MOZAMBIQUE‐MADAGASCAR GEOSYNCLINE, I: DEPOSITION AND ARCHITECTURE

Abstract

The Mozambique‐Madagascar geosyncline consists of an open‐ended non‐compressional NNE‐SSW downwarp 1,700 km long in the area of the present Mozambique Channel and its bordering coastal regions. Exposed basement marks the lateral boundaries of the geosyncline, and embayments rather than discrete basins modify its regional geometry. A sedimentary volume of more than 5,000,000 cu km resulted from subsidence and hypersubsidence, not only in the Mozambique Channel but also on the coastal borders, in places relatively close to exposed basement.Continental facies of South African Karroo type dominates the sedimentary columns of the geosynclinal flanks in the Upper Carboniferous to Middle Jurassic interval, but is not yet documented beneath the bathyal and abyssal waters of the intervening Mozambique Channel. Marine facies, mixed facies, and a few continental facies occupy the sedimentary columns of the flanks above the Middle Jurassic. Pelagic facies may be dominant in contemporaneous strata now below the bottom of the Channel.As part of the vast non‐compressional province of the Indian Ocean the Mozambique‐Madagascar geosyncline has been subjected only to epeirogeny. The structural response has been normal faulting, in some places, with major downthrow and in others antithetic with patterns of graben and horst. Volcanism is known to have accompanied the faulting from Late Triassic time onward.Modern consensus invokes a separation of Madagascar from Africa followed by drift along southward, eastward, or NE paths, depending upon individual tectonophysical interpretation, each of the three of which displays weakness. The alternative of separation by subsidence only needs the test of deep drilling under the present Mozambique Channel for support orproof Until then, a tentative scheme of stratal succession is proposed, namely, a relatively‐thin post‐Karroo column in pelagic facies and a relatively‐thick Karroo column in continental and mixed facies.Confirmation of the succession by drilling would provide strong support of a model presenting paleo‐Madagascar as the eastern sector of an Africa that bulged more than once toward the east in post‐Carboniferous time. The corollary of a broad band of reservoir Karroo, under a broad cover of pelagic post‐Karroo, all under the waters of the present Mozambique Channel would be of considerable interest to explorationists.