Madagascar is composed of five basins covering a total 636.185 km² extending from the border of the basement till to the Exclusive Economic Zone (EEZ) in offshore.
The three most important basins, Ambilobe, Majunga and Morondava, lay in the western part of the island. The two remaining smaller basins, are both offshore : the Cap Sainte Marie basin in the South and the East Coast basin.
The sediments of Madagascar are constituted by two major systems:
the Karroo formation Permian to Eo-Lias in age containing thick continental sediments
the Post Karroo mainly deposited in marine environment.
The western basins have a common geological history, initiated by intra-cratonic rifts during the Neo Permian/Eo-Triassic period. They contain thick continental clastic sequences then overlied by Triassic to Lias sag deltaic to fluvial sequences.
Subsequent Toarcian rifting and southward drifting of Madagascar away from Somalia coast to its present location have controlled the Jurassic to Neocomian deposition. They have created a passive margin along the coast.
Continued subsidence and westward tilting in response to the rifting and drifting of India North-eastward during the Late Cretaceous, have led to a thickly developed Mesozoic and Cenozoic sequence in each basin.
The two latter basins are poorly understood:
- the Cap Sainte Marie basin could have Permian and Mesozoic fills underlying the Neogene cover,
- the East Coast basin may have thick Cretaceous section beneath the lava flow, cropping out along the onshore narrow sedimentary basin.
DRIFTING OF MADAGASCAR
The Majunga salt basin setting would have occurred just before Madagascar South-Westwards drifting.
Madagascar left the Somalia coast 165 millions years ago.
The drifting event lasted 65 million years, Madagascar has reached its present location since Aptian-Albian. (100 MY ago).
India North-Eastward drifting started at about 84 million years ago.
Madagascar, Somalia and Kenya share a common geological history through the Karroo system deposition : Permian to Early Lias.
Majunga and South-East Somalia basins were conjugate and Morondava basin were joint with Kenya and North Tanzania.
Sedimentation of Madagascar basins is controlled by the main tectonic events which have shaped Madagascar.
They are responsible of the major faulting trends. All Madagascar fault systems are inhe-rited from the basement. They have reacted through the geological times.We record four main directions
PRIOR THE DRIFTING
modified from Reeves & al. (1987)