Continuing
NPA’s Central African Rift System Study we have now completed
our West and Central African Rift System Study covering the
Termit, Lake Chad, Bongor, Salamat, Doba and Doseo basins of
Chad.
Combining satellite image interpretation, geophysical
data sets and structural models in these basins has provided
valuable information
on the underlying basin architecture. The spatial distribution
of seismic imaged faults have been traced for 10’s of
kilometres. Analysis of breaks in slope, drainage anomalies,
differences
in vegetation and other geomorphological variations enables
areas of low or no outcrop to be structurally mapped.
The Doba, Doseo, and Bongor and
basins: Transtensional rifts
Tertiary sediment fill and vegetation in the rift basins do
not entirely mask the structural control on the geomorphology.
The Landsat imagery clearly shows ENE-WSW trending topographic
highs in the Doseo basin. The interpretation shows the association
of these highs (which are related to transpressive flower structures)
with the Kome oilfields.
Integration of
Seismic, Grav/Mag and Structural Models
Integration of geophysical data and structural models help
to better define the basin architecture and enable the
prediction of structural trends and areas of possible structural
closure such as roll-over anticlines, transpressional flower
structures.
The Landsat ETM extract shows a NE-SW trending
high dividing lake Chad. This is also known to be present
at depth. Based upon the plate reconstructions and geo-seismic
cross-sections for the Cad basin it is apparent that the
basin is dominantly extensional (opposed to the transtensional
basin to the south). The model above shows the progressive
deformation of a 600 oblique rift and the formation of half-graben
(dark blue indicates narrow, local depo-centres, possibly
lakes). Accommodation zones offset the graben and are highlighted
by diagonal line shading, their 3D geometries are shown to
the right. The topographic high on the ETM is thought to
represent such an accommodation zone.
The dry and wet season drainage regimes
of the two rivers have been mapped. As Landsat ETM is inhibited
by cloud cover during the wet season ERS radar has been utilized
(below). The images cover the same area and show the variations
in river course from one data set to the next and also the
areas of potential flooding as mapped from the radar data.
On the Landsat ETM imagery rivers and river bluffs can be
easily mapped. However, the ability of ERS to highlight variations
in the nature of surface roughness, moisture levels and relief
enables boggy/standing water areas or areas prone to flooding
to be mapped.
