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Types and development of stream terraces in the Marche Apennines (central Italy): a review and remarks on recent appraisals

Le développement de terrasses fluviatiles dans les Marches du Nord (Italie centrale) : synthèse et remarques appuyées sur des résultats récents
Olivia Nesci, Daniele Savelli et Francesco Troiani
p. 215-238

Résumés

La très bonne conservation des terrasses fluviatiles quaternaires et des dépôts qui les constituent a fait de la région septentrionale des Marches (partie centrale des Apennins) un objet privilégié depuis longtemps pour les recherches géomorphologiques. L’étagement des terrasses fluviatiles observé dans cet espace est révélateur de la succession des terrasses observées dans la plupart des bassins versants drainant le versant adriatique des Apennins. Appuyé sur l’examen des données publiées ou acquises récemment et de résultats inédits, l’article propose une réflexion sur la succession des terrasses alluviales dans le Nord des Marches. Dans les Apennins centraux, le tracé général des principales vallées qui se jettent dans la Mer Adriatique a été acquis dès le début du Pléistocène moyen, conduisant ensuite à la mise en place d’un système de terrasses alluviales étagées. Dans un premier temps, la dynamique d’incision a entraîné la formation de niveaux d’érosion. Ils correspondent à plusieurs replats situés sur les parties hautes des versants et affectés par la tectonique. Dans un second temps, des terrasses d’accumulation sont observables le long des vallées principales en relation avec la « Révolution du Pléistocène moyen ». Ainsi, quatre principales terrasses d’accumulation, comprenant de très nombreuses unités stratigraphiques plus ou moins imbriquées, se succèdent et correspondent aux cycles climatiques de 100 ka. Les phases successives d’incision des terrasses d’accumulation et du substrat géologique sont corrélées aux périodes de réchauffement et de refroidissement.

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Notes de la rédaction

Article soumis le 1er février 2011, accepté le 17 juin 2011.

Texte intégral

Introduction

1The northern Marche, set in the Adriatic side of the Apennines, is a non-glaciated mid-latitude Neogene-Quaternary fold-and-thrust belt characterised by a relief largely matching uplifting tectonic structures. Deep gorges are being cut by trunk-valleys through pronounced anticline ridges in a sub-parallel pattern reflecting the regional topographic dip and markedly contrasting the rectangular drainage arrangement of the opposite (Tyrrhenian) side of the Apennines (Mazzanti and Trevisan, 1978). Main valleys, coastal areas and piedmont sectors show well-preserved terrace staircases consisting of several levels of both terrace alluvium and erosional terraces (Bisci and Dramis, 1991; Nesci et al., 2005). In other both Mediterranean and extra-Mediterranean contexts, similar terrace staircases have been already described (e.g., Brigdland, 2000; Litchfield and Berryman, 2005; Brigdland and Westway, 2008). Both the peculiar drainage arrangement and the excellent preservation of late Quaternary terraces, together with a large number of exposures in terrace alluvium and related deposits, made this area an object of targeted scientific work since a long time (e.g., Lipparini, 1935; Sacco and Bonarelli, 1936; Lipparini, 1939; Selli, 1954; Coltorti et al., 1991; Nesci et al., 1995; Wegmann and Pazzaglia, 2009, to mention only a few). Actually, the terraces of the northern Marche area can be taken as representative of terrace-staircases arrangement and development in several river basins of the Adriatic side of the Apennines (Fanucci et al., 1996; Cyr and Granger, 2008; Della Seta et al., 2008). The aim of this paper is to focus on types, distribution and development of terrace staircases in this specific sample-area. In this frame, both a short review of pre-existing data and new specific appraisals on recent, partly unpublished achievements are presented and discussed. In this respect, the Upper Pleistocene-Holocene terrace-related events, as a rule, being better preserved and adequately constrained, are crucial to deciphering the older suites of terrace alluvium (Nesci et al., 2010) and are thus emphasised. Moreover, dealing mainly with the development of major fill-terraces (sensu Bull, 1991), risers included, this paper only briefly reports both local tectonic deformations of terraces, an issue however also discussed in reported areas (e.g., Di Bucci et al. 2003; Wegmann and Pazzaglia, 2009), and minor recent terraces related to anthropic factors (Coltorti, 1997) or complex and local behaviours.

Geological setting

2The Marche Apennines (fig. 1) are part of the Northern Apennines, an imbricate, E- to NE-vergent fold-and-thrust belt developed during the Neogene because of convergence between the Sardo-Corsican block (to the W) and Adria microplate (to the E; Dewey et al., 1973; Channell et al., 1979). Accretion on Adriatic foreland and extension on Tyrrhenian back-arc area accompanied the Miocene-Early Pleistocene eastward thrust-front migration (Malinverno and Ryan, 1986; Mazzoli and Helman, 1994). Compression reached the present coastal sector in the uppermost Early Pliocene (Calamita et al., 1994), being the most external, offshore thrust-folds the product of the final stages of contractional deformation that occurred approximately at the end of Early Pleistocene (Di Bucci and Mazzoli, 2002). Because of thrust-front migration, foredeep/thrust-top basin sedimentation progressed toward the Adriatic foreland up to the Middle Pleistocene (Ricci Lucchi, 1986). Along the NE margin of the belt, the Po Plain-Adriatic Sea (fig. 1) is the present-day remnant of the foredeep. A major geodynamic change occurred at 700-800 ka, and a new tectonic regime established in the Apennine chain and adjacent foothills, producing normal, oblique- and strike-slip faults that dissect the thrust belt, are still seismically active, and characterised by a NE-SW oriented maximum extension (Di Bucci and Mazzoli, 2002). The folds and thrust-fronts display an overall arc-shape (Speranza et al., 1997), roughly paralleling the modern Adriatic coastline/internal margin of the Po Plain, with a marked north-eastward convexity culminating in the central Marche sector (fig. 1), and are cross-cut by transversal tectonics (e.g., Fazzini and Gelmini, 1982; Boccaletti et al., 1983; Di Bucci et al., 2003). The northern Marche sector of the belt in issue consists of NW-SE trending asymmetric, thrust-folds (Calamita and Deiana, 1987). Thrust-faults, affecting a Meso-Cenozoic mainly carbonate and marly, peritidal to pelagic succession (Deiana and Pialli, 1994), propagated upward and northeastward into overlying Mio-Plio-Pleistocene hemipelagic, turbiditic and evaporitic terrains of foredeep and/or thrust-top basins (De Donatis et al., 1998). In the NW of the area in issue (fig. 1), allochthonous terrains (Val Marecchia Sheet) consisting of a large variety of lithologies tectonically overlie all the above-described units (Conti, 1989). The most part of the emerged foredeep consists of terrains of the Plio-Pleistocene peri-Adriatic succession (sensu Mazzoli et al., 2002), reflecting basinward migration of prograding systems, and represented by pelitic-arenitic-conglomeratic marine/non marine sequences (Cantalamessa and Di Celma, 2004) with an overall regional NE-dip, although deformed in broad, gentle synclines. An exception to such arrangement is the coastal sector roughly between the rivers Conca and Esino we are dealing with (fig. 1). It is distinguished by broad synclines cored by Pliocene-Lower Pleistocene pelitic and arenitic marine terrains; they are bounded to the NE by Miocene formations forming narrow anticlines parallel to the present coastline and cut to the NE by foreland-vergent thrusts (De Donatis et al., 1995). Soon after its definitive emersion, the Adriatic onshore underwent a generalised tectonic uplift (e.g., Dramis, 1992), with an increase in rate of topographic growth towards the end of Early Pleistocene (Argnani et al., 1997; Bartolini, 1999). Long-term uplift rates assessed for the studied areas gradually decrease from inland sectors, where maxima of about 0.5 mm/a over the last 1 Ma have been derived (Bigi et al., 1995; Cyr and Granger, 2008), to present-day coastal areas for which G. Calderoni et al. (2010) estimated minima of about 0.15 mm/a. Despite a general agreement in assessing active deformation and uplift (Frepoli and Amato, 2000; D’Agostino et al., 2001; Di Bucci and Mazzoli, 2002; Vannoli et al., 2004), a major debate concerns the activity of the outermost sectors of the orogenic wedge. It is worth underlining that former analyses of terrace distribution has shown that no plan relationship may be established between terrace height and related anomalies and uplift of coastal anticline structures (Di Bucci et al., 2003; Mayer et al., 2003).

Fig. 1 – Geological sketch of the northern Marche Apennines
Fig. 1 – Schéma géologique des Apennins dans les Marches du nord

Fig. 1 – Geological sketch of the northern Marche ApenninesFig. 1 – Schéma géologique des Apennins dans les Marches du nord

1: main gorges; 2: Holocene wave-cut scarp; 3: topographic transects of fig. 4; 4: valley topographic profiles of fig. 3; 5: location of the section of fig. 2; 6: gravel and sand of the Middle Pleistocene-Holocene fluvial and coastal terraces; 7: Plio-Pleistocene pelitic and arenitic marine deposits; 8: Eocene-Miocene marly-calcareous, evaporitic and terrigenous units; 9: Jurassic-Paleogene formations of the carbonatic ridges; 10: calcareous, marly-calcareous and pelitic allochthonous units of the Val Marecchia Sheet; 11: anticline axis; 12: thrust fault (dashed where buried); 13: normal fault; 14: strike-slip fault.
1 : gorges principales ; 2 : falaise holocène ; 3 : profils topographiques de la fig. 4 ; 4 : coupes représentées sur la fig. 3 ; 5 : localisation de la coupe de la fig. 2 ; 6 : dépôts sablo-graveleux des terrasses fluviatiles et côtières du Pléistocène moyen-Holocène ; 7 : sédiments gréso-pélitiquees du Plio-Pléistocène marin ; 8 : dépôts marno-calcaires, évaporitiques et terrigènes de l’Éocène et du Miocène ; 9 : formations calcaires du Jurassique et du Paléogène des monts ; 10 : unités calcaires, marno-calcaires et pélitiques allochtones de la Nappe du Val Marecchia ; 11 : axe anticlinal ; 12 : chevauchement (ligne en pointillés : n’affleurant pas) ; 13 : faille normale ; 14 : décrochement.

Geomorphologic setting

3The northern Marche Apennines consist of an inland hilly-mountain area merging to the NE into a hilly coastal zone (fig. 1). The first pertains to the Meso-Cenozoic, autochthonous Umbria-Marche geological domain and subordinately to the allochthonous Val Marecchia, while the latter zone is shaped in Neogene formations. The inland Umbria-Marche autochthonous domain is characterised by two regionally-extended carbonate anticline ridges, i.e. Umbria-Marche (to the SW, culminating in Mounts Catria, 1701 m, and Nerone, 1525 m) and Marche Ridge (to SE, culminating in Mount San Vicino, 1479 m), separated by a broad terrigenous syncline depression, where several minor carbonate anticline reliefs also rise. The fold-shape is actually imprinted in the planform of the derived morphostructures (fig. 1). Isolated resistant spurs and reliefs protruding from undulating hillscapes and culminating in Mount Carpegna (1415 m), typify instead the Val Marecchia allochthonous domain, where “chaotic” clays with rocky inclusions of varying lithology and sizes are found. Evidence for Pleistocene glaciers is well known on the highest mountains of central Apennines (Huges et al., 2006). Glacier occurrence in the Marche Apennines has been reported for a long time for the more than 2400 m high Sibillini Mountains (Ricci, 1907); small glaciers on the lower, northernmost reliefs have also been considered (Selli, 1954, Mount Nerone; Savelli et al., 1995, Catria massif). However, in Pleistocene times northern Marche inland was quite completely glacier-free, therefore the influence of local small glaciers on stream-terraces must be regard as irrelevant.

