Northwestern Venezuela Research Articles

Allozyme diversity and genetic structure of the leafy cactus (Pereskia guamacho [Cactaceae]).

We examined levels of genetic variation and genetic structure in the leafy cactus (Pereskia guamacho) in arid and semiarid zones in Venezuela. We surveyed genetic diversity within 17 populations using 19 allozyme loci. Genetic diversity was relatively high at both the species (P(s) = 89%, A(s) = 3.26, AP(s) = 3.53, H(es) = 0.24) and population (P(p) = 63%, A(p) = 1.90, AP(p) = 2.42, H(ep) = 0.20) levels. A significant deficit of heterozygote individuals was detected within populations in the Paraguana Peninsula region (F(IS) = 0.301). Relatively low levels of population differentiation were detected at macrogeographic (G(ST) = 0.112) and regional levels (G(ST) = 0.044 for peninsula region and G(ST) = 0.074 for mainland region), suggesting substantial genetic exchange among populations; however, gene flow in this species seems to be regulated by the distance among populations. Overall, estimates of genetic diversity found in P. guamacho are concordant with the pattern observed for other cacti surveyed, namely high levels of polymorphism and genetic diversity with one common allele and several rare alleles per locus. Differences in gene dispersal systems between this species and other cacti studied were not reflected in the patterns of genetic diversity observed. The concentration of the highest estimates of genetic variation in northwestern Venezuela suggests a potential reservoir of plant genetic diversity within xerophilous ecosystems in northern South America.

A high-frequency sequence study: a Miocene deltaic and estuarine succession in the Eastern Maracaibo Composite Foreland Basin, Western Venezuela

ABSTRACT The Maracaibo Basin in northwestern Venezuela, one of the most prolific oil-producing areas in the world, is a composite foreland basin limited by foredeeps both to the south and west, as well as by piggy-back basins and half-grabens in the east. The western foredeep is part of a retro-foreland basin associated with the Oligocene-Miocene uplifted Sierra de Perija. The La Rosa and the lower Lagunillas formations (lower-middle Miocene) were deposited in the distal portion of this retro-foreland basin. Cores and well logs allow the identification of tidally influenced fluvial, tidal flat, central basin and embayment, shoreface and offshore, lagoonal and organic rich deposits, as well as zones of subaerial exposure within regressive deltaic or strandplain, and transgressive barrier bar or estuarine environments. Core-log calibrations and the identification of stacking patterns and key stratigraphic surfaces allow two types of high-frequency stratigraphic sequences to be recognized. Most sequences are of a simple transgressive-regressive type, implying continuous but irregular rates of relative rise of sea level. However, some have a lowstand segment, implying a relative sea level fall associated with the regressive-to-transgressive turnaround. The repetition of high-frequency sequences, each one of duration less than 0.5 m.y., is believed to have been controlled by high subsidence rates in the basin. The basinward and landward stepping of sequences is interpreted in terms of milder and strong advance of the western thrust-belt wedge and the accompanying variable migration of a forebulge to the east. End_Page 3--------------------------

Rubiacearum Americanarum Magna Hama Pars X. New Species and a New Subspecies of Faramea (Coussareae) from Central and South America

Descriptions and illustrations are presented for the new species Faramea accumulans C. M. Taylor and E areolata C. M. Taylor of the San Blas region of eastern Panama, F guaramacalensis C. M. Taylor from cloud forests in northwestern Venezuela, and E longistipula C. M. Taylor from montane forests in northwestern Colombia. The new subspecies E tamberlikiana subsp. sessifolia (P. H. Allen) C. M. Taylor from southern Costa Rica represents the northernmost range extension of this

New discoveries of vertebrates from a near-shore marine fauna from the Early Miocene of northwestern Venezuela

New discoveries from a recently described nearshore marine fauna from northwestern Venezuela of presumed early Miocene age are reported. The fossils consist of a cranial portion of a crocodile assigned to the Tbmistominae, confirming the presence of this group in South America, and the scapula of a cetacean with affinities to the Platanistoidea. The stratigraphic section of the fossil locality ‘Cerro La Cruz’ consists of ca. 87 m of clayey marls interbedded with thin hardground units, with the upper strata being gypsiferous. The fossils were found in sandstones stratigraphically above this sequence.

