Granulometric evidence indicates the dynamics of transport of and/or distance from the detrital material sources during deposition. Clay minerals deposited in the fluvial environment reflect the intensity of chemical and physical weathering of their source area (Chamley, 1989). This weathering depends on the lithology, climate and morphology of the environment under consideration and results in the formation of superficial soils and sediments. In temperate and warm humid and semi-arid climates, chemical weathering prevails, and kaolinite or smectite appear. Under weakly hydrolytic conditions in dry climates, physical weathering prevails, and illite and chlorite often appear, since the production of clays depends essentially on increased direct rock erosion. The abundance of illite can also indicate strong erosion related to tectonically rejuvenated relief or to deforestation and not necessarily to weak hydrolitic climatic conditions. Post-depositionary processes can also modify detrital clay minerals.

Magnetic parameters allow estimation of the ferromagnetic minerals present in the sediment. The presence of these minerals (e.g. iron oxides: magnetite Fe₃O₄, maghemite γFe₂O₃, haematite αFe₂O₃, oxyhydroxides: goethite FeOOH and sulphides: greigite Fe₃S₄) depends on the source area, and conditions during deposition and after, where early diagenetic processes can modify the detrital fraction, and new minerals can appear. Low field magnetic susceptibility (χ) depends on the ferromagnetic minerals (which can carry remanent magnetisation) present in sediment, if their content is very low, then χ depends on the paramagnetic and/or diamagnetic minerals which cannot carry remanent magnetisation. The magnetic hysteresis parameters: coercivity (Bc), remanent coercivity (Bcr), saturation magnetisation (Ms) and remanent magnetisation (Mrs), allow discrimination between different ferromagnetic materials. The thermomagnetic behaviour of the sediment allows identification of ferromagnetic minerals through their Curie temperature, the temperature at which they are demagnetised. Some of these minerals are unstable on heating, but can still be identified because of their specific thermomagnetic behavior (maghemite, goethite). This is the basis of environmental magnetism as defined by Thompson and Oldfield (1986) and it is used by many authors.

During fieldwork in spring 2014 and 2015, a stratigraphic framework based on lithological studies was designed, and magnetic susceptibility measurements and sampling for laboratory analysis performed in the area of the Masol village where the Masol Formation is most complete. This was possible in the western part of the Pichhli choe stream, starting from its confluence with the principal stream Patiali Rao and proceeding until the top of the sequence passing through the Masol 6 paleonto-archeological locality (black line in Fig. 2b and Fig. 3a).

The low field magnetic susceptibility was measured during the fieldwork directly on each identified lithological unit (after cleaning to obtain fresh surface) with a Bartington MS-2 susceptibility meter. Samples were collected near to the places where magnetic susceptibility was measured, and then analysed in the laboratories of National Museum of Natural History and GEOPS-University Paris-Sud (GEOPS-UPS) for magnetic and clay minerals, Ca and Mg carbonate content and grain size. Particle size analyses of samples were performed by laser granulometry on fractions of sediment under to 2 mm with a Malvern Mastersizer 2000 instrument. Carbonate contents were measured by titration with bromophenol blue solution. X-ray diffraction (XRD) on oriented mounts of non-calcareous clay-sized (< 2 μm) particles was used to identify clay minerals with a PANalytical diffractometer, following the laboratory routine of the GEOPS-UPS (Liu et al., 2004 and Liu et al., 2008). Three XRD runs were conducted following air-drying, ethylene-glycol solvation for 24 h, and heating at 490 °C for 2 h. Identification of clay minerals was based on the position of the (001) series of basal reflections on the three XRD diagrams. Semi-quantitative estimates of peak areas of the basal reflections for the main clay mineral groups of smectite (including mixed layers) (15 to 17 Å), illite (10 Å) and kaolinite/chlorite (7 Å) were carried out on the glycolated curve (Holtzapffel, 1985) with MacDiff software (Petschick, 2000). Relative proportions of kaolinite and chlorite were determined based on the ratio 3.57/3.54 Å of the peak areas. Evaluation of each clay mineral had an accuracy of 5%. In order to identify magnetic minerals in sediment, the thermomagnetic behavior and magnetic hysteresis of several bulk sediment samples were measured. The thermomagnetic behavior was obtained by using a horizontal force translation balance in an air atmosphere and a magnetic field of 0.375 T. Magnetic hysteresis measurements were made on discrete samples (5 mg to 15 mg of dry, bulk sediment) with an alternating gradient magnetometer (AGM 2900 – Micromag) at the LSCE – CNRS/CEA laboratory at Gif-sur-Yvette, France. A peak-applied field of 1 T was used for hysteresis measurements and the results were corrected for the paramagnetic and diamagnetic contribution of the matrix minerals. After removal of this contribution, saturation magnetisation (Ms), saturation remanent magnetisation (Mrs) and coercive force (Bc) were obtained from the hysteresis loop. Remanent coercivity (Bcr) was obtained through step-wise application of backfields to remove the saturation remanent magnetisation.

