Geoscience Reference
In-Depth Information
upper member appears to have been deposited quite rapidly during a return to a
stronger monsoon.
The Khetaunhi formation forms a low alluvial terrace up to 10 m thick and is
banked against the older formations. It consists of interbedded silts, clays and fine
sands and is overtopped during present-day floods. It contains Neolithic artefacts and
was laid down between 5.5 ka and 3.5 ka.
There is some evidence in support of another formation informally termed the
Khunteli formation (Williams et al.,
2006b
). This formation contains reworked vol-
canic ash from the 74 ka Toba super-eruption discussed in
Section 19.5.4
. The ash
overlies at least 6 m of medium sand, is up to 4 m thick and is overlain by up to 20 m
of interbedded clays, sands and rolled carbonate gravels. The ash is a channel-fill
deposit and has been locally eroded and replaced by several lenses of alluvial quartz
and carbonate gravels. The absence of any deep weathering and iron precipitation
in the alluvial sands and gravels suggests that it post-dates the Patpara formation.
Carbonate cementation of the gravels above the ash points to drier conditions follow-
ing the eruption, consistent with the
13
C values in pedogenic carbonate nodules in
fossil soils above the ash layer, which indicate that after the eruption grassland and
open woodland replaced the former forest cover in this region (Williams et al.,
2009a
;
Williams et al.,
2010a
).
While it is not possible to draw precise climatic inferences from the Quaternary
alluvial deposits in the Son and Belan valleys, some provisional conclusions can be
drawn. The phase of widespread and prolonged aggradation in the two valleys between
around 39
±
±
3 ka ended with sustained vertical incision and a
return to warmer, wetter conditions. Renewed aggradation starting at around 5.5 ka
marks a return to a weaker monsoon, more erratic rainfall and the onset of more
frequent ENSO events (see
Chapter 23
) in this region (Williams et al.,
2006b
).
Enigmatic features of the landscape of the northern Thar Desert and Indus Basin
are the abandoned cities of Harappa and Mohenjo-Daro. These cities formed part of
the once flourishing Indus Valley Culture but were abandoned some 3,500 years ago.
Located along the banks of now dry river systems, they have aroused debate as to why
they were abandoned. Among the favoured hypotheses are invasion from the north-
west, river capture, tectonic diversion of drainage and climate change (see
Chapter 12
).
What seems reasonably well-established is that the Ghaggar-Hakra drainage system
dried out after around 3.5 ka, at about the same time that the Thar Desert lakes were also
drying out. Climatic desiccation therefore seems a likely cause of the abandonment
of these cities.
9 ka and around 16
19.5.4 Regional impact of the 74 ka Toba volcanic eruption
Few matters have been more widely debated in recent years than the possible impact
on regional and global climate of the great explosive eruption of Toba volcano in
