AMQ 31(1) 21-35 Giraudi ProofCopy Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 31 (1), 2018, 21 - 35 THE PLIOCENE AND EARLY PLEISTOCENE DRAINAGE NETWORK EVOLUTION IN THE MONFERRATO HILLS (PIEDMONT, NW ITALY) Carlo Giraudi ENEA C.R. Saluggia, Saluggia (VC), Italy Corresponding author: C. Giraudi ABSTRACT: The drainage network in the Monferrato Hills is, in general, quite complex. However, in the western and eastern end of the Monferrato there are areas where the drainage is less complex. Morphological analysis suggests that the differences in drainage were mainly produced by captures that have affected the catchments of some streams. Geological and geomorphological studies have shown that during the early Pliocene, a portion of the Monferrato, along with part of the Torino Hill, was an island in which there were some catchment basins draining towards the southern and northern sea. The tectonic evolution caused the capture of rivers that have been dated from the Piacenzian onwards using stratigraphic and morphological data. The head of the Stura, Colobrio, Rotaldo and Grana valleys, that flowed southwards after the emersion of the island, were captured by headward erosion of streams draining towards the northern basin. The post-Zanclean uplift of the western Monferrato, some gentle post-Zanclean anticlines lying south of the hills, the subsiding basin lying north and the tectonic structures transversal to the Monferrato thrust front, played a dynamic role in the evolution of the drainage. On the contrary, the tectonic structures displacing the sediments that form the core of the Monferrato hills, active mainly before the Pliocene, influenced, mostly passively, the drainage network when the area emerged from the sea. In fact, the initial direction of the streams was conditioned by the different strike, dip and erodibility of the pre-Pliocene sediments. Keywords: structurally-controlled drainage network, river capture, Pliocene, Pleistocene, Monferrato Hills, NW Italy 1. INTRODUCTION The present paper deals with the Pliocene-Early Pleistocene evolution of the drainage network of the Monferrato Hills. Since the paper by Sacco (1889), it has been known that during the Pliocene the Monfer- rato was an island, and therefore the study of the pre- sent drainage network can highlight the structure of the drainage that developed on the just emerged island and the influence of the Pliocene and Pleistocene tectonic activity on its evolution. The Monferrato and Torino Hills are the north- western termination of the Apennine chain, and are surrounded by the Savigliano and Alessandria basins, to the south, and by the Po basin to the north (Fig. 1). The post-Messinian geological and tectonic evolution of the hills can be considered fairly similar (Irace, 2009; Mosca et al., 2010). The drainage network in the Monferrato and Torino Hills is formed by small streams, that are tributaries of the Po river north and SW of the hills, and of the Tanaro river south of the hills. In the Monferrato, the drainage is far more complex than in the Torino Hills. The geological literature reports various hypothe- ses regarding the evolution of some portions of the drainage network (Biancotti & Franceschetti, 1979; Carraro et al., 1980; Giraudi, 1981; Carraro & Valpreda, 1991; Carraro et al., 1995; Dela Pierre et al., 2003b; Forno & Lucchesi, 2005; Vezzoli et al., 2010; Giraudi, 2015; Forno & Lucchesi, 2016): the picture that emerges from the morphology of the valleys, the presence of fluvial sediments of Alpine origin and the deformation of Plio-Pleistocene sediments and terraces is clear in indi- cating that the drainage is strongly influenced by the tectonic evolution connected with the activity of the Mon- ferrato thrust front and some transverse strike slips or transtensive faults and deformation zones. The aim of the present paper is to analyze and discuss the morphology of the valleys of the main streams of the Monferrato Hills in order to recognize the features that originated during the Pliocene and the evo- lution of the drainage network until the Early Pleisto- cene, and to hypothesize the forcing factors. 2. MATERIAL AND METHODS In the present paper it is assumed that, as sug- gested by the results of the studies of Sacco (1889), Carraro et al. (1980) and Dela Pierre et al. (2003a,b), the hills bounded by Pliocene sediments of littoral facies indicate approximately the extent of the land surrounded by the sea during the Pliocene, in particular during the https://doi.org/10.26382/AMQ.2018.02 22 Giraudi C. Zanclean. The stratigraphy and tectonic structures af- fecting the Monferrato Hills have been outlined because of their possible impact on the drainage evolution and some interpretations of peculiar geological and morpho- logical features have been discussed. The geology of the area drawn in Fig. 2 is based on the Geological Map of Italy at the scale of 1:100,000, Sheets 57, 58, 69 and 70 (Anfossi et al., 1969; Boni & Casnedi, 1969; Bonsignore et al., 1969; Bortolami et al., 1969; Braga & Ragni, 1969; Corsi et al., 1969; Montra- sio et al., 1969). In fact, only part of the Monferrato lies in the Trino Sheet of the Geological Map of Italy at the scale 1: 50,000 (Dela Pierre et al., 2003a), which repre- sents an advancement in both the cartography and the interpretation of the stratigraphic and tectonic evolution. However, some tectonic structures identified by Dela Pierre al. (2003a) and the Fontanetto Po, Trino, Salera Line, Giarole-Lu and Valenza deformation zones, evi- denced by Giraudi (2014; 2015; 2016), have been in- cluded in Fig. 2. The evolution of the drainage network of the Mon- ferrato Hills was outlined using geomorphological analy- sis. Some morphological features have been used in order to identify areas in which the drainage network has the same characters. The presence of elbows of capture, mostly already reported in literature (Biancotti & Franceschetti, 1976; Carraro et al., 1995; Dela Pierre et al., 2003; Giraudi, 2015; 2016), and strong valley asym- metries have been taken into account in order to estab- lish the most important drainage variations. The drainage network has been interpreted in or- der to determine the approximate location of the Plio- cene divide between basins draining to the south and to the north and in order to have some information on the Pliocene landscape that was later modified by the geo- logical and geomorphological evolution. 