The Geophysical Signatures of the Kgagodi Basin, Central Botswana

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 the 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 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.