The studied Late Pliocene sequence contains cyclically deposited layers of sands (sandstones now) and finer grain-sized sediments – silts/clays (Fig. 3b). Layers of sandstones are more or less homogeneous; their thickness varies from several tenths of centimeters to several meters. They are rich in quartz and in white mica and therefore in their fresh state they are grey; nevertheless, in the outcrops, some of them stay grey, but others are yellow or rose-colored. They are not strongly consolidated, they become hard when they are dry and in humid conditions, they disintegrate into sand. Sometimes, they contain oblique stratification, nodules or other consolidated forms. Especially in the upper part of the sequence, these layers of sandstones/sand contain levels with gravel and pebbles, and are even locally replaced by conglomerate (Fig. 3c). Gravel and pebbles are heterogeneous in size, nature (including quartzitic) and degree of rounding. Generally, the sandstones/sand are quite well resistant to weathering and erosion. They form cliffs and are interlaid by often thinner layers of finer silty to clayish sediments, which are evidently eroded more easily and often form platforms between cliffs (Fig. 3a). These silty/clayish sediments are mostly brownish (rarely grey) and sometimes colored (yellow, orange, reddish) because of the oxydo-reduction of iron (e.g. c6 in Fig. 3a).

The sequence contains a series of 17 pairs of layers defined during the fieldwork as silty/clayish and sandstone/sandy units (Fig. 5a). The adopted numbering starts at the bottom (c1-s1, c2-s2 …), i.e. at the junction of Patiali Rao and Pichhli choe (Fig. 2b), “c” meaning silty/clayish and “s” sandstone/sandy units. Thicknesses are approximate because of lateral variation. The 17 silty/clayish and sandstone/sandy pairs counted represent the first order of the lithological changes. In these layers, the second order of lithological changes is often observed, and fine silty levels can appear locally in sandstone/sandy units and sandy levels can also be present in silty/clayish units (Fig. 3b and Fig. 3d).

A more detailed lithological description of the sediment is not the focus of this work, with the exception of unit c6, which is thicker than other silty/clayish units and is ∼20 m thick. We distinguished four sub-units: (1) c6-1, a clear brown silty sequence containing locally one or two sandy levels (Fig. 3d); (2) c6-2, very characteristic and well observable in the landscape, a grey/white silty layer, ∼1 m thick (Fig. 3d and Fig. 4a), and in the upper part of this sub-unit a ∼10 cm thick level is present, which enhances the stratification; (3) c6-3, brownish-reddish silt with traces of the first step of the lateritisation and some concretions enriched in the iron minerals found here (Fig. 4a), contains, as in sub-unit c6-2, ∼10 cm thick levels which enhance the stratification; and finally (4) c6-4, more than 10 m thick, which is formed by several silty and sometimes sandy fine levels, each one differentiated by yellowish, reddish, orange, brownish colors which follow the stratification (c6 in Fig. 3a and c6-4 in Fig. 4a).

Samples for granulometry and analysis of clay minerals and carbonate contents were collected in 2014 and 2015 close to the layers where the magnetic susceptibility measurements were taken directly in outcrops. The samples of 2014 have been analysed.

Granulometry performed on particles with a diameter under 2 mm reflects mostly changes between sandstones and silty/clayish units identified during the fieldwork (Fig. 5a). Proportions of clays and silts as well as main grain sizes are presented in Fig. 5c and Fig. 5d. The considered grain sizes present higher and more scattered values in the lower part of the sequence than in the upper one.

Ca and Mg carbonate content varies between 10% and 50% (Fig. 5e); it is slight in the lower part of the sequence, increases, but changes between units c5 and c6 (45 m to 70 m), and becomes higher above.

Among clay minerals, illite is dominant, and its content is frequently as high as ∼60% (Fig. 5f). Nevertheless, in the lower part of the sequence until unit c6-2 it is lower and it even became below 10% in unit s3 and in one point above, in unit c11. When illite content is low, smectite presents higher values, and becomes dominant at 80% for units s3 and c11. Variations in the illite and smectite contents are independent of the grain size of the rock. Chlorite and kaolinite present values lower than 20% all along the sequence (Fig. 6a).