4Seaward landscape is characterised by rounded, northeastward declining hills with summit elevations usually not exceeding 500 m to the SW and 200 m close to the present coastline. Resistant arenites embedded into pelites folded in gentle synclines generate a series of characteristic homoclinal ridges striking NW-SE. A series of prominent hills partly cut by both active and relict sea-cliffs matches the narrow coastal anticlines, constraining the shape of the present shoreline. The Apennine hillsides join rather sharply the Adriatic coastal plain/Po Plain flats. Their emerged sectors consist of both active and relict coastal flats and alluvial plains (Elmi et al., 2003), while the modern off-shore is a rather flat sandy to muddy shelf not exceeding in depth -50/-70 m. Owing to its physiography, the Adriatic shelf experimented repeated emersions responding to 100-ka eustatic sea-level fluctuations driven by major glacials (Trincardi and Correggiari, 2000; Ridente et al., 2009), thus becoming repeatedly an extension of the alluvial Po Plain, where many of the rivers flowing down the Apennines reliably joined the palaeo-Po (De Marchi, 1922; Vai and Cantelli, 2004; Fagherazzi et al., 2008). The sea-level rise, following the end of the last glacial shifted the shoreline to its present position, ca. 250 km to the SE of the Last Glacial Maximum position (Correggiari et al., 1996; Lambeck et al., 2004). In the subsiding Po Plain, the Holocene palaeoshoreline reached locations up to 20-30 km landward with respect to present position; landward shifting steadily decreased to ca. 1 km to the S of Rimini, where the shoreline reached the pede-Apennines hills (Elmi et al., 2003; Calderoni et al., 2010). Here, in addition, rapid erosion on Gabicce-Pesaro and Monte Conero promontories (fig. 1) shifted the shoreline 500-1000 m landward of its position at the time of Holocene maximum flooding (Elmi et al., 1994, 2001; Colantoni et al., 2004).

5The progressive emergence of the Adriatic side of the Apennines from Messinian to Early Pleistocene (e.g., Castellarin et al., 1985; Cantalamessa et al., 1986), and the regional tilting to the NE following the main thrusting (D’Agostino et al., 2001) forced the drainage to a sub-parallel arrangement of main valleys, flowing to the NE perpendicularly to the structural grain of the chain (e.g., Bisci and Dramis, 1991). The main rivers often maintain their course cross-cutting both the major anticline ridges of the internal areas and the narrow coastal anticline-reliefs, a behavior stressed by a long-lasting scientific debate (Mayer et al., 2003). Whatever the causes and evolution model advocated (Marinelli, 1926; Giannini and Pedreschi, 1949; Sestini, 1950; Mazzanti and Trevisan, 1978; Ciccacci et al., 1985, 1989; Alvarez, 1999; Mayer et al., 2003), the first steps in drainage development are usually retained as old as the Messinian, partly inherited as fluvial valleys from previous Tortonian submarine canyons (Ciccacci et al., 1985; Alvarez, 1999). However, from Mid-Upper Pliocene to Lower-Mid Pleistocene, the emergence of the Apennine piedmont greatly extended eastward a drainage formed by transverse trunk-valleys joining arrays of strike-, dip-, and antidip-tributaries (Mayer et al., 2003). The composite frame thus established, where opportunistic evolutive behaviors are likely to be set (Mayer et al., 2003); particularly, O. Nesci and D. Savelli (2003) demonstrate that both entrenching of distributary channels and establishment of inter-fan streams were reliable mechanisms in drainage development in coastal areas where fan deltas were formed. Ultimately, starting in Lower-Middle Pleistocene, a generalised tectonic uplift forced the drainage to entrench allowing terrace formation (e.g., Bisci and Dramis, 1991; Bigi et al., 1995; Fanucci et al., 1996).

6In the next sections a synthetic description of stream valley evolution since Lower Pliocene will be given. A detailed description and discussion of terrace arrangement, formation and development will follow.

Erosional surfaces and strath terraces

7Reliable reconstructions of relief evolution assume earliest stages as old as late Lower Pliocene with low-relief gently undulating landscapes, preserved as relict palaeo-surfaces throughout the central-northern Apennines on modern divides or atop flattened reliefs also exceeding 1000 m in height (e.g., Bernini et al., 1977; Ciccacci et al., 1985; Dramis, 1992; Calamita et al., 1999). Below the highest “summit surface”, further surfaces are reported for central Apennines (Dramis et al., 1991, 1992): actually, erosional surfaces arranged on several levels have been recognised in northern Marche at heights less than 500-600 m (Nesci et al., 1992; Veneri et al., 1995), thus highlighting a complex, stepped evolution in both dissection of primitive low-relief landscapes and valley entrenching (Nesci et al., 1995). It was an areal and chronologically complex drainage entrenchment to fix the overall late Quaternary position and planform of trunk-valleys. A key-evidence and a result is the passage from extra-vallive (erosional surfaces at higher elevations atop hills and divides) to intra-vallive erosional terraces (i.e., strath terraces; Nesci et al., 1992; Fanucci et al., 1996). The transition to strath terraces (i.e., fixing of valley-throughs/axes at about the present-day positions) far from being synchronous, is rather related to the timing in development of specific onshore sectors, becoming younger eastward, where the Plio-Pleistocene peri-Adriatic succession is found (Nesci et al., 1995). Accordingly, in this latter sector transition to intravallive forms is often found at relatively low heights, i.e. close to the highest and oldest fill terrace (Nesci et al., 1995; see also Dramis et al., 1992, p. 290, who fix the position of a “young” erosional surface close to the oldest fill terrace). Hence, since in northern Marche a strath terraces staircase always precede fill terraces (see hereafter), above these latter only one or few strath terraces can be found in such areas (Nesci et al., 1995). Similar arrangements, stressing ongoing valley deepening and/or confinement (Nesci et al., 2002), sound quite similar to the array described by K.W. Wegmann and F.J. Pazzaglia (2009) for oldest terraces of the Bidente valley in the Romagna Apennines. Nonetheless, examples are also known of valley-axes fixing in even more recent stages, as stressed by the oldest fill terraces placed atop hills or divides (Nesci et al., 1990; Coltorti et al., 1991), unless such behaviour does not indicate dissection of alluvial fans/pedimentary surfaces (Savelli et al., 1994; Guerra and Nesci, 1999; Nesci and Savelli, 2003). Despite the mentioned exceptions, the first stages of trunk-valleys incision are characterised by the development of strath terraces, lacking any preserved alluvial cover or palaeosoil (Nesci et al., 1992, 1995), that have been interpreted (Fanucci et al., 1996) as related to unsteady vertical stream incision driven by tectonic factors, according to genetic mechanisms already described by W.B. Bull (1991). Individual valley cross-sections, depending on preservation potential and/or on position inside the valley, may evidence up to six strath terraces (fig. 2A) that can be correlated up to 10 km in the along-valley direction. However, only the two lowermost terraces can be reasonably correlated over greater distances and are likely to occur throughout different basins a few meters above to the oldest fill terrace (Nesci et al., 1992), being conceivably the younger of them the same erosional surface described by F. Dramis et al. (1991) for southernmost areas. Besides, some of the strath-terraces reported for the northern Marche could also be correlated with the oldest strath terraces already recognised in the Romagna (Bidente valley) and there constrained by means of numeric ages to the end of Early Pleistocene-early Middle Pleistocene by K.W. Wegmann and F.J. Pazzaglia (2009), although available data are not yet sufficient to satisfactorily support such correlation.

Fig. 2 – Metauro river valley
Fig. 2 – Vallée du Metauro

Fig. 2 – Metauro river valleyFig. 2 – Vallée du Metauro

A: Schematic section across the lower Metauro river valley: narrowing and transition from strath- to fill-terraces are apparent (after Nesci et al., 1992, redraw; location on fig. 1). B: The terrace staircase of the Metauro River sketched on fig. 2B (T1a-T3: fill-terraces; s: strath terraces; for further explanations, see the text).
A : Coupe schématique à travers la basse vallée du Metauro : rétrécissement de la vallée et transition des niveaux d’ablation aux terrasses d’accumulation (d’après Nesci et al., 1992, redessiné ; localisation sur la fig. 1). B : Succession des terrasses étagées dans la vallée du Metauro esquissées sur la fig. 2B (T1a-T3 : terrasses d’accumulation; s : terrasses d’érosion ; pour de plus amples explications, voir le texte).

Fill terraces

8The initial “only erosional” terraces are followed by a better known and documented fill terraces staircase (fig. 2A and B and fig. 3; e.g., Nesci and Savelli, 1986, 1990; Coltorti et al., 1991b), often variously associated with minor suites of strath terraces on bedrock, fill-strath terraces on previous alluvium (Nesci and Savelli, 1991a; Wegmann and Pazzaglia, 2009), and alluvial fans (Nesci and Savelli, 1991a; Nesci et al., 2010).

Fig. 3 – Fill-terraces staircases on transversal profiles of the Metauro and Foglia valleys; cross-sections also show Pleistocene alluvial-fan deposits stratigraphically overlying the trunk-valley alluvial fill
Fig. 3 – Profils transversaux de vallées Metauro et Foglia montrant la série de terrasses d’accumulation ; les profils montrent également les dépôts de cônes alluviaux du Pléistocène superposés aux remplissage alluviaux dans la vallée principale

Fig. 3 – Fill-terraces staircases on transversal profiles of the Metauro and Foglia valleys; cross-sections also show Pleistocene alluvial-fan deposits stratigraphically overlying the trunk-valley alluvial fill Fig. 3 – Profils transversaux de vallées Metauro et Foglia montrant la série de terrasses d’accumulation ; les profils montrent également les dépôts de cônes alluviaux du Pléistocène superposés aux remplissage alluviaux dans la vallée principale

1: fluvial deposits; 2: alluvial fan deposits; 3: strath and fill-strath terraces; 4: major terrace-levels; 5: Holocene slope-waste and alluvial-fan deposits.
1 : alluvions fluviatiles ; 2 : alluvions de cônes de déjection ; 3 : terrasses d’érosion dans le lit rocheux ou aux dépens du remplissage alluvial ; 4 : niveaux de terrasse principaux ; 5 : dépôts de pente et de cônes de déjection de l’Holocène.

After Nesci et al., 2010; location on fig. 1.
D’après Nesci et al., 2010 ; localisation sur la fig. 1.

Historical background

9In late 19th c., scientists became aware of the occurrence of ancient terraced alluvium, thus starting to explain its distribution, extension and thickness through more or less reliable scenarios: for instance, the hypothesis of somehow catastrophic beaching through mountain ridges followed by formation of lacustrine basins later filled-up by alluvium (Scarabelli Gommi Flamini, 1880). Non-catastrophist interpretations of terraced valley-profiles, anyway, were soon drawn around the influential Davisian concepts, highlighting on the Adriatic side of the Apennines valley narrowing following stream rejuvenations (Di Sawicki, 1909). In the first decades of the 20th c., the international debate about marine and fluvial terraces at a world-wide scale involved Italy, including alluvial terraces of the Adriatic side of the Apennines (e.g., Gortani, 1928, 1938) and systematic field surveying also started (e.g., Sacco, 1904). The highest and oldest terrace-alluvium characterised by reddish weathering colors and consisting of one or more levels was distinguished and, both acknowledging its wider extension and attributing it to Pleistocene glaciations, was named Diluvium. At lower elevations, fluvial terraced deposits, sometimes identified as “Terrazziano” to indicate “postglacial terrace-alluvium” (Sacco, 1904), were distinguished. Lastly, the lowermost Holocene fluvial and littoral deposits were ascribed to the “alluvium”, usually intended as inclusive of both slightly terraced and modern deposits, being the latter sometimes set apart as “recent”.

