Abstract U/Pb zircon and 40 Ar/ 39 Ar hornblende ages of medium- to high-grade metamorphic basement rocks from La Vela Bay in the Falcon Basin and of granitic gneiss rocks from a basement inlier in the southeastern Guajira Peninsula, northwestern Venezuela, measured by laser ablation–inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe, document the presence of Meso- to Neoproterozoic crust related to the Putumayo orogen, which probably correlates with the Chibcha terrane, in Colombia. Metawackes, metapelites, and granitic gneisses from these localities showed a high-grade metamorphism peak between 1.0 Ga and 0.98 Ga. Ages of inherited zircon cores of 1212 ± 11 Ma, 1214 ± 10 Ma, 1227 ± 9 Ma, 1366 ± 38 Ma, 1519 ± 9 Ma, and 2707 ± 8 Ma, are interpreted as relicts of detrital igneous grains that were originally incorporated from the Amazon craton into the sedimentary protolith. In addition, 40 Ar/ 39 Ar ages of hornblende crystals in metapelites preserve younger events between 923 Ma and 916 Ma, which indicate cooling under 530°C after the granulite facies peak. Radiometric ages presented in this study support the idea that these basement rocks probably formed part of a single orogenic belt in western South America, that formed in an active margin located in the northwestern part of the Amazon craton, which gave rise to a continental collision with Baltica during the Neoproterozoic assemblage of Rodinia.

Abstract A multi-event tectonic episode that affected the Caribbean and South American Plate boundaries as well as Cenozoic oil generation is based on new structural and geochemical data from the western Falcón Basin, Venezuela. It involves Late Cretaceous to Middle Eocene emplacement of the Lara Nappes followed by Late Eocene to Early Miocene tectonic collapse and graben formation, Middle Miocene inversion and out of sequence thrusting. Oil-source rock correlation of seeps in the northern part of the basin suggests a Cenozoic siliciclastic source rock deposited under suboxic to anoxic conditions. Potential Cenozoic source rocks and Late Cretaceous La Luna Formation were used to evaluate the generation conditions using one- and two-dimensional thermal modelling. A heat flow of c . 190 mW m −2 was reached during the Oligocene–Early Miocene in the central part of the basin. As a result the Cretaceous source rock is overmature, while the primary Cenozoic source rocks are in the oil window. The thermal modelling also suggests that hydrocarbon accumulations are mainly located on the flanks of the graben, with small amounts possible in the centre, due to erosion during basin inversion. This modelling is highly consistent with the proposed polyphase tectonic model.

ABSTRACT The Falcon basin, located in northwestem Venezuela, has been intermittently explored since 1912. Since 1912, 200 exploratory wells have been drilled and 12,000 km of seismic lines have been acquired. This exploration effort has resulted in the discovery of eight small producing fields in both onshore and offshore areas. The geologic history of the basin began in the late Eocene, and deposition continued through the Pliocene to the Recent. Because the basin is located at the boundary between the Caribbean and South American plates, sedimentation was controlled primarily by tectonism as evidenced by seismic and well data. Three structural systems developed as a result of east-west dextral crustal movement. The first, consisting of a set of normal faults and associated horsts and grabens, forms the northern extension of the Oligocene-Miocene basin. The second system, known as the Falcon anticlinorium, includes east-northeast- striking parallel folds located in the center of the basin. The third structural system encompasses the active east-striking right-lateral strike-slip faults of which the Oca fault is the most relevant, owing to its regional extent. The stratigraphic discontinuity within the basin is one of its principal features. The two stratigraphic stages that have been recognized are the result of a late Eocene to early Miocene transgression and a middle Miocene to Recent regression. The northern flank of the basin, including the offshore area, has generated hydrocarbons from Oligocene and lower Miocene marine source rocks. However, small quantities of crude oils of terrigenous origin have been generated from Eocene source rocks. Based on the tectonic and stratigraphic framework of the Falcon basin, a new conceptual model is proposed that can be applied to future hydrocarbon exploration in the area.

