Microsoft Word - 04_Mozzi_LM02.doc Available online http:/amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 26 (1), 2013, 41-54 INTEGRATING DIGITAL ELEVATION MODELS AND STRATIGRAPHIC DATA FOR THE RECONSTRUCTION OF THE POST-LGM UNCONFORMITY IN THE BRENTA ALLUVIAL MEGAFAN (NORTH-EASTERN ITALY) Paolo Mozzi1, Francesco Ferrarese2, Alessandro Fontana1 1 Dipartimento di Geoscienze, Università degli studi di Padova, Italy 2 Dipartimento di Scienze Storiche, Geografiche e dell’Antichità, Università degli studi di Padova, Italy Corresponding author : Paolo Mozzi ABSTRACT: Stratigraphic cores and sections, geological maps and Digital Elevation Models (DEMs) were used for the three- dimensional reconstruction of a portion of the regional unconformity, which separates the Last Glacial Maximum (LGM) sedimentary complex from the post-LGM one. The investigation focused on the Brenta megafan, a ca. 3000 km2 wide alluvial system that extends from the Alpine piedmont to the Venice Lagoon. During LGM the Brenta megafan experienced a major aggradation phase, which led the system to achieve its maximum extent. Fluvial downcutting started after 17.5 cal ka BP and led to the formation of incised valleys (IVs) in the plain, which reached depths up to 20-30 m. The interfluves between IVs experienced soil formation (Calcisols, Luvisols) until their burial by later deposits, or until present in the case of wide exposed stretches of LGM plain. A DEM of the present surface of the Brenta megafan and a geological map of the LGM deposits were created in a Geographic Infor- mation System (GIS). Seventeen cores and sections with information on the nature (erosive vs. hiatal with soil formation) and depth of burial of the post-LGM unconformity, as well as on the stratigraphy of the alluvial succession, were introduced in the GIS as stratigraphic control points (SCPs). Contours of the base of post-LGM sediments with 2-m interval were available in the Venice Lagoon and its main- land. Different interpolation and statistical methods were applied for modeling the unconformity in the pedogenized interfluves and in IVs. Contours were interpolated with Triangulated Irregular Network (TIN) technique. SCPs were interpolated with Inverse Distance Weight (IDW) algorithm with the function cos3. Linear trend equations between the exposed top of the valley fill, i.e. the topographic surface in the DEM, and the unconformity elevation were calculated in different reaches of IVs. Data processing resulted in the production of a DEM of the post-LGM unconformity in the whole Brenta megafan with a cell size of 30x30 m. Morphometric analysis of the DEM enabled to evaluate the gradients of the unconformity in long and transverse profiles, as well as to calculate its overall extension (1491 km2), the areas characterized by buried soils (970 km2), and those which correspond to an erosive surface in IVs (521 km2). The difference between the DEM of the unconformity and the present topographic surface allowed for the first calculation of the volume of alluvial and coastal sediments which lie on top of the unconformity (10.4 km3), and of those which fill the IVs (4.3 km3). The unconformity was traced and modeled across a whole range of sedimentary environments, from braided gravel-bed channel belts (in the proximal sector) down to coastal barrier-and-lagoon systems. The method may be applied to neighboring megafans of the Vene- tian-Friulian and eastern Po Plain. Moreover, it can be extended offshore, in the shelf of Northern Adriatic Sea, to the tip of the deltaic systems, providing a valuable tool for basin-scale stratigraphic correlations and quantitative estimation of erosive processes and sedi- mentary storage. Keywords: Last Glacial Maximum, unconformity, allostratigraphy, DEM, alluvial megafan, Venetian Plain. 1. INTRODUCTION An unconformity is a surface between rock bodies that represents a significant hiatus or gap in the strati- graphic succession (Salvador, 1994). The importance of an unconformity is related to its geographical extent (e.g., regional-interregional traceability and/or correlata- bility) and the temporal interval of the stratigraphic hia- tus. Whilst the concept of unconformity has a long histo- ry, starting from Hutton’s and Jameson’s works between the end of 18th and the beginning of the 19th century (Tomkeieff, 1962), the use of unconformities to formally subdivide and organize sedimentary successions has been receiving increasing attention only in more recent times. Allostratigraphy has been incorporated in the North American Stratigraphic Code (NACSN, 1983) and the International Stratigraphic Guide (Salvador, 1994). Several state geological surveys in the USA use an