Quaternary tectonics and present stress tensor of the inverted northern Falcón Basin, northwestern Venezuela

The Tertiary Falcón Basin in northwestern Venezuela has a privileged position in the geodynamic puzzle of northwestern South America, occurring in a region where several major plates (Caribbean, South America and Nazca) and minor lithospheric blocks (Maracaibo, Bonaire and Western Colombia) are interacting. A combination of good exposures due to aridity and a near-continuous sedimentary record in a now inverted basin helps to unravel the Neogene and Quaternary geodynamic evolution of this region. A neotectonic and microtectonic investigation of the Plio-Quaternary sedimentary rocks of the northern Falcón Basin reveals that this region is subject to a compressive to transpressive regime at present. This regime is characterized by a NNW–SSE oriented maximum horizontal stress, and a ENE–WSW trending intermediate (or minimum) horizontal stress, as is confirmed by focal mechanism solutions. This stress field is in agreement both with the NNE-directed extrusion of the Maracaibo and Bonaire blocks in Western Venezuela, where the Falcón Basin is located, and present-day transpression along the Caribbean-South America plate boundary zone.

A new near-shore marine fauna and flora from the Early Neogene of northwestern Venezuela

A diverse near-shore marine fauna existed during the early Miocene in what is today an arid inland region about 90 km south of the Caribbean coast of northern Venezuela, a poorly known area geologically and paleontologically. The fossil locality consists of more than 100 m of section exposed in an area of about 1 km2. We report the discovery of 20 molluscan species, one crab (Portunus oblongus), at least three sharks (Hemipristis serraandCarcharhinusspp.), one turtle (“Podocnemis” venezuelensis), one crocodile (Crocodylidae), two whales (Odontoceti) and a three dimensional cast of the mesocarp or endocarp of a palm fruit. Several taxa are reported for the first time in Venezuela or in northern South America. The fauna indicates, or at least is consistent with, an early Miocene age for the locality, and a near-shore and shallow water marine depositional environment. We suggest that the earliest mammal previously reported from Venezuela, the pyrothereProticia venezuelensis,was collected in Miocene rocks of the Castillo Formation instead of Eocene rocks of the Trujillo Formation.

Temporal variability of macroinvertebrate assemblages in a neotropical intermittent stream in Northwestern Venezuela.

Changes in macroinvertebrate assemblages were identified among different flow phases during the dry and rainy seasons in a neotropical intermittent stream in Northwestern Venezuela. Invertebrates and physicochemical parameters were sampled during 9 months to assess temporal changes in biotic and abiotic variables. Macroinvertebrate densities decreased following either a high-flow or a stream drought. Three invertebrate assemblages were identified via PCA and cluster analysis. PCA ordination showed a temporal sequence between the sampling dates. A low flow association occurred in August, a period of rewetting after the stream drought in late July. A high-flow association was established by sampling dates (September and November) with the highest flows registered during the study, and an intermediate-flow association occurred during the longer dry period after the rainy season. The low-flow group was dominated by the Philopotamidae (65 %). The intermediate-flow group showed a more even distribution of macroinvertebrate abundances. The high-flow group was dominated by Philopotamidae, Leptophlebiidae, Chironomidae and Baetidae. The lowest richness and highest density values were observed for the low-flow group. Average physicochemical parameters also were different between the PCA groups. A gradient of physicochemical parameters was observed from the low-flow group to the high-flow group. Temporal flow changes probably structure macroinvertebrate communities during extreme episodes of drought or spates, but during intermediate flow periods some biotic processes may determine community structure.

Change and Persistence in Aboriginal Settlement Patterns in the Quíbor Valley, Northwestern Venezuela (Sixteenth to Nineteenth Centuries)

Traditional history in Venezuela held that during the sixteenth century all traces of the aboriginal societies were erased, particularly in regions where no indigenous societies have survived. In this article I explore the changes observed in the use of the space by the population that has settled, since the fifth century B.C. until the present, in the Quíbor Valley in northwestern Venezuela. I provide an analysis of sociocultural change over a long time period, with special emphasis on the cultural transformations that were set in motion after the colonial encounter of the sixteenth century. Also explored are the responses of the indigenous societies in their process of change into what are now known as criollo peasant communities. The article's conclusion is that this cultural transformation has not yet ended.

