Toarcian–Kimmeridgian depositional cycles of the south-western Morondava Basin along the rifted continental margin of Madagascar

After rifting and final breakup of Gondwana along the former East-African-Antarctic Orogen during the Toarcian–Aalenian, passive margins formed around the Proto-Indian Ocean. Sedimentological and stratigraphic studies in the southern Morondava Basin contribute to an improved reconstruction of palaeoenvironmental changes during the syn-rift and post-rift margin formation. Depositional models based on outcrop and literature data in combination with subsurface data sets provide a stratigraphic framework of four transgressive-regressive (T-R) cycles. The incorporation of stratal architecture models derived from seismic images is essential. After a syn-breakup T-R cycle (T-R1), the post-breakup succession commenced with a Bajocian – Early Bathonian carbonate platform (T2). Middle–Late Bathonian sandstones (R2) formed when a global sea-level fall forced the shoreline to move basinward. Incised valleys and palaeokarst known from seismic lines are typical for forced regression cycles. In the Early Callovian again a widespread transgression occurs (T3). During a short regressive phase from the Late Callovian(?) to Early Oxfordian (R3), the siliciclastic shoreface deposits prograded onto the shelf. From the Early Oxfordian onwards a transgressive trend continued (T4). T1–T3 can be explained as the response to the structural development of the breakup rifting but they follow sea-level changes observed in other parts of the world. R3 and T4, in contrast, reflect eustacy.