5.3 Detrital Zircon U-Pb
We examined the range of zircon U-Pb ages using a kernel density estimate (KDE) diagram (Fig. 7) to assess similarities between different sampled sediments and potential source regions (Fig. 7). All of the sediments analyzed in this study show a significant zircon U-Pb component younger than 200 Ma. In addition, we see significant components dated at 350–1250 Ma and 1500–2300 Ma. The abundance of these older age components overall increases with decreasing sample depositional age. The 350–1250 Ma age component appears to increase in all sediment samples dated at 5.87 Ma or younger compared to the older sediments. A particularly prominent age mode at ~1800 Ma first occurs in sediments deposited at 3.43 Ma and becomes extremely prominent in all samples younger than 1.92 Ma. This age mode is also observed in the modern sediment from the Indus river mouth [Clift et al. , 2004].
Examining the <200 Ma zircon U-Pb ages in detail, we see that the vast majority of grains are younger than 120 Ma with prominent age peaks at around 100–120 Ma and 40–70 Ma (Fig. 8). In the youngest samples, especially those deposited starting at 3.02 Ma, we see another age mode at ~20 Ma, although this is also seen in the sample dated at 5.78 Ma. One sample deposited at 3.17 Ma differs in its <200 Ma age spectra from the other samples as it is characterized by a prominent age peak at 100–120 Ma, with a general lack of other young zircon grains.
6 Discussion
Major element discrimination diagrams (Fig. 5) suggest that the Laxmi Basin sediments are most similar to deposits found in the Quaternary and modern Indus River/delta/canyon, as well as the older sedimentary rocks from Indus Marine A-1 (Fig. 5). However, they are distinctly different from sediments sampled from the modern western Indian shelf, and largely derived from the Deccan Plateau and underlying units [Kurian et al. , 2013]. These geochemical data suggest that the Laxmi Basin sediments, most likely originated from the Indus River mouth.
We assessed the overall geochemical characteristics of the sediments by plotting the major element composition of each sample normalized to the upper continental crust (UCC; Fig. 9)[Taylor and McLennan , 1995]. Most of the samples display a relatively uniform topology in these diagrams and are broadly similar to both post-LGM sediments from the Indus Delta (KB-40-4), the Holocene delta (TH-10-1) and, the modern Indus river (Thatta TH-1). Most of the samples show a similar major element composition compared to the UCC, with a consistent enrichment in TiO2, suggestive of a higher content of Ti-bearing heavy minerals (e.g., rutile, anatase, brookite, ilmenite, titanite). This enrichment is particularly strong in the 0.93 Ma sample which apatite fission track data indicate to have a unique provenance [Zhou et al. , 2019]. There are also relative depletions in CaO and Na2O, as well as P2O5, implying both a lower plagioclase and apatite content relative to the UCC. This relative depletion in CaO is strongest in the modern river mouth sediment and weakest in the post-glacial delta sediments, with the fan sediments plotting between these extremes. The systematically lower abundance of plagioclase and apatite likely reflects chemical weathering in the floodplains prior to deposition in the ocean, because these phases are less stable under conditions dominated by chemical weathering [Guidry and Mackenzie , 2000; White and Brantley , 1995]. However, all samples show this effect and there is a general consistency in the overall composition, we conclude that we are comparing sediments of a similar bulk character. All fan sediments show Zr abundances relatively close to the UCC average