Low field magnetic susceptibility presents low values throughout the studied sequence, indicating low content of magnetic minerals in the sediment as a whole. Nevertheless, there is clear difference between sandstone units “s”, with lower magnetic susceptibility, and finer grain size units “c”, where magnetic susceptibility is higher (Fig. 5b). This relation is systematic, and although sometimes its value is low in the thin level of unit “c”, it indicates that the level is rich in sand, as confirmed by laser granulometry measurements. Clearly higher values of magnetic susceptibility were obtained for concretions found in sub-unit c6-3 (these concretions are hereafter called Fe-rich concretions), which represents the early step of lateritisation, and in unit c16.

Magnetic hysteresis and Curie balance experiments were performed for Fe-rich concretions and for samples from units c2, s3 and c6-2. Results for units c2, s3 and c6-2 are similar. Magnetic hysteresis loops, as well as hysteresis characterising parameters (Bc, Bcr, Mrs/Ms) for samples c2 and Fe-rich concretions, are presented in Fig. 6b and Fig. 6c. Both samples present hysteresis loops, which are constricted in the middle section, and wider above and below the middle section; in sample c2 (Fig. 6b) the constriction is more pronounced. Such “wasp-waisted” hysteresis loops in geological materials indicate the coexistence of at least two different magnetic components with contrasting coercive fields (e.g. Day et al., 1977, Roberts et al., 1995, Tauxe et al., 1996 and Wasilewski, 1973). In such conditions, the coercive field (Bc) is largely controlled by “soft”, low-coercivity components such as ferrimagnetic magnetite or maghemite, whereas the coercivity of remanence (Bcr) is controlled by “hard”, high-coercivity components such as canted antiferromagnetic haematite or goethite. The degree of the constriction in the middle section of the magnetic loop depends on the relative contribution of each component. Wasp-waisted hysteresis loops may reflect (1) a mixture of different grain sizes of a single magnetic mineral, (2) a mixture of different magnetic materials with largely different coercivities, or (3) a combination of both (Roberts et al., 1995). In the presence of “soft”, high magnetic moment minerals (magnetite or maghemite), “hard” minerals with weak magnetic moments (haematite, goethite) must be abundant to cause wasp-waisted magnetic loops. The hysteresis loop form, low Bc and high Bcr for Fe-rich concretions from sub-unit c6-3 (Fig. 6c), indicate the mixture of the “soft” mineral(s) such as magnetite and/or maghemite and “hard” mineral(s) such as haematite and/or goethite. The hysteresis loop form and Bc and Bcr values in sample c2 indicate the dominance of high-coercivity minerals such as goethite and haematite.

The thermomagnetic behavior of samples from unit c2 shows the mixture of a mineral, which undergoes demagnetisation before 200 °C and haematite, with Curie temperature of 680 °C (Fig. 6d). The decrease of the magnetisation until ∼200 °C is probably owed to the presence of an iron oxyhydroxide, goethite, which loses water in such temperatures, and is transformed into haematite (e.g. Tarling, 1983). The concretion from sub-unit c6-3 shows also a small decrease of magnetisation until 200 °C (Fig. 6e). The second total demagnetisation, which occurs at 400 °C, and the irreversible cooling curve suggest the presence of maghemite (e.g. Tarling, 1983 and Thompson and Oldfield, 1986). This is consistent with magnetic hysteresis results; the use of these methods allows identification of the goethite and haematite in samples from unit c2 and goethite and maghemite in the Fe-rich concretion from sub-unit c6-3.

The series obtained for the sediment passing through the Masol 6 paleonto-archeological locality contains a repetitive sequence corresponding to a cyclical pattern of influx of detrital material in fluvial environmental conditions. This lithology and geomorphological features of the Masol anticline make it difficult to establish the precise stratigraphy and correlations between different Masol paleonto-archeological localities. Nevertheless, some layers present specific characteristics, which are easily identifiable. Unit c1, the sequence of units c3, s3 and s4, unit c6, and unit s13 are such examples.

