AMQ_28(1) coloph 3.pub


 

   

 

 

 

 

Available online http://amq.aiqua.it 
ISSN (print): 2279-7327, ISSN (online): 2279-7335 
 
 
 

Alpine and Mediterranean Quaternary, 28 (1), 2015, 71 - 92 

QUATERNARY EVOLUTION OF THE PLAIN BETWEEN CASALE MONFERRATO AND VALENZA: 
IMPLICATIONS FOR THE TECTONICS OF THE MONFERRATO THRUST FRONT 

(PIEDMONT, NW ITALY) 
 

Carlo Giraudi 
ENEA C.R. Saluggia, Vercelli, Italy 

Corresponding author: C. Giraudi <carlo.giraudi@enea.it> 
 
 
ABSTRACT: The present paper highlights the Quaternary evolution of the plain which lies on the eastern portion of the Monferrato Thrust 
Front, in order to identify and provide evidence of recent tectonic deformations. The study has made it possible to detect and date late 
Middle Pleistocene or early Upper Pleistocene tectonic deformations related to the activity of the front, and to suggest the hypothesis of 
more recent ones. It can be observed that some parts of the plain, located north of the thrust fronts and the area corresponding to some 
buried synclines, mainly subsided during the Quaternary. On the other hand the area of the isolated hills and terraces suffered an almost 
continuous uplift until the late Middle Pleistocene or early Upper Pleistocene. The areas where the uplift prevailed were affected by faults 
both transversal and parallel to the thrust fronts. The overall interpretation of the tectonic structures lying below the plains and on the hills 
has enabled us to assume that there are two deformation zones where strike-slip or oblique-slip displacements took place. In the first one 
(Giarole-Lu Deformation Zone) some left-slip tectonic deformations should exist, while in the second one (Valenza Deformation Zone) 
some right-slip displacements may be present. The movements along the two deformation zones would result in an advance towards the 
north of the thrust front near the isolated Pomaro-Montevalenza Hill. 
In the Eastern Monferrato contractional tectonics related to the Monferrato Thrust Front was more recent than in the Central and Western 
Monferrato. 
The subsidence of the northern Alessandria piggyback basin, that lasted until the late Middle Pleistocene or the early Upper Pleistocene, 
would be a consequence of the shift towards the north of the thrust front.  
The horizontal and vertical deformation rates obtained by the Alessandria GPS station seem compatible with the hypothesis of a shift to the 
north and with a subsidence still in progress. The morphology and distribution of the most recent terraces suggest that part of the Po Plain 
may have been affected by Upper Pleistocene and Holocene tectonic movements. These areas include Casale Monferrato, which under-
went a remarkable aseismic uplift during the twentieth century.  
 
KEYWORDS: Terraces, Quaternary evolution, Monferrato thrust front, transverse fault, Po Plain, Piedmont, Italy. 

1. INTRODUCTION 
 

Recent studies on the buried tectonic structures 
covered by fluvial sediments of the Po Plain have sug-
gested that the Monferrato Thrust Front, despite being 
subject to very low seismic activity, may be considered 
a seismogenic structure capable of producing earth-
quakes of high magnitude. 

According to Burrato et al. (2012) and Michetti et 
al. (2012), the present seismicity of the area could be 
characterized by earthquakes with long return periods. 
However, according to other authors (Costa, 2003; Pieri 
& Groppi, 1981; Cassano et al., 1986; Galadini et al., 
2012; Galli et al., 2012), the compressive tectonics of 
the Monferrato front ended in the Lower or Middle Pleis-
tocene. 

In order to verify which of the two hypotheses is 
more reliable, a detailed study (Giraudi, 2014) has al-
ready been conducted on the evolution of the Vercelli 
Plain, below which the central portion of the Monferrato 
Front is present. 

The data have indicated that the compressive 
tectonics could have ended before 280-300 ka BP, 
probably around 400 ka BP, but that tectonic move-
ments, related to structures transversal to the front, 
probably lasted until the late-Holocene. 

The present paper is intended to highlight the evo-
lution of the plain lying on the eastern portion of the 
Monferrato Thrust Front (Fig. 1), in order to identify and 
provide evidence of recent tectonic deformations and to 
check whether tectonic mobility is comparable to that of 
the Vercelli Plain. The investigated area is located south 
of the Po river between Casale Monferrato and the area 
SE of Valenza and lies almost entirely between the bur-
ied Monferrato Thrust Fronts, to the north, and the Mon-
ferrato Hills to the south. The presence of some large 
terraces makes the plain of the study area markedly 
different from the plain lying on the northern slope of the 
westernmost Monferrato-Turin Hills system. 

The geomorphological and geological characteris-
tics of the Casale Monferrato-Valenza area, therefore, 
may have been conditioned by a different tectonic mobil-
ity. For this reason, the geological structure of the terti-
ary bedrock lying below the Quaternary cover and some 
peculiar geomorphological and geological features of the 
Eastern Monferrato Hills are presented and discussed 
before the data on Quaternary sediments.  

The interpretation of the evolution of the plain re-
veals Quaternary tectonic deformations, related to the 
activity of fronts, which are difficult to prove in the Mon-
ferrato Hills because the outcrops are very scarce and of 
poor quality. 



 

   

 

 

 

2. GEOLOGICAL CHARACTERISTICS OF THE BED-
ROCK 
 

The Monferrato, together with the Turin Hills, 
represents the northernmost part of the Apennines and 
is formed by folded marine Tertiary formations. The 
Monferrato Hills overlie the Plio-Quaternary thrust belt 
separating the Tertiary Piedmont Basin from the Meso-
zoic and Cenozoic succession underlying the fluvial Po 
Plain (Fig. 1).  

To the north of the Monferrato, the Po Plain corre-
sponds to a foredeep developed as a result of the 
propagation of north-verging tectonic fronts located on 
the southern side. According to Dela Pierre et al. (1995) 
the Po Plain foredeep was subject, until the Quaternary, 
to subsidence which reflects the deformation of the front 
of the Apennine thrust belt. 

The sedimentary sequences lying south of the 
Turin and Monferrato Hills were deposited inside piggy-
back basins (Irace et al., 2009; Mosca et al., 2010). 
South of the study area, a deep piggyback basin was 
formed around Alessandria. 

 
2.1. Stratigraphy and tectonics of the tertiary sedi-
ments forming the bedrock of the plain and the iso-
lated hills based on bibliographic data 

The bedrock lying below the fluvial sediments of 
the plain, i.e. the sediments that form the buried Monfer-
rato Thrust Front, consists of the same tertiary marine 
sediments (Braga & Ragni,1969; Corsi et al., 1969) that 
form the adjacent hills and outcrop in the three isolated 
hills of Occimiano (RIO), Mirabello (RIM), 
and Pomaro-Montevalenza (RIP). 

Some seismic sections (Corsi et al., 
1969; Montrasio et al.,1969; Pieri & 
Groppi, 1981; ENEL, 1984; Cassano et 
al., 1986) indicate the position of the Mon-
ferrato Thrust Fronts buried below the 
Quaternary deposits (Fig. 2). The north-
ernmost and oldest front was active during 
the Lower Pliocene while others are 
known to have been active also during the 
Quaternary (ENEL, 1984). 

The northernmost front is always 
placed beyond the Po river, that is, outside 
the study area, except in a small stretch. 
One of the seismic sections (Corsi et al., 
1969) shows the presence of a buried 
(thrust?) fault near Occimiano (Occimiano 
Fault, FO in Fig. 2), linked to the struc-
tures of the front. Costa (2003) suggests 
that in the subsurface of the plain of the 
area studied there is also a SW-NE trend-
ing transcurrent or transpressive fault 
crossing the Monferrato Front (Giarole 
Fault, FG in Fig. 2).  

Cassano et al. (1986) show, in their 
section N. 4, a syncline (called Villabella 
Syncline in the present paper, VS in Fig. 
2) involving Tertiary and Quaternary sedi-
ments lying in the area between the RIP 
and the Monferrato Hills. The syncline is 
also proved by the geological structure of 

the RIP, which forms its northern margin (Casnedi, 
1975), and by the Late Messinian-Early Pliocene (?) 
“Conglomerati di Cassano Spinola” sediments, outcrop-
ping on the slope of the Valenza Plateau at Castel Me-
nada (NE of Valenza; Fig. 2), which are part of the 
southern margin. 

The studies reported in ENEL (1984) exclude that 
the sediments exposed at the base of the escarpment of 
the Valenza Plateau, at least from Castel Menada east-
wards, are marine, as supposed in previous papers 
(Martinis, 1954; Corsi et al., 1969) but suggest that they 
are continental and can be correlated to the Villafran-
chian succession. 

The bedrock of the isolated hills located west and 
east of the Giarole Fault is made up of different sedi-
ments (Corsi et al., 1969). The isolated hills RIO and 
RIM, located to the west of the fault, are formed by Plio-
cene marine sediments while the isolated hill RIP, lo-
cated to the east, consists, according to Casnedi (1975), 
of Tortonian, Messinian and Pliocene sediments. The 
area where Pliocene and Messinian deposits outcrop at 
the SE slope of the RIP is very small and cannot be 
represented at the scale of Fig. 2. 

From subsurface data, reported in the Structural 
Model of Italy (Bigi et al., 1990), it is possible to estimate 
the depth of the bottom of the Pliocene in the area be-
tween the hills of the Eastern Monferrato and the Po 
river. Fig. 3 shows the contour lines of the depth of the 
Pliocene base, modified according to surface data and 
the interpretation in section 4 of Cassano et al. (1986) 
showing that the Pliocene is lacking in the area just 

 
 
72 

 
 

Giraud C. 

Fig. 1 - Geological sketch of the Monferrato-Turin Hills and depth of the base of the 
Pliocene (redrawn from Dela Pierre et al., 2003a). 

 



 

   

 

 

 

north and NE of the RIP. 
The base of the Pliocene reaches a depth of about 

3000 m north of the outermost and older thrust front, 
while the depth slightly exceeds 500 m south of the 
front. The contour lines reported by Bigi et al. (1990) 
show that the Pliocene base is deeper in correspon-
dence with the axis of the Villabella Syncline and that 
the axis trends NW-SE and W-E. The contour lines as-
sume a SW-NE direction in correspondence with the 
SW-NE trending Giarole Fault, which crosses the Mon-
ferrato Front.  

 
 

2.1.1. Extended tectonic interpretation based on the 
contour lines of the base of the Pliocene 

The trend of the contour lines of the Pliocene base 
(Fig. 3) can be interpreted in order to hypothesize the 
presence of other buried structures. As reported above, 
the contour lines evidence the thrust fronts, the trends of 
the Villabella Syncline and of the Giarole Fault. It follows 
that the contour lines showing a depression of the Plio-
cene base indicate the presence of synclines or tectonic 
troughs, while the contour lines crossing the Monferrato 
Front could suggest the presence of faults. 

Near Casale Monferrato and NE of the RIP, the 
contour lines of the depth of the Pliocene base are 

 
 

73 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 

Fig. 2 - Geological map of the Central-Eastern Monferrato and surrounding plains (modified from: Montrasio et al.,1969; Corsi et al.,1969). 
The fronts drawn in the figure are reported in Montrasio et al. (1969), Corsi et al. (1969), Pieri and Groppi (1981), ENEL (1984), Cassano 
et al. (1986). 



