Geoscience Reference
In-Depth Information
conditions was subsequently buried beneath alluvial and colluvial sands laid down
during a wetter climatic interval (Fairbridge and Finkl,
1984
). If the stone layer is
developed in situ as a result of termites removing sand, silt and clay from a deeply
weathered mantle rich in vein quartz or pegmatite, as in many areas of basement
complex rocks, then there is no need to invoke climatic change. This example neatly
illustrates the concept of
equifinality
, or convergence of form, in which landscapes of
similar form may arise from the operation of entirely different sets of processes.
15.7 Identification and interpretation of paleosols
Paleosols, or buried fossil soils, are widespread in every arid and semi-arid region of
the world (
Figure 15.4
). Not all such soils are buried; many crop out at or near the
surface, often in truncated form. Within dunes, the presence of buried soils denotes
a phase of stability that is usually associated with a wetter climate and widespread
vegetation cover (Singhvi et al.,
2010
). Similarly, the regular alternation of loess
and intercalated paleosols developed on weathered loess denotes an alternation of
cold, dry, windy climatic intervals in which loess is deposited downwind and warm,
wet intervals that allow plant growth to thrive and soils to develop (Liu and Ding,
1998
). Red, clay-rich soils and dark cracking clay soils ('vertisols') in the heart of
the Sahara are likewise diagnostic of wetter times (Rognon,
1967
; Williams et al.,
1987
). However, because soil development depends on five independent variables, it
is very hard to quantify the temperature and precipitation regime under which those
soils developed. Awell-defined textural B-horizon may denote an abundance of wind-
blown dust and an efficient leaching regime under seasonally wet conditions, or it
may reflect a very long interval of moderate soil formation. In the absence of other
independent means of quantifying past climatic or other environmental attributes,
usually including the detection of vegetation type using pollen and phytolith analysis
(
Chapter 16
) or carbon isotopic analysis, one is inclined to agree with Galloway
(
1971
) that soil scientists are more likely to be consumers than producers of paleocli-
matic data. With this cautionary note in mind, paleosols can still provide information
that is not always obtainable by other means, as the following examples serve to
illustrate.
In the far north-west of Sudan, there is a huge volcanic massif, Jebel Marra, which
covers an area of 13,000 km
2
. The highest summit on this mountain attains an elevation
of 3,042m. The massif lies at the intersection of twomajor tectonic lineaments that run
across North Africa from south-west to north-east and from south-east to north-west,
and had an estimated original volume of about 8,000 km
3
, compared to 3,000 km
3
for
Tibesti. The flora of Jebel Marra is unique and contains a mixture of plant species from
the southern tropics and from the northern Mediterranean realms (Wickens,
1975a
;
Wickens,
1975b
;Wickens,
1976a
;Wickens,
1976b
). Along the western and southern
margins of the volcanic massif, there are cliff sections exposed where rivers have cut
