Clay Mineral Assemblage of the Lower Siwalik Nahan Formation in the Type Area Nahan, Northwestern Himalaya

Clay mineral suits incorporated in fluvial deposits are mostly detrital in nature and are a useful tool to understand the provenance of fine grained sediments, composition and climate of the source terrains (Chamley, 1989). The fine grained sediments are composed mainly of clay minerals and amorphous materials. The composition and relative abundance of the clay minerals are controlled by their source rocks and weathering conditions. Clay mineral assemblage of the Nahan Formation of the type area Nahan in northwestern Himalaya have the significant importance to understand the palaeoclimatic and palaeotectonic conditions , modes of sediment transport, sedimentation environments and post-depositional changes experienced by the sediments. Clay minerals are hydrated aluminium silicates with very fine particle size, usually >2µm (Moore and Reynolds, 1989). Modern spectroscopic techniques can investigate the sediment surface on a molecular scale, but are not yet used routinely for sediment analysis (Linge, 2008). Mineralogical investigations were generally performed by using X-ray powder diffraction technique (Petschick et al., 1996; Preda and Cox, 2005), often in combination with other analytical techniques, like SEM, XRF. These techniques are accurate and have found widespread use. In this paper we report clay mineral assemblage of selected represented samples from the measured three sections from the Lower Siwalik, Nahan Formation of the type area. Lithologically the Formation consists of alternating sequences of sandstone and clays. The sandstones are fine grained and show dominantly purple and grayish- purple colour in the lower part while in upper part grayish- purple colour is dominated. The clay are dominantly maroon in colour.

The Siwalik group of northwestern Himalaya in India is very well exposed in a linear fashion along the Himalayan foothills for a distance of about 2400 km from near Jammu in West to near Tripura in the East. It represents a huge thickness of sediments ranging from 3300m to 6300m which were deposited in a foredeep. The Siwalik group is further divided into three sub-groups, The Lower Siwalik (Middle Miocene), Middle Siwalik (Late Miocene- Early The Lower Siwalik is also known as Nahan Formation. The age ranges from 13 Ma to 9.2 Ma. During the 9.2 Ma – 10.7 Ma the maximum erosion is takes place and from 11.5 Ma – 12.5 Ma upliftment takes place (Gautam and Wolfgang, 1999). The Middle Siwalik ranges from 9.2 Ma to 5 Ma and The Upper Siwalik ranges from 5 Ma to 0.8 Ma. Chaudhri (2000) observed that the Siwalik group of the western Himalaya is a product of two coarsening up mega cycle. The first mega cycle is represented by the Lower Siwalik / Nahan Formation which is characterized by a slow pace of erosion and sedimentation and stable palaeotectonic conditions. The second mega cycle comprising the Middle and Upper Siwalik Formation is marked by coarsening of the sediments which eventually indicates a fast rate of degradation process.

Fig. Geological map of Nahan and surrounding regions (modified after Rohtash et al., 2002)

35 represented samples from the measured sections and random samples were identified for clay mineral analysis. Clay minerals require special techniques because of their extremely small size. These techniques are X- ray diffraction, infrared spectroscopy, differential thermal, thermo gravimetric; scanning electron microscope; neutron scattering, electron spin resonance, Mossbauer spectroscopy and ultraviolet and visible light spectroscopy. X- ray diffraction technique and Scanning Electron Microscope (SEM) has been used for analyzing the clays of the Nahan formation mainly because of availability and reliability of the technique. The area of investigation spreads over 504 sq. km in the frontal Himalayan terrain. Samples showing variations in grain size and colures are selected for this analysis. The samples were first crushed to -250 mesh ASTM sieve size. About 100 gm of the sample powder was and then allowed to settle down. The stirring was done after every 6 – 8 hour. After that the clear water was decanted off from the jar and fresh distilled water was added. The process was repeated till the suspension of clay particles appeared in the standing water column. 100 ml of suspension clays were separated out from a depth of 5 cm from the top of the standing water column with the help of pipette. The suspension so collected was put on glass slides of oriented aggregates of clay minerals. The incident X- ray beam from the X- ray machine can be directed down the Z axis of the flat lying plate- shaped phyllosilicate minerals in the oriented aggregates slides thus facilitating the recording of diagnostic basal diffractions. The Z axis depicts the intensity of d- spacing indicative of different clay minerals during glycolation and heating. Table 1 - Coordinates of sampling stations in the study areas

Knowing the clay minerals in sediments, X-Ray Diffraction was used. Identification of clay minerals was based on the position of the (001) series of basal reflections on the XRD diagrams. This evaluation was based on mineral standard data following the formula nλ = 2dsinФ. Table shows the result of minerals present in the study area. Result shows that through the bulk mineral analysis, we identified illite, vermiculite, kaolinite and mixed layers clay minerals. The mineral assemblage of the Shambhuwala – Nahan section, Renuka – Nahan section and Sataun – Rajban section are almost identical.

