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Fluvial landforms in relation to the geologicalsetting in the “Murge Basse” karst of Apulia (BariMetropolitan Area, Southern Italy)Dario Gioia a , Luisa Sabato a , Luigi Spalluto a & Marcello Tropeano aa Dipartimento di Geologia e Geofisica , Università di Bari “Aldo Moro” , Italy E-mail:Published online: 23 Jan 2012.

To cite this article: Dario Gioia , Luisa Sabato , Luigi Spalluto & Marcello Tropeano (2011) Fluvial landforms in relation tothe geological setting in the “Murge Basse” karst of Apulia (Bari Metropolitan Area, Southern Italy), Journal of Maps, 7:1,148-155, DOI: 10.4113/jom.2011.1136

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Journal of Maps, 2011, 148-155

Fluvial landforms in relation to the geological setting inthe "Murge Basse" karst of Apulia (Bari Metropolitan Area,Southern Italy)

DARIO GIOIA, LUISA SABATO, LUIGI SPALLUTO and MARCELLO TROPEANO

Dipartimento di Geologia e Geofisica, Università di Bari “Aldo Moro”, Italy; dario.gioia@unibas.it.

Abstract

An integrated geological and geomorphological study was performed in order to describe the mainlandform-features of the Bari Metropolitan Area ("Città Metropolitana di Bari", Apulia, Southern Italy).The study is focused on the coastal sector of the "Murge Basse" area. This latter is characterized by alow-relief carbonate landscape gently dipping toward the Adriatic Sea, and represents the eastern andlowermost part of "Murge", a karstic region belonging to the South Apennines foreland.

Results coming from field survey, aerial-photo interpretation, and digital terrain model analysis weresummarized in a 1:50,000 scale map which can contribute to land management and to assessment offlood hazards in a metropolitan area. In fact, although the "Murge" area shows some features typical ofkarst landscapes, it exhibits a well developed drainage-network, formed by a dense dendritic pattern inthe headwater zone ("Murge Alte") which evolves into regularly spaced, incised valleys cutting througha staircase of marine terraces moving towards the coastal area ("Murge Basse"). Some of these valleys arejoined to sub-horizontal areas (corresponding to some marine terraces), and represent relict landformsrelated to ancient sea-level changes that occurred during middle and late Pleistocene times. These flatsurfaces located downstream of incised valleys may represent sectors of high flooding hazard in the BariMetropolitan Area.

(Received 29th April 2010; Revised 28th February 2011; Accepted 7th March 2011)

ISSN 1744-5647doi:10.4113/jom.2011.1136

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1. Introduction

Karst landscapes are generally not dominated by surface runoff and related subaerialfluvial landforms (Ford and Williams, 2007, and references therein). In contrast, the"Murge" karstic area in Apulia (Southern Italy) (see Figure A on the map) exhibits awell developed drainage-network, formed by a dense dendritic pattern in the headwa-ter zone ("Murge Alte") which evolves into regularly spaced, incised valleys movingtowards the coastal area ("Murge Basse") (Pieri, 1975; 1980; Pennetta, 1983) (see Figure Bon the map). These valleys are locally named "lame", and show subvertical rocky flanksand a flat bottom. The fluvial network deeply incised a staircase of marine terraces;these latter are linked through small scarps (i.e. ancient cliffs), and gently dip towardthe NE (Ciaranfi et al., 1988). Valleys cutting the Murge area act as water channels onlyduring and immediately after heavy rainfall (Andriani and Walsh, 2009), and can beclassified as episodic (ephemeral) channels. Catastrophic floods occurred after extremerainfall events in the last century and affected the Bari Metropolitan Area (Moretti, 2005;Mossa, 2007). Focusing our study on the hinterland of the Bari coast, and consideringthat the analysed drainage network merges towards a densely inhabited district whereinteraction between human activities and karst environments may lead to dangerouseffects (e.g. Parise et al., 2008), the aim of this work is to describe the relationships bet-ween some geomorphological features of Murge Basse and the geological setting of thewhole Murge. In particular, in order to contribute to the understanding of the genesisand evolution of a drainage basin on a karstic area, stratigraphical and structural dataderived through a geological survey were integrated with information about the fluviallandforms obtained by means of aerial-photo interpretation, map analysis and digitalterrain model study (see Figure C in the map). This approach allowed the recognitionof several relict valleys that end sharply, crossing sub-horizontal areas located at dif-ferent elevations above the sea level. Finally, these landforms were compared with thespatial and altimetric distribution of Quaternary marine deposits, suggesting a geneticlink with relative sea-level changes.

