Sedimentary facies and sequence stratigraphy of

Sedimentary Geology 210 (2008) 87–110

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Sedimentary facies and sequence stratigraphy of coarse-grained Gilbert-type deltas within the Pliocene thrust-top Potenza Basin (Southern Apennines, Italy) Sergio G. Longhitano Dipartimento di Scienze Geologiche, Università degli Studi della Basilicata, Campus di Macchia Romana, 85100 Potenza, Italy

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Article history: Received 24 July 2007 Received in revised form 16 July 2008 Accepted 17 July 2008 Keywords: Coarse-grained Gilbert-type braid deltas Depositional processes High-frequency relative sea-level changes Tectonic uplift

a b s t r a c t Coarse-grained, fluvially-dominated Gilbert-type braid deltas represent a volumetrically significant component of the Potenza Basin sedimentary infill, a small thrust-top structural depression that developed during the Pliocene in the Southern Apennines (Italy). Excellent exposure shows vertical and lateral relationships among facies assemblages that identify a suite of deltaic and non-deltaic depositional environments occupying semi-confined marine embayments along the southern–western basin margin. The succession is around 50 m thick and consists mainly of shoreface and offshore sands and clays, moderately wave-worked and covered by shoal-water deltaic clinostratified gravels. The deltas are organized into two vertically-stacked sequences that display well-developed angular-totangential foresets and poorly-preserved topsets, and a suite of internal depositional architectures consisting of alternating progradational and aggradational geometries that were controlled by high-frequency, relative sea-level changes in a relatively slowly subsiding basin. The common element that characterizes all the depositional architectures detected within the two main studied sections is the constant influence of coastal uplift on the deltaic systems during sediment accumulation. The tectonic control of the basin margin from which the deltas were sourced forced a forward-stepping (basinward) arrangement of stacked Gilbert-type braid deltas, and these produced clinoforms that become progressively younger toward the basin depocentre. The deltaic sequences show different offsets in their along-dip arrangement suggesting that the coastal margin was inclined at varying angles depending on the rates of the tectonic uplift. Two end-members of Gilbert-type delta architectures are represented by concave- and flat-bottom deltas, prograding onto a mudstone and calcareous substrate, respectively. These two different Gilbert-type models reflect a different style of delta accretion during progradation, due to the response of the substrate to the erosion exerted from gravel avalanches. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Shallow-marine depositional systems are usually good potential indicators of sediment input to the coast, relative sea-level changes and rates of tectonic movements (e.g., Schlager,1993; Kamp and Naish,1998). Coastal environments where muds and sands prevail record marine hydrodynamics and physical processes more faithfully than those where coarse-grained sediment dominates. Furthermore, the interplay among vertical tectonic movements of coastal areas and changes in accommodation space, sediment supply and relative sea-level can be more easily detected within sandy systems, whilst it is usually difficult to assess glacio-eustatic (4th- and 5th-orders) control on gravelly sublittoral deposits. This is because sediments of gravelly coastal depositional systems: (i) do not record small-scale sedimentary structures, (ii) form lithofacies assemblages in which changes are not always easy to

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recognize, (iii) have higher erosional potential during deposition and (iv), in the case of steep depositional profiles, favour gravity-driven, mass-transport and re-sedimentation of primary beds, initially formed by river, wave, and/or tidal processes. For these reasons, reliable analyses of gravel or conglomeratic successions need rigorous mapping and logging of well-exposed and undeformed outcrops. Gilbert-type deltas have often been described from coarse-grained coastal systems. The scientific literature of recent decades offers a large number of studies on coarse-grained deltas (e.g., Nemec and Steel, 1984; Colella, 1988a,b; Massari and Parea, 1988; Nemec, 1990b; Prior and Bornhold, 1990; Oti and Postma, 1995; Muto and Steel, 1997; Falk and Dorsey, 1998; Young et al., 2000; Uličný, 2001; Sohn and Son, 2004; McConnico and Bassett, 2007). These works have emphasised shoreline trajectory paths during delta accretion, their controlling mechanisms and fluvial–deltaic morphology (climate change, sediment supply/discharge variation, etc.) (e.g., Postma, 1990, 1995, 2001), importance of sediment supply vs. sea-level changes influencing systems tract development (e.g., Blum and Törnqvist, 2000), delta vertical stacking induced by basin-floor subsidence triggered by earthquake

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clustering (Dorsey et al., 1997) and 2-D vs. 3-D geometries of coarsegrained fluvial–deltaic deposition (e.g., Lunt et al., 2004). Rarely, however, have studies focused on the role of ‘substratum competence’ in relation to progradation mechanisms. Coarse-grained delta 2D characteristics have also been considered (e.g., Falk and Dorsey, 1998; Young et al., 2000; Uličný, 2001; Sohn and Son, 2004; McConnico and Bassett, 2007). However, geometric features, depositional processes, basin dynamics and lithofacies assemblages have rarely been summarized in a single model. Coarse-grained deltas have been described from very different tectonic settings (both extensional and contractional, transcur-

rent, etc.) (e.g., Colella, 1988a,b; Gupta et al., 1999; Young et al., 2000; Gawthorpe and Leeder, 2000; Mortimer et al., 2005; García-García et al., 2006a,b), but few examples have been reported of coarse-grained deltas developed within thrust-top or piggyback basins (e.g., Lopez-Blanco et al., 2003; Ghinassi, 2007). Coarse-grained Gilbert-type deltas crop out extensively within the Pliocene Potenza Basin of the Southern Apennines (Italy) (Fig. 1). They represent an example of gravelly coastal systems developed along thrust-top basin margins that were geologically diverse and affected by different rates of tectonic uplift during sedimentation.

Fig. 1. (A) Southern Apennines and location of the study area. (B) Simplified geological map of Southern Apennines and location of the Potenza Basin. (C) Geological profile across the Southern Apennines showing the geometrical relationship between the various units (modified, after Piedilato and Prosser, 2005). (D) Geological map of the Potenza Basin, showing the location of the two main study sections and the measured stratigraphic logs. White arrows indicate the progradational direction of the Ariano unit coastal systems. (E) Stratigraphic cross-section (along line A–A′) showing the depocentres of the two main Pliocene Altavilla and Ariano units and the position of the Lowstand Prograding Complex that is the focus of this study (‘lower’ and ‘middle’ Pliocene unconformity refers to the assumed age of their formation).


In the present study, a sedimentological and high-resolution sequence-stratigraphic analysis of Middle to Upper Pliocene deltaic sediments from the southern Potenza Basin coastal margin is presented, in order to estimate the role that regional tectonic movements, sediment supply, and eustatic sea-level fluctuations assumed during sedimentation. The Potenza Basin Gilbert-type deltas prograde toward the basin depocentre and are represented by two main 4th-order depositional sequences, organized into a series of stacked coarse-grained wedges that display a suite of internal geometries recording the influence of cycles of relative sea-level changes of different amplitude and duration. The discussion of the data analyzed in this study provides new insights into: (i) uplift control on internal architecture and the stacking pattern of component deltaic sequences in a thrust-top Pliocene basin; (ii) the processes responsible for the depositional facies of the coarsegrained deltas; (iii) the architecture of the component lithosomes; and (iv) an evaluation of how high-frequency eustasy and long-term regional uplift interacted to construct a specific sequence stacking pattern. 2. Geological setting of the Potenza Basin The Potenza Basin (Fig. 1) originated as one of a number of small thrust-top basins (sensu Bulter and Grasso, 1993), mostly of Pliocene age,

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overlying the deformed units of Mesozoic to Cenozoic age that form the axial part of the Southern Apennine belt (Fig. 1A, B and C). These thrusttop basins formed during the last orogenic phase of the Southern Apennines and underwent substantial deformation (Vezzani, 1967; Casero et al., 1988; Roure et al., 1988; Hippolyte et al., 1994; Pieri et al., 1994; Bonini and Sani, 2000; Patacca and Scandone, 2001, 2004). The basin fills developed during various stages, were deformed by NE-verging contractional tectonics, and subsequently dissected by normal- and strike–slip fault systems trending NNE–SSE and NE–SW, respectively. Contemporaneously with the orogenic construction, the Tyrrhenian Basin (Fig. 1A) developed in the adjoining up-dip (southwestern) area in a regime related mainly to back-arc extension associated with the subduction of the Apulian foreland (e.g., Malinverno and Ryan, 1986; Royden et al., 1987; Ben Avraham et al., 1990; Boccaletti et al., 1990; Patacca et al., 1990; Roure et al., 1991; Hippolyte et al., 1995). Normal faults, developed within the fold-and-thrust belt encompassing the Potenza Basin, are commonly related to this extensional regime, and postdate the progressive northeastward migration of the compressive thrust-front toward the Apulian foreland (Fig. 1B). A thrust-top-foredeep basin system migrating toward the Apulian foreland developed during the Neogene at the front of the advancing chain (Casnedi et al., 1982; Pescatore, 1988; Boccaletti et al., 1990; Patacca et al.,1990; Roure et al.,1991). Sedimentation continued at the front of the