Birth and death of the Late Cretaceous “La Luna Sea”, and origin of the Tres Esquinas phosphorites

Deposition of organic carbon-rich intervals of the La Luna and Navay formations of northwestern Venezuela was governed by the development of key paleobathymetric barriers (Santa Marta and Santander massifs, Paraguana Block, and ancestral Mérida Andes). These enhanced the development of anoxia in the “La Luna Sea” by causing poor circulation and limited ventilation. Anoxia was also promoted by high evaporation and low precipitation rates (high salinity bottom water), and high levels of marine algal productivity (high organic matter flux). Nutrient supply was augmented by infrequent fluvial sources. Bottom water oxygen levels increased from the Late Santonian through the end of the Cretaceous. Ventilation of anoxic bottom waters may have been enhanced by more frequent or intense seasonal upwelling (caused by higher wind stress) and catastrophic overturn, as well as the removal of a key paleobathymetric barrier. Common byproducts of overturn events were massive phytoplankton blooms, which produced red tides. Fish and marine reptile bone beds within the Tres Esquinas Member (La Luna Formation) are attributed to massive mortality during these events, and are correlative with similar Campanian units in eastern Colombia. During the Maastrichtian, increasing ventilation, combined with siliciclastic dilution, ultimately produced sediments with lower total organic carbon (TOC) content.

Herpetofauna of Estado Falcon, northwestern Venezuela: a checklist with geographical and ecological data

Herpetofauna of Estado Falcon, northwestern Venezuela: a checklist with geographical and ecological data

Metallogenic zoning in Northwestern Venezuela

Metallogenic zoning in Northwestern Venezuela

Neotectonic and paleoseismicity studies on the Urumaco Fault, northern Falcón Basin, northwestern Venezuela

The northern Falcón Basin in northwestern Venezuela is affected by several small active faults, subordinated to the major right-lateral east–west-trending Oca–Ancón Fault System. A set of prominent NW–SE right-lateral faults — synthetic shears — such as the Urumaco, Rı́o Seco, Lagarto and La Soledad faults, stands out among those. The Urumaco Fault, located between the Lagarto and Mitare rivers (in the Urumaco Trough, west of Coro), presents a rather complex active fault trace that comprises two NW–SE fault segments linked by an ENE–WSW reverse echelon, all showing a restraining stepover geometry. Its western segment seems to continue to the north at sea. Conversely, the eastern one dies out on land and its northern tip ends in a transtensive horse-tail structure, that disrupts an Early Pleistocene conglomerate. This same unit is flexed and upheaved some 30 m at the restraining overlap. The kinematics and present stress tensor, the latest activity and the seismogenic potential of the eastern segment of the Urumaco Fault, have been assessed at a set of three river cuts of an ephemeral tributary stream of the Urumaco River, 3 km north of the Urumaco village, where the Urumaco Formation is truncated by a Late Pleistocene terrace ( 14C date of 20,700±950 yr BP at the base) of the Urumaco River. On the one hand, one of these outcrops features the Urumaco Fault affecting the Late Miocene Urumaco Formation, which comprises two prominent fault planes disposed as a wedge. The southwestern bounding plane juxtaposes two different sequences whereas the northeastern one does not, implying different slip behavior. In fact, the northeastern plane shows oblique-slip striations (29°N, normal-dextral), whereas the other one shows perfectly horizontal striations (right-lateral). On the other hand, both updip plane prolongations in the overlying alluvial unit are not so sharp, if the 17-cm throw of the erosive bottom of such terrace measured at the lowermost part of the southwestern plane is regarded as an artifact. However, a mudflat deposit within this unit is bent with a 14-cm throw right above the northeastern fault plane clearly affecting the underlying Miocene unit. The estimated total offset per event allows to infer the occurrence of two individual events of magnitude ranging between M s 5.8 and 6.4 on this strand of the eastern segment of the Urumaco Fault in the last 20,000 yr.

Vector competence of three Venezuelan mosquitoes (Diptera: Culicidae) for an epizootic IC strain of Venezuelan equine encephalitis virus.

