--> --> Trap Formation by Turonian Sill Complexes in the Deepwater Morondava Basin, Madagascar

2018 AAPG International Conference and Exhibition

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Trap Formation by Turonian Sill Complexes in the Deepwater Morondava Basin, Madagascar

Abstract

When Madagascar separated from the greater Indian plate, vast amounts of lava erupted during the late Cretaceous (Turonian). Remnants of these extrusives and many intrusive bodies are preserved in the Mesozoic sedimentary sequence along the western coast of Madagascar, mainly in the onshore Morondava Basin. In the undrilled deepwater segment of the Morondava Basin, hundreds of seismically imaged igneous sills were analyzed on recently acquired and vintage 3D reflection seismic data sets. The age of these sills is Turonian (91-93 Ma, K/Ar dating) based on well penetrations on the shelf and in the nearby onshore part of the basin. The sills are typically saucer-shaped, but other types, such as tabular and transgressive sills were also documented. Besides the large number of sub-volcanic intrusive complexes, a relatively small number of submarine paleo-volcanoes with associated lava flows and hydrothermal vents were also mapped corresponding to an inferred Turonian unconformity. Like in many other volcanic basins, where the geometry of the sills have been analyzed in great detail, the vertical magnitude of the 4-way structural closures observed in the forced folds above the sills is the function of the emplacement depth below the paleo-seafloor and thickness. Whereas individual sills may not always result in very large forced folds above them, a vertical stacking of several sills clearly provide large, compound structural traps with considerable vertical closure (up to 300 m) and fairly large areal extent (up to 200km2). Differential compaction above these traps resulted in simpler but lower amplitude structural closures higher up in the Senonian to Cenozoic sequences. Exceptionally thick (up to 400 m), “inflated” intrusions were also mapped on 3D seismic and these singular sills alone can produce large enough traps for deepwater exploration.