10The first influential, systematic studies about the Marche-Romagna terraces are by T. Lipparini (1935, 1939). He distinguished (1935) in Emilia Apennines four “orders of terraces” (i.e., I to IV from the highest) relating the three oldest aggradation episodes to as many glacial periods. Later on, in the light of concepts about effects of glacio-eustatic baselevels on streams (De Marchi, 1922), for the Marche region he recognised (1939, p. 6-7) “three orders of terraces, whose risers were caused by erosional renewals, as a primary and direct consequence of negative eustatic movements”, and interpreted aggradation as due to “baselevel rising, it is to say positive eustatic movements”. Specifically, T. Lipparini (1939, p. 21), hinted at two major “gravel-levels” of “mindel-rissian and riss-würmian attribution”, plus a “broad valley-floor bench in which the thalweg is engrooved” that attributed to “epiwürmian or flandrian” positive eustatism. Notably, since Marche rivers flow directly into the Adriatic Sea, he “inverted” the interpretation key adopted for Emilia terraces attributing riser formation to glacials (negative eustatism) and aggradation to interglacials. Shortly after, G.M. Villa (1942) recognised four main terraces, associating downcutting and aggradation with negative and positive eustatic fluctuations respectively, in turn related with glacials and interglacials. Actually, G.M. Villa (1942, p. 62-63) recognised an over-aggradation of terminal valley-reaches “on the third and fourth level” of terraces (numeration from the top, I order being the highest), assuming eustatic fluctuations as necessary to explain the origin of Marche terraces. He remarked however a downvalley convergence of terraces, interpreted as “naturally due to a greater uplift suffered by the most internal sector of the different fluvial basins with respect to the more external” sectors, thus anticipating forthcoming more reliable interpretations of genetic mechanism for terrace formation in the Marche river valleys. Hence, G.M. Villa (1942, p. 65-68) ascribed the oldest terrace to “a first marine transgression” Sicilian in age, followed by a further alluvial plain construction forced by a second Sicilian (Mindel-Riss) eustatic rise. A successive “enormous over-alluviation in the third level of alluvial sedimentation” was attributed to the Tyrrhenian (Riss-Würm) eustatic rise and, eventually, the “fourth alluvial bench” is attributed to the “flandrian transgression”. Hence, G.M. Villa (1942, p. 65-68) ascribed aggradation of the oldest terrace alluvium to “a first marine transgression” Sicilian in age, followed by a second (Sicilian, Mindel-Riss), a third (Tyrrhenian, Riss-Würm), and a fourth (“flandrian”) eustatic rise forcing as many episodes of alluvial plain construction. R. Selli (1954, p. 207-209) took on the distinction for the Marche area of “four orders of terraces whose surfaces mark the last most important episodes of stasis in valley deepening”. Henceforth, this fourfold subdivision of terraces definitely became an unquestionable reference for most part of the scientific work dealing with Marche terraces (Carloni et al., 1971). Basing on then “new acquaintances on marine Quaternary”, he challenged the chronological attributions of Lipparini and Villa, founded “essentially on the Quaternary eustatic oscillations”: he rejuvenated the whole terrace staircase, attributing to the oldest “1st level” an “age not older than the Mindel-Riss (Milazzian) interglacial”. In spite of persuasive argumentations of L. Trevisan (1946, 1949), he also affirmed that the “various movements of chain uplift have been the essential determining elements of valley fluvial terracement of the Quaternary”, in a context in which “the eustatic oscillations of sea level, of undoubted importance (…) may only have altered the effect of the first”. Later on, R. Selli (1962) assuming roughly equal rates of tectonic uplift and eustatism, and based on the subdivision of the last two glacials in several “glacials”, i.e. Riss I and II, Würm I, II and III, assigned to the respective “interglacials” the aggradation stages of the four “orders of terraces”.

11It is only in the ‘70s that radiometric acquiring, pedostratigraphic data, appraisals of prehistoric lithics and loess deposits, brought about a substantiated re-assessment of genetic models, relating the principal basin-wide aggradation episodes not so much to eustatic and/or tectonic causes, but to climatic controls (Damiani and Moretti, 1969; Alessio et al., 1979; Coltorti, 1979, 1981). Since that time, the major basin-wide aggradation events have been ascribed to cold stages (e.g., Coltorti et al., 1991; Nesci and Savelli, 1991b; Nesci et al., 1995), thus stressing the strong influence of climate in generating the principal fill terraces of the Marche region. Further, effects of eustatic sea-level rises have not been disregarded, recognising their crucial importance for aggradational events close to modern river mouths (Elmi et al., 2003; Mencucci et al. 2003). Tectonic control also found a reliable collocation as responsible for both altimetrical distribution of terraces (e.g., Bisci and Dramis, 1991; Fanucci et al., 1996) and terrace-levels displacement and/or unevenness (Coltorti and Nanni, 1987; Nesci et al., 1990; Coltorti et al., 1996; Di Bucci et al., 2003; Della Seta et al., 2008).

Arrangement and hierarchy of fill terraces

12Early studies on Marche terraces referred aggradation to glacials (i.e., Mindel, Riss, Würm, being the Günz usually unemployed) and risers to interglacials-postglacials, also admitting low “epiwürmian” and Holocene minor terraces. Natural corollary to this approach was, as in every other country, the custom of differentiating terraces according to their altimetrical distribution and correlating them by means of morphostratigraphic criteria, as always apparent in geological maps. However, since the very first researches, several efforts were also made to chronologically constrain fill-terrace staircase, basing on both indirect inferences and artefacts found on terraces (e.g., Lipparini, 1939). At present, since the only Upper Pleistocene-Holocene sediments can be profusely dated with 14C, only the youngest terraces provided a most assuredly chronology. Conversely, only a few isotopic and numerical ages are available for older terraces (e.g., Cyr and Granger, 2008). Nonetheless, a reliable terrace chronology has been recently proposed by K.W. Wegmann and F.J. Pazzaglia (2009) also based on a notable amount of numerical ages. It is the scheme on which O. Nesci et al. (2010), accordingly with evolution steps hereafter described, tentatively framed the oldest terraces, the alluvial fans and the downcutting stages (fig. 11).

13After Lipparini’s works, regardless of different interpretations of terrace origin and age, his fourfold subdivision was basically maintained for the successive three decades. Nonetheless, the progress of research took both number and hierarchy of alluvial terraces to be repeatedly discussed, rearranged and re-assessed. As regards the oldest fill terraces, it has been stressed (Nesci et al., 1990, 1995; Fanucci et al., 1996) that the traditional “I order” (Lipparini, 1935; Villa, 1942 and followers) actually accounts for two distinct episodes of climate-driven fill-terrace development (named T1a and T1b levels after Nesci et al., 1990) with the same “hierarchic range” of the already distinguished “II and III order of terraces” (i.e., T2 and T3 levels). Such array of Pleistocene terraces has been firstly introduced by O. Nesci et al. (1990) for the Metauro and Foglia river basins, where the traditional “I order” revealed to be unsuitable to describe the altimetrical distribution of the oldest terrace-alluvium. Given the relatively old age of the earlier fill terraces, tectonic deformation may be responsible for more or less localised height discrepancies (Nesci et al., 1990) and/or for local increase in the number of treads, as already recognised in the central-southern Marche (e.g., Coltorti and Nanni, 1987; Bisci and Dramis, 1991, p. 100-101). Nonetheless, this is far from invalidating the subdivision of “I order” into two distinct units, which has been validated in trunk valleys of the Adriatic sector throughout the northern Abruzzo and Marche regions (Nesci et al., 1995; Fanucci et al., 1996; Della Seta et al., 2008).

14More complex arrangements resulted instead for late Pleistocene-Holocene terraces. Since the first systematic work on fluvial terraces of the Adriatic and Po Plain side of the Apennine the lowermost terraces (“IV order”) have been acknowledged for being different from the older ones. T. Lipparini (1935) himself, though separating a “IV order” neglected it in its evolutive model, thus implicitly relegating it to a somehow irrelevant role, following in this Sacco’s idea (Sacco, 1904, p. 161) of a frame where Holocene (“Terrazziano”) terrains have a “not very great importance from the scientific standpoint”. Anyhow, it is only in relatively recent times that the intrinsic difference of genetic mechanisms and morphoevolutive significance between such terraces and older ones has been definitely assessed (Ciccacci et al., 1985; Bisci and Dramis, 1991; Nesci and Savelli, 1991 a and b). More in detail, studies of outlet areas substantiated by subsurface data and radiometric dating demonstrated that the alluvial suites rapidly thicken seaward and consist of Pleistocene alluvium disconformably overlain by thick Holocene deposits at least in part related to eustatic sealevel rise (e.g., Elmi et al., 1981, 1983; Nesci et al., 1995; Mencucci et al., 2003). Furthermore, anthropogenic causes proved to be effective in triggering both Holocene aggradation and terrace development (e.g., Gentili and Pambianchi, 1987; Coltorti, 1991; Coltorti et al., 1995; Coltorti, 1997; Elmi et al., 2003) not only in downstream valley-reaches but also in inland sectors. In addition, with a special reference to inland areas, other causes have been claimed as responsible in the Holocene for the development of, often local, terraces throughout the northern Marche trunk-valleys; among such causes there are stream adjustment following meander cutoff (Nesci and Savelli, 1991a), complex responses (sensu Schumm, 1977; Calderoni et al., 1991a), and knickpoint upstream migration (Troiani et al., 2009).

15The staircase of major fill terraces (fig. 2B and fig. 4) from I-to-III order (or T1a-T3 terrace after O. Nesci et al., 1990) is characterised by an overall marked downstream convergence (Elmi et al. 1987; Mayer et al., 2003) already noticed and discussed by the earliest Authors. Accordingly with more general achievements, such configuration has been usually retained a result of the general decreasing rate of crustal uplift from the chain axis towards the Adriatic. Regardless of overall downwalley decreasing in heights, major fill terraces are found above the present thalweg at heights ranging from few meters (recent terraces in downstream areas) up to 130-150 m, with maxima of 200-210 m that are reported for the oldest units of upper Metauro valley by O. Nesci et al. (1990). Anomalies in both downstream and transverse valley-profiles are well known, and usually related with tectonic displacement (e.g., Coltorti et al., 1996; Di Bucci et al., 2003; Della Seta et al., 2004, 2005, 2008). Also noticeable is the unequal altimetric distribution of the same terrace order (to retain as roughly contemporaneous given the climate-driven origin; Bull, 1991) within adjacent valleys (fig. 4). In this concern, as a general statement such extra-basin “up-and-down” configuration of terraces on composite topographic profiles parallel to the coast, must be retained to account for different physiography of river basins (i.e., drainage area, different geometry of slopes, drainage patterns and density, stream gradients, etc.), rather than by active tectonics (Troiani and Della Seta, 2011).

Fig. 4 – Topographic transects orthogonal to the orientation of trunk-valleys showing the height distribution of fill-terraces throughout the northern Marche Apennines
Fig. 4 – Profils topographiques perpendiculaires à l’orientation des vallées principales dans les Marches septentrionales, montrant l’étagement des terrasses d’accumulation

Fig. 4 – Topographic transects orthogonal to the orientation of trunk-valleys showing the height distribution of fill-terraces throughout the northern Marche Apennines Fig. 4 – Profils topographiques perpendiculaires à l’orientation des vallées principales dans les Marches septentrionales, montrant l’étagement des terrasses d’accumulation

Location of transepts in fig. 1 and fig. 12. 1: “I order” terrace (T1); 2: “II order” terrace (T2); 3: “III order” terrace (T3).
Position des profils sur la fig. 1 et la fig. 12. 1 : terrasse d’« ordre I » (T1) ; 2 : terrasse d’« ordre II » (T2) ; 3 : terrasse d’« ordre III » (T3).

The complexity of aggradation and incision

16In a frame of a “rhythmical succession of cycles” (in the words of T. Lipparini, 1939, p. 20), where since the ‘70s some earlier “diluvial” ideas found reliable support, O. Nesci and D. Savelli (1986, 1990) highlighted a systematic recurrence of peculiar erosional-depositional steps in each cycle of fill terrace development. Despite some obvious limitations due to the lesser degree of preservation of the older terraces, such steps have been recognised to repeat quite unvaried into the four main cycles of aggradation-dissection (T1a, T1b, T2, and T3 according to O. Nesci et al., 1990) to such an extent that O. Nesci and D. Savelli (1986 and following) called them “guide stages”/“guide deposits” to signify something hinting at a precise position in the cycle itself. The cold-climate driven aggradation of main valleys has been acknowledged to consist of two steps (fig. 3, fig. 5 and fig. 6), with coarse braided-river sediments deposition (up to 30-35-m thick) systematically followed by alluvial fans construction at tributary junctions and within piedmont areas (e.g., Savelli et al., 1984; Nesci and Savelli, 1986, 1991b). Hence, sceneries for the alluvial plains during the glacial stages (fig. 5) are those of trunk-valley filled by gravel alluvium and crossed by braided streams redistributing large amounts of sediments supplied by tributaries. In a second time, a threshold (sensu Schumm, 1977) has been somehow exceeded modifying functional interactions between trunk-streams and tributaries/mountain streams (Nesci et al., 2010), so that alluvial fans began to be constructed in both piedmont areas and at tributary junctions on trunk valleys.

