Evaluating the mechanical compaction of quartzarenites: The importance of sorting (Llanos foreland basin,Colombia)

Mechanical compaction is the main porosity-reducing process in sandstones, including high-reservoir-quality rigid-grain sandstones. For such sandstones, the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories. Evaluating the compaction of these rocks is achieved by comparing current intergranular volume (IGV) with depositional IGV, which is strongly dependent on sorting. However, because sandstone sorting is difficult to measure accurately, its impact on depositional porosity and compaction state is largely underestimated. We use the quartzarenites of the Oligocene Carbonera Formation in the subsurface of the hydrostatically-pressured Llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid-grain sandstones. IGV and sorting were measured in core samples using a combination of transmitted-light and cathodoluminescence images, resulting in improved accuracy over standard procedures. The compaction state of clean quartzarenites at given depths is best described using IGV-versus-sorting plots, which are used to derive compaction curves for specified sorting values. The IGV-versus-sorting trends are displaced to lower IGV values with increasing burial depth. The differences in IGV caused by differences in sorting exceed the differences in IGV resulting from 1000 m of burial, illustrating the high impact of sorting when evaluating compaction. Contrasting with published experimental results, the compaction of the Llanos basin ductile-grain-poor quartzarenites is independent of grain size, and grain rearrangement is the main compaction mechanism during the first ∼1.6 km of burial. Based on the Llanos data, we have generated IGV-versus-depth curves for clean pure quartzarenites of specific sorting, which can be used to predict their maximum primary porosity up to moderate burial depths. Differences with other published burial curves are probably related to unaccounted variations in sorting, ductile-grain content and framework-strengthening cements. However, the Llanos basin quartzarenites contain virtually no cements, explaining their high degree of compaction relative to other rigid-grain sandstones, and making them ideal to isolate the effects of compaction on the IGV of quartzarenites. The Llanos basin data suggest that, below ∼2.5 km of depth, clean well- to moderately well sorted quartzarenites continue reducing their IGV by mechanical compaction below the 26% limit, which should apply only to extremely well sorted, rigid grain, uncemented sandstones.     

Seasonal inundation patterns in two large savanna floodplains of South America: the Llanos de Moxos (Bolivia) and the Llanos del Orinoco (Venezuela and Colombia)

Inundation patterns in two of the largest savanna floodplains of South America were studied by analysis of the 37‐GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer (Nimbus‐7 satellite). Flooded area was estimated at monthly intervals for January 1979 through to August 1987 using mixing models that account for the major landscape units with distinctive microwave emission characteristics. Results are presented separately for five subregions in each of the two floodplain regions to show the spatial as well as temporal variability in inundation patterns. The total area inundated during the 9 years varied between 2069 and 78 460 km² in the Llanos de Moxos (also spelled as Mojos; median area, 23 383 km²) and 1278 and 105 454 km² in the Llanos del Orinoco (median, 25 374 km²), not including the open‐water area of permanent lakes and river channels. The correlation between flooded area and river stage was used to extend the inundation records over a 30‐year period in the Moxos (1967–97) and a 58‐year period (1927–85) in the Orinoco. Interannual variability in inundation is greater in the Moxos than the Orinoco. Comparison of these data, however, with a previously published analysis of the Pantanal wetland shows that inundation patterns in these two floodplain regions are not as variable across years as they are in the Pantanal. Copyright © 2004 John Wiley & Sons, Ltd.     

Quantitative morphometric analysis of lakes using GIS: rectangularity R,ellipticity E,orientation O,and the rectangularity vs. ellipticity index,REi

Quantitative measures of polygon shapes and orientation are important elements of geospatial analysis. These kinds of measures are particularly valuable in the case of lakes, where shape and orientation patterns can help identifying the geomorphological agents behind lake formation and evolution. However, the lack of built-in tools in commercial geographic information system (GIS) software packages designed for this kind of analysis has meant that many researchers often must rely on tools and workarounds that are not always accurate. Here, an easy-to-use method to measure rectangularity R, ellipticity E, and orientation O is developed. In addition, a new rectangularity vs. ellipticity index, REi, is defined. Following a step-by-step process, it is shown how these measures and index can be easily calculated using a combination of GIS built-in functions. The identification of shapes and estimation of orientations performed by this method is applied to the case study of the geometric and oriented lakes of the Llanos de Moxos, in the Bolivian Amazon, where shape and orientation have been the two most important elements studied to infer possible formation mechanisms. It is shown that, thanks to these new indexes, shape and orientation patterns are unveiled, which would have been hard to identify otherwise.     

