Geoscience Reference
In-Depth Information
detrital thorium from wind-blown dust, for example, can again lead to inaccuracies in
dating.
One especially useful tool in determining the likely provenance of the initial calcrete
parent material involves the use of stable strontium (Sr) isotope analysis. The
87
Sr/
86
Sr
ratio in the calcrete reflects the geology of the initial source of the calcium in calcium
carbonate and does not alter over time. Dart et al. (
2004
;
2007
) have used this technique
to demonstrate that valley-floor calcretes located more than 400 km from the present
coast in arid and semi-arid inland Australia, once thought to have formed by in situ
weathering of local rocks, were in fact derived primarily from wind-blown dust blown
from coastal eolianites and from the exposed continental shelf during times of lower
sea level.
15.8.2 Equivocal climatic significance of laterite and lateritic soils
Although in popular usage laterites are generally associated with the hot, wet tropics
or the seasonally wet tropics, laterite remnants occur sporadically in many deserts
and semi-deserts. The horizontally bedded Mesozoic sandstone hills conspicuous in
the deserts of Africa, Arabia, India and Australia have been preserved from erosion
by a resistant caprock of ferricrete, or laterite, often silicified. If we can narrow down
the conditions under which such ferruginous duricrusts formed and determine their
geological ages, we will be able to identify what types of environments (and, perhaps,
climates) were once prevalent in those presently arid regions, although such data are
very unlikely ever to yield high-resolution information.
Many authors have considered laterite and lateritic soils to be good indicators of
a hot, wet tropical climate (Cooke,
1958
; Flint,
1959b
). As a generalisation, laterites
are rich in the hydrated sesquioxides of iron and/or aluminium, and are depleted in
bases, alkali earths and silica (Harrison,
1933
; Sivarajasingham et al.,
1962
). Because
they are thought to require a tropical climate for their formation, so the argument runs,
then in those localities where they occur outside the tropics, the climate must once
have been tropical. There are, however, a number of problems with this conclusion.
Paton and Williams (
1972
) have provided a detailed historical review of how the term
laterite has been so changed over time that it now applies to a whole variety of materials
and has little diagnostic value. Originally laterite was defined by Buchanan (
1807
,
pp. 440-441) as a clay located on the uplifted coastal plain of Malabar in India which,
on exposure to the atmosphere, hardened to a brick-like consistency (hence 'laterite',
from the Latin word
lateritis
, or 'brickstone'). This definition excluded high-level
ironstone cappings, which many later workers regarded as laterite (McFarlane,
1976
;
McFarlane,
1983
). Subsequent investigators drewa distinction between autochthonous
(primary) and allochthonous (secondary) forms of laterite, that is, between profiles
in which there was a relative enrichment in iron as a result of weathering processes
that led to leaching and removal of bases and silica and those in which there was