Experimental studies were undertaken to evaluate the vector competence of selected mosquito species [Aedes taeniorhynchus (Wiedemann), Culex declarator Dyar and Knab, and Mansonia titillans (Walker)] from northwestern Venezuela for the epizootic (IC) strain of Venezuelan equine encephalitis (VEE) virus that was responsible for the 1995 outbreak of VEE in this area. Ae. taeniorhynchus was highly susceptible to infection (94% of 35), and 89% had a disseminated infection. Virus-exposed Ae. taeniorhynchus that refed on susceptible hamsters readily transmitted virus, confirming that this species was an efficient vector of VEE virus. In contrast, only 1 of 28 (4%) Cx. declarator was infected, and that individual did not develop a disseminated infection. Ma. titillans was moderately susceptible (3 of 8 infected, 38%), and 2 (25%) of these had a disseminated infection. These data indicate that Ae. taeniorhynchus was an important epizootic vector during the 1995 VEE outbreak in Columbia and Venezuela.

Rhodothyrsus, A New Genus of Euphorbiaceae from Tropical South America

The new genusRhodothyrsus is proposed, based on the AmazonianSenefeldera macrophylla Ducke but also containing another newly described species,Rhodothyrsus hirsutus from northwestern Venezuela. The genus is a member of the tribe Hippomaneae of the Euphorbiaceae and apparently related toSenefeldera, but its closest relationships are still obscure, because the phylogeny of the tribe is poorly known and most of the significant characters ofRhodothyrsus are probably autapomorphies or symplesiomorphies. The restriction to tropical lowland rain forests is rare within the Hippomaneae, and several floral characters, pointing to a specialized pollination possibly by butterflies, are unique in the tribe.

Novas espécies e notas sobre Euglossinae do Brasil e Venezuela (Hymenoptera, Apidae)

Eumorpha danielis (Schrottky, 1907) and Centris nigrita var. raymondi Schrottky, 1907, are trasferred to Euplusia Moure, 1943 and validated as Euplusia danielis and Euplusia raymondi comb.n.. Specimens from Santa Isabel do Morro, Ilha do Bananal, Tocantins, and from Santarem and Cachimbo, Para, are described as Euplusia danielis fuscatra subsp.n., Euplusia fragocara Kimsey, 1977, was discovered in Ouro Preto d'Oeste, Rondonia; its female shows bluish colors as E. caerulaescens (Lepeletier, 1841), and E. violacea (Blanchard, 1840). The true meaning of E. smaragdina (Perty, 1833) as different from E. surinamensis (Linnaeus, 1758) is restored according to the study of the type specimen in Munich, Germany. E. xantha Kimsey, 1977 is a new synonym of E. vidua Moure, 1976. The following species from Brasil are described as new: Euplusia faceta from Mucuje, Bahia, Euplusia nordestina from Santa Luzia, Paraiba, and Euplusia velutina from Merida, Northwestern Venezuela.

Microseismicity evidence for subduction of the Caribbean plate beneath the South American Plate in northwestern Venezuela

Over 1100 microearthquakes with body wave magnitude mb<4 have been located in western Venezuela and the southwestern Caribbean region since the installation in 1980 of the Venezuelan Seismological Array, together with 120 events of mb≥4, one of them with surface wave magnitude Ms∼6. This tectonically complex region is part of the boundary between the Caribbean and the South American plates. The main seismically active feature inland in western Venezuela is the northeast striking, 600‐km long, 100‐km wide, right‐lateral strike‐slip Boconó fault zone along the Venezuelan Andes. About 80% of the earthquakes located in the entire region in the period 1980‐mid‐1995 have occurred on this fault zone, at focal depths <20 km. Microearthquake activity at lower rates also occurs northwest of the Venezuelan Andes, both in the continental and Caribbean sea regions. Part of this activity takes place at depths down to ∼150 km. Northwest oriented seismicity depth profiles show the existence of a Benioff zone dipping to the southeast beneath northwestern Venezuela and northern Colombia. This indicates the presence of a northeast striking, southeast dipping subducted slab of the Caribbean plate beneath the South American plate. Hypocentral locations show that the northeastern end of this subduction occurs northwest of the Curaçao‐Aruba region, in the vicinity of a northwest trending, right‐lateral strike‐slip fault zone that joins up with the northeastern end of the Boconó fault zone. This latter place turns out to be the western end of the east‐west striking San Sebastián fault along the Venezuelan coast.

