TRACKING SOURCES OF SEDIMENTS IN GREAT RANN OF KACHCHH, WESTERN INDIA DURING LAST 17 KA USING SR AND ND ISOTOPE COMPOSITION

This study attempts to identify possible sediment sources and track their temporal variations by analyzing Sr and Nd isotope compositions of sediments from two continuous cores obtained from Dhordo and Berada in the central and marginal part of the Great Rann, a dried up palaeo-gulf at the western continental margin of India. ⁸⁷Sr/⁸⁶Sr values range from 0.72545 to 0.73249 and εNd from -14.84 to -12.63 in the Dhordo core however the Berada core ⁸⁷Sr/⁸⁶Sr ratio varies from 0.72508 to 0.73219; and εNd from -15.11 to -12.71. The radiogenic nature of Sr and less radiogenic εNd of sediments diverge with proximal lithologies of Mesozoic, Cenozoic sedimentary rocks as well as the Deccan trap formation. Sr-Nd isotope records indicates mixed origin of the sediments i.e. via Indus (characterized by lower⁸⁷Sr/⁸⁶Sr, higher εNd) and, Ghaggar/Saraswati river (with higher ⁸⁷Sr/⁸⁶Sr, lower εNd) and are mainly sourced from the different parts of the Himalayan (Higher/Lesser Himalaya) and the Trans-Himalayan lithologies. The Aravalli range, due to its proximity could be a possible source (with high ⁸⁷Sr/⁸⁶Sr and lower εNd), however considering its lower relief, this proposition seems unlikely. Temporal variations in Sr-Nd isotope compositions with available age control show stable signatures in Late Pleistocene. However, concomitant changes in ⁸⁷Sr/⁸⁶Sr and εNd are observed during Late Pleistocene/Early Holocene which is also seen in Berada core during Early Holocene. These records are in contrast with those observed in the Indus delta. The decreasing ⁸⁷Sr/⁸⁶Sr and increasing εNd patterns since Late Pleistocene/Early Holocene hints towards decreasing sediment supply from the Higher/Lesser Himalaya via the Ghaghar/Saraswati river. Reduction/increase in ⁸⁷Sr/⁸⁶Sr/εNd isotopes of sediments is unlikely to be attributed to increased input from the Indus distributary and/or alongshore sediment transport as these variations are opposite in the Indus. Consistent decrease/increase in ⁸⁷Sr/⁸⁶Sr/ εNd isotopes of sediments during Early to Mid Holocene times indicates gradual decline in sediment supply from the Ghaggar/Sarswati.