Geoscience Reference
In-Depth Information
an absolute enrichment in iron from external sources (Paton and Williams,
1972
;
McFarlane,
1976
; McFarlane,
1983
).
Lithology plays an important role in weathering, and its influence may outweigh
that of climate. For example, Harrison (
1910
;
1933
) noted that primary laterite failed to
develop on granite bedrock inBritishGuiana, but did formon basic rocks in that region.
The notion that laterite invariably demanded a tropical climate was effectively refuted
by Goldschmidt (
1928
) who found it actively forming on labradorite in Norway, by
Robinson (
1932
) who observed it forming on steep slopes in North Wales and by
Crompton (
1960
) who found it forming on basic to intermediate lavas on very steep
slopes under high rainfall in the English Lake District. In all three cases, an efficient
leaching regime was characteristic. Milne (
1938
;
1947
) reached a somewhat similar
conclusion from his soil mapping in East Africa but considered that laterites were
more widespread in tropical regions because higher soil temperatures speeded up
the processes of chemical weathering. In fact, measurements of dissolved river loads
have shown that rates of chemical erosion are no faster in the tropics than they are
in temperate and periglacial regions (Livingstone,
1963
;Davis,
1964
; Gibbs,
1967
;
Douglas,
1969
).
Extensive regions in the Sahara and inland Australia consist of horizontally bedded
Mesozoic sandstones capped by resistant beds that are rich in iron (Bourman,
1995
;
Twidale and Bourne,
1998
). These caprocks have been variously termed 'ferricrete',
'ferruginous duricrusts', 'cuirasses ferrugineuses' and 'ironstone'. Some authors have
claimed that these ferruginous duricrusts are genetically linked to the final stages of
peneplanation and therefore imply prolonged tectonic stability (Woolnough,
1927
).
Despite this questionable assumption, the presence of laterite surfaces at varying
elevations has been used as a guide in reconstructing the Cenozoic tectonic and
denudational history of large tracts of southern Africa and northern Australia (Wright,
1963
; Hays,
1967
; Maud,
1968
; Twidale and Bourne,
1998
). In addition, Glassford and
Semeniuk (
1995
) have proposed that the 'lateritic sandplains' of south-west Australia
are in fact allochthonous eolian deposits and not the result of deep weathering and
laterite formation in situ.
The use of laterite to correlate erosion surfaces assumes that the laterite is the same
age as the surface it overlies, which may not be true (Bourman,
1993
; Bourman,
1995
).
Williams (
1969b
) mapped the distribution of morphologically distinct types of laterite
on a tract of seasonally wet tropical Australia 40,000 km
2
in area and found that in
certain sites laterites were actively forming, in others they were stable and in others
they were disintegrating. In addition, profiles once regarded as matching the so-called
standard monogenetic lateritic profile as defined by Walther (
1915
;
1916
) (consisting
from the base up of a pallid zone, a mottled zone and a ferruginous zone) were seen,
on close examination, to consist of a younger, transported layer of ironstone gravel
lying unconformably above a slightly mottled and kaolinised pallid zone (Williams,
1969b
).
