Piaui Basin, A jambalaya of geologic structures

Abstract

Although the Piaui basin was the last of the Brazilian equatorial marginal basins to be studied in detail, it is the one that presents the most fascinating structural geology of them all. The superposition of two major tectonic events, quite distinct in nature (rifting and wrenching), produced an abundance and diversity of geologic structures unparalleled by any other Brazilian basin. The first tectonic event, the rift-stage (early Aptian), created the basin through a series of normal faults predominantly oriented N40/sup 0/-45/sup 0/E in the western part, and N75/sup 0/-85/sup 0/E in the eastern part. Clastic continental sediments constitute the rift depositional sequence. Continental drifting followed (late Aptian-early Cenomanian), and a thick clastic sequence of transitional to marine sediments was deposited. Wrenching was already taking place, but on a small scale. During the middle Cenomanian (approx. = 95-93 m.y.B.P.) the separation between South America and Africa in this area changed from north-south to east-west along the oceanic Romanche fracture zone. Shear stresses developed. The right-lateral motion, the large bend in the Parnaiba platform, and the different average trends of rift faults from west to east resulted in convergent wrenching. Transpression was greatly enhanced. Rift faults were reactivated as dextral strike-slip faults.more » Synthetic (N65/sup 0/-70/sup 0/W) and antithetic (N20/sup 0/W and NS) strike-slip faults were formed. Flower structures occurred along the fault trends. Abundant en echelen folds and shale ridges (N20/sup 0/E) were created. By the time wrenching ceased, a transpressive belt of significant dimensions had emerged in the Piaui basin. Sedimentation resumed in the area during the Oligocene-Miocene.« less