CENOZOIC SEDIMENTOLOGICAL AND PALAEOCLIMATIC STUDIES IN THE NW HIMALAYA, INDIA

Abstract

The Cenozoic Himalayan Foreland Basin (HFB) is divided into a number of subbasins separated from each other by sub-surface basement highs and ridges. The Siwalik Group is developed all along the southern margin of the Himalayan mountain belt as the folded, faulted and uplifted part of the HFB, ranging in age from the Mid-Miocene to the Lower Pleistocene. During this Cenozoic period, besides floral and faunal changes in the history of Himalayan orogen, it witnessed major changes in the climate of the Asian continent because of large scale uplift of the Himalayan ranges during the Miocene. Very limited studies have been carried out towards palaeopedological and palaeoclimatic studies in the Indian part of the Siwaliks. Also the work done in this regard in the Indian part of Siwaliks covers the Kangra, Subathu and Dehradun sub-basins and no such detailed work carried out in the Ramnagar sub-basin. Keeping in view of the importance of this sub-basin as a link between the extensive works done in the Pakistan Siwaliks, the present study was carried out in the NW part of Siwaliks in India to bridge these gaps in our knowledge. In this study detailed pedological and sedimentological investigations have been done to deduce the basin tectonics, depositional history and palaeoclimatic implications during the deposition of the Siwalik sediments. The Lower Siwalik and the stratigraphically older Dagshai sediments are conspicuously red in colour. This redness in colour of these Cenozoic sediments has also been investigated in the present study. Detailed field and laboratory studies were carried out in four sections of the Siwalik Group alongwith, a section of the Dagshai Formatiom. Jammu-Nandni (J-N), Purmandal- Uttarbehani (P-U) and Samba-Mansar (S-M) Siwalik sections were studied in the Ramnagar sub-basin covering a lateral stretch of- 40-50 km along strike in the Jammu & Kashmir state. In a similar manner, the Katilu Khad (KK) Siwalik section in the Kangra sub-basin in the Punjab state and the Dagshai (DD) section in the Dagshai Formation of the Subathu sub-basin in the Himachal Pradesh state have been taken. In general, the Siwalik Group is divided in to three sub-groups from bottom to top: Lower, Middle and Upper Siwalik. On the basis of sedimentological characters and change in dominant lithology, the Upper Siwalik sub-group in the Ramnagar sub-basin is subdivided from bottom to top into the Purmandal sandstone, Nagrota and Boulder Conglomerate formations. Likewise, Middle Siwalik sub-group of KK section is subdivided in to two formations: lower part as Middle Siwalik Fm. A and upper part iii as Middle Siwalik Fm. B. Stratigraphic units in the studied region have been assigned ages on the basis of published palaeomagnetic dates from the studied sections or nearby sections. The Lower Siwalik and Middle Siwalik boundary falls at 10 Ma whereas the Middle Siwalik and Upper Siwalik boundary falls at 5.5 Ma in the KK section (Raiverman, 2002). The Lower- Middle Siwalik and Middle-Upper Siwalik boundaries in the J-N and P-U sections fall at 10.8 Ma and 4.92 Ma, respectively whereas, the same boundaries fall at -8.4 and 5.68 Ma respectively in the S-M section (Ranga Rao, 1993; Pandita, 1996). The measured Dagshai section in the present study has been assigned an approximate age range of 33 Ma to 27 Ma (Jain et al., 2009). Lithofacies associations and characteristic architectural elements of different formations in the Ramnagar and Kangra sub-basins have been deduced through the study of -2740 m thick J-N section, -1140 m thick P-U section and -2935 m thick KK section. Four architectural elements of Miall (1985) have been recognized in these studied sections. These are Channel belt deposits, Levee sandstones and siltstones, Floodplain deposits (with weak pedogenesis) and Palaeosol assemblages. Detailed morphological and micromorphological (i.e. 177 thin-sections) studies bring out six major pedotypes (i.e. I to VI), showing increasing degree of development from I to VI, in the studied Cenozoic palaeosols. First independent or dominant argillic or spodic horizon is found in the Pedotype V palaeosols and the thickness increases in the Pedotype VI palaeosols. Pedotypes I and II, III and IV and V and VI are classified as weakly, moderately and strongly developed palaeosols, respectively. The Lower Siwalik sub-group is marked by mainly floodplain facies with multiple, moderately to well-developed palaeosols and minor channel facies. The Middle Siwalik sub group in the Ramnagar sub-basin and the Middle Siwalik Fm. A in the Kangra sub-basin are marked by multistoried sandstones and minor mudstone, with largely weakly developed palaeosols. However, in the Kangra sub-basin, the Middle Siwalik Fm. B is marked by a fair amount of finer sediments and weakly to moderately