Morphostructure of the Lucania Apennines front between the Basento and Salandrella rivers (Southern Italy)

Abstract

We have produced a morphostructural map to explore the tectonic geomorphology of the Lucania part of the southern Apennines mountain front to assess the relative role that tectonic and erosion processes played in shaping the topography of this part of the orogen. Data were collected mainly through field work and airphoto interpretation. The main morpho-tectonic elements seem to be affected by out-of-sequence thrusting and wide regional uplift which played an important role in the latest stage of orogenic wedge evolution.

The geomorphic expression of the local tectonics of individual structures reflects the structural and stratigraphic heritage and were overprinted in the Middle Pleistocene when regional rock uplift lifted the entire mountain front and its foredeep above sea level, imparting a new tectonic slope that dictates erosion and landscape evolution to the present day. Distinct geomorphological features such as asymmetrical ridges, relict valleys and paleosurfaces, have developed on rocks of variable durability and are exposed by regional uplift and erosion along non-cylindrical structures, fault transfer zones, and lateral ramps.

This map also identifies for the first time a relict strike valley at the mountain front and a set of geomorphic markers that highlight the particular relationship between drainage evolution and thrusting.

Keywords:

• morphostructural map
• mountain front geomorphology
• Lucanian Apennines
• paleovalley
• fluvial terraces

1. Introduction

It is now well established that surface processes and the redistribution of mass by erosion and deposition have dynamic feedbacks to tectonic processes that deform and uplift rocks across a range of spatial and temporal scales (Burbank, 1992; Whipple, 2009). These feedbacks are expressed in the topography of a growing orogen, specifically in the development of a mountain front and subsequent orogen drainage patterns (Alvarez 1999; Capolongo, Cecaro, Giano, Lazzari, & Schiattarella, 2005; D'Alessandro, Miccadei, & Piacentini, 2008; Della Seta et al. 2008). The Apennines of southern Italy have a complex topography incongruent with the primary modern tectonic forcing which appears to be dominated by active normal faulting in the interior of the chain and peninsula-wide uplift since the Middle Pleistocene. The main river valleys and related drainage pattern, rather, appear to follow erodible rock-types and outline asymmetric folds of a formerly compressive regime parallel to the modern mountain front. This setting, therefore, offers a glimpse into a disequilibrium landscape, with numerous transients in the geomorphology acting to transform the modern topography from one that developed in response to past tectonic forcing to the modern deformation field (Boenzi et al., 2004). This map has been built using a field and digital topographic-based tectonic geomorphology approach to document the topographic disequilibria and identify the key transients that are actively modifying the landscape to the modern tectonic forcing.

The map, which straddles the front of the chain and the inner margin of the Bradanic area, in the middle portion of the Basento and Salandrella rivers drainage basins was produced in order to find morphological and geological evidence of the Quaternary tectonic evolution of the external portion of the Southern Apennines, focusing on features that record breaching thrust and vertical movement.

2. Fieldwork and data handling

Several excursions in the study area were carried out in order to collect geological and geomorphological data both in the Apennines and in the Bradanic trough. In particular, starting from the published geological and geographical maps at different scales, an integrated dataset was implemented within a geographical information system (GIS) to allow a multi-layer spatial analysis and quantitative geomorpho-tectonic analysis. To integrate field collected data we mainly used topographic maps (1:25,000 and 1:50,000 scale), digital elevation model (DEM; 20 m pixel) and by digital orthophotos (1:10,000 scale) and stereo airphotos (1:33,000 scale).

A geomorphological survey of the study area was of key relevance to assess the large-scale observations achieved through the remotely sensed data and the topographic maps analyses. Moreover detailed geological surveys have been of primary importance in order to find suitable outcrops to collect samples for future dating and to obtain stratigraphic and tectonic data within the study area.

3. Regional geology and tectonic setting

The area represented on the Main Map is located in the Basilicata Region, which straddles the Apulian foreland, Bradanic Trough, and the Lucanian Apennines.

The topographic expression of the mountain front is a combination of tectonic relief generated during active shortening, and subsequent exhumation as the softer sediments in the Bradanic Trough have been uplifted and eroded during Middle Pleistocene – Holocene (Doglioni, Mongelli, & Pieri, 1994; Patacca & Scandone 2001; Pieri, Festa, Moretti, & Tropeano, 1997).