Fig. 2. (A) General stratigraphy of the Pliocene infill of the Potenza Basin and the interval studied in this paper. The succession comprises two cycles, the Altavilla and Ariano units, each interpreted here as 3rd-order depositional sequences. The studied interval represents a lowstand prograding complex and is composed of two stacked progradational 4th-order units. Letter symbols: ts = transgressive surface; mfs = maximum flooding surface; sfr = surface of forced regression; FSS = forward-stepping (regressive) sequence set; BSPS = backstepping (transgressive) parasequence set; APS = aggradational parasequence set; LST = lowstand systems tract; TST = transgressive systems tract; HST = highstand systems tract. (B) Simplified relative sea-level curves representing the two transgressive 3rd-order cycles TB3.4-3.5 and 3.6 of Haq et al. (1987) of the Early–Middle and Middle–Late Pliocene, respectively. The Lowstand Prograding Complex occupies the lowstand segment of the second cycle and is composed, in turn, of two 4th-order cycles during which sequences P1 and P2 formed. In turn, sequence P1 is composed of a series of 5th-order deltaic sequences (see Fig. 12 for further details).

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Table 1 Facies associations, depositional processes and main environments depicted for the study successiona

S.G. Longhitano / Sedimentary Geology 210 (2008) 87–110 a Facies association have been grouped into ‘non-deltaic’ and ‘deltaic’ depending on their origin. F and S represent clay and sandy deposits of distal and deeper environments, whilst G1, G2, G3 and G4 represent gravelly-dominated deposits of shoaling-upward environments forming bottomset, foreset and topset of Gilbert-type deltas. Facies codifications have been modified after Miall (1978, 1999).


chain in the Pliocene–Pleistocene Bradanic foredeep (Fig. 1B). However, several syntectonic marine basins developed during the Pliocene and Pleistocene and have been interpreted by various authors as piggyback or thrust thrust-top basins within the tectonic accretionary prism (Vezzani, 1967; Hippolyte et al., 1994; Pieri et al., 1994). From the Early Pliocene onwards, the Apulian Platform was involved in thrusting and formed a duplex geometry that resulted in the burial of the Apulian Chain (Fig. 1C) (Cello et al., 1989, 1990; Lentini et al., 1990; Roure et al., 1991; Catalano et al., 1993). The generation of syntectonic piggyback basins is attributable to this duplexing process which, transferring part of the deformation to the overlying Apennine chain, caused the development of outof-sequence thrusting and folding (Roure et al., 1991). The Potenza Basin (Fig. 1D) represents one of several tectonic depressions located in the internal domain of the tectonic wedge of the Southern Apennines (Vezzani, 1967; Patacca et al., 1990). It is a small basin measuring about 5 km long and elongate in a roughly east–west direction within an internal sector of the Lucanian Apennine in southern Italy (Fig. 1). Previous works have focused on the general stratigraphy of the area (Vezzani, 1967; Lazzari et al., 1988; Amato and Cinque, 1992; Hippolyte et al., 1994; Pieri et al., 1994; Bonini and Sani, 2000). The fill of this basin unconformably overlies a folded substratum, composed of siliceous marls and shales of the Lower to Middle Cretaceous Galestri Formation, pelagic varicoloured clays and shales of the Middle Cretaceous to Oligocene Groppa d'Anzi Formation, marls and shales with calcarenites and calcirudites of the Upper Cretaceous to Oligocene Flysch Rosso Formation and calcilutites, marls and shales of the Eocene to Oligocene Corleto Perticara Formation (Pescatore, 1988; Pescatore et al., 1999). All these stratigraphic units represent the sedimentary infill of the Meso-Cenozoic ‘Lagonegro Basin’ (Scandone, 1975) (Fig. 1B and C). According to the calcareous plankton content, the Potenza Basin succession encompasses the Early and Middle–Late Pliocene (Lazzari et al., 1988). The complete basin-fill, ~ 200 m in thickness, is composed of two units, known as the Altavilla and Ariano units (D'Argenio et al., 1973; Di Nocera et al., 1988; Lazzari et al., 1988) (Fig. 2). The older Altavilla Unit is of Early to Middle Pliocene age, consists of conglomerates and sands, and occurs dominantly in the northern part of the basin. The younger Ariano Unit, Middle to Late Pliocene in age, occurs in the southern part of the basin (Fig. 1E). This unit is composed mainly of clinostratified conglomerates, marine sandstones and calcarenites, and passes upwards into diatomites and shelf mudstones (Vezzani, 1967; D'Argenio et al., 1973; Longhitano, 2008). The Altavilla and Ariano units coincide with the Pliocene TB3.4-5 and TB3.6 (3rdorder) eustatic cycles in the standard chronostratigraphy of Haq et al. (1987). The Ariano Unit unconformably overlies both the older Altavilla Unit and the deformed pre-Pliocene bedrock (Fig. 1D and E). Its lowermost part is represented by a series of basinward-dipping clastic wedges (40–50 m thick), and exhibits overall shallowing trends from slope clays to shallow-marine and non-marine conglomeratic deposits, comprising two major sequences (P1 and P2). The clinoforms are interpreted as a series of vertically-stacked coarse-grained delta systems and represent a ‘lowstand prograding complex’ (Longhitano and Colella, 2004; Longhitano, 2006, 2008), composed of two 4th-order progradational sequences. These deposits are transgressively onlapped by diatomites which record a sudden basin-wide marine flooding, in turn overlain by open-marine neritic mudstones (Fig. 2). Overall, the Middle Pliocene coarse-grained deltas of the southern Potenza Basin record changes in the stratal stacking patterns that can be referred to a series of influencing factors, such as delta-lobe switching, climatically-driven variations in sediment input and tectonic subsidence. In this study, the examined data lead to the interpretation of the studied succession as the sedimentary record of two 4th-order relative sea-level oscillations, punctuated by repeated, high-frequency (5th-order) relative sea-level changes.

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3. Description and interpretation of sedimentary facies associations The sedimentary facies associations within the studied succession of the Potenza Basin are distinguished by: (i) lithology, grain-size and sedimentary structures; (ii) stratal organization and lithosome geometries, and (iii) fossil content. Grain-size was estimated using a graincomparator for sandstone facies, and subdividing sediment into classes of fine, medium, coarse and very coarse gravels for conglomerates, by determining the diameter of their long axes from scattered sample points along the stratigraphic log (Nemec, 1996). A modified version of Miall's (1978, 1999) lithology coding scheme was used to define lithofacies in this study (Table 1). The facies associations (f.a.) recognized in this study succession are documented below in ascending stratigraphic order (Fig. 3) and, on the basis of their interpretation, they have been grouped into ‘deltaic’ and ‘nondeltaic’ facies associations. This division distinguishes sediments of fluvial–deltaic origin from sediments derived from littoral longshore drift and distal fines fallout (Dalrymple et al., 1992). 3.1. Non-deltaic’ facies associations The ‘non-deltaic’ facies associations comprise two facies associations F and S, although they are locally interbedded with some thin deposits deriving from distal ‘deltaic’ deposition. 3.1.1. Facies association F (Fb, Fl): offshore-transition deposits 3.1.1.1. Description. This facies association is composed of mudstones, which crop out in small scarps and represent the lowermost part of each clastic wedge. It consists primarily of massive, intensely bioturbated (Thalassinoides) grey and silty clays with rare shell fragments (Ostrea lamellosa, Pecten flabelliformis, C. foliaceolamemmosus, Callista sp., N. tigrina), variably interbedded with fine to very fine sands (facies Fb). Fine sand layers are up to about 10 cm thick, sharpbased and normally graded, and contain horizontal planar lamination. Upward, this facies passes into fine- and medium-grained sandstone layers, interbedded with massive and bioturbated silty clays with abundant shell fragments (mainly pectinids and venerids) (facies Fl). In some cases, sand beds are characterized by gently undulating and lowangle cross-lamination. These beds are up to 50 cm thick, have sharp bases, and some pass upwards into silty sands with symmetrical ripples (Fig. 4A and B). 3.1.1.2. Interpretation. This facies association records deposition in an open-marine setting, below storm-wave base. Sand layers reflect the waning flow deposits of gravity-generated turbidity currents, whilst the clay portion results from fallout deposition of suspended fine material. In the upper part, cross-lamination and symmetrical ripples suggest deposition during waning oscillatory flows and periodic storm events, indicating deposition of sediment fallout occurring between fairweather and storm-wave base and reflecting accumulation in an offshore-transition environment under the influence of sporadic storms. 3.1.2. Facies association S (Sm, St, Sr, Shc, Gt): shoreface deposits 3.1.2.1. Description. This facies association consists mainly of thoroughly amalgamated and intensely bioturbated fine- to mediumgrained sandstone, organized into alternating massive (facies Sm) and laminated beds (facies St, Sr) (Fig. 4C). The laminated beds show concave-up geometry and normally-graded particles (Fig. 4D). Waveripple laminated sands (facies Sr) and rare lenticular gravel layers with pronounced concave-up erosional bases (facies Gt) also occur. Commonly, thin horizontal and discontinuous pebbly layers, up to 5–7 cm, composed of well-rounded pebbles and granules, and subhorizontal erosional surfaces are present. In the upper portion, these beds consist of