developed palaeosols. The Upper Siwalik sub-group shows very different facies in the Ramnagar and Kangra sub-basins. In the Ramnagar sub-basin, the Purmandal sandstone Fm. is marked by thin sandy channel deposits alternating with weakly to moderately developed palaeosols/mudstones whereas the Nagrota Fm. is characterized by moderately to few well developed palaeosols, a patch of gleyed palaeosols along with lacustrine mudstones and siltstones and minor small channel deposits. The Boulder Conglomerate Fm. comprises thick conglomerates with thin weakly developed iv palaeosol units. In the Kangra sub-basin, the lower part of Upper Siwalik sub-group consists of sequence of mudstones/weakly to moderately developed palaeosols, thin sandstones and conglomerate facies, whereas the upper part comprises thick conglomerate beds. The Dagshai Fm. is dominated by weakly to moderately developed palaeosols with some fine to medium sandstone channel facies. Detailed micromophological studies revealed the abundant presence of diagenetic features such as dissolution and recrystallisation of quartz, feldspar and mica; complex patterns of birefringence fabrics; calcite spars and veins as major carbonate fabrics; pigmentary ferric oxides, largely hematite; coarse thick cutans and cementation by calcite, in the Dagshai and Lower Siwalik palaeosols, indicating the diagenesis of sediments on a large scale. Mineralogical analyses of 80 palaeosol samples (i.e. 27 for thermal analysis and 53 for SEM and SEM-EDX studies) have been carried out using DTA-TG-DTG, SEM and SEMEDX techniques, particularly for the Dagshai palaeosols, which have dark micromass for identification of its nature. Carbon and oxygen isotope analyses of 301 pedogenic calcretes from the palaeosols of J-N, P-U, S-M, KK and DD sections and 72 non-pedogenic carbonates in the form of calcareous cement and nodules in sandstones of the J-N section have been carried out for palaeoclimatic interpretations on the basis of palaeovegetational and palaeoprecipitational reconstructions. SEM and Thermal analyses indicate that the Dagshai palaeosols are dominated by 1:1 clay minerals, like kaolinite and halloysite, beidellite, chlorite and minor mixed layer clays (like illite-smectite) and iron oxides. Beidellite, however, maintains its place as the dominant clay mineral in the well developed profiles. As we move towards younger age sediments (i.e. from Dagshai to Upper Siwalik), mixed layer clays become dominant. SEM studies also revealed crumb microstructure typical of oxic horizons and common chemical weathering features like dissolution pits and neoformed amorphous aluminosilicates in the Dagshai palaeosols. SEM-EDX analyses show abundant presence of C, Fe, Al, O in the Dagshai palaeosols suggesting larger concentration of humus and sesquioxides (i.e. Fe & Al oxides and hydroxides). The exchangeable basic cations are almost absent in the Dagshai palaeosols, indicating the absence of clay minerals having high base cation exchange capacity and the dominance of low cation exchange capacity clay minerals. In contrast, the Siwalik palaeosols, particularly Middle and Upper Siwalik palaeosols, show varied amounts of Ca, Mg, Na, K, indicating presence of a wide variety of clay minerals in different palaeosols. With a view reconstruct palaeovegetation scenario, 8 C values of pedogenic carbonates form the basis. The 813C values are highly depleted (i.e. -18.52%o to -7.48%o) indicating exclusive dominance of C3 vegetation in the sediments ranging in age between 33- 27 Ma (i.e. Dagshai) and 13-7 Ma (i.e. Lower Siwalik and part of Middle Siwalik). It is named as Zone I. Similarly, the younger part ranging in age between 5-0.4 Ma (i.e. Upper Siwalik) named as Zone III consists of 813C enriched values (i.e. -6.18%o to +2.42%o) indicating thereby exclusive presence of C4vegetation. This change in vegetation from C3 to C4 plants was not abrupt rather gradual comprising of both C3 and C4plants and ranging in age between 7-5 Ma (i.e. Zone II). • • • • • IS The palaeoprecipitational reconstruction in the studied sections on the basis of 8 O values of pedogenic carbonates, show progressive increase in aridity from the last -13 Ma to Recent except for a significant increase in rainfall or monsoon intensification at around 10 Ma, 5 Ma and 1.8 Ma. During the deposition of the Lower Siwalik sub-group in both Ramnagar and Kangra sub-basins, stable tectonic conditions prevailed which alongwith humid or sub-humid climate provided fine sediment dominated load of streams and development of meandering streams with broad floodplains. Multiple moderately to strongly developed palaeosol assemblages suggest that the basin was marked by Upland set-up of Thomas et al. (2002) and activity of intra-basinal faults on the uplands and deposition of terminal fans at different times caused the development of multiple soils (Singh et al., 