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Mineralogy of the Ganges and Mahanadi Rivers: Implication of Provenance Change in the Offshore Bengal Basin, India

Nabarun Pal1, Sudipta Dasgupta1, and Andrew Morton2
1Petroleum Business (E&P), Reliance Industries Ltd, Mumbai, India
2HM Reserach, West Midlands, United Kingdom

Heavy mineral assemblages are sensitive indicators of sediment provenance. The north-eastern coast of India is fed by two major river systems viz. the Ganges and the Mahanadi. Subsurface seismic data exhibit little difference in the observed patterns in sediments derived from the rivers. Here we aim to determine the relative dominance of the two river systems as sediment suppliers to the Neogene succession. Modern riverbed grab samples are studied for their heavy mineral assemblages. They are found to be extremely diverse, which is considered to be the result of differences in hydrodynamic conditions and probable influences of tributaries. These variations are reduced by using provenance-sensitive ratios, which show that both river systems form relatively tight clusters on ratio crossplots. A number of ratios offer means for discriminating the river systems, since sands from them show little or no overlap on the crossplots. For e.g. garnet:zircon and rutile:zircon values are lower in the Mahanadi than in the Ganges, and monazite:zircon values are higher in the Mahanadi than in the Ganges. In addition to the above ratios, there are also major differences in the relative fraction of the metasedimentary constituent mineral group; e.g. sillimanite forms >88% of the metasedimentary mineral group in the Mahanadi, but <33% in the Ganges. Comparison of heavy mineral data from few wells drilled by RIL in the area provides evidence for change in provenance during deposition of the Miocene-Pleistocene succession. The Ganges appears to have been the sole sediment source during Tortonian to Piacenzian Stage. In Late Gelasian-Pleistocene supply from the Mahanadi was dominant, but there is evidence for variable Ganges influences within this interval. There is one sand unit within Messinian Stage with slightly lower GZi values, and this may have been influenced by the Mahanadi.

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