3. GEOLOGICAL OUTLINE OF THE HILLS AND BURIED MONFERRATO Detailed knowledge of the geological features of the Monferrato Hills is limited by the extensive vegeta- tion cover and the scarcity of outcrops. Paradoxically, there are more detailed data on tectonic structures that affect the tertiary sediments covered by alluvial deposits of the Po Plain (the Buried Monferrato) thanks to the geophysical data and the extensive outcrops along the Po riverbed. The geological structure of the Monferrato is out- lined in Fig. 2. 3.1. The Monferrato hills The current geological structure of the Monferrato Hills was determined by the Pliocene and Pleistocene northward migration along thrust fronts of Tertiary sedi- mentary units already deformed by earlier tectonic phases (Dela Pierre et al., 2003a,b; Galadini et al., 2012; Giraudi, 2014; 2015) and by an uplift that lasted, at least in some places, until the Late Pleistocene (Carraro et al., 1980; Carraro & Valpreda, 1991; Carraro et al., 1995; Michetti et al., 2012; Giraudi, 2014; 2015). According to several authors (Costa, 2003; Giraudi, 2014; 2016), the thrust front is intersected by transverse strike slip or transtensive faults and deformation zones. The thrust front, buried under Quaternary fluvial sediments, is located at a variable distance from the northern slope of the Monferrato and Torino Hills. In general, the core of the Monferrato Hills consists of strongly deformed pre-Messinian sediments and is bounded to the south by gently folded Messinian and Pliocene sediments. The new stratigraphic studies reported in Dela Pierre et al. (2003b) have led to the recognition of sedi- ments of littoral facies, Pliocene (Zanclean) in age, and have demonstrated that, after the Zanclean, a marine regression began. The area surrounded by the Zanclean sediments corresponds, at least approximately, to the area not submerged by the sea during that period. However in the Moncalvo area (Fig. 2) there are two small anticlines affecting Messinian and Pliocene sediments and the Pliocene that completely surround the Messinian core (Sacco, 1889). The post-Zanclean anticlines (Pieri & Groppi,1981; Cassano et al., 1986), create a unique case in Monferrato, where the Messin- ian sediments always surround unconformably the older marine formations. Near San Salvatore, the marine sedimentary se- quences do not contain the Ligurian Units, outcropping Fig. 1 - Geological sketch of the Monferrato and Torino hills and surrounding Pliocene basins, with location of the study area. 23 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... in other areas of the Monferrato (Fig. 2), and the anti- cline that forms the core of the hills consists of continen- tal and marine Messinian and early Pliocene marine sediments. Elsewhere, except in the Moncalvo area, Messinian and Pliocene sediments outcrop just on the edge of the highest hills. On the northern slope of the hills west of Casale Monferrato, near Verrua Savoia, there is just an isolated outcrop of Pliocene (Zanclean) sediments (Martinis, 1954, Zappi, 1960; Trenkwalder & Violanti, 2001; Bove Forgiot et al., 2005). In the eastern Monferrato, between Casale Mon- ferrato and Conzano, Pliocene marine littoral sediments are quite extensive and indicate the approximate bound- ary of the emerged land. Regarding the tectonic features, it can be observed that the hills west of the Grana Valley consist of a series of uplifted complex structures and synclines oriented mainly NW-SE and WNW-ESE, although some struc- tures lying in the northern portion of the area are ori- ented approximately W-E and, seldom, SW-NE and approximately N-S. While many of the sediments that form the core of the Monferrato hills were already folded and faulted before the Pliocene, and partly emerged during the Zan- clean, the area south of Moncalvo, where the anticlines Fig. 2 - Geological map of the Monferrato Hills (modified from Anfossi et al., 1969; Boni and Casnedi, 1969; Bonsignore et al., 1969; Bor- tolami et al., 1969; Braga and Ragni, 1969; Corsi et al., 1969; Montrasio et al., 1969). Some faults are from Dela Pierre et al. (2003), while the Fontanetto Po, Trino, Salera Line, Giarole-Lu and Valenza deformation zone are from Giraudi (2014; 2015; 2016). RIT, RIO, RIM and RIP indicate the isolated hills of Trino, Occimano, Mirabello and Pomaro. with a Messinian core involving Pliocene littoral sedi- ments are located, was deformed and uplifted later (Sacco, 1889; Pieri & Groppi, 1981; Cassano et al., 1986). According to Dela Pierre et al. (2003b), tectonic movements started during the sedimentation of the de- posits of Villafranchian facies (Piacenzian in age) and caused the uplift of part of the area between Moncalvo and Asti. Also the anticlines that involve Messinian and early Pliocene marine sediments forming the core of the high- est hills near San Salvatore were produced by tectonics active during or after the Pliocene. According to ENEL (1984) these sediments lie on a thrust front and their displacement occurred after the lower-middle Pliocene. Therefore, the deformation of the Pliocene marine sediments and the emersion of the areas near Moncalvo and San Salvatore probably took place at the same time after the Zanclean, while the sea was still present in the Alessandria and Po basins. Because the emersion occurred later than in the core of the Monferrato, this area from now on will be called Neo-Monferrato. In the lower Cerrina Valley (Fig. 2) there is a very peculiar syncline, whose axis runs first from NW to SE, then W-E and afterwards S-N. The stretch of the