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31.
In this study, we discuss the first cosmogenic 10Be dating of river terraces located in the lower reaches of the Santo Domingo river (Southeastern flank of the Mérida Andes, Western Venezuela). The geomorphic observations and dating allowed the restoration of the temporal evolution of incision rate, which was analysed in terms of tectonic, climatic and geomorphic processes. The long-term incision rate in the area has been constantly around 1.1 mm/a over the last 70 ka. Taking into account the geologic and geomorphologic setting, this value can be converted into the Late Pleistocene uplift rate of the Southeastern flank of the Mérida Andes. Our results show that the process of terraces formation in the lower reaches of the Santo Domingo river occurred at a higher frequency (103–104 years) than a glacial/interglacial cycle (104–105 years). According to the global and local climate curve, these terraces were abandoned during warm to cold transitions. 相似文献
32.
Rita Pez Gilia Ruiz Rmulo Mrquez Luz Marina Soto Marynes Montiel Carlos Lpez 《Limnologica》2001,31(2)
Limnological features of Tulé Reservoir, a large and shallow reservoir in western Venezuela were studied from May 1992 to December 1993. This reservoir is polymictic, with a low water transparency. Levels of nitrogen were high and phytoplankton productivity was low. The phytoplankton productivity values may be explained by the low concentrations of orthophosphate during the larger part of the sampling period and the frequent mixing and resuspension of sediments. Heterotrophic bacteria were between 1651.67 and 4365.00 Colony-forming Units/ml. Species composition of the crustacean zooplankton community was similar to other neotropical eutrophic reservoirs. 相似文献
33.
Eugenio Sanhueza Kathleen H. Octavio Alfonso Arrocha 《Journal of Atmospheric Chemistry》1985,2(4):377-385
A one-year set of surface ozone measurements in a four-station network located in the Venezuelan savannah is reported. The diurnal ozone variation is typical of continental stations with a maximum in the afternoon, when vertical turbulent mixing is strongest. The annual O3 average concentration, based on the monthly averages of daily maxima, was 17±2 ppb, which is in good agreement with values reported for similar latitudes. The boundary-layer ozone levels did not fall below 8 ppb, in contrast with previous sporadic measurements made in tropical latitudes. No evidence was found that mesoscale O3 downdrafts in the ITCZ in the South American continent are an important source of surface ozone. Finally, it is suggested that the relatively high ozone levels observed at the end of the dry season are probably of photochemical origin. 相似文献
34.
Alessandro Ellero Alessandro Malasoma Michele Marroni Luca Pandolfi Franco Urbani 《Island Arc》2007,16(1):105-123
Abstract The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid-oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep-sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4 . Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4 ) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel ( D2 and D4 phases) and normal ( D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike-slip and pure compressional or pure extensional tectonics occurred through time. 相似文献
35.
Maximiliano J. Bezada Michael Schmitz María Ins Jcome Josmat Rodríguez Franck Audemard Carlos Izarra the BOLIVAR Active Seismic Working Group 《Journal of Geodynamics》2008,45(4-5):191-200
The Falcón Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a 450 km long profile. The final model shows thinning of the crust beneath the Falcón Basin where depth to Moho decreases to 27 km from a value of 40 km about 100 km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcón Basin. 相似文献
36.
Jean Carlos Montero-Serrano Manuel Martínez Armelle Riboulleau Nicolas Tribovillard Gonzalo Márquez José Vicente Gutiérrez-Martín 《Marine and Petroleum Geology》2010
The early Miocene Pedregoso Formation is one of the numerous formations rich in organic matter within the stratigraphic record of the Urumaco Trough, in the central area of the Falcón Basin. Due to its lithological characteristics and stratigraphic position, this formation is of great interest regarding the basin's petroliferous systems. The evaluation of various inorganic and organic geochemical parameters indicates that the organic matter is primarily of marine origin, deposited in a marine carbonate environment typical of reefal systems, under oxic-to-dysoxic conditions. The low variability in the TOC concentrations and in the distributions of the biomarkers extracted from the samples suggests that the paleoenvironmental conditions and the organic-matter supply remained approximately constant throughout the sedimentation of this unit. The Pedregoso type-II organic matter (marine origin) and initial organic richness value (∼1.8%) suggest that this unit has probably generated hydrocarbons within the Urumaco Trough. However, present-day thermal maturity parameters reveal that the Pedregoso organic matter is overmature (dry gas window), indicating that this unit is only capable to generate gas. In addition, the geothermal gradient, maturity parameters, and the maximum paleotemperature estimated in this study suggest that the Pedregoso Formation reached a maximum burial depth the ∼6.5 km, consistent with the value obtained from data of stratigraphic thickness in the Urumaco Trough. This implies that the thermal anomaly that affected the basin during the Late Eocene–Early Miocene did not reach the central part of the basin, and therefore, the organic matter maturation in this unit is due to the sedimentary burial. 相似文献
37.
