Siwalik Gr

The Siwalik Group is the youngest deposit of the Himalaya and formed by the erosion of the pre-existing rocks. Medlicott, (1864) was the first to propose the nomenclature. A tripartite classification of the Siwalik Group based on Vertebrate fossils was first proposed by Pilgrim (1913). Pilgrim (1910, 1913, 1934) classified the Siwalik Group into Lower, Middle and Upper divisions and further into subdivisions, which are now referred as Subgroup and Formation respectively. However, the generally unfossiliferous nature of the Siwalik severely limits the usage of this classification on a regional scale. More recently ONGC geologists have divided the Siwalik on the basis of heavy mineral assemblages, into three lithostratigraphic units, namely Lower Siwalik Fm, Middle Siwalik Fm and Upper Siwalik Fm. These subdivisions are marked by distinct lithology, bedding characteristics, degree of induration and texture (Sahni & Mathur, 1964) and can be recognized in the field. In Pakistan, these are further subdivided, which would imply an elevation of the three India formations to subgroup status. The type sections of the most of those Siwalik formations are located in Potwar Plateau, Pakistan viz., Kamlial Fm and Chinji Fm (Lower Siwalik Fm Subgroup), Nagri Fm and Dhok Pathan Fm (Middle Siwalik Fm Subgroup), and Tatrot Fm (the lower part of the Upper Siwalik Fm Subgroup). However, the type sections of the Pinjor Fm and the Boulder Conglomerate Fm (the middle and upper parts of the Upper Siwalik Fm Subgroup) are best exposed in the vicinity of Chandigarh in India. As most of the type sections are recognized in Potwar Plateau, Pakistan and are not easily accessible to Indian workers, the workers in India have tried to propose equivalent reference sections for different Siwalik formations in India. Karunakaran and Ranga Rao (1979) published a detailed map of the Tertiary belt of Himalayan foothills and distinguished various divisions of the Siwalik Group.

Claystone to conglomerate. Siwalik Group comprises more than 7000 meters thick succession of fresh-water molasse sediments. In the field, it is fairly easy to recognize the Lower, Middle and Upper Siwalik subgroups on the basis of lithology. Lower Siwalik Fm is characterized by red mudstones. Middle Siwalik Fm is characterized by multistoried, grey, medium to coarse grained, salt and pepper sandstones. Upper Siwalik Fm are fully conglomeratic in most sections, however in Jammu region brown, pink grey mudstones along with greenish grey, brown, soft sandstones are also well developed.

The palynofloral assemblage is represented by Schizaeacea sp., Magnoliaceae sp., Hibiscus sp., Parkeriaceae sp., Monocalpistes sp., Tricolpittes sp., Monoporate sp., Nymphaeaceae sp. and Intertaessedis sp. The faunal assemblage is highly rich in gastropods. The forms identified are Tectus sp., Sinum sp., Calliostroma sp., Clathrodrialla sp., Cyramidella sp., and Brittum sp., Presence of Calliostromes indicates that the sediments are not older than Miocene.

Pilgrim (1913) used lithology as the main parameter for identifying the various divisions and subdivisions of the Siwalik Group. This caused considerable confusion to later workers, as the lithology used in identifying these subdivisions were found to be time transgressive because the contained fauna indicated different ages. This problem was solved to an extant with the introduction of magnetostratigraphic studies, clubbed with vertebrate paleontology. The magnetostratigraphic studies were initially taken up for the Siwalik Group of Potwar Plateau, and the pioneers were Opdyke et al. (1977), Barry et al. (1982), Johnson et al. (1985), Barry and Flynn (1990) and Hussain et al. (1992). In India, scanty magnetostratigraphic studies have been carried out so far, and these include Tandon et al. (1984), Ranga Rao et al. (1988), Sangode and Kumar (2003) and Kumaravel et al. (2005). The magnetostratigraphic studies have given new dimensions to various faunal events (such as first and last appearance, abundance, range, migration, dispersal events etc. of various taxa). On the basis of magnetostratigraphic studies, various stratigraphic boundaries are precisely dated and these studies give better resolution to the problems concerning the time-transgressive nature of litho-boundaries with respect to the fauna.

Barry et al., (1982) proposed a new biostratigraphic classification based on magnetostratigraphy and marked four biostratigraphic interval-zones for the Middle and Upper Siwalik subgroups of Pakistan. These interval zones are "Hipparion s.l" Interval-Zone (9.5-7.4 Ma), "Selenoportax lydekkeri" Interval-Zone (7.4-5.3 Ma), Hexaprotodon sivalensis Interval Zone (5.3-2.9 Ma) and Elephas planifrons Interval-Zone (2.5-1.5 Ma). Later Hussain et al. (1992) and Nanda (1997) suggested some modifications in this classification. Raiverman (2002) proposed another classification based on energy sequences.

The Siwaliks molasses-like fluvial sedimentary deposits comprising a coarsening-upwards sequence as a whole, which reflects the rising history of the Himalayas (Gansser, 1964). Palaeocurrent and petrographic data from the sandstone and conglomerate indicate that these rocks were derived from the fold-thrust belt, and deposited within the flexural foredeep of the Himalayan foreland basin.

In addition to the best natural repository of the vertebrate fossils, the Siwalik Group is an important succession for the study of structural evolution of foreland basin. The Siwalik Group is the youngest deposit of the Himalaya and formed by the erosion of the pre-existing rocks. Thus a great depth of information pertaining to Himalayan evolution, migration of various structural units, antecedent history of rivers and the nature of prevailing flora and fauna are hidden in this succession. The placer gold resources have been reported from the Siwalik Group. Sharma et al. (1981) have also reported uranium distribution in the Siwalik vertebrates.