syn- cline oriented NW-SE might correspond to the continua- tion of the syncline having the same direction which extends from the area of Cereseto to Treville and Ron- caglia. The syncline of the Cerrina valley was also ac- tive from the Early Pleistocene (or late Pliocene) to the Middle Pleistocene and (according to Giraudi, 2016) may correspond to a subsiding basin produced by the distension that occurred behind the thrust front (Fig. 2) located north of the Trino isolated hill (RIT in figures). In correspondence with the Giarole-Lu Deforma- tion Zone (GLDZ), some diapiric structures, with WNW- ESE oriented axes, are folded (Fig. 2), assuming a SW- NE trend. Between the San Salvatore area and the Po- maro isolated hill (RIP in figures), the folds assume a WNW-ESE direction and then, at the eastern end of the Monferrato, in correspondence with the Valenza Defor- mation Zone (VDZ), a direction approximately N-S. The lithology of the Tertiary sediments of the Mon- ferrato is predominantly marly and clayey, but the Plio- cene marine sediments have sandy and calcarenitic facies, the Miocene ones contain interbedded calcaren- itic horizons and sandstones, while very cemented con- glomerates and sandstones are common in the Oligo- cene sediments. In particular, the latter, which generally form the highest hills, are the most resistant to erosion (Fig. 2). In the core of the Monferrato Hills, continental sedi- ments older than the Late Pleistocene are scarce and outcrop in a patchy fashion, with the exception of the Cerrina Valley. Some alluvial sediments, probably Plio- cene in age, lie in the area east of the Colobrio Valley, at an elevation similar to the hilltop, many tens of metres above the lake sediments outcropping in the Cerrina Valley, dated at the Early Pleistocene sub-chron Olduvai (Giraudi, 2016). Characteristic of the lower Cerrina Val- ley (Fig. 2) is the presence of a significant amount of Early and Middle Pleistocene alluvial sediments, formed by clasts of Alpine origin, demonstrating the presence of a river flowing from the area sited NW of the valley head (Giraudi, 1981; Carraro et al., 1995; Giraudi et al., 2003; Dela Pierre et al., 2003a,b; Giraudi, 2016). The lithology of some patches of alluvial sediment suggest that the river continued SE of the valley reaching the Buried Monferrato lying north of the easternmost Mon- ferrato Hills (ENEL, 1984; Carraro et al., 1995; Giraudi, 2015; 2016). 3.2. Buried Monferrato The Buried Monferrato, located north of the hills between the hill slope and the thrust front, is mainly formed by the same pre-Pliocene marine sediments forming the hills, underlying Late Early Pleistocene to Holocene fluvial and glaciofluvial deposits of the Po plain (ENEL, 1984; Dela Pierre et al., 2003 a, b; Giraudi, 2014) which, normally, are less than 20 m thick. The marine sediments have been observed in a number of wells and boreholes drilled for oil exploration, or related to studies for a nuclear power plant, and water wells (ENEL, 1977; 1984; Pieri & Groppi, 1981; Cassano et al., 1986; Giraudi, 2014; 2016). Pliocene and Pleisto- cene (Gelasian) marine deposits of littoral facies have been found not only in the Po and the Alessandria ba- sins at the top of a thick successions of older Pliocene sediments, but also in some places in the Buried Mon- ferrato lying unconformably on older Pliocene neritic and pre-Pliocene marine sediments (Fig. 2). In general, in the Buried Monferrato, the Pliocene marine sediments lie near the thrust fronts, but north of the Eastern Mon- ferrato they have been found between the hillside and the Pomaro isolated hill, i.e. in an area well south of the Monferrato thrust front (Pieri & Groppi, 1981; Cassano et al., 1986). The tectonic structures affecting the Buried Monfer- rato, shown in Fig. 2, take into account mainly syn- clines, thrust fronts, faults and deformation zones active also during the Pleistocene and which fit better with borehole and well data already discussed in Giraudi (2016). In point of fact, such structures could have con- ditioned the Early Pleistocene drainage evolution on the Buried Monferrato. Therefore, some thrust fronts active before the Late Pliocene or of uncertain activity, as- sumed in geological literature (Montrasio et al., 1969; Pieri & Groppi, 1981; ENEL, 1984a; Cassano et al., 1986; Bigi et al., 1990; Michetti, et al., 2013; Giraudi, 2014; 2015; 2016) have not been drawn in the figure. The area of the Buried Monferrato (Fig. 2) is af- fected by strike-slip or transpressive faults transverse to the front (Costa, 2003) both in the areas west and east of the Trino and Pomaro isolated hills. According to recent studies (Giraudi, 2014; 2015; 2016) in places roughly corresponding to the transpres- sive faults reported by Costa (2003), but also in other areas, there are the Fontanetto Po (FPDZ), Trino (TDZ), Salera Line (SLDZ), Giarole-Lu (GLDZ) and Valenza (VDZ) deformation zones. The deformation zones, ac- tive during the Pliocene, Early and Middle Pleistocene, are transverse to the thrust fronts, and show left-slip west of the Trino and Pomaro isolated hills, and right- slip to the east. 24 Giraudi C. 3.3. Connection between topography of the hills and tectonic structures The connections between the topography and the geological structures seem very clear in the core of the hills. The map of Fig. 3 shows the elevations of the study area with 50 m contour lines. The maximum ele- vations, just below 500 m a.s.l., are reached towards the western end of the area. In the same area the height of the hills often exceeds 400 m a.s.l. The elevations de- crease from NW to SE. From a comparison between Fig. 2 and Fig. 3, it is evident that the maximum elevations coincide with the uplifted complex structures and the lower ones with the synclines. It is also easy to observe that the hills lying NE of the head of the Stura, Grana and Rotaldo valleys are, in general, higher than those lying SW and that, just after changing their direction from NW-SE and WNW- ESE towards NE, the same valleys lie on SW-NE trend- ing faults and cut deeply through the uplifted structures oriented NW-SE (Fig. 2). Making a comparison between the maximum heights of the hills and the portions where the Zanclean sediments of littoral facies are present, next to the divide between the Po and the Tanaro basins (Fig. 3), the difference is of about 100 m in both the Western Mon- ferrato (400-440 m and 330 m respectively) and the Eastern Monferrato (250-260 m and 160 m). In Fig. 3 it can be observed also that east of Moncalvo, the Plio- cene littoral sediments outcropping north of the Po- Tanaro divide sometimes reach elevations higher than the hills forming that divide, but do not reach the eleva- tion of the northernmost hills. 