al- lostratigraphic approach in geological mapping (e.g., McCulloh et al., 2003; Heinrich et al., 2006; Brecken- bridge et al., 2005). The Geological Survey of Italy has adopted unconformity-bounded stratigraphic units for the production of 1:50,000 scale geological maps (CARG project) (CNR, 1992; Pasquarè & Venturini, 2005). The strength and weakness of allostratigraphy vs. lithostratigraphy or morphostratigraphy in subdiving Quaternary continental successions have been widely debated (e.g. Gibbard, 1985; Rawson et al., 2002; Hughes, 2007, 2010; Räsänen et al., 2009). A signifi- cant advantage of allostratigraphy in respect to lithostra- tigraphy is that it enables the setting up of a sequence framework which is particularly effective in basin-scale reconstructions, as it allows the definition of units with comparable ages but encompassing deposits with very varied sedimentary facies (Miall, 1996). An allostrati- Mozzi P. et al. 42 graphic approach enables genetically-related hetero- genous deposits, e.g. interfingering alluvial and lacus- trine deposits, to be grouped within a single stratigraphic unit (Hughes, 2007). Whilst allostratigraphy is a form of sequence stratigraphy (Hughes, 2007), it provides the possibility of separating sedimentary bodies of similar lithology basing on the presence of a significant uncon- formity (Rawson et al., 2002). The hierarchic organization of a sedimentary suc- cession in allomember, alloformation, allogroup, or sub- synthem, synthem, supersynthem strictly depends on the magnitude of each bounding unconformity (e.g., Mi- all, 1988, 1991). To this respect, the analysis of uncon- formities is crucial for the definition of an allostratigraph- ic framework. Nevertheless, the recognition, tracing and ranking of unconformities is particularly problematic in Quaternary continental successions, where unconformi- ties often correspond to erosive surfaces with irregular geometries, and need to be correlated across different sedimentary environments and basins that can be phys- ically separated. This paper proposes the integration of boreholes, geological maps and Digital Elevation Models (DEMs) to reconstruct the three-dimensional geometry and analyze the lateral variability of a portion of the regional uncon- formity, which separates the sedimentary complex of the Last Glacial Maximum (LGM) from the post-LGM one. The investigation focuses on the Brenta megafan (Fig. 1), a ca. 3000 km2 wide alluvial system which was chosen in order to test the method across a whole range of sedi- mentary environments, from braided gravelly channel belts in the Alpine piedmont to clayey-silty distal plain, and extending also to the related deltas and lagoon. This unconformity formally represents the base of the post- glacial synthem (“Po synthem”) in several 1:50,000 scale CARG geological maps in the Friulian Plain (Zanferrari et al., 2008a, b, c; Fontana et al., 2012) and in the Venetian area (Tosi et al., 2007a, b; Cucato et al., 2012). It has been recognized in the underground of the Venetian- Friulian Plain (Mozzi et al., 2003; Primon & Fontana, 2008; Fontana et al., 2008, 2010, 2012), as well as in the Po Delta (Amorosi & Marchi, 1999; Correggiari et al., 2005; Stefani & Vincenzi, 2005), in the Northern Adriatic shelf (Trincardi & Argnani, 2003; Trincardi et al., 2011) and along its western coast (Amorosi et al., 2008). This investigation provides new insights for basin-scale corre- latability of unconformities related to major Pleistocene glacial-interglacial cycles. Fig. 1 - The megafans of the Venetian-Friulian Plain (modified from Fontana et al., 2008). Integrating DEM and stratigraphic data for the reconstruction of the post-LGM unconformity … 43 2. SETTINGS 2.1. Geology and geomorphology The Brenta megafan is part of the foreland basin of the uplifting Eastern Southalpine chain (e.g., Castel- larin et al., 1992; Carminati et al., 2003; Stefani et al., 2007; Barbieri et al., 2007). Within about 20 km from the mountain front the megafan is relatively steep (average gradient 3-5‰), consisting of gravel deposits, hundreds of metres thick, related to braided channel sedimenta- tion (Mozzi, 2005). Further downstream, where topo- graphic gradient gradually decrease to less than 1‰, the geomorphic features mainly consist of sandy fluvial ridges separated by silty-clayey floodbasins (Bondesan & Meneghel, 2004; Mozzi, 2005; Bondesan et al., 2008). Climate change and eustasy forced by glacial- interglacial cycles appear to be the main driving factors in the Upper Pleistocene and Holocene evolution of the Brenta megafan (Mozzi, 2005; Mozzi et al., 2010), as well as of the other megafans of the Venetian-Friulian Plain (Fontana et al., 