ABSTRACT: Sedimentary Provinces: Plate Tectonic Classification and Hydrocarbon Habitat, Africa, Middle East, and South America

Frank J. Picha

Worldwide exploration for hydrocarbons requires compilation and evaluation of geological data from numerous sedimentary provinces. To see the critical aspects of the oil habitat and to predict the potential of various provinces, diverse data must be organized into manageable systems. Sedimentary provinces of whole continents have been mapped using a simple but geologically well-supported classification system. These maps have proven to be a helpful tool in achieving these manageable systems.

The sedimentary provinces are grouped into five classes that correspond to principal plate tectonic settings: (1) intracratonic basins (rifts, sags), (2) divergent margins, (3) oceanic basins, (4) convergent margins, and (5) cratonic forelands. The history of each province is determined by critical stages of the plate tectonic cycle: (1) rifting, (2) drifting and sagging, and (3) subduction

and continental collision. Simple basins, such as the Cenozoic East African rift, formed during a single stage of a plate tectonic cycle. Polyhistory provinces can comprise several stages of the same cycle (e.g., the intracratonic Sirte basin with both rift and sag stages) or extend over two cycles (e.g., northwest Africa with stages of the Paleozoic cycle ending at the Hercynian orogeny overlain by the Mesozoic and Cenozoic cycle, which ends with the Alpine orogeny).

In all, 243 sedimentary provinces (basins, fold belts) were analyzed, classified, and graphically displayed on continental maps of Africa and South America. An elaborate system of symbols and colors shows the plate tectonic history of each province, the stratigraphic extent and thickness of the sedimentary fill, the type of basin-forming and basin-modifying tectonics, and the distribution of oil and gas fields. Thus, the maps reveal many aspects of the depositional and tectonic history of continents and subsequently help in understanding the hydrocarbon habitat of various provinces.

The African continent, including the Arabian Peninsula, is flanked on both the eastern and western sides by Mesozoic to Cenozoic divergent margins. The interior of the continent shows both Paleozoic intracratonic basins (e.g., Ghadames, Kalahari) and three generations of intracratonic rifts: the late Paleozoic to early Mesozoic Karroo rifts in southeastern Africa, the Early and Late Cretaceous rifts in central Africa and the Sirte basin, and the Cenozoic rifts in east Africa, Red Sea, and Gulf of Aden. The northwestern part of Africa and northern Arabia belong to the Mesozoic to Cenozoic Alpine orogenic system displaying both thin-skinned thrust belts (Rif, Tell) and the folded and faulted forelands (High Atlas, Zagros fold belt). Superimposed on the orogenic belts are numerous interm ntane basins that resulted from relaxation of compressional stresses or strike-slip motion. Foreland basins are rather discontinuous in North Africa and the Mediterranean, but very prominent in front of the Zagros fold belt.

The South American continent is rimmed on the eastern Atlantic side by the Mesozoic to Cenozoic divergent margins. On the western side, the Andean convergent margin displays a system of trenches, accretionary complexes, forearc basins, and magmatic arcs. On the cratonic side, the Mesozoic to Cenozoic foreland fold and thrust belt is flanked by a chain of sub-Andean foreland basins, which developed on the faulted foreland. Numerous intermontane basins are found within the Andes. The northern side of South America belongs to the Mesozoic to Cenozoic Caribbean system with a history of rifting, opening of oceans, subduction, magmatism, development of fore-arc and back-arc basins, and finally the formation of pull-apart basins related to the translation of the Caribbean plate. The interior of South America contains large intracratonic Paleozoic basins (e.g., Chaco, Maranhao) whereas Mesozoic rift basins are few and mostly restricted to aborted arms of the Atlantic margin (Tucano).

Distribution of hydrocarbons in various depositional settings, as defined by the classification system, shows that the Mesozoic to Cenozoic folded and faulted forelands and foreland basins of both the Middle East and South America are by far the most prolific habitats, comprising 89% of oil discovered in those continents. The widespread divergent margins, apart from a few prominent exemptions such as Niger Delta, and Lower Congo and Campos basins, are to date disappointingly low (5%). The Paleozoic intracratonic sags of North Africa, rift basins (Sirte), and few intermontane basins share the remainder of the discovered oil.

The hydrocarbon potential of each principal setting is directly related to its tectonic and depositional history. The classification system and its application on continental maps attempt to improve the understanding of these relationships. However, it is not the classification itself but the process of analyzing data within a simple geological framework that enriches our knowledge and enhances creative thinking.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990