SOURCE TO SINK STUDY OF THE CONGO SYSTEM SINCE 40 MYR : A MEASUREMENT OF THE RATIO BETWEN MECHANICAL AND CHEMICAL EROSION.
Résumé
The erosional processes are of two types; mechanical and chemical. If the first one are extensively
studied, the chemical ones are less known whether it is for the coupling between geomorphological
processes and chemical alteration or two the knowledge of the ratio between
mechanical and chemical erosion. In 1995 Gaillardet and coauthors proposed based on global
geochemical mass budget of the modern Congo River, a ratio between physical and chemical
weathering of 1.5.
To test this ratio, we here performed a source to sink study of the Congo System (catchment
and depositional system up to the abyssal plain) since 40 Myr, time interval during which the
Congo catchment did not experience major climate changes.
The continental denudation was measured by subtracting the present-day topography
from the so-called African Surface, etchplain (weathering surface) shaped between 70 and 40
Ma. This etchplain is today preserved as remnant planation surfaces from the eastern limb of
the East-African Dome to the high plateaus of Cameroon and Angola. The deposited volumes
were measured from seismic line and petroleum wells. Dated time-lines were drawn based on
the principles of the seismic stratigraphy. The measured volumes from the coastal to the abyssal
plains were compacted for a better comparison with the eroded volumes.
From 40 Ma to today, 799 000 km3 of rocks were denudated over the Congo River catchment
and 488 000 km3 of (compacted) sediments were deposited along the delta and the
deep-sea-fan. The ratio between eroded and deposited volumes is around 1,64 (1,3 – 2,1 including
uncertainties).
In the absence of any type of buffer effect with temporary sediments storage along the
Congo Basin, we interpret this difference of volume between erosion and sedimentation as
the amount of chemical erosion. The value is quite similar to this measured by Gaillardet et al.
(1995).