2008, 2010; Carton et al., 2009). A phase of maximum aggradation took place during the peak of LGM, that in a global perspective lasted be- tween 26.5-19 cal ka BP (Clark et al., 2009). In that pe- riod the Eastern Southern Alps were occupied by exten- sive glacial systems (Ehlers & Gibbard, 2004): the Piave and Tagliamento glaciers reached the plain, building terminal moraine ridges (Carton et al., 2009; Monegato et al., 2007); the Astico glacier, connected by transflu- ence to both the Adige and Brenta glaciers, had its front few kilometres from the plain (Cucato, 2001; Rossato et al., 2013). The Brenta glacier front is regarded to have been located about 10 km upstream of the Brenta valley mouth, near Valstagna, but evidence of its precise posi- tion is lacking (Trevisan, 1939). In this period the Vene- tian-Friulian megafans achieved their maximum areal expansion (Fontana et al., 2008). The LGM Brenta megafan extended from the Sile R. to the Berici Hills (Mozzi et al., 2003; Mozzi, 2005; Monegato et al., 2011) (Fig. 1). The thickness of LGM deposits in the distal sec- tors of the Brenta megafan is about 20 m (Tosi et al., 2007a; Bondesan et al., 2008; Cucato et al., 2012), which is coherent with the thickness of 20-30 m ob- served in the lower tracts of most Venetian-Friulian megafans (Fontana et al., 2010). Aggradation in the Brenta megafan continued until about 17.5 cal ka BP, with apparent lower rates than at the peak of LGM (Mozzi, 2005; Fontana et al., 2010). In the Tagliamento megafan and in the other alluvial systems connected with the Tagliamento glacier (Corno, Cormor and Torre outwashes) the late LGM was instead characterized by an early phase of downcutting at the fanhead and the formation of telescopic fan lobes in the distal plain (Fon- tana, 2006; Fontana et al. 2008, in press). A dramatic erosive phase took place in all the Ve- netian-Friulian megafans at the end of the LGM, during the deglaciation of the lowest sectors of the Southern Eastern Alps (Fontana et al. 2008, 2010). Fluvial downcutting in the Brenta megafan started after 17.5 cal ka BP and led to the formation of incised valleys (sensu Darlymple, 2006) in the plain (from now abbre- viated as IV), which reached depths of 10-20 m (Iliceto et al., 2001; Mozzi, 2005; Mozzi et al., 2010; Cucato et al., 2012). The interfluves between IVs consist of the terraced LGM alluvial plain. These surfaces experi- enced soil formation until their burial by later deposits or, in the case of large outcropping stretches of LGM, until present (for further details on soils, see following paragraph 2.2.). The proximal sector of the Brenta megafan was deactivated due to the entrenching of the river near the valley outlet (Mozzi, 2005). IVs are still detectable in the geomorphology of the upper tract of the megafan, near its apex, as they are laterally bounded by alluvial scarps which are more than 15 m high (Figs. 1 and 2) (Casti- glioni, 1997; Mozzi, 2005). These scarps gradually de- crease downstream, finally disappearing at a distance of about 40 km from the mountain front. At the North- Western outskirts of city of Padova, the scarps cut in LGM deposits are about 2-3 m high (Ferrarese et al., 2006; Mozzi et al., 2010; Ninfo et al., 2011). Here the oldest meander belts at the top of the valley fill were ac- tive between 12.0 and 6.5 cal ka BP (Mozzi et al., 2010). This suggests that the infilling of the IV took place during Late Glacial and early Holocene, and was basically completed by middle Holocene. South of Padova, 2-5 m thick alluvial deposits dat- ed between 6.5 and 4.5 cal ka BP cover the LGM de- posits (Cucato et al., 2012). Towards SE, the Holocene Brenta deposits have ages varying between 4 and 1 cal ka BP and thickness of 2-4 m (Bondesan et al., 2008; Cucato et al., 2012). East of the Brenta R. and in other extensive stretches north of Padua and around the Eu- ganei Hills, LGM portions crop out (Mozzi, 2005; ARPAV, 2005; Cucato et al., 2012). The distal Brenta megafan is attached to the barri- er-and-lagoon system of the Venice Lagoon, which started to form around 7.5-6.0 cal ka BP in response to sea-level rise (Favero & Serandrei Barbero, 1978; Brambati et al., 2003; Amorosi et al. 2008). The Venice Lagoon is presently bounded by dykes and large por- tions of the surrounding alluvial plain are reclaimed land below mean sea level (Fig. 2). The lagoonal and littoral deposits are part of the high-stand sedimentary wedge, consisting of sandy beach ridges, lagoons and deltas that rim the western sector of the northern Adriatic Sea. This sedimentary wedge covers the LGM alluvial depos- its but it rapidly pinches out moving offshore (Trincardi et al., 2011). In wide portions of the northern Adriatic floor the fluvial LGM deposits are cropping out or cov- ered by just a few-decimetres to 1-m-thick veneer of ma- rine deposits, separated by a ravinement surface (Cor- reggiari et al., 1996; Gordini et al., 2002, 2003; Trincardi et al., 2011). 