4. PRELIMINARY DATA ON THE DRAINAGE NET- WORK IN THE MONFERRATO AND TORINO HILLS The Torino and Monferrato Hills are affected by a drainage network with different characteristics (Fig. 4A). All the streams on the northern slopes are tributaries of the Po river , while the streams that drain the southern slopes flow into two different basins: from the Torino Hills they flow mostly to the E-W trending river Banna, which is a tributary of the Po, while those draining the Monferrato flow to the W-E trending Tanaro river or the Triversa stream, its tributary. North of the hills, the Po river and the small streams flow through the Buried Monferrato. The drainage network in the hills is shown in Fig. 4A. It can be observed that the smaller streams flowing from the hills to the Po river are sometimes not drawn in the figures when they reach the alluvial plain. These streams are in fact connected with the network of irriga- tion ditches and therefore are difficult to follow and their shape and direction are wholly artificial. In the context of Fig. 4A, various areas have been highlighted in which the drainage has common features. Zone A: corresponds to the Torino Hills and the portion of Monferrato located W and SW of the confluence of the river Dora Baltea and the river Po. The drainage network in this zone has, in general, the following characteristics: the head of the basins of most streams begins at the divide between the northern and southern slopes of the hills, and the streams, with directions transverse to the divide, flow directly into the river Po (to the north) or into other streams roughly flowing E-W or W-E, tributaries of the main 25 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... Fig. 3 - Elevations of the Monferrato Hills and of the surrounding plains, evidenced by 50 m contour lines, and elevation of the Pliocene littoral sediments north and south of the Po-Tanaro divide. rivers (to the south). In zone A, the Po-Tanaro divide reaches its northernmost point. To the north of the hills, the river Po runs close to the hillslope, on the Buried Monferrato, where tertiary sediments are predominantly pre-Pliocene. Studies conducted by Forno & Lucchesi (2005; 2016) in the Torino Hills indicate the presence of small remnants of terraces on the NW and N hillside. The terraces and the lithology of the alluvial sediments suggest that, in the period between the Middle Pleistocene and the present time, the drainage at the margin of the hills migrated towards N and NW. Zone A1: corresponds to the eastern end of the Mon- ferrato. Its drainage is similar to that of the Zone A, and the head of most basins reaches the divide be- tween the northern and southern slopes of the hills, that is, the Po-Tanaro divide. The streams have a direction roughly transverse to the divide and flow directly into the Po and the Tanaro rivers. Zone B: corresponds to most of the Monferrato Hills, except at its western and eastern ends. Starting from the western edge of Zone B, the Po-Tanaro divide heads southeast. Only the head of the valleys of three streams (Stura, Colobrio, Grana), pertaining to the Po basin, reaches the Po-Tanaro divide. At first, the streams run NW-SE or WNW-ESE, parallel to the divide, but at some point they abruptly change direc- tion, flowing generally towards N and NE and reach the Po river. Other small streams lying at the northern slope of the hills flow directly into the Po river. In the Tanaro basin, several streams having catchments that reach the divide flow into the Versa, a tributary of the Tanaro, in the area where the stream runs NW-SE and therefore is parallel to the divide. On the Buried Monferrato, the streams are sub-parallel to each other. Zone C: includes the hills between the town of Valenza and the Grana Valley. The head of the small catch- ment basins reaches the Po-Tanaro divide and the direction of the streams is generally transverse to that of the divide. In the hills the drainage pattern is similar to that of the A1 Zone, but on the Buried Monferrato the streams are sub-parallel to each other and do not drain directly into the Po but flow into other tributary streams of the Po. The streams of the southern slope of the hills drain directly into the Tanaro. In Fig. 4B, it can be observed that the boundaries between the A, B, C and A1 zones correspond to or are located in the vicinity of some deformation zones: the Rio Freddo Deformation Zone between A and B, the Giarole-Lu Deformation Zone between B and C, and the Valenza Deformation Zone between C and A1. The A, A1, B and C zones correspond, roughly, to portions of the hills having different geological features. The portion of the Monferrato Hills where the drain- age is more complex (zones B and C) will be discussed and interpreted below. 5. THE DRAINAGE NETWORK IN THE MONFER- RATO HILLS The characteristics of the Monferrato drainage network have already been partially discussed in a num- ber of papers. One of the major basins, that of the Stura stream, was studied by Biancotti & Franceschetti (1979). Also Dela Pierre et al. (2003b) discussed the character- istics of the drainage in part of the area covered by the present study, while in Giraudi (2015) some features of the drainage of the eastern Monferrato were taken into account. The studies made it possible to recognize some stream captures or diversions but only some of the variations that occurred in the Middle-Late Pleisto- cene have been chronologically framed (Carraro et al., 1980; Giraudi, 1981; Carraro & Valpreda, 1991; Carraro et al., 1995; Dela Pierre et al., 2003b; Giraudi, 2015; 2016). 