Geoscience Reference
In-Depth Information
stronger and gustier than today, particularly during colder, drier intervals in the past
100 ka, when the dune fields and alluvial outwash areas east of the Pilbara were far
more sparsely vegetated, considerable volumes of wind-blown dust were blown from
the arid centre of Australia in a north-west direction out to sea (Bowler,
1973
;Bowler,
1976
). Wind-blown dust is as a rule very well sorted, with the grains rounded to sub-
rounded and a high proportion of quartz particles. More locally derived dust can be
bimodal in terms of particle size but generally better sorted than fine alluvial sediments.
The Pilbara would have been in the pathway of this very wide dust plume. Dust
scavenged from the atmosphere during sporadic rains would be deposited downwind
of the source, and it would therefore be available for reworking by local streams. The
particle size of dust is usually in the silt to clay fraction, that is, generally finer than
about 15 to 20
m, but within the longitudinal axis of the dust plume, particles as
coarse as very fine sandy silt (20-60
μ
m) can be transported for several hundreds of
kilometres downwind before deposition from suspension. Fine sand is more likely to
have been transported by saltation near the base of the dust plume (Pye,
1987
). It is
therefore quite possible that the ephemeral streams were carrying a mixed load of both
locally derived ironstone sand and gravel and very fine sand of fluvio-eolian origin
that had undergone multiple cycles of reworking. The resulting weakly developed
fluvisols/entisols (
Tables 15.1
and 5.2) on the terrace surfaces in the Pilbara region
reflect this polygenic inheritance.
Certain late Pleistocene valley-fill deposits in arid mountainous areas are derived
almost entirely from reworked desert dust or loess. Examples include the Matmata
Hills in Tunisia (Coude-Gaussen et al.,
1987
)(
Figure 15.6
), the Flinders Ranges
in South Australia (Williams et al.,
2001
; Haberlah et al.,
2010a
; Haberlah et al.,
2010b
), the Sinai Desert (Rogner et al.,
2004
) and the Namib (Eitel et al.,
2001
;
Eitel et al.,
2005
). In all four of these regions, the fine-grained alluvial silts and clays
display one or more intercalated paleosols within the main alluvial sequence, which
is indicative of a brief halt to valley aggradation with sufficient time for plants to
colonise the alluvial surface and for organically enriched soils to develop, a process
that might take decades or centuries. In the seasonally wet Son Valley in north-central
India, a vertically stacked sequence of soils has formed on late Pleistocene and early
Holocene silty levee deposits, with pedogenic carbonate nodules common within each
soil (Williams and Clarke,
1995
).
The vertical alternation of parent material-soil-parent material in a variety of geo-
morphic contexts (eolian, alluvial, colluvial) prompted Butler (
1959
;
1967
)tocoin
the term K-cycle for each such couplet identified in south-east Australia (Butler,
1967
). In descending stratigraphic order, each couplet was labelled K
1
,K
2
,K
3
and so
on. Soils were presumed to have formed during intervals of landscape stability and
to have been buried during ensuing phases of landscape instability. Valiant attempts
to correlate individual K-cycles were ultimately thwarted by poor chronologic con-
trol, by the realisation that many factors can contribute to local erosion and by the
μ