2.2. Soils on LGM deposits Recent soil surveys carried out in the Brenta mega- fan (Ragazzi et al., 2004; ARPAV, 2005; Dalla Rosa et al., 2012) evidence that the soils with the highest degree of development are those formed on the exposed LGM plain. Soils developed on LGM gravels show a charac- teristic profile differentiation, with calcium carbonate leaching in the epipedon and clay illuviation to deep Bt horizons. These well-drained soils have 30-40 cm thick argillic Bt horizons, with hue 7.5YR (Munsell Soil Color Charts), and are classified as Cutanic Luvisols in the FAO World Reference Base for Soil Resources (WRB) classification (Ragazzi et al., 2004). Soils on the silty- clay floodplain of the distal sector of the LGM megafan are characterized by marked vertical mobilization of car- Mozzi P. et al. 44 bonates. Typically, 20-30 cm thick leached Bw horizons overlie some-decimetres-thick Bk and Ck calcic horizons (WRB Calcisols), often displaying gley pedofeatures (mottling, iron-manganese nodules) in relation to the poor drainage of the soils (WRB Gleyic Calcisols). The loamy-sandy soils of the adjacent fluvial ridges have leached epipedons but, depending on the local hydro- topographic conditions, do not always have Bk horizons and are classified as WRB Hypereutric(-Gleyic)- Cambisols. On the sandy channel deposits located in well-drained position on fluvial ridges, there are evi- dences of decarbonation of the top horizons, occasion- ally with traces of clay illuviation in the Bt/Bw horizons. In the distal sector of the Brenta megafan the soils developed at the top of the LGM succession are often buried by Holocene deposits. In the Venice Lagoon the LGM deposits are covered by the Holocene deltaic and barrier-and-lagoon sedimentary complex. Though these sedimentary bodies may locally have erosive basal boundaries (e.g. channel scours, ravinement surface), the soils on top of LGM alluvium are largely preserved. The most known of these soils is the so-called “caranto paleosoil” (Gatto & Previatello, 1974; Mozzi et al., 2003; Donnici et al., 2011), which is characterized by well de- veloped calcic and gley horizons on silty-clay parent material. From the geotechnical point of view the caran- to is regarded as an “overconsolidated clay”, as soil ho- rizons are particularly stiff (in the average 3-5 kg/cm2) if compared to overlying and underlying layers. This ge- otechnical property makes the caranto palaeosoil rela- tively easy to be recognized in cores, penetrometric tests and seismo-acoustic soundings (Stefanon, 1980; McClennen & Ammerman, 1997; Zecchin et al., 2009), thus allowing for reliable stratigraphic correlation and mapping. In fact, it has been commonly used as the stratigraphic marker of the boundary between the Upper Pleistocene fluvial sequence and the Holocene barrier- and-lagoon deposits (e.g., Tosi, 2007a, b). The LGM plain which was buried by middle and late Holocene al- luvial sediments, S and SE of Padua, also displays Cal- cisols and Cambisols which correlate with the caranto paleosoil (Cucato et al., 2012). To all effects, the buried caranto palaeosoil corre- sponds to the post-LGM unconformity and it is in physi- cal continuity with the soils developed on the stretches of the presently exposed LGM alluvial plain. The typical fine-grained caranto palaeosoil is easily recognized in corings due to overconsolidation, abundance of car- bonate nodules and evidence of iron mottling. Where sandy parent material is present, the soil properties are far less expressed and diagnostic, thus in the cores the detection of the post-LGM unconformity is less easy and probably often underestimated (Mozzi et al., 2003). 3. DATASETS AND PROCESSING 3.1. Synopsis In the first step of the research, a DEM of the pre- sent topography of the Brenta megafan (Fig. 2) and a Fig. 2 - Hillshaded DEM of the Brenta megafan, with 2-m contours. The present Venice Lagoon is in light green. Integrating DEM and stratigraphic data for the reconstruction of the post-LGM unconformity … 45 geological map of the LGM deposits (Fig. 3) were cre- ated. Subsequently, boreholes and sections with infor- mation on the nature and depth of burial of the post- LGM unconformity, as well as on the stratigraphy of the alluvial succession, were introduced in the Geographic Information System (GIS) as stratigraphic control points (SCP) (Fig. 3; Tab. 1). Additional information were de- rived from maps of the base of