Abstract:
Petrographical and geochemical data of modern coastal and estuarine sediments from the Gulf of Khambhat, western India, illustrate the problem of inferring tectonic setting from petrographic and geochemical discrimination diagrams. The passive margin basin at the western coast of India receives sediments from diverse sources from the stable craton of peninsular India, in which a considerable input is from the Cretaceous Deccan basalt that occupies a major part of the hinterland. Tectonic setting discrimination diagrams do not correctly identify the passive margin signature of sediments from the Gulf of Khambhat in most cases. Tectonic setting is correctly interpreted only in the absence of basaltic derivatives in modern sediments. The sediments predominantly derived from the Deccan basalt are wrongly placed in the oceanic island arc setting in the tectonic discrimination diagrams. Mixing of basaltic sediments derived primarily from the southern rivers with felsic sediments supplied from the northern rivers in the gulf tends to reflect active continental margin and oceanic island to continental island arc settings. Discrimination of the tectonic setting is complicated in estuaries because of the mixing of mafic and felsic derivatives in different proportions. Correct identification of the tectonic setting is possible in the inner estuaries, lying beyond the range of tidal influence, because of negligible mixing of sediments supplied by rivers and by tidal currents. Tectonic setting discrimination diagrams are therefore misleading for modern shallow marine basins situated close to continental flood basalt and, by implication, should be used carefully for ancient successions, given the common occurrence of basalts in the geological record. .