A Comparative Heavy Mineral Study of the Cenozoic Sediments of Assam and Siwalik Foreland Basins, Northeast Himalaya
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A Comparative Heavy Mineral Study of the Cenozoic Sediments of Assam and Siwalik Foreland Basins, Northeast Himalaya Priti Rai1, Biraj Borgohain1, Nisha Chettri1,2 and George Mathew1* 1
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India Presently at Department of Geology, St. Anthony College, Shillong -793 001, India * E-mail: [email protected] 2
ABSTRACT Heavy mineral assemblages of sedimentary units are used as an essential provenance constraint. The sedimentary successions of the southern Assam and Siwalik foreland basins were analyzed for their heavy mineral suits and provenance determination. Heavy minerals were identified using the petrological microscope, X-ray Diffraction (XRD) and Electron Probe Micro Analyzer (EPMA) analyses. The study demonstrates that heavy minerals weight percentage in the southern Assam basin is lower than that of the northeast (NE) Siwalik basin. In the Assam basin, the weight percentage of dense minerals varies from 0.08% to 1.31%; however, in the Siwalik sediments, it varies from 2.27% to 5.78%. The relative dominance of opaque minerals over transparent heavy minerals is observed throughout the Cenozoic rocks of Assam basin, except for the Tipam unit. At the same time, the Siwalik basin successions show a low percentage of opaque minerals and high amount of non-opaque heavies. Detritus in the Assam basin reveals a distinct change in the occurrence of heavy mineral assemblages (amphibole, aluminosilicates, staurolite and a high percentage of epidote, pyroxene and sphene) in the Mio-Pliocene unit (Tipam sandstone) compared to Barail and Surma older units, implying involvement of an additional orogenic source. This study suggests that NE Siwalik sediments were primarily derived from Himalayan rocks of the Siang window, while heavies in the detritus of the southern Assam basin exhibits a mixed provenance both from the eastern Himalaya, chiefly from Dibang and Lohit valley and IndoBurman-Ranges (IBR). INTRODUCTION Adequate knowledge of the heavy mineral assemblages in sediments has always provided vital information for provenance reconstruction, as they divulge the nature of the source rocks from which they are derived (e.g., Mange and Maurer, 1992; Mange and Wright, 2007; Kripnner et al., 2016; Fossum et al., 2019). The study of heavy minerals in the foreland basin systems is useful to decipher composition, tectonic history and character of the provenance (Mange and Maurer, 1992; Najman, 2006; Zhang et al., 2017; Yang et al., 2019). The foreland basin deposits reflect events associated with the erosional history of the orogenic belts (Mange and Maurer, 1992). A variation in heavy mineral distributions illustrates that there may be distinct provenances (Krynine, 1942; Morton et al., 1992, 2003; Singh et al., 2004; Fossum et al., 2019). The analysis of heavy minerals has also been an effective means in the reconstruction of sediment’s source, course of the ancient river systems and unroofing history of the orogeny (Uddin et al., 2007a).
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