Summary of Tectonic History of Venezuela: ABSTRACT

Present-day Venezuela may be divided into the following major structural provinces: Perija Mountains, Goajira-Paraguana arch, Maracaibo basin, Falcon, Venezuelan Andes, Carribbean Mountains (Coast Range and Serrania del Interior), Barinas-Apure basin, Eastern Venezuela basin, and Guayana Shield. With the exception of the shield and possibly the Goajira-Paraguana arch, all these provinces owe their essential development to Late Mesozoic or Tertiary tectonic events. The pre-Cretaceous history is little known. Mid-Ordovician (?), Late Devonian or Early Carboniferous, and Permian (?) deformations may be postulated, but their trends and importance can not be evaluated at present. Toward the end of the Paleozoic, widespread uplift, accompanied by faulting and volcanism, raised the whole of the country above sea-level. Subsequent erosion resulted in positive features of structural origin, such as the Central Goajira massif, the Maracaibo platform, and the El Baul swell, which had an effect on Late Mesozoic and Tertiary sedimentation. Beginning in the Late Mesozoic, the Caribbean Sea began an extensive transgression of northern and western Venezuela. The east-west-trending Caribbean orthogeosyncline developed in the area now occupied by the Caribbean Mountains; shelf conditions prevailed elsewhere. Volcanism and major deformation of the Caribbean geosyncline began about Middle Cretaceous time, leading to the folding, faulting, and metamorphism of the previously deposited Cretaceous rocks. However, sedimentation, volcanism, and deformation, although on decreasing scale, continued throughout Late Cretaceous and Paleocene time and perhaps into the early Eocene. Dominant structural features in the Coast Range trend N. 70°-80° E.; wrench (strike-slip) faulting with horizontal displacements of several kilometers is common. It has been suggested that the over-all deformation of the Coast Range resulted from an east-west shear couple related to the development of the Caribbean sea area. Renewed uplift and south-southeastward thrusting, with related N. 60° W. strike-slip f ulting, which reached a maximum in the Late Tertiary have caused the crumpling, faulting, and over-riding of the northern edge of the Eastern Venezuela basin to the south. The northern oil fields of the Eastern Venezuela basin are controlled either directly or indirectly by the consequent structures. The Eastern Venezuela basin is an east-plunging asymmetrical elongate feature which was initiated in the late Eocene with the downsinking of the area south of the Caribbean ranges and east of the El Baul swell. Greatest depths are in the north close to the mountains. In addition to the north flank structures related to Coast Range uplift, east-west faulting with minor cross-faulting on the south flank, probably associated with the sinking of the basin, is important in the localization of oil fields. The Perija, Maracaibo, Andes, and Barinas-Apure provinces owe their character as distinct units to the Tertiary Andean deformation, with the basins sinking as the mountains rose. The Cretaceous and Eocene oil fields of western Venezuela owe their existence to the Andean orogeny; the Middle to Late Tertiary fields are also linked to it. There is some indication that movements began first in the northwest (Sierra de Perija) during the Eocene, progressed southeastward across the more-or-less stable Maracaibo platform, reached the Andes at the close of the Eocene, and culminated in the Mio-Pliocene. The mountains, with dominant trend of N. 30° E. for the Perija and N. 45° E. for the Andes, are essentially complexly folded and faulted structural arches with high-angle reverse, no mal, and wrench faults. Mountainward-dipping reverse faults are thought to bound their flanks. Both the Maracaibo and Barinas-Apure basins have asymmetrical cross sections with deepest zones close to the flanks of the Andes. The Falcon province shows some relation to the Maracaibo basin and the Andes both in its sedimentation and in its deformational history. Originally a narrow trough, it received great thicknesses of sediments during the Middle and Late Tertiary and was finally folded and uplifted in the Pliocene, as well as in earlier Tertiary time. An outstanding tectonic feature of northern Venezuela is a series of long east-west-trending right-lateral wrench faults that are located close to and roughly parallel with the coast. Best known of these are the End_Page 410------------------------------ Oca fault in the west and the Pilar fault in the east. It is possible that these faults are at least as old as Cretaceous, that they are related to the tectonic history of the general Caribbean area as suggested by Bucher and others, and that they have played a major role in the deformation of northern Venezuela. The displacement on other large wrench faults, such as the northeast-trending Bocono fault in the Andes and the northwest-trending Urica and San Francisco faults in the eastern Serrania del Interior, must be taken into account in reconstructing past structural and sedimentological trends and relationships. The two most prominent tectonic features of Venezuela, the Coast Range (northern Caribbean Mountains) and the Andes, differ rather markedly from each other in the following respects: (a) the Coast Range had its origin in a trough of more-or-less geosynclinal character; the Venezuelan Andes did not; (b) many of the Coast Range rocks were metamorphosed during deformation; no metamorphism took place during the Andean orogeny; (c) volcanism was common in the Coast Range area both somewhat before and during deformation; no post-Lower Mesozoic volcanism is known in the Andes; (d) the Coast Range has a belt of serpentinites; the Andes does not; (e) the major deformation of the Coast Range was Middle to Late Cretaceous; that of the Andes was latest Eocene to Miocene. The contact between the Coast Range and Andean trends at the Barquisimeto Gap is abrupt and may be modified by later faulting. Genetic and structural continuity of the Venezuelan Andes and the Caribbean Ranges as suggested by many geologists is questionable. End_of_Article - Last_Page 411------------