Abstract
The clay mineralogy of the Late Pliocene-Early Pleistocene Pinjor Formation of the type area, northwestern Himalaya, India has been investigated to understand the paleoclimatic conditions and paleotectonic regime prevailing in the frontal Himalayan terrain during 2.5 Ma to 1.7 Ma. The clay minerals were investigated by X-ray diffraction analysis and scanning electron microscope studies. Study of the oriented aggregates of 47 representative clay samples of the Pinjor Formation of the type area reveals that illite is the most dominant mineral followed by chlorite, kaolinite, vermiculite and mixed layer clay minerals. The distribution of the clay minerals in the three lithostratigraphic units of the Formation, namely the Kona Clay Member, the Tanda Bhagwanpur Wacke Member and the Chauki Nadah Pebbly Bed Member which are well exposed along the Berwala-Mandhna section, the Kona-Karaundanwala section and the Ghaggar River-Chauki Nadah section, is nearly uniform suggesting thereby the prevalence of similar sedimentation environments in the Himalayan foreland basin. The presence of illite and kaolinite suggests their derivation from crystalline rocks containing felspar and mica as also from pre-existing soils and sedimentary rocks. Further, the paleoclimatic conditions were moderate. Presence of chlorite suggests the weathering of intermediate and basic crystalline rocks and low grade metamorphic rocks in the positive areas. The presence of kaolinite in the Pinjor Formation is mainly attributed to the weathering and subsequent leaching of the mineral from granitic and basic rocks in the hinterland. Vermiculite has been mainly formed by weathering and transformation of biotite. Warm and humid climatic conditions prevailed for a major part during the deposition of the detritus which favored weathering and transformation of minerals. During the terminal phase of sedimentation there was renewed tectonic activity which had a significant impact on climate as precipitation and mechanical weathering rates increased substantially. Post 1.7 Ma there was a marked shift in temperature patterns and subsequent cooling of the landmass, which resulted in a decreased vegetation cover and a subsequent decrease in animal population thriving on it.
Highlights
Clay mineral assemblage of the Late Pliocene-Early Pleistocene Pinjor Formation of the type area in northwestern Himalaya is of significant importance in understanding the paleoclimatic and paleotectonic conditions prevailing in the positive and negative areas and their interrelationship; types of source rocks, modes of sediment transport, sedimentation environments and post-depositional changes experienced by the sediments.The Himalayan orogenic belt is topographically divided into three major sub divisions, namely the southernmost frontal Himalaya comprising the low lying Siwalik Hills, the middle Lesser Himalaya and the northern Great Himalayan belt
The Himalayan foreland basin has formed due to flexure of the Indian plate caused by the high mass of the evolving Himalayan mountain belt
The Himalayan tectonic activity reached its acme during the Quaternary period
Summary
Clay mineral assemblage of the Late Pliocene-Early Pleistocene Pinjor Formation of the type area in northwestern Himalaya is of significant importance in understanding the paleoclimatic and paleotectonic conditions prevailing in the positive and negative areas and their interrelationship; types of source rocks, modes of sediment transport, sedimentation environments and post-depositional changes experienced by the sediments. From near Jammu in the West to near Tripura in the East It represents a huge thickness of sediments ranging from 3300 m to 6300 m [1] which were deposited in a foredeep. Reference [1] observed that the Siwalik Group of the western Himalayan is a product of two coarsening up mega cycles. The second mega cycle comprising the Middle and the Upper Siwalik formations is marked by coarsening of the sediments which eventually indicates a fast rate of degradation processes. Some of the more important contributions during the past decade have been made by [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
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