The Falcón Basin in northwestern Venezuela and adjacent offshore basins developed within a zone of extensional tectonics during Oligocene and Miocene times. Extension resulted from right-lateral motion along offset, east-west-trending, transcurrent faults, including the Oca fault in western Venezuela, the Cuiza fault in northern Colombia, and the San Sebastián fault along the coastal areas of central Venezuela. On both local and regional scales, transcurrent and normal faults were active during the early evolution of the basins. These faults define rhomb-shaped pullapart basins in map plan. Extension occurred in a northeast direction causing normal faulting along north-west trends. Basin subsidence was accompanied by crustal thinning and injection of basaltic magmas. Evidence of Oligocene magmatic activity is found in the central part of the Falcón Basin where volcanic rocks and hypabyssal intrusions are exposed. These rocks are similar to other suites of continental igneous rocks typically associated with rifting environments. Basaltic rocks of both alkalic and subalkalic affinities are present. Xenoliths of the underlying crust and mantle are abundant. Felsic igneous rocks are relatively rare. Other structures within the Falcón and Bonaire basins formed during later stages of basin development. These include folds and reverse faults of northeast trend and conjugate sets of small transcurrent faults. These structures were amplified by greater compressional stresses during late Miocene and Pliocene time. A similar Tertiary tectonic regime is postulated for the larger area of the Bonaire Crustal Block, a block that includes the Falcón and Bonaire basins. This block is a broad region of extensional pullapart structures which developed during the Oligocene to Miocene, right-lateral motion between the Caribbean and South American crustal plates. Minimum extension within the northern part of the block, along the Venezuelan and Netherlands Antilles and the Paraguaná and Guajira peninsulas, is estimated at 35 to 45 km in an east-west direction. The extension within this nonrigid block should be considered when determining Tertiary movements between the Caribbean and South American plates.

Paleomagnetic data for basaltic intrusions in the central Falcón Basin document a secondary magnetization associated with alteration during Miocene time. The rocks are extensively altered; however, most of this alteration probably accompanied cooling soon after intrusion. Comparing these secondary magnetic signatures to those of other Cenozoic sites in South America, we conclude that magnetization occurred before the late Miocene folding in the basin and soon after intrusion. A later stable partial overprinting of this secondary magnetization occurred in which the present geomagnetic field was superimposed on the stable Miocene remanence. At many sites it is difficult to remove this overprinting during demagnetization. The resultant magnetic signature has a steep positive inclination, which in the past has been difficult to interpret.

The type area of the Tertiary Agua Salada group is at El Mene de Acosta and Pozon, State of Falcón, Venezuela; it is named after Quebrada Agua Salada, Pozon and is approximately 1442 meters thick. It is subdivided lithologically into the San Lorenzo formation (two members), approximately 400 meters thick, type locality El Mene de Acosta, and the Pozon formation (three members), approximately 1042 meters thick, type locality Pozon. Fossil evidence indicates that the Agua Salada group is Middle Oligocene (Rupelian) to Middle Miocene (Tortonian). The San Lorenzo formation is considered Middle to Upper Oligocene and the Pozon formation Upper Oligocene to Middle Miocene. Biostratigraphically (foraminiferal) the sequence comprises the “Uvigerinella” sparsicostata zone (Middle Oligocene), Acostian stage (mainly Upper Oligocene), Araguatian stage (Upper Oligocene to Lower Miocene), and Lucian stage (Lower to Middle Miocene). Type locality of the “Uvigerinella” sparsicostata zone and Acostian is El Mene de Acosta; type locality of the Araguatian and Lucian is Pozon. Correlation between Eastern Falcón and certain Oligocene-Miocene foraminiferal sequences in the Caribbean region and tropical America is displayed in Table 15. Ecologically, the enrivonment—in comparison with present-day conditions-appears to represent deposition on a Continental Shelf in tropical latitudes at depths ranging from 30 to 200–600 meters. Estimates of depths are as follows: “Uvigerinella” sparsicostata zone, less than 100 meters; Acostian and Araguatian, between 200 and 600 meters; Lucian, at depths gradually decreasing from 200–600 to less than 30 meters. Two hundred and thirty-nine species and varieties of Foraminifera, of which four are described as new, are listed and discussed in the part of the report devoted to systematic paleontology.