The Tectonic and Paleogeographic Context of Madagascan Petroleum Systems

Hoult, R.J.¹, P.J. Markwick¹, K.L. Wilson¹, J.P. Armstrong¹, D.G. Wright¹, A.J. Bourne¹ (1) Petroleum Systems Evaluation Group, GETECH, University of Leeds, Leeds, United Kingdom

Models of a pre-Jurassic Gondwana fit range from a palaeo location for Madagascar off Mozambique (Flores, 1970) to that of Reeves et. al. (2004) who like most current authors, place Madagascar off the Kenya/Somali coast. The precise tightness of fit is still a matter of debate. The two main Madagascan basins are different in terms of the main controls on sedimentation, with the Majunga Basin being an obliquely rifted passive margin with a significant salt section wherein basin fill is largely influenced by sedimentation rates. The Morondava Basin is a strike-slip margin with a basin architecture and fill driven by tectonics and reactivation of old structural grains. Two phases of rifting (failed Permo-Trias; successful Early Jurassic) followed by up to five phases of reactivation and/or neotectonics (Turonian to Recent), have controlled landscape evolution and the spatial distribution of drainage systems.

Controversy exists as to the magnitude of rift shoulder uplift, which has a direct impact on regional drainage patterns and the spatial variation in reservoir and source rock quality. Marine incursions during Gondwana rifting culminated with the opening of the Somali Basin as Madagascar translated southward along the Davie Transcurrent Deformation Zone. It was not until the Late Cretaceous, through the combined effects of India-Madagascar rifting and plume-related (?) volcanism, that the ‘modern' topographic expression was initiated. Consequently, short-headed rivers flowed into the developing rift to the east and larger drainages flowed west. A change to seasonally wetter conditions also contributed to an increased clastic flux into the Morondava and Majunga Basins. Tertiary reactivation of pre-existing structures resulted in modifications to drainage networks, with a concomitant change in sediment flux into the offshore basins.