2. Methods

The data presented in this work were acquired through an integrated geological andgeomorphological study. Geological data were collected during field survey performedto produce the new Geological Map of Italy at a 1:50,000 scale (CARG Project). In par-ticular, geological data derived from sheet 438 "Bari" (CARG-Puglia Project) (ISPRA, inpress) and related products (Pieri et al., 2009; in press; Sabato et al., 2010; Spalluto andCaffau, 2010; Spalluto et al., 2010; Spalluto, 2011), while geomorphological data derived

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from new field-survey and photo-aerial interpretation at a 1:33,000 scale, in addition todigital terrain model analysis.

Both geological reviewed data and geomorphological collected data were transferred toa simplified topographic map at 1:50,000 scale (see map). As a co-product, in accordancewith methods suggested by Gioia and Schiattarella (2010), a morphometric analysis ofthe drainage network was performed in order to facilitate the identification of the roleof tectonic structure on the planimetric arrangement of the hydrographic net in MurgeBasse. In particular: i) all the channels were rectified using a selected module of 250m; ii) all the azimuthal values of every rectified segment within every channel of thedrainage network were computed. Successively, these data were statistically treatedthrough rose diagrams and compared with data from fault orientation (see map).

Finally, in order to better visualize the geometric arrangement and extension of thewhole drainage pattern of channels flowing towards the Bari coast, photo-aerial analy-sis (1:33,000) and topographic map interpretation (1:25,000) was performed over a widerarea (see Figure B on the map), using photos and maps from the Military GeographicalInstitute (I.G.M.) of Italy.

3. Geological and geomorphological setting

The Murge (see Figure A on the map, from Pieri et al., 1997) is a karstic area which repre-sents the central part of the Southern Apennines foreland (Apulian Foreland), and pri-marily comprises Cretaceous limestones. It corresponds to a series of NW-SE trendingplateaux comprising Murge Alte, a morphostructural high, and Murge Basse, some dis-placed blocks flanking Murge Alte to the NE (Pieri, 1980; Boenzi and Caldara, 1990); theMurge Basse structure is better described as an asymmetric NW-SE trending horst andgraben system (Iannone and Pieri, 1983) (see geological section in Figure A on the map),with faults mainly inherited from late Cretaceous tectonics (Festa, 2003). Subsidence ofthe Murge area during the Pliocene and early Pleistocene led to the deposition of theBradanic Trough cycle (carbonates passing upwards to clays) onto limestones of MurgeBasse; remnants of these transgressive deposits mainly occur inside grabens (Iannoneand Pieri, 1983). Successively, since middle Pleistocene times, the Murge area expe-rienced uplift, which led to the progressive emersion of this karstic landscape and tothe formation of rocky shorelines and terraced marine-deposits, today discontinuouslyoutcropping at different heights (Ciaranfi et al., 1988; Spalluto et al., 2010). Accor-ding to Sauro (1991), a relict tropical karst landscape, successively partially modifiedby the headward migration of the hydrographic network during uplift, characterizesthe Murge area. According to Mastronuzzi and Sansò (2002), valleys cyclically origi-nated by sapping processes along coasts of the uplifting Murge during sea-level high-

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stands; as a consequence, several generations of relict valleys characterize the presentday hydrographic network of the Murge area. Beneduce et al. (2004), studying one ofthe active valleys of Murge, hypothesized superimposition processes for its origin, sincethe fluvial net took place on Pleistocene covers before cutting bedrock limestones.

4. Results

The drainage basin flowing towards the Bari coast covers an area of about 1,300 squareskilometres (see Figure B on the map) and primarily develops on Cretaceous limestones.Due to geomorphological features and morphometric properties, the hydrographic net-work can be roughly subdivided in two main morphological units. The first one is loca-ted in the headwaters sector, where a dense dendritic pattern is carved into the MurgeAlte karstic plateau. In this sector, the drainage network displays a clear hierarchy andis characterized by small dry valleys with flat bottoms ("lame") that converge into relictkarstic depressions described by Sauro (1991).