5.1. Drainage network features Most of the main streams in the Monferrato, north of the divide between the Po and the Tanaro, show a common trend (Fig. 5A): at the head of the valleys the streams flow NW-SE and NNW-ESE, then they turn toward NE or NNE. In the case of the Ponara stream, a tributary of the Rotaldo, at the head of the valley it flows from NNE to SSW, and then takes the same direction as the other streams. A more detailed observation of the drainage net- work in the Monferrato Hills and the Buried Monferrato allows some areas with different features (Fig. 5B) to be identified. Area 1: the heads of the main valleys are parallel to the divide between the Po and Tanaro catchments and their southern slopes correspond to the divide. In gen- eral, the prevailing direction of the streams is towards south-east and east. The tributary basins of the main valleys are mainly elongated in the directions from NW to SE and from N to S. The valleys of the main streams (Stura, Colobrio, Grana, Rotaldo) trending from NW and WNW to SE and ESE at the head, turn N and NE at the boundary of the area. Their basins are clearly asymmetric and the tributary basins facing the northern quadrants are much narrower than those facing the southern ones (Fig. 5A). The asymmetry of the slopes is very clear near the places where the change in direction of the streams occurs, that is in correspondence with elbows of capture of the upper Stura and Colobrio valleys reported in previous pa- pers (Biancotti & Franceschetti, 1979; Dela Pierre et al.,2003b). The capture of the upper Stura valley, which originally drained towards the SW, was as- sumed by said Authors after the evaluation of morpho- logical parameters, while that of the Colobrio is repre- sented on a map without any discussion. The capture of the valleys and the abrupt changes in direction occur in correspondence with faults trending mainly SW-NE, as reported in Dela Pierre et al (2003b). Abrupt changes of direction occur also in the high Grana and Rotaldo valleys, in correspondence with SW-NE trending faults and of strong asymmetries of the valleys. The change of direction of the valleys, from NW-SE to SW-NE, takes place in an area formed by Pliocene sediments in the Alessandria basin. Those sediments reach elevations higher then the hills forming the divide between the Po and Tanaro rivers but lower than that of the hills lying to the north 26 Giraudi C. heads of all the basins begin at the divide between the northern and southern slopes of the hills, and the streams flow in directions transverse to the divide. Area 2: this includes part of the Stura, Rotaldo and Grana catchments (Fig. 5B) where the main streams flow, in general, from SW to NE. The Stura stream direction of flow is more complex: it first flows from SW to NE, then W - E, and finally forms an elbow and starts heading NNE. The main and secondary streams flow in many differ- ent directions (Fig. 5A;B). The tributaries of the Stura stream drain both SE- SSE and NE-NNE, i.e. towards the axis of the lower Stura valley (Cerrina Valley). In this area, while the slopes of the main valley are quite symmetrical (Fig. 6A), the slopes of the small valleys of the secondary streams can be asymmetrical. The small valleys often have slopes facing west much more extensive than those facing east or vice versa. In Area 2, also the (Fig, 3). Other sharp variations in the direction of the Stura and Grana valleys (Fig. 5A) are present in cor- respondence with stretches of valley having charac- teristics very similar to the elbows of capture evi- denced in literature Area (1): it is located south of the Po-Tanaro divide (Fig. 5B) and corresponds to the catchment of the Versa stream, mostly developed on Pliocene marine sediments, draining towards south or south-east, and was studied by Carraro & Valpreda (1991). In Area (1) the drainage is more complex than in the other catch- ments draining south (Fig. 4A, 5A): the Versa stream flows from NW to SE, running parallel to the divide, then turns eastwards, then southwards, and then, once again, flows from NW to SE. The heads of some smaller catchments in this area do not reach the di- vide between the Po and the Tanaro. All the other basins lying south of the divide have a drainage sys- tem similar to that of the A Zone (Fig. 4A): in fact the 27 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... Fig. 4 - A: morphological outline of the Monferrato and Torino Hills and the Buried Monferrato drainage network, showing areas with differ- ent drainage patterns. B: geological outline of the main tectonic structures and sediments forming the Monferrato and Torino Hills and the Buried Monferrato. Rotaldo and Grana streams flow from SW to NE (Fig. 5A). On the left side of the lower catchment of the Rotaldo stream, the direction of the tributary streams is from WNW to ESE. The tributaries of the Ponara stream flow mainly from WNW to ESE but some flow from WSW to ENE. In the south-eastern portion of 28 Giraudi C. Fig. 5 - Drainage network and areas with a different drainage pattern in the Monferrato Hills and the Buried Monferrato. A: drainage net- work and catchment basins. B: areas with different drainage patterns. Area 2, the secondary streams flow mainly NW-SE and SE-NW into the Rotaldo and Grana streams. Both the mouth of the Rotaldo valley and the catch- ments of some secondary streams are very asymmet- ric (Fig. 6A), with slopes facing the western quadrants much less extensive than those facing east. In a few cases the slopes facing the southern quadrants are much narrower than those facing north. A divide separates the major catchments forming Area 2: the