Abstract:
The Kgagodi basin is located at 22°28'26.67"S and 27°34'54.10"E, about 7 km South of the
Kgagodi village, Central Botswana. This structure is buried by sediments thus it has no
pronounced surficial expression. The Kgagodi basin has been suggested to represent an
intersection of two fault lines thereby making it a prospective hydrological target. Investigating
the Kgagodi basin allows understanding of the impact process and its effects on geological
formations. The current study aimed at delineating the geologic structures within the basin and
determining the 3-D geometry of the Kgagodi basin using high resolution aeromagnetic data,
ground gravity data, and Vertical electrical sounding (VES) data. Results of the study will aid
future scientific investigations and guide drilling initiatives in the basin.
Aeromagnetic data analysis showed a smooth textured magnetic signature over the basin
corresponding to younger sediment fill as compared to the noisy signature of the surrounding
metamorphic terrain. Two major cross cutting lineaments were identified. The first lineament
has a NW-SE trend and is characterised by a magnetic high and was presumed to be a dyke. The
second lineament observed is a low magnetic signature anomaly trending NE-SW which can be
attributed to possible faulting. Results of the gravity survey showed two main signatures on the
Bouguer Anomaly (BA) map: (1) a pronounced density low (-127.8 to – 119.4 mGal) over the basin
which is associated with the basin fill and (2), high density anomalies ( -119.5 to -112.2 mGal)
associated with the metamorphic basement terrain. The 3-D density distribution model of the BA
data showed a bowl-shaped structure with a diameter of ̴3.5 km. The density model revealed
predominant gravity low anomaly features (1 to 2.75 g/cm3
) at the centre, which is the suspected
Abstract
v
impact crater region. From the surface until approximately 970m deep, the model showed
medium density contrasts (2.0-2.75 g/cm3
) and were interpreted to be the later crater fill possibly
made up of allochthonous material. A parabolic lens of low densities ranging between 1 to 1.9
g/cm3 was observed at depth of 700-900m and was proposed to be the breccia lens made up of
both allochthonous material and highly shocked and melted target material. Beneath this layer
high densities of (2-3 g/cm3
), suggested to represent the parautochthonous material were also
observed.
Results from 1-D modelling of the VES data showed that the predominant field curve in the
area is the H-type combination curve with mostly three and four geoelectrical layers identified.
The centre of the basin is characterised by a low resistivity zone (6-193 ohm.m), corresponding
to conductive basin fill and saturated sediments. The constructed geoelectrical cross sections
along East-West profiles indicated that the shallow basin fill (about 100m depth) is made up of
alternating layers of different sediments. From this investigation, it was concluded that the
magnetic signature over the Kgagodi basin is smooth textured. The two cross cutting lineaments
in the Kgagodi basin could be an intersection of two fault lines. The dolerite dyke in the area
could be older or relatively the same age as the Kgagodi basin. This structure has a diameter of
approximately 3. 5 km and its depth to the basement is 900m. The shallow basin fill is made up
of possibly faulted low resistive materials, with signatures corresponding to those of saturated
sediments. The inside of the basin is considered to have good groundwater potential.