At the beginning of the investigated sequence, unit c1, which is about 1.8 m thick, is formed by lower brownish and upper grey silt/clay, and is only one such situation observed. The sequence of units c3, s3 and s4 presents a unique change multicolored silt c3 several meters thick, clear grey sandstones/sand s3 ∼1 m thick and usually, without thin c4, immediately above, sandstone s4, which is some meters thick. The c3 silt starts with a brownish colored sub-layer which is followed by grey, reddish and finally orange ones. The sandstone s4 has a clear rose color on the surface and is underlined by lying below the clear/grey sandstone unit s3. The sandstone s4 is called elephant sandstone, because of the fossilized head of an elephant found in situ in its upper part (Fig. 4b). Unit c6 presents a thick sequence (∼20 m) composed of 4 different and characteristic sub-units, c6-1, c6-2, c6-3 and c6-4, as described above. The particularity of this set is enhanced by the presence of the fine grey/white c6-2 and thick multicolored c6-4 sub-units (Fig. 4a). Unit s13 constitutes thick, clear grey sandstone rich in white mica, and it is composed of second order layers including fine-grained silty layers. Unit s13 is clearly visible in the landscape as an important cliff (Fig. 3a) above the Masol 6 paleonto-archeological locality and is present only in this part of the site and to the southeast of Masol village.

In the section obtained for lithological log, Masol 6 paleonto-archeological locality occupies unit c6 (Fig. 5). The lithological characteristics of the sedimentary sequence allowed the identification of the stratigraphic position for other Masol paleonto-archeological localities. Fig. 4a, Fig. 4c and Fig. 4d illustrate Masol 6, Masol 1, Masol 5 and Masol 3 and Fig. 5 summarizes the identifications, which were made. All identified Masol paleonto-archeological localities are positioned between units c3 and c6. High fossil contents were found from unit c3 to unit c7; above unit c7, they became scarce. Most of the fossils between c3 and c7 were redeposited and some of them come from upper layers. Nevertheless, numerous fossils are well preserved (for more details, see Moigne et al., 2016), and were collected on the slopes of the layer from which they were exhumed by erosion. That and the presence of fossils which were found in situ (e.g. in units c3, s4 and c6; for more details see Chapon Sao et al., 2016a) highlight recent and local redeposition, which is the result of a very young erosion. Such erosion related to high exhumation rate was also pointed out by Gargani et al. (2016).

The Himalayan Foreland Basin (HFB) characterized by the fluvial deposits, Siwalik molasse, is subdivided into several sub-basins, including Subathu (our study area is in its western part), Kangra to the northwest of Subathu, and to the east Dehra Dun (Fig. 1b). These sub-basins were extensively studied in particular for magnetostratigraphy, the evolution of the fluvial system during Pliocene and the Quaternary, the origin of these fluvial deposits and their nature, including clay and magnetic minerals (Kumar et al., 2003, Kumaravel et al., 2005, Kumaravel et al., 2010, Ranga Rao, 1993, Ranjan and Banerjee, 2009, Sangode and Bloemendal, 2004 and Suresh et al., 2004).

Kumar et al. (1999, cited in Suresh et al., 2004) documented transverse trunk drainage flowing southwest between 5.5 Ma and 1.77 Ma, and piedmont drainage flowing southeast between 4.8 Ma and 0.5 Ma. The first was attributed to the major rivers draining through Higher and Lesser Himalaya, while the piedmont drainage, tributaries of major rivers, drained through the Sub-Himalayan region. Interweaving of different sandy channel deposits was evident, but nevertheless the floodplain deposits of the two types of drainage were difficult to distinguish.

Our results show the changing sequence of finer and coarser grain-sized deposits, which reflects this fluvial environment. Magnetic susceptibility increases in silty sediments and decreases in sandy sediments. On the whole, there are lower contents of carbonate in the lower part of the sequence, where smectite is also present, and higher main grain sizes were measured in the fraction of the sediment below 2 mm. In the upper part of the sequence, illite is clearly dominant, carbonate content is higher and increased contents of gravel and pebbles can be observed. The middle part of the sequence, which constitutes the Masol 6 paleonto-archeological locality, represents the transitional zone between the two parts of the sequence. A more detailed study of some levels from Masol 6, as well as from Masol 1 and Masol 13 paleonto-archeological localities, is presented in Abdessadok et al. (2016) and in Chapon Sao et al. (2016a).