basin of the Stura (to the west) and the portion of the Rotaldo basin west of the stream (to the east). How- ever, until the Middle Pleistocene, the Stura basin and a portion of the Rotaldo basin were part of a single catchment crossed by a river from its north-western edge to the south-eastern one (ENEL, 1984; Carraro et al., 1995; Dela Pierre et al., 2003b; Giraudi, 2015; 2016). Area 3: here the Stura flows from SSW to NNE, but the main streams of this area run approximately W-E and E-W and their tributaries flow from NW, N, S, SW, and SE (Fig. 5B). Fig. 6A shows that the valley slopes are usually asymmetrical and that generally the sides facing north are narrower than those facing south. In only two cases are the slopes facing east much more extensive than those facing west (Fig. 6A). In the final stretch of the Cerrina Valley, where the Stura runs SSW-NNE, this kind of asymmetry is in correspon- dence with a northward elbow of capture of a river, mentioned above, which until the Middle Pleistocene drained SE towards the Rotaldo valley, as evidenced before. Area 4: here the streams drain mainly towards N, NNE and NE and flow directly into the river Po. Only in the eastern area are there any asymmetries in the valleys (Fig. 6A), with slopes facing the eastern quadrants far larger than the opposite ones. Also in this area, in two cases out of three, the asymmetry is in correspondence with the elbow of capture of a stream flowing towards NNE but draining originally towards ESE (Carraro et al., 1995; Dela Pierre et al., 2003b; Giraudi, 2015; 2016). - Area 5: the streams here flow into small valleys in a NNE direction, but when they reach the plain, they do not flow directly into the river Po, but into a series of secondary streams, tributaries of the river. In this area there are asymmetries in the basins (Fig. 6A): the slopes facing south are far less extensive than the opposite ones. Area 6: isolated hills with radial drainage that cannot be represented at the scale of the figures. 6. DISCUSSION The hypotheses of captures and changes in the drainage network reported in previous papers were based in one case (Cerrina Valley) on the lithological composition of the alluvial sediments, and in the other cases (Stura and Colobrio valleys) on morphological features, mainly the presence of elbows of capture. The presence of a different drainage in the lower Cerrina Valley is indicated by the presence of a signifi- cant amount of Early and Middle Pleistocene alluvial sediments (Fig. 2), formed by clasts of Alpine origin. These sediments demonstrate the presence of a river flowing from the area sited NW of the valley head (Giraudi, 1981; Carraro et al., 1995; Giraudi et al., 2003; Dela Pierre et al., 2003a,b; Giraudi, 2016). The river continued SE of the Cerrina Valley (Fig. 6A) reaching the Eastern Monferrato Hills and probably the Buried Monferrato north of the hills, as suggested by the lithol- ogy of some patches of alluvial sediment found in ter- races and in boreholes (ENEL, 1984; Carraro et al.,1995; Giraudi, 2015; 2016). The old drainage net- work, mainly elongated in a NW-SE direction, and run- ning in drainage Area 2, was strongly modified around 600 ka ago. The capture of the upper Stura and Colobrio val- leys which, according to Biancotti and Franceschetti (1976) and Dela Pierre et al (2003b), in origin drained towards SW, was assumed from the presence of an elbow of capture. Before capture, the divide between the catchments flowing towards the Savigliano-Alessandia and Po basins was near the northern divide of the upper Stura and Colobrio valleys (Fig. 6A). The presence of Pliocene sediments in a small portion of the catchment testifies that part of the head of the present Colobrio valley was covered by the Pliocene Sea in the Alessan- dria basin. After emersion, as a consequence of the Zanclean regression, the Colobrio excavated its bed in the newly emerged marine sediments in a SE direction. Later, the stream was captured towards the NE. In the same figure, it can be observed that in corre- spondence with the elbows of capture of the Cerrina, Stura and Colobrio valleys there is pronounced asym- metry of the slopes and therefore also the asymmetry could be linked to the morphological evolution that con- tributed to or followed the captures. The captures and the abrupt change in direction occur in correspondence with faults (Fig. 6A) trending mainly SW-NE reported in Dela Pierre et al (2003b). Abrupt changes of direction occur also in the high Grana and Rotaldo valleys, in correspondence with SW- NE trending faults and of strong asymmetries of the valleys (Fig. 6A; B). Having the same geological and morphological characteristics as the Stura and Colobrio valley elbows of capture, it can be assumed therefore that also the Rotaldo and Grana valley elbows were due to captures (Fig. 6A; B). The elbows of capture of the Grana and Rotaldo valleys occur in an area formed also by Pliocene sedi- ments of the Alessandria basin that reach elevations higher than the hills forming the divide between the Po and Tanaro rivers but lower than that of the hills lying just north (Fig. 3). It follows that, before the capture and until a period following the post-Zanclean regression, the Rotaldo and Grana streams flowed towards the Alessan- dria basin and the divide was roughly in correspondence with the hills forming the northern divide of the head of the Rotaldo valley. It can be observed also that the Stura, Colobrio, Grana and Rotaldo streams show the same trend, both in areas 1 and 2, and therefore a common evolution can be assumed. The former divide between the basins draining 29 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... 30 Giraudi C. Fig. 6 - Asymmetry of the valleys and elbows of capture (6A) and relations between the drainage network and major tectonic features (6B). towards the Alessandria and the Po basins was near the boundary between areas 1 and 2. Other sharp variations in the direction of the Stura and Grana valleys (Fig. 6A) are present in correspon- dence with stretches of valley having characteristics very similar to the elbows of capture evidenced in litera- ture and discussed before. It is likely that said elbows were produced by other captures. 