The Maranhao basin is an approximately circular intracratonic depression filled with middle and upper Paleozoic marine and Triassic continental beds, mildly deformed, extensively intruded by Jurassic diabase, and overlain by Jurassic basalt and Cretaceous lacustrine and transgressive marine deposits. Graben faulting along the Ferrer arch on the north flank of the basin in Cretaceous time formed the Sao Luis and Barreirinhas coastal basins and the intervening Rosario horst. Pre-Pennsylvanian strata of the Maranhao basin include potential petroleum source and reservoir rocks, some perhaps in structurally favorable sites, but the presence of widespread diabase intrusions militates against effective use of geophysical or other exploration techniques. Source beds are not known to occur in the Sao Luis basin, although reservoir rocks are present. In the Barreirinhas basin, thick marine shales that are potential source rocks intertongue northwestward with porous sandstones, and a

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