Litho-structure of the Oltrepo Pavese, Northern Apennines (Italy)

In this article we present a detailed litho-structural map of the Oltrepo Pavese, a sector of the Northern Apennines, Southern Lombardy, Italy. Lithology and geological structures are an important basis for di ﬀ erent disciplines of Earth Sciences. In particular, for the assessment of earth surface processes such as soil erosion, mass movements, ﬂ ooding, etc. The Oltrepo Pavese is characterised by a complex geology and related tectonic settings. In this study, we conducted a comprehensive lithological mapping approach considering existing geological maps, and detailed ﬁ eld surveys. The lithotypes have been subdivided into 11 classes based on the dominant outcropping lithologies. Integrating bibliographic data and a detailed Digital Terrain Analysis of a high-resolution DTM (5 m) we detected faults, folds and tectonic lineaments in the study area. The ﬁ nal result is represented by a litho-structural map of the Oltrepo Pavese-area, consisting in two shape ﬁ les elaborated in an open source GIS environment.


Introduction
The Oltrepo Pavese area is located in the Northern Apennines and is belonging to the southern part of Lombardy, Italy (Figure 1a). The Oltrepo Pavese covers about 1110 km 2 dominated by a hilly landscape. The elevation ranges from 60 m.a.s.l. close to the Po River up to 1724 m.a.s.l. at the top of Monte Lesima ( Figure  1b). The region is characterised by a typical agricultural land use (Figure 1c) consisting in permanent grassland (12.4%), bushes (12.45%), broad-leave forests, vineyards (13.65%), simple agricultural fields (14.54%) and, bushes in abandoned agricultural areas (14.55%). The Oltrepo Pavese area is one of the most important agricultural and vine-growing regions of Italy (DUSAF, 2015 from GEOPORTALE DELLA LOMBARDIA available at http://www.geoportale.regione.lombardia.it/). From the geological point of view the study area is a mosaic of different Mesozoic and Cenozoic sedimentary formations. The geology of the Oltrepo Pavese has been studied and mapped by several authors in the past (e.g. Bellinzona, Boni, Braga, & Marchetti, 1971;Boni, 1967;Braga et al., 1985;Di Dio, Piccin, & Vercesi, 2005;Marroni, Ottria, & Pandolfi, 2010;Meisina, Zucca, Fossati, Ceriani, & Allievi, 2006;Panini, Fioroni, Fregni, & Bonacci, 2002;SERVIZIO GEOLOGICO D'ITALIA, 1965;SERVIZIO GEOLOGICO D'ITALIA, 1969;SERVIZIO GEOLOGICO D'ITALIA, 2005; SER-VIZIO GEOLOGICO D'ITALIA, 2010; SERVIZIO GEOLOGICO D'ITALIA, 2014;Taramelli, 1882;Vercesi et al., 2014). However, there is no accurate and homogeneous subdivision of lithologic formations available or a comprehensive regional litho-structural map for further geological assessments and modelling approaches. Generally, litho-structural maps yield information on different rock types as well as geological structures and play a fundamental role in understanding the history of the study region (Ali & Ali, 2013). Therefore, we generated a new litho-structural map of the Oltrepo Pavese. The Main Map was developed based on historical maps, field surveys and DTM analysis, and represents 11 lithologies as well as the main faults, folds, thrust and tectonic lineament systems present in the study area. Finally, the litho-structural map gives important information about the lithology of the substrates and the structural setting of the study area. Thus, this map provides basic information for the assessment of processes i.e. flooding, soil erosion etc. and features i.e. gully, Badlands etc. related to earth sciences.