post-LGM sediments in the Venice Lagoon and its mainland (Tosi et al., 2007b; Primon & Fontana, 2008) (Fig. 4). Different correlation and interpolation methods were applied for modeling the unconformity in the pedogenized interfluves and in IVs. GIS operations were carried out using Microsoft ExcelTM for calculation, ArcGIS 10.0TM for digitizing and interpolation of spot heights, and IDRISIgisTM for inter- polation, filtering, and feature extraction. Processing procedures were carried out in a raster dataset with cells of 30 m. 3.2. The Digital Elevation Model The DEM of present topography was produced within a collaborative project of the Department of Ge- ography of the University of Padova and the Environ- mental Protection Agency of the Veneto Region (ARPAV), regarding the whole Venetian plain. The Ve- Fig. 3 - Geological sketch map of the Brenta megafan with location of SCPs (see Table 1), geological and geomorphological maps. The present Venice Lagoon is in light blue. Tab. 1 - The stratigraphic control points (SCPs). Mozzi P. et al. 46 neto Region produces maps at scale 1:10,000 and 1:5,000 (CTR - Carta Tecnica Regionale), which contain elevation points with declared range of error ±1.2 m, mostly deriving from analog stereo restitution of aerial images at average scale 1:18,000. The accuracy and density of 20-70 points per km2 allow for a fairly detailed description of the main alluvial landforms (Castiglioni et al., 1987; Castiglioni, 1999; Castiglioni, 1997; Bondesan & Meneghel, 2004; Mozzi, 2005; Ferrarese et al., 2006; Ninfo et al., 2011). Contour lines with the spacing of 1 m for heights >5 m a.s.l., and 0.5 m in the areas below, were drawn on hard copy CTR maps. A manual interpo- lation was preferred to other automatic methods in the drawing of the contour lines, as this procedure allows the operator to discard all those points on modern arti- facts which are regarded not to be representative of the surrounding topographic surface, such as roads and railway tracks on embankments, bridges, dikes, gravel and sand pits, farm yards, etc. The DEM was originated from interpolation of the contour lines with an optimized version of the Triangulated Irregular Network (TIN). The comparison of the CTR-derived DEM with LiDAR data in the surroundings of Padua shows that the CTR DEM provides a good approximation of the mean natural sur- face of the alluvial plain, even though it tends to “over- smooth” landforms (Ninfo et al., 2011). In the Venice Lagoon, the DEM results from the interpolation of 0.5-m bathymetric contours from Bondesan & Meneghel (2004). 3.3. Geological and geomorphological data Geological and geomorphological surveys at scale 1:50,000 cover the whole distal sector of the Brenta megafan and some portions of its middle part (Bonde- san & Meneghel, 2004; Tosi, 2007a, b; Cucato et al., 2012; Bondesan et al., 2008), providing indications on the extent of the LGM exposed surfaces (Fig. 3). Major fluvial landforms (e.g. fluvial scarps, incised valleys, al- luvial ridges) were recognized and mapped on the DEM. Soil maps are also available in the whole study area at scale 1:250,000 (ARPAV, 2005), and scale 1:50,000 in the Venice and Padova Provinces and small parts of the Fig. 4 - Map of the base of post-LGM sediments in the Province of Venice (Modified from Primon & Fontana, 2008). Integrating DEM and stratigraphic data for the reconstruction of the post-LGM unconformity … 47 Vicenza Province (Ragazzi et al., 2004; Ragazzi & Za- marchi, 2008; Dalla Rosa et al., 2012). Routine physi- cal-chemical analyses, chemical indexes of different iron forms (oxalate-extractable, dithionite-extractable and total iron) and micromorphological analyses of diagnos- tic horizons were carried out on selected soil samples during these surveys. These investigations helped in the recognition of soil chronosequences and the detection of the alluvial surfaces exposed to soil formation since LGM. The definition of the general stratigraphic frame- work benefits of recent papers on the late Quaternary evolution of the Venetian-Friulian megafans and coastal areas (Amorosi et al., 2008; Fontana et al., 2008, 2010; Zecchin et al., 2009; Piovan et al., 2012) and, more specifically, of the Brenta megafan (Mozzi, 2005; Mozzi et al., 2010; Rossato et al., 2012). Maps of the depth of the post-LGM deposits in the underground of the Venice Lagoon, based on seismic soundings and corings, are provided by Tosi et al. (2007a,b). Primon & Fontana (2008) integrate these data with a large dataset of bore- holes and correlate the caranto palaeosoil across the Venetian mainland, presenting the elevation a.s.l. of the post-LGM unconformity through 2 m contour lines. Cucato et