Southwestern Venezuela Research Articles

Trace elements of Paleocene Táchira coals, southwestern Venezuela: a geochemical study

The concentration and distribution of 35 elements in 78 coal samples from the Táchira State coal fields belonging to Los Cuervos Formation (Paleocene) of western Venezuela were obtained using atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission (ICP-AES). The aim of this work was to determine, through a multivariate statistical approach (factor analysis), if there is a correlation between the trace element content in coal beds and the provenance rocks at the time of peat deposition. Comparison with world averages and geometric means for trace element concentrations in coal-bed samples shows that B, Ba, La, V, Mn, Zn, and Pb are depleted. However, Táchira coal samples show a perceptible enrichment in Bi, Sb, As, Cd, and Mo, and they are highly enriched in Ag and Co. Nicholl's plot suggests that only B and Co show a distinctive organic affinity. Enriched elements (Ni, Ag, Cd, Mo, Co) are both statistically and genetically related, and we attribute their origin to a volcaniclastic Jurassic unit. Other statistical factors (Th–V–P and Ca–Mg–Mn) reveal a different provenance, indicating a felsic plutonic source and a sedimentary limestone, respectively. A fourth factor (K–Mo–Th–S) is composed mainly of clay minerals and authigenic sulfides.

Movement of Captive-Released Orinoco Crocodiles (Crocodylus intermedius) in the Capanaparo River, Venezuela

To test the feasibility of using captive-reared Orinoco crocodiles to restock depleted wild populations we used radio-telemetry to monitor eight crocodiles (103.3-139.3 cm total length) released in the Capanaparo River in southwestern Venezuela. Crocodiles were located every 1-2 d from April 1991 to March 1992 to determine movement patterns, survivorship, and growth rates. The crocodiles moved consid- erably during the first month following release, and the maximum distance moved was 11.6 km upstream by one crocodile four months after release. However, following an initial period of movement, crocodiles became more sedentary and by the end of the study the maximum distance from the point of release was less than 8 km. Six of the eight crocodiles moved upstream while two moved up to 3.3 km downstream. Crocodiles remained principally along the main course of the river in areas with mixed shallow and deep water habitats and abundant sand beaches. As the river rose during the wet season, crocodiles remained in the same areas, but moved into shallow-water areas among flooded riparian vegetation. The mean growth rate of released crocodiles (0.079 cm TL/day) was similar to that of smaller wild-born juvenile crocodiles. One animal was accidentally killed by a local resident who was hunting spectacled caiman. Based on the results of this study, we feel that a carefully designed program of releases of captive-reared crocodiles can be an effective conservation tool to speed the recovery of depleted populations of Orinoco crocodiles. How- ever, care must be taken to insure that the release program is designed as one component of an overall crocodile strategy and not an excuse to avoid the onerous issues of the protection of wild crocodile popu- lations and their habitat

Study of magnetic contrasts in the Guafita oil field (south-western Venezuela)

We have carried out magnetic susceptibility (MS) studies of near-surface samples (drill cuttings) from 11 wells at the Guafita oil field and environs (southwestern Venezuela). In order to characterize magnetic mineralogies and lithological contrasts, MS results have been complemented by S-ratio logs, scanning electron microscopy (SEM), X-ray energy dispersion (EDX), isothermal remanent magnetization (IRM) acquisition curves, visual examination of mineral compositions using a stereomicroscope of 8X to 66X and X-ray diffraction studies of whole rock, clay and magnetic fractions. Two different kinds of MS anomalies have been recognized (i.e. A and B). Anomaly A was observed only in three producer wells and seems to be related to the presence of Fe-rich spherical aggregates and low coercivity magnetic phases, probably of authigenic origin, whereas B appears in most of the producer and non-producer wells. Although B is not associated to framboids of magnetic minerals, X-ray diffraction analyses reveal the presence of quartz, kaolinite, magnetite and hematite at those depth levels where it has been detected. We argue that anomaly A could be linked to a reducing environment induced by the underlying reservoir, whereas B probably reflects a lithological contrast. This hypothesis also seems to be supported by preliminary visual examination of mineral compositions. A contour map of anomaly B amplitude values appears to indicate the sediment input and transport directions in the field. On the other hand, an average-depth contour map of this same anomaly shows the existence of a major fault that has been previously identified by geological studies in the region.