Geology of the study area
The Oltrepo Pavese is located in the northern parts of the Apennines and shows typical features of a foldthrust belt landscape structure (Carmignani, Conti, Cornamusini, & Meccheri, 2004;Cibin, Spadafora, Zuffa, & Castellarin, 2001;Toscani, Seno, Fantoni, & Rogledi, 2006). Several authors (e.g. Ciarapica & Passeri, 1998;Decarlis et al., 2014;Finetti et al., 2001;Maino, Decarlis, Felletti, & Seno, 2013) link the evolution of the Apennines with the formation of the Balearic basin. According to this theory the opening of the basin, caused by the anticlockwise rotation of the Corsica-Sardina blocks, is setting up the Apennines orogen (Maino et al., 2013;Maino, Dallagiovanna, Gaggero, Seno, & Tiepolo, 2012). The origin of the Apennines can be associated with two main accretionary steps: (i) the initial Cretaceous western subduction, of the Ligurian Piemonte Ocean, under the European Plate -Eoalpine Phase and (ii) the Eocenic collision between Europe and the Adriatic microplate -Mesoalpine Phase. During these phases the chain acquired its dominant eastward vergence. Finally, during the Oligocene the formation of a double vergence accretionary wedge occurs, which is related to the late eastward subduction of the Adria region (Vercesi et al., 2014).
The Northern Apennines is historically subdivided in several tectono-stratigraphic Units belonging to the two continents (Adria and Europe) and the ocean between them. From the base to the top the relevant Units are structured as follows: Tuscan-Umbrian, Subligurian, Ligurian, and Epiligurian (e.g. Elter, 1975;Elter, Grasso, Parrotto, & Vezzani, 2003;Piazza, Artoni, & Ogata, 2016). In the study area different allochthonous units crop out such as the Subligurian (SUB) and Ligurian Units (LIG), that are often covered by Epiligurian Units (EPI). These, in turn, are sealed by post-Messinian deposits (PMess) and finally covered by Quaternary alluvial deposits (Q).
The Subligurian Units represent the interface between the External Ligurian domain and the Tuscan domain. In the Oltrepo Pavese the Subligurian Units are characterised by the Monte Penice Flysch as well as interstratified limestones; the so called Canetolo clay-and limestones. These Units lay on the thinned boundary of the Adriatic Continent. From a lithological point of view, they are composed by turbiditic deposits of interstratified and massive limestones.
Above the Subligurian Units the Ligurian Units crop out (Elter, Elter, Sturani, & Weidmann, 1966). The latter units were historically subdivided in Internal and External Units. The External Units are widely distributed in Oltrepo Pavese area and are characterised by the presence of resedimented ophiolites. They are considered fragments of the Ligurian-Piemonte ocean (Elter & Marroni, 1991;Molli et al., 2010). The Unit is represented by flysch, terrigenous interstratified rocks and claystones that were deposited between the Adriatic plate and the Ligurian-Piemonte Ocean. The Internal Ligurian Units do not crop out in the Oltrepo Pavese. They are distinguished from the External Ligurian Units by the presence of ophiolites still in their original stratigraphic position with an Upper Jurassic/Lower Cretaceous sedimentary cover (Molli et al., 2010).
The Epiligurian Units, deposited on the top of the Ligurian Units, represent sedimentary depositions in thrust-top basins (Ricci Lucchi & Ori, 1985). These Units are related to the erosion and sedimentation of the Ligurian Units during the Eocene-Miocene uplift phases of the Apennines belt. The synorogenetic deposition of the Epiligurian Units causes the quite complex geology of the area, bringing these Epiligurian Units in stratigraphic and unconformable contact with the Ligurian Units. Subsequently, during the Oligo-Miocene rising phases of the Apennines, the Epiligurian Untis acted like preferential path in the overtrusting bringing the Ligurian Unit in tectonic contact with the Epiligurian one ( Figure 2a).
From a lithological point of view the Epiligurian formations are very heterogeneous (Panini et al., 2002). In fact, they are mainly composed by: Furthermore, in the Oltrepo Pavese area the sedimentary formations of the Tertiary Piedmont Basin (BTP) crop out. The BTP is an episutural basin like initially described by Bally and Snelson (1980) covering both the Alpine and Apennines Units. These formations are represented by continental and marine rocks consisting of conglomerates, sandstones, flysch and hemipelagic deposits. Moreover, evaporitic and terrigenous formations were deposited during the Messinian in small shallow marine basins. Finally, the area is characterised by Quaternary alluvial, colluvial and landslide deposits. From a geomorphological point of view the area is highly influenced by shallow landslide, in unconsolidated or weakly consolidated substrates. Moreover, Badlands (Calanchi) landforms are present in the soft sedimentary formations and show certain relation to tectonic lineaments.

Tectonic features of the study area
From a tectonic point of view the Oltrepo Pavese is a very complex mosaic of tectonic units. During the middle Eocene-late Miocene emplacement phases of the Apennines chain the Internal Ligurian Units were thrusted on the External ones and during the same time sin-tectonic deposits, i.e. turbiditic sequences and submarine landslides, were deposited on the underlying Ligurian Units generating the Epiligurian Units. Even if the Oltrepo Pavese represents a delimited study area, it is very difficult to observe directly the above-mentioned sequences in open sections. Thus, especially the available described outcrops reveal important tectonic information.
The faults and tectonic lineaments detected in the field fit to three main directions as defined by Panini et al. (2002): (i) NW/SE directions: defined as 'Apenninic direction' which frequently juxtapose the Epiligurian Units with the Ligurian ones. In other words, these tectonic lineaments follow the direction of the overlapping Apennines strata; (ii) NE/SW directions: defined as 'Anti-Apennines direction' that is orthogonal to the Apennines lineaments and interrupt the lateral continuity of the tectonic Units; and (iii) N/S directions: usually represented by left lateral strike-slip faults.