 

   

 

 

 

roughly parallel to the hillside and to the thrust front, 
while in the intermediate area they assume a transver-
sal direction (Fig. 3) and indicate the presence of a de-
pression, a syncline (Borgo San Martino syncline, 
SBSM in Fig. 3) with the axis oriented from SW to NE. 
The Borgo San Martino Syncline is transversal to the 
Villabella Syncline (Fig. 3). The eastern edge of the 
Borgo San Martino Syncline, in position and direction, 
coincides with the SW-NE trending Giarole Fault. The 
western edge of the syncline, which is parallel to the 
eastern one, could suggest the presence of another 
fault (Roncaglia Fault, FR in Fig. 3). It cannot be ex-
cluded that the tectonic depression called Borgo San 
Martino Syncline was a trough bounded by the faults 
trending SW - NE. The Giarole Fault acts as a boundary 
between the synclines trending SW-NE and NW-SE. 
The change in the direction of the tectonic structures 
occurring in correspondence with the Giarole Fault con-
firms the importance of the fault for the tectonic evolu-
tion of the area. 

The NW-SE trend of the isobaths in correspon-
dence with the Occimiano Fault suggests that south of 
the (thrust?) fault the base of the Pliocene is uplifted. 
The features of the Pliocene base near the Occimiano 
Fault are quite similar to those observed near the north-
ernmost thrust fronts (Fig. 3) and suggest that the fault 
is a stretch of a buried front.  

East of the RIP (Fig. 3), the contour lines of the 
depth of the Pliocene base assume a direction transver-
sal to the thrust fronts, as in correspondence to the 

Giarole Fault: therefore the N-S contour lines may evi-
dence a fault trending N-S (Valenza Fault). 
 
2.1.2. The “platea” below the Po Plain fluvial sedi-
ments 

In the plain between Casale Monferrato and 
Valenza, as in all the whole plain surrounding the north-
ern Monferrato, there are areas where the Tertiary sedi-
ments lie less than 20 m below the ground surface (Fig. 
3). In those areas, the top of the Tertiary bedrock, ero-
sional in origin and shaped by rivers, is represented by a 
nearly flat surface, or by different surfaces located at 
altitudes slightly different, known in the Italian literature 
as "platea" (ENEL, 1984b; Carraro et al., 1995; Dela 
Pierre et al., 2003a,b; Festa et al., 2009). Quaternary 
fluvial sediments of various ages overlie the platea. 

The stratigraphic data from boreholes indicate the 
extent of the platea, already partly represented in ENEL 
(1984), Dela Pierre at al. (2003a,b), ARPA Piemonte 
(2014) and Giraudi (2014). Figure 3 shows that north of 
the Monferrato, but also around the Turin Hills (Festa et 
al., 2009), the limits of the platea are always next to the 
thrust fronts or structures crossing them.  

Outside the platea, the Quaternary fluvial and flu-
vioglacial sediments become thicker, indicating that the 
platea edge is a boundary between areas that have 
undergone a different evolution. Hence, during the Qua-
ternary, the area outside the platea was mainly in subsi-
dence, as demonstrated by studies on the westernmost 
plain (ENEL, 1984a; Dela Pierre at al., 2003a,b; Giraudi, 

 
 
74 

Fig. 3 - Tertiary bedrock covered by Quaternary fluvial sediments less than 20 m thick (platea), contour lines of the base of the Pliocene, 
and tectonic structures north of the Central and Eastern Monferrato Hills.  

 
 

Giraud C. 



 

   

 

 

 

2014). 
Fig. 3 shows that in the west, from Crescentino to 

Tricerro, the platea ends near the thrust front, reaching 
a distance of 10 km from the northern Monferrato hill-
slope. The eastern margin of the platea lies on the 
Salera Line, a complex N-S striking tectonic structure, 
according to Giraudi (2014). 

In the area of Casale Monferrato and south of 
Balzola, the platea ends almost in coincidence with a 
trust front very close to the Monferrato hillslope. 

In the Occimiano-Giarole area the northern margin 
of the platea lies on the Occimiano Fault, while, from 
Giarole to Bassignana, the platea ends near the thrust 
front north of the RIP, narrowing toward Bassignana. 

In the area SE of Casale Monferrato, Occimiano 
and Giarole, the margins of the platea are also oriented 
SW-NE, transversal to the direction of the fronts. Near 
Giarole, the position of the margin of the platea almost 
coincides with the NE part of the SW-NE trending 
Giarole Fault.  

 
2.1.3. Hyphotheses regarding the tectonic origin of 
the platea boundaries  

The data reported above show that the platea 
edge is a boundary between areas that have undergone 
a different evolution, and that the area outside the pla-
tea is mainly in subsidence. Moreover, some stretches 
of the platea edge lie near some buried thrust fronts and 
correspond to the Occimiano and Giarole faults.  

SE of Casale Monferrato, the margin of the platea 
is parallel to that corresponding to the Giarole Fault, and 
lies on the SW-NE striking Roncaglia Fault, hypothe-
sized using the contour lines of the base of the Plio-
cene. The coincidence between two different data al-
lows us to consider the presence of the Roncaglia Fault 
to be very likely. 

The distance between the outer edge of the platea 
and the hillslope (Fig. 3) indicates a great difference 
between the areas separated by the Giarole Fault: to 
the east of the fault the platea expands strongly north-
wards beyond the Po river, seven kilometres further 
than in the western area. The activity of the fault, there-
fore, had an important role in determining the platea 
boundary. The position of the boundary may have been 
produced by left-slip displacements along the Giarole 
Fault or by the uplift of the area east of the fault. 
Transcurrent or oblique movements may indicate a con-
tinuation, even in the Quaternary, of the faults activity 
linked to the northward migration of the front of the Mon-
ferrato suggested by Costa (2003).  

West of the Roncaglia Fault, the platea expands 
and its limit is about 5 km further north than in the area 
of the isolated hills RIO and RIM. The position of the 
edge of the platea is opposite to that produced by the 
Giarole Fault. The Roncaglia Fault could have been 
characterized by a right-slip activity (Fig. 3) and have 
been involved in the northward migration of the thrust 
front or could have produced the uplift of the area lying 
NW of the fault. 

The Giarole and Roncaglia faults, thus, coincide 
with margins of the platea transversal to the direction of 
the fronts. Based on this datum, the correspondence 
between some stretches of the margin and other buried 

tectonic structures can be hypothesized.  
Also near the NW margin of the RIO, the edge of 

the platea, west of the Rotaldo Stream, is about 1 km 
further south. This suggest the possible presence of a 
fault directed SW-NE near the streambed corresponding 
to the Rotaldo Fault (FdR in Fig. 3). Given the limited 
shift, it could be a fault with a left-slip component or a 
normal fault. 

Overall, the subsurface data indicate that the 
structures which affect the margins of the platea were 
active during the Quaternary and that the whole area 
external to the platea has undergone a certain degree of 
subsidence.  
 
2.2. Morphology, stratigraphy and tectonics of the 
Eastern Monferrato Hills 

Some observations useful to understand and con-
firm the hypothesized geological structures buried below 
the Quaternary sediments derive from the interpretation 
of the morphology and the known geological structure of 
the adjacent hills (Fig. 2). 

The divide between the Po and Tanaro river ba-
sins is arcuate in shape, first running from WSW to ENE, 
then approximately W-E and finally NW-SE, and is sub-
parallel to the trend of the boundary between the Ales-
sandria Plain and the southern slope of the hills.  

However, the general trend of the divide under-
goes noticeable local variations (Fig. 2): there are in fact 
stretches with a SW-NE direction in the west, and a 
NNW-SSE direction in the east. 

The directions of the valleys of the Rotaldo and 
Grana streams are quite peculiar (Fig. 2). The high val-
leys are directed NW-SE and WNW-ESE, but the Ro-
taldo Valley, north of Vignale, changes direction toward 
NE, the same occurs at the Grana Valley SW of Vignale 
and west of Lu. 

The geological features of the Eastern Monferrato 
Hills are reported in Bonsignore et al. (1969), Braga & 
Ragni (1969), Corsi et al. (1969) and Montrasio et al. 
(1969). The sediments that form the hills ranging from 
SSW to W of Mirabello (Fig. 2) include almost all the 
formations recognized in the Monferrato Hills and their 
age is between Upper Cretaceous and Pliocene. The 
oldest sediments are attributed to the Ligurian Units. It 
should be noted that the scarcity of outcrops in the hills 
allowed the Authors to recognize the general geological 
features but hardly to analyze in detail the characteris-
tics of the structures. 

According to Braga & Ragni (1969) in the lower 
Grana Valley there is a fault (the Grana Fault in the pre-
sent paper, FdG in Fig. 2) having the same direction as 
the valley (SW-NE), and therefore transversal to the 
main structures of the Monferrato area. 

On the northern hillslope, between Terruggia and 
the Rotaldo stream, the Pliocene sediments always 
overlie the Messinian ones, and are folded by synclines 
having axes trending SW-NE (Castello di Lignano Syn-
cline- SCL in Fig. 4) and NW-SE (Ponara and Terruggia 
Synclines, SRP and ST in Fig. 4).  

Between the Rotaldo and Grana Valleys the Plio-
cene sediments overlap on pre-Messinian sediments 
(Fig. 4), their eastern boundary lies near the Grana Fault 
and the western one close to the possible southern pro-

 
 

75 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

 

 
 
76 

Fig. 4 - Geological map of the Po Plain, Monferrato Hills and northern Alessandria Basin. The geological structures active during the Qua-
ternary lie from the Po Plain to the southern boundary of the Monferrato Hills, probably reaching the northern Alessandria Basin. 

 
 

Giraud C. 



 

   

 

 

 

longation of the Rotaldo Fault (Fig. 4). 
The hills to the east of the Grana Valley (south of 

Mirabello) and as far as the area of Valenza are com-
posed mostly of Miocene sediments (mainly Tortonian 
and Messinian) and there are no outcropping Pliocene 
marine sediments on the northern slopes. 

In addition, a seismic section through the area, 
reported in Cassano et al. (1986), highlights the lack, 
even in the subsoil, of the Ligurian Units outcropping a 
few kilometres to the west (Fig. 4). Starting from south 
of Valenza, the hills are formed by Eocene to Miocene 
sediments affected by diapirs and by Messinian and 
Pliocene marine sediments. The Pliocene sediments are 
lacking on the northern hillslope.  

According to ENEL (1984b), NW of Rivarone lie 
also calcareous marls pertaining to the Ligurian Units. 

Overall, there is a clear difference in the stratigra-
phy of the areas located SW and W of Mirabello, those 
located from SE of Mirabello and Valenza, and from 
Valenza to Rivarone, and the boundaries lie on the pos-
sible south-western prolongation of the Giarole Fault, 
and southern prolongation of the Valenza Fault.  

It can be observed that the stratigraphy of the 
sediments forming the easternmost hills appears similar 
to that of the western hills, while the stratigraphic anom-
aly is restricted to the area between the southern pro-
longation of the Giarole and Valenza faults.  

West of the possible prolongation of the Giarole 
Fault a stratigraphic anomaly can be observed also in 
the southern slope of the Monferrato hills, between the 
area south of Vignale and Lu. The Pliocene sediments 
outcropping on this stretch of hills overlie pre-Messinian 
sediments (Fig. 4), a case almost unique in the whole 
southern slope of the Monferrato and Turin Hills. Only 
near Castelnuovo Don Bosco and Passerano-
Marmorito, at the southern boundary of Rio Freddo De-
formation Zone (Dela Pierre et al., 2003a; Festa et al., 
2009) a similar contact was observed. The eastern limit 
of the area of anomalous contact between Pliocene and 
pre-Messinian sediments lies on the possible prolonga-
tion of the Giarole Fault while the western one is on the 
possible southern prolongation of the Rotaldo Fault. A 
deviation of the divide of the Rotaldo and the change in 
the direction of the Grana and Rotaldo Valleys, occur in 
the possible southern prolongation of the Rotaldo Fault, 
north and south of Vignale.  