17In the last decades, along with researches dealing with the entire flight of terraces (e.g., Coltorti, 1981; Nesci and Savelli, 1990; Coltorti et al., 1991; Nesci et al., 1992), lithostratigraphic and sedimentological issues came also into light, generally addressed to analyse Upper Pleistocene-Holocene both alluvial and slope-waste deposits (e.g., Nesci and Savelli, 1986; Coltorti and Dramis, 1987; Nesci and Savelli, 1991b), often taking cue from radiometric dates (e.g., Alessio et al., 1979, 1987; Calderoni et al., 1991 a and b, 1994). The cyclical repetition of alluvial environments and facies in the main fill terraces makes detailed analyses of terraced deposits of the last climatic cycle effective for unraveling the oldest terraced suites, as already highlighted for the Marche region (Nesci and Savelli, 1986; Nesci et al., 2010) and other extra-glacial areas (e.g., Lewin and Gibbard, 2010). In this regard, the mid sectors of the main trunk-valleys have been deemed as representative area to unravelling the evolution stages (Nesci and Savelli, 1986 and following), since influenced neither by sea level effects nor upstream climato-tectonic controls on fluvial system behavior (see also Blum and Tornqvist, 2000; Bridgland and Westway, 2008; Lewin and Gibbard, 2010). In this reading, the main achievements can be summarised as follows. According to D. Savelli et al. (1984) and O. Nesci and D. Savelli (1990, 1991b), the alluvial deposits of fill terraces usually rest unconformably above deeply-incised pre-Quaternary bedrock both in inland areas and in coastal sectors, where, as discussed in next sections, the last aggradation cycle accounts for incised valleys deeply entrenched below present sea level (Calderoni et al., 2010). A stream deepening stage (“entrenching stage”, according to O. Nesci and D. Savelli, 1986, 1990) thus predates any major trunk-valley aggradation. According to a majority of authors (e.g., Merrits et al., 1989; Veldkamp and Van den Berg, 1993; Maddy et al., 2000; Merrits, 2007; Bridgland and Westway, 2008; Wegmann and Pazzaglia, 2009; Lewin and Gibbard, 2010), stream entrenchment has been enabled by uplift of inland areas, forced downstream by baselevel lowering and likely favoured by climate-controlled enhanced weathering rates (Nesci and Savelli, 1991 a and b; Nesci et al., 1995). The onset of the aggradation regime - also based on radiocarbon datings from Conca (Calderoni et al., 1993), Metauro (Alessio et al., 1987; Calderoni et al., 1994), Cesano (Calderoni et al., 1991a), and Esino rivers (Calderoni et al., 1991b) - has been constrained around the onset of Upper Pleistocene full-glacial conditions (Nesci et al., 1995). Such behavior accounts for a shift from incision to aggradation during glacial stages because of cold climate-driven increase of sediment supply at the basin scale. It matches similar mechanisms already proposed for both Marche river valleys (Nesci and Savelli, 1990; Coltorti et al., 1991; Wegmann and Pazzaglia, 2009; Nesci et al., 2010 limited to the Musone River) and other non-glaciated areas (e.g., Bull, 1991; Blum and Tornqvist, 2000; Bridgland, 2000; Litchfield and Berryman, 2005), and can also be effective for streams that alike the principal Marche rivers during glacio-eustatic lows crossed emerged shelf areas (e.g., Schumm, 1993).

Fig. 5 – Principal evolution steps in the development of a major fill terrace in the northern Marche
Fig. 5 – Principales étapes du développement d’une terrasse d’accumulation majeure dans le nord des Marches

Fig. 5 – Principal evolution steps in the development of a major fill terrace in the northern Marche Fig. 5 – Principales étapes du développement d’une terrasse d’accumulation majeure dans le nord des Marches

For palaeoclimatic and chronostratigraphic assumptions see fig.11; further explanations on the text.
Pour les hypothèses paléoclimatiques et chronostratigraphiques, voir la fig. 11 ; pour de plus amples explications, voir le texte.

After Savelli et al., 1984, modified.
D’après Savelli et al., 1984, modifié.

Fig. 6 – Fill-terraces fluvial deposits
Fig. 6 – Dépôts de terrasses fluviatiles d’accumulation

Fig. 6 – Fill-terraces fluvial depositsFig. 6 – Dépôts de terrasses fluviatiles d’accumulation

A: Upper Pleistocene braided-stream alluvium downstream Fossombrone, Metauro valley. B: Upper Pleistocene fine-grained floodplain deposits characterising the very first stages of aggradation unconformably overlain by braided-stream gravel alluvium; the arrow indicates a conifer stump with 14C age > 44,000 BP (Nesci et al., 2005); Cesano valley downstream Pergola. C: Upper Pleistocene braided alluvial deposits (A) unconformably overlain by a thin sheet of early Holocene meandering-stream alluvium, hinting at ongoing re-incision leading to underfit streams; relete to fig. 5 (4), where de Fs alluvium is the thin layer in black covering the inclined terrace surface on the left; lower Metauro valley.
A : Alluvions des cours d’eau tressés du Pléistocène supérieur à l’aval de Fossombrone, vallée du Metauro. B : Alluvions fines des plaines inondables des premières phases d’alluvionnement du Pléistocène supérieur, recouvertes en discordance par des graviers alluviaux des cours d’eau tressés ; la flèche indique une souche de conifères ayant livré un âge 14C > 44.000 BP (Nesci et al., 2005); vallée de Cesano aval Pergola. Graviers alluviaux des cours d’eau tressés du Pléistocène supérieur : (A) recouverts en discordance par une mince couche d’alluvions déposées par le cours d’eau sinueux du début de l’Holocène, indiquant un creusement en cours ; voir sur la fig. 5 (4) la fine bande noire recouvrant la surface inclinée de la terrasse sur la gauche ; vallée inférieure du Metauro.

18Alluvial fill of trunk valleys typically consists of gravelly braided-stream alluvium (Savelli et al., 1984; Nesci and Savelli, 1990, 1991b; fig. 6). Valley reaches where channels developed relatively high-sinuosity patterns have also been identified, as along the Cesano River (Nesci and Savelli, 1991b, 1995) and along the upper Esino River (Calderoni et al., 1991b). Anastomosed patterns have also been pointed out in the upper Esino River (Calderoni et al., 1991b) and along the Arzilla stream (Nesci et al., 1995). Certain variability in both alluvial lithofacies and channel patterns comes even from this short summary, and can be related to extension, relief, hydrology, lithology, and physiography of the basins. In addition, a vertical complexity of the alluvial suites has been also established, underlining that the overall braided-stream aggradation (Savelli et al., 1984; Nesci and Savelli, 1986), consists of several subordinate cut-and-fill episodes and buried terraces (Calderoni et al., 1991a), also showing local occurrence in time of different channel patterns (Calderoni et al., 1991b). Two major findings are worth to be here emphasised in this regard. i) Accurate dating of the last-glacial alluvium of the upper Esino River coupled with the analysis of its architectural styles, allowed G. Calderoni et al. (1991b) to assess several depositional stages which, for the first time on the Adriatic side of the Appennines, were recognised to match the stadial and interstadial phases already known for the last glacial period of northern Europe. ii) The onset of Upper Pleistocene aggradation regime (“III order” terrace-alluvium) is usually characterised by clayey-silty-sandy alluvium associated with subordinate gravels (fig. 6 and fig. 7). Such lithofacies, although rather discontinuous because repeated cut-and-fill processes (Calderoni et al., 1991a), occur throughout northern Marche (Nesci et al., 1995), from the Conca (Calderoni et al., 1993), to the Metauro (Alessio et al., 1987; Calderoni et al., 1994), Cesano (Calderoni et al., 1991a) and Esino (Calderoni et al., 1991b) basins, with up to 10-m thickness (Metauro valley). Hinting at a somehow generalised depositional phase of “low energy-fluvial deposits” (Alessio et al., 1987), this phase seems important in detailed reconstructing of aggradation events, although further studies are needed for both better understanding such deposits and assessing their possible occurrence also in older aggradation cycles.

Fig. 7 – Idealised cross-sections outlining the arrangement of Upper Pleistocene-Holocene alluvial suites in downstream sectors of the northern Marche trunk-valleys
Fig. 7 – Profils transversaux schématiques soulignant l’architecture des formations alluviales du Pléistocène supérieur et de l’Holocène dans les basses vallées principales des Marches septentrionales

Fig. 7 – Idealised cross-sections outlining the arrangement of Upper Pleistocene-Holocene alluvial suites in downstream sectors of the northern Marche trunk-valleysFig. 7 – Profils transversaux schématiques soulignant l’architecture des formations alluviales du Pléistocène supérieur et de l’Holocène dans les basses vallées principales des Marches septentrionales

A: Close to the present river mouth. B: 4-6 km upstream from river mouth. C: 10-15 km upstream from river mouth. Numbers refer to radiocarbon ages, expressed in ka BP (Elmi et al., 2003, modified). Vertical and horizontal scales are only indicative. 1: Upper Pleitocene fine-grained alluvium (multichannel; anastomosed stream); 2: Upper Pleistocene gravel and sand (braided stream); 3: upper Pleistocene mud and gravel (alluvial fan); 4: Holocene gravel and sand (from meandering to braided river).
A : A proximité de l’embouchure du fleuve actuel. B : 4-6 km en amont de la bouche de la rivière. C : 10-15 km en amont de l’embouchure du fleuve. Les nombres désignent les âges obtenus par la méthode du radiocarbone, exprimés in ka BP (Elmi et al., 2003, modifié). Les échelles verticales et horizontales sont indicatives. 1 : alluvions fines (chenaux multiples, cours d’eau anastomosés) du Pléistocène supérieur ; 2 : graviers et sables alluviaux (chenaux en tresses) du Pléistocène supérieur ; 3 : dépôts fins et graviers (cônes de déjection) du Pléistocène supérieur ; 4 : graviers et sables alluviaux (chenaux à méandres ou tressés) de l’Holocène.

Alluvial fans and piedmont aprons

19Despite the depositional complexity, the fluvial aggradation of trunk valleys is followed by generalised alluvial fan formation at both tributary junctions and piedmont areas (fig. 3 and fig. 5). Clearly, this doesn’t mean that a large number of “out-of-cycle” alluvial fans, sometimes grading to scree cones (e.g., Savelli et al., 1984), cannot form anytime and in any suitable place, as it happens for the coastal fans reported in the next section, or the recent fans related with both land use and deep-seated slope gravitational deformations (fig. 3). It simply states that major episodes of alluvial fan development in the northern Marche are cyclical (fig. 11), climate-controlled, and form part of the alluvial fill suites constituting the main valley terrace, as clearly demonstrated by heteropic relationship between the lowermost fan and uppermost fluvial deposits (e.g., Nesci et al., 2010).

