process interesting Osoppo Conglomerate and early Pleistocene Tagliamento basin deposits. Basal boundary is a disconformity and locally an angular unconformity of 15 degrees with the substratum. Boundary existing on the top is a weathered surface on gravel facies, characterised by a paleosol 3.5 m thick, with an 1 m thick argillic horizon with a yellowish red (5YR4/6) color. 44..22.. PPllaaiinnoo ssyynntthheemm ((PPLLII)):: this unit is here made by a “loose skeletal till”, characte- rised by isolated, weathered boulders and cobbles scat- tered in Ragogna hill southern side, between 340 and 310 m a.s.l. This is the evidence of the older glacial expansion so far observed in Tagliamento morainic amphitheatre. 44..33.. SSaannttaa MMaarrgghheerriittaa ssyynntthheemm ((SSNNMM)):: this unit, whose morphology was strongly reshaped by late Pleistocene sedimentary events, is situated just next to some soft ridges outwardly to the main ridges of the amphitheatre. Unit is made by a sequence of glacial depo- sits; in the study section they can be distinguished in: - Silty massive diamicton, matrix-supported and heavily consolidated (Dmm), rich in chaotic clasts, from suban- gular to subrounded, striated and glacially shaped (flat- iron and bullet shaped). Matrix is silt with sand, dark yellowish brown (10YR4/5) to brownish yellow (10YR6/6). The facies presents pods and irregular sha- ped rafts of sorted sedi- ment, likely originated by c o m p r e s s i o n of fine lenses in subglacial environment, and linear or cluster con- centration of clasts, pebble to fine boul- der-sized. The whole deposit can be defined as a subgla- cial lodgment till. Fig. 2 - Schematic cross section of Rio Ponte section (A-AI in Fig. 1). MON: Montello Conglomerate; OSP: Osoppo Conglomerate; URP: Rio Ponte Unit; PLI: Plaino Synthem; SNM: Santa Margherita Synthem; NDS: Canodusso Synthem; a: alluvial deposits; b: lodgment till; c: ablation till; d: “loose skeletal till”; AR: Arba–Ragogna thrust. Sezione schematica della sezione del Rio Ponte (A-AI in Fig. 1). MON: Conglomerato del Montello; OSP: Conglomerato di Osoppo; URP: Unità del Rio Ponte; PLI: Sintema di Plaino; SNM: Sintema di Santa Margherita; NDS: Sintema di Canodusso; a: depositi alluvionali; b: till d’alloggiamento; c: till di ablazione s.l.; d: “morenico sche- letrico sparso”; AR: faglia Arba-Ragogna. G. Paiero & G. Monegato m a.s.l. 189 - Massive diamicton, matrix-supported to clast-suppor- ted (Dmm-Dcm), rich in subangular boulders; this deposit is normally consolidated and locally cemented; it is rich of striated clasts in a silty-sandy matrix. It can be classified as an ablation till. Lodgment till is about 10 m thick; it is easily obser- ving in Ponte brook incision and in the head of Fos brook valley, where is heteropic with fluvioglacial gravel (see the further on “Santa Margherita Synthem” in “San Pietro - Aonedis Section”). Whereas ablation till has maintained a thickness of 15 m, moreover it must taken to account several isolate boulders and pebbles situated at 250÷275 levels on Ragogna Hill and that can be relate to this unit. Lower boundary is an erosion surface on Rio Ponte Unit deposits. Upper boundary with Canodusso Synthem is an erosion surface sometimes marked by a weathered surface having brown to strong brown (7.5YR4/4-5/6) color. 44..44.. CCaannoodduussssoo SSyynntthheemm ((NNDDSS)):: this unit represents last main sedimentary event in Friuli piedmont plain, and is characterised by a glacial-fluvio- glacial sequence morphologically evident in the whole morainic amphitheatre. In the study area these glacial deposits make up a double end morainic ridge, the outer more discontinuous and lower (quotes from 250 to 270 m), the inner more defined and higher (quotes from 260 to 300 m). Fluvioglacial deposits are made by clast-supported massive gravel (Gcm) to crudely bedded gravel (Gh), weakly cemented, with subrounded clasts, medium peb- ble to boulder sized, with silty-sandy matrix. Thickness is variable and not