al. (2012) provide indications on the thickness of post-LGM deposits in the plain S and SE of Padova, ranging between 2-5 m. Pellegrini et al. (1984) allow to identify the unconformity in the middle tract of the mega- fan. Data on the stratigraphy of the post-LGM valley fill and channel belts are presented in Iliceto et al. (2001), Mozzi et al. (2010) and Cucato et al. (2012). In SCP 10 (Fig. 3 and Tab. 1) the unconformity was recognized basing on lithofacies changes and its elevation is fully consistent with nearby SCP 1 from outcrop data (Pelle- grini et al., 1984). SCP 17 is located in the terminal tract of the Brenta valley, where the post-LGM fluvial depos- its directly lie on the limestone bedrock. 3.4. Subsurface data processing The contour lines of the base of the post-LGM de- posits from the maps of Primon & Fontana (2008) and Tosi et al. (2007b) were digitized and geo-referred (Fig. 4). The contour lines were interpolated with TIN tech- nique, in order to produce an elevation surface of the unconformity in this distal reach of the megafan. This surface is after indicated as Udist_1. Elsewhere, the depth and nature of the unconformi- ty (erosional vs. hiatal with soil formation) were derived from cores and stratigraphic sections (Tab. 1; Fig. 3). Each SCP provides the elevation and characteristics of the unconformity, as well as the stratigraphy of the sedi- mentary successions. In the south-western corner of the megafan, the buried interfluve is limited to the N and E by IVs. Eleva- tion data in this sector of the unconformity were provid- ed by SCPs 9, 14, 2, and 6. The elevations of the un- conformity were interpolated with an Inverse Distance Weight algorithm (IDW, applying the function cos3), which was chosen because of the relatively small di- mensions of the area (221 km2) and the even distribu- tion of elevations. This procedure allowed to reconstruct the elevation surface of the unconformity, to which we refer as Udist_2. Upstream from Padova the incised valleys could be mapped in detail thanks to the morphological evi- dence of their lateral slopes. These correspond to the original fluvial scarps which are still recognizable in the DEM, even if they have been partly covered by middle and late Holocene sediments and largely remodelled by local erosive processes. Basing on the interpretation of contours in the map of Primon & Fontana (2008) (Fig. 4), it was possible to trace the floor of the IV in this very dis- tal reach of the megafan between -10 and -16 m a.s.l. Nevertheless, a critical area is present between Padova and the northern boundary of the map of Primon & Fon- tana (2008): here the valleys are completely filled, loos- ing morphological evidence in the plain topography, and no stratigraphic data were available. The highest uncer- tainty concerns the valley width, which has been as- sumed as comparable to the upstream and downstream reaches. As regards the reconstruction of the longitudinal profiles of IVs, simple linear trends between the ex- posed top of the valley fill, i.e. the topographic surface in the DEM, and the unconformity elevation were calculat- ed for each SCP. The linear trend equation applied in the more up- stream reach between SCPs 17, 10, 1 is the following: UIV_1 = PS * 1.034783306465 – 7.540390897056 R² = 0.998378264792 Where: UIV_1 = unconformity surface corresponding to the ero- sive valley floor upstream of SCP 1; PS = present topographic surface corresponding to the top of the post-LGM valley fill. Between SCPs 10 and 1 and the cluster of SCPs in the area of Padova (Fig. 5) the equation is: UIV_2 = PS * 1.2685794626 – 13.7052467081 R² = 0.9229633297 Where: UIV_2 = unconformity surface corresponding to the ero- Fig. 5 - Linear regression of the elevation of the post-LGM uncon- formity vs. the present topographic surface in the middle reach of IV in the Brenta megafan (for location of SCPs see Fig. 3). Mozzi P. et al. 48 sive valley floor downstream of SCP 1; PS = present topographic surface corresponding to the top of the post-LGM valley fill. The gradient of UIV_2 was maintained downstream of SCP 15 to the intersection at -13 m a.s.l. with the floor of the incised valley recognizable in the map of Primon & Fontana (2008). The processing of subsurface data allowed for the production of elevation surfaces relative to specific sec- tors of the unconformity: i. pedogenized top of LGM de- posits in the interfluves (Udist_1 and Udist_2); ii. base of post-LGM deposits in IVs upstream of SCP 1 (UIV_1); iii. base of post-LGM deposits in IVs downstream of SCP 1 (UIV_2). UIV_1 and UIV_2 were merged with a max (maximum between two values) cell-by-cell func- tion. The resulting DEM was then merged with