Methods
The detection and identification of the sedimentary formations is a complex task since only a few outcrops are available in the study area. This is caused by the hilly landscape and the rolling morphology, which makes it difficult to observe directly the bedrocks. Furthermore, the bushy vegetation cover does not allow the direct analysis to extract the lithology, such as classic remote sensing i.e. mineral mapping using ASTER data (i.e. Omran, Hahn, Hochschild, El-Rayes, & Geriesh, 2012). However, a complete lithological and structural map is a prerequisite to assess surface earth processes and to simulate these processes through models at local and regional scales. To carry out the litho-structural map of the Oltrepo Pavese we followed subsequent steps, (Figure 3).
To discriminate the lithologies outcropping in the study area we analysed and combined available geological maps covering different parts of the Oltrepo Pavese.  Vercesi and Scagni (1984), Braga et al. (1985), Meisina et al. (2006). Moreover, we mapped areas of the Oltrepo Pavese not covered or that have a too coarse scale, (e.g. areas covered by the old 1:100.000 scale sheets). The geological formations were grouped in 11 classes (Table 1)   lithological unit was described and photo-documented ( Figure 4). Moreover, for the 11 lithological classes the coverage area and the percentage of the covered area was determined through statistical analysis.
Lineaments are defined as 'simple or composite linear features of a surface, whose parts are aligned in a rectilinear or slightly curvilinear way, which differs distinctly from the pattern of adjacent features and presumably reflect a subsurface phenomenon' (O'Leary, Friedman, & Pohn, 1976). However, tectonic lineaments are often represented by linear features of the earth surface usually connected with zones of structural weakness. Moreover, scarps, linear ridges, joints, and faults etc. can be interpreted as lineaments (Pandian, Shruthi, & Pavithra, 2016). Lineaments were traditionally identified and extracted from topographic maps using a visual procedure (Zhumabek, Assylkhan, Alexandr, Dinara, & Altynay, 2017). Despite traditional methods an automatic lineament extraction can be used to identify lineaments, especially at broader scales.
The semi-automatic extraction of lineaments was carried out using the PCI Geomatica software package (PCI Geomatics Ltd., 2017). The combination of the semi-automatic extraction was refined with a visual interpretation and lineament identification. Initially, the DTM is pre-processed and clipped to the extent of the Oltrepo Pavese study area. The pre-processing was performed with SAGA GIS and consists in the application of a simple filter and fill sink algorithm (Planchon & Darboux, 2002) to correct the DEM hydrologically and to eliminate major artefacts. Furthermore, four hillshadings were generated using four illumination angles (0°, 45°, 90°, 135°) and a vertical exaggeration of 4. We chose the following parameter setting for the PCI analysis: Filter Radius = 20; Edge Gradient Threshold = 200; Curve length Threshold = 10; Line fitting Error threshold = 3; Angular difference threshold = 30; and Linking Distance Threshold = 20.
Subsequently, a rose diagram of faults and tectonic lineaments was derived using GeoRose (version 0.5.0). The final step consists in the QGIS elaboration of the main map.