The Southern Apennine Orogen has been driven by the west-dipping subduction of Tethyan realms and Adria Plate beneath Europe and the eastward roll-back of the subduction hinge from late Oligocene up to the Quaternary (e.g. Doglioni, Moretti, & Roure, 1991; Malinverno & Ryan, 1986;). The architecture of the southern Apennines is broadly characterized by two distinct orogenic wedges (e.g. Patacca & Scandone, 2007; Piedilato & Prosser, 2005). The upper and older wedge outcrops forming the relief of the chain; it consists of allocthonous units which represent the Oligocene-Miocene frontal accretion and thin-skinned shortening of palaeogeographical domains belonging both to the Mesozoic – Paleogene divergent margin of the Adria (Apenninic Platform and Lagonegro-Molise Basin) and to the Miocene – Pliocene convergent setting of the Southern Apennine Foredeep; these structural units are imbricated beneath the Liguridi and Sicilidi nappes, a western-traveled structural lid offscraped from the subducting oceanic crust of Neo-Thetys (Lentini, Carbone, Di Stefano, & Guarnieri, 2002). In contrast, the lower and younger wedge is buried and represents underplating of the Apulian carbonate platform which evolved with imbrication and duplex structure in the Plio-Pleistocene. During this time the upper orogenic wedge overthrusted the subducting Apulian wedge along the roof thrust of the duplex structure. Anticlinal stacks and thrust from the lower wedge (Apulia duplex) punched up into the upper wedge causing complex deformation that propagated toward the foreland as breaching thrust, asymmetric folds and strike slip faults (e.g. Mostardini & Merlini, 1986; Patacca & Scandone, 2007; Piedilato & Prosser, 2005). Middle Pleistocene–Holocene shortening has resulted in the subduction of thicker, more buoyant Adria lithosphere which has driven peninsula-wide regional uplift of the foreland and Apennine wedge (e.g. Doglioni et al., 1994). The buried leading edge of the southern Apennine mountain front marks the location of the most forward fault-propagation folds while the Bradanic Trough began to be exposed from the Middle Pleistocene.

4. Geology of the study area

The area represented on the Main Map includes, to the west, a portion of the Lucanian Apennines front, eastward of the regional divide, in a section between Accettura, San Mauro Forte, Tricarico and Castelmezzano villages and, to the East, a portion of the Bradanic trough between Salandra and Grassano villages.

4.1. Lucanian mountain front

In the analyzed sector of the chain three tectonically superimposed structural units outcrop; these are represented from the higher geometrical position by: Sicilide Unit, Sannio Unit and Tufillo-Serra Palazzo Unit, sensu Patacca and Scandone (2007).

The Sicilide Unit is localized in the westernmost part of the study area. This structural unit is mainly represented by the Argille Variegate Group (Cretaceous–Oligocene in age, APAT, 2007) unconformably covered by a Miocene wedge-top turbidite unit known as the flysch di Gorgoglione formation (Ogniben, 1969). This unit shapes a East-verging homoclinal structure which shows outcrops of several hundred meters with extensive lateral continuity (e.g. Boiano, 1997). The Sicilide Unit overlies towards the East the Sannio Unit through a Miocene Est verging thrust.

The Sannio Unit crops out in the central portion of the Apennine part of the study area. This unit, also named the Campomaggiore tectonic unit in the Geological Map of Italy (ISPRA, 2011; Pieri et al., 2011) is represented essentially by the flysch numidico formation, a very thick succession of Burdigalian-Langhian quartzarenites (e.g. ISPRA, 2011; Pieri et al., 2011) which lie stratigraphically on the Flysch Rosso Formation (Cretaceous-Lower Miocene, Sabato et al., 2007; APAT, 2007). The flysch numidico formation crops out with a thickness of several hundred meters along an impressive mountain ridge having NW-SE strike and passing through Monte La Croccia and Monte Cupolicchio. This unit overthrusts east toward the Tufillo-Serra Palazzo Unit.