Between Conzano and Lu, according to Corsi et 
al. (1969) and Montrasio et al. (1969) the axes of the 
anticlines, oriented WNW-ESE, are folded, assuming a 
SW-NE direction in correspondence with the possible 
SW prolongation of the (left transcurrent) Giarole Fault 
(Fig. 4).  

To the west of the possible southern prolongation 
of the Giarole Fault, the Tortonian and Messinian sedi-
ments surround to the south the oldest sediments that 
form the Monferrato and dip towards the Alessandria 
basin, but east of the possible prolongation of the fault 
they advance considerably northwards forming also the 
northern slope of the hills and the southern slope of the 
RIP (Fig. 4).  

East of the possible prolongation of the Giarole 
Fault until the area south of Valenza, the Tortonian and 
Messinian sediments form an anticline with an axis cor-

responding to the highest part of the hills. In no other 
sector of the Monferrato or the Turin Hills do late Mio-
cene sediments form the highest hills. The anticline has 
a WSW-ENE trend, but to the east it is oriented WNW-
ESE. The anticline lying north of Montecastello is formed 
by diapiric structures aligned NNW-SSE. This anticline 
therefore forms a nearly symmetrical structure with re-
spect to the anticlines lying near Lu (Fig. 4). Near the 
Montecastello anticline, the divide between the Po and 
Tanaro basins undergoes some noticeable local varia-
tions assuming NNW-SSE directions. 

On the northern prolongation of this anticline lies 
the hypothesized Valenza Fault having a similar direc-
tion. 
 
2.2.1. Tectonic interpretation of the geological and 
morphological features 
 

The joint interpretation of the morphological and 
geological features of the Eastern Monferrato Hills sug-
gests that the hills are interested by the same kind of 
deformations affecting also the bedrock of the Po Plain. 

As a matter of fact the anomalous contacts be-
tween Pliocene and pre-Messinian sediments, the distri-
bution of the Tortonian and Messinian sediments, the 
presence or absence of the Ligurian Units, the folded 
anticlines, the sudden deviations in the divide and the 
change in the direction of the Grana Valley, are consis-
tent with a left-slip displacement produced by tectonic 
deformations lying on the southern prolongation of the 
Rotaldo and Giarole faults and with one or more right-
slip faults or plastic deformations lying on the southern 
prolongation of the Valenza Fault. But, according to 
Corsi et al. (1969) and Montrasio et al. (1969) there are 
not important surficial faults that produce the tectonic 
deformations and therefore we can assume the pres-
ence in the hills of plastic deformations induced by deep 
tectonic structures. 

The Giarole Fault, and the tectonic deformations 
lying on its SW prolongation, stretches from the Monfer-
rato Thrust Front NE of the RIP to the boundary be-
tween the hills and the Alessandria Basin: acting as a 
boundary between geologically different areas both be-
low the plain and on the hills it can be considered the 
most important tectonic structure affecting the area. The 
complexity of the tectonic deformations observed in this 
area suggests the presence of a deformation zone 
(Giarole-Lu Deformation Zone, GLDF in Fig. 4). 

Also the tectonic structure formed by the Valenza 
Fault and the associated tectonic deformations lying 
south of it reaches the boundary between the hills and 
the Alessandria Plain and suggests the presence of a 
deformation zone (Valenza Deformation Zone, VDZ in 
Fig. 4).  

The Rotaldo Fault probably is a normal fault that 
starts near the Occimiano (thrust) Fault, continues to-
ward SW, and reaches the southern slope of the Mon-
ferrato Hills.  

Changes in the axes of the synclines similar to 
those occurring in correspondence to the Giarole Fault 
occur also on a possible southern prolongation of the 
SW-NE trending Roncaglia Fault (Fig. 4). In fact, the 
axis of the Ponara Syncline (lying west of the fault) is 

 
 

77 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

oriented WNW-ESE, while the axis of the Castello di 
Lignana Syncline (east of the fault) is SW-NE. 

The Eastern Monferrato is not the only place in the 
Monferrato-Turin Hills where the southern slope is af-
fected by deformation zones transverse to the hills: also 
the T. Traversola Deformation Zone, shows a strike slip 
nature (Forno et al., 2015; Gattiglio et al., 2015). 
 
3. MORPHOLOGICAL FRAMEWORK AND QUATER-
NARY STRATIGRAPHY OF THE PLAIN BETWEEN 
CASALE MONFERRATO AND VALENZA 
 

General indications on the Quaternary cover of the 
study area are shown by Anfossi et al. (1969), Bon-
signore et al. (1969), Braga & Ragni (1969) and Montra-
sio et al. (1969). During the 80’s of the twentieth century 
(ENEL, 1984), the area was also included in a mor-
phostratigrafic study conducted during the research for 
the seismotectonic characterization of nuclear sites, and 
the results are partly discussed in Carraro (2012). 

The main streams flowing in the area investigated 
are the Rotaldo and its tributary the Ponara, and the 
Grana, which flow into the Po river. 

In the Rotaldo and Grana Valleys, clear remains of 
fluvial deposits different than those that form the valley 
floor have not been found. Outside the valleys, the 
streams cut into terraces lying at different altitudes. 

The presence of some large terraces makes the 
plain north of the Eastern Monferrato Hills markedly 
different from the plain lying at the northern slope of the 
westernmost Monferrato-Turin Hills system (Fig. 2). 
Moreover in the area covered by this study, the Po lies 
at some distance from the hills while from Turin to 
Casale Monferrato it always flows near the foot of the 
hills.  

The presence of the isolated hills at the boundary 
between the belt of high terraces and the Po Plain is 
also exceptional. The only other isolated hill to the north 
of the Monferrato area is located further west, near 
Trino. 

These features thus make the plain to the north of 
the Eastern Monferrato peculiar compared with the 
westernmost plain. The same conclusion was reached 
observing the stratigraphy and the tectonics of the bed-
rock.  

The morphology of the plain makes it possible to 
identify three areas with different characteristics: the 
isolated hills, the high terraces, and the lower terraces 
of the Po river. 

 
3.1. The isolated hills 

The isolated hills of Pomaro-Montevalenza (RIP), 
Mirabello (RIM) and Occimiano (RIO) are small hilly 
areas, consisting mainly of Tertiary sediments, isolated 
among the fluvial terraces (Fig. 2; 4). 
 
The isolated Pomaro-Montevalenza hill (RIP) 

The RIP is a hill, elongated in a NW-SE direction 
for about 3.5 km having a width of 1.5 km, reaching an 
altitude of 165-170 m a.s.l., that is ca. 60-70 m above 
the surrounding terraced plain. It is located on the north-
ern edge of the Villabella Syncline (Fig. 4) and consists 
mainly of Late Miocene marls and clay, of marine origin 

(Casnedi, 1975). The RIP is divided into two parts by a 
valley that separates the hill of Pomaro from that of 
Montevalenza (Fig. 5 B). 

The NW area (Pomaro) has steep slopes having a 
NW, SW, SE and NE orientation, and a nearly flat top, 
located at altitudes between 150 and 155 m, and is 
eroded by small valleys. The valleys are present only in 
the SE and southern slopes. In the summit area some 
patches of very weathered silty colluvial sediments were 
found (colour index up to 2.5 YR of the Munsell Soil 
Colour Chart-MSCC). In the colluvia, sand and small 
pebbles have also been observed, and their lithology 
suggests an Alpine origin. In particular the pebbles are 
lithologically similar to the gravel outcropping in the Cer-
rina Valley (ENEL, 1984), outside of the study area, 
described by Giraudi (1981), Carraro et al. (1995), 
Giraudi et al. (2003), Dela Pierre et al. (2003a,b). 

The heterogeneous lithology of the deposits, their 
variable grain size and the presence of a fraction of 
gravel of Alpine origin, indicate that the colluvia derive 
from sediment of fluvial origin. The nearly flat summit is, 
therefore, the remnant of a terraced surface, originally 
shaped by a stream, for which there are no homologous 
terraces either to the north or on the Monferrato Hill 
slope. 

The part of the RIP on which Montevalenza lies 
reaches 165-170 m, and is extremely asymmetrical 
since it is limited by very steep slopes to the NW, NE 
and east, while it slopes gently towards the south. The 
stream network is developed only on the southern slope. 
The Quaternary deposits identified are colluvia consist-
ing of weathered silt, with a colour index reaching 2.5 
YR MSCC. The silts are present however only at alti-
tudes below 155-150 m, corresponding to the altitude of 
the top of the Pomaro hill.  

It is possible that at the time of the shaping of the 
flat top of the Pomaro hill the highest part of the Monte-
valenza area was already a small isolated hill. 

 
The isolated Mirabello hill (RIM) 

The RIM, formed by Pliocene sediments (Corsi et 
al., 1969), is elongated for about 1.5 km in a W-E direc-
tion, about 0.6 km wide, and divided into two parts by a 
small valley. The largest slopes run W-E, and are ex-
posed to the south and north (Fig. 5 A).  

The western part of the RIM reaches a height of 
161 m while the eastern one reaches 155 m. Some thin 
patches of weathered fluvial and colluvial silt, whose 
colour index reaches 2.5 YR MSCC, lie on top of the hill. 

Also the surface of the RIM could correspond to 
the remains of a terrace which has no homologous ter-
race on the Monferrato hillslope. According to Corsi et 
al. (1969), the surface of the RIM corresponds to the top 
of the oldest terrace in this area (see below) dated at the 
Mindel.  

 
The isolated Occimiano hill (RIO) 

The RIO is an approximately rectangular hilly area, 
consisting of Pliocene marine sediments, elongated for 
about 3.5 km in a NE-SW direction, about 1.5 km wide, 
which reaches a maximum height of 166 m, ca. 25-40 m 
above the surrounding terraced plain (Fig. 5 A). 

The slopes shaped by the Rotaldo and Grana 

 
 
78 

 
 

Giraud C. 



 

   

 

 

 

streams corresponding to the longer 
sides of the rectangle, are oriented 
SW-NE like the axis of the Castello di 
Lignano and Borgo San Martino syn-
clines, and the NW slope is parallel 
and lies near the Rotaldo Fault (Fig. 
4); the eastern one is parallel to the 
Grana Fault and the NE one is paral-
lel to the Occimiano Fault.  

The hill is eroded by small val-
leys directed mostly towards NW and 
SE, but also to the S and NNE, due to 
a centrifugal drainage pattern. Near 
the summit of the hill discontinuous 
outcrops of very weathered silty sedi-
ments have been observed (colour 
index up to 2.5 YR MSCC). The silt 
sometimes contains a gravel compo-
nent and the lithology of the gravel is 
similar to that of the fluvial sediments 
of the Cerrina Valley, Alpine in origin 
(ENEL, 1984). Artefacts from the 
Lower Paleolithic were collected to-
wards the base of one of these silty 
deposits (Giraudi & Venturino, 1983). 

Some colluvial patches with a 
different degree of weathering and 
colour index between 2.5 YR and 10 
YR MSCC have been observed on 
the slopes of the RIO. The redder 
colluvia have the same grain size as 
the summit colluvia having the same 
degree of weathering. 