exceeding 5 m. The reworked top of these deposits makes the terraced surface on right side of Ponte brook. Glacial deposits can be distinguished in: - Silty massive diamicton, matrix-supported and heavily consolidated (Dmm), rich of chaotic clasts, from subangular to subrounded, striated and glacially sha- ped (flat-iron and bullet shaped). Matrix is made by silt with sand, dark greyish brown (2.5Y5/2) to light olive brown (2.5Y5/4). This facies is rich of pods and irregu- lar shaped rafts of sorted sediment, likely originated by compression of fine lenses in subglacial environment, interbeds of thin bedded to massive silty sand and linear or cluster concentration of clasts, pebble to fine boulder-sized, are present. These elements are often glaciotectonically deformed. Basal level of this deposit, because of its general deformation, can be classified as a deformation till; it can be defined as a subglacial lodgment till. - Massive diamicton, matrix-supported to clast-suppor- ted (Dcm-Dmm), normally consolidated; clasts are generally subangular, many of which are striated, gra- nule to boulder sized; they are characteristic of Tagliamento basin with very rare exotic pebble of austro-alpine gneiss; rich of striated clasts in a silty- sandy matrix pale yellow (2.5Y7/3) colored. It is a supraglacial ablation till. Lodgment till has a kept thickness of 20 m, while ablation till has a kept thickness of 40 m. The basal boundary consists of an erosion surface over Rio Ponte Unit and Santa Margherita Synthem deposits; near Canodusso hamlet, a sample of wood fragment was found in silty-clayey sediments next to basal surface (Fig.3); this sample yielded the conventio- nal 14C age of 1199..007755±±116600 yr BP (Rome-1393). The upper boundary is the topographic surface marked by a weathered surface having dark yellowish brown (10YR4/4) to brown (7.5YR4/4) color. 55.. SSAANN PPIIEETTRROO -- AAOONNEEDDIISS SSEECCTTIIOONN In San Pietro - Aonedis section (left Tagliamento fluvial slope) is displayed a series of Pleistocene sedi- mentary bodies, laid down by watercourses flowing out from Pinzano narrow, and fluvioglacial deposits connec- ted with glacial end moraines of Tagliamento amphitheatre; in this site, 4300 m long and 60-70 m high, 7 units are distinguishable (Fig. 4). 55..11.. BBoorrggoo ddii MMeezzzzoo SSyynntthheemm ((BBGGMM)):: outcropping from Pinzano narrow to Ponte brook valley, it is made by a sequence of clast-supported massive gravel (Gcm) to crudely-bedded gravel (Gh), generally lithified, with carbonate (more than 95% of white or grey carbonate) alterated subrounded clasts, medium pebble to boulder sized. Its matrix is muddy sand, altered and Fig. 3 - Canodusso Synthem lower boundary with the location of sample dated (Rome-1393). SNM: Santa Margherita Synthem; NDS: Canodusso Synthem. Limite inferiore del Sintema di Canodusso con l’ubicazione del campione datato (Rome-1393). SNM: Sintema di Santa Margherita; NDS: Sintema di Canodusso. The Pleistocene evolution of Arzino ... 190 F ig . 4 - a ) S ch e m a tic c ro ss s e ct io n o f S a n P ie tr o – A o n e d is s e ct io n ( B -B I in F ig . 1 , ve rt ic a l e xa g g e ra tio n x5 ), n o te d iff e re n t th ro w s ca u se d b y A rb a – R a g o g n a g ro w in g t h ru st ( A R ). M O N : M o n te llo C o n g lo m e ra te ; O S P : O so p p o C o n g lo m e ra te ; B G M : B o rg o d i M e zz o S yn th e m ; V M I: V ia d i M o lin S yn th e m ; A O N : A o n e d is S y n th e m ; P L I: P la in o S y n th e m ; V L Z : V il lu z z a S yn th e m ; S N M : S a n ta M a rg h e ri ta S yn th e m ; N D S : C a n o d u s s o S y n th e m . 1 -1 6 : e v o lu ti o n e p is o d e s o f e xa m in e d p la in ( fo r d e sc ri p tio n s e e c p t. 6 ). b ) D e ta il p ic tu re o f V ia d i M o lin s lo p e . a ) S e zi o n e s ch e m a tic a d e lla s e zi o n e d i S a n P ie tr o – A o n e d is ( B -B I in F ig . 