The Tufillo-Serra Palazzo Unit, also known as the San Chirico tectonic Unit in the Geological Map of Italy (ISPRA, 2011; Pieri et al., 2011), is in the lower geometric position and crops out in the outer margin of the chain. It consists mainly of foredeep turbidite succession consisting of siliciclastic and calciclastic arenites referable to Serra Palazzo Formation (late Burdigalian-Serravallian; e.g. Gallicchio & Maiorano, 1999). This unit lies continuously on the flysch numidico formation. The Serra Palazzo Formation is made up essentially of medium cemented arenites which give rise to a slightly accentuated relief.

In this portion of the chain Pliocene sandy-conglomeratic and clayey deposits, known in the literature as the Ariano Unit (e.g. Ippolito, D'Argenio, Pescatore, & Scandone, 1975), overlie the Cretaceous to Miocene units with an angular unconformity. This unit covers wedge-top basin deposits characterized by two overlapping sedimentary cycles having an age ranging from Early Pliocene to Late Pliocene (e.g. Caldara, Ciaranfi, & Marino, 1993; Longhitano, Sabato, Tropeano, & Gallicchio, 2010; Maiorano, 2000; Pepe & Gallicchio, 2013).

A Pleistocene exhumed breaching thrust marks, toward the East, the outcropping boundary between the Apenninic front and the Bradanic Trough.

4.2. Bradanic trough

The Plio-Quaternary stratigraphic units cropping out in the Bradanic area can be referred to two sedimentary stages: trough stage and post-trough continental stage, occurring after the emergence of the foredeep basin. The units of the two contexts are separated by regional erosional disconformities.

4.2.1. Trough stage units

The outcropping Bradanic trough units are represented by a continuous regressive sedimentary succession consisting from the bottom to the top of the argille subappennine formation and the Monte San Marco formation (Cilumbriello, Sabato, & Tropeano, 2008; ISPRA, 2011; Pieri et al., 2011).

The argille subappennine fm. crops out widely in the Bradanic study area and consists of a very thick succession of gray silty and marly clayey deposits. This unit, contains in the outcropping lower part a conglomerate-sandy deltaic body, known as the conglomerato di Serra del Cedro member (e.g. ISPRA, 2011; Loiacono & Sabato, 1987; Pieri et al., 2011). The outcropping thickness of the member ranges from 400 meters in the Serra del Cedro locality to a few tens of meters at M. Morrone. The age of the argille subappennine ranges from Gelasiano to Siciliano (Pieri et al., 2011).

The argille subappennine pass upward with alternanting, or locally with an erosive contact, to sandy-conglomerate coastal deposits called the Monte San Marco formation. The Bradanic succession is disconformably covered by Middle Pleistocene-Holocene post-trough stage continental deposits.

4.2.2. Post-trough continental stage units

These units are represented by alluvial deposits subdivided in two groups: a) continental deposits belonging to a drainage network different from the actual and b) fluvial terraced deposits linked to the actual drainage network.

Continental deposits belonging to a drainage network different from the actual, characterize the top of the tabular relief present in the study area; in particular, they outcrop in correspondence with paleosurfaces developed from 600 to 550 m a.s.l. in the area of Salandra village and from 525 to 455 m a.s.l. in the area of Grottole village; generally in the Bradanic area the surfaces are characterized by a deeping of about 0.2°–0.4° toward the SE, but along the boundary between the apenninic front and the foredeep it shows a steeper deep. These deposits are represented by conglomerate and sands with matrix of a typical red color. The conglomerate shows a crude stratification and is represented by polygenic clasts that indicate paleocurrents from a westerly direction. The sandy deposits are massive or crudely laminated and contain lenticular bodies of conglomerates. These deposits have been linked to alluvial fan and braided river type environments. The age of the unit is generally dated to the Lower – Middle (?) Pleistocene (Pieri et al., 2011; Sabato, 1996).

Another unit belonging to this group is represented by alluvial deposits outcropping along a relict valley identified along the border between the Apenninic front and Bradanic trough. This unit lies unconformably on the argille subappennine fm. and is represented by small and fragmented outcrops at about 400 m a.s.l. and stands on the actual Basento reach at about 200 m. The unit is mainly constituted by 5–6 m thick polygenic conglomerates with lenticular sand intercalations. The unit shows a sedimentary arrangement linked to a braided channel, and clast vergence indicate a northward provenience. Based on its elevation compared to the Fosso Macello unit (Pieri et al., 2011), the terrace is linked to the Middle-Lower Pleistocene.