20As already observed elsewhere (regardless of dimensions; Bull, 1977; Kochel, 1990; Mukerji, 1990; Guzzetti et al., 1997; Sorriso-Valvo et al., 1998; Harvey et al., 2005) the typical fan-shape can develop or not, being in most cases the radial growth of fans constrained by topographic obstacles (fig. 8; e.g., Savelli and Ballerini, 1991) and/or tight between adjacent fans on alluvial aprons (e.g., Nesci et al., 2002). A progressive confinement of fans, locally so strong that no alluvial fan could develop in the youngest cycles (Savelli et al., 1994) has also been noticed passing from the older fill terraces to the younger ones. This accounts for enhancing rates of stream entrenchment, likely related to increased uplift rate (Nesci et al., 2002, 2010). Alluvial fans lithology is greatly heterogeneous, depending on the geology of the drained area (Nesci and Savelli, 1991b) according to what already recognised in different contexts (e.g., Bull, 1977; Lecce, 1990; Blair and McPherson, 1994; Harvey et al., 2005). Specifically, O. Nesci and D. Savelli (1991b) recognised two different fan-types, e.g. “fine-grained fans” and “pebbly fans” among which a wide range of intermediate types exist, varying in thickness, respectively, from a few meters up to 25-30 m. As a result, where along a trunk-valley a given major fill terrace consists of both alluvial fan and braided-river sediments (fig. 3), the cumulative thickness often exceeds 40-50 m (Nesci and Savelli, 1991b; Nesci et al., 2010), and sidewards expansion of alluvial top-surfaces occurs. In this frame, also the remark by M. Coltorti (1997, p. 316) of “a thick gravelly unit (...) indicating a higher rate of sedimentation, and a dramatic expansion of the alluvial plain (...) during the Upper Pleniglacial” in the upper Esino valley, is likely to be justified by localised increased of sedimentation rate of tributary supplied gravels (Nesci et al., 1995). Moreover, where a complete alluvial suite occurs the undissected braided-fill is systematically topped by alluvial fan deposits (e.g., Nesci and Savelli, 1991b), and the stratigraphic boundary is always rather sharp (fig. 8), ranging from unconformable erosional or sharp conformable to heteropic (Nesci et al., 2010). If heteropic, the boundary however consists in a thin alternation of fluvial sediments and locally-derived alluvial fan deposits, usually less than 2-3-m thick, thus less than 5% of cumulative fluvial + fan thicknesses.

Fig. 8 – Alluvial fans and related piedmont alluvium
Fig. 8 – Cônes de déjection et alluvions des zones de piémont

Fig. 8 – Alluvial fans and related piedmont alluviumFig. 8 – Cônes de déjection et alluvions des zones de piémont

A: Upper Pleistocene terraced fan confined by topographic constraints (arrows) on the NE side of the Catria massif (Acquaviva fan, after Savelli and Ballerini, 1991). B: Middle Pleistocene piedmont terrace-alluvium derived from the dissection of a previous glacis on the NE side of the Catria massif (Serra S. Abbondio). C: Stratigraphic superposition of alluvial-fan alluvium (af) on braided alluvium (f) of a trunk-valley; latest middle Pleistocene, Metauro valley downstream Urbania.
A : Cônes de déjection incisés du Pléistocène supérieur. L’incision a été initialement confinée par des contraintes topographiques (flèches) ; nord-est du massif du Catria (cônes de déjection d’Acquaviva ; d’après Savelli et Ballerini, 1991). B : Alluvions des terrasses du Pléistocène moyen soulignant la dissection d’un ancien glacis au nord-est du massif du Catria (Serra S. Abbondio). C : Superposition stratigraphique des dépôts de cône de déjection (af) sur les graviers alluviaux déposés par un cours d’eau en tresses (f) de la vallée principale ; dernière phases du Pléistocène moyen, vallée du Metauro à l’aval d’Urbania.

21Along the inner margins of trunk-valleys and/or toward the fan apex areas, alluvial fan deposits can rest directly above the bedrock and, proceeding further upstream along the tributary (fig. 3), the fan apex often merges into terraced alluvial fill of the feeding stream (Nesci and Savelli, 1991b). Such arrangement clearly indicates a progressive backfilling into the tributary catchment, thus highlighting an overall upstream shifting of the depositional loci. This behaviour, according to O. Nesci et al. (2010), is better evident in the piedmont alluvial aprons, that either outside the trunk-valleys and/or at the foothills of the major anticline ridges, are the only significant alluvial contributors to terrace formation (e.g., Savelli and Ballerini, 1991; Savelli et al., 1994; Nesci et al., 2002).

22Moving towards the upper sectors of trunk-valleys, fill terraces gradually disappear and are locally substituted by strath terraces (e.g., Nesci and Savelli, 1991a; Wegmann and Pazzaglia, 2009). Significantly, in several upstream reaches of trunk-valleys alluvium supplied from lateral tributaries also occurs (i.e., referable, in our meaning, to alluvial fan deposits), representing the bulk of “glacial” aggradational stages. In this area deposits of the main stream are lacking or reduced to a very thin layer upon rock-cut straths (Nesci et al., 2010). Here, the strath terraces have also to be regarded as an equivalent of the cut-and-fill episodes characterising the downstream aggradation during full glacial stages. In this frame, unravelling relationships between the development of upstream strath terraces and downstream fill/cut-and-fill stages is definitely a crucial topic which, excluding some examples reported by K.W. Wegmann and F.J. Pazzaglia (2009), remains hitherto quite univestigated.

23Moving to piedmont sectors (fig. 9), fan construction has been often accompanied, sometimes substituted, by areal erosional processes that, smoothing the foothills all around the fans during the coldest morphogenetic stages, have generated more or less developed erosional-depositional glacis (Nesci et al., 1993; Savelli et al., 1994; Guerra and Nesci, 1999; Nesci et al., 2002). Similarly to trunk-valley fills glacis and related deposits have been re-incised and terraced (fig. 9) during interglacial stages. As a result, remnants of ancient glacis at present rest high above the valley floors, either on the divides or along the hillslopes (Savelli et al., 1994). The best and widest examples of ancient erosional glacis and related piedmont alluvial fans, extended enough to coalesce blanketing wide foothill areas, predate enhanced entrenching already discussed, and correlates to the traditional “II order” of terraces (Nesci et al., 2002).

Fig. 9 – The piedmont area of the upper Musone basin, where valley deepening and narrowing prevented the formation of alluvial-fans in the last glacial cycle
Fig. 9Le piémont du bassin supérieur du Musone ; l’approfondissement et le rétrécissement de la vallée a empêché la formation de cônes de déjection au cours du dernier cycle glaciaire

Fig. 9 – The piedmont area of the upper Musone basin, where valley deepening and narrowing prevented the formation of alluvial-fans in the last glacial cycle Fig. 9 – Le piémont du bassin supérieur du Musone ; l’approfondissement et le rétrécissement de la vallée a empêché la formation de cônes de déjection au cours du dernier cycle glaciaire

Further information in the text. 1: principal latest Middle-Pleistocene alluvial deposits; 2: depositional glacis (alluvial-fan terrace alluvium); 3: erosional glacis.
Pour de plus amples explications, voir le texte. 1 : dépôts alluviaux principaux du Pléistocène moyen final ; 2 : glacis d’accumulation (alluvions résultant de l’incision des cônes de déjection) ; 3 : glacis d’érosion.

After Savelli et al. (1994), redrawn and simplified.
D’après Savelli et al., 1994, redessiné et simplifié.

Downcutting and terrace development

24The dissection of “glacial” alluvial suites has been discussed chiefly for the upper Pleistocene-Holocene terraces (Nesci et al., 1995), although some indications have been given even for older fill terraces advocating similar mechanisms (Nesci and Savelli, 1990; Coltorti, 1991, p. 85; Nesci and Savelli, 1991b), thus once more hinting - apart from anthropogenic controls - at a cyclical repetition of both aggradational and erosional styles in development of main terraces. Actually, the surface of oldest terraces is often extensively concealed by colluvial slope-waste coverings and/or deeply remolded by erosion. However, it is proved to be frequently stepped by unpaired both rock-cut and fill-strath terraces, commonly resulting in nearby terrace-patches at different heights without any tectonic displacement could be advocated.

25Although in the Marche area little is yet known about timing of the transition from aggrading to downcutting regimes, the onset of dissection of Upper Pleistocene alluvium forming the bulk of the traditional “III order” terraces is to be regarded as early as Late Glacial (Coltorti, 1997, p. 317). Actually, an important downcutting stage occurred ever since Lateglacial-early Holocene (Cilla et al., 1996, p. 463): hereafter downcutting, albeit with up and downs, progressed throughout the Holocene (Calderoni et al., 1991b; Nesci and Savelli, 1991a; Coltorti, 1997).

26O. Nesci and D. Savelli (1991b, p. 161) suggest that along trunk-valleys the onset of downcutting regimes should have produced at first a rapid removal of previous sediment bodies (e.g., alluvial fans) encumbering the alluvial floor. Minor straths lying at about the same height of the fluvial-fan transition have been thus generated, also fostered by meander shifting and highlighting a rapid re-adjustment of trunk-stream long-profiles (fig. 5 and fig. 6). In this frame, according to O. Nesci et al. (1995), the important aggradation phases of the inland sector of the Esino basin attributed by means of radiocarbon ages to the late Pleistocene (Pre-Bölling; Alessio et al., 1979; Calderoni et al., 1991b) may be taken as representative of a secondary, yet remarkable, aggradational interruption of an already-started “re-incision” (sensu Nesci and Savelli, 1991b) of the former “glacial” alluvial fill. Evidence for this is several terraces definitely “out of range” as to altimetrical distributions of “terrace orders” throughout the Esino river basin and adjacent valleys (Centamore et al., 1979). Such seeming anomaly is easily explained if the occurrence of wide and thick alluvial-fans is taken into account, thus rising the depositional top-surface of Upper Pleistocene alluvium well beyond the top of the Pre-Bölling deposits at issue. Such interpretation accounts for fan dissection, which has been proved to be very effective in producing local altimetrical differentiation of terrace-heights (fig. 9) by generating unpaired terraces, as already described both for Marche region (since Savelli et al., 1984) and other areas (e.g., Wegmann and Pazzaglia, 2009, Bidente River).

27The longer interval of dowcutting regime was characterised by generalised sinuous/meandering channels, following the typical braiding of glacial stages (Savelli et al., 1984; Nesci and Savelli, 1991b; Coltorti, 1997). Downcutting regime soon proved its inherent complexity, going through an array of different processes and local behaviors that shaped risers that only locally consist of sub-vertical, unique scarps separating the “glacial” plain from the deepening valley-floor (Nesci and Savelli, 1991a). Actually, overcoming climate-related geomorphic thresholds brought the typical braiding of trunk-streams to change into generalised sinuous/meandering planforms (fig. 6), which persisted for the most part of downcutting stage (Nesci and Savelli, 1990; Coltorti, 1997). The development of “manifest” underfit streams (sensu Dury, 1964) with ingrown meanders (Chorely et al., 1984, p. 312) thus became a principal topic of valley deepening (fig. 5 and fig. 10), stressing the simultaneous occurrence of both vertical and lateral components in stream erosion. A principal geomorphologic result was (fig. 10) the generation of both minor unpaired strath terraces (rock-cut and fill-strath units) and streamward inclined terrace-surfaces (Savelli et al., 1984): anyhow, in shaping of both strath terraces and scarps segmenting the inclined terrace-surfaces, an intervening of headward erosion, rather than downcutting is most likely to be retained a dominant process (e.g., Troiani et al., 2009), a behavior already observed (e.g., Seidl and Dietrich 1992) and disputed (e.g., Wegmann and Pazzaglia, 2002) in different settings.

Fig. 10 – Idealised section across the intermediate valley sector of a northern Marche underfit stream
Fig. 10 – Profil en travers schématique dans une vallée moyenne des Marches septentrionnales

Fig. 10 – Idealised section across the intermediate valley sector of a northern Marche underfit streamFig. 10 – Profil en travers schématique dans une vallée moyenne des Marches septentrionnales

Explanation in the text.
Pour de plus amples explications, voir le texte.

After Nesci and Savelli, 1991b, modified.
D’après Nesci et Savelli, 1991b, modifié.

28The shaping of risers between the principal “terrace orders” was accomplished by relatively scant alluvial deposition throughout river basins, the thick alluvial deposits accumulated in downcutting regime in lowermost valley sectors being major exceptions (fig. 7 and fig. 12). It follows that major concerns definitely exist on distinction between “glacial” and younger forms in the upstream sectors of trunk-valleys, where only strath-terraces developed: unfortunately, unless numerical ages are obtained (Wegmann and Pazzaglia, 2009), reliable differentiations are here problematic, being only presumable from strath wideness.