2006). During the deposition of largely thick (> 15 m and some reaching more than 30 m) multistoried sandstones with minor mudstones and weakly developed palaeosols in the Middle Siwalik sub-group of the Ramnagar sub-basin and Middle Siwalik Fm. A of the Kangra sub-basin, it can be interpreted that deposition took place by large braided rivers in the form of megafans in a Lowland set-up (Thomas et al., 2002). The Middle Siwalik Fm. B in the Kangra sub-basin with thick multiple palaeosols alternating with thick sandstones are considered to have been deposited in Upland set-up, suggesting that this sub-basin acted altogether differently from the Ramnagar sub-basin. The presence of alternations of mudstone, sandstone and conglomerates in the lower part of Upper Siwalik in the Kangra sub-basin with weakly to a few moderately developed palaeosols represent distal-braided stream deposits and thick conglomerates with weakly developed palaeosols in the upper part represent proximal braided stream deposits. These Boulder Conglomerates were deposited by small rivers, as in the modern piedmont. vi The presence of mudstone, a few thin gravel beds and dominant sandstone lithology with weakly to moderately developed palaeosols in the Purmandal sandstone Fm. in the Ramnagar sub-basin indicates prevalence of shallow braided fluvial streams. The deposition of mudstone dominant Nagrota Fm. with moderately to some well developed palaeosols and a zone of Gleysols with laminated mudstones and thin sandstones took place in an environment marked by numerous small lakes, water-logged regions and small streams in an environment just south of the Piedmont zone, as is happening presently in the Upland region/the Upper Gangetic plain. This area is locally called the 'Trai region'. Deposition of Boulder Conglomerate Fm. took place by gravelly braided river system close to the Himalayan Ranges. Activity along the Main Boundary Fault led to progradation of these environments distal-ward and led to development of on the whole a coarsening upward sequence. In the last phase, a large river may have been active in this area. Integrating all the field and laboratory observations, the following major conclusions can be made: 1. During the deposition of the upper half of the Middle Siwalik sub-group, the basin was segmented in nature. During the deposition of the Upper Siwalik in the Ramnagar subbasin, an interesting environmental set-up prevailed. Close to the Himalayan Ranges, a piedmont zone with small streams was present. Distalward a wet zone marked by numerous small lakes and water-logged regions and small streams was present. Further distalward, a stable upland with small streams and large regions subjected to pedogenesis was present. Significant orogenic activities at about 10 Ma and 5 Ma in the Himalaya also affected the HFB, causing a change from the Upland set-up to Lowland set-up over major parts of the basin at -10 Ma and back to Upland set-up at -5 Ma in western parts of the basin. 2. Palaeovegetational reconstructions indicate a significant change in vegetation from the exclusive C3 vegetation pre-7 Ma to dominance of C4 vegetation by 5 Ma. The threshold value necessary for C3 photosynthesis was globally achieved for a considerable time during Late Miocene and seems to be the probable cause of the Late Miocene global expansion of C4 grasslands irrespective of their time of first appearance on different parts of the Earth. 3. Climate was cool and humid to sub-humid in the pre-7 Ma period and by 5 Ma there was change in climatic conditions to warm and semi-arid to arid on the evidences of Vll palaeovegetational and palaeoprecipitational reconstructions during the last - 13 Ma. There was a significant increase in rainfall or monsoon intensification at around 10 Ma, 5 Ma and 1.8 Ma. 4. The stable isotope analysis of non-pedogenic carbonates showed their little importance in I ^ palaeovegetational and palaeoprecipitational reconstructions. By comparison of the 8 C and 8180 values between non-pedogenic and pedogenic carbonates, we find that that the post-6 Ma non-pedogenic carbonates were largely formed by shallow groundwater, whereas pre-6 Ma, non-pedogenic carbonates were formed by deep groundwater conditions. * 5. Following classification given by Mack et al. (1993), the Siwalik palaeosols in both the Ramnagar and Kangra sub-basins are classified into three major types: Spodosols, Argillisols and Calcisols. A thin patch of gleysols is observed in the middle part of Upper Siwalik of the Ramnagar sub-basin. On the other hand, the Dagshai palaeosols can be classified in two major types: Oxisols and Spodosols. 6. The bright red coloration of abundant palaeosols of the Dagshai and Lower Siwalik sediments is due neither to inheritance from source rock nor due to presence of a cool and sub-humid palaeoclimate, but has been, caused by burial diagenesis on a large scale.