The altitude of the various 
ridges is fairly constant (between 
about 160 and 165 m) and indicates 
that the top of the RIO was originally 
almost flat. If we consider also the 
fluvial origin of the gravel we can 
agree with Corsi et al. (1969) and 
Braga & Ragni (1969) that the summit 
of the RIO is a remnant of a mainly 
erosional fluvial terrace. Depending 
on the summit altitude, the terrace 
could have no homologous terrace 
either north or south. 

The survey of some artificial 
exposures has also shown a se-
quence of colluvial phases (Fig. 6). 
The oldest colluvia, up to 3.5-4 m 
thick, overlying a nearly flat erosion 
surface lying at an altitude of approximately 130 m, are 
very weathered, with a colour index of 2.5-5 YR MSCC. 
An intermediate colluvial layer, with a colour index of 7.5 
YR MSCC, rests on the older sediments. Some colluvial 
sediments that are little weathered (colour index 10 YR 
MSCC) overlie the intermediate layer. At the foot of the 
slope another sequence of colluvia was exposed. The 
oldest colluvia, about 3 m thick, with a colour index of 
5YR MSCC, lie on an erosional surface about 6 m be-
low the valley floor: they are covered by a second collu-
vial deposit with a colour index of 7.5 YR MSCC, on 
which lie the last, poorly weathered colluvia. 

The sequence of erosions and sedimentation of 
the colluvia, enables us to recognize the evolution of the 
slope. The stratigraphy indicates that, after the sedimen-
tation of the fluvial deposits preserved at the top of the 
hill, when the weathering was already very strong 
(2.5YR MSCC), a phase of erosion began which carved 
up the slope to an altitude of about 130 m a.s.l.. The 
erosion was followed by the emplacement of the collu-
vium. 

Later, at the top of the RIP, a new paedogenesis 
took place which produced a soil with a colour index of 5 
YR MSCC. The soil in turn was colluviated after the 

 
 

79 

Fig. 5 - Map of the terraces lying north of the Eastern Monferrato Hills. 1 - Monferrato 
Hills; 2 - Fluvial and Lacustrine Unit exposures; 3 - summits of the RIO, RIM and RIP 
isolated hills; 4 - S1 terrace; 5 - S2 terrace (a: covered by aeolian sediments; b: covered 
by flat fluvial fans); 6 - terrace S3; 7 - studied colluvial series; 8 - river diversion; 9 - 
anomaly in the hillslope-terrace S2 boundary; 10 - Villafranchian succession outcropping 
in the S2 terrace scarp; 11 - A-A’, B-B’, C-C’, D-D’ cross sections through the terraces S1 
and S2, and the top of the RIO reported in Fig. 8; 12 - boundary between hills and ter-
races. 

 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

erosion of the slope, produced by the 
presence of a small stream at its foot. 
Later a soil with a colour index of 7.5 YR 
MSCC evolved; this was eroded and collu-
viated, and the new colluvium covered the 
older one. Finally the last soil, with a col-
our index of 10 YR MSCC, developed and 
was later eroded producing the colluvia of 
the same colour. 

It is evident that the phase of erosion 
of the base of the slope (Fig. 6) decreased 
or ceased after the sedimentation of colluvium with a 
colour index of 5 YR MSCC and that the later phases of 
erosion and aggradation were weaker. 

 
3.2. The Quaternary sediments between the Eastern 
Monferrato hills and the Po river  

The sequence of Quaternary sediments lying be-
tween the northern edge of the Eastern Monferrato area 
and the Po river, in spite of the information derived from 
water wells, is rather difficult to study and date mainly 
because of the scarcity of exposures and sections that 
can provide adequate stratigraphic and chronological 
data. The sediments will be described starting from the 
oldest. 

 
Fluvial-lacustrine Unit  

The poorly exposed sediments that form this unit 
have been identified in two places: near the Castello di 
Lignano and in the area of Coppi (the Ponara Valley- 
Fig. 5A). 

Near the Castello di Lignano, about 140 m a.s.l., 
clay, silt and sand of fluvial-lacustrine origin, 5-6 m thick, 
have been identified, surrounded by Pliocene marine 
sediments (Sabbie di Valleandona, according to Montra-
sio et al.,1969). 

Given the poor quality of the exposure, the nature 
of the basal surface is not clear but, since the Pliocene 
sediments lie at higher altitudes, it is likely that the base 
of the fluvial-lacustrine sediments was carved in the 
underlying marine deposits. 

The mineralogical composition of the sands is 
similar to that of the fluvial deposits of the Lower and 
Middle Pleistocene of the Cerrina Valley (quartzites of 
various colours, gneiss and mica schist, fine-grained 
metabasites) studied by Giraudi (1981), ENEL (1984), 
Giraudi et al. (2001) and Dela Pierre et al. (2003b).  

The sediments described above are covered, with 
a clear erosional contact, by weathered sandy silt, col-
oured 5 YR MSCC, about 1-1.5 m thick. 

In the area of Coppi, at an altitude of about 160-
170 m, in the ploughed ground gravelly sand is ex-
posed, whose lithology is very similar to that of the sand 
found near the Castello di Lignano. Also in this area the 
bedrock is formed of Pliocene marine sediments. 

The fluvial-lacustrine and fluvial sediments of the 
two areas are isolated and there is no direct dating. 
However, the lithological composition (quartzites of vari-
ous colours, gneiss and micaschist, limestone, medium-
fine grained calcarenite) shows similarities to fluvial 
sediments of Alpine origin observed in the Cerrina Val-
ley, whose sedimentation began about 1 MA ago 
(Giraudi et al., 2003). 

We can therefore assume (in accordance with 
ENEL, 1984; Carraro et al., 1995) that the sediments 
were deposited by the same river that flowed through 
the Cerrina Valley and continued eastwards reaching 
the area under consideration. Sedimentation would have 
ceased at the time of the deviation to the north of the 
river, which occurred in the Cerrina Valley, around 600 
ka BP (Dela Pierre et al., 2003b). It follows that the sedi-
ments of the fluvial unit may be between about 1 and 0.6 
MA old. 

 
The terrace S1 and its sedimentary succession  

The terrace S1, recognized in the Ponara Valley 
and on the left side of the Rotaldo Valley (Fig. 4; 5A), is 
the oldest (Corsi et al., 1969), heavily eroded and 
formed by relict surfaces preserved as narrow and elon-
gated ridges covered by strongly weathered sediments. 

In the Ponara Valley the surface of the terrace S1 
dips from north to south and from NW to SE, that is to-
wards the axis of the Rio Ponara Syncline, while on the 
left side of the Rotaldo Valley it dips from W to E and 
from SW to NE, that is, towards the axis of the Castello 
di Lignano and Borgo San Martino Synclines. The rem-
nants of the terrace lie at altitudes between about 190 
and 135 -140 m a.s.l., ca. 30- 40 m above the valley 
floor of the Rotaldo and Ponara Streams. 

The sediments that form the surface of the terrace 
S1 consist mostly of weathered silt, with a colour index 
up to 2.5 YR MSCC. Horizons of fine gravel are some-
times interbedded in the silt, their lithology being similar 
to that forming the Fluvial-Lacustrine Unit but mixed with 
clasts of local origin. The sediments are fluvial and 
linked to a non-local river, according to Corsi et al. 
(1969). It can be assumed, therefore, that the clasts of 
Alpine origin derive from the erosion of the deposits of 
the older sedimentary unit.  

The estimated thickness of the sediments is al-
ways less than 2-3 m. Colluvia with the same lithology 
as the fluvial sediments are also present on the terrace 
scarps.  

According to Corsi et al. (1969) and Braga & Ragni 
(1969), fluvial sediments, weathered into a typical 
“ferretto", are covered by brick-red coloured loess a few 
metres thick. This results from outcrops no longer visible 
and indicates the presence of aeolian sediments much 
more weathered than those recognized further east (see 
below) described by Sacco (1889), ENEL (1984) and 
Corsi et al. (1969). The heavily weathered loess may be 
correlated to that found on the higher surfaces of the 
Isolated Trino hill (RIT in Fig. 3), reported by the Gruppo 
di Studio del Quaternario Padano- GSQP (1976) and 
ENEL (1984), assigned to the MIS 8 by Giraudi (2014) 

 
 
80 

 

Fig. 6 - Colluvial deposits on the RIO NW slope. 

 
 

Giraud C. 



 

   

 

 

 

on the basis of the stratigraphy. 
Therefore, sediments and soils similar to those 

found on the summit of the isolated hills RIO, RIM and 
RIP lie on the top of the terrace S1. The altitude of the 
eastern remnants of the S1 surface is slightly less than 
that of the RIO summit. According to Corsi et al. (1969), 
the sediments of S1 correspond to those found at the 
top of the isolated hills RIO and RIM. The age of the 
fluvial sediments, that date back to the Mindel according 
to Corsi et al. (1969), must therefore be more recent 
than the Fluvial-Lacustrine Unit, dated between about 
1000 and 600 ka BP, and older than the weathered 
loess (MIS 8, dated between about 300 and 270 ka BP). 
According to Arduino et al. (1984), soils with a colour 
index corresponding to 2.5 YR MSCC may have devel-
oped from the early Middle Pleistocene.  

 
The terrace S2 and its sedimentary succession 

The terrace S2 is significantly lower than S1 and is 
divided into three parts (Fig. 4; 5), being cut crosswise 
by the Rotaldo and Grana Streams. 

The first area where S2 is present (Fig. 4; 5A) 
corresponds to the left side of the Ponara and Rotaldo 
Valleys. The surface of the terrace dips from SW to NE, 
from about 160 to 140 m a.s.l., and must have been 
shaped by rivers that flowed in that direction. On the 
surface of the S2 terrace some quite extensive patches 
of weathered fluvial silts are preserved, with a colour 
index of 5YR MSCC, less than 1.5 m in thickness. Ac-
cording to Arduino et al. (1994) soils with a colour index 
of 5 YR MSCC would have developed during the Middle 
Pleistocene. 

The second area is between the RIO and the hill-
side, limited to NW and SE by the Rotaldo and Grana 
streams (Fig. 4; 5A). The position and dip of the surface 
of the terrace suggest that S2 has been shaped by the 
Rotaldo and by smaller hill streams. The altitude of the 
S2 surface is between 150 and 135 m a.s.l. 

The surface of the terrace S2 in this area is mainly 
eroded by streams that drain towards the NE and then, 
after forming an elbow, flow towards ESE. There is also 
a remnant of the S2 terrace on the western side of the 
RIO indicating that, in the course of the shaping of S2, 
the Rotaldo Stream changed direction and began to flow 
N. 

The third area where the terrace S2 is present, the 
largest one, is located east of the Grana stream, and 
corresponds to the area known as the Valenza Plateau. 
The plateau is strongly dismembered as a result of the 
incisions of streams active after its shaping. About the 
streams, we can distinguish the longitudinal ones (with a 
bed extending in the direction of elongation of the ter-
race S2) and the transversal ones. The longitudinal 
streams form the collectors into which flow the transver-
sal streams. They have approximately a WSW-ENE 
direction in the area south of RIM, W-E in the area south 
of the RIP, and then WSW-ENE from the RIP to 
Valenza. South of the RIP some stretches of longitudi-
nal streams overlap almost exactly the axis of the Villa-
bella Syncline (Fig. 4).  

The transversal streams that cut the terrace from 
the area south of the RIM to the area to the SW of the 
RIP, flow NNE and NE, while from the area south of the 

RIP to Valenza, they flow N and NNW. The streams on 
the RIP slopes, flow from NNW to SSE.  

East of Valenza the small valleys are generally 
directed NE and NNE. 