Framboidal magnetic minerals and their possible association to hydrocarbons: La Victoria oil field, southwestern Venezuela

En los últimos años, las anomalías detectadas en las propiedades magnéticas totales (e.g. Susceptibilidad (MS) y Magnetismo Remanente Natural (NRM)) medidas en estudios aeromagnéticos o en muestras de superficie sobre campos petrolíferos, han sido atribuidas a la presencia, en niveles someros, de agregados esferoidales de magnetita autigénica, producto del ambiente reductor inducido por el reservorio subyacente. De verificarse la relación causal entre las anomalías medidas en las propiedades magnéticas y la presencia de hidrocarburos, este tipo de estudio podría ser utilizado como una herramienta rápida, efectiva y económica para estimar la distribución de crudos en zonas en explotación. En este trabajo se realiza una validación de la técnica, utilizando muestras en profundidad de tres pozos (dos productores, uno no productor), pertenecientes al campo La Victoria, Edo. Apure, Venezuela. Los registros de MS y NRM correspondientes a niveles someros (hasta 1500 metros de profundidad), se complementan con estudios de difracción de Rayos X, Microscopía Electrónica (Transmisión y Barrido), curvas de Termoremanencia, Remanencia Magnética Isotermal y Experimentos de Lowrie. Los resultados obtenidos confirman una concentración mayor de magnetita en los niveles anómalos, presente en forma de agregados esferoidales, respecto a otros minerales magnéticos. Se estaría confirmando así una relación causal entre anomalías en las propiedades magnéticas, medidas en niveles someros, y la presencia de hidrocarburos causantes de un ambiente reductor que favorece la nucleación y recristalización de agregados esferoidales de magnetita autigénica, planteándose la potencialidad de este tipo de estudios como una herramienta para la exploración y posible evaluación de zonasprospectivas.

Paleomagnetic and rock magnetic evidence for inverse zoning in the Parguaza batholith (southwestern Venezuela) and its implications about tectonics of the Guyana shield

We report paleomagnetic and rock magnetic data from the rapakivi granites of the Parguaza batholith (Guyana Precambrian Shield, southwestern Venezuela). These results suggest that the pluton is inversely zoned with respect to the cooling ages. In order to explain such an age pattern, tentative structural settings are proposed placing the Parguaza intrusions in a plate tectonic context. Six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith. Thermomagnetic curves, X-ray diffraction and fluorescence, hysteresis loops, thermal and alternating field (AF) intensity plots, transmission (TEM) and scanning (SEM) electron microscope analyses and Königsberger ratios (Qn) values were used to identify the different magnetic mineralogies and their distribution of grain sizes. Magnetite, titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations (NRMs) in these rocks. Magnetic granulometry indicators such as Königsberger ratios (Qn values) suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect. The paleomagnetic results reveal the existence of two primary thermoremanent and/or thermochemical magnetizations (TRMsTCRMs) for sites CSP-3, PI-2 and PI-4 (Decl. = 328°, Incl. = −21°, k = 15, α95 = 11.4°) and site CSP-2, PI-1 and PI-3 (Decl. = 16°, Incl. = 87°, k = 10, α95 = 13°), respectively. There is also a poorly defined G3R2 magnetization (Decl. = 284°, Incl. = −86°, k = 6, α95 = 27°) found in sites CSP-2, PI-1 and PI-3. The overlap of coercivities and the unblocking temperatures spectra, probably resulting from the coexistence of primary single-domain magnetic mineralogies with secondary exsolutions of single-domain-like Ti-poor (Fe-rich) regions (almost pure magnetite) in multidomain titanomagnetite grains, in most cases precludes the complete resolution of hybrid G1 (N + R) or G3 (N + R). The relative ages for these components were determined using a map of RbSr model age ‘chrontours’. G1 and G3 are the older and the younger TRMsTCRMs, respectively, and were acquired at two discrete moments of the batholith's geological history. The final map resulting from integrating the paleomagnetic and RbSr data, shows an age pattern for this batholith, and the rest of the intrusions that belong to the Parguaza Igneous Complex, that could be explained either as the effect of inverse cooling of a single intrusive body, the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of NE-trending horsts and grabens cutting through the pluton. Because of its feasibility and agreement with the most recent theories about the tectonic evolution of the Guyana Shield, we favor the latter hypothesis.

