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Sedimentation in an intracratonic extensional basin: the Karoo of the Central Morondava Basin, Madagascar

Published online by Cambridge University Press:  01 May 2009

G. J. Nichols  and
M. C. Daly
G. J. Nichols
Affiliation:
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.
M. C. Daly
Affiliation:
B.P. Petroleum Development Ltd, Britannic House, Moor Lane, London EC2 9BU, U.K.
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Abstract

The late Carboniferous to Triassic Karoo Supergroup of Madagascar is a sequence of predominantly continental clastic sediments deposited during a long period of regional crustal extension. In the Morondava Basin of western Madagascar the lower two divisions of the Karoo sediments – the Sakoa and the Sakamena – are deposits of fluvial and lacustrine sedimentation systems supplied from the Precambrian metamorphic basement terrain to the east. East–west crustal extension produced a series of graben and half-graben structures after the Sakoa period which were reactivated after the Sakamena. The position and orientation of these half graben, which were marginal to a larger rift system to the west, were partly controlled by the steep NNE–SSW mylonitic fabric in the metamorphic basement. Palaeocurrents in the braided river deposits of the Sakoa and Lower Sakamena indicate flow to the southwest and west in both sequences. The rivers followed a regional palaeoslope to the southwest/west and were apparently not significantly influenced by the local structural trends which were oriented perpendicular to this slope. The absence of local structural control is attributed to extensive erosional events which followed each tectonic episode and preceded the onset of further sedimentation which took place on an essentially peneplained surface. The tectonic episodes brought about changes in base level which caused this part of the basin to fluctuate between erosion and deposition.

Type
Articles
Information
Geological Magazine , Volume 126 , Issue 4 , July 1989 , pp. 339 - 354
Copyright
Copyright © Cambridge University Press 1989

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