Fig. X- ray diffractograms of clay minerals of selected samples of Nahan Formation of Renuka Nahan section (RNR) and Shambhuwala Nahan section (SNR). Fig. X- ray diffractograms of clay minerals of selected samples of Nahan Formation of Sataun – Rajban section (SRR).

Illite (K1-1.5 Al4[Si7-6.5Al1-1.5O20](OH)8) is the most dominant clay mineral. The basal spacing at 10 Å shows the most dominant peak corresponding to basal therefore organic liquids do not have any effect on the mineral. Therefore it does not show any change on glycolation and remains unaffected on heating up to 500°C. Kaolinite – Kaolinite (Al2Si2O5(OH)4) is The reflection at (001) plane corresponding to 7.16 Å is more intense than those from (020) and (110) planes. The peak at 4.36 Å is stronger than the peak at 4.46 Å. The peaks of the dioctahedral kaolinite remain unaffected upon ethylene glycol treatment but get destroyed on heating above 450°C. Heat treatment helps in distinguishing kaolinite from chlorite which too has the basal reflection at 7.12 Å.

Vermiculite (Mg3(Si3Al)O10(OH)2Mg0.5(H2O4)4) forms about % of the total clay mineral assemblage. The peak at 14.4 Å corresponding to (002) plane is stronger than the peak corresponding to 2.39°. The reflection along the (021, 111) planes and along (060, 330, 332) planes are not distinct. The 14.4 Å peak of the trioctahedral vermiculite, containing strongly linked water molecule to the layer structure, expands to 15 Å - 16 Å on glycolation but collapses to 10 Å on heating up to 500°C.

Based on the SEM result, the morphological character or microstructure of clay minerals in the sediment sample can be seen clearly. In figure- shows morphological features of clay minerals observed in the study area. Kaolinite normally formed in acidic environment like weathering of granitic rocks because it contains lot of feldspar that will turn into kaolinite after the weathering process. Clay minerals that can be observed under SEM machine are illite, vermiculite and kaolinite.

Clay minerals are the products of weathering and soil formation processes. The variation in the clay mineral assemblage is indicative of paleoclimatic changes, the cyclicity of tectonic activity and digenetic modifications experienced by the sedimentary horizon. Illite in Nahan Formation is primarily a product of detrital origin being derived from weathered crystalline rocks containing felspars and micas in the source areas and from soils and pre-existing sedimentary rocks in the drainage basin of fluvial channel(s) that deposited the sediments of the Nahan Formation. Kaolinite is formed by weathering of igneous rocks which contain feldspar and mica that experienced weathering processes from the upstream and settle down in basin. The presence of kaolinite in the Nahan Formation is mainly attributed to the weathering and subsequent leaching of the mineral from these rocks in the source regions. Silicon and aluminum are the major chemical elements needed for kaolinite formation and these are derived by leaching of potassium felspars and micas present in the pre-existing rocks. The presence of kaolinite in the Nahan Formation indicates the prevalence of acidic conditions and presence of relict organic matter in the source areas and near neutral pH conditions in the basin of deposition. weathering regime following high precipitation and chemical weathering of feldspar and other intermediate weathering products has resulted in its development in the Nahan Formation. Mixed layer clay minerals are present in the all the samples of the Pinjor Formation. In nature, clay minerals usually occur intermixed with each other. Environments rich in Na+ or K+ ions help in the formation of mica and hydrous aluminosilicate mixed layers.

Note: +++ dominant mineral (>50% observed), ++ frequent (21% - 50% observed), + few or less (< 21% observed), - none.