The use of underground plant organs and its relation to habitat selection among the Pumé Indians of Venezuela

The use of underground plant organs and habitat selection by the Pume Indians of southwestern Venezuela is described. Detailed information is presented on 18 species in 10 families. The three most significant uses identified are food, medicine and/or ritual, and fish poison; the dry forest habitat contains the largest number of exploited species. The taxonomic and conservation relevance of ethnobotanical investigation in dryland macrohabitats in South America is addressed.

The evolutionary context of chronic allergic conditions : The Hiwi of Venezuela.

The question of why populations with ecologies that resemble our evolutionary past rarely experience allergic conditions such as asthma has intrigued many biomedical scientists. Here we present descriptive data on the ecological context of allergic sensitization among the Hiwi of southwestern Venezuela and suggest reasons for why this and other lowland South American Indian groups do not express the allergic response at levels seen in industrialized contexts. Allergic sensitization among the Hiwi appears to be negligible. This absence occurs in the context of high exposure to macroparasites (mainly hookworm), nutritional stress, frequent and prolonged breastfeeding, low indoor allergen deposition, and few hours spent per day indoors. We conclude that seeking unidimensional answers to the question of why isolated human groups generally experience few allergic conditions is potentially flawed because allergies are produced by a multifaceted immunoglobulin E (IgE) system that responds in complex ways to the environmental and behavioral exposures we examined. Instead, we propose a general model of physiological trade-offs in energy allocation between production of IgE of undefined specificity and production of allergen-specific IgE. In addition, we consider the simultaneous effects that exposures such as nutritional stress, allergen exposure, and breastfeeding may have on these trade-offs.

Relationships between carbon and hydrogen isotope ratios and nitrogen levels in leaves ofClusia species and two other Clusiaceae genera at various sites and different altitudes in Venezuela

Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (Δ), hydrogen isotope ratio (δD) and N-levels, and with altitude were assessed. In the samples, where values of Δ above 15‰ indicate predominant performance of C3 photosynthesis, there were slight tendencies of increasing Δ, δD and N-levels with increasing altitude and of increasing Δ with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of Δ with altitude in the samples, where values of Δ below 10‰ indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C3-CAM transitions in C3/CAM intermediate species, low availability of water is the most important.

The changing course of the Orinoco River during the Neogene: a review

Recent studies have revealed that the rich fresh-water vertebrate fossil fauna of the Urumaco Formation in northwestern Falcón, of Late Miocene age, belongs biogeographically to the Orinoco River system. This finding, together with other evidence, can be used to chart the changing course of the Orinoco River during the Neogene and thus date the uplift of various mountain ranges that affected the change in this course. Presence of the Misoa delta in the Middle Eocene of the Maracaibo Basin has been suggested as evidence for a large river, running in a south-north direction and draining the Central Cordillera of Colombia and the Guayana Highlands. The late Middle Eocene uplifting of Western Venezuela changed the paleogeographic setting, and a new delta-building shifted to the south, represented by the extensive Carbonera Formation of Late Eocene to Oligocene age in the Llanos Basin of Colombia and Southwestern Venezuela. In the earliest Miocene, the Falcón Basin, situated to the east of the Maracaibo Basin, was primarily the site of marine shale sedimentation. Stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto-Orinoco River in the western margin of the basin. In any case, from the mid Early Miocene until the end of the Middle Miocene, a fluvio-deltaic sequence was deposited in the northwestern Falcón, with a total thickness of 4.7 km. The Urumaco Formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto-Orinoco. The deformation and uplift of the Eastern Cordillera of Colombia and of the southwestern end of the Mérida Andes caused the distal course of the river to be deflected to a west-east direction. A late Middle Miocene age for this change of course is precisely dated on marine microfaunal evidence. The Late Miocene sedimentary record of the Orinoco River delta appears east of Maturín, in the Eastern Venezuela Basin, and Pliocene and Pleistocene deltaic sediments are especially evident in Trinidad and the Columbus Basin.