Udist_1 and Udist_2 through a "covering" GIS operation, in order to produce the final DEM of the unconformity surface with cell size set at 30 m. This DEM was after merged with that of the outcropping LGM surface, as it can be seen in Fig. 6. The production of the DEM of the unconformity al- lowed for the calculation of the volume of sediments which lie above it, separating IVs from interfluves. The operation was carried with the following work flow: i. ex- traction of the areas of deposition of post-LGM sedi- ments on interfluves and in IVs; ii. subtraction of the el- evations of the unconformity from the present topo- graphic surface in each 30x30-m cell. 4. RESULTS AND DISCUSSION Data processing allowed for the three-dimensional modeling of the post-LGM unconformity (Fig. 6). The low density of SCPs in the areas where isopach maps are not available represented the most problematic aspect. Nevertheless, the unconformity on pedogenized inter- fluves represents the buried continuation of the LGM plain, and it can be assumed that it maintains similar morphologies and regular gradient. This was checked in the Udist_2 sector, where IDW interpolation proved to be effective. The recognition and modeling of the uncon- formity in IVs was also critical. Notwithstanding, the re- markably high correlation between the elevations of the unconformity (i.e., the bottom of the valley fill) and the topographic surface (i.e., the top of the valley fill) sup- ports the application of linear trend equations in the in- terpolation procedure. In the lower reach (UIV_2), where there are several SCPs, the correlation coefficient R² of the regression analysis is 0.923. The post-LGM unconformity resulted to extend on Fig. 6 - Hillshaded DEM of the post-LGM unconformity and exposed LGM deposits in the Brenta megafan, with 2-m contours. The white line indi- cates the margin of the present Venice Lagoon. Integrating DEM and stratigraphic data for the reconstruction of the post-LGM unconformity … 49 about half of the Brenta megafan; the hiatal unconformi- ty on interfluves represents one third of the total, while the erosive unconformity is significantly smaller (Tab. 2; Fig. 7). The total volume of sediments which lie above the post-LGM unconformity is 10.4 km3: 6.1 km3 cover the interfluves and 4.3 km3 fill the IVs. The unconformity DEM evidences the continuity between the exposed LGM surface and the unconformi- ty on the buried pedogenized interfluves (Fig. 6). In IVs, the unconformity corresponds to the erosive valley flanks and floors. Valley slopes are steep and range in height from about 20 m at the fanhead to 4-6 m in the distal sector; valley floors are rather flat and range in width around 2-6 km. This geometry is coherent with the IVs observed with good detail in the Tagliamento mega- fan, thanks to the availability of a large stratigraphic da- taset and the marked contrast in sediment grain size of valley fills (Fontana, 2006; Fontana et al. 2008, 2012). To be noted that the Tagliamento and Piave megafans dis- play multiple incisions in the distal sector. In the Brenta megafan this was not evident from available stratigraphic data, but the possibility that other IVs cross the distal reaches downstream of Padova cannot be excluded. The relations between the unconformity and the present topographic surface across the megafan can be observed in the cross sections of Figs. 8 and 9. Cross section AA’ and the downstream end of CC’ are particu- larly effective in evidencing the continuity between ex- Fig. 7 - Sketch map of the post-LGM unconformity in the Brenta megafan, with location of cross sections of Figs. 8 and 9. The black line indicates the margin of the present Venice Lagoon. Tab. 2 - Extension of the post-LGM unconformity and volume of overlying sediments in the Brenta megafan. Mozzi P. et al. 50 posed and buried LGM top surface, as well as the onlap of post-LGM coastal deposits on the interfluve uncon- formity. Cross section BB’ and the upper and middle por- tions of CC’ intersect the erosive unconformity and the top surface of the sedimentary fill in IVs. In BB’ it can be seen that both surfaces display a major knickpoint at around 30 m a.s.l., which corresponds to the transition from the gravelly piedmont sector to the fine-dominated lower portion. Upstream of this knickpoint both surfaces have similar gradient around 3.5‰. Downstream, the unconformity is slightly steeper (1‰) than the top sur- face of the valley fill (0.7‰). The unconformity keeps the same gradient below sea level and it dips to -18 m a.s.l. The present alluvial plain has a minor knickpoint around sea level, about 5 km upstream of the inner margin of the Venice lagoon, where the topographic