Geoscience Reference
In-Depth Information
(Miller and Tevesz,
2001
), with permanent water gastropod shells accumulating as
beach deposits as the lake level drops (Williams et al.,
2003
; Williams et al.,
2006c
).
Fossil ostracods are another valuable indicator of water chemistry and temperature (De
Deckker and Williams,
1993
; Holmes,
2001
). If aquatic pollen grains and charophytes
are preserved within the lake sediments, they may also be used as indicators of
water depth and salinity (Livingstone,
1980
). In addition, the calcareous shells of
snails and ostracods and the calcareous oogonia of charophytes can all be used
for radiocarbon dating (Bj orck and Wohlfarth,
2001
), provided there has been no
recrystallization of the original carbonate and that the fossils are no older than about
50 ka. Other methods of dating lake sediments include luminescence techniques
[although problems of partial bleaching of quartz grains being dated will need to be
resolved (Lian and Huntley,
2001
)], electron spin resonance (Blackwell,
2001
a) and
amino acid racemisation dating (Blackwell,
2001
b). Paleomagnetic correlation is also
useful but needs independent calibration against other dating methods (King and Peck,
2001
).
In some instances, it may be difficult to distinguish between former fluvial and
former lacustrine sediments, especially when both consist of horizontally bedded silts
and clays rich in aquatic gastropods and ostracods. The finely laminated slackwater
silts in the arid valleys of the Sinai and Namib deserts, as well as those in the presently
semi-arid Flinders Ranges of South Australia, were all initially interpreted as former
lake deposits until detailed topographic surveys established that the gradients of the
upper surface of these late Pleistocene fine-grained, valley-fill deposits were parallel
to those of the present-day channel floors eroded down to bedrock and were too steep
to have been tilted by epeirogenic uplift to that degree in the brief time available
(Williams et al.,
2001
; Williams et al.,
2006a
; Haberlah et al.,
2010a
; Haberlah et al.,
2010b
).
On occasion a river may become a lake. Consider, for example, the lower White
Nile, which had an unregulated flood gradient of 1:100,000 prior to the completion
of the Jebel Aulia Dam in 1935 near the distal end of that river. As a consequence
of its gentle gradient, erosion has been minimal, so that it has a remarkably well-
preserved sedimentary flood record. This record spans at least the last 240,000 years
and covers two full glacial-interglacial cycles (Williams et al.,
2003
). During the last
interglacial 125,000 years ago, the White Nile formed a lake at an elevation of 386 m
(relative to the Alexandria datum) that was more than 650 km long from south to
north and up to 80 km wide from east to west (Barrows et al.,
2014
). This lake was
stable at that level long enough for a series of beach ridges several hundred metres
wide to develop between rocky headlands located 20-40 km apart. The ridges consist
of sandy gravels derived from erosion of the local Precambrian Basement rocks. A
second White Nile lake came into being soon after flow resumed from the Ugandan
lakes in the headwaters of the White Nile some 14,500 years ago, after a long dry
interval spanning the Last Glacial Maximum (21
±
2 ka) (Williams et al.,
2006c
).