J.C. Weber J. Saleh S. Balkaransingh T. DixonW. Ambeh T. LeongA. Rodriguez K. Miller 《Marine and Petroleum Geology》2011,28(1):200-211
We estimated horizontal velocities at 25 sites first surveyed in a 1901-1903 British Ordnance Survey triangulation and then resurveyed with GPS in 1994-1995 to identify Trinidad's principal active on-land faults, quantify fault-slip-rates, and test for elastic locking. Our best-fit single-fault elastic dislocation model put 12 ± 3 mm/yr of dextral strike-slip on the Central Range Fault (1-2 km locking depth), an apparently aseismic active fault. The estimated motions also showed statistically insignificant horizontal motion (2.2 ± 1.8 mm/yr of right-slip; 2.7 ± 2.0 mm/yr of N-S shortening) on the eastward on-strike extension of the El Pilar Fault, known to be the active transform fault in Venezuela. Repeat GPS measurements made between 1994 and 2005 at two sites spanning the island north to south showed a 14 ± 3 mm/yr eastward (plate-motion-parallel) dextral velocity differential, consistent with our best-fit historic (1901-1995) fault-slip-rate. Paleoseismology trenching demonstrates that the Central Range Fault cuts <5000-year-old sediment and is capped by ∼550-year-old sediment, suggesting that it may be locked and may have ruptured at least once during this time interval. About ∼5 mm/yr of slip could be taken up on the Los Bajos Fault and additional faults in the offshore south of Trinidad. The existing 1901-1995 and 1994-2005 geodetic data alone cannot resolve whether the Central Range Fault is essentially creeping (≤1-2 km locking depth) or locked to a more standard depth of 10 km. 相似文献
38.
Francesco Sauro Nicola Tisato Jo De Waele Stefano M. Bernasconi Tomaso R. R. Bontognali Ermanno Galli 《Sedimentology》2014,61(5):1433-1451
Gypsum (CaSO4·2H2O), alunite (KAl3(SO4)2(OH)6), and rare phosphate–sulphate sanjuanite Al2(PO4)(SO4)(OH) 9(H2O) and rossiantonite (Al3(PO4)(SO4) 2(OH)2(H2O)14) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H2S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments. 相似文献
39.
The Eastern Venezuela Basin serves as a world-class petroliferous basin, with the sedimentary environment controlled by interactions between the Caribbean and South American plates. This complex tectonic movement could have an important impact on the unsynchronized sedimentary evolution process and heterogeneous marine transgression of different regions within the basin. Thus, it is important to clarify the impact of interactions between the fluviatic- and marine-deposits on the development of the sedimentary environment, the transition of different delta types, and sand-body distribution within this complicated tectonic movement. Based on interpretation of 3D seismic data, electrical well-logging facies and analysis of the sedimentary characteristics of the core, we were able to identify the different sedimentary association types and understand the evolutionary process of the sedimentary system within the study area. We found that the differential sedimentary process in the eastern and western Carabobo block in the Eastern Venezuela Basin was caused by differential uplift and the corresponding spatial discrepancy transgression process in the Middle and Early Miocene foreland basins in this region. The flat geomorphology in the eastern Carabobo block contributed to the obvious sea level transgression process from North to South, but the uplift in the center of the western Carabobo block continuously blocked the sea level transgression. The different transgression processes led to the different sedimentary evolution processes in the eastern and western Carabobo blocks. In the eastern Carabobo block, braided river deltas (23.3–20.43 Ma) gradually changed into meandering river deltas (20.43–15.97 Ma) and finally into tide-influenced estuaries (15.97 Ma). However, in the western subsiding Carabobo block, impacted by the uplift in the center of this area, braided river deltas (23.3–15.97 Ma) in front of the uplift (adjacent to the sea) quickly changed into tide-influenced estuaries (15.97 Ma) with the generation of meandering distributary channels. However, the relatively weak transgression impacting the area behind the uplift resulted in braided channels being retained. Along with the evolution of the sedimentary environment, the tide-related deltaic distributary channel began to change from a braided channel to a meandering one. After rebuilding the paleo-hydrodynamic field, ancient geomorphology, and combining the hydrodynamic equations, we were able to quantitatively explain why sausage-shaped mouth bars occurred in the West and isolated mouth bars occurred in the East. Differential sedimentary environment evolution in eastern and western Carabobo is related to the spatial discrepancy of transgression, which resulted in the rate of change in hydrodynamic intensity, the direction of water flow confluence, and paleo-geomorphology. 相似文献
40.
Simulating Movement-Related Resource Dynamics to Improve Species Distribution Models: A Case Study with Oilbirds in Northern South America 总被引：1，自引：0，他引：1
Paul Holloway 《The Professional geographer》2013,65(4):528-540
A better understanding of the current and future distributions of organisms is a critical facet of biodiversity conservation, and species distribution models (SDMs) are an important framework for achieving this. Despite the potential of SDMs to address an array of biogeography questions, they are subject to a number of conceptual and methodological uncertainties, such as the role of animal movement processes in determining geographic ranges. Movement processes have only recently been incorporated in SDMs, predominantly conceptualized as broad-scale movement processes (e.g., dispersal), while finer scale ambulatory movements of mobile animals (e.g., foraging) have been omitted. This research addresses this gap by developing a model that simulates the dynamic relationship between movement and biotic resources (e.g., food sources) for oilbirds (Steatornis caripensis) in Venezuela. This simulation represented the sustainability of an oilbird’s neighborhood, based on the connectivity, accessibility, and viability of its biotic resources. These dynamic variables improved the accuracy and ecological realism of the SDM projection compared to other commonly applied SDM scenarios. Integration of a Lagrangian (individual-level) form of movement in SDM with step-selection functions to parameterize biased-correlated random walks provides a new empirical framework for applying geographic context to simulation. 相似文献