1 , e sa g e ra zi o n e v e rt ic a le x 5 ), d a n o ta re i d if fe re n ti r ig e tt i ve rt ic a li ca u sa ti d a lla fa g li a i n c re s c it a A rb a -R a g o g n a ( A R ). M O N : C o n g lo m e ra to d e l M o n te llo ; O S P : C o n g lo m e ra to d i O so p p o ; B G M : S in te m a d i B o rg o d i M e zz o ; V M I: S in te m a d i V ia d i M o lin ; A O N : S in te m a d i A o n e d is ; P L I: S in te m a d i P la in o ; S in te m a d i V ill u zz a ; S N M : S in te m a d i S a n ta M a rg h e ri ta ; N D S : S in te m a d i C a n o d u ss o . 1 -1 6 : e p is o d i d e ll’ e vo lu zi o n e d e lla p ia - n u ra i n e sa m e ( p e r la d e sc ri zi o n e v e d i ca p . 6 ). b ) F o to d e lla s ca rp a ta d i V ia d i M o lin . G. Paiero & G. Monegato m a .s .l. light reddish brown (5YR6/4) colored. Stratification is til- ted and has an average dip of 30 degrees southea- stward. The basal boundary is characterised by an angular unconformity of 15 degrees with Osoppo Conglomerate strata. The top of unit is an erosion surface. This unit represents a buried early alluvial fan sequence of Arzino stream. 55..22.. VViiaa ddii MMoolliinn SSyynntthheemm ((VVMMII)):: outcropping next to Via di Molin, this unit is made by crudely-bedded gravel (Gh) normally consolidated and clast-supported; clasts are subrounded, carbonate, gra- nule to coarse cobble sized. Matrix is silty sand. Massive (Sm) to planar cross-beds (Sp) sandy levels, about 2 m thick, are interbedded. Lower boundary does not outcrop; upper boundary is given by a weathered surface, developed on the deposits of this unit, with an argillic horizon about 50 cm thick and strong brown (7.5YR5/6) colored (Fig. 5). The unit represents an Arzino stream alluvial deposit. 55..33.. AAoonneeddiiss SSyynntthheemm ((AAOONN)):: outcropping from Pinzano narrow to Aonedis, it is made by stratified sandy gravel (Gt), clast-supported to partial- ly open-work, with subangolar to subrounded carbonate clasts (more than 95% of white or gray carbonate), medium pebble to cobble sized; massive sandy levels (Sm) are common and have an average thickness of 1 meter. Next to these ones some discontinuous horizons, rich of organic matter, are joined; one of these (Rome- 1428) was dated >>4433..000000 yr BP. The lower boundary is an erosion surface, that northernside of Ponte brook is emphasized by an angular unconformity that reaches 30° with Borgo di Mezzo Synthem’s strata; the upper boundary is an alluvial erosion surface. Unit is 15÷20 m thick and represents an Arzino stream alluvial deposit. 55..44.. PPllaaiinnoo SSyynntthheemm ((PPLLII)): this unit, outcropping from Borgo di Mezzo terrace slo- pes to Aonedis slope, is made by crudely bedded (Gh) sandy gravels, clast-supported; with subrounded Tagliamento's basin clasts (polychromatic carbonates, siliceous sandstones, volcanites and metamorphites), medium pebble to fine boulder sized. Deposits of this unit are easily distinguishable from the bottom and top Arzino basin's ones for polychromic clasts typical of Tagliamento basin. Moreover, as notified by Feruglio (1929a), in Borgo di Mezzo terrace slopes a strongly weathered massive and matrix-supported diamicton, rich in striated clasts, is present. Both boundaries are erosion surfaces. In Villuzza slope of Ponte brook valley upper boundary is made by a buried soil 3.2 m thick, with a 180 cm argillic reddish brown (5YR5/4) colored horizon; the soil is developed on the deposits of this unit. Average thickness is of 5-7 m. These deposits represent the evidence of the older glacial expansion so far observed in San Pietro - Aonedis section. According to their stratigraphic position they are probably related with glacial ones of Rio Ponte section and here credited to the same unit. 55..55.. VViilllluuzzzzaa SSyynntthheemm ((VVLLZZ)): outcropping from Pinzano narrow to Aonedis, this unit is made by crudely-bedded stratified gravel (Gh) to strati- fied gravel (Gt), clast-supported to partially open-work, with some interbedded lenses of massive silty sand (Sm); carbonate clasts, fine pebble to cobble sized, point out an Arzino basin alluvial deposit. Both bounda- ries are erosion surfaces. Villuzza Synthem is 5÷10 m thick and represents last alluvial depositional event clearly ascribing to Arzino stream. 