Nd and Sr isotope characteristics of Quaternary Indo-Gangetic plain sediments: Source distinctiveness in different geographic regions and its geological significance

Quaternary sediments of the Himalayan foreland basin, known also as the Indo-Gangetic plains, have been studied for their Sr and Nd isotope geochemistry to constrain their sources and to infer their geological significance. The data show a large variation in the 87Sr/86Sr values and relatively a minor variation in the εNd(0) values, for compositionally homogeneous sediments. Sediments of the Ganga–Yamuna and Satluj systems have high 87Sr/86Sr (> 0.75) and low εNd(0) (<− 16) values, characteristics of High Himalayan Crystallines. Sediments of the Ghaggar river and the Thar desert in the western part of the Indo-Gangetic plains in India have distinctly lower 87Sr/86Sr (< 0.75) and higher εNd(0) (>− 16), all relative to the previous group, suggesting their derivation from the Sub-Himalayan sediments including Tertiary Subathu Formation. The Subathu was likely derived from the Indus-Tsangpo Suture Zone basaltic volcanics and Tibetan Sedimentary Series. The Thar desert/Ghaggar river sediments are isotopically similar to those of the Indus plain indicating a shared provenance, either immediate or ultimate. The exposed sediments of the incised Yamuna river at Kalpi in the Ganga–Yamuna plains were derived from Peninsular India. These southern Ganga–Yamuna plain sediments are isotopically similar to the Thar Desert/Ghaggar river sediments although the juvenile components of their provenance differ — the Vindhyans–Deccan traps and Indus-Tsangpo suture zone volcanics, respectively. In the southern Ganga plain the Peninsular wedge of sediments could be present at depths below about 12 m, interbedded or mixed with Himalayan sediments. The isotope geochemistry of sediments also shows that the earliest influx of the Himalayan sediments to the Ganga–Yamuna interfluve occurred around 68–70 ka. This indicates that the Yamuna river has been flowing in the region at least since the Late Pleistocene. The dynamic relationship between the Yamuna and Chambal rivers in the region negates the idea of Yamuna being a tributary of Ghaggar–Saraswati river system during the Harappan time (3500 to 1900 BC).