Geoscience Reference
In-Depth Information
Climatic frameworks: legacies from the past
David S.G. Thomas and Sallie L. Burrough
3.1
Introduction
evolved. Sedimentary and tectonic events as far back as
the Precambrian have played a role in the nature of some
deserts today, such as the Sahara (Williams, 1984).
There is also an economic significance today for under-
standing the ancient aspects of deserts, as ancient evapor-
ite and aeolian sediments can be important sources of hy-
drocarbons. The elucidation of the whereabouts and nature
of ancient deserts is therefore of paramount economic im-
portance and will be considered briefly. The sporadic exis-
tence of deserts has been identified from the rock record as
far back as the Proterozoic (Glennie, 1987) (Figure 3.1).
Various criteria may be used to credit an ancient sediment
with an arid zone origin, including mineralogy, grain-size
distribution, bedding structure and the micromorphology
of individual particles (Table 3.1). These criteria may al-
low the depositional context or landform association of
sediments to be determined. The utility of these crite-
ria for palaeoenvironmental reconstruction is dependent
upon the application of results from studies of today's arid
zone phenomena, the so-called uniformitarian or modern
analogue approach.
Studies of ancient desert sediments have led, for ex-
ample, to the identification of different dune types from
bedding structures in the rock record (see, for example,
Ahlbrandt and Fryberger, 1982) and to the recognition of
complicated depositional systems in ancient sandstones
(Crabaugh and Kocurek, 1993). Perhaps most signifi-
cantly, it has also been recognised that ancient desert
systems were as variable and complex as those of today.
However, the environmental significance of pre-Devonian
aeolian sediments is likely to be rather different from that
of modern sand seas, as prior to the colonisation of land
surfaces by plants wind activity may have been a more
potent geomorphological agent, even in relatively humid
environments (Wilson, 1973; Glennie, 1987).
In Chapter 1 the broad climatic controls on the distribu-
tion of drylands and deserts are presented. Morphological
and sedimentary evidence indicate that the world's deserts
have experienced significant episodic expansions and con-
tractions during the Quaternary Period, and that these
changes are primarily due to global climate changes. The
geological record further indicates that the positions of
deserts have changed in response to major global tectonic,
as well as climatic, developments throughout Earth's his-
tory. Therefore, to appreciate fully the development of
drylands and the landscapes, landforms and sediments
that they possess, it is necessary not only to understand
the geomorphological processes that occur within these
areas today but also to develop a record of the environ-
mental changes that these regions have experienced in
the past.
3.2 The significance of arid zone
fluctuations in the past
3.2.1
Ancient arid zones
The emphasis in this chapter is on changes in the arid zone
during the Quaternary Period of geological time, partic-
ularly the Late Quaternary (last
250 000 years). How-
ever, changes over longer geological time periods have
also been important in desert evolution. Williams (1994)
has noted that as a generalisation, erosional landscapes
in deserts tend to be significantly older than the deposi-
tional landforms they possess: in other words, very ancient
events have set the scene in which modern deserts have
Search WWH ::




Custom Search