The Himalayan foreland basin has formed due to flexure of the Indian plate caused by the high mass of

the basin dynamics. The Siwalik sediments were deposited in a fore deep and represent the detrital products of the rather rapidly evolving Himalaya. The Himalayan tectonic activity reached its acme during the Quaternary period. During the period of deposition (Mid Miocene to Early Pleistocene) of the Siwalik sediments, the tectonic activity manifested itself in the formation of numerous fold and thrust structures by Chaudhri (2012), which intermittently increased the local relief triggering heightened erosion of the pre-existing rocks located adjacent to fore deep. The growth of the Himalaya also contributed significantly in altering the atmospheric circulation pattern and consequent climatic readjustments. The Indian Summer Monsoon (ISM) is perhaps the single most significant factor influencing the sediment supply of the foredeep sediments. Tectonic control of climate is best documented in the development of high relief intercontinental topographic barriers like the Himalayan mountain belt (Sakai et al., 2006). The rise of the Himalaya had a significant impact on the surface albedo and the atmospheric pressure regime which augmented the monsoon precipitation in the Himalayan terrain. Raymo et al., (1988) and Ruddiman (1997) suggested that the weathering processes acting in young orogenic belt consume carbon dioxide and drive global cooling. Deep sea sediment studies carried out in the Arabian Sea suggest that the ISM had established itself as a consequence of the Himalayan Mountain rise around 10 Ma and the monsoon intensity strengthened itself from about 8 Ma (Kroon et al., 1991 and Rea, 1992). Evidence from deep sea drilling in Bengal fan suggested an accelerated uplift of the Higher Himalaya around 10 Ma and subsequently after 0.9 Ma by Amano and Tyra (1992) and Derry and Lanord (1997). Studies from the Indus delta in the Arabian Sea carried out by Clift et al., (2008) suggest the intensification of the Indian summer monsoon after 14 ka. Sediment supply in mountain regions is a complex interplay of tectonically related rock uplift and degradation processes influenced by precipitation. Hodges et al., (2004) and Wobus et al., (2005) favor precipitation as the primary controlling factor for sediment supply while Burbank et al., 2003 favor tectonic uplift. Clift et al., (2008) linked the volume and exhumation of the western Himalayan to the intensity of the summer monsoon. In northwestern Himalaya rapid uplift of the Siwalik frontal range during Early Pleistocene to Middle Pleistocene has been reported by Burbank and Johnson (1983). Sakai et al., (2006) suggested that coeval uplift and consequent degradation of the frontal Lesser Himalaya since 1 Ma in Nepal had a perceptible influence on the composition of clay The characteristic of clay minerals in terms of the mineral type and abundance is dependent upon three major factors viz, detrital inheritance, transformation and neo- formation. Clay minerals serve as reliable indicator of provenance and environments of sedimentation. Clay minerals are susceptible to alteration during transportation and accumulation in varying environments. The mineral illite is formed by weathering of felspathic and micaceous rock which is stable in moderate climatic conditions by M. L. Jacson (1959) and remains unaltered during transportation by fluvial agencies over a short distance of transport by Hurley et al (1961). A temperate environment and moderate weathering conditions have been recorded for the formation of Illite by Foth and Truck in 1973. Milne and Earley in 1958 suggested that illite is formed as a consequence of absorption of potassium from sea water by montmorrillonite. Digenetic origin of illite has been suggested by Chaudhri (1985, 1988), Chaudhri (1997), Velde (1983) and Shaw (1980). The clay mineral assemblage recorded from the Lower, Middle and Upper Siwalik subgroups in the northwestern Himalaya show variation in the mineralogy and abundance of different clay minerals given by Chaudhri (2000). In Lower Siwalik sediments, illite is the most dominant clay mineral followed by kaolinite, mixed layers and vermiculite. Chlorite and montmorillonite are nearly absent. In Middle Siwalik sediments a similar scenario persists with the difference that the abundance of vermiculite is severely reduced and montmorillonite is present in a few samples. In Upper Siwalik Pinjor Formation illite is the most dominant mineral followed by chlorite, kaolinite, vermiculite and mixed layer clay minerals. This variation in abundance of clay minerals reflects variation in source rocks, change in precipitation regime consequent to tectonism, variation in weathering pattern and distance of transport of the detritus and post depositional alterations/neoformation of the clay minerals. Siwalik sediments record reveals a slow pace of sedimentation for the Lower and Middle subgroups and a rather fast pace of sedimentation for the upper Siwalik subgroup. Degradation of the Himalayan hinterland during the deposition of this thick foreland stratigraphic unit resulted in successive unroofing of the evolving Himalaya and exposure of new hitherto buried lithologies to denudation processes. A variation in rates of tectonic uplift at local and regional scales as also uplift along thrusts and faults caused a change in precipitation levels and erosional patterns. The clay minerals/sediments of the Lower and the Middle Siwalik subgroups which were buried under the load of the overlying sediments underwent transformation/neoformation. Dr. A. R. Chaudhri for his constant support and guidance. The work was carried out at Department of Geology , Kurukshetra University, Kurukshetra.

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