gradient av- erages zero. Cross sections represented in Fig. 9 are trans- verse to the longitudinal axis of the megafan. DD’ evi- dences the convex morphology of the LGM megafan apex and the wide post-LGM incision. The overall con- vexity of the distributive systems is evident also in EE’, which shows the almost complete infilling of IVs and their characteristic morphology with flat floor and steep slopes. Cross sections FF’, GG’, and the lower part of CC’ document the complete infilling of IVs by post-LGM sediments, which also buried the surrounding inter- fluves. To be noticed that the eastern IV in FF’ is signifi- cantly wider than the ones in EE’; this is due to the merging of two IVs and the slightly transverse direction of the cross section in respect to the valley. Fig. 8 - Longitudinal cross sections in the Brenta megafan, showing the present topographic surface (red line) and the post-LGM uncon- formity (blue line) (for locations see Fig. 7). Integrating DEM and stratigraphic data for the reconstruction of the post-LGM unconformity … 51 In the middle portion of the megafan, nearby Pa- dova, the unconformity in the IVs is close to sea level (Fig. 6 and Fig. 9 - cross section BB’). This is due to the above-mentioned steeper gradient of the erosive uncon- formity in respect to present topographic surface, and implies that the river was related to a base level which was significantly lower than present. In fact, this is con- sistent with the timing of IVs downcutting, which took place not later than Late Glacial (Mozzi et al., 2010) when the Adriatic Sea was still lower than -50 m a.s.l. (Lambeck et al., 2004; Amorosi et al., 2008). The Holocene relative sea-level rise conditioned the deposition of the coastal wedge on top of the uncon- formity, as evidenced at the downstream end of cross section AA’, where the lagoonal sediments covering the unconformity pinch out landwards at 0 m a.s.l. Also the minor knickpoint around 0 m a.s.l. in the present alluvial plain in cross section BB’ can be interpreted as the re- sponse of the fluvio-deltaic system to sea level. In the most distal sector, the unconformity is buried by barrier- and-lagoon deposits, which are up to 25 m thick, as evi- denced in cross section GG’. 5. CONCLUSIONS The interpolation and linear regression of strati- graphic and topographic data in the Brenta megafan resulted in the production of the DEM of the post-LGM unconformity from the eastern Southalpine piedmont to the Adriatic coast. This means that the depth of burial, the elevation a.s.l., and the nature (erosive vs. hiatal with soil formation) of the unconformity were mapped on a 30x30-m grid base on an area of ca. 3000 km2. The reconstructed unconformity surface is coherent with lo- cal stratigraphic settings, showing the consistency of processing procedures. Nevertheless, the accuracy of results is evidently related to the number and geograph- ic distribution of available SCPs. Thus, in the piedmont and medium sector of the megafan, it was possible to check the model only in few sites. Moreover, from Pa- dova to the margin of Venice Lagoon the estimation of the width and precise path of the IV was only hypothe- sized. Morphometric analysis of the DEM enabled to evaluate the gradients of the unconformity in long and transverse profiles, as well as to calculate its overall ex- tension (1491 km2), the areas where it is characterized by buried soils (970 km2), and those where it corre- sponds to an erosive surface in IVs (521 km2). The sub- traction of the DEM of the unconformity from the present topographic surface allowed for the first calculation of the volume of post-LGM alluvial and coastal sediments (10.4 km3), differentiating those which bury the inter- fluves (6.1 km3) from IVs fills (4.3 km3). The three-dimensional modeling of this surface proved to be a valuable aid for the analysis of the forc- ing of sea level on the post-LGM progradation of the Fig. 9 - Transverse cross sections in the Brenta megafan, showing the present topographic surface (red line) and the post-LGM unconformity (blue line) (for locations see Fig. 7). Mozzi P. et al. 52 Brenta megafan. The results evidence the correlatability of the unconformity across a wide distributive alluvial system and connected deltaic and barrier-and-lagoon systems. The method can potentially be applied to neighboring megafans of the Venetian-Friulian and eastern Po Plain and extended offshore to the tip of the deltaic systems. This provides a tool for basin-scale stratigraphic correlations and quantitative estimate of erosive processes and sedimentary storage during the post-LGM on the northern Adriatic shelf. 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