Among the streams cutting the Valenza plateau, 
the longest one is the Rio Anda (Fig. 4; 5). It flows in a 
small valley which, SW of the RIP, has a flat bottom 
formed by sandy silty sediments, while starting from S of 
the RIP, it becomes very narrow and deep, as all the 
streams in the area.  

Overall, the surface of the terrace S2, between the 
altitudes of 145 and 90 m, indicates a general slope 
from west to east and towards the northern quadrants.  

The greater height reached by the surface S2 at 
the foot of the hills is mainly due to the presence of a 
belt of flat fluvial fans and a layer of colluvial deposits 
that stretches from the Grana Valley to the eastern end 
of the plateau. One of the most extensive fluvial fans is 
located on the right bank at the mouth of the Grana val-
ley, showing that this stream also took part in the shap-
ing of S2 (Fig. 5). 

However, the presence of S2 at the western and 
northern edges of the RIM and on the right side of the 
Grana stream indicates that, during the shaping of the 
terrace, the stream changed its bed, heading N, bypass-
ing the RIM and flowing eastwards reaching the longitu-
dinal collector, oriented WNW-ESE, lying south of the 
RIP. 

Overall, the sector of S2 corresponding to the 
Valenza plateau was therefore shaped by the Grana and 
other smaller streams, but it is possible that, before a 
diversion, even the Rotaldo and its tributary Ponara 
flowed eastwards forming a much longer collector.  

The boundary between the S2 terrace and the 
hillslope shows some peculiarity. At the mouth of the 
Rotaldo and Grana valleys and of the valley of a minor 
stream, south of the RIM (Fig. 5A), in the eastern side of 
the valleys the boundary between S2 and the hillslope is 
located 800, 1000 and 500 m, respectively, further north 
compared to the western side. Instead, starting from the 
area SE of the RIM till the eastern end of the Valenza 
plateau (Fig. 5B), in correspondence with some small 
valleys, the boundary between surface S2 and the hill-
side is located, west of the streams, 500 or 600 m fur-
ther north if compared to the east. Hence the position of 
the boundary between the surface S2 and the hillslope 
does not seem random; the peculiarities of the boundary 
observed S and SE of the RIM, lie in the Giarole-Lu 
Deformation Zone (Fig. 4), while those lying near 
Valenza are located in the Valenza Deformation Zone. 

The sediments that form the terrace S2 consist, 
from top to bottom, of: 
- Coarse yellowish quartz sands of aeolian origin, de-

scribed by Corsi et al. (1969) and ENEL (1984), 
which form some small dunes less than 2-3 m high, 
present at the NE margin of the terrace. The shape of 
the dunes is no longer visible, probably because of 
the fields having been levelled for agriculture. The 
sands lie on the underlying sediments by means of 
an erosional contact. Similar dunes have been re-
ported lying on the fluvial sediments to the east of the 
Po river, a few kilometres from the Valenza plateau, 
and their age is considered as Upper Pleistocene 

 
 

81 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

Corsi et al. (1969) although it could reach the Holo-
cene (ENEL, 1984). 

- Unstratified silt, close to 2 m thick, already recog-
nized as loess by Sacco (1889), which can be di-
vided into two horizons between which there is a 
fairly sharp boundary (ENEL, 1984). The more surfi-
cial horizon, characterized by porosity and vertical 
prismatic fissures, is weathered by a soil with an 
average colour index of 10 YR MSCC. In the darker 
lower horizon, the prismatic structure is lacking, and 
there are carbonate concretions towards the base. 
Although there are no elements for a direct dating of 
these sediments, the loess of the Valenza plateau 
can be correlated to that widespread in Piedmont in 
the following areas: the Turin Hills, the Poirino pla-
teau, the Rivoli-Avigliana Moraine Amphitheatre, the 
isolated Trino hill, the area near Novi Ligure, and the 
Cerrina valley (Forno, 1979; Biancotti & Cortemiglia, 
1981; Giraudi et al., 2003; Festa et al., 2009; 
Balestro et al., 2009). The loess is generally attrib-
uted to the late Pleistocene. On the isolated Trino hill 
(Giraudi, 2014) loess with the same features contains 
some Upper Paleolithic artifacts while, in the Cerrina 
Valley, it contains Mousterian artifacts that can be 
dated to the very late Middle Pleistocene or to the 
Upper Pleistocene. We can therefore assume an 
Upper Pleistocene age for the loess of the Valenza 
plateau. 

- Silt and sandy silt, from 1.5 to 5 m thick, sometimes 
containing thin layers of fine gravel at the bottom, of 
fluvial origin; the lithological composition of the gravel 
shows that the pebbles have a mixed origin, Alpine 
and local, and that they probably derive also from the 
reworking of the oldest fluvial sediments. The depos-
its are weathered and the soil colour index is 7.5 YR 
MSCC. The sediments lie, through an erosion sur-
face, on the Tertiary marine and Villafranchian suc-
cession. Having been weathered before the sedimen-
tation of the loess, the fluvial sediments must be sig-
nificantly older than the aeolian deposits. The greater 
pedogenesis (5YR MSCC colour index) of the sedi-
ment lying on S2 in the valley of the Ponara and Ro-
taldo streams, where the described loess are lacking, 
should be connected, according to Arduino et al. 
(1984), to a soil that developed before the end of the 
Middle Pleistocene. 

The fluvial sedimentation in the Valenza plateau 
was followed by a phase of pedogenesis, an erosion 
phase and finally by the sedimentation of the loess. The 
evolution of the soil buried by the loess was therefore 
halted during the Upper Pleistocene. For this reason, 
the palaeosol is less evolved than the soil preserved on 
S2 near the Rotaldo Stream. It is therefore likely that the 
fluvial sediments lying on S2 date back to the latest 
Middle Pleistocene. 

 
The terrace S3 and its sedimentary succession 

The terrace S3, recognized at the mouth of the 
Rotaldo and Grana valleys, is poorly preserved and 
discontinuous, and its surface lies between 127 and 120 
m a.s.l. (Fig. 4; 5). No exposures indicate the stratigra-
phy and facies of the sediments. Only observation of the 
ploughed sediments has allowed us to establish that the 

top of the terrace is formed of silt, sometimes sandy, 
weathered by soil with a colour index of 7.5 YR MSCC. 

 
The sediments of the valley bottoms and the plain 
south of the Po River 

Between the Po river and the northern edge of the 
terraces S1, S2 and isolated hills, is the floodplain to 
which are connected the valley bottoms of the streams 
with catchments in the hills. The plain consists, basi-
cally, of a main surface (terrace T4) and a set of sur-
faces located at altitudes very similar to each other (for 
simplicity grouped under the acronym T5) that corre-
spond to the area shaped in recent times by the Po 
river, and in which abandoned meanders and river beds 
are clearly visible (Fig. 4). 

The bottoms of the Rotaldo and Grana valleys are 
connected with the terrace T4 at the mouth of the val-
leys, but further downstream the streams cut this sur-
face joining the terrace T5. All the streams that cut the 
Valenza plateau SE of the RIP are connected to the 
terrace T5. 

The morphological and geological features of the 
terraces, based on a field survey, small artificial expo-
sures, and stratigraphic data from quarries and bore-
holes, are the following. 

 
Terrace T4: It forms the largest sector of the area be-
tween the Po river and the Valenza plateau and isolated 
hills but is not present SE of the RIP. Its altitude ranges 
from about 120-125 m in the area of Casale Monferrato, 
to less than 100 m in the area near the RIP.  

The surface T4 is correlated, according to its alti-
tude, with a terrace lying north of the Po river formed by 
fluvio-glacial deposits, linked to the morainic amphithea-
tre of Ivrea, dated to the late Upper Pleistocene 
(Montrasio et al., 1969; Corsi et al., 1969; Giraudi, 
2014). 

The morphology of the terrace and the grain size 
of the sediments indicate that it has been shaped by the 
Po river. 

The surface consists of sandy gravel covered by 
sand and silty sand. It is possible that some of the silty 
sands that form the surface were deposited by the 
streams with catchments in the hills, and date back to 
the Holocene. 

The thickness of gravelly sandy sediments in the 
subsurface of T4 ranges from less than 10 m in some 
areas (Fig. 4) up to 50-60 m in the area north of the 
edge of the platea. 

In the subsurface, two fossil elephant teeth were 
found which unfortunately do not provide any significant 
chronological information. In a quarry on the outskirts 
SE of Casale Monferrato, under the groundwater level, a 
tooth (right second molar) of E. antiquus was found 
(determined by V. Borselli, pers. comm., 1983, in ENEL, 
1984). The depth is not known, but in the area the thick-
ness of the fluvial sediments is less than 20 m. On the 
western outskirts of the same town, a few metres below 
ground level, an upper left molar of E. primigenius 
(Negri, 1884) was instead found in the fluvial sediments. 

The age of the base of the sediments forming T4 is 
unknown, but it is very likely that, at least in the area 
where the sediments are thicker, that is north of the 

 
 
82 

 
 

Giraud C. 



 

   

 

 

 

edge of the platea, it is considerably older than that of 
the superficial deposits.  

The top of the T4 terrace is partially eroded and it 
is formed by three different surfaces that can be hardly 
recognized being separated by discontinuous scarplets. 

SE of Casale Monferrato (Fig. 4), there is a flat-
topped ridge (T4-1), about 2 km long and 500-600 m 
wide, slightly higher than the T4-2 surface. The ridge is 
limited by discontinuous very gentle scarps that do not 
reach one metre in height. 

In a temporary quarry (ENEL, 1984) it has been 
observed that the ridge is formed, at least to a depth of 
4 m, by slightly weathered fluvial sandy gravels quite 
similar to those which form the other sectors of T4.  

The best preserved portion of the T4 surface (T4-
2) surround the T4-1, but north and south of T4-2 lie 
surfaces whose altitudes are slightly lower (T4-3N; T4-
3S) (Fig. 4). T4-2 is between Casale Monferrato and 
Borgo San Martino-Ticineto; it has a NW-SE direction 
and its northern and southern limits, in places, coincide 
with or are parallel to the buried thrust fronts (Fig. 4). A 
section of the boundary between the T4-2 and T4-3N 
surfaces corresponds to the northern limit of the buried 
platea and the width of T4-2 decreases east of the east-
ern edge of the platea. 

The boundary between T4-2 and T4-3N is not 
represented by a real scarp, but just by a hardly notice-
able scarplet in the plain having a nearly straight direc-
tion. Although it is possible that the scarplets in the plain 
were connected with the erosion of the Po river, their 
morphology is totally different from that of the escarp-
ment, certainly shaped by the Po river, that separates 
T4-3N from the most recent T5 terrace. The latter is, in 
fact, much more evident and, especially, has been 
clearly shaped by a meandering river (Fig. 4). 

The erosion of T4-3S is certainly due to the Gat-
tola, Rotaldo and Grana streams. The streams gently 
eroded T4-2 and deposited surface silty sediments. The 
southern edge of T4-3S, especially where it reaches the 
scarp of the terrace S2 (Fig. 7), has a curvilinear shape 
as a consequence of fluvial erosion (Fig. 4). 

The direction of the beds of the Gattola and Ro-
taldo streams, now artificially diverted, was from SW to 
NE, but before they reached the southern edge of the 
T4-2 surface, they diverged sharply to the SE and ESE 
(Fig. 4).  

The mainly silty-sandy deposits of the Rotaldo and 
Grana valley bottoms are connected with the T4-3S 
surface. 