92 S.G. Longhitano / Sedimentary Geology 210 (2008) 87–110 Fig. 3. A. Sedimentological logs measured across the southern part of the studied area. The two main depositional sequences (P1 and P2) form a lowstand prograding complex of Middle–Late Pliocene age (Fig. 2). B. Sedimentological logs of the northern outcrops of the studied succession. For the location, see Fig. 1.


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Fig. 3 (continued ).

normally-graded structureless sand, locally bearing plane-parallel stratification at the top. Individual lenticular layers are 1–2.7 m wide, up to 0.4 m in thickness and have axes oriented normal to the main depositional dip. Where unbioturbated, the dominant sedimentary structures within amalgamated sand units are cross-cutting sets of 20– 30 cm deep concave-upwards depressions (swales) filled by gently undulating, broad concave-up laminae gradually flattening out upwards (Fig. 4C and D). In rare instances, the swales pass laterally into hummocks and the infilling concave-upwards laminae, concordantly draping the lower bounding surface, gradually change style passing into convex-upwards laminae. Bivalves, such as Glycimeris glycimeris, Pholodomia sp., Polimesoda sp. are present, both in fragments and in life position. 3.1.2.2. Interpretation. Through analogy with outcrop study (Leckie and Walker, 1982), the sandy facies characterized by swale filling concave-up laminae associated with rare hummocks is interpreted to

represent swaley cross-stratification (SCS) which is commonly thought as having been storm-produced in slightly shallower water than HCS. Occasionally, fairweather waves reworked the tops of the storm deposits and formed wave ripples. The presence of swaley and hummocky cross-stratification, the intense bioturbation and the limited amount of mud and silt layers formed by the fallout of material put into suspension during storms, indicate a lower shoreface zone, characterized by fairweather waves reworking the tops of the storm deposits and forming wave ripples. The coarser beds are interpreted as the tails of turbidity current deposits; observed features indicate the more distal and deepest environments of subaqueous delta slopes (toesets of Gilbert-type delta lobes; Colella, 1988a; Massari and Colella, 1988; Sohn et al., 1997), where preferential dumping of sand from high-density turbidity currents bypassing the delta slope accounts for the high sand content. These deposits may have been generated after the occurrence of storm events, without the preservation of fairweather deposits, whereas the

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Fig. 4. Outcrop photos of the studied succession. (A) Facies associations (f.a.) F, S and G1 (HST and FSST of depositional sequence P1 in the lowermost part of Serra Ciciniello 3 section of Fig. 3A). (B) Mudstones passing upwards to cross-stratified and massive sandstones (HST of depositional sequence P2). (C) Vertically-stacked f.a. F–S–G corresponding to the basinward stratigraphic interval in Fig. 4A (lowermost part of the Falcianella section in Fig. 3A; dotted line indicates erosional contact of the system that progrades somewhat to the right). (D) Swaley and hummocky cross-stratification of f.a. S. (E) Foreset to bottomset transition in the section Serra Ciciniello 2 (Fig. 3A). These gravelly foresets are tangential and merge basinwards (to the left) with sandy bottomsets, where isolated pebble clusters (pc) occur. In this outcrop, local slides and accumulations of deformed foreset strata occur. This clinoform constitutes the lowermost part of depositional sequence P2 (woman in the circle as scale). (F) Incised valley fills (f.a. G4) occur at the top of the LST (f.a. G2) and are overlain by the TST (f.a. G1) of depositional sequence P2, at the Falcianella section (Fig. 3A). Note the different progradation directions between the two deltaic sequences (dotted lines and arrow; woman in the circle as scale).

lenticular gravel beds record the infilling of longshore troughs. The fossil content and their specific assemblage also confirm a depositional zone characterized by low-energy hydrodynamics, with periodic high-energy deposition.

3.2. ‘Deltaic’ facies associations The ‘deltaic’ deposits have been distinguished into facies associations G1, G2, G3 and G4.


3.2.1. Facies associations G1 and G2 (Gm, Gg, Gmm, Gms, Gt, Sm): bottomset and foreset deposits of Gilbert-type deltas 3.2.1.1. Description. Clinostratified conglomerate and subordinate sandstone layers are the dominant features of this facies association. The conglomerate layers consist of more than 60% pebbles with a matrix made of poorly to well-sorted yellowish-grey sandstone. Clasts are rounded to subrounded and largely consist of calcareous and arenaceous rocks. In the lower part of the studied sections, gravels (f.a. G1) are generally organized into gently-inclined (b10°) beds that pinch-out basinwards, and interfinger with thinner sandy beds (facies Gmm and Sm). The clasts are mostly oriented parallel to the bedding, although some are randomly oriented or grouped into isolated pebble clusters (pc in Fig. 4E). Upwards through the succession, the gravels occur in steeply-inclined (N20°) beds, forming clinostratified sequences (f.a. G2) up to 10–12 m thick (Fig. 4E). The clasts range in size from granule to boulder, and are either matrix- or clast-supported in a poorly- to wellsorted sand matrix with a clay content of less than 1% (facies Gmm and Gms). The clinostratified gravel layers are mostly sharp-based, up to a few decimetres thick, and many are composed of normally graded, wellrounded and imbricated clasts (facies Gg). Foresets of the clinoforms are locally truncated by concave-up surfaces. Above these surfaces, a few metres down-dip of the truncation point, soft-sediment-deformed strata occur (Fig. 4E). Where the succession is thick and well-developed, this facies association can be divided into two subfacies (G2a and G2b). In the lowermost part of the clinoforms, the foresets show tangential geometry and are formed by beds organized into couplets of sand and gravel in regular alternation, with higher contents of sand (sub-f.a. G2a). Upwards, the uppermost part of the clinoforms is characterized by an increase in the gravel content (sub-f.a. G2b), and shows a more complex internal geometry. The overall architecture of the clinoforms is angular (no bottomsets occur) and the foresets appear irregularlybased, variably truncated by local scours, chute-fills and indistinct sets of upslope-dipping cross-strata up to 0.5 m thick (‘backsets’ sensu Massari, 1996) (Fig. 5A and B). Along the foresets, thin (up to 1.5–2 m) units occur, characterized by sigmoidal geometry and internal crossstratification with a local increase of the foreset dip angle (shadowed unit in Fig. 5B). Fragments of oysters and bivalves, such as Arca striata and Ostrea edulis, sparsely occur, the latter often with articulated valves and encrusting the surface of large pebbles. 3.2.1.2. Interpretation. The deposits forming f.a. G1 and G2 record sedimentation occurring along a subaqueous slope, where the flux of sediment was influenced by gravity, favouring avalanches of coarser material (debris flows). The basinward increase in interbedded sandy facies within the clinoforms led to a progressive decrease in the palaeoslope inclination. The progressive basinwards transition of clinoform geometries from angular to sigmoidal–tangential corresponds to a clear relationship between proximality and distality along each sequence (Mortimer et al., 2005). The lowermost facies association G1 defines a toeset geometry. It contains less well-oriented clasts with respect to the upper, more steeply-inclined part, reflecting a decrease in the dispersive pressure occurring between particles during their deposition which, instead, characterizes the clinostratified beds. The uppermost facies association G2 forms a foreset geometry. The relatively high pebble content suggests that dispersive pressure played an important role in maintaining the clasts in a dispersed state (Hwang and Chough, 1990). The orientation of pebbles within each single stratum unit furthermore implies that grain collision was important during the last stages of flow propagation (Allen, 1982; Gravenor, 1986). According to the classification proposed by Dasgupta (2003, p. 277), based on the trend of change in character and rheology of the flow with the proportion and composition of the grains in a subaqueous flowing system, coarse-grained sediment avalanches can

