4.1. Presentation of New Residual Bouguer Anomaly map
The residual anomaly map presented in Fig. 6 shows two anomaly sectors: positive sector and negative sector.
The first sector is located in west of Garoua, northeast of the study area and east of Dourbey. The values of anomalies are between (0 to 15mGal). It would be due to the presence of heavy rocks in granitic environment. The analyze of this map also shows another positive sector which extend from Bibémi to Léré in Chad and include the Mayo Oulo-Lére sedimentary basin which is located inside positive anomaly zone (15 to 35mGal). These anomalies show that the Mayo Oulo-Léré basin does not have the morphology of sedimentary basin. It would correspond to a lake basin with a spectacular rise of heavy rocks probably basaltics.
The second sector is constituted of negative anomalies. These anomalies are located in south of Garoua and around Dourbey where the Babouri-Figuil sedimentary basin is located. The values of these anomalies are between − 15 and − 5mGal. This could correspond to the sedimentary deposits of the Garoua trough in general and the Babouri-Figuil basin in particular. This basin constituted essentially of sandstone would be linked to the Benue trough. The Analysis of the map shows also another negative sector located in south of Léré on Cameroon-Chad border. In this sector the minimum value of anomalies is -25mgal. This would indicate the presence of weak formations compared to the surrounding formations.
4.2. Estimation of Mean Depth of Density Interfaces
In this part, we used the spectral analysis to determine the depths of geological structures source of anomalies. Six profiles P1, P2, P3, P4, P5 and P6 have been traced on densified residual Bouguer anomaly map. P1, P2 and P3 were plotted on Babouri-Figuil sedimentary basin and P4, P5 and P6 on Mayo-Oulo-Léré sedimentary basin. All these profiles are executed perpendicularly to the main elongation of the structure to be studied. When we plot the energy spectrum logarithm as a function of frequency, the spectral curve presents two characteristic slopes. The first slope located in the low frequencies corresponds to the deep structures. The second slope which represents the high frequencies corresponds to the bodies near surface.
In Babouri-Figuil sedimentary basin, two major discontinuities have been obtained by spectral analysis on profiles P1, P2 and P3 (Fig. 7).
The first discontinuity corresponds to deep structures with depths estimated at 4.70km, 4.55km and 5.46km respectively for profiles P1, P2 and P3. These depths could correspond to the sediment-granite contact zone. The second discontinuity is associated with bodies near surface. The estimated depths are: 1.48km, 1.44km and 1.58km respectively for profiles P1, P2 and P3. The average value of depth in this basin is around 1.50km. This result agrees with those obtained by (Schowoerer 1965; Ndjeng and Brunet 1998). According to these authors the depth of the sedimentary series does not exceed 1500m. Therefore the boundary between the lower crust and the upper crust of Babouri-Figuil sedimentary basin would be shallow.
In Mayo-Oulo-Léré sedimentary basin, two major discontinuities have been obtained on profiles P4, P5 and P6 (Fig. 8).
The first discontinuity possesses the following depths 4.27km, 4.62km and 5.32km. These depths could correspond to the crust-mantle interface. The second discontinuity presents the following depths 1.48km, 1.54km and 1.72km. These depths are associated with intracrustal structures with an average depth of 1.55 km. This value probably corresponds to the near surface layer. It indicates that the Mayo Oulo-Léré basin would be deeper than that of Babouri-Figuil.
4.3. Density and Density Contrast of Structures
To determine the characteristics and shapes of geological structures of suspected bodies in Babouri-Figuil and Mayo oulo-Léré sedimentary basins, six profiles were modelized. P1, P2 and P3 of SE-NW direction were modelized in Babouri-Figuil sedimentary basin and P4, P5 and P6 of SW-NE direction in Mayo Oulo-Léré. The average densities of sediments, granites and basaltic rocks present in the study area are respectively: 2.45g/cm3; 2.65g/cm3; 3g/cm3 (Telord et al.1990). The corresponding density contrasts are respectively: -0.2g/cm3, 0g/cm3 and 0.3g/cm3. In Babouri-Figuil sedimentary basin, we obtain three models of structures corresponding to profiles P1, P2 and P3. These models are constituted of two formations of different density contrast (Fig. 9).
- The first formation has density contrast and density respectively − 0.2g/cm3 and 2.45g/cm3. This formation is present throughout the profile. Its depth varies and reaches a spectacular value of 5km, this formation would probably be responsible for a vast zone of negative Bouguer anomaly observed in the sedimentary basin. The density contrast associated with this formation permits to identify along the continental sediments.
- The second formation with density of 2.65g/cm3 is associated to granites. It constitutes the substratum of the basin and it is presented as a rooted structure that extends to great depth.
In Mayo Oulo-Léré sedimentary basin, we obtain three models corresponding to profiles P4, P5 and P6. These models are constituted of three formations of different density contrast (Fig. 10).
- The first formation of density contrast − 0.2g/cm3 has an average density of 2.45g/cm3. It is associated with continental sediments. This formation is present throughout the profile. It depth varies and reach a maximum depth of 3km.
- The second formation with an average density of 2.65g/cm3 is associated to granites. The depth is an extension of this formation probably constitutes the substratum of the basin.
- The third formation with density contrast of + 0.3g/cm3 has an average density of 2.95g/cm3. It is associated with basaltic rocks. To the SW of the profile, these basalts are near surface. The roof of this formation is decreasing and stabilizes at 1.5km. This roof drops to a depth of 3km to the NE of the profile. This formation would be formed during the cooling of magma inside the earth's surface during the volcanic eruption.