Landscape transformations in savannas of northern South America: Land use/cover changes since 1987 in the Llanos Orientales of Colombia

This study presents a detailed spatial, quantitative assessment of the land use/cover changes (LUCC) in the savanna region of Llanos Orientales in Colombia. LUCC was determined from multitemporal satellite imagery (Landsat and CBERS) from 1987 to 2007. Systematic landscape transitions were identified and put in the context of population change and economic activity. The results showed that during the period 1987 to 2007, 14% of the study area underwent some kind of land use/cover change, with most change occurring in the last decade. Systematic transitions were observed from flooded savannas to crops and exotic pastures. An important land cover change was linked to the expansion of palm oil plantations from 31 km² in 1987 to 162 km² in 2007. The observed changes are shown to be related to the economic and market-oriented-development from before 1970 to the present day. Based on the future economic development plans, the Llanos Orientales will continue to undergo significant change as an estimated 70% of the 17,000 km² have been identified for conversion to plantation, or for petroleum and mining purposes. We provide recommendations for future economy integrated conservation, by proposing the implementation of a Llanos ecological network.     

哥伦比亚Llanos盆地东部油田Carbonera组C5段河道砂岩地震识别与预测

哥伦比亚Llanos盆地东部的中新世Carbonera组属于三角洲前缘相沉积。Carbonera组内部的水下分流河道砂体是良好的储层,因此对河道的精细刻画有利于指导下一步的油气勘探和开发。研究通过波动方程波场正演模拟建立了河道砂岩储层的地震识别模式,并利用地震沉积学RGB混频体地层切片以及沿层地震振幅属性对河道的形态、发育期次、以及砂体的平面分布进行了详细的研究。在此基础上,利用约束稀疏脉冲反演和地质统计学反演研究定量描述了河道砂体的孔隙度和厚度。最终对五条有利的河道砂岩岩性圈闭进行了详细的描述,研究区河道的宽度为322~830 m、砂岩最厚可达到35.05 m,这些河道砂体可为有利勘探目标区。区内钻探的W2井证实了该技术方法的有效性和储层描述结果的可靠性,为研究区下步的油气勘探开发提供新方向。      

A new Late Cretaceous crocodyliform from the western margin of Gondwana (La Rioja Province,Argentina)

The Cretaceous witnessed the greatest diversity of Gondwanan notosuchian crocodyliforms, which displayed high levels of diversity and a notable array of specialized forms that developed in different ecological niches. Among this diversity, the advanced notosuchians are a clade of mid body sized forms which displays a remarkable abundance but is restricted to two lithological units from the Late Cretaceous of South America, the Adamantina Formation in southeastern Brazil and the Bajo de la Carpa Formation in Patagonia (Argentina). The only exceptions known so far were the Late Cretaceous Labidiosuchus from the Marilia Formation in Brazil and Yacarerani from the Cajones Formation in Bolivia. Herein we report a new Cretaceous crocodyliform, Llanosuchus tamaensis gen. nov. et sp. nov., found in the Los Llanos Formation (Campanian?) in northwestern Argentina (La Rioja Province). The small specimen includes well preserved fragments of the cranium and mandible with an estimated skull size of about 9 cm lengths. This crocodyliform shares several derived characters with Notosuchus terrestris from the Bajo de la Carpa Formation (Patagonia) and it was found in a weakly developed sandy paleosol profile formed in wet local conditions but in a region characterized by semi-arid climate and active eolian sedimentation. The presence of a new advanced notosuchian in the Cretaceous of western Gondwana, and its intermediate geographical region between the known South American species (Brazil, Argentina and Bolivia), has interesting implications, and adds another record of an advanced notosuchian in deposits with inferred warm climates and semi-arid paleoenvironments. Finally, Llanosuchus tamaensis supports a Late Cretaceous age for Los Llanos Formation with important geological implications for La Rioja Province.     

Environmental history of the Colombian savannas of the Llanos Orientales since the Last Glacial Maximum from lake records El Pinal and Carimagua