6.1. Structural and lithological forcing in the drain- age network In Fig. 4B, it can be observed that both in the Mon- ferrato and Torino Hills the boundaries between zones A, B, C and A1 correspond to or are located in the vicin- ity of some deformation zones: the Rio Freddo Deforma- tion Zone between A and B, the Giarole-Lu Deformation Zone between B and C, and the Valenza Deformation Zone between C and A1. Tectonics probably played a role in determining the drainage evolution in the Monferrato Hills as observed by previous Authors (Biancotti & Franceschetti, 1979; Carraro et al., 1980; Giraudi, 1981; Carraro & Valpreda, 1991; Carraro et al., 1995; Dela Pierre et al., 2003b; Forno & Lucchesi, 2005; Vezzoli et al., 2010; Giraudi, 2015; Forno & Lucchesi, 2016). The comparison between the morphology of the valleys, the position of the elbows of capture and the tectonic structures (Fig. 6B) suggests the following con- siderations: - the widest valleys correspond to synclines active also during the Late Miocene and sometimes during the Pleistocene; - the highest ridges are located in the western hills and in general coincide with the presence of coarse ce- mented sediments involved in complex uplifted struc- tures active before the Pliocene; - changes in direction of the valleys and elbows of cap- ture towards the north mostly occur in correspon- dence with faults trending SW-NE and deformation zones (FPDZ; SLDZ; GLDZ; VDZ) trending SW-NE, WSW-ENE, and N-S, active also during and after the Pliocene. The tectonic structures and the lithology also affect the boundaries between the areas in which the drainage network shows different characteristics: - the boundary between Areas 1 and 2 mostly coin- cides with ridges formed by strongly cemented sedi- ments present in complex uplifted structures; - the boundary between Areas 2 and 3 corresponds to the transition between hills with pre-Pliocene tectonic structures trending from NW to SE and hills where W- E trending structures prevail; - the boundary between Areas 2 and 4 lies in the south- ern extension of the Fontanetto Po Deformation Zone active also during the Pleistocene; - part of the boundary between Areas 2 and 3 lies in the southern extension of the Trino and Salera Line Deformation Zones, active also during the Pleisto- cene; - the boundary between Areas 2 and 5 coincides with the Giarole-Lu Deformation Zone active until the Pleistocene; - the boundary between Areas 5 and 4 is in correspon- dence with the Valenza Deformation Zone active also during the Pleistocene. Comparing between the maximum heights of the hills and the elevation reached by the Zanclean sedi- ments of littoral facies, next to the divide between the Po and the Tanaro basins (Fig. 3), the difference that can be seen is of about 100 m in both the Western Monfer- rato (400-440 m and 330 m respectively) and the East- ern Monferrato (250-260 m and 160 m). It may be con- cluded that most of the differences in the maximum heights of the hills were due to a differential uplift that occurred after the Zanclean, and that the western por- tion of the study area underwent a greater uplift, esti- mated at about 150-170 m. The structural and lithological influence on the drainage network seems therefore real. However in the core of the Monferrato Hills, the directions of the streams, not affected by SW-NE trending faults, follow the trend of the main tectonic structures active before the Pliocene. It can be hypothesized that, when the core of the Monferrato emerged from the sea, the initial NW- SE and WNW-ESE orientation of the streams was con- ditioned by the different strike, dip and erodibility of the already deformed pre-Pliocene sediments, and therefore the influence of the older tectonic structures was mostly passive. The flow towards SE or ESE of the streams was also affected by the stronger post-Zanclean uplift of the western Monferrato Hills. 7. THE PLIOCENE DRAINAGE NETWORK AND ITS EVOLUTION The pre-Zanclean emersion of the Monferrato core formed mainly by Tertiary marine sediments, already strongly deformed, and the clear structural forcing on the drainage network, allow speculation about the original drainage. Fig. 7A shows the possible drainage network on the just emerged lands, before the capture to the north of the main streams. The drainage must have been con- ditioned by the pre-Pliocene tectonic structures and by lithology, with the valley bottoms in correspondence with synclines or formations consisting of softer sediments easy to erode, and the top of the ridges and the main divides lying on strongly cemented sediments forming the uplifted structures. The boundary between Areas 1 and 2, lying near the elbows of capture of the main streams and in correspondence with the highest hills formed by strongly cemented uplifted sediments, was probably the divide between the Zanclean catchment basins draining towards the Saluzzo-Alessandria and the Po marine basins. This hypothesis is corroborated by the presence of Zanclean sediments of the southern basin north of the present-day Po-Tanaro divide. It can be observed that the head of the Grana val- ley, cut into Zanclean marine sediments (Fig. 2), was still below the sea during the early Pliocene and emerged only during the post-Zanclean regression (Fig. 7A). During the regression, the Colobrio, Grana and 31 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... Fig. 7 - The Pliocene and Gelasian drainage evolution (A) and uplifted and subsiding areas (B). Rotaldo streams, already existing during the Pliocene, excavated their beds in the newly emerged marine sedi- ments. South of the hills the streams drained mainly NNW -SSE, that is towards the Saluzzo-Alessandria basin while east of the hills they drained to the east and NE