Results
We identified, homogenised and classified the main formations outcropping in the Oltrepo Pavese based on the different lithotypes. The main geological formations are illustrated in (Table 1). They are described with the common abbreviations and the geological Units they belong to, as reported in Boni (1967) (1) Alluvial Deposits (Figure 4a): The unit is lithologically composed by rounded gravel, sand, silt, and clay deposited by the Po river and its tributaries. It consists in rounded gravel in a sandy matrix and is often orientated.
(2) River Terrace Deposits (Figure 4b): The unit is lithologically composed by flat and rounded centimetric pebbles scattered in a brown silt, sandy silt and clay silt matrix. Usually gravel layers are found at the base of the group or layers of red fine sediments. Pedogenetic processes occurred generating a sandy clay loam texture. The average colour is defined by a Munsell soil colour chart of 2.5Y4/3. This deposit can be associated to river terrace deposits, from local river system at the margins of the Apennines.   ( pedogenetically modified. In outcrops diffuse planar layers are visible. The average Munsell soil colour range between 7.5YR and 10YR (e.g. 10YR5/4 represented in Figure 3l). This deposit can be associated with colluvial and mass flow processes. (4) Landslide Deposits (Figure 4d): The unit is lithologically composed by unlithified deposits generated by landslides and denudation processes. The unit is characterised by incoherent material without stratification, metric and centimetric blocks in fine matrix.  (Orthoconglomerate) and (ii) matrix dominated conglomerates (Paraconglomerate) characterised by a sandy/silty matrix. The pebbles are mainly composed by limestones and sandstones with variable size, from sub centimetric to centimetric size. These deposits represent the river sediments. (6) Melange (Figure 4f): The unit is lithologically composed by centimetric to metric chaotic blocks in a fine grey or reddish clay matrix. The blocks are mainly composed by sandstone, limestone and conglomerates, corresponding to the Ligurian and Epiligurian Formations. The smectite clay matrix comprises minerals like montmorillonite, and illite. Moreover, low percentages of fine silt are present. The texture appears chaotic without stratifications. These deposits represent submarine landslides (Olistrostroms). (7) Sandstones ( Figure 4g): The unit is lithologically composed by stratified or massive deposits of yellow to grey bioclastic sandstones, biocalcarenites, rich in fossils (i.e. foraminifers and platform dweller). These rocks are more or less cemented and represent platform deposits. The fluvial sandstones of this group occasionally are interbedded with silty claystones usually characterised by cross beddings or a poorly defined stratification. (8) Claystones (Figure 4h): The unit is lithologically composed by dark red, dark grey, grey and violet claystones, with subordinated siltstones and sandstones. In outcrops usually, pop-corn structures appear or other desiccation forms like mud cracks. Generally, typical characteristics of run off processes are present. In outcrops alternated bands of coloured clays are often visible, representing a zonal change in chemical composition. In general, the inherited stratification is not preserved due to the high tectonic deformation and shrinking-swelling phenomena of active layer clays. These rocks represent ocean floor sediments. (9) Interstratified rocks (Figure 4i): The unit is lithologically composed by an alternation of conglomerates, sandstones and pelites in variable proportions. In general, we found centimetric to sub-metric grey or light brown sandstones interbedded with grey silty marly siltstones. Groove cast, flute cast and sparse bioturbation on the top of the strata can be observed. Run off processes are evident in the outcrops characterised by thick fine-grained strata interbedded with thin sandstone layers. These rocks might crop out in a folded and deformed way due to tectonic activity. These deposits can be associated with terrigenous turbiditic sediment deposited in small basins.
(10) Interstratified limestones and limestones ( Figure 4l): The unit is lithologically composed by yellow or white massive limestone, and limestone interbedded with calcareous marls and pelites. Centimetric and metric strata of limestone are observed occasionally with sandstone layers at the base. Ripple and flute cast rarely occur. This unit can be associated with calcareous turbiditic sediments deposited in small basins. (11) Marls and Interstratified Marls (Figure 4m): The unit is lithologically composed by grey, light blue to greenish marls, chalky marl, marls interbedded with silty marls, flint, pelites and clayey marls. The lithology crops out with a typical conchoid or flaked fracturing. On the exposed surface weathering processes and runoff features are presents. Usually scarce bioturbations are evident. These deposits represent pelagic sediments deposited regularly in shallow basins, often combining the marine sedimentation with evaporitic conditions.
Once the lithological units were classified, we determined the coverage area and the percentage of coverage area for each lithological unit (see Table 2).
The final map (Main Map) was completed adding the faults, folds and tectonic lineaments. Even though faults and folds have been reported partly in bibliography we identified the tectonic lineaments with a combination of visual (DTM interpretation) and semi-automatic procedures (analysis PCI output). The PCI output was combined with a classic visual method modifying the lineaments in QGIS in order to extract only the major tectonic lineaments.
The direction of the main faults and tectonic lineaments is represented in a rose diagram. Basically, two main directions: NW/SE-NE/SW and N/S are highlighted in the rose diagram. These directions correspond to the main Apennines and anti-Apennines directions. Moreover, a N/S direction typically represents strike-slip faults ( Figure 5).
Finally, the main map was elaborated in QGIS joining lithology and structural elements.

Conclusions
In the complex landscape of the study area, characterised by a dense vegetation cover, a consistent lithologic classification was not existing and hence, also a corresponding homogeneous map was missing so far. Thus, in this paper we present a litho-structural map of the Oltrepo Pavese area. The main lithologies were obtained combining bibliographic information and own field surveys. The lithologies are grouped in 11 representative classes based on their intrinsic characteristics. The map is enhanced with the main structural elements (faults, folds and tectonic lineaments) present in the study area. The latter were derived with a combination of a semi-automatic GIS-based method and a traditional visual method. The lineaments extracted can be associated with faults and disturbance zones. The final map (Main Map) represents the litho-structure of the study area. Moreover, we provide two vector files characterising the lithology and the linear structures of for the Oltrepo Pavese. This study provides basic information for further assessment of earth surface processes in the Oltrepo Pavese Area.

Software
The maps were generated and realised using open source GIS (QGIS 3.0). The DTM was elaborated with SAGA GIS and PCI Geomatica. The Rose diagram was generated with GeoRose (version 0.5.0). The statistical analysis has been conducted in Microsoft Excel.