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be represented basically by two type of flows: (i) low- and (ii) highdensity debris flows. Low-density flows contain low amounts of transported materials and, along a subaqueous slope, they may evolve into turbidity currents (Postma, 1986). When the amount of transported particles increases, a high-density — or hyperconcentrated — flow may occur, especially with a high content of clay (above 27%) that gives rise to a cohesive debris flow (sensu Lowe, 1982; Nemec and Steel, 1984; Falk and Dorsey, 1998). To category (i) can be related all the sedimentary structures generated by low-density gravity currents, characterized also by sudden changes in the flow velocity. This condition may have produced the formation of hydraulic jumps generating scour and contemporaneous filling of ‘backset’ laminae. These structures, very common in this kind of depositional environment, have been widely discussed in the past by several authors (e.g., Nemec, 1990b; Massari, 1996) and suggest that the surge of granular material along the delta slope consisted of rapid cohesionless debris flows (sensu Lowe, 1982; Nemec and Steel, 1984). The steep gradient of the delta slope together with the energy of gravity currents can locally produce incised sinuous channels (chutes). Deposition along these zones may, or may not, have formed progradation of small ‘point-bar type’ sigmoids (Fig. 5A and B) with the same significance as the microdeltas of Hwang and Chough (1990). The processes causing accumulations of gravelly deformed strata at the toe of the foresets represent the second category of debris flows (ii), characterized by cohesive behaviour during their propagation (Fig. 4E). The truncation of foresets is interpreted as the result of internal, syn-sedimentary slides (slump scar) removing a coherent mass of coarse-grained material. The movement may have been quite slow, so that non-avalanching falls of cohesive sediment occurred, producing plastic, slump-type collapses (Postma, 1984; Nemec, 1990b), as indicated by the deformed remnants of foreset strata (Fig. 4E). Avalanche, debris falls (Nemec, 1990b) and subaqueous rock falls (Dott, 1963) are the other possible types of granular flow responsible of the accumulation of pebble clusters at the delta toe (pc in Fig. 4E). 3.2.2. Facies association G3 (Gms, Gg): beach deposits of Gilbert-type deltas 3.2.2.1. Description. Throughout the study area, the top of the large clinoforms of f.a. G2 are in many places truncated by an irregular erosion surface (s in Fig. 5C, D and E). In rare cases, below this surface clast-supported and disk-shaped edgewise-stacked pebbles form small accretionary units, with sigmoidal- to wedge-like external geometry up to 2 m thick (Fig. 5E). The clasts are arranged to form either planar, tangential or trough cross-bedding, in sets up to 2 m thick. The foreset, which varies in inclination from 10° to 20°, consists of alternating gravels of different grain-size, commonly openwork and normally graded (Fig. 5F) (Lunt and Bridge, 2007). 3.2.2.2. Interpretation. The characteristics of this facies assemblage indicate beachface deposits of a reflective shoreline (Massari and Parea, 1988; Postma and Nemec, 1990; Bluck, 1999), as is typical of wavedominated shorelines (e.g., Ilgar and Nemec, 2005). The parallel stratification (Fig. 5E and F) represents deposition from swash and backwash traction: the former predominant in fairweather conditions and the latter during higher energy events (storms), as is indicated by the variable imbrication of clasts in conglomerate beds. The undulatory scours (s in Fig. 5E) and stratification pattern reflect the cusp-and-trough beach morphology created by storm waves (Sallenger, 1979), when the beach profile is abruptly altered and the shoreward-coarsening cusps and shoreward-fining intercusp troughs were formed (Caldwell and Williams, 1985; Walker and Plint, 1992; Sherman et al., 1993; Bluck, 1999). The stratigraphic position of this unit, with a highly variable preservation rate in the observed sections, implies that this facies may have developed in a frontal and shoal sector of the depositional system, where the sediments debouched by the river were immediately redistributed and sorted by constant wave motion.

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Fig. 5. Northern outcrops of the study succession. (A) Foresets of the clinoforms of the depositional sequence P1. (B) Line-drawing of A. The unit is composed of two vertically-stacked sub-units made of subfacies G2a and G2b, respectively. In the centre of the outcrop a small sigmoidal micro-delta (in shadow) occurs. Chute-fill structures (cf), small truncation surfaces (t) and backset laminae (b) are also present. (C) Angular foresets of the clinoforms within the deltaic depositional sequence P1. (D) Topset-foreset transition of the Gilberttype delta sequence P1. The topset is composed of normally-graded strata of multi-storey fluvial braided channel fills. (E) Detail of f.a. G2, G3 and G4 from the top of sequence P1 in outcrop section Serra Ciciniello 3 (see Fig. 3A). (F) F.a. G2 shows alternating layers of well-sorted, fine- to coarse-pebble and granule conglomerate, gently-inclined seawards, interpreted as progradational beachface deposits; note the good roundness of clasts and the shoreward imbrication of large pebbles; the hammer is 30 cm.


3.2.3. Facies association G4 (Gmm, Gh, Gt, Sc): topset deposits of Gilberttype deltas 3.2.3.1. Description. This association is characterized by horizontallybedded gravels and conglomerates, and constitutes a minor proportion of the studied succession. Where present, this facies association consists mainly of poorly-sorted conglomerates and sandstones (facies Gmm). These are commonly organized into horizontally- to subhorizontally-stacked massive beds (facies Gh). These beds are several decimetres thick and are truncated locally by small channels, a few decimetres thick and 1.5–3 m wide. The channels are concave– planar in shape, have erosional bases and are filled with cross-bedded conglomerates (facies Gt) and lenticular and stratified sandstones (facies Sc). The total thickness of these deposits ranges from 2 m to 5 m. Locally, some lenticular beds of red sands with plant root structures are also present. The base of the horizontally-bedded layers of the f.a. G4, overlying foreset beds (f.a. G2 and G3), is a widespread erosional/depositional surface, whereas the upper limit outlines the smooth present-day topography (Fig. 5D). Sediments preserved between these two surfaces are associated with small bivalves with articulated valves and in life position, such as Cuspidaria cuspidata and G. glycimeris. 3.2.3.2. Interpretation. F.a. G4 is composed of aggradational units in the study sections and is interpreted as the result of continental sedimentation in a river braidplain, where repetitive pulsatory delivery and deposition of a gravelly river bedload causes vertical accretion (Bridge, 1993, 1995, 2003). Facies Gt and Sc form channel-fill deposits, which may represent the expression of braided braided-fluvial systems, and the interchannel zones may record deposition of overbank gravel sheets and/or bar accretion (Hein and Walker, 1977). According to this model, a crude horizontal stratification typically with imbricated clasts develops under conditions of rapid gravel transport when gravel sheets lengthen downstream faster than they aggrade. Locally, these in-channel accumulations are partly eroded by cutand-fill structures, which is clearly indicated by the U- or V-shaped erosional base (cf. Ramos and Sopeqa, 1983; Morison and Hein, 1987; Smith, 1990). The erosional phase is followed by the filling of the carved structures by foreset deposition at the upstream end and lateral accretion along the flanks (Siegenthaler and Huggenberger, 1993). Small clinostratified units within the subhorizontal beds possibly represent scour-pool fills occurring at channel bends or intersections of minor channels (Ashmore, 1982). The internal structure of the topset suggests the development of a highly dynamic river system which was characterized by unstable and low-sinuosity channels and a high sediment discharge pattern, typical of braided streams (Marr et al., 2000). The fossil content, represented by small-sized molluscs commonly related to brackish-water environments (Buatois et al., 2005), confirms sedimentation within restricted inundated zones on an alluvial plain located close to the sea.

