Contrasting Geological Evolution of the Ambilobe, Majunga and Morondava Basins, Offshore Madagascar

Hyden, Graham¹, Edward P. Blunt², Douglas Winstone³, Mark Enfield⁴, Matthew Watkinson⁵ (1) TGS-NOPEC Geophysical Company (UK) Ltd, MK40 3JY , Bedford, United Kingdom (2) Sterling Energy plc, (3) ACECA Geologica, (4) PDF Ltd, (5) PDF Fellowship, University of Plymouth,

The Ambilobe, Majunga and Morondava basins show contrasting geological evolution despite their contiguous location and similar ages. The basins cover an area of more-than 275,000 square kilometres (approximately 160,000 square kilometres lie offshore). TGS has acquired over 20,000 kilometres of marine long-offset seismic and potential fields data since 2000, in water depths up to 4,000 metres. We present results of our regional seismic studies and show how changing geological controls on genesis and evolution of the basins influences their structure and stratigraphy.

The three basins have evolved due to the Mesozoic break-up of Gondwana and the separation and southward translation of East Gondwana along the Davie Fracture Zone. The basins exhibit contrasting geological styles indicating changes in the relative importance of strike-slip to normal extensional tectonics along the margin of East Gondwana during the break-up of Gondwana. Further variation in overall architecture of the basins during subsequent coeval passive margin evolution and East Gondwanan break-up shows (i) the long-lived effect of early structuring on subsequent basin development and (ii) contrasts in the susceptibility of basins to deform during later tectonic episodes.

The newly acquired seismic and potential field data have allowed a detailed examination of the three basins to take place. Examples taken from these data demonstrate the contrasting styles of the stratigraphy and structure of each basin. It is also shown that despite all three basins having similar tectono-stratigraphic frameworks their genetic association with divergent or transform plate boundaries has played a significant part in their subsequent differential evolution.