Arid environments - Environmental Sedimentology - page 144

5

Arid environments

Anne Mather

5.1 within the sediment transfer system (Fig. 5.1).

This chapter examines the processes involved in
this transfer of sediment within the arid envir-
onment, and the environmental hazards that
occur en route. It provides an insight into the
recent sedimentary deposits of arid environments
and how they are affected by changing external
environmental controls.

INTRODUCTION

Within arid landscapes sediment transfer begins
with the breakdown of bedrock to transportable
material. This can be achieved by weathering
processes, which in arid areas are dominated by
moisture and salt or insolation. Owing to the role
of gravity in the transportation processes, most
source areas will be located within the upland
areas of the arid landscape. These areas are
typically dominated by zones where the erosive
capability (erosivity) of water is high, such as steep
slopes. Other sediment source areas are domin-
ated by zones with sediment that is susceptible
to erosion owing to its weak material charac-
teristics (its 'erodibility'), such as exposed lake
sediments (Fig. 5.1). These available materials
will be transported by wind or water to (i) areas
of temporary storage within transport zones, such
as rivers, or (ii) a more permanent storage area of
net sediment accumulation, such as an untrenched
alluvial fan, a sand sea or a lake. These sediment
stores will themselves be susceptible to recycling

5.1.1 Definition of arid environments

In this chapter arid environments will be con-
sidered to embrace regions described both as
'drylands' and 'deserts'. Deserts are more qual-
itatively defined by a range of physical criteria,
whereas drylands are quantitatively defined
and classified using meteorological data. As
the definition of drylands is based on modern
climate data records, however, it is difficult
to rigorously apply this definition to older
historic to geological time-scales. As a result
both terms (drylands and deserts) have tended
to be used interchangeably in the literature,

Sediment sources Sediment transport Sediment storage

Lakes/playas/sabkhas

Slopes Water

Ephemeral channels

Alluvial fans

Fig. 5.1 Simplified
flow diagram of
sediment production
and routing in arid
environments.

Stored
sediment

Wind

Aeolian dunes

Weathering