EVOLUTION OF THE INDOGANGETIC FORELAND BASIN DURING PLIO-PLEISTOCENE PERIOD AS INFERRED FROM HARIPUR-KOLAR SECTION, H.P., INDIA

Abstract

The 2535 mthick Upper Siwalik (Pliocene-Pleistocene) and 271 mof the upper part of the Middle Siwalik sequence consisting of mainly fluvial clastic sediments exposed in Haripur-Kolar section have been studied with a view to deduce environmental set-up, depositional processes, paleoclimatic changes and role of eustatic changes and tectonics on sedimentation. In this study stress is on detailed variation in type and degree of development of soils and use recent studies on geomorphology, soils and sedimentation in the Indogangetic Plains in interpretation of the Siwalik sediments. The stratigraphic sequence of the Siwalik exposed in Haripur-Kolar section was measured and logged. Various characters of the sandstone and paleosols were noted down and samples were collected for laboratory analysis. Lithofacies analysis, architectural element analysis (Miall, 1978) and Markov analysis of transition of these elements were carried out for the Siwalik sediments. Samples from 14 profiles of typical paleosols were analysed for major elements, dithionite extractable iron, clay minerals and micromorphological features. For paleoclimatic and paleoecological inferences 96 calcrete samples were analysed for their carbon and oxygen isotopic composition. The Upper Siwalik is subdivided into four Units l-IV on the basis of lithofacies, architectural elements and degree of development of paleosols. The finer scale stratigraphic restoration from the available broad spaced magnetostratigraphic dates (Sangode et al., 1996) and considering the landscape stability times shown by paleosols give ages of 5.26 Ma, 4.12 Ma, 2.6 Ma, 1.76 Ma respectively for boundaries between Middle and Upper Siwaliks, units HI, ll-lll and lll-IV and 0.1 Ma for the top of the section. The estimated rates of deposition for Units l-IV, considering the soil development, are 61.6, 60.2, 75.8 and 31.3 cm/kyr. Six pedotypes l-VI, with soil increasing from I to VI are recognised. By comparison with surficial soil of the Gangetic Plains, estimated times taken for their formation are: 10-100 yrs, 100-500 yrs, 500-2,500 yrs, 2,500-5,000 yrs, 5,000-8,000 yrs Ml and 10,000-13,000 yrs respectively for pedotypes I to VI. The variation in the degree of soil development from pedotype III to VI over small stratigraphic intervals such as a few meters was probably due to tilting and subsidence/uplift of tectonic blocks. The Upper Siwalik sediments were deposited mainly by basin transverse fluvial system. Two types of river systems, which differ in their sizes can be documented in units I and II : one trunk river system similar to the modern Kosi and the other smaller river system, which formed tributaries to the former. The large rivers were mainly braided in nature. During the deposition of the Middle Siwalik and units I and II, Upland and Lowland set-ups were involved. Lowland set-up involved deposition on mega alluvial fans and interfan areas and which resulted in sand-rich and mud-rich sequences with poorly developed soils. The Upland set-up allowed large tracts to act as upland for 1000's of years giving rise to sandstone poor intervals with moderately to well developed soils. The unit III was deposited in the piedmont zone by large and small braided streams and the unit IV was deposited during an arid climate by debris flows, hyper-concentrated flows, and braided fluvial streams. Tilting/subsidence of the tectonic blocks leading to frequent flooding of downtilted/subsided regions followed by avulsion of rivers to these areas is inferred. The lithofacies/petrographic changes indicate an activity of the Main Boundary Thrust at ca. 3.0 Ma and Nahan Thrust at ca. 1.5 Ma. Tectonics seems to have controlled variation in lithofacies and degree of soil development over 100's of meters and significant changes in the degree of soil development over short intervals as well. The paleoclimate at the time of Siwalik deposition was warm and semi-aridic. Precipitation was around 1000 mm. The monsoonal climate (periodical) was existing in the Pliocene period also. Some cooling is discerned at about 2.7 Ma. At ca. 0.9 Ma a significant change to arid and cooler climate is inferred.