I post a link to my new review of about the long-term Geomorphology and Quaternary dynamics of the Namib Sand Sea, which follows the presentation I gave at Gobabeb for the SAAG conference. It is called: "Age and dynamics of the Namib Sand Sea: a review of chronological evidence and possible landscape development models" and is available online now.
Abstract
The
Namib Sand Sea constitutes a major physiographic feature of the Namib
Desert on the west of Namibia, covering a 50-160 km wide region on the
west coast of Namibia between Lüderitz and Walvis Bay. It is widely
considered to be one of the oldest desert regions, with a Tertiary-aged
fossil desert underlying the modern sand sea. The sand sea has been well
studied, benefiting from the presence of the Gobabeb Training and
Research Centre over the past 50 years. Whilst much is understood about
its sediments and geomorphology, it is only recently that new
chronological information, using cosmogenic-nuclide burial dating and
optically stimulated luminescence dating have offered new insights, and
these call for an updated review of the age and landscape development of
the sand sea. This assessment of the geomorphological and Quaternary
dynamics of the region is complemented by developments in description
and analysis of sediment composition.
New age control
from cosmogenic dating indicates that the sand sea is in excess of a
million years old, whilst the initial data from luminescence dating
yield depositional ages for dune sediments in three broad areas of the
sand sea that include MIS 5, later in the Pleistocene around the Last
Glacial Maximum and the Holocene, although it is not expected that these
will be the only, or discrete age groupings. Detailed dating and
application of ground penetrating radar in the far northern reaches
reveals extensive dune migration and deposition during the Holocene. It
is important to stress that the upper limit of luminescence dating here
is about ∼200 ka (depending on the environmental dose rate of the site)
and that migration and reworking of dunes reset the luminescence signal
(so what is recorded is the last phase(s) of preserved sediment
accumulation).
Whilst there are three potential
sources of material for the Sand Sea (reworked Tsondab Sandstone (TSS),
material from the Great Escarpment derived by rivers and water and
wind-derived material from the Orange River delta) the weight of
evidence points towards the dominance of an Orange River source, with
localised contribution from fluvially-derived escarpment material close
to river courses. Despite the fact that it remains difficult to
definitively distinguish between recent Orange River sediment and
recycled TSS because of a great mineralogical similarity, an Orange
River source contemporaneous with the accumulation of the sand sea
appears to be favoured. Models of landscape development rely on
understanding source region, and an Orange River source suggests growth
and extension from south to north (a wind-displaced Orange Delta),
rather than localised reworking of sediment from the TSS. One model,
proposed for the southern part of the sand sea, divides accumulation
into two distinct phases with different palaeoenvironmental conditions:
large draas accumulating under enhanced Pleistocene trade winds and
superimposed features on the eastern dune flanks formed by westerly
winds moving material over the crest. However, the latter phase could
equally be explained by a northerly migration of the superimposed
features, and there is still too little in the way of chronological
control to construct a coherent picture of dune accumulation and
migration for the sand sea as a whole. There are also interesting
insights from conceptualising dune bedform patterning in sand seas as a
time-dependent, self-organising, complex system, rather than necessarily
requiring changing palaeoenvironmental conditions for different scales
of features, with some of this research referring directly to the Namib
Sand Sea. Refining the details of the accumulation of the Namib Sand Sea
requires both detailed site-specific studies and joined-up analysis.
Highlights
►
Review of age and models of landscape development in the Namib Sand
Sea. ► Approaches to assessing the age of dune forms are reviewed. ►
Ideas about development of sand sea tightly connected to sediment
sources. ► Sand sea is > 1 million years old, with later Pleistocene
and Holocene deposition. ► Landscape development may include northerly
migration and bedform self-organisation.
Keywords
- Namib Sand Sea;
- Geochronology;
- Geomorphology;
- Quaternary;
- Namibia
Thank yous!
I'd like to take the opportunity to again thank a number of people for fruitful discussions about the Namib Sand Sea: Andrew Goudie, Charlie Bristow, Kevin White, Mary Seeley, John Ward, Ian Livingstone, Frank Ekhardt, David Thomas and Mark Bateman. The author thanks two anonymous reviewers for their constructive comments, which have improved the manuscript.
And to sources of funding - the paper was drafted whilst in receipt of a John Fell Fund Small Award at the University of Oxford (School of Geography and the Environment), which partly funded attendance of the SAAG and SASQUA conferences in September 2012 at which these ideas were presented. Funding to attend the UK aeolian research meeting ‘Windy Day’ in October in Leicester where revised ideas were presented, came from St John’s College.
Best wishes
Abi