into the Po basin. The capture of the high Colobrio, Grana and Ro- taldo valleys was probably due to the withdrawal of the heads of streams draining towards the northern basin, in correspondence with faults cutting through the uplifted structures forming the earlier divide. The headward ero- sion of the stream flowing to the north and the capture of streams flowing towards the south can be explained considering the known geological evolution of the Mon- ferrato. The flow of the streams towards the southern basin may have been hindered by the uplift of the Neo- 32 Giraudi C. Monferrato with respect to the northern hillslope. This interpretation is supported by the comparison between the elevation of the Pliocene sediments of the southern and northern basins (Fig. 3). According to the Geologi- cal Map of Italy at the scale 1: 100,000, Sheets 57, 58, 69, 70 (Anfossi et al., 1969; Boni & Casnedi, 1969; Bon- signore et al., 1969; Braga & Ragni, 1969; Corsi et al., 1969; Montrasio et al., 1969), the Pliocene sediments of the southern basin reach elevations between 290 and 250 m a.s.l. between the Colobrio and Grana valleys, while those of the northern basin lie between 205 and 150 m a.s.l. In the same period, the headward erosion of the streams flowing northwards was increased by the lowering of the base level due to the subsidence of the northern basin which, until the Gelasian, was still cov- ered by the sea (ENEL, 1977; 1984; Pieri & Groppi, 1981; Cassano et al., 1986; Violanti & Sassone, 2008; Giraudi, 2014; 2016). The subsidence lasted at least until the Middle Pleistocene (Giraudi, 2015; 2016). The uplift of the Neo-Monferrato and the subsi- dence of the northern basin could have favoured also the headward erosion of the streams and the capture of the high Stura valley. However, also the subsidence of the Cerrina valley, which acted as a local base level of the Stura and Colobrio streams, could have played an important role. The subsidence of the Cerrina valley (Giraudi, 2016) started before the sub-chron Olduvai (age 1.9-1.7 MA). During the Olduvai, the sedimentation started as lacustrine silty clay, followed, during the Jaramillo sub- chron and the early Matujama chron, by the sedimenta- tion of alluvial deposits consisting mainly of clasts of Alpine origin. It can be ruled out that the headward ero- sion and lake sedimentation were coeval. In fact it is supposed that during the phases of headward erosion the streams had to carry downstream many clasts con- sisting of marine sediments outcropping in their catch- ments. But the lacustrine sediments have an extremely fine grain size and no coarse alluvial sediments of local origin, interfingered with lacustrine sediments, have been observed. The alluvial deposits (dated from about 1 to 0.7 MA), contain few clasts of local origin, and it is unlikely that they were contemporary with the phases of headward erosion of the streams. A younger age of the captures is ruled out by the stratigraphy, lithology and grain size of the later alluvial sediments and by the mor- phological features. We can assume, therefore, that the erosion phase, to which the capture of the heads of the Stura and Colo- brio valleys was connected, was mostly completed dur- ing the Gelasian, before the Olduvai sub-chron. To sum up, the headward erosion that led to the capture of the valleys draining towards the southern basin during the Zanclean and the following regression started with the Piacenzian uplift of the Neo-Monferrato and ended, probably, before the Olduvai sub-chron. Although some important captures and diversions occurred later, during the Middle and late Pleistocene (Giraudi, 1983; Carraro et al., 1995; Dela Pierre et al., 2003; Giraudi, 2015; 2016), the Middle Pliocene-Early Pleistocene captures shaped the most evident anoma- lies in the Monferrato drainage network. 8. CONCLUSIONS During the Pliocene, a portion of the Monferrato emerged as an island on which a drainage network developed. Analysis of the morphological and stratigraphic data has made it possible to reach a comprehensive interpretation of the drainage evolution in the Monferrato from the Pliocene to the Early Pleistocene. The heads of the basins of the Stura, Colobrio, Rotaldo and Grana streams drained southwards, to- wards the Savigliano and Alessandria synclines. The flow towards the south of those streams, that followed the post-Zanclean regression, was hindered by the Piacenzian uplift of the Neo-Monferrato, south of the core of the hills. The headward erosion of the streams draining towards the Po basin, that produced the capture of the heads of said streams, possibly started during the Zan- clean regression and was strengthened by the subsi- dence of the northern basin, occupied by the sea until the Gelasian. The headward erosion was favoured also by the presence of tectonic structures (faults and defor- mation zones) cutting into the Pliocene divide. According to the stratigraphic data, it can be as- sumed that the capture of the high valleys of the Stura, Colobrio, Rotaldo and Grana streams occurred between the Piacenzian and before the Early Pleistocene Olduvai chron. The correlation between the development of the hydrographic network and tectonic structures, assumed by previous Authors, is confirmed and even strength- ened by the relations between the drainage and the tectonic structures recognized in more recent investiga- tions. Tectonic structures played different roles. The gen- tle post-Zanclean anticlines that form the Neo- Monferrato, the Pleistocene subsidence of the northern basin and the deformation zones transverse to the Mon- ferrato front, played a dynamic role in the evolution of the drainage network. 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Atti Socie- tà Italiana di Scienze Naturali, Museo Civico di Storia Naturale Milano, 100, 73-204. 35 Ms. received: November 22, 2017 Final text received: July 25, 2018 The Pliocene and Early Pleistocene drainage network evolution in the Monferrato Hills ... 36