 
Terrace T5: as noted above, this is formed by a series 
of low river terraces shaped during the changes of the 
bed of the Po river (Fig. 4). In the contiguous Vercelli 
plain and just north of the Po river, the sediments that 
form these terraces contain Bronze Age, Roman and 
Middle Age artifacts and remains (Tropeano & Olive, 
1989; Giraudi, 1998; Giraudi, 2014). Therefore, the 
shaping of T5 took place during the late Holocene. 

From the area of Casale Monferrato to the RIP, 
the T5 surface develops closely parallel to the current 
bed of the Po river and lies between T4 and a terrace, 
equivalent to T4, sited N and NE of the Po river (Fig. 2). 
Only downstream of the RIP, where T4 is lacking, does 

T5 reach the base of the scarp of the Valenza plateau 
and the river flows in an asymmetric position, very close 
to the scarp. 
 
4. DISCUSSION 
 

The data presented above will be interpreted and 
discussed in order to highlight the sedimentary and tec-
tonic evolution of the area during the Quaternary. The 
results will be compared with geodetic data, derived 
from topographic measurements of precision and GPS 
stations, in order to verify the compatibility between tec-
tonic interpretations and data relating to the ongoing 
deformation. Finally, we will discuss the distribution of 
the epicentres of earthquakes recorded in the area and 
the correspondence between epicentres and areas af-
fected by tectonic movements. 
 
4.1. Sedimentary and erosive Quaternary evolution 

The data and the interpretations of the succession 
of phases, erosional, depositional and pedogenetic, 
enable the evolution of the area between the Po and the 
Eastern Monferrato Hills to be outlined. 

It should first be pointed out that, apart from the 
coarse fluvial sediments of the Po plain, on the whole 
the sediments found are quite scarce and lie on planar 
erosional surfaces shaped on the bedrock, and this sug-
gests that the terraces were mainly erosional.  

The terraces can be regarded as remnants of an 
uplifted and eroded platea.  

The erosional terraces ranging from S1 to S3, 
therefore, show the different stages of erosion. Also the 
base of the fluvial sediments in the southern area of 
terrace T4 is erosional. The sediments allow us to as-
sess that the shaping of the erosional terraces occurred 
during the period between the beginning of the Middle 
Pleistocene and the Upper Pleistocene. 

The incision of the valleys during the Quaternary 
may be linked to the gradual deepening of the erosion 
surface north of the hills, while the infilling of the Rotaldo 
and Grana valley bottoms may be connected mainly with 
the aggradation of the fluvial sediments that form the 

 
 

83 

Fig. 7 - The scarp between the S2 and the T4-3S terraces 
viewed from the S2 top. The scarp is in the foreground, while in 
the background lies the Pomaro isolated hill (RIP).  

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

surface of the terrace T4. 
During the late Upper Pleistocene or the Holo-

cene, the Po river, which flowed S and SE of Casale 
Monferrato (Fig. 4), eroded the S2 terrace scarp and 
captured some stretches of the streams that flowed 
across the Valenza plateau between the RIM and the 
RIO (Fig. 5). 

During the late Upper Pleistocene or the Holo-
cene, the river underwent a diversion to the east and 
began shaping the T5 surface. During the Holocene the 
river eroded the northern side of the RIP and the es-
carpment which limits the Valenza plateau to the N, and 
induced a deepening of the tributary valleys which cut 
into the plateau in that area. 

 
4.2. Evidence of tectonic deformations 

The existence of Quaternary tectonic deformations 
is proved by the Quaternary evolution and by the rela-
tions between some morphological features and the 
structures affecting the bedrock. 

The Monferrato Thrust Front is the most important 
tectonic structure having a regional extension that af-
fects the area. Since the limits of the buried platea 
mostly coincide with the younger thrust front or transver-
sal structures (Fig. 3) and outside the platea the Quater-
nary sediments become thicker, it can be assumed that 
compressive tectonics were active during the Quater-
nary. In particular, the stretch of the fronts situated near 
Casale Monferrato and NE of Pomaro would be the 
ones most involved in the deformation of the base of the 
Quaternary fluvial sediments, while the Giarole and 
Roncaglia Faults acted as boundaries of the subsiding 
Borgo San Martino Syncline or tectonic trough. 

Among other structures, the Giarole-Lu Deforma-
tion Zone is the one that produced the most evident 
effects because it separates stretches of plain and hills 
with different geological and geomorphological features.  

The interaction between the fluvial and tectonic 
Quaternary evolution could have produced the clearest 
morphological anomalies that are essentially the hills 
made of Tertiary sediments isolated between Quater-
nary fluvial deposits. 

The only other isolated hill present north of the arc 
formed by the Monferrato and Turin Hills is that of Trino 
(RIT in Fig. 3). It is located in the platea area just south 
of the Monferrato buried front, and it was definitely af-
fected by movements related to compressive tectonics, 
at least, up to part of the Middle Pleistocene (GSQP, 
1976, ENEL, 1984; Giraudi, 2014) and, according to 
Burrato et al. (2012) and Michetti et al. (2012), even in 
more recent times. 

In correspondence to the RIT, the bedrock is 
shaped by a series of planar erosion surfaces, covered 
by fluvioglacial deposits, located at altitudes gradually 
lower, both to the north and to the south, separated by 
buried scarps. The topographically higher surfaces are 
those covered by the older deposits. 

The isolated hills RIP, RIO, and RIM are also lo-
cated to the south of thrust fronts (Fig. 2; 4) and their 
edges are sub-parallel to thrust fronts or lie in corre-
spondence of faults transversal to the fronts. Also in the 
area studied, the bedrock is shaped by a series of 
nearly flat erosion surfaces (corresponding to the top of 

the isolated hills) lying at different altitudes and sepa-
rated by scarps. The higher surfaces were most likely 
produced by the Alpine river flowing from the Cerrina 
Valley while, after the diversion of that river (see above) 
dated around 600 ka BP, the erosional terraces S1, S2, 
S3 were shaped by local streams. 

There is little doubt, therefore, that the develop-
ment of the isolated hills is also connected with the Qua-
ternary tectonic evolution of the Monferrato, as already 
hypothesized in ENEL (1984) and Carraro (2012). The 
existence of Quaternary tectonic mobility makes it diffi-
cult to accept the hypothesis that near-flat surfaces 
forming the top of the RIO and RIM isolated hills and the 
S1 surface were shaped simultaneously as assumed by 
Corsi et al. (1969). The chronological framework of the 
sediments lying on such surfaces is too loose to support 
a clear correspondence between them. 

In the presence of tectonic movements, the ero-
sion surface that was evolving in the area between the 
thrust front and the hillslope was certainly subject to 
deformations and these deformations could have been 
different in areas bordered by the various buried struc-
tures. 

Some sectors could have been uplifted and in 
these the remnants of the erosional surface, covered by 
thin layers of fluvial sediments, were raised to form ter-
races higher than the surrounding area. The uplift in-
duced the streams to migrate to topographically lower 
places where they began to shape new portions of the 
erosional surface. 

The interference between tectonics and fluvial 
erosion would have produced an almost continuous 
evolution of the erosional surface and the different alti-
tudes at which the erosion terraces lies demonstrate 
this.  

A possible confirmation of the continuous erosion 
is also given by the almost total absence of terraces in 
the inner portion of the Rotaldo and Grana valleys. If the 
modelling of S1, S2 and S3 had occurred during three 
phases of erosion interrupted by periods of stasis, the 
three terraces would have formed also within the val-
leys. 

Thus the continuous down-cutting of the valley 
bottoms and of the erosion surfaces that formed the 
terraces allows us to hypothesize a nearly continuous 
tectonic uplift, common to the hills and to the adjacent 
terraced plain. 

Figure 8 shows a SSW-NNE section (A - A’) of S1 
on the left of the Rotaldo stream and the projection on 
the same section of the top surface of the RIO (section 
B - B’). From the B - B’ section we note that the top of 
the RIO, apart from being located at higher altitudes, 
does not dip towards NE as S1. 

This situation does not allow for an unambiguous 
interpretation. One could speculate that tectonic move-
ments have induced changes in the river network. After 
the shaping of the erosional surface that corresponds to 
the top of the RIO, tectonic deformation induced a diver-
sion towards the NE of the Ponara and Rotaldo streams 
that started to shape S1. Equally acceptable is the as-
sumption that both the top of the RIO and of S1 were 
shaped by a drainage network having the same direction 
and that the current slopes of the terraces were pro-

 
 
84 

 
 

Giraud C. 



 

   

 

 

 

duced subsequently by differential tectonics. In any 
case, the evolution implies Quaternary tectonic defor-
mations.  

Because the RIO’s north-western side lies near 
the Rotaldo Fault and is parallel to it and other major 
faults transverse to the Monferrato Front (Terruggia and 
Giarole faults) we can assume that the trend of that side 
was conditioned by the Quaternary activity of the Ro-
taldo Fault. 

The lack of dip towards NE of the top of the RIO 
(Fig. 8) may be due to the activity of the Occimiano 
Thrust Fault, located at the base of the north-eastern 
slope of the RIO, which would have produced the uplift 
of the area. 

After the shaping of the top of the isolated hills and 
of terrace S1, a stream fed at least by the Rotaldo and 
Grana valleys flowed to the ESE and east, in the area 
between the hills and the southern slopes of RIO, RIM 
and RIP, and started to form the S2 terrace. The S2 
terrace lies, at least up to Valenza, on the axial zone of 
the Castello di Lignano and Villabella Synclines. It can 
therefore be assumed that the synclines were active 
and affected the drainage network at least until the late 

Middle Pleistocene. 
The presence of several sectors of the S2 surface, 

separated by transverse river incisions and located at 
different altitudes, also suggests that the shaping of the 
terrace may have been complex, non-uniform and, in 
detail, not even contemporary in all its extension. In fact, 
a topographic profile oriented WNW to ESE (Fig. 8) 
shows that S2 is located at altitudes not compatible to 
the west and to the east of the Rotaldo and Grana faults.  

The diversion towards the NE of the Rotaldo and 
Grana streams shown by the S2 terrace, could evidence 
that the differential tectonics started to prevail. After the 
diversions and the sedimentation of the fluvial deposits 
that form the surface S2, the streams started to down-
cut the valleys. The colluvia observed on the RIO slope 
confirm the erosion and suggest that the main erosion 
phase ended with the aggradation of the present valley-
bottom and the alluvial sediments that occurred during 
the late Upper Pleistocene-Holocene. The erosion was 
probably triggered by the uplift of all the area where S2 
lies. In fact, if the shaping of S2 and of the older terraces 
was produced by the climatic cycles, a near continuous 
belt of terraces having the same age should surround 
the Northern Monferrato hillslope, from Casale Monfer-
rato to Turin: as explained before, such terraces do not 
exist. 

Therefore, the evolution of the drainage network 
suggests that the phase of deformation, due to struc-
tures connected to the thrust front, likely continued after 
the late Middle Pleistocene. 

The Tertiary sediments outcropping in the Po river-
bed (Fig. 4) near the thrust front north of the RIP (Fig. 
9), and west of Casale Monferrato seem to confirm that 
an erosion surface on the bedrock is evolving even to-
day and that the uplift near the front could still be active. 

The plain east and south-east of Casale Monfer-
rato deserves separate discussion because the evi-
dence of tectonic deformation is more ambiguous.  

The presence of an erosion surface (the platea) 
also under the sediments of the southernmost T4 ter-
race could suggest that the erosion, and then the uplift, 
acted up to the late Upper Pleistocene. However, since 
the shaping of this erosion surface is due to the Po river, 
it could be just lateral erosion of Tertiary sediments easy 
to erode.  