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The model (Fig. 6) shows a dip-parallel, 2D profile of a Gilbert-type delta system, from the proximal (subaerial) to distal (submerged) sectors. Geometrically, the delta clinoforms are subdivided into the classical tripartite topset, foreset and bottomset components (Gilbert, 1885), which correspond to the depositional environments of delta plain, beach, front, slope and delta toe or prodelta. Moving from offshore landwards, the ‘non-deltaic’ lithofacies associations represent the offshore transition and lower shoreface (sensu Reading and Collinson, 1996). The latter is characterized by sedimentary structures (swaley and hummocky cross-stratification) implying deposition below the fairweather wave-base (Fig. 6). Rare current ripples, dunes and trough cross-stratification suggest that longshore bottom currents were occasionally active, probably because of the protection from significant seawater movements, similar to the setting of the fjord- or ria-type fan deltas (Corner et al., 1990; Breda et al., 2007), which implies confinement of the sedimentation areas. Landwards, the system passes to the distal ‘deltaic’ depositional zones which represent the delta toe, where the gravity-driven massflows derived from the up-dip part of the delta slope (cohesive and non-cohesive debris flows) lost their energy causing deposition of bedload and suspended load (f.a. G1 and G2). The main processes which characterized the delta slope were hypo- and hyper-pycnal flows, causing non-cohesive and cohesive debris flows and debris falls, respectively (Nemec, 1990b). These processes are particularly prominent on the proximal foreset slope, with only minor influence of gravity slides and traction currents. Shearing and loading forces of moving, overpassing sediment masses partly reworked the substratum thus generating prominent erosional surfaces. Locally, flows assumed the characteristics of eroding streams, forming low-sinuosity gullies along the delta slope (e.g., Uličný, 2001). The shallow-water proximal depositional zones of the system are represented by the f.a. G3 and G4. These associations derive from the reworking action of the waves along the beach swash zone and from sediment distribution and local erosion on a delta plain, respectively. The channel-fill structures suggest the occurrence of braided, poorlydeveloped fluvial systems, indicating high-gradient, narrow coastal plains and proximity to the sediment source. The vertically-stacked facies assemblage observed within the Gilbert-type delta sequences shows erosional contacts between foreset and topset units, and an absence of sigmoidal units. The occurrence of sigmoidal geometries at the top of the foreset units indicates a transitional subaerial/subaqueous zone that is usually better preserved in wave-dominated deltas (Colella, 1988b). In the present case, the model of Fig. 6 shows a poorly-preserved or very small beach face, wave-reworked deposits, the prevalence of fluvial, braided-type sediments and an abrupt, erosional contact between topset and foreset units, with no preservation of any sigmoidal strata. This characteristic, observed in most of the study sections, suggests a high rate (chiefly uninterrupted) of sediment supply that was distributed along narrow deltaic plains and that strongly influenced delta progradation. These observations allow us to distinguish those of the Potenza Basin as mainly fluvially-dominated braid deltas.

4. Sedimentary processes of Potenza Basin Gilbert-type deltas The data derived from the facies analysis and external/internal geometry of the observed lithosomes suggest the presence of three main geometrically distinct stratal units of bottomset, foreset and topset. This tripartite architecture and the overall depositional features are inferred to be the product of the Gilbert-type braid delta sedimentation, similar to those described for coarse-grained deltas (Postma, 1984; Colella, 1988a,b; Syvitski and Farrow, 1989; Nemec, 1990b; Prior and Bornhold, 1990; Falk and Dorsey, 1998; Young et al., 2000; Uličný, 2001; Sohn and Son, 2004; McConnico and Bassett, 2007). For the studied systems, a new model summarizing facies features, depositional processes and architectures is proposed in this paper.

5. Depositional architectures and sequence stratigraphy of the Potenza Basin coarse-grained deltas Coarse-grained Gilbert deltas represent highly constructive, dominantly progradational systems (Ethdrige and Wescott, 1984; Colella, 1988a; Nemec, 1990a; Oti and Postma, 1995). Their spatial/ temporal development can be subject to the influence of (i) climatedriven factors (sediment supply fluctuations, wave reworking of the delta front, mass-processes on the delta slope and storm-induced tractive currents at the delta toe); (ii) tectonics (uplift and subsidence); and (iii) glacio-eustatic relative sea-level changes (e.g., Nemec and Steel, 1984; Colella, 1988a; Dart et al., 1994; Gawthorpe

98 S.G. Longhitano / Sedimentary Geology 210 (2008) 87–110 Fig. 6. 2D depositional model of the Gilbert-type deltas observed within the study succession. The model shows the three main geometric components (topset, foreset and bottomset) that comprise the different depositional zones. The inferred sedimentary processes occurring along the system's profile produce distinct ‘deltaic’ and ‘non-deltaic’ lithofacies associations (see Table 1 for letter symbols used and text for explanation).


et al., 1994; Gupta et al., 1999; Marr et al., 2000; Viseras et al., 2003; Mortimer et al., 2005; Breda et al., 2007; Swenson and Muto, 2007). The deltaic sequences recognizable within the Potenza Basin sedimentary infill (southern deposits of the Potenza Basin, Pliocene Ariano unit of Di Nocera et al., 1988) show the three classical depositional elements of topset, foreset and bottomset, which typify the well-known Gilbert-type delta models (Colella et al., 1987; Leeder et al., 1988; Massari and Colella, 1988; Gawthorpe and Colella, 1990). The Gilbert-type deltas are marked by the distinctive lithofacies assemblages previously described, the depositional architecture and relative position with respect to each other. To better understand all of these components, two main sections have been described and interpreted with the aim of reconstructing the sedimentary dynamics of the Potenza Basin during the Middle–Late Pliocene. Both sections are parallel to the progradation direction, and have been subdivided into 4th-order depositional sequences (Fig. 2B) and these, in turn, into component systems tracts. Facies-based systems tracts (Swift et al., 1991) have been defined on the basis of the nature and geometry of their bounding surfaces and the architecture of the facies associations. These criteria allowed the subdivision of individual sequences into incised valley fills (IVF), lowstand (LST), transgressive (TST), highstand (HST) and falling stage systems tract (FSST). In this paper, the sequence boundaries are placed above the sharp-based HSTs and, where present, above the FSSTs (Plint and Nummedal, 2000) (Fig. 3). 5.1. The Serra Ciciniello section 5.1.1. Description The SSW–NNE-oriented Serra Ciciniello hill (Fig. 1D) shows a wellexposed part of the study succession. This natural cliff (Fig. 7A) shows two main vertically-stacked clinoform sets, prograding towards the NNE (basin depocentre) and downlapping onto an unconformity surface (SBI). These deposits form a wedge-shaped, compound progradational body, thickening basinwards and marked by internal discontinuity surfaces (Fig. 7B). The two bodies (P1 and P2) are formed by conglomeratic clinoforms and appear separated by a discontinuity surface (Fig. 7B). The conglomeratic foresets of the older unit P1 range in thickness from 10 to 40 m, and downlap onto an unconformity surface, which lies directly above the pre-Pliocene mudstones of the Groppa d'Anzi Formation (Pescatore et al., 1988, 1999). The deposits forming this sequence are characterized by the vertically-stacked f.a. F–S–G1–G2– G4 and are organized in progradational clinoforms in the inner part of the wedge, rapidly migrating downwards (Fig. 7C). The younger unit P2, up to 25 m in thickness, shows onlapping geometries onto the top of the lower sequences and evolves to progradational clinoforms in its upper part. The unit is characterized by the vertically-stacked f.a. S–G1–G2 and is base-bounded by a transgressive condensed shell lag, locally onlapping the top of unit P1. The boundary separating the two sequences (SBII) abruptly divides the prodelta facies association from the delta front facies association (Figs. 4E, F and 5B). Along the Serra Ciciniello section, the clinoforms of unit P1 occur prominently and show cyclical variation in the foreset dip angle (the inclination ranges from 20°–35° to 19°–20°) (Fig. 7B). This geometric repetition occurs along the progradation direction over a few tens of metres, so that the clinoforms appear cyclically prograding and slightly aggrading (black arrows in Fig. 7B). The variations in the foreset dip and the coincident changes in the prograding/aggrading behaviour correspond with alternating increases and decreases in clinoform thickness. These changes are underlain by concave-up, erosional basal surfaces on the substrate. In the Falcianella section (Figs. 1D and 3A), the boundary between sequences P1 and P2 can be seen. This contact is marked by the oc-

