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Stratigraphy and Structural Setting of the Western Diego Basin, Northern Madagascar

Ressetar, Robert 1; Schelling, Daniel D.2
1 Consultant, Salt Lake City, UT.
2 Structural Geology International, LLC, Salt Lake City, UT.

The Diego basin of northwestern Madagascar formed during the breakup of Gondwana and is the location of a current hydrocarbon exploration program. On the Ampasindava Peninsula in the western Diego basin, approximately 6000 m of Triassic to Middle Jurassic strata record shifts from fluvial to marine to fluvial deposition. The Upper Triassic to Liassic Isalo Group consists almost entirely of coarse-grained fluvial sandstone. The overlying, Liassic to Bajocian Jangoa Limestone and Ampasimena Formation form a progradational succession of marine-shelf carbonates and siliciclastics to shoreface and fluvial sandstone, with several well-developed, shallowing-up parasequences, and abundant oil stains. The Bajocian-Bathonian Andrahibo and Komamery Formations consist of fluvial sandstone and minor lacustrine/floodplain mudstone.

In contrast, in the Ambilobe area about 140 km to the northeast, the equivalent strata are thinner (about 1500 m) and, above the Isalo Group, nearly devoid of coarse siliciclastics. The succession here consists of alternating units of calcareous sandstone, shale, and limestone, the latter ranging from nodular, argillaceous mudstone to fossiliferous, oolitic grainstone.

These relationships suggest a paleogeographic setting in which the Ambilobe area was a semi-isolated, low- to moderate-energy, mixed carbonate and siliciclastic platform while the Ampasindava Peninsula was possibly a deeper basin, receiving a significantly greater clastic sedimentary input. At present, however, the Ampasindava area is located along a structural uplift that separates the Diego basin from the Mahajunga basin to the south, whereas a tilted, extensional fault-block geometry characterizes the Ambilobe area.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009