55..66.. SSaannttaa MMaarrgghheerriittaa SSyynntthheemm ((SSNNMM)): outcropping from Rio Ponte valley to Aonedis slope; this unit is made by clast-supported massive gravel (Gcm) to crudely bedded gravel (Gh) with sandy matrix; clasts are subrounded, pebble to cobble sized; deposit is normally consolidated and locally weakly cemented. Petrography of clasts point out a Tagliamento basin origin. Boundaries are erosion surfaces; between them the lower is easily locating by petrographic composition of clasts. It has an average conserved thickness of 5 meters. These deposits represent the outwash plain of glacial event, whose glacial deposits were described in Santa Margherita Synthem in Rio Ponte Section. Inside Ponte and Fos incisions physical relationship of these deposits with glacial ones is visible; in these outcrops, gravels are coarser next to glacial deposits. 191 Fig. 5 - Paleosol at Aonedis Synthem lower boundary in Via di Molin slope. VMI: Via di Molin Synthem; AON: Aonedis Synthem. Paleosuolo in corrispondenza del limite inferiore del Sintema di Aonedis sulla scarpata di Via di Molin. VMI: Sintema di Via di Molin; AON: Sintema di Aonedis. The Pleistocene evolution of Arzino ... 55..77.. CCaannoodduussssoo SSyynntthheemm ((NNDDSS)): this unit outcrops at the top of sequence from San Pietro to Aonedis; it is made by clast-supported massive gravel (Gcm) to crudely bedded gravel (Gh) with Tagliamento's basin subrounded clasts, medium pebble to boulder sized, with silty-sandy matrix; deposits are normally con- solidated. Basal boundary is an erosion surface over every present units; upper boundary is the topographic surfa- ce marked by a soil having dark yellowish brown (10YR4/4) to brown (7.5YR4/4) color. Thickness of this unit increases downstream from 10 to 20 meters. These deposits represent the sandur of last glacial advance and they are heteropic with glacial deposits observed in Ponte brook site. 66.. DDIISSCCUUSSSSIIOONN AANNDD CCOONNCCLLUUSSIIOONN Stratigraphic units and discontinuities recognised during geologic survey, supported by some chronologi- cal data, allow to distinguish several depositional and erosion episodes, from which it is possible to go back to the Pleistocene evolution of this area. Tectonic influence on deposits architecture nowa- days visible is not marginal; presence of Arba-Ragogna thrust (Zanferrari et alii, in prep.), traceable next to Ponte brook valley (Fig. 1), contributed to determine the stratigraphic differences existing between the two slo- pes of the same valley. A certain dating of ancient units, because of lack of chronological data and tectonic deformation undergone, seems not possible; then a relative chronology is presen- ted; reliability became better for Late Pleistocene units. Sequence of deposits outcropping in left Tagliamento slope and its tributaries’ ones is held between two main boundaries: the lower is the erosion surface on Osoppo Conglomerate, an angular unconfor- mity on which Pleistocene sedimentation of Arzino allu- vial fan began, this boudary can be combined with Miall (1996) 8th-order type (episode 1 in Fig. 4a); the upper is topographic surface relative to last glacial maximum event in Tagliamento basin and the late Pleistocene - Holocene incision (6th-order type). Borgo di Mezzo Synthem represents an apex allu- vial fan sequence of Arzino stream (episode 2 in Fig. 4a), a part of the dip of the strata has probably a tecto- nic origin (Fig. 4a). Alteration of deposits, general stee- pness and its stratigraphic position at the base of the pleistocenic sequence allow to give to this synthem an early-middle Pleistocene age. The southern side of Ragogna Hill has been con- temporary interested by early stage of small alluvial fans, according to the classification after Blair & McPherson (1994); these fans were characterised by colluvial slides, debris flows and incised channel flows sedimentary processes (Rio Ponte Unit). Afterwards the trenching of Borgo di Mezzo Synthem’s alluvial fan (episode 3 in Fig. 4a) an aggra- dation took place, witnessed by Via di Molin Synthem; it likely represents a distal fan which partially backfilled the shaped valley (episode 4 in Fig. 4a); afterwards its deposits were pedogenized during a sedimentary stasis (episode 5 in Fig. 4a). Then Aonedis Synthem was deposited (episode 6 in Fig. 4a). The sedimentary facies suggests a fan apex located more upstream than Borgo di Mezzo one, with several sandy beds and more sorted clast average. Aonedis Synthem lower boundary, clearly diachronic, is an angular unconformity, as to Borgo di Mezzo deposits, concerning a 7th order bounding surface. Whereas weathered surface, that emphasizes boundary with Via di Molin Synthem, can be combine with a 6th order one. Stratigraphic position, besides chronological data (>43.000 yr BP), gives to Aonedis Synthem a middle Pleistocene age, when Arzino stream was still free to growing up its fan downstream of Pinzano narrow. In southern side of Ragogna hill went on Rio Ponte Unit deposition. Then first documented local glacial advance took place, represented by Plaino Synthem (episode 8 in Fig. 4a); its basal boundary is an erosion surface (episode 7 in Fig. 4a) and it does not give any information about relationship with Aonedis Synthem deposits; conse- quently the rank of boundary becomes vague. The deep evolution of upper boundary's paleosol and the strong reshaping allow to give a middle Pleistocene age to this glacial unit. At the same time Rio Ponte Unit aggradia- tion was stopped and buried by this glacial event, afterwards evolution of these small brooks have been constrained inside incisions in glacial deposits. Later on a likely trenching phase happened (episo- de 9 in Fig. 4a), on the terraced surfaces the weathering growing on; Arzino stream probably still flew down Pinzano narrow. Then the backfilling of Arzino and its scattering over the plain (Villuzza Synthem) represent last docu- mented Arzino stream sedimentary event (episode 10 in Fig. 4a) out of Pinzano narrow (Zanferrari et alii, in prep.). Lower surface of this synthem, marked by a deep paleosol, can be credited to a 7th order boundary. Afterwards Santa Margherita Synthem represents the second local glacial event (episode 12 in Fig. 4a) well documented in the whole Tagliamento morainic amphitheatre, even though deposits’ morphology is strongly reshaped. Its basal erosion surface (episode 11 in Fig. 4a) gives unfit informations about the relationshi- ps with deposits of Villuzza Synthem; however these alluvial deposits lay in the same stratigraphic position, then a partial heteropy cannot be excluded. These two synthems can ascribed to middle-late Pleistocene for their stratigraphic position. After this another entrenchment took place (episo- de 13 in Fig. 4a), consequently a sedimentary stasis is documented, inside Ponte valley, by a paleosol on gla- cial deposits of Santa Margherita Synthem, that can be related to a 6th order bounding surface. Then the last local glacial event (episode 14 in Fig. 4a) covered deposits of the whole area (Canodusso Synthem). This depositional episode represents the last glacial maximum in Tagliamento amphitheatre and here its age fits in 14C calibration range 23.040-22.240 (±1σ, yr BP) after Stuiver et alii (1998). 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(in prep.) – Note illustrative della Carta Geologica d’Italia alla scala 1:50.000: Foglio 065 “Maniago”. Servizio Geologico d’Italia – Regione Autonoma Friuli – Venezia Giulia. Zenari S. (1927) - Carta Geologica delle Tre Venezie, Foglio "Maniago". Uff. Idrogr. R. Magistrato Acque Venezia. 193 Ms. ricevuto il 6 maggio 2003 Testo definitivo ricevuto il 3 settembre 2003 Ms. received: May 6, 2003 Final text received: September 3, 2003 The Pleistocene evolution of Arzino ...