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currence of laterally confined multi-storey 3–4 m thick cross-stratified gravels (f.a. G4) encased within the marine sediments of the underlying sequence P1 (f.a. G3) and exhibiting onlapping geometries (Fig. 3A). The top of these deposits is truncated by an erosional surface (ts in Fig. 3A). 5.1.2. Interpretation The base of the older sequence P1 represents a transgressive lower surface of marine erosion (or ravinement surface of Nummedal and Swift, 1987; sequence boundary SBI in Fig. 7B). The absence of transgressive deposits may indicate very rapid relative sea-level rise (‘non-accretionary’ transgression, sensu Helland-Hansen and Gjelberg, 1994). At the base of subsequent regressive deposits there may be a complex polygenetic surface originating from subaerial erosion during times of relative sea-level lowstand (a sequence boundary) and subsequent reworking during the ensuing transgression. In this case, the same surface of transgression must also be considered as coinciding with a sequence boundary (Cattaneo and Steel, 2003). The majority of sequence P1 is interpreted to result from a HST evolving basinward to an FSST, composed of highstand-to-falling stage, NNE-prograding coarse-grained deltas. The top of sequence P1 is bounded by a surface of relative sea-level fall (sequence boundary SBII in Fig. 7B), that translates the depositional system basinwards to form a new lowstand deltaic system pertaining to the younger depositional sequence P2 (out from the section of Fig. 7; see Falcianella section in Fig. 3A). The deposits that occur at the top of sequence P1 represent incised valley fills (IVF in Fig. 3A). According to their sequence-stratigraphic significance, IVFs develop in response to periods of fluvial entrenchment due to base-level fall and lowstand, become filled when sea-level rise resumes slowly, and are transgressively overrun as the rate of accommodation generated at the shoreline progressively outpaces the rate of sediment supply. These features are well recognizable at the P1/P2 boundary and can also be observed for several hundreds of metres in other sections far from those of Serra Ciciniello (Fig. 4F). The sequence P2 is composed of a progradational LST, retrogradational TST and again a progradational HST of deltaic systems (the following FSST has not been recognized in outcrop), and records a relative sea-level lowstand, followed by a subsequent rise (simplified relative sea-level curve in Fig. 7C). Onlapping stratal geometries present on top of sequence P1 are indeed characteristic of transgressive deposits, whereas progradational clinoforms typify the highstand deltaic sedimentation. 5.2. The Torrente Tora section This section crops out along the left flank of the Torrente Tora river, a few kilometres north of the Serra Ciciniello section, and forms a WSW–ENE-oriented cliff (Fig. 1D). It is composed of a series of wedgeshaped, coarse-grained, ENE-elongate clinoforms, from 10 to 27 m thick (Fig. 8A). The succession is characterized by vertically-stacked f.a. F–S–G1–G2–G4 and lies directly on a pre-Pliocene substratum, here represented by the tectonically deformed limestones of the Galestri Formation (Pescatore et al., 1988, 1999). The clinoforms show offlapping progradational geometries, and are exclusively formed by conglomeratic foresets, marked at the top by a truncation surface of modern exposure, so that no trace of the topset is preserved. The three main wedge-shaped units that comprise the section are organized into a clear forward-stepping arrangement, so that each single unit appears translated to the ENE with respect to the underlying unit, forming an ‘imbricate-stacked’ architecture (Fig. 8B). The excellent outcrop continuity of these units has permitted recognition of the facies tracts characterizing the deltas. Very coarsegrained conglomerates constituting the inner portion of every unit are organized into steep (up to 45°) angular clinoforms, composed of the sub-f.a. G2b. This subfacies association grades basinwards into the alternating conglomerates and sands of the sub-f.a. G2a, forming

100 S.G. Longhitano / Sedimentary Geology 210 (2008) 87–110 Fig. 7. (A) Photomosaic of the Serra Ciciniello section and location of stratigraphic logs. (B) Line-drawing of the section showing the two 4th-order depositional sequences, their internal architectures and main bounding surfaces. (C) Sequencestratigraphic interpretation of the Serra Ciciniello section and simplified curve of relative sea-level changes. For more details on the high-frequency curve, see Fig. 9.

S.G. Longhitano / Sedimentary Geology 210 (2008) 87–110 Fig. 8. (A) Photomosaic of the Torrente Tora section. (B) Line-drawing of the section showing the two depositional sequences, their internal architectures and main bounding surfaces. (C) Sequence-stratigraphic interpretation of the Torrente Tora section and simplified curve of relative sea-level changes. For more details on the high-frequency curve, see Fig. 10.

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tangential clinoforms (Fig. 8B; see again Fig. 5A and B for details). These three clinoform sets differ in the amount of dip, and the erosional surfaces encompassing each unit show a progressive forwarddecrease in the dip angles. In the northeastern part of the Torrente Tora section, a succession formed by the vertically-stacked f.a. F–S–G1–G2 onlaps the top of sequence P1 above an evident truncation surface (SBII in Fig. 8B). 5.2.1. Interpretation The Torrente Tora section also records two complete cycles of relative sea-level changes represented by depositional sequences P1 and P2, which correspond with those of the Serra Ciciniello section. In turn, the three clinoform units represent a set of minor (5th-order) sequences (P1e, P1f and P1g), forming the bulk of the 4th-order depositional sequence P1 (Fig. 8C). In this outcrop, a downlap truncation surface, eroding the base of the first unit P1e, allows differentiation of two clinoform units, which can be regarded as HST and FSST respectively, suggesting correlative elements with the analogous sequences of Serra Ciciniello section. The sequence P2, preserved only in the northeastern part of the Torrente Tora section, is composed of the TST and the HST and occurs in a lower position with respect the older sequence P1 (Fig. 8C). Therefore, the study succession observed within Serra Ciciniello and Torrente Tora sections represents the result of the superimposition of two 4th-order cycles of relative sea-level changes recorded within the two depositional sequences P1 and P2. These two sequences may represent, in turn, the ‘lowstand prograding complex’ forming the base of the 3rd-order Ariano depositional sequence (Fig. 2B). 6. Tectonic vs. eustatic control on sedimentation: different pathways on the delta shoreline trajectory In published 2D models of Gilbert-type deltas, the shoreline is assumed to be positioned on the offlap-break point of the sigmoidal unit along the depositional dip (Colella et al., 1987). This point, not always preserved especially after regressive marine erosion (Plint and Nummedal, 2000), generates pathways during relative sea-level changes and contemporary sediment accretion. The trajectory assumed by the cross-sectional shoreline migration is considered a diagnostic tool to discriminate the interaction between relative sea-level changes, sediment supply and basin physiography (Helland-Hansen and Gjelberg, 1994). During the development of sequences P1 and P2, the relative sealevel oscillations might have occurred contemporaneously with respect to an overall uplift of the southern basin margin. This would have produced a distinctive stratigraphic arrangement of the depositional systems, so that older bodies occupied the most internal position and younger ones basinward (e.g., Postma, 1995). This depositional architecture generated a distinctive pathway in the delta shoreline trajectory (Figs. 9 and 10). Tectonic tilting affecting the internal area of the southern basin margin is suggested by the differences in elevation (50 m) that occur between the two subsequent highstands of the sequences P1 and P2. The quantification derives from the respective altitudes at which the beach facies crop out (874.5 m and 825.7 m asl, respectively). Observing the sequence of P1 internal architectures exposed in the Serra Ciciniello section, the signal of higher-rank (5th-order) relative sea-level oscillations can be detected. The Gilbert-type deltas show alternating episodes of aggradation and progradation, so that the clinoform wedge is composed of a stack of unconformity-bounded shingles consisting of oblique to sigmoidal clinothems. This architecture can be summarized in a sinusoidal pathway that the delta shoreline trajectory followed during the sedimentary accretion (HellandHansen and Martinsen, 1996). In Fig. 9, this feature is attributed to the effect of high-frequency cyclicity of alternating highstands and lowstands and intermediate stages of rising and falling of the relative