P wave velocity tomography of the Venezuelan region from local arrival times

Arrival times from the local seismological network of Venezuela were used to estimate a three‐dimensional P wave velocity model for the region between longitude 60°–74° W and latitude 6°–14° N to a depth of more than 80 km. The inversion was carried out by damped least squares, describing the media by homogeneous velocity blocks. The resolved lateral velocity variations in the first layer (0–30 km depth) showed a correlation with the main stratigraphic features of the area, while second layer (30–50 km depth) showed the influence of Moho depth variations through the region, generating a pattern well correlated to the Bouguer Anomaly Map. Lithospheric seismic velocities below the Moho appear to be influenced by the major crustal fault systems. An important low‐velocity zone is present below the triple junction of the fault systems of Oca, Boconó, and Morón in northwestern Venezuela. Farther south, a similar low‐velocity zone is present below the junction of the Boconó and the Santa Marta fault systems. Those are the two continental corners of the triangular Maracaibo Block. Below 80 km depth (the fourth layer) the low velocity zones show a connected pattern that follows or is adjacent to the crustal fault zones. The presence of subducted Atlantic lithosphere below the Eastern Venezuelan Basin could explain the high‐velocity zone at this location. A similar interpretation emerges from the tectonic wedging model, previously proposed to explain the pronounced minimum of the gravity anomaly.

Active fault recognition in northwestern Venezuela and its seismogenic characterization: Neotectonic and paleoseismic approach

Los tiempos de recurrencia obtenidos de trincheras en el NW de Venezuela son 1752 ± 133 años para la falla de Ancón y 4300±1000 años para la falla de Oca, con magnitudes de Ms 7.4-7.5. Las fallas menores en el norte del estado de Falcón pueden generar sismos de Ms 6.2 a 7.0 cada 1,500 años o más.

Paleoseismicity studies on the Oca-Ancón fault system, northwestern Venezuela

The E-W-trending, right-lateral Oca-Ancón fault system extends eastward for 650 km, from the Colombian Caribbean coast near the city of Santa Marta to the town of Boca de Aroa (located on the eastern coast of Falcón State, northwestern Venezuela), across the Goajira peninsula, the outlet of Lake Maracaibo, the coastal plains of Buchivacoa and the central Falcón range. This system is very conspicuous west of the city of Maracaibo where its single trace sharply truncates the northern ends of the Santa Marta block and the Perijá range. Fault-related landforms along the system trace (e.g., displaced beach strandlines at Sinamaica and fault scarplets in the Buchivacoa plains) suggest that it may have been active during Holocene times. A first trenching attempt carried out north of Maracaibo (at Sinamaica) in 1969 only confirmed its Holocene tectonic activity. On the western coastal plains of Falcón State and east of Lake Maracaibo, the system is less simple as it is composed of two parallel fault strands but they are clearly evidenced by Holocene paleoseismic scarplets. Recently, this system has been paleoseismically evaluated by means of two trenches placed across each of the two parallel branches known as the Oca and the Ancón faults. Dimensions of these bulldozer-dug trenches are 80 m long and 8 m deep. Various lines of evidence obtained from these exploratory trenches indicate that: (a) three surface rupture events, dated slightly prior to 7755 ± 320, 6240 ± 39 and 1945 ± 630 yr B.P., have occurred along the Oca fault; (b) one surface rupture event has occurred within the past 3125 ± 185 yr on the Ancón fault; (c) the Holocene slip rate of the system is close to 2 mm/yr; and (d) either of the faults can generate a maximum event of magnitude 7.4–7.5 Recurrence of such earthquakes on the Ancón fault is close to 1900 years while it is about 4300 years on the Oca fault.

Venezuelan Equine Encephalitis — Colombia, 1995

An outbreak of Venezuelan equine encephalitis (VEE) that began in northwestern Venezuela in April 1995 has spread westward to the Guajira peninsula and to Colombia (Figure 1), resulting in an estimated minimum of 13,000 cases in humans and an undetermined number of equine deaths. Governments of both countries have initiated efforts to control the spread of this outbreak by quarantining and vaccinating equines and applying insecticides. This report summarizes the ongoing investigation of the outbreak in Colombia.