Chronostratigraphy and Regional Distribution of the Cretaceous-Tertiary and Upper Cretaceous (Maastrichtian-Campanian) Unconformities, Barinas Basin, Southwestern Venezuela: ABSTRACT

Chronostratigraphy and Regional Distribution of the Cretaceous-Tertiary and Upper Cretaceous (Maastrichtian-Campanian) Unconformities, Barinas Basin, Southwestern Venezuela: ABSTRACT

Revisions of Pterogastra and Schwackaea (Melastomataceae: Melastomeae)

The closely related neotropical melastomean genera Pterogastra and Schwackaea are revised, the species illustrated, and their distribution mapped. Schwackaea is maintained as a monotypic genus because it is readily distinguished from all other melastomes by its 8‐winged capsule. This weedy little plant occurs from Mexico throughout Central America, chiefly on the Pacific slope, to northern Colombia and on Cocos Island and grows in natural or man‐made savannas from sealevel to 2000 m altitude. Two species are recognized in Pterogastra and one new combination, P. divaricata spp. glabra, is made. Pterogastra is characterized by distinctly 4‐ or 5‐winged capsules, unlike those of Schwackaea or any other New World melastome. The more widespread species occurs in Colombia, Venezuela, Guyana, Ecuador, and Peru at altitudes between sealevel and 2600 m in natural or disturbed grasslands. The second species is endemic in southwestern Venezuela and is confined to natural savannas at low altitudes.

A biostratigraphic study of Paleogene sequences in southwestern Venezuela

Abstract Palynological analysis of the Los Cuervos, Mirador and Carbonera formations in western Venezuela suggests that changes in the composition and diversity of the microfloral assemblages are related to the nature and dynamics of the paleoenvironments of deposition. The palynological assemblages of the Los Cuervos and Mirador formations are characterized by the mixing of different palynological assemblages, related to the proximity to coastal areas of the low‐delta plain environments (Los Cuervos Formation) and the energy and vegetation present in the drainage area of the fluvial system (Mirador Formation). On the other hand, the assemblages of the Carbonera Formation are considered to be representative of the autochthonous vegetation growing nearby extensive swamps in a low delta plain located far behind coastal areas. The palynological data also permitted the age assignment for each of the stratigraphical units; Paleocene for the upper part of Los Cuervos Formation, Early to Middle Eocene for the Mi...

Tectonic Events and Structural Configuration of the Barinas to Apure Basin

Among the Sub-Andean basins of northern South America is located the Barinas-Apure basin, in southwestern Venezuela. Through the analysis of high-resolution seismic-profiles and surface geology, three different tectonic events have affected the sediments of this basin: (a) a late Cretaceous-Paleocene event, related with the Larmidian orogenesis, (b) a late Middle Eocene event, related to north-Venezuelan flexure, and (c) a Miocene to Pleistocene event, under the influence of the Andean (Merida) orogenesis. Is the last one, the responsible of the present-day structural configuration on the basin, i.e., an assymetric syncline. Several complex structures and fault-systems are recognized in this basin, they are grouped in pre-Oligocene and Miocene-Pleistocene in origin. Among these, only the pre-Oligocene structures have accumulated important oil reservoirs.

Strategic procurement of fish by the pum�: A South American ?fishing culture?

Information is presented on fishing effort, efficiency, techniques, and catch composition for Pume men, women, and children along with a conceptual model of fishing as a food procurement strategy. The Pume are a native lowland South American group living in the topical savanna region of southwestern Venezuela characterized by seasonal flooding. Results are discussed in relation to the Pume environmental and social situation, and briefly compared with results from other lowland South American groups.

Main aspects of petroleum exploration in the Apure area of Southwestern Venezuela, 1985–1987

Abstract This paper gives an update of exploration advances made in Apure during the years 1985–1987. It begins with a brief history of exploratory activities in the area, and then reviews present concepts as to its tectonic-sedimentary evolution. New seismic surveys have notably extended structural knowledge and recent geochemical studies have given new insights into the origin and migration of hydrocarbons; these, together with results of all exploratory drilling, allow new ideas as to additional prospects in the area.

Petty-Capitalist Production in Agriculture

The mid-1940s are considered to be the starting point in Venezuela of an oil-financed model of economic development based on a general policy of import substitution and economic diversification to satisfy a rapidly growing internal market. The massive devaluation of the national currency in 1983 revealed the inadequacy of this already exhausted development path and led to its replacement. During these years, the hitherto practically virgin forests and extensive savannas of Acarigua in southwestern Venezuela were transformed into the country's main grain-producing area with secondary crops of cotton, sugarcane, and tobacco. A highly mechanized farm, the socalled family farm employing varying amounts of salaried labor, was the predominant type of enterprise in this process. In the same period, Perija, a plain bordering Lake Maracaibo in northwestern Venezuela, became the main dairy-producing district in the country. Mixed dairy and beef ranching, combining both family labor and wage labor, was the most common type of farm enterprise throughout this time. At the same time, in the north-central region, at the margin of an accelerated urbanization process, many former sugarcane plantations and cattle ranches were divided into lots to create small poultry farms. In this case, the family farm was linked to a semiautomated technical poultry package.