Fluvial terrace deposits, linked to the actual drainage network, outcrop typically along the valleys of the rivers that cross the area. The presence of fluvial terrace deposits along the Basento river valley were identified several decades ago (Boenzi, Digennaro, & Pennetta, 1978; Neboit, 1975). In the present study they have been revisited and studied in detail with a new field survey and on the basis of modern geomorphic concepts. Two orders of fluvial terraces can be recognized on the left flank of the Basento river near the village of Grassano where the river coming out from the mountainous area flows into the Bradanic trough. At the outcrop scale each terrace consists of a planar bedrock strath surface, above which lies 5–10 m of upward fining alluvial successions, consisting mainly of gravels and sands, which suggest an alluvial plain with channels evolving from braided to floodplain type. As a whole the sediment deposition of the depicted different terraces would have occurred during cold periods (Boenzi et al., 1978, Boenzi et al., 2011).

Terrace (T1), develops at an elevation of 350 m a.s.l. and stands at 160 m on the actual Basento reach. Their deposits, well outcropping near Mass. Lotrionte are 10–15 m thick and consist of polygenic conglomerates of variable size which pass upward to alternating sands and conglomerates and finally to argillaceous sands. The age is linked to the Middle-Upper Pleistocene (Boenzi et al., 1978).

Terrace (T2) develops at an elevation of 250 m a.s.l. and stands at 65 m on the actual Basento reach. The lithostratigraphic characteristics studied in the neighborhood of Mass. Decuzzi are represented by a basal polygenic conglomerate arranged in lenses and beds passing upward to sandy clays and sands. The age of these deposits should be the Upper Pleistocene (Boenzi et al., 1978).

5. Morphostructural setting of the Lucania mountain front

Based on morphology, lithology and relief we broadly identify two different areas of the mountain front: an innermost part characterized by higher relief and an outermost part characterized by lower relief. The boundary is represented by an important Pleistocene breaching thrust known in the literature as the Stigliano Ramp (see the main map).

The innermost area presents well-defined asymmetric ridges extending from NNW to SSE, reaching and sometimes exceeding 1000 m in elevation and mainly consisting of sandstones belonging to the Gorgoglione Flysch and to the Numidian quartzarenites. Instead its outer sector is characterized by NNW-SSE trending ridges as well but with an elevation of about 600 m and mainly formed of sediments belonging to the Serra Palazzo formation that are more easily eroded.

The most elevated peaks represent the morphological expression of the ‘Stigliano ramp’ thrust and are characterized by a homoclinal ridges elongated from NNW to SSE with steep escarpments to the East. Those homoclines are locally extended between ‘Monte Cupolicchio’ to the North and ‘Monte Cortaglia’ to the South including ‘Monte La Croccia’ which rises up to 1150 m in elevation.

West of the those crests there is another homoclinal ridge, the Pietrapertosa ridge, related to an older thrust with an eastward steep escarpment characterized by differential erosion in relation to the litostratigraphic conditions.

Even the morphology of the external sector of the mountain front is characterized by homoclinal ridges elongated from North to South but present a lower relief. These are older thrust structures produced during the evolution of the accretionary prism in the Middle-Upper Miocene and, in some cases, during the Middle-Upper Pliocene. The relief of these ridges is lower compared with the innermost part of the front because the erosional processes have been more intense due to the specific lithostratigraphic conditions of the formations affecting the area represented mainly by the silty sandstones of the Serra Palazzo formation.

The Basento river and the Salandrella stream deeply dissect these homoclinal ridges resulting in narrow valleys set mainly along strike-slip faults which have displaced these ridges. In this external sector there is a clearly identifiable crest related to the ‘Oppido ramp’ developed in the Middle-Pleistocene. This ramp is morphologically identified by a mildly sloping mountainside from NNE and SSE and in part clearly exhumed. Along the Oppido ramp the Tufillo Serra Palazzo Unit terrains are in contact with the Fossa Bradanica units which in some places are deformed, as at ‘Serra del Cedro’.

