Reappraisal of the timing of the breakup of Gondwana based on sedimentological and seismic evidence from the Morondava Basin, Madagascar

Abstract

The breakup of Gondwana along the former East African Orogen is widely interpreted to have started in the Late Palaeozoic. The present study indicates that the Permo-Triassic or Karoo phase of rifting was not responsible for the separation of Madagascar from Africa. This rift system failed in the Late Triassic. The separation of Madagascar occurred in the Late Liassic. This event was relatively short-lived, with a distinct episode of rifting followed by separation and continental drift. Previous workers have interpreted the rift and drift phases as a transitional process, starting in the Late Permian and finishing in the Callovian. The pre-rift phase in the Morondava Basin is represented by the fluvial Isalo Sandstones which are Late Triassic in age, and the syn-rift phase is recorded by the marine shales of the Andafia which are Toarcian–Aalenian in age. The Early Bajocian unconformity is interpreted as the breakup unconformity. The post-rift or drift phase is represented by the Bajocian–Bathonian carbonates of the Bemaraha Formation, and by the marls, sandstones and carbonates of the Sakaraha Formation. These two Formations are considered to be lateral equivalents of one another, with the Sakaraha Formation representing a coastal plain environment and the Bemaraha Formation a coastal barrier/lagoon complex.