Coupled study of geomorphology - sedimentary geometry of the South Namibian margin : constraints on the kinetics of of the surrection of the South African Plateau
Étude couplée géomorphologie - géométrie sédimentaire de marge sudnamibienne : contraintes pour la cinétique de la surrection du Plateau sud-africain
Abstract
The timing of the uplift of the Southern African (or Kalahari) Plateau is
debated, with four scenarios of uplift : (1) at time of rifting (Early Cretaceous),
(2) during Late Cretaceous, (3) around the Eocene-Oligocene
boundary and (4) during Pliocene times. This knowledge is of primary
importance for a better understanding of the mantle processes at the origin
of this very long wavelength structure.
To answer this question, we studied the key area of the northern Orange
Margin (sequence stratigraphic analysis of seismic lines and wells), offshore
of the Sperrgebiet. This study is coupled with an analysis of the
landforms dated with associated sediments and volcanics.
(1) The first uplift of the Plateau was during Late Cretaceous times, with
a first increase of siliciclastic sediments supply during Late Cenomanian
(95-90 Ma) in response to the beginning of the uplift along the Indian
Ocean side that propagates eastward around the Campanian (85-70Ma).
(2) Because of the humid climatic conditions prevailing since Santonian
(85 Ma), most of the relief of this first plateau is removed by erosion.
At the end of the planation of this topography, chemical erosion is dominant
with the growth of thick lateritic profiles (55 ?-45 Ma).
(3) A second uplift of the Plateau occurred during Late Eocene-Early
Oligocene (30-40 Ma - paroxysm) and Miocene with the incision of at
least three generations of pediments that shape the present-day « Great »
Escarpment.
(4) This « old » Oligocene relief is probably preserved because of
the decrease of the erosion due to the climate aridification, starting at
Middle Miocene times (around 15 Ma).