In this external sector of the Apennines portions of erosional paleosurfaces rising at about 600–700 meters a.s.l. are identified which develop at the base of the escarpment that bounds to the West the monocline extending between Monte La Croccia and Monte Cortaglia. The features and altimetric distribution of this paleosurface suggests it was a pediment formed in relation to denudational processes, during a phase of relative base level stability, which took place at the base of the margin of the Apennine chain due to the action of weakly incised streams forming easily floodable anastomised channels.

This pediment can be linked to the erosional paleosurface recognized in some areas of the Lucanian Apennines and interpreted, hypothetically, as a glacis forming in a periglacial environment during the Middle-Pleistocene (S3 paleosurface in Martino & Schiattarella, 2006).

Along the western edge of the Bradanic foredeep, just at the mountain front, a long depression trending with an azimuth of 160° is present. The uppermost part of this depression is represented by a relict fluvial valley, today represented by a windgap between Serra del Cedro and Grassano, on whose flanks strips of strath terraces of Plio–Pleistocene clayey bedrock crop out at 400 meters a.s.l. The lowermost part corresponds to the highest Salandra river valley whose watershed rises from the peaks of Monte Impiso, west of Accettura village.

The eastern part of the study area, corresponding to the Bradanic Foredeep, is characterized by the presence of tabular relief extending from NW to SE with a decreasing elevation from 600 to 500 meters. The slopes of this relief are made of Pleistocene clays (Argille Subappennine) affected by landslides and badland morphologies (calanchi) whereas the hilltops are characterized by a wide suspended regressive Pleistocene caprock made of sandy-conglomeratic sediments representing the topping and closing unit of the Bradanic foredeep sedimentary cycle.

More recent stratigraphic studies (Pieri et al., 1996; Sabato, 1996) have interpreted the uppermost portion of this formation, called ‘Fosso Macello’ sands and conglomerates (Sabbie e Conglomerati di Fosso Macello), as an alluvial deposit lying on top of an erosional surface (‘relict of braided plain’ in the main map). In fact, this uppermost conglomeratic succession morphologically forms an alluvial plain fed by braided rivers.

This former plain, which was deeply incised by the Basento river and by the Salandrella torrent, can be correlated upstream to the pediment incised in the pre-Pliocene sediments which form the external sector of the Apenninic edge. Along the Basento river which cuts this area, between Grassano and Grottole villages, two orders of well-preserved strath terraces are present, respectively at 250 and 300 meters a.s.l.. Both strath fluvial terraces consist of silty–sandy deposits overlaying imbricated polygenic conglomerates suggesting a current from the West. These terrains whose age is linked to the Upper Pleistocene (Boenzi et al., 1978), are sometimes (like at Masseria Lotito site) affected by small displacement normal faults striking N45.

6. Conclusions

This study has highlighted the geological evolution of the lucanian Apennines mountain front trough, and analyses and maps of geomorphological landforms. In particular several thrusts sheets that were active during the Early–Middle Pleistocene characterize the inner part of the chain. As shown in the main map, differential erosion processes, acting on these thrusts, resulted in a series of linear ridges constituted mainly of sandstone turbidite successions and clayey flysch units.

Along the frontal part of the chain along the contact with the Bradanic foreland basin and the Apennine front we identified for the first time a relict longitudinal valley that strikes roughly N –S, probably formed during the thrust tectonics active in the early Pleistocene.

In the eastern part the Bradanic trough is characterized morphologically by several tabular relief features that represent relicts of a former alluvial plain that extended eastward from the foot of the chain and then uplifted starting from the Middle Pleistocene. This uplift has been unsteady and was punctuated by a series of strath terraces along the Basento river, developed during the Middle-Late Pleistocene.

Software

Digitization, DEM and GIS analyses have been performed using Esri ArcGIS 9.3. The final map layout was assembled using Corel Draw® 12.

Acknowledgments

We thank Silvio Takashi Hiruma, Patrick Kennelly and Tommaso Piacentini for their constructive criticisms and insightful suggestions. D. C. has been financially supported as part of the Research Project COFIN MIUR 2010–2011 ‘Response of morphoclimatic system dynamics to global changes and related geomorphological hazard’ (National Coordinator: Prof. Carlo Baroni, Università degli Studi di Pisa).