The second morphological unit, representing the focus of this work (see map), is locatedin the Murge Basse area. Geomorphological photointerpretation, field work and mor-phometric analyses show that the drainage basin develops either on Cretaceous limes-tones or, crossing filled tectonic depressions, on Pleistocene units (see map). Our studyarea is characterized by a low-relief carbonate landscape (see Figure C on the map)with typical karst features and by a well-defined drainage network. This shows fluviallandforms (i.e. fluvial terraces, relict valleys) and, locally, along the main river valleys,thin fluvial deposits (either clast- or matrix-supported gravels, made up of pebbles toboulder limestone fragments) and reddish-brown residual soils ("terre rosse") occur.

The analyzed region comprises the Bari Metropolitan Area, and is characterized by lowdrainage density values (sensu Horton, 1945) and a low hierarchic organization. "Lame"in the area are roughly rectilinear, regularly spaced, characterized by a flat bottom, andwith a N25-30◦ preferred orientation, which represents the trend (i.e. the dip) of theregional slope. Statistical comparison between orientation of the streams and azimuth ofthe faults demonstrates that faults have only a limited control on the planimetric patternof the drainage network. This interpretation emphasizes the role of the regional slope inthe evolution and migration of drainage networks during progressive emergence of thecarbonate landscape of the area during the Quaternary. However, some segments of themain streams follow the N20-30◦ and N120-130◦ trending tectonic lineaments of the area(see for example the western part of the map, in the vicinity of Bitonto and Modugnotowns), suggesting a possible local tectonic control on the direction of some segmentsof the main valleys.

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It should be emphasized that main valleys are generally connected to the absolute base-level of erosion. Nevertheless, several "lame" do not reach the absolute base-level, flo-wing into relict valleys not joined with either the coast or the thalweg of main valleys.Coming out from relict valleys, streams sharply flow, without a well-defined bed, to-wards sub-horizontal areas, representing either ancient abrasion (wave-cut) platformsor flat tops of Quaternary units (terraced marine-deposits). Since both kinds of sub-horizontal areas could be interpreted as marine terraces, the inner border of these flatareas should represent either ancient rocky coastlines or the position of the innermosthighest coastal onlap during relative high-stands of the sea level. In the studied area,five generations of marine terraces, distributed at about 4, 12, 48, 92, and 125 m (seesub-horizontal areas in the map and schematic section in Figure A of the map), wererecognized by Pieri et al. (in press), ISPRA (in press) and Spalluto et al. (2010); sincerelict valleys end sharply in front of these sub-horizontal areas, their origin can be in-terpreted as related to different relative sea-level high-stands that occurred during themiddle and late Pleistocene.

5. Conclusions

This paper describes the drainage network in the low-relief carbonate landscape of theBari Metropolitan Area, located along the Adriatic coast of Murge, a karstic morpho-structural high belonging to the foreland (Apulian Foreland) of the Southern Apen-nines chain. The peculiar character of the fluvial system suggest that eustatic sea-levelchanges and regional uplift, responsible for the formation of a staircase of marine ter-races, controlled the geomorphological evolution of the investigated area during Qua-ternary times. The older karst landscape was partially modified by the regressive mi-gration and growth of the Quaternary drainage network, and detailed mapping of thedownstream end of relict valleys could reveal the position of uplifted Quaternary coast-lines. The map provides information about the main geomorphological landforms andthe drainage network delivering water to the Bari metropolitan area. Gregory et al.(2008) suggests that this kind of map represents a basic requirement for land mana-gement and hydrogeological risk assessment because, as broadly observed by Gilvear(1999), it is worthy of note that sub-horizontal areas located downstream of relict valleysmay represent sectors of high flood hazard.

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Software

ESRI ArcGIS 9.2 was used to digitize the data collected during field survey and aerialphoto-interpretation and to perform spatial analysis. The DEM was derived from analtimetric dataset obtained from Regional Technical Cartography (CTR) at a scale of1:5,000 for the Apulia region.

Acknowledgements

The authors wish to thank Professor Pieri (Bari University) and Professor Schiattarella(Basilicata University) for interesting discussions. Thorough and constructive reviewsby Professor Gregory and Dr Rice helped to improve the manuscript and the map. Thestudy has been supported by Bari University ("PostDoc 2009-2011" grants to D. Gioia,and "ex 60% 2009" funds to L.Sabato and G.F. Andriani).

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