Cenozoic Landscape Evolution in and Around the Congo Basin: Constraints from Sediments and Planation Surfaces - Geology and Resource Potential of the Congo Basin - page 276

Fig. 14.8 The pediments of the
CB and surrounding highs

PEDIMENTS

PIEDMONT PEDIMENTS

Pediment (x10 2 - 10 3 km 2 )

Depositional
Erosional : pediplain

or

(x10 3 - x10 4 km 2 )

PEDI-VALLEYS

Pediment

50-300m
up to 500 m

x10 km - up to 100 km

PEDIMENT SYSTEM

INCISED
RIVERS

PEDI-VALLEY

50-300m
up to 500 m

x10 km

most of the weathering products (saprolite), they are
called stripped etchplains.
Stripped etchplains define either the base of the
weathering profile (exhumed weathering front, Mabbutt
1961 ) or a surface with remnants of weathering profiles
dissected by incised rivers or pediments (Twidale and
Bourne 2013 ). Remnant relief called inselbergs, tors or
bornhardts can have hillslopes hillslopes still weathered
(B¨del 1957 ; Thomas 1989a , b ) or are baren. Stripped
etchplains cut by incised rivers and pediments may be the
result (1) of simultaneous chemical weathering and physical
stripping (B¨del 1957 , 1982 ; Bremer 1993 ) or (2) of
alternations of chemical and mechanical erosion periods
due to climate changes (Thomas 1989a , b ; Millot 1980 ;
Beauvais et al. 2008 ; Beauvais and Chardon 2013 ).
All the large planation surfaces ( u1 , u2 , l ) and most of the
pediments, are partly to fully duricrusted. Iron duricrusts

form in two different settings, (1) capping a complete later-
itic profile (with saprolite) as the end product of the
weathering or (2) encrusting (cuirassing) thin ( 1m)

conglomerates, veneering a poorly to unweathered

basement.
Type (1) iron duricrusts are characteristic of mantled
etchplains or stripped etchplains by pediments under rela-
tively stable climatic conditions, while type (2) records
alternative periods of mechanical erosion under more arid
climatic settings (pediments incision) and chemical erosion
(weathering) under very humid conditions. Upper planation
surfaces ( u1 and u2 ) are etchplains. The youngest pediments
(X, Y, Z) record contrasted climatic conditions, as suggested
by the occurrence of iron duricrusted conglomerates
(mechanical erosion) directly deposited on unweathered
pediments (e.g. the Cabatuquila iron duricrust in the large
pedi-valley of the Kwango in Angola; Marques 1992 ).