sea level (Johannessen and Steel, 2005). Similar stratigraphic architectures can also be generated by other physical processes, such as (i) autocyclic delta-lobe switching and/or (ii) climatically controlled fluctuations in sediment input (e.g., Elliott, 1989). Lobe switching is common especially in braid delta systems with very active channel avulsion (e.g., Dorsey et al., 1995). This hypothesis predicts that palaeocurrent directions should vary substantially from one deltaic sequence to the next, because the direction of input would have to change in order to shift deltaic lobes laterally to different depocentres through time (Postma, 2001). However, in the present case palaeocurrent data collected from foreset strata reveal overall uniform N–NE (Serra Ciciniello) and E–NE (Torrente Tora) transport (see palaeocurrents in Fig. 3A and B), although the greatest variations occur within rather than between foreset units. Analogously, stacked deltaic geometries can also be produced by variations in sediment discharge influenced by wet/dry shifts in local climate that may be driven by global cycles (Postma, 2001). Wet periods produce high sediment discharge and delta progradation, whilst dry periods favour slow sediment input and transgression. Although climatic fluctuations are known to occur at high frequencies (6th to 7th-order; e.g., Bardaji et al., 1990) and considering the dimensions of each delta unit presented in this study, the hypothesis of climate perturbations producing large variations in rainfall capable of turning on and off the deltas is not satisfactory to explain the magnitude of delta nucleation in the Potenza Basin. In this study, the hypothesis of high-frequency sea-level changes that influenced the Potenza Basin delta progradation is thus preferred (e.g., García-García et al., 2006b). Accordingly, during the initial highstand, the delta system was predominantly progradational, even given that minor aggradation occurred through the delta plain facies accretion (Fig. 9A). The subsequent sea-level fall produced a shift of the deltaic system, probably through a series of basinward down-stepping, incising river valleys on top of the previous system that was above sea level and, therefore, in a phase of deep erosion. The erosion was not equal everywhere, because the sediment varied from sandstone (delta plain) to conglomerate lithofacies (delta front) offering a different response to erosion. The consequence was the production of a typical undulating morphological profile that characterizes the present hilly relief (Fig. 9B). During the high-frequency falling stage and lowstand, the delta prograded, producing erosion of the mudstone substrate and creating concaveup basal surfaces. The successive transgression produced a gravelly lag on top of the previous high-frequency sequence and aggradation of the deltaic body (Fig. 9C). When the high-frequency cycle was repeated, the overall architecture that the deltaic system assumed was identical to the previous cycle (Fig. 9D, E and F). The regressive surfaces are characterized by pinch-out (often truncation) of the underlying clinoforms and have similar significance to the ‘internal downlap surfaces’ that Pomar and Tropeano (2001) recognized within Pliocene carbonatedominated coastal deposits of southern Italy. These surfaces represent lower-rank sequence boundaries, separating high-frequency, minor deltaic sequences. The final shoreline trajectory (dotted line in Fig. 9) is reconstructed simply by plotting the offlap-break points for each deltaic sequence, and this shows an undulating geometry with steep inclination during transgressions and gentle inclination during regressions. The Torrente Tora palaeo-margin was also being uplifted during sedimentation but was characterized by low vertical movement rates. The uplift directly controlled sedimentation, producing slight progressive unconformities between the forward-stepping deltaic sequences. The reconstruction of Fig. 10 shows the development of a first unit (P1e), via progradation of a steep coarse-grained, Gilbert-type delta along a slightly inclined sea-bottom profile. During the highstand, a new delta prograded (P1f) and shifted basinwards during the subsequent falling stage. No erosion or excavation of the substrate


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Fig. 9. Interpretative model of the Gilbert-type fan delta sequence observed for the Serra Ciciniello section. The final stage in F is the present-day outcrop of Serra Ciciniello (its profile is the surface of modern exposure) and represents a set of 5th-order deltaic sequences developed (from A to E) during a 4th-order regression within a 3rd-order lowstand of the relative sea level. Note: (i) how the 5th-order sea-level oscillations produce alternating stages of delta progradation and aggradation; (ii) how gravelly avalanches along delta slopes scoured the pre-Pliocene bedrock, which was formed by non-lithified deposits, during the 5th-order sea-level falls; and (iii) how the delta front facies progressively shifted basinwards under the influence of the increasing coastal uplift.

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Fig. 10. Interpretative model of the deltaic sequences observed for the Torrente Tora section. In this coastal setting, characterized by a gentle inclination of the depositional profile as a result of a minor uplift rate, 5th-order relative sea-level changes induce the deltas to develop in forward-stepping vertical stacking arrangements. The deltas prograded significantly without any substantial aggradation. Also in this case, the Gilbert-type deltaic sequences in F records a 4th-order regressive stage punctuating a 3rd-order lowstand. No erosion or significant scours occur along the delta base during progradation. Shoreline position describes a zigzag-shaped trajectory during delta accretion.


occurred at this stage, because the delta prograded onto a more resistant substrate (limestones of the Galestri Formation), and thus it gained more accommodation longitudinally (along-dip) rather than vertically. During the sea-level fall, the top of the previous systems tract was eroded by waves and then by rivers (Fig. 10B). Thus, progradation continued during the lowstand, accreting a further deltaic unit (P1g) longitudinally. In this stage, the sea-level rise was not accompanied by vertical accretion of the deltaic unit (aggradation), as in the case of the Serra Ciciniello section (see again Fig. 9D), but by sudden flooding of the previous lower unit, so that the new delta prograded on top of the previous one. The trajectory that the shoreline assumed during the influence of the high-frequency relative sea-level changes was quite different from those detected within Serra Ciciniello. Here, the moderate gradient of the basin physiography enhanced the effects of transgressions, producing retrogradational-to-progradational shorelines (dotted line in Fig.10) and so forming a ‘zigzag-shaped’ trajectory (Kim et al., 2006). This behaviour suggests a different character of transgression, probably triggered by the variation in coastal gradient of the substrate (Cattaneo and Steel, 2003), which was less inclined with respect to the Serra Ciciniello area. The diversity of the coastal gradient may be related to the differential tectonic displacement affecting the substratum during sedimentation (Johannessen and Steel, 2005). The difference in the basin basin-floor gradient between the two study sectors is also confirmed by the different preservation of the topsets in the two deltaic sequences. Indeed, the Serra Ciciniello Gilbert-type delta shows preserved topsets, which are completely absent in the Torrente Tora Gilbert-type delta. As argued by HellandHansen and Gjelberg (1994), this feature is characteristic of high- and low-gradient physiography of the basin floor that might have characterized the two localities, respectively. 7. Discussion Thanks to the recognition of Globorotalia aemiliana which distinguishes the Globorotalia gr. crassaformis subzone (Lazzari et al., 1988), the lowermost part of the Middle–Upper Pliocene Ariano unit (lowstand prograding complex) records an overall relative sea-level

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lowstand during sedimentation that can be temporally constrained between the uppermost Middle Pliocene and the lowermost part of the Late Pliocene. This phase represents the initial stage of a subsequent southwards widening of the Potenza Basin, after the Middle– Upper Pliocene transgression (TB3.6 3rd-order cycle of Haq et al., 1987; Fig. 2B). The coastal sedimentation during this lowstand is interpreted to have been controlled mainly by torrential-type, braided-fluvial streams, draining uplifting highlands along the southern basin margins during the Middle–Late Pliocene. This setting produced narrow deltaic braidplains and adjacent, non-deltaic coastal strips, confined into narrow gulfs or embayments, protected from the effects of marine longshore currents and/or wave motions. This confinement may also have been influenced the high-progradational rate of the deltaic sequences which, in some cases, developed longitudinally for several kilometres basinwards. The overall geometry of the lithosomes and their internal depositional architectures show no similarities with the models used to describe fault-controlled, vertically-stacked Gilbert-type deltas (i.e., Colella, 1988a; García-García et al., 2006a). For the braid deltas in the Potenza Basin, deposition might have occurred in a tectonic setting characterized by low accommodation space and differential tectonic uplifts along the basin margins. Many recent studies have discussed the influence of the rate and magnitude of sea-level changes vs. the influence of local controls in the development of deltaic geometries (e.g., Viseras et al., 2003; Ritchie et al., 2004a,b), but no consideration has been given to the influence of substrate lithology on submarine erosion rates by gravelly avalanches. The depositional architectures of deltaic sequences observed in the two sections of Serra Ciciniello and Torrente Tora (Figs. 9 and 10) represent the end-members of a range of braid deltas occurring within the Potenza Basin. Following these outlines, and on the basis of the overall delta shape and internal architectures, two main groups of coarse-grained braid deltas are presented (Fig. 11): the first group (type-1 delta) is represented by the Serra Ciciniello deltas (see Figs. 7 and 8). In this section (Fig. 11A), the Gilbert-type braid delta developed both vertically and longitudinally (down-dip), and downlapped onto a muddy substrate. The gravel sediment exerted an erosional effect on

Fig. 11. Two different inferred groups of deltas. (A) ‘Type-1’ corresponds to Gilbert-type deltas prograding and eroding a substrate composed of mudstones. The erosion during deposition produces a concave-up downlap scour surface during falling stage and lowstand progradation. The repetition in forward-stepping of these deltaic sequences gives rise to the depositional architecture observed for the Serra Ciciniello section. (B) ‘Type-2’ illustrates a delta prograding onto a rocky substrate, more resistant to erosion. During sea-level lowering the delta progrades onto a flat base, gaining more accommodation basinward. Also the syndepositional uplift of the coastal areas controls the vertical-stacked architecture of both delta models during successive stages of progradation. The differences in the rates of vertical movement may explain the smaller offsets between two successive delta sequences calculated for the Serra Ciciniello section (Ω′) compared with those of the Torrente Tora section (Ω″).

