The Maranhão intracratonic basin of northern Brazil lies in the states of Maranhão and Piauí, southeast of the mouth of the Amazon River, and is about 600,000 sq. km. in area. Adjacent on the north, are the very much smaller São Luís and Barreirinhas coastal basins.

Three great sedimentary cycles in the Maranhão basin’s history deposited a little more than 3,000 meters of sediments, 2,500 meters of which are Paleozoic and the remainder, Mesozoic. Two major erosional unconformities separate the three cycles, the first between Mississippian and Pennsylvanian time, the other during the Jurassic.

The lower cycle transgresses folded Cambro-Ordovician and pre-Cambrian rocks and consists of Upper Silurian (?), Lower, Middle, and Upper Devonian marine gray sandstones and dark shales, with continental sediments in the basal part. Marine, deltaic, and finally continental gray Mississippian sandstones form the top of the sequence. A humid and temperate climate prevailed during this deposition.

Resting in slight angular unconformity on this sequence is a second cycle deposited during a warm and semi-arid climate, consisting of Pennsylvanian eolian sandstones, anhydrites, red dolomites, thin marine limestones, and continental redbeds; followed by Permian chert beds, anhydrites, dolomites, eolian sandstones, and redbeds; ending with Triassic fluvial and eolian sandstones.

Jurassic basalt overlies the Triassic. Diabase of the same age intrudes all rocks of Triassic or older age.

A third and final cycle of Cretaceous strata onlaps Paleozoic, Triassic, and Jurassic beds along the north flank of the basin. The Lower Cretaceous sequence begins with lacustrine black shales carrying thin anhydrites and marine limestones, passing to marine sandstones and gray shales, and finally to reddish, continental clastics.

Early Cretaceous subsidence in the northern Maranhão basin downwarped the Jurassic unconformity surface, giving the basin an over-all tilt toward the north.

The axis of greatest sediment accumulation during the Silurian-Devonian-Mississippian cycle lies along the eastern and southeastern margins of the basin, which is also the area of greatest Jurassic basic igneous intrusive activity. A strong northwestward shift of the focus of subsidence occurred during the Permian and Triassic. The greatest thickness of these younger rocks coincides with the area of basalt extrusion and relatively mild intrusive activity. Fissures to the surface permitting Jurassic extrusion apparently occurred only in areas which had strong subsidence during Late Triassic, whereas the areas of greatest intrusion were those with thick sediments, but no appreciable Triassic subsidence.

Only mild pre-Jurassic deformation occurred in most of the Maranhão basin. Jurassic diabase intrusives have created very numerous, large domes that have altered the smaller pre-Jurassic structure and seriously hampered exploration methods. Surface geological, seismic, and gravity methods can not differentiate domes caused by diabase sills and prospective pre-diabase folds. The magnetometer may be able to distinguish the two types. Only in the southwest corner of the Maranhão basin, where moderate compressional folding occurred, are the pre-diabase structures not likely to be masked by diabase domes.

The São Luís coastal basin has a maximum sedimentary thickness of about 4,500 meters, 2,500 meters of which are Lower-Middle Cretaceous deltaic and continental clastics. The Cretaceous lies unconformably on about 2,000 meters of Devonian-Mississippian (?) dark marine shale. The basin is an ovate graben with bounding normal faults which may have 2,000 meters of displacement. Some of the faulting was contemporaneous with deposition. Although exploration methods function well in this diabase-free basin, source rocks in the Cretaceous and reservoir rocks in the Paleozoic are lacking.

In the Barreirinhas basin, maximum sedimentary thickness may exceed 10,000 meters, 8,000 meters of which are probably Lower, Middle, and Upper Cretaceous. More than 3,000 meters of Lower-Middle Cretaceous dark marine shales of the southeastern part of the basin change facies toward the northwestern part to mostly continental and deltaic sandstones. This unit lies deformed and tilted beneath more than 3,500 meters of horizontal Albian marine limestone. Then Turonian marine limestone, shale, and sandstone overlie the Albian limestones, possibly unconformably. Unconformably overlying Turonian rocks are marine limestone, shale, and sandstone of Late Campanian age. No diabase sills have been found.

The Barreirinhas basin is an elongate graben bounded on at least three sides by faults with throws exceeding 3,000, perhaps 4,000 meters. Subsidence occurred along these faults contemporaneously with deposition. The best oil shows in the area of this study were from Lower Cretaceous sandstones and Albian limestones of this basin. Strong facies changes, penecontemporaneous deformation, and thick, interfingering source and reservoir rocks make the Lower Cretaceous Itapecurú-Tutóia Formation the most attractive in the most prospective basin of the three studied.

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