29Concerning details of cut and fill episodes in downcutting stage, while the youngest phases are rather well known and constrained (Coltorti, 1991, 1997), and some work exists about the earliest alluvial units (Cilla et al., 1996), no specific paper deals specifically with “intermediate” minor terraces, that are only synthetically described by more general papers on strath-terrace distribution (Wegmann and Pazzaglia, 2009) or barely reported by some papers dealing with local problematics (Coltorti, 1991; Dall’Aglio et al., 2004; Savelli et al., 2004). The series of strath terraces generated in downcutting regime, realistically associates with fill-units better and better developed and thick approaching the present river mouths. However, also in this case, no specific study or correlations is available yet for highlight chronologic-functional relations between alluvial bodies and straths. Conversely, some data exist about channel-pattern metamorphoses in recent times, also driven by anthropogenic controls (Coltorti, 1997). Shortly, because beyond the scope of this paper, a destruction of meandering with a return to braided and/or wandering planforms is recorded in post-Roman terrace-alluvium (Coltorti, 1991, 1997; Dall’Aglio et al., 2004; Savelli et al., 2004); a renewed stream-deepening and a return to overall single-channel configurations instead occur in the last 50-100 years (Coltorti 1997; Dall’Aglio et al., 2004).

River mouths and coastal areas

30Following the onset of the Lastglacial, sea-level lowering responsible for the emersion of the north Adriatic shelf also drove incision along the lowermost reaches of northern Marche trunk-streams (e.g., Trevisan, 1949, p. 78; Elmi et al., 2003), likely coupled with an inland acceleration of incision rates in bedrock due to ongoing cooling (Merrits, 2007). In the near-coast offshore incision took most likely advantage of preceding highstand depositional topography (i.e., interglacial mud belts; Trincardi and Correggiari, 2000) that, according to P.J. Talling (1998), alone can explain an incision as deep as 20-70 m. Incised valleys thus formed, deepening up to 30 m below present sealevel into pre-Quaternary bedrock (Calderoni et al., 2010). Although possible mechanisms of incision have not been clearly stated yet, headward erosion (e.g., Schumm, 1993; Van Heijst and Postma, 2001; Fagherazzi et al., 2004) can be a likely solution, possibly through headward propagating “erosion waves” (pulses of accelerated incision; Merrits, 2007), a mechanism also fit for explaining the propagation of incision through bedrock. This hypothesis can be also substantiated by a knick-zone (likely one of the several expected along trunk-streams in pre-aggradation stages) always noticeable close to the present shoreline at the base of the alluvial fill (Nesci et al., 1995; Mencucci et al., 2003; Calderoni et al., 2010). Furthermore, despite the uncertainty of any correlation, the deepening stage of incised-valleys (sensu Dalrymple at al., 1994) can be associated with the “entrenching stage” already documented for more inland areas (Nesci and Savelli, 1986, 1990). The incised valleys were filled-up by an over 50-m thick pile of upper Pleistocene-Holocene mostly alluvial deposits unconformably resting on bedrock (Elmi et al., 1981, 1983, 2003; Calderoni et al., 2010). Accordingly, generalised climate cooling was the major forcing for aggradation of incised valleys (Nesci et al., 1995; Elmi et al., 2003) at about the onset of upper Pleistocene full-glacial conditions, despite lowstand sealevel throughout the central-north Adriatic shelf (Lambeck et al., 2004). At least until the end of Upper Pleistocene-early Holocene the aggradation style was the same outlined for trunk-valleys: as for inland areas, mechanic drilling and electric geotechnical tests actually demonstrated that late Pleistocene-early Holocene deposition in downstream zones is mostly related with braided rivers (Nesci et al., 1995; Calderoni et al., 2010), and is likewise characterised by several cut-and-fill episodes, each showing a typical fining upward succession of facies, as apparent (Elmi et al., 1991) in the Foglia river-mouth area.

31The partial “re-incision” of former deposits following downcutting regime at the end of the Last Glacial Maximum, extended far downstream, most likely well beyond the present coastline. As already mentioned, recurrent episodes of alluvial filling complementary to “re-incision” have occurred with increasing magnitude downvalley and in particular on the lowermost 5-10 km of present valleys (fig. 7). Although interrupted by prolonged stages of stream downcutting and terrace formation, a thick aggradation of the lowermost sectors of trunk-valleys could then continue throughout the Holocene: up to over 20-m thick alluvial suites were thus deposited (Nesci et al., 1995; Mencucci et al., 2003), forced by both natural (primarly eustatic and climatic) and anthropic causes (e.g., Coltorti, 1997; Elmi et al., 2003).

32A major finding contributing to unravel the heap of depositional episodes intervening in downcutting regime is the recent detection of uppermost Pleistocene-early Holocene large fans at the mouths of the principal northern Marche rivers (fig. 12). First T. Lipparini (1939, p. 13) reported a coastal terrace at the mouth of the Tenna River (southern Marche) that “could be regarded as a costal terrace if it were not covered by fluvial alluvium”, but which “represents undoubtedly the relic of a terraced fan, originally much more extended eastward, then frontally demolished by a slight marine ingression”. However, for a long time this report remained the only citation of coastal-fans in the Marche region, until D. Di Bucci et al. (2003, p. 227) highlighted an early Holocene “fan-like sedimentary body at the river mouth” of the Metauro River. Later on, O. Nesci et al. (2008) and G. Calderoni et al. (2010) provided a detailed description and reconstruction of the fan, constraining by means of radiometric ages at early Holocene (Preboreal) the intermediate portion of the fan deposits, evaluated on a whole some 10-20 m thick. G. Calderoni et al. (2010) basing mainly on morphological and lithostratigraphic data also inferred the occurrence of similar fans at the mouths of other northern Marche rivers, from the Foglia to the Misa; additional coastal fans have been recently recognised also at the mouth of the northernmost Conca River (unpublished data).

33In the Metauro, Cesano and Conca valleys the coastal fans are high-relief, marked landforms; conversely, they are low-relief landforms with scarce morphologic evidence in the lowermost reaches of the Foglia and Misa valleys. The more or less pronounced fan-relief reflects primarily the geology of the river basin: G. Calderoni et al. (2010) demonstrated that well developed fans with relatively high relief (i.e., Metauro and Cesano) pertain to basins where abundant resistant rock outcrop, whilst catchments with predominance of poorly resistant rock provide more abundant fine alluvium, originating flat and sometimes hardly detectable fans. Some questions arise for the Esino valley, where despite relatively high amounts of resistant rocks in the catchment, no fan is detectable by surface surveying. It is indeed possible that no coastal fan formed in this area; it is likewise possible that huge amounts of later Holocene sediments (Coltorti, 1991, 1997) buried such landforms, perhaps controlled by local subsidence (Elmi et al., 1987) and/or local physiography, possibly characterised by sea embayments as already suggested by M. Coltorti (1997). The construction of fans although occurring in conditions of still low eustatic sealevel, undoubtedly post-dates the “glacial” aggradation (Baldelli et al. 2009). Postglacial rise of sea level progressively destroyed the outer sectors of the fans, thus originating landward migrating wave-cut scarps that with the maximum Holocene ingression attained their overall present position (fig. 1 and fig. 12), i.e. some 200-500 m inland with respect to the present shore (Calderoni et al., 2010), maintaining it despite minor oscillations since Roman-Medieval times (Veggiani, 1982; Elmi et al., 2003). These peculiar fans are common to all northern Marche valley-mouths: their origin, however, have not been proven yet. They are most likely related to an increased sediment supply because of ongoing upstream downcutting into previous “glacial” valley fills, a mechanism already stressed of general effectiveness in driving complex stream behaviors (e.g., Schumm, 1977, 1993), that in the areas in issue found favorable physiographic conditions in the almost flat, still largely emerged shelf (Lambeck et al., 2004). Seemingly, any tectonic control is to be excluded because of fan occurrence in basins of different lithology and physiography (Bull 1991); nonetheless it remains unclear what the role climate and related parametres may have had.

34Some important corollaries follow directly the occurrence of coastal fans and related landforms:

35– First, the aforementioned wave-cut scarps (fig. 12) are remarkable landforms striking parallel to the present coastline of northern Marche (e.g., Veggiani, 1982; Elmi et al., 1994, 2001): they are related to marine wave erosion of fluvial deposits and are found only where fan-like coastal deposits do occur. Delimitating seaward previous fan remnants (i.e., the apex areas), such scarps are strictly fan-dependant, varying in height as a function of the original fan relief and upward convexity, that is the higher and convex the former fan, the higher the scarp (Calderoni et al., 2010). Noteworthy, in such regard also K.W. Wegmann and F.J. Pazzaglia (2009, p. 143), in order to explain irregularities observed on coastal terrace treads, advocate the occurrence of lowstand fan deltas at the river mouths.

36– Second, roughly at the same time of wave-cut coastal fans have been also dissected by trunk streams and minor streams (Nesci et al., 2008). Later on, trunk streams generated new, rather narrow, alluvial plains largely related to the sedimentary construction of a coastal plain and consequent re-advance of the shoreline (Coltorti, 1997; Elmi et al. 2003). The treads of already terraced-fans have been thus separated by more recent alluvial sediments that, where flattened fans occur (Foglia, Misa), also partially buried some treads: however, such recent alluvial plains always merge into the coastal plain (fig. 12). Moreover, immediately upslope the wave-cut scarps the outer fan margins also make transition to an older coastal plain (fig. 12) whose age and modes of development still remain object of controversy (Veggiani, 1982; Elmi et al., 1994, 2001, 2003). Switch-points fan-coastal plain and/or fluvial plain-coastal plain can be thus identified by means of quantitative terrain analysis and geostatistical topographic reconstructions, thus allowing highlighting both geometrical evidence for fans and occurrence of possible older similar landforms (Calderoni et al., 2010). Based on this evidence either an older coastal fan relating to the traditional “II order” of terraces, and previous switch-points (“II order” and “I order”) have been recognised in the terminal sectors of the Cesano and Misa valleys (Calderoni et al., 2010; Troiani and Della Seta, in press), thus constraining overall position of previous coastal areas. Since interglacial alluvial deposition dramatically increases seaward (fig. 7), direct evidence in the field for marked seaward thickening of alluvial deposits in ancient terraces (fig. 12) usually confirmed the approaching to former shore zones.

37Based on the above “chains of landforms” as indicators for vicinity to former shorelines, an overall maintenance of the relative position of the target river mouths, realistically associating with minor modification of the local arrangement of the shore zones, has been assessed since middle Pleistocene for the Cesano-Esino coastal sector (Nesci et al., 2008, 2009). Different conclusions arise from the northernmost sector (unpublished), where such chains are missing, likely confirming for this sector a marked erosional coast retreat (Veggiani, 1982; Elmi et al., 1994, 2001; Colantoni et al. 2004) in highstand conditions.

Concluding remarks

38Stream terraces of northern Marche provide useful information about development of terrace staircases in extra-glacial areas. The reported staircases consist of transitions from wide flat erosional surfaces to strath terraces and lastly to fill terraces. For the first time, it has been possible to state that the north Marche terrace staircase evolution matches other well known arrays that have been related with an apparently worldwide increase in uplift rates and consequent valley deepening and narrowing following the “Mid Pleistocene Revolution” (MPR; Bridgland and Westway, 2008). Accordingly, also the onset of thick aggradational suites (fig. 11) appears to respond to increased weathering and erosion caused by cold stages in 100-ka climatic cycles. However, since the definition MPR indicates a marked intensification and prolongation of glacial-interglacial cycles initiated between 900 and 650 ka (e.g., Mudelsee and Schulz, 1997), and given an overall scarcity of numerical ages, some trouble arise on attributing to a specific cycle the beginning of fill stages. Actually fill terraces, also by means of a evident cyclical repetitiveness of characterising erosional-depositional stages (“guide stages”, since Nesci and Savelli, 1986), stress a marked dependence on 100-ka climatic cycles, most likely with one major terrace per cycle. In this concern, the upper Pleistocene-Holocene cycle provides suitable tools for deciphering stages and modes of older fill terrace formation. Considering that (i) the traditional “I order” of terraces actually accounts for two distinct fill terraces, and (ii) the traditional “IV order”, despite its elevated downstream thickness, consists of assemblages of discontinuous, often unpaired and diachronous units incomparable with previous major fill terraces: then it results that all along the Marche main valleys four principal climate-driven fill terraces are found at approximately one per 100-ka cycle, to which an undefined number of minor (sensu Bull, 1991, p. 26) and/or local recent units are overlapped. Modes of both previous fill “re-incision” and stream entrenchment in bedrock during downcutting regime also suggest that both warming- and cooling-limb forcing of incision were effective.

