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Structure and evolution of the Malagasy sedimentary belt were controlled, during Paleozoic and Mesozoic (Karroo) times, by north-northeast-south-southwest (East Coast)- and north-northwest-south-southeast (Bongolava)-trending faults. These two fault trends correspond to old lines of weakness (faults and flexures), which had a role in the Precambrian evolution of the Malagasy basement.
In southwest Madagascar, the Manalobe, Ilova, and Iakomby (East Coast trend) fault lines are known to strike east-west, whereas the Andranopasy-Befandriana fault lines (Bongolava trend) strike southwestward. They were active during Karroo deposition, and they frame the region where Malagasy Karroo rocks are found.
No Karroo formations are known southwest of the Andranopasy-Befandriana fault lines. On the northeast side of the fault lines a Lower Karroo basin (Sakoa) lies between the crystalline basement on the east and Manalobe en echelon lines on the west; Middle and Upper Karroo basins (Sakamena and Isalo) are between the basement on the east and the Ilova fault line on the west; a basin of mixed-facies sedimentary rocks of Middle Jurassic age (Dogger; Sakaraha basin) lies between the Ilova and Iakomby fault lines. West of the Iakomby fault a completely marine Liassic to Cretaceous series lies directly on the crystalline basement.
The Sakoa, Sakamena, and Isalo basins originated by successive downward movements of the blocks outlined by the crystalline Precambrian backbone of Malagasy Island on the east and the Manalobe, Ilova, and Iakomby, Andranopasy-Befandriana faults, which always determined the western and southwestern boundaries of these basins.
The Sakoa and Sakamena basins were of a mediterranean type, of restricted circulation. As indicated by several surface and subsurface hydrocarbon shows, naphthogenic conditions certainly existed within the Sakamena basin, and probably within Sakoa basin as well.
From the structural and paleogeographic interpretation of Sakoa and Sakamena basins here presented, these basins, and particularly their cul-de-sac southern part, seem to deserve a reexamination from the point of view of oil exploration.
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