Sedimentary and stratal patterns in Jurassic successions of western Madagascar : Facies, stratigraphy, and architecture of Gondwana breakup and drift sequences

The breakup of Gondwana along the former East African Orogen is widely interpreted to have lasted from the Late Palaeozoic to the Callovian. The present study indicates that the Permian-Triassic or Karoo phase of rifting was not responsible for the separation of East- and West-Gondwana, since that rift system failed in the Late Triassic. Instead the breakup of Gondwana occurred in the Late Liassic. The pre-rift phase in the Morondava Basin is represented by the Karoo deposits, and the syn-rift phase is recorded by Toarcian marine shales, locally overlain by Aalenian sandstones. A major Early Bajocian unconformity is interpreted as the breakup unconformity. The initial post-rift or drift phase is represented by the Bajocian-Bathonian carbonates, marls and sandstones of coastal plain environment and a coastal barrier/lagoon complex. During the Bathonian the siliciclastic shoreface system moved basinward. Callovian-Early Kimmeridgian shales with interbedded iron-oolites represent a shallow-deeper shelf system, in which a Lower Oxfordian shoreface sandstone is intercalated. Based on outcrop and literature data in combination with subsurface data sets four transgressive-regressive (T-R) cycles within the syn-and post-breakup successions were recognised: T-R1 cycle is represented by the Toarcian shales and the Aalenian sandstones. After breakup a widespread flooding formed the Bajocian carbonate platform (T2), followed by the Bathonian sandstones (R2) when a sea-level fall forced the siliciclastic shoreline to move basinward. In the Early Callovian again a widespread transgression established shelf conditions (T3). During a short regressive phase during the Early Oxfordian (R3), siliciclastic shoreface deposits prograded onto the shelf. From the Early Oxfordian onwards a transgressive trend continued (T4). The recognised T-R cycles are generally consistent with sea-level changes observed in other parts of the world and are therefore interpreted to reflect eustacy.