For this area (Fig. 4) the coincidence between the 

 
 

85 

Fig. 8 - Cross sections through the terraces S1, S2 and the top 
of the RIO. 

Fig. 9 - The Po river bed NE of the Pomaro-Montevalenza isolated hill. The small waterfalls in the river bed (right) evidence the plane-
parallel layers of Tertiary sediments. In the background (left), the northern slope of the isolated hill can be observed. 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

almost straight trend of a stretch of the boundary be-
tween T4-2 and T4-3N, and the buried structures at the 
margins of the platea, have already been highlighted. It 
is possible that the boundary between T4-2 and T4-3N 
is the consequence of a tectonic deformation and that 
the higher surface (T4-1) derives from the tectonic de-
formation of the top of T4. In this case, one should as-
sume a tectonic mobility more recent that the Upper 
Pleistocene. 

Even though it is parallel and in part superimposed 
on thrust fronts, the possibly uplifted area crosses the 
Roncaglia Fault and extends also in an area of prevail-
ing subsidence during the Quaternary. The uplift could 
have been produced by tectonic movements at least in 
part different from those linked to the Monferrato Front. 

The diversion to the east and NE of the Po river at 
Casale Monferrato (abandoned bed in Fig. 4) is com-
patible with the hypothesis of tectonic deformation of T4. 
However, there are no clear morphological elements 
that can confirm the uplift of the area in periods before 
the Upper Pleistocene. The absence of traces of an 
older uplift may be due to river erosion, but it can also 
be explained by assuming that the tectonic deformation 
started or intensified during the Upper Pleistocene. 

The location of T5 shows some elements worth 
being discussed. First, from Casale Monferrato to the 
area of the RIP, T5 is located between T4, to the south, 
and a terrace, equivalent to T4, to the north of the Po 
river (Fig. 2). From the RIP area to the SE, T4 is lacking 
and it may have been eroded or it was never formed as 
a result of the evolution of the hydrographic network. In 
this area T5 reaches the escarpment of the Valenza 
plateau (S2). 

Precisely in the area where it crosses the Monfer-
rato Front NE of the RIP, the Po river, which flowed into 
the central portion of T5, migrates to the SW following 
the scarp of the Valenza plateau to its end, and then 
flows to the east, again crossing the front. 

The distribution of the terraces and the position of 
the Po river suggest that the Late Pleistocene and Holo-
cene river migration could also be influenced by tecton-
ics and that the migration of the river to the S and SW 
was triggered by the uplift of an area east of the RIP. 
This uplift concerns an area partially outside the buried 
Monferrato Front and it is not clear whether the as-
sumed relative uplift is an indication of the ongoing com-
pressive tectonics or if it is linked to other causes. 

Overall, the Upper Pleistocene and Holocene mor-
phological elements that may suggest tectonic mobility 
are open to more than one interpretation. If they were in 
fact produced by tectonics, they would not be fully com-
patible with some older morphological features clearly 
conditioned by tectonics. 

The observations about the edge between the 
terrace S2 and the hillslope, mentioned earlier (Fig. 4; 
5), can be used in order to prove the relations between 
the tectonic activity and the morphology. The different 
position of the boundary between the terrace and the 
hillslopes at the mouth of some valleys could be due to 
the displacement of the hillslope produced by transver-
sal tectonics active before and, probably, also after the 
shaping of the T2 terrace. 

In correspondence with the Giarole-Lu Deforma-

tion Zone, at least two of the changes in the position of 
the boundary between S2 and the hillslope (Fig.4) occur 
NE of two abrupt changes in direction of the divide be-
tween the Po and Tanaro basins and of the deformation 
of the axes of two anticlines (Fig. 4). All these morpho-
logical and geological features are compatible with the 
movements of the left-slip Giarole-Lu Deformation Zone 
(GLDZ in Fig. 4). The Deformation Zone runs from the 
thrust front to the hills south of the watershed and 
reaches at least 15 km in length. 

The anomalies of the boundary between S2 and 
the hillslope in the area SSE of the RIP lie on the 
Valenza Deformation Zone (Fig. 4): the westernmost 
one lies north of a divide deviation and near the change 
of the direction of the anticline formed by Messinian and 
Tortonian sediments, while the easternmost one lies on 
the southern prolongation of the Valenza Fault, on the 
northwards prolongation of the NNW-SSE anticline and 
north of a noticeable deviation of the divide. These mor-
phological and geological features are all compatible 
with dislocations produced by right-slip Valenza Defor-
mation Zone (VDZ). The length of the deformation zone 
reaches, at least, 13-14 km. 

One of the anomalies in the boundary between S2 
and the hillslope is at the Rotaldo Valley mouth and lies 
on the Rotaldo Fault. South of this anomaly, on the pos-
sible prolongation of the same fault, there is a clear de-
viation of the divide and the western boundary of the 
area of anomalous contact between Pliocene and pre-
Messinian sediments.  

The joint activity of the Giarole-Lu (left-slip) and 
Valenza (right-slip) Deformation Zones would have pro-
duced the northward advance of the thrust front north of 
the RIP and of the area located between them.  

The direction of many streams across the Valenza 
plateau, connected with the Holocene T5 terrace, is the 
same as the geological structures forming the Giarole-
Lu and Valenza Deformation Zones. It follows that con-
tractional tectonics may have been still active during the 
Upper Pleistocene and Holocene or, more likely, that the 
streams have retained the direction induced by tectonic 
structures that were active during the late Middle Pleis-
tocene or early Upper Pleistocene. 

In summary, the phase of deformation related to 
tectonic contraction of the Monferrato Front lasted, most 
likely, until the late Middle Pleistocene or early Upper 
Pleistocene. Only in the areas of Casale Monferrato and 
of Frascarolo did some tectonic movements, which may 
not be linked with certainty to the Monferrato Front, pos-
sibly occur during the Upper Pleistocene and Holocene. 

Even in the adjacent Vercelli Plain, Upper Pleisto-
cene and Holocene deformations have been observed, 
partially or totally independent from the Monferrato Front 
(Giraudi, 2014). In the Vercelli Plain, where the sedi-
mentary units are much more abundant and better 
dated, the end of tectonic contraction took place mostly 
around or before 400 ka BP, and in any case could not 
be more recent than 280 ka BP, while the deformations 
independent from the thrust front were particularly in-
tense during the last 140 ka BP. 

The chronology of the Eastern Monferrato Front 
seems to indicate a significantly more recent age for the 
end of tectonic contraction. The morphological differ-

 
 
86 

 
 

Giraud C. 



 

   

 

 

 

ences between the two zones confirm the 
different tectonic evolution, excluding the 
risk that the different chronology of the 
movements was caused by the approxi-
mation of the dating of the sediments and 
not by a really different evolution. 

 
4.3. Tectonic zonation of the Po Plain 
and the Eastern Monferrato area 

Overall, the area where the higher 
terraces and isolated hills lie could thus be 
divided into four parts with different Qua-
ternary tectonic features (Fig. 10).  
- The easternmost area, from the 

Valenza Deformation Zone to Ri-
varone, is formed only by the terrace 
S2 and was uplifted at least after the 
late Middle Pleistocene or early Upper 
Pleistocene.  

- The eastern area, formed by the plain 
and the hills, situated between the 
Giarole-Lu and Valenza Deformation 
Zones and the thrust front north of the 
RIP, underwent a migration to the 
north, at least from the Lower Pleisto-
cene to the end of the late Middle 
Pleistocene or early Upper Pleistocene 
and an uplift that lasted until the period 
following the shaping of S2. In this 
framework, the Villabella Syncline was 
able to influence the late Middle Pleis-
tocene or Upper Pleistocene drainage 
network that developed on the S2 ter-
race.  

- The central area, i.e. the plain located 
between the Giarole-Lu Deformation 
Zone and the Rotaldo valley and the 
adjacent hills, is affected by the Ro-
taldo, Grana and Occimiano faults and 
the Borgo San Martino Syncline, and is 
disjointed into various sectors. The 
tectonic activity lasted until the late 
Middle Pleistocene or early Upper 
Pleistocene. The Roncaldo Fault forms 
the western boundary of the area. The 
Grana Fault seems a normal fault, be-
cause it is not associated with morpho-
logical anomalies suggesting horizontal 
displacements. In this tectonic frame, 
the Occimiano Fault may correspond to 
the thrust front segment that seems to 
be lacking between the fronts lying 
near Casale Monferrato and NE of the RIP. 

- The western area, i.e. the plain lying NW of the Ron-
caglia Fault and the adjacent hills, would have been 
subject to an uplift that started around 600 ka BP, 
after the sedimentation of the Fluvial-Lacustrine Unit 
dated between 1000 and 600 ka BP. Precisely that 
uplift could have halted the flow of the Alpine river 
that drained the Cerrina Valley. The uplift continued 
until the period subsequent to the shaping of S2. 
Even if the Roncaglia Fault could be of strike-slip or 
oblique type, in this area there are no morphological 

features suggesting the activity of strike slip or 
oblique-slip faults during the late Middle Pleistocene 
or later.  

The disarticulation in tectonic blocks of the area 
between the Giarole-Lu Deformation Zone and the Ron-
caglia Fault, and the subsidence of the Borgo San 
Martino syncline, may indicate that the central area 
represents a junction between the NW part, free of late 
Middle Pleistocene strike-slip movements and the east-
ern zone where such movements were active.  

While the activity of strike-slip faults ended earlier 

 
 

87 

 

Fig. 10 - Tectonic movements affecting the area studied until the late Middle Pleisto-
cene or early Upper Pleistocene. 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 

Edi
Evidenziato



 

   

 

 

 

in the western part (Roncaglia Fault) and did not occur 
east of the Valenza Deformation Zone, the uplift of the 
terraces and the hills seems mainly coeval from west to 
east.  

However the presence of the older morphological 
features (isolated hills and S1 terrace) only in the area 
west of the Valenza Deformation Zone suggests that the 
uplift of the (now) terraced plain started in the west and 
that the deformation zone probably played a role as a 
boundary of the uplifting area. 

The Po Plain, corresponding to the T4 and T5 
terraces (Fig. 4), shows Quaternary subsidence north of 
the thrust fronts and on the Borgo San Martino syncline. 
The subsidence is known to have affected the Po basin 
foredeep during the advance of the thrust fronts (Dela 
Pierre et al.,1994). However some morphological fea-
tures suggest the possibility of late Upper Pleistocene 
vertical movements near or outside of some stretches of 
the thrust fronts. 
 
5. TESTING THE HYPOTHESIS OF LATE MIDDLE 
PLEISTOCENE OR EARLY UPPER PLEISTOCENE 
THRUST FRONT ACTIVITY: OUTLINES OF THE 
QUATERNARY EVOLUTION OF THE ALESSANDRIA 
BASIN 
 

In order to test the hypothesis that compressive 
tectonics continued at least until the late Middle Pleisto-
cene or early Upper Pleistocene, we can analyze the 
Quaternary evolution of the Alessandria Basin sug-
gested by bibliographic data. 

As noted before, the Alessandria piggyback basin 
subsided as a result of the northward migration of the 
Monferrato Thrust. If the migration northward had con-
tinued until at least the late Middle Pleistocene or Upper 
Pleistocene, south of the Eastern Monferrato hills the 
geological evolution of the basin is expected to show 
evidence of subsidence up to the same period. 

The data for the interpretation of the evolution of 
the Alessandria Basin are reported in Anfossi et al. 
(1969). 

