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Stratigraphy of Special Areas
The Devonian System in Western Venezuela
The type section of the Devonian in western Venezuela is named the Rio Cachiri Group, and the type locality is near the headwaters of the Rio Cachiri, on the east flank of the Perija Mountains, some 89 kilometers west of the city of Maracoibo The base of the Group is unconformable and possibly in fault contact with Precambrian schists and quartzites of the Perija Series, and the top of the Group is unconformably overlain by the Palmarito Formation of Permo-Carboniferous age. The thickness of the Rio Cachiri Group is estimated at 2,438 meters, but because of structural complications and incomplete sequences, the present thickness measurement will undoubtedly be revised after a more detailed study has been made. The Rio Cachiri Group is made up of three formations, with transitional boundaries between them: the Cano Grande Formation below, the Cano del Oeste Formation in the middle, and the Campo Chico Formation above. The Cano Grande Formation is approximately 762 meters thick and consists of sandy shales, argillaceous sandstones, and calcareous shales, the latter locally fossiliferous, especially so near the top of the Formation. The Cano del Oeste Formation is about 1,067 meters thick, and is composed of blackish quartzitic sandstones, dark gray shales, and blue-black argillaceous limestones, occasionally containing corals and crinoid columnals. The uppermost formation — the Campo Chica — is an estimated 609 meters thick, and composed of quartzitic sandstones, sandy shales, and sandy limestones, all of which are generally unfossiliferous.
The fossils in the Cano Grande and Cano del Oeste Formations, and the recurrence of similar strata in both the Cano del Oeste and Campo Chico, suggest that the Rio Cachiri Group was deposited in late Early Devonian to early Late Devonian time. In western Venezuela the Rio Cachiri Group occurs in a faulted and disturbed zone some 7 kilometers in length between Cano Grande and Cano del Norte, but the extent to the northeast and southwest beyond these limits is still not known with certainty. In Colombia the Group would seem to be correlative, at least in part, with the Middle Devonian reported by Schuchert in the Guajira Peninsula, but the Guajira location has not been confirmed. Elsewhere in Colombia, however, Middle Devonian fossils have been identified east of Manaure and south of Santa Isabel on the west flank of the Perija Mountains, and at Floresta in the Cordillera Oriental. Farther south, the Erere Formation of Brazil and the Sicasica Formation of Bolivia are said to be contemporaneous with the Mesodevonian deposits of Venezuela and Colombia.
The lithofacies and biofacies of the South American Mesodevonian deposits are reminiscent — often strikingly so — of the Mesodevonian deposits of northeastern North America. This similarity has led to two alternative premises: that ecologic conditions must have been fairly uniform in a vast sea connecting North America and South America, or, that the Mesodevonian rocks and fossils are similar because the two continents were very much closer then than they are now. The latter hypothesis, based on palaeomagnetic and structural data, would have it that eastern North America was separated from northern South America by only a narrow, east-west Devonian seaway, located perhaps in the present Caribbean Sea; that from late Palaeozoic time on to the Recent North America has been drifting away from South America with a counterclockwise movement; and that the present Gulf of Mexico is the relict of this ever-widening rift.
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