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.