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the unlithified sea bottom. This process became accentuated during the late high-frequency sea-level fall, forming concave-up downlap surfaces of submarine erosion. During lowstand of the relative sea level, the topmost part of the delta emerged, becoming incised from braided river entrenchment; during the subsequent sea-level rise, this part of the delta underwent wave reworking, remaining preserved only in part (Ritchie et al., 2004a). The amount of erosion was controlled by the facies lithology, so that the finest topset lithofacies (channel fills and sandy overbank deposits) were more easily eroded than the coarsest ones (beach and delta front gravels), forming an undulating morphological profile. This delta is here informally called ‘type-1’ or ‘concave-bottom’ delta (Fig. 11A). During a generalized phase of relative sea-level lowstand contemporaneous with the margin uplift, the progradation of some of these types of deltas gave rise to forward-stepping type-1 delta sequences. The second model (type-2 delta) is represented by the deltaic architectures observed within the Torrente Tora section (see Figs. 9 and 10). In this model, progradation represented the prevailing component of the delta accretion, but it developed along a rocky substrate, which was not subject to significant erosion due to gravel avalanches. Uplift produced a narrow or absent coastal plain so that little or no topset built up during progradation. This setting gave rise to Gilberttype deltas prograding on to a gently-inclined basin margin, and producing the different longitudinally-stacked arrangement of ‘type-2’ or flat-bottom deltas (Fig. 11B). The overall depositional architectures described above suggest some similarities with the models proposed by Postma and Cruickshank (1988) and Corner et al. (1990) and observed for the Pleistocene– Holocene fjord-head Gilbert-type deltas of Finnmark (northern Norway). The authors refer these architectures to a relative sea-level fall, punctuated by short periods of sea-level rises. The sea-level curve profile (Fr sea-level curve of Postma, 1995) results from high-frequency oscillations occurring in an uplifting marginal area, producing a ‘timeinclined’ falling curve, punctuated by lower-rank sea-level rises (Fig. 12).

To produce normal regression, exceptionally high sediment supply rates are required; sea-level change alone cannot explain the amount of sediment added to the coast, and which increased exponentially with sea-level rise (e.g., Muto and Steel, 1992, 2002; Porebsky and Steel, 2006). Increase in the amount of sediment supply is also suggested by the development of entrenchment of river channels in braided systems during the stages of delta progradation (Bridge, 2003; Ashworth et al., 2007). For all these reasons, uplift can be considered as the strongest influencing factor during delta accretion. In this case, the regression stages and the consequent forward-stepping architectures of the delta sequences represent the influence of the coastal uplift that enhanced the sediment supply to the coast. The reconstruction of the longitudinal relative position (offset) of proximal facies along the Serra Ciciniello type-1 stacked deltas (Ω′ in Figs. 11A and 13A) can be interpreted in terms of a high-frequency sealevel curve characterized by an inclined profile with a progressive lowering of the elevations of each relative highstand. This shape implies that a high uplift rate occurred during sedimentation in this part of the southern margin of the Potenza Basin, producing ‘attached’ (sensu Ainsworth and Pattison, 1994; Ritchie et al., 2004a) forwardstepping coarse-grained deltaic sequences, becoming younger basinwards (Fig. 13A). In the same way, the Torrente Tora type-2 stacked deltas show larger offsets (Ω″ in Figs. 11B and 13B) between proximal facies, and the less-inclined sea-level curve shows minor differences in the elevations of the ‘highstand points’ (Fig. 13B). The overall depositional architecture represents the result of control of the lower rate coastal uplift on the sedimentation, influencing a gently-inclined sea-bottom profile. Also in this case, this setting produced forwardstepping deltas. These considerations might discriminate the effects of long-term uplift movements from the influence of glacio-eustatic perturbations: the former accounts for the offset longitudinal arrangement of the deltaic sequences, while the latter explains the clinoform nucleations

Fig. 12. Reconstructed depositional profile, obtained by the correlation of the main outcrop localities (Torrente Tora/Serra Ciciniello and Falcianella). The succession represents the complete depositional sequence P1 and the lowermost part of P2, formed by prograding deltaic units (P1e–P1f–P1g and P2a). The Gilbert-type deltas are ‘attached’ in the initial part of the profile, and are ‘detached’ in the central part, because of the difference in amplitude (duration) of the relative sea-level falling stages. The relative sea-level curve (Fr on the left side), considered as qualitative function of sea-level oscillations and local uplift rate, shows that the general regressive trend was punctuated by a series of short rises during which the deltas developed. Note that the older units have a more significant inclination than the younger units forming progressive unconformities (modified after Longhitano, 2008).


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Fig. 13. 3D models summarizing the depositional features characterizing the two studied sections. (A) For the Serra Ciciniello section the basin margin may have been affected by higher syn-sedimentary uplift rates, producing a high-gradient (θ′) coastal sector. This morphological setting controls the overall forward-stepping architectures of the prograding fan deltas. (B) In the Torrente Tora section, the coastal gradient of the substrate (θ″) may have been less inclined with respect to the previous case, favouring the development of more offset delta sequences. The delta progradation rates are inferred at maximum during the highstand stages (e.g., Ritchie et al., 2004b).

internal to each deltaic sequence, and these are identical in the two study sections. 8. Conclusions The major conclusions derived from this study are: 1. The conglomeratic clinoforms of Middle to Late Pliocene age, exposed in the southern–western region of the Potenza thrust-top basin, are interpreted as coarse-grained, Gilbert-type braid deltas, showing distinctive facies assemblages and varying depositional architectures. The depositional model derived from the two studied sections is extended to other clinostratified gravelly bodies in the basin that show analogous facies and depositional geometries. 2. The deltas exhibit a wide suite of sedimentary structures that allow definition of the main depositional processes characterizing the coastal systems sourced from the southern margin of the Potenza Basin. The deltas are typically tripartite. In the present state of outcrop, the least preserved component is the topset, especially for the deltas prograded from less-inclined coastal margins (Torrente Tora section). The topset deposits may have been mainly formed by braided torrential-type rivers forming channel-fill, gravel bars and overbank sandstones. The depositional processes, mainly responsible for marked progradation and formation of foresets, were cohesionless debris flows and debris falls, forming alternating gravels with openwork normal-graded strata. Gravity slides and cohesive flows, forming deformed gravelly and

inverse-graded beds respectively, also occur in the foreset lithofacies assemblage. Low-density turbidity currents, generated from sediment accumulation and consequent destabilization from wave motion at the delta front, eroded gullies and chutes along the delta slope, frequently bypassed the slope and deposited stratified sands that dominantly represent the bottomset component. This deeper depositional zone was frequently characterized by occasional stormgenerated waves, forming swales and hummocks, or by longshore bottom currents producing trough cross-stratified sands. 3. The Gilbert-type deltas show cyclical variation of foreset dip angle and variable aggradation/progradation rates. Although alternative mechanisms, such as delta-lobe switching and climate variations can produce similar delta architectures, the amplitudes of delta accretion observed within the study succession cannot be sufficiently explained by invoking such mechanisms; however, highfrequency (5th-order) repetitions of relative sea-level changes are reasonable explanations, and are here believed to have influenced the Potenza Basin delta development. Sedimentation occurred during an uninterrupted phase of tectonic uplift that drove the delta progradation. 4. The depositional architecture of the deltas was also strongly controlled by the change in coastal gradient, as the reciprocal arrangement between successive deltaic units demonstrates; coastal gradient was influenced by different uplift rates along the southern margin of the Potenza Basin. The two study sections represent the sedimentary responses to two different coastal uplifts, since the stratigraphic arrangement of the deltaic complexes shows a varying

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amount of offset between two successive delta sequences, in response to the minor or major inclination of the substrate. 5. Two end-members of Gilbert-type deltas are represented by concaveand flat-bottom deltas, prograding onto a mudstone and calcareous substrate respectively, with different but peculiar depositional geometries. Since significant increases in sediment supply to the delta front areas are inferred only during relative sea-level stages of falls and lowstands, the difference in the geometric shape of the delta base may be related to the delta's capacity to erode the substrate and to increase additional accommodation space by eroding down the substrate rather than simply prograding basinward. Acknowledgements Financial support for this research was provided by Ministero dell'Università e della Ricerca Scientifica (CoFin 2005 grant, Scient. Resp. M. Schiattarella). 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