The magnetic susceptibility obtained for all the studied sequence indicates on the whole rather low contents of magnetic particles. Nevertheless, the distribution of the magnetic particles changes between sandy and silty granulometric fractions, with clearly higher contents in the finer sediment units “c” and lower contents in the sandy sediment from units “s”. Preliminary results of the mineralogical identifications were obtained only to samples from four units from the lower part of the sequence. They show that goethite and haematite are dominant magnetic minerals. Moreover, they show that these two minerals are present in both granulometric – sandstone/sandy and silty/clayish sedimentary units and their presence indicates that oxic conditions dominated during deposition and after. The reddish, yellowish, orange and brown colors of sediment in different parts of the sequence indicate that other more or less amorphous forms of iron oxyhydroxides are present, too. The composition of magnetic minerals was obtained for soils and parent rocks (sandstones, siltstones and mudstones) in previous studies on the Late Pliocene part of the Upper Siwalik sequence in Subathu sub-basin (Kumaravel et al., 2010, Sangode and Bloemendal, 2004 and Sangode et al., 2007). These results showed that haematite and goethite are dominant, with increased magnetic susceptibility in soil horizons, which are on the whole weakly developed, and lower magnetic susceptibility in parent rocks. Our study shows that the content of magnetic minerals varies even through silty/clayish and sandstone/sandy rocks, which can become parent rocks for soil development. In our study, c6-3 sub-unit shows early step of the lateritisation. The Fe-rich concretions from this sub-unit are composed of goethite and maghemite. The presence of the maghemite suggests locally reducing conditions during or after concretion formation. Further studies will allow accurate identification of the magnetic minerals in the whole sequence.

Chaudhri and Gill (1983), Rainverman and Suresh (1997) and Rainverman (2002) investigated Dehra Dun and the Kangra and western Subathu sub-basin's Siwalik sediments for clay minerals. They found abundant illite and smectite with wide zigzag pattern in Kangra and Dehra Dun but in the Subathu sub-basin they found no smectite. They suggested that smectite was supplied by major rivers, and documented two independent paleo-drainage systems, similar to the present-day Indus and Ganga systems (see also Clift and Blusztajn, 2005), separated by a water divide along the Subathu sub-basin. They suggested that the systems did not cross each other and that this water divide is an ancient feature. However, the study by Suresh et al. (2004) shows high proportions of smectite in the eastern part of the Subathu sub-basin. Indeed, Suresh et al. (2004) identified grey sand/sandstone in the Upper Siwalik sequence as deposited by trunk drainage, and sands that they named “buff ribbon sandstone bodies” as deposited by piedmont drainage. They found changing but high contents of smectite in grey sandstones; in buff ribbon sandstones, illite was largely dominant. Contents of illite and smectite in mudstone depend on tributaries or major rivers flooding. The distribution of clay minerals observed in our study (western part of the Subathu sub-basin), and presented in Fig. 5f and in Fig. 6a, is very close to that obtained by Suresh et al. (2004), including the presence of the smectite.

Rainverman and Suresh (1997) and Rainverman (2002) observed higher contents of illite in the older part of the studied sequence, and smectite in the younger one. They suggested that illite could be of diagenetic origin. Nevertheless, Suresh et al. (2004) did not confirm this distribution and identified illite as being of detrital origin, the same result as that for higher contents of illite in the upper part of the sequence, which we obtained in this study. As detrital illite in sediments is caused by physical weathering, and is usually very abundant in high-relief regions (Chamley, 1989), Suresh et al. (2004) suggested Higher Himalaya was its source area in the studied region.

Smectite is a result of the chemical weathering of volcanic rocks and of other rocks in warm, contrasting wet/dry climates (Chamley, 1989). Rainverman and Suresh (1997) suggested that smectite in the studied sub-basins is a result of the weathering of volcanic rocks in the Lesser Himalaya; nevertheless, sediments rich in smectite studied by Suresh et al. (2004) content only rare volcanic rocks. Another possible source of the smectite could be pedogenesis in situ; nevertheless, some mudstones studied by Suresh et al. (2004) are rich in smectite but do not have traces of pedogenesis. Therefore, Suresh et al. (2004) suggested that smectite formed in the Lesser Himalaya from volcanic and other rocks in the favorable climate and supplied the studied sub-basins. This is also suggested by our results, since we found very high contents of smectite in the sandy unit s3.

Our results and these obtained by Suresh et al. (2004) indicate the presence of smectite in the Subathu sub-basin, contrary to the previous studies, and the smectite here is of detrital origin. Therefore, it had to be supplied from its source area by a major river system, as indicated by Rainverman and Suresh (1997) and Rainverman (2002) for the Dehra Dun and Kangra sub-basins. Since the supply of smectite was attributed to major river activity, it indicates that the Subathu sub-basin was still paleo-drainage area for major river(s) during the Late Pliocene considered in our study before definitive water division.

The smectite in the Lesser Himalaya and Siwalik is of major importance for studies on the sediments from the Indus and Ganga/Brahmaputra deltas. Indeed, in paleoclimate reconstructions, sources of different clay minerals have to be well recognized. In sediments from the Bay of Bengal for instance, smectite can reflect changes in its sources (Deccan plateau or Himalaya) or changes in the weathering conditions (Derry and France-Lanord, 1997, cited in Suresh et al., 2004).