Observing the terraces in the Alessandria Basin 
(Fig. 11), one can notice that they were formed by rivers 
that flowed towards the centre of the basin, that is, to-
wards the area where the thickness of the Pliocene-
Quaternary sediments is greater. Figure 11 shows that, 
generally, only the oldest terraces of the tributary rivers 
of the Tanaro (FL1, of undefined Pleistocene age, ac-
cording to Anfossi et al., 1969) join the hillsides formed 
by marine Tertiary and Villafranchian sediments, and 
that the lower terraces (FL2, FL3-a1 a2-1; a3) are in-
cluded between the older terraces and the centre of the 
basin. This means that the centre of the basin subsided 
more than its borders. 

The contact zone between the northern margin of 
the Alessandria Basin and the southern slopes of the 
Eastern Monferrato, an area which is located just south 
of the hills lying between the Giarole-Lu and Valenza 
Deformation Zones, is an exception. 

In this area the oldest terrace is lacking and it is 
the FL2 terrace, attributed to the "Middle Fluvial", Pleis-
tocene in age, by Anfossi et al. (1969), that joins the 
hillslope. The anomaly can be explained by the subsi-

dence of this part of the plain which lasted, at least, until 
the sedimentation of the fluvial deposits which form the 
FL2 terrace. 

The subsidence of the area is also confirmed by 
the thickness of the Quaternary fluvial sediments. Ac-
cording to the maps shown in Irace et al. (2009), the 
thickness of the fluvial sediments forming FL2 south of 
the hills lying between the Giarole-Lu and Valenza De-
formation Zones is greater than 20 m, while in the adja-
cent western area the thickness is 10 m or less.  

The contour lines show that the base of the Plio-
cene reaches its maximum depth SE of Alessandria, 
exactly south of the hills lying between the Giarole-Lu 
and Valenza Deformation Zones.  

If the subsidence is due to the northward migration 
of the Monferrato thrust front, we can assume that the 
thrust was active at least until the shaping of the FL2 
terrace. 

The dating of the terrace FL2, then, is the key to 
determining the age of the last phases of subsidence. 
However there is no direct dating of deposits that form 
the terraces and the chronological framework reported in 
Anfossi et al. (1969) is based on the relative position of 
the terraces and on the degree of pedogenesis.  

In a study on the surroundings of Asti, just west of 
the area reported in the present paper, Carraro & Val-
preda (1991) show that the terraces corresponding to 
FL2 of Anfossi et al. (1969) have, at their top, a yellow-
ish soil with a colour index of 7.5 YR MSCC. In the area 
at the northern edge of the Alessandria Basin according 
to Ajassa et al. (1997), the thickness of the soil does not 
exceed one metre. 

According to Arduino et al. (1984), this kind of soil 
can be compared to that which developed in the late 
Middle Pleistocene or early Upper Pleistocene. It follows 
that the aggradation of the sediments finished before 
that period. Cortemiglia (2012) assumes that the sedi-
ments forming the terrace FL2 were deposited between 
220 and 128 ka BP and weathered in the period be-
tween 128 and 115 ka BP. 

The age of the sediments FL2, therefore, is not 
very well determined, ranging from late Middle Pleisto-
cene to early Upper Pleistocene, but it is similar to that 
of the fluvial deposits that form the surface S2 of the 
Valenza Plateau. The subsidence of the Alessandria 
piggyback basin in the area south of the Giarole-Lu and 
Valenza Deformation Zones continued at least until the 
late Middle Pleistocene or early Upper Pleistocene and 
seems to confirm the age of the northward compressive 
tectonics and of the uplift of the area just north of the 
hills. 
 
6. DATA ON SEISMICITY AND ASEISMIC DEFORMA-
TIONS  
 

In some areas covered by this study, geodetic 
measurements have been carried out and stations of the 
GPS network are located. The seismic catalogues 
(Rovida et al., 2011; RSNI, 2014 a, b) report the earth-
quakes recorded in the area. 

In the paper by Arca & Beretta (1985), the uplift 
rate of the Casale Monferrato area is reported. The uplift 
rate was obtained by means of a comparison of the data 

 
 
88 

 
 

Giraud C. 



 

   

 

 

 

Fig. 11 - Terraces surrounding the Alessandria Basin (redrawn from Anfossi et al., 1969) and contour lines of the base of the Pliocene 
(redrawn from Bigi et al. (1990). 

from detailed geodetic measures carried out in 1897 
and in 1957 AD. 

In particular, the points lying on a section from 
Casale Monferrato to Vercelli, crossing the Monferrato 
Thrust Front, indicate an uplift rate of 4.32 mm/yr-1 in the 
town of Casale Monferrato (south of the front) on the T4 
surface, and of 1.37 mm/yr-1 at Villanova Monferrato, 
located north of the thrust front. The area south of the 
Monferrato Front has an uplift rate three times greater 
than the area north of it. 

However, the section does not permit a three-
dimensional interpretation and it is therefore not possi-
ble to determine whether the deformation took place 
exactly on the front, or along other structures transver-
sal to the front.  

In any case, between the two surveys no strong 
earthquakes occurred in the area and thus the deforma-
tion was completely aseismic or connected with non-
recorded earthquakes of very low magnitude. The geo-
detic datum of Arca & Beretta (1985) would be consis-

tent with the hypothesis of a recent uplift of the area 
corresponding to the T4 terrace provided by morphologi-
cal evidence (Fig. 10). 

The same paper also reports the uplift rate (1.2 
mm/yr-1) measured in Quattordio, on the boundary be-
tween the hills and the western edge of the Alessandria 
Plain (Fig. 10). Also in this area no earthquakes oc-
curred between 1897 and 1957, although one of magni-
tude 4.2, with its epicentre in Quattordio, struck the area 
on July 18, 2001 (Rovida et al., 2011; RSNI, 2014 a, b). 

The seismic hazard of the area under investigation 
is considered low because the earthquakes reported by 
the catalogues are scarce and of magnitude less than or 
equal to 5 (Rovida et al., 2011; RSNI, 2014 a, b). Some 
of the epicentres of earthquakes (Fig. 10) of magnitude 
between 2 and 4.3 are in the Giarole-Lu Deformation 
Zone or their prolongation to the SW, while only three 
epicentres, including that of the earthquake that struck 
Alessandria in 1369 AD (estimated magnitude around 5, 
the strongest that occurred in the area) are inside, or 

 
 

89 

 
 
Quaternary evolution of the Casale Monferrato-Valenza Plain 



 

   

 

 

 

near, the Valenza Deformation Zone. 
Some very low magnitude earthquakes occurred 

near the buried thrust fronts east of the Valenza Defor-
mation Zone. Few other earthquakes with magnitude 
less than 3 occurred in the study area or the surround-
ings. 

Studies on the data of the GPS network (Devoti et 
al., 2011) have shown that the Alessandria Basin has a 
negative vertical speed slightly greater than -1 mm/yr-1 
and a horizontal speed towards the northern-western 
quadrant a little less than 2 mm/yr-1. The movements 
seem compatible with the migration to the N of the area 
located between the Giarole-Lu and Valenza Deforma-
tion Zones and with the subsidence of the Alessandria 
Basin, and could support the hypothesis that tectonic 
activity still occurs in the Monferrato thrust. 
 
7. CONCLUSIONS 
 

The plain between the Po river and the Eastern 
Monferrato Hills consists of fluvial sediments lying on 
the Tertiary marine bedrock. The Tertiary sediments are 
displaced by thrusts and strike-slip or oblique faults 
produced by the compressive tectonics of the Monfer-
rato Front, the westernmost of the Northern Apennine 
fronts. 

The presence of a nearly continuous belt of ter-
races at the base of the northern hillslope, of isolated 
hills formed by tertiary sediments, and the distance be-
tween the Po river and the hills are peculiar characteris-
tics of the Eastern Monferrato and immediately suggest 
some geological differences between the eastern and 
the westernmost Monferrato-Turin Hills system. 

Study of the stratigraphy and morphology has 
enabled us to advance several hypotheses about tec-
tonic mobility during the Quaternary. However, the ab-
sence of well dated morphological and stratigraphic 
features suitable for the correlation of the terraces af-
fected by tectonic movements precludes any reliable 
estimates of the deformation rate. The assumptions 
regarding the type and age of tectonic deformations are 
summarized below. 
- The areas located north of the youngest thrust fronts 

and the area corresponding to the Borgo San Martino 
Pliocene Syncline mainly subsided during the Qua-
ternary. On the other hand, starting at about 600 ka 
BP, the area of the isolated hills RIO, RIM and RIP 
and S1, S2, S3 terraces suffered an almost continu-
ous uplift until the late Middle Pleistocene or early 
Upper Pleistocene. The dataset makes it possible to 
assume that compressive tectonics were active at 
least until that time, and could still be active. 

- The tectonic interpretation of the contour lines of the 
depth of the Pliocene base and the comparison of the 
altitudes of the top of isolated hills and terraces sug-
gest that the areas where the uplift prevailed were 
affected by faults transverse to the thrust fronts, and 
subject to differential movements. 

- The overall interpretation of the tectonic structures 
lying below the plains and affecting the hills allow us 
to assume (Fig. 4) that there are normal faults 
(Roncaglia, Rotaldo and Grana faults) and two defor-
mation zones (Giarole-Lu and Valenza Deformation 

Zones) where strike-slip or oblique-slip faults that 
make up the thrust ramps were active until the late 
Middle Pleistocene or the early Upper Pleistocene. In 
the first deformation zone (Giarole-Lu Deformation 
Zone) ccurred some left-slip displacements, while in 
the second one (Valenza Deformation Zone) some 
right-slip deformations would be present. The tectonic 
movements along the two deformation zones would 
result in an advance towards the north of the thrust 
front near the RIP. The Villabella Syncline evolution 
and the subsidence of the northern Alessandria Basin 
between the two deformation zones would be con-
nected with the northward shift of the thrust front.  

- The age of the sediments affected by subsidence in 
the Alessandria piggyback basin corroborate the hy-
pothesis that the activity of the Monferrato Front con-
tinued at least until the late Middle Pleistocene or the 
early Upper Pleistocene.  

- The horizontal and vertical deformation rates ob-
tained by the Alessandria GPS station seem compati-
ble with the hypothesis of a shift to the north and with 
a subsidence still in progress. 

- The morphology and distribution of the most recent 
terraces lying north of the hills suggest that some 
sectors of the Po Plain may have been affected by 
upper Pleistocene and Holocene tectonic move-
ments. These areas include Casale Monferrato which 
underwent a remarkable aseismic uplift during the 
twentieth century. The possible tectonic mobility of 
these areas could have been independent from the 
main structures of the Monferrato Front. 

 
The possible end of the compressive tectonics and 

the beginning of the deformations related to structures 
wholly or partly different from those that form the buried 
Monferrato Front seem to confirm the findings of the 
study of the Vercelli Plain (Giraudi, 2014). In the plains 
north of the Eastern Monferrato area, however, contrac-
tional tectonics would be more recent or still active. 

It was already known (Pieri & Groppi, 1981; Cas-
sano et al., 1986; Costa, 2003; Burrato et al., 2012; 
Galadini et al., 2012; Galli et al., 2012; Michetti et 
al.,2012) that tectonic activity on the three main fronts of 
the Northern Apennines increased from west 
(Monferrato Front) to east (Emilian and Ferrarese 
Fronts). The results of the present study indicate that 
also in the Monferrato Front the eastern portion shows 
greater mobility in a very recent period.  
 
AKNOWLEDGEMENTS: 

The manuscript has been greatly improved by the 
suggestions and accurate revision of Andrea Festa, 
Andrea Sposato and Maria Gabriella Forno. 
 
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