Fig. 11 – Main stages in the development of the principal stream terraces in the northern Marche Apennines: the main cyclical episodes of downcutting and aggradation discussed in the text are plotted against a representation of late Quaternary oxygen isotopic fluctuations
Fig. 11 – Principales étapes de l’évolution des terrasses alluviales dans les Apennins des Marches septentrionnales : les principaux épisodes de creusement et d’aggradation ont été datés par des mesures isotopiques (18O) et sont discutés dans le texte

Fig. 11 – Main stages in the development of the principal stream terraces in the northern Marche Apennines: the main cyclical episodes of downcutting and aggradation discussed in the text are plotted against a representation of late Quaternary oxygen isotopic fluctuationsFig. 11 – Principales étapes de l’évolution des terrasses alluviales dans les Apennins des Marches septentrionnales : les principaux épisodes de creusement et d’aggradation ont été datés par des mesures isotopiques (18O) et sont discutés dans le texte

After Nesci et al., (2010) ; Wegmann and Pazzaglia (2009), simplified.
D’après Nesci et al. (2010) ; Wegmann et Pazzaglia (2009), simplifié.

Fig. 12 – 3D surface (derived from a 40-m gridded DTM) of the northern Marche coastal area
Fig. 12 – Représentation en trois dimensions de la surface topographique (provenant d’un MNT de 40 m de résolution) de la zone côtière des Marches septentrionales

Fig. 12 – 3D surface (derived from a 40-m gridded DTM) of the northern Marche coastal areaFig. 12 – Représentation en trois dimensions de la surface topographique (provenant d’un MNT de 40 m de résolution) de la zone côtière des Marches septentrionales

The principal morphologic elements reported in the text are shown. 1: Upper Pleistocene-early Holocene coastal fan; 2: reconstructed inland border of the coastal fan; 3: Holocene wave-cut scarp; 4: fluvial-coastal plain switch-point; 5: track of the topographic transept shown in fig. 4; 6: Middle Pleistocene fluvial-terrace deposits (T1); 7: latest Middle Pleistocene fluvial-terrace deposits (T2); 8: Upper Pleistocene-early Holocene fluvial-terrace deposits (T3); 9: marked seawards thickening of terrace deposits.
Les principaux éléments morphologiques mentionnés dans le texte sont présentés. 1 : cônes de déjection côtiers du Pléistocène supérieur et du début de l'Holocène ; 2 : restitution de la limite intérieure des cônes de déjection côtiers ; 3 : falaise holocène ; 4 : point d’inflexion entre la plaine alluviale et la plaine côtière ; 5 : tracé du profil topographique montré dans la fig. 4 ; 6 : dépôts des terrasses fluviatiles du Pléistocène moyen (T1) ; 7 : dépôts des terrasses fluviatiles du Pléistocène moyen final (T2) ; 8 : dépôts des terrasses fluviatiles du Pléistocène supérieur et du début de l'Holocène (T3) ; 9 : épaississement rapide des dépôts de terrasse vers la Mer Adriatique.

The authors would like to thank Paola Fredi and two other anonymous reviewers for their helpful comments and suggestions that enhanced the results reported in this manuscript and improved readability of the paper. The authors are also grateful to the Editor-in-Chief G. Arnaud-Fassetta. A special thank to A. Fontana.

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Wegmann K.W., Pazzaglia F.J. (2009) – Late Quaternary fluvial terraces of the Romagna and Marche Apennines, Italy: Climatic, lithologic, and tectonic controls on terrace genesis in an active orogen. Quaternary Science Reviews 28, 137-165.

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Annexe

Version abrégée en français

La très bonne conservation des terrasses fluviatiles quaternaires et des dépôts qui les constituent a fait de la région septentrionale des Marches (partie centrale des Apennins) un objet privilégié depuis longtemps pour les recherches géomorphologiques sur le Quaternaire. L’étagement des terrasses fluviatiles observé dans cet espace reflète la succession des terrasses observées dans la plupart des bassins versants drainant le versant adriatique des Apennins (Cyr et Granger, 2008 ; Della Seta et al., 2008). Appuyé sur l’examen des données publiées ou acquises récemment et de résultats inédits, l’article propose une réflexion sur la distribution spatiale et la succession des terrasses alluviales dans le Nord des Marches. Dans les Marches, les Apennins forment une chaîne de plis plus ou moins chevauchant ayant entrainé la formation d’un relief de type jurassien, organisé en monts dérivées dans les anticlinaux et en vals dans les synclinaux (fig. 1). Sur le versant adriatique des Apennins, les principaux fleuves s’écoulent vers le nord-est, traversant transversalement les structures géologiques et dessinant de profondes cluses. Le développement du réseau de drainage, initié au cours du Messinien suite à l’émergence progressive de la chaîne, s’est poursuivi selon des mécanismes complexes impliquant des processus d’antécédence, de surimposition et de « percées » dans les monts (Mayer et al., 2003). Le tracé des vallées principales était déjà acquis à la fin du Pléistocène ancien et au début du Pléistocène moyen, permettant ainsi le passage à une dynamique alluviale caractérisée par la mise en place de terrasses (fig. 2A). Les terrasses d’érosion qui se sont initialement formées sont aujourd’hui observables sur les hauts de versant des vallées où elles forment plusieurs niveaux de replats affectés par la tectonique. Puis des terrasses d’accumulation ont pu être observées le long des vallées principales en relation avec la « Révolution du Pléistocène moyen ». Ainsi, quatre terrasses d’accumulation principales correspondant à quatre périodes prolongées d’aggradation alluviale se sont formées durant les périodes glaciaires. Elles ont été suivies de périodes d’incision et de contraction des plaines alluviales pendant les périodes de transition climatique conduisant aux périodes interglaciaires (Nesci et Savelli, 1991b, 1995). Ainsi, chaque épisode majeur d’aggradation a été suivi par une incision du lit du fleuve et le creusement des formations alluviales de la période précédente. Du fait de la poursuite du soulèvement tectonique, ce processus a abouti à la production de terrasses d’accumulation qui se distribuent actuellement depuis des hauteurs de 200 m au-dessus du thalweg actuel jusque vers un peu plus de 2 m (Bisci et Dramis, 1991 ; Fanucci et al., 1996). Dans les grandes vallées où la présence d’affluents a favorisé la formation de cônes alluviaux, les terrasses d’accumulation se composent de formations fluviatiles et de celles issues des cônes alluviaux. Quand ces formations sont également présentes, les dépôts de cônes alluviaux recouvrent systématiquement les graviers des nappes alluviales mises en place par les écoulements issus de la vallée principale (fig. 3 ; Nesci et al., 2010).

Par ailleurs, au pied des principaux reliefs calcaires des espaces montagnards intérieurs, les cônes alluviaux ont formé de larges glacis qui sont les seules aires contributrices du matériau des terrasses. Après la dernière phase d’aggradation, l’incision puis la formation de la dernière terrasse alluviale s’est accompagnée de la mise en place de dépôts alluviaux qui n’ont jamais eu une épaisseur et une extension comparable à celle des périodes précédentes contrôlées par le climat froid des périodes glaciaires. En revanche, à l’aval des vallées principales, au cours de l’Holocène comme au cours des interglaciaires précédents, on retrouve les témoignages d’importants épisodes d’aggradation (fig. 7 et fig. 10 ; Calderoni et al., 2010) déclenchés par des causes complexes et probablement liés à un accroissement temporaire de la charge grossière, à la remontée du niveau de la mer, à la construction des plaines littorales et, au cours de l’Holocène, au facteur anthropique. La succession des terrasses d’accumulation, témoignant de l’alternance de période d’érosion et d’aggradation (Nesci et Savelli, 1986), exprime le contrôle joué par les cycles climatiques de 100 ka, probablement avec une terrasse majeure par cycle. Le dernier cycle, c’est-à-dire celui du Pléistocène supérieur-Holocène, qui est également le mieux préservé, a fournit des éléments permettant de déchiffrer les étapes et les modalités de la formation de ces terrasses d’accumulation (Nesci et al., 2010). Ainsi, 1) les formes majeures de terrasses se composent en réalité de deux terrasses d’accumulation distinctes et 2) les unités sédimentaires principales, d’une épaisseur très importante à l’aval, se composent en fait d’assemblages d’unités sédimentaires élémentaires discontinues, souvent non-appariées et diachroniques et qui ne sont donc pas comparables avec les terrasses d’accumulation principales des périodes précédentes. En conséquence, les quatre principales terrasses d’accumulation ont bien été formées lors de chaque cycle de 100 ka et induites par les changements climatiques (fig. 11) mais les sédiments observables représentent un nombre indéfini d’unités sédimentaires mineures et/ou locales imbriquées. Finalement, la mise en évidence des modes complexes de creusement et d’accumulation suggère que les périodes de réchauffement et de refroidissement climatiques ont été tout aussi efficaces pour engendrer les incisions présidant à l’étagement des terrasses alluviales des Marches.