Nitrification Rates in Two Undisturbed Tropical Rain Forests and Three Slash-and-Burn Sites of the Venezuelan Amazon

Nitrification was measured during the rainy season in two undisturbed tropical rain forests and three slash-and-burn sites in the Amazon region of southwestern Venezuela. Nitrification was higher in the soils of the slash-and-burn sites than in the undisturbed forests. Rates of nitrate production were 14.9 mg/kg/30 days in a forest on an Oxisol, 3.5 mg/kg/30 days in a forest on Ultisol, 23.2 mg/kg/30 days in a 1.5-yr-old slash-and-burn agricultural site on an Oxisol, 12.9 mg/kg/30 days in a 0.5-yr-old slashand-burn agricultural site on Ultisol, and 12.1 mg/kg/30 days in a 3.0-yr-old pasture on Ultisol. In the undisturbed forests, rates of nitrate production in the root mat averaged 93.5 mg/kg/30 days on Oxisols and 11.2 mg/kg/30 days on Ultisols. Nitrification rates dosely followed total nitrogen mineralization rates. Ammonium availability appears to regulate nitrification in both the undisturbed forests and in the slash-and-burn sites. Rates of nitrification in the undisturbed forests in this Amazon site on poor soils were lower than in other tropical forests on more nutrient rich soils.

The genusEnderleina(Plecoptera: Perlidae)

Three new species of Enderleina Jewett, 1960, are described from Venezuela and several additional generic characters are suggested for this rarely collected group. E. flimi sp. n. and E. yano sp. n. are known from Cerro de la Neblina in southwestern Venezuela, and E. bonita sp. n. is known from the Rancho Grande area of Venezuela.

Reservoir Heterogeneity and Hydrocarbon Production in Mixed Dolomitic-Clastic Sequence: Escandalosa Formation, Barinas-Apure Basin, Southwestern Venezuela: ABSTRACT

Widespread dedolomitization and differential leaching occur in the Turonian O Member of the Escandalosa Formation, Barinas-Apure basin. Within this dolostone-dominated succession, calcite was developed through a dedolomitization process occurring in deeply buried dolomitized lime sediments previously deposited on a carbonate platform as well as dedolomitization on the associated glauconitic-quartzose sandstones of small-scale channels that scoured the platform. The dolomitized intervals have a strata-bound nature, and their original fabric is totally obliterated. The dolomitization process generated a sucrose-textured mosaic of saddle dolomite. Initial dolomite was of the scattered type, but progressive replacement of the host produced a mosaic dolostone with both idiotopic and xenotopic textures. A general increase occurred in the iron and manganese content, and goethite was exsolved from the curved rhombs of saddle dolomite. Calcite usually postdates dolomitization, except in the highly fossiliferous packstones; calcite veins develop in both dolostones and limestones. Leaching is restricted essentially to glauconitic sandstones where calcite and some clay have been leached. This has produced very low intercrystalline porosity within the dolostones and partially dissolved, corroded and floating grains with oversized pores in the sandstones. These sandy intervals exhibit maximum potential for hydrocarbon storage, due to contrasting diagenetic influence associated with reservoir heterogeneity.

Use of integrated geological, geophysical and remote sensing methods in the finding of economic minerals for open pit mining, southwestern Venezuela : Rodriguez, S E Proc 13th World Mining Congress, Improvement of Mine Productivity and Overall Economy by Modern Technology, Stockholm, June 1987V1, P143–149. Publ Rotterdam: A A Balkema, 1987

Use of integrated geological, geophysical and remote sensing methods in the finding of economic minerals for open pit mining, southwestern Venezuela : Rodriguez, S E Proc 13th World Mining Congress, Improvement of Mine Productivity and Overall Economy by Modern Technology, Stockholm, June 1987V1, P143–149. Publ Rotterdam: A A Balkema, 1987