Late Quaternary environments have been studied by pollen analysis of lake sediments from the savannas of the Colombian Llanos Orientales at 180 m elevation. The pollen record form Laguna El Pinal (4°08N, 70°23W), dated by 6 AMS radiocarbon dates, starts at 18,290 ¹⁴C yr B.P. The record from Laguna Carimagua (4°04N, 70°14W), also dated by 6 AMS dates, starts at 8270 ¹⁴C yr B.P. Both records show a landscape dominated by grassland savanna with only few woody savanna taxa, such as Curatella and Byrsonima, frequent fires, and little occurrence of forest and/or gallery forest along the rivers. The savanna ecosystem at the studied sites was relatively stable during the last 18,000 yrs, but minor changes in floral composition, and in the proportion of savanna/forest, have been recorded. Very little gallery forest and the non permanent lake conditions of Laguna El Pinal reflect the driest period, interpreted to reflect low rainfall rates and long dry seasons during the Last Glacial Maximum until 10,690 ¹⁴ C yr B.P. During the Late Glacial, Laguna El Pinal was a permanent shallow lake, and changed into a lake with higher water levels during the Holocene, indicating wetter conditions. Expansion of regional gallery forest also started at around 10,690 ¹⁴C yr B.P. Little vegetational change observed in Laguna Carimagua at 5570 ¹⁴ C yr B.P., in combination with a simultaneous decrease of savanna observed in previously studied lakes, suggest a change to regional wetter conditions. Thus, the Holocene before 5500 ¹⁴ C yr B.P. was somewhat drier than the following period until about 3850 ¹⁴C yr B.P. In both records, Late Holocene lake deposits are incomplete. Shore vegetation of Laguna Carimagua always included a minor contribution of the palms Mauritia and Mauritiella. The marked increase of palms during the last c. 3800 yrs points to increased human impact on the vegetation under the wettest Holocene climate regime.     

Geomorphological and sedimentary evidence for late Pleistocene to Holocene hydrological change along the Río Mamoré, Bolivian Amazon

Aerial photography and satellite imagery reveal manifold geomorphological evidence of a dynamic evolution of past and present rivers in the Bolivian Amazon. Comparison of oxbow lake and meander scar dimensions along an inactive meander belt of the Río Mamoré (Llanos de Moxos, north-eastern Bolivia) and its modern counterpart suggests significant regional paleohydrological variability. We used these features as geomorphological and sedimentary archives to enhance our understanding of longer-term variations of the fluvial system. Late Pleistocene to Holocene hydrological changes of the Río Mamoré are inferred from: (i) the analysis of satellite imagery, (ii) discharge estimates from meander morphology, (iii) stratigraphic, and (iv) chronological information based on luminescence and radiocarbon dating. The combined data from three oxbows indicate that the now abandoned meander belt – the paleo-Mamoré – continued to be active at least until ∼5 ka, and likely even postdating 3 ka. An up to threefold increase in discharge is estimated for the modern Río Mamoré versus the paleo-Mamoré. The altered runoff regime may have triggered an avulsive shift towards the currently active Río Mamoré. The preceding increase in discharge in turn, was possibly related to a shift in climatic conditions, which changed markedly between the mid- and late Holocene in tropical South America. In addition, it may have been the indirect result of capturing the avulsive Río Grande system to the east of the Río Mamoré. Alternative explanations for the differences in dimensions of the paleo versus the modern Río Mamoré, i.e. contemporaneous activity of both rivers or alteration of site factors such as the channel/floodplain relationship, are considered to be unlikely.     

The Las Chacras-Potrerillos batholith (Pampean Ranges,Argentina): structural evidences,emplacement and timing of the intrusion

Within the southern part of the Sierra Pampeanas (the Sierra de San Luis, Argentina), a series of extensive intrusive bodies are regarded to post-date the Famatinian cycle but were emplaced during the Achalian, a period of heterogeneous deformation along crustal scale fault zones. The largest of those is the Las Chacras-Potrerillos batholith that is situated at the northern end of the transpressive, sinistral Guzmán shear zone. This composite pluton exhibits three sub-domains that comprise two granitoid sub-units each: The southern Potrerillos stock (muscovite-bearing red granite and biotite-bearing red granite) and the central (biotite porphyritic granite and giant porphyritic granite) and northern domain (equigranular granite and porphyritic granite) of the Las Chacras stock. The crystallisation ages of the biotite porphyritic granite is around 381 Ma (U/Pb on zircons and Pb/Pb on sphene), while the host rock was already cooled below 350 °C at 420 Ma. Thermal modelling approaches favour a pulsed intrusion with a duration of 1.5 Ma. The emplacement was followed by rapid cooling below the muscovite cooling temperature. Biotite cooling ages in different sub-units reflect either a long-lasting cooling history of approximately 30 Ma (which is supported by the modelling) or a reheating effect at around 350 Ma. Devonian-age determinations on the fault rocks and granitoids point to a syn-tectonic emplacement of the batholith. The pluton is interpreted to be positioned at the crossover of sinistral shear zones. The origin of this NNE directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of NNW-trending sinistral faults. The final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation. This model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data. Our results indicate that the emplacement is syn-kinematic with respect to the Achalian deformation event.