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Table des illustrations

Titre Fig. 1 – Geological sketch of the northern Marche ApenninesFig. 1 – Schéma géologique des Apennins dans les Marches du nord
Légende 1: main gorges; 2: Holocene wave-cut scarp; 3: topographic transects of fig. 4; 4: valley topographic profiles of fig. 3; 5: location of the section of fig. 2; 6: gravel and sand of the Middle Pleistocene-Holocene fluvial and coastal terraces; 7: Plio-Pleistocene pelitic and arenitic marine deposits; 8: Eocene-Miocene marly-calcareous, evaporitic and terrigenous units; 9: Jurassic-Paleogene formations of the carbonatic ridges; 10: calcareous, marly-calcareous and pelitic allochthonous units of the Val Marecchia Sheet; 11: anticline axis; 12: thrust fault (dashed where buried); 13: normal fault; 14: strike-slip fault.1 : gorges principales ; 2 : falaise holocène ; 3 : profils topographiques de la fig. 4 ; 4 : coupes représentées sur la fig. 3 ; 5 : localisation de la coupe de la fig. 2 ; 6 : dépôts sablo-graveleux des terrasses fluviatiles et côtières du Pléistocène moyen-Holocène ; 7 : sédiments gréso-pélitiquees du Plio-Pléistocène marin ; 8 : dépôts marno-calcaires, évaporitiques et terrigènes de l’Éocène et du Miocène ; 9 : formations calcaires du Jurassique et du Paléogène des monts ; 10 : unités calcaires, marno-calcaires et pélitiques allochtones de la Nappe du Val Marecchia ; 11 : axe anticlinal ; 12 : chevauchement (ligne en pointillés : n’affleurant pas) ; 13 : faille normale ; 14 : décrochement.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-1.png
Fichier image/png, 526k
Titre Fig. 2 – Metauro river valleyFig. 2 – Vallée du Metauro
Légende A: Schematic section across the lower Metauro river valley: narrowing and transition from strath- to fill-terraces are apparent (after Nesci et al., 1992, redraw; location on fig. 1). B: The terrace staircase of the Metauro River sketched on fig. 2B (T1a-T3: fill-terraces; s: strath terraces; for further explanations, see the text).A : Coupe schématique à travers la basse vallée du Metauro : rétrécissement de la vallée et transition des niveaux d’ablation aux terrasses d’accumulation (d’après Nesci et al., 1992, redessiné ; localisation sur la fig. 1). B : Succession des terrasses étagées dans la vallée du Metauro esquissées sur la fig. 2B (T1a-T3 : terrasses d’accumulation; s : terrasses d’érosion ; pour de plus amples explications, voir le texte).
URL http://geomorphologie.revues.org/docannexe/image/9838/img-2.png
Fichier image/png, 301k
Titre Fig. 3 – Fill-terraces staircases on transversal profiles of the Metauro and Foglia valleys; cross-sections also show Pleistocene alluvial-fan deposits stratigraphically overlying the trunk-valley alluvial fill Fig. 3 – Profils transversaux de vallées Metauro et Foglia montrant la série de terrasses d’accumulation ; les profils montrent également les dépôts de cônes alluviaux du Pléistocène superposés aux remplissage alluviaux dans la vallée principale
Légende 1: fluvial deposits; 2: alluvial fan deposits; 3: strath and fill-strath terraces; 4: major terrace-levels; 5: Holocene slope-waste and alluvial-fan deposits.1 : alluvions fluviatiles ; 2 : alluvions de cônes de déjection ; 3 : terrasses d’érosion dans le lit rocheux ou aux dépens du remplissage alluvial ; 4 : niveaux de terrasse principaux ; 5 : dépôts de pente et de cônes de déjection de l’Holocène.
Crédits After Nesci et al., 2010; location on fig. 1. D’après Nesci et al., 2010 ; localisation sur la fig. 1.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-3.png
Fichier image/png, 74k
Titre Fig. 4 – Topographic transects orthogonal to the orientation of trunk-valleys showing the height distribution of fill-terraces throughout the northern Marche Apennines Fig. 4 – Profils topographiques perpendiculaires à l’orientation des vallées principales dans les Marches septentrionales, montrant l’étagement des terrasses d’accumulation
Légende Location of transepts in fig. 1 and fig. 12. 1: “I order” terrace (T1); 2: “II order” terrace (T2); 3: “III order” terrace (T3).Position des profils sur la fig. 1 et la fig. 12. 1 : terrasse d’« ordre I » (T1) ; 2 : terrasse d’« ordre II » (T2) ; 3 : terrasse d’« ordre III » (T3).
URL http://geomorphologie.revues.org/docannexe/image/9838/img-4.png
Fichier image/png, 72k
Titre Fig. 5 – Principal evolution steps in the development of a major fill terrace in the northern Marche Fig. 5 – Principales étapes du développement d’une terrasse d’accumulation majeure dans le nord des Marches
Légende For palaeoclimatic and chronostratigraphic assumptions see fig.11; further explanations on the text. Pour les hypothèses paléoclimatiques et chronostratigraphiques, voir la fig. 11 ; pour de plus amples explications, voir le texte.
Crédits After Savelli et al., 1984, modified. D’après Savelli et al., 1984, modifié.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-5.png
Fichier image/png, 1,1M
Titre Fig. 6 – Fill-terraces fluvial depositsFig. 6 – Dépôts de terrasses fluviatiles d’accumulation
Légende A: Upper Pleistocene braided-stream alluvium downstream Fossombrone, Metauro valley. B: Upper Pleistocene fine-grained floodplain deposits characterising the very first stages of aggradation unconformably overlain by braided-stream gravel alluvium; the arrow indicates a conifer stump with 14C age > 44,000 BP (Nesci et al., 2005); Cesano valley downstream Pergola. C: Upper Pleistocene braided alluvial deposits (A) unconformably overlain by a thin sheet of early Holocene meandering-stream alluvium, hinting at ongoing re-incision leading to underfit streams; relete to fig. 5 (4), where de Fs alluvium is the thin layer in black covering the inclined terrace surface on the left; lower Metauro valley.A : Alluvions des cours d’eau tressés du Pléistocène supérieur à l’aval de Fossombrone, vallée du Metauro. B : Alluvions fines des plaines inondables des premières phases d’alluvionnement du Pléistocène supérieur, recouvertes en discordance par des graviers alluviaux des cours d’eau tressés ; la flèche indique une souche de conifères ayant livré un âge 14C > 44.000 BP (Nesci et al., 2005); vallée de Cesano aval Pergola. Graviers alluviaux des cours d’eau tressés du Pléistocène supérieur : (A) recouverts en discordance par une mince couche d’alluvions déposées par le cours d’eau sinueux du début de l’Holocène, indiquant un creusement en cours ; voir sur la fig. 5 (4) la fine bande noire recouvrant la surface inclinée de la terrasse sur la gauche ; vallée inférieure du Metauro.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-6.png
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Titre Fig. 7 – Idealised cross-sections outlining the arrangement of Upper Pleistocene-Holocene alluvial suites in downstream sectors of the northern Marche trunk-valleysFig. 7 – Profils transversaux schématiques soulignant l’architecture des formations alluviales du Pléistocène supérieur et de l’Holocène dans les basses vallées principales des Marches septentrionales
Légende A: Close to the present river mouth. B: 4-6 km upstream from river mouth. C: 10-15 km upstream from river mouth. Numbers refer to radiocarbon ages, expressed in ka BP (Elmi et al., 2003, modified). Vertical and horizontal scales are only indicative. 1: Upper Pleitocene fine-grained alluvium (multichannel; anastomosed stream); 2: Upper Pleistocene gravel and sand (braided stream); 3: upper Pleistocene mud and gravel (alluvial fan); 4: Holocene gravel and sand (from meandering to braided river).A : A proximité de l’embouchure du fleuve actuel. B : 4-6 km en amont de la bouche de la rivière. C : 10-15 km en amont de l’embouchure du fleuve. Les nombres désignent les âges obtenus par la méthode du radiocarbone, exprimés in ka BP (Elmi et al., 2003, modifié). Les échelles verticales et horizontales sont indicatives. 1 : alluvions fines (chenaux multiples, cours d’eau anastomosés) du Pléistocène supérieur ; 2 : graviers et sables alluviaux (chenaux en tresses) du Pléistocène supérieur ; 3 : dépôts fins et graviers (cônes de déjection) du Pléistocène supérieur ; 4 : graviers et sables alluviaux (chenaux à méandres ou tressés) de l’Holocène.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-7.png
Fichier image/png, 435k
Titre Fig. 8 – Alluvial fans and related piedmont alluviumFig. 8 – Cônes de déjection et alluvions des zones de piémont
Légende A: Upper Pleistocene terraced fan confined by topographic constraints (arrows) on the NE side of the Catria massif (Acquaviva fan, after Savelli and Ballerini, 1991). B: Middle Pleistocene piedmont terrace-alluvium derived from the dissection of a previous glacis on the NE side of the Catria massif (Serra S. Abbondio). C: Stratigraphic superposition of alluvial-fan alluvium (af) on braided alluvium (f) of a trunk-valley; latest middle Pleistocene, Metauro valley downstream Urbania.A : Cônes de déjection incisés du Pléistocène supérieur. L’incision a été initialement confinée par des contraintes topographiques (flèches) ; nord-est du massif du Catria (cônes de déjection d’Acquaviva ; d’après Savelli et Ballerini, 1991). B : Alluvions des terrasses du Pléistocène moyen soulignant la dissection d’un ancien glacis au nord-est du massif du Catria (Serra S. Abbondio). C : Superposition stratigraphique des dépôts de cône de déjection (af) sur les graviers alluviaux déposés par un cours d’eau en tresses (f) de la vallée principale ; dernière phases du Pléistocène moyen, vallée du Metauro à l’aval d’Urbania.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-8.png
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Titre Fig. 9 – The piedmont area of the upper Musone basin, where valley deepening and narrowing prevented the formation of alluvial-fans in the last glacial cycle Fig. 9Le piémont du bassin supérieur du Musone ; l’approfondissement et le rétrécissement de la vallée a empêché la formation de cônes de déjection au cours du dernier cycle glaciaire
Légende Further information in the text. 1: principal latest Middle-Pleistocene alluvial deposits; 2: depositional glacis (alluvial-fan terrace alluvium); 3: erosional glacis.Pour de plus amples explications, voir le texte. 1 : dépôts alluviaux principaux du Pléistocène moyen final ; 2 : glacis d’accumulation (alluvions résultant de l’incision des cônes de déjection) ; 3 : glacis d’érosion.
Crédits After Savelli et al. (1994), redrawn and simplified. D’après Savelli et al., 1994, redessiné et simplifié.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-9.png
Fichier image/png, 183k
Titre Fig. 10 – Idealised section across the intermediate valley sector of a northern Marche underfit streamFig. 10 – Profil en travers schématique dans une vallée moyenne des Marches septentrionnales
Légende Explanation in the text.Pour de plus amples explications, voir le texte.
Crédits After Nesci and Savelli, 1991b, modified. D’après Nesci et Savelli, 1991b, modifié.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-10.png
Fichier image/png, 543k
Titre Fig. 11 – Main stages in the development of the principal stream terraces in the northern Marche Apennines: the main cyclical episodes of downcutting and aggradation discussed in the text are plotted against a representation of late Quaternary oxygen isotopic fluctuationsFig. 11 – Principales étapes de l’évolution des terrasses alluviales dans les Apennins des Marches septentrionnales : les principaux épisodes de creusement et d’aggradation ont été datés par des mesures isotopiques (18O) et sont discutés dans le texte
Crédits After Nesci et al., (2010) ; Wegmann and Pazzaglia (2009), simplified.D’après Nesci et al. (2010) ; Wegmann et Pazzaglia (2009), simplifié.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-11.png
Fichier image/png, 333k
Titre Fig. 12 – 3D surface (derived from a 40-m gridded DTM) of the northern Marche coastal areaFig. 12 – Représentation en trois dimensions de la surface topographique (provenant d’un MNT de 40 m de résolution) de la zone côtière des Marches septentrionales
Légende The principal morphologic elements reported in the text are shown. 1: Upper Pleistocene-early Holocene coastal fan; 2: reconstructed inland border of the coastal fan; 3: Holocene wave-cut scarp; 4: fluvial-coastal plain switch-point; 5: track of the topographic transept shown in fig. 4; 6: Middle Pleistocene fluvial-terrace deposits (T1); 7: latest Middle Pleistocene fluvial-terrace deposits (T2); 8: Upper Pleistocene-early Holocene fluvial-terrace deposits (T3); 9: marked seawards thickening of terrace deposits.Les principaux éléments morphologiques mentionnés dans le texte sont présentés. 1 : cônes de déjection côtiers du Pléistocène supérieur et du début de l'Holocène ; 2 : restitution de la limite intérieure des cônes de déjection côtiers ; 3 : falaise holocène ; 4 : point d’inflexion entre la plaine alluviale et la plaine côtière ; 5 : tracé du profil topographique montré dans la fig. 4 ; 6 : dépôts des terrasses fluviatiles du Pléistocène moyen (T1) ; 7 : dépôts des terrasses fluviatiles du Pléistocène moyen final (T2) ; 8 : dépôts des terrasses fluviatiles du Pléistocène supérieur et du début de l'Holocène (T3) ; 9 : épaississement rapide des dépôts de terrasse vers la Mer Adriatique.
URL http://geomorphologie.revues.org/docannexe/image/9838/img-12.png
Fichier image/png, 310k
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Olivia Nesci, Daniele Savelli et Francesco Troiani, « Types and development of stream terraces in the Marche Apennines (central Italy): a review and remarks on recent appraisals », Géomorphologie : relief, processus, environnement, vol. 18 - n° 2 | 2012, 215-238.

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Olivia Nesci, Daniele Savelli et Francesco Troiani, « Types and development of stream terraces in the Marche Apennines (central Italy): a review and remarks on recent appraisals », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 - n° 2 | 2012, mis en ligne le 02 novembre 2014, consulté le 26 mai 2016. URL : http://geomorphologie.revues.org/9838 ; DOI : 10.4000/geomorphologie.9838

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Auteurs

Olivia Nesci

University of Urbino “Carlo Bo” - Department of Earth, Life and Environmental Sciences - Italy (olivia.nesci@uniurb.it)

Daniele Savelli

University of Urbino “Carlo Bo” - Department of Earth, Life and Environmental Sciences - Italy

Francesco Troiani

University of Urbino “Carlo Bo” - Department of Earth, Life and Environmental Sciences - Italy

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Droits d’auteur

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