5.2 Bulk Sediment Chemistry
The general geochemical character of the sediments can be seen on a CN-A-K ternary diagram [Fedo et al. , 1995](Fig. 5A). The IODP samples plot in an array with a Chemical Index of Alteration (CIA) of ~65 to 73 [Nesbitt et al. , 1980]. They form a roughly linear array trending towards the illite end member and suggestive of its progressive involvement as the primary mineral breakdown product. The Laxmi Basin samples can be compared with sediments from the Quaternary Indus delta [Clift et al. , 2010], Indus Canyon [Li et al. , 2018], the Indus Marine A-1 borehole, as well as modern sediments from the western Indian shelf and slope between the Saurashtra peninsula and Bhatikal [Kurian et al. , 2013](Fig. 1). Rivers south of Bhatikal have a different composition and are unlikely to be sources to the Laxmi Basin sites.
The Laxmi Basin sands have very similar bulk compositions to the Quaternary Indus canyon and delta, as well as the Indus Marine A-1 samples (Fig. 5A), but plot below or to the right of the array of the western Indian shelf sediments. Only the shelf sample taken near Bhatikal (the southernmost shelf sample that overlies the Precambrian crystalline basement of India, rather than the Deccan Traps), plots below the Laxmi Basin sediments, with a lower CIA value. This plot confirms that the analyzed sands have little in common with material eroded from peninsular India and appear consistent with an Indus River origin. Likewise, the sediments plot close to the Quaternary Indus sediments and those of Indus Marine A-1 on the discrimination diagram of Herron [1988](Fig. 5B). The IODP samples plot with slightly lower Fe2O3/K2O values compared to the proximal sediments. The Laxmi Basin sediments form an array defined as shales and wackes, while the western Indian shelf sediments fall into the Fe shale, litharenite, sublitharenite and Fe sand fields.
We further assess whether grain size has any relationship to zircon concentrations by plotting Zr contents against median grain size (Fig. 6). Although the original source characteristics and hydrodynamic sorting of the sediment might be expected to concentrate zircons in certain size fractions this does not appear to be a significant factor within the range of grain sizes considered here.