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