Taphonomy of guanaco bones in Tierra del Fuego     

Luis Alberto Borrero 《Quaternary Research》1990,34(3)

Guanaco carcasses are deposited in great quantities in Cabo San Pablo, Tierra del Fuego (Argentina), as a result of winter stress. Taphonomic studies indicate that the gnawing action of foxes on guanaco (Lama guanicoe) carcasses produces only very tenuous marks on the bones. Lack of sustained interest in the carcasses by carnivores results in slow disarticulation. The articulated and disarticulated bones are exposed to heavy trampling by guanacos, a process that produces vertical migration of small/dense bones and fracturing of the most weathered bones. An understanding of this ongoing process is important for local archaeology, since modern bones are migrating into archaeological contexts. A regional approach to taphonomy is the most appropriate instrument to solve this and other related problems.  相似文献   

Cu, Cd and Zn in Blanca Bay surface sediments, Argentina     

JosLuis Sericano  Adan E. Pucci 《Marine pollution bulletin》1982,13(12):429-431

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Metamorphic gradients in burial metamorphosed vesicular lavas: Comparison of basalt and spilite in Cretaceous basic flows from central Chile     

Beatriz Levi  Luis Aguirre  Jan Olov Nyström 《Contributions to Mineralogy and Petrology》1982,80(1):49-58

Partial spilitization of a 9 km thick pile of flood basalts with highly vesicular flow tops gave rise to patterns of secondary mineralogy at different scales: (a) a local pattern of mineralogical variation from the almost unaltered bottom towards the altered top of each flow, and (b) an overall pattern, comparing flow tops throughout the pile, with changes in mineralogical composition within a sequence of metamorphic zones and facies. The local patterns mimic the trend of the overall pattern, but are of opposite direction and telescoped. Thus, a gradual ordering and Andepletion of the secondary albite and increases in the Fe^*/Al ratio of epidote and pumpellyite upwards within individual flows are comparable in range to corresponding overall changes downwards throughout several kilometres. The mineralogical changes within the flows diminish in range towards the more altered deeper part of the pile.The local and overall patterns cannot be interpreted in terms of grade. They represent trends from metastable towards stable equilibrium, this latter only approached in the flow tops of the lower part of the pile. The patterns of secondary mineralogy were formed by an interplay of metamorphic gradients at different scales at any given time, and as burial proceeded. The overall pattern was caused by depth-controlled gradients: increasing P _fluid, temperature and temperature-induced increase of reaction rates, and decreasing fO₂ (downwards in the pile). The local patterns resulted from permeability-controlled gradients: increasing reaction rates, fO₂ and contrast in chemical activity between different domains, and decreasing P _fluid (upwards in each flow). The mineralogical observations reported in this paper fall into line if the overall temperature-induced increase of reaction rates and the local permeability-controlled rate factors played the leading role during burial metamorphism of the pile.  相似文献   

Dynamics of lava flow fronts, Arenal Volcano, Costa Rica     

Andrea Borgia  Scott Linneman  Daniel Spencer  Luis Diego Morales  Jose Brenes Andre 《Journal of Volcanology and Geothermal Research》1983,19(3-4)

Arenal Volcano has effused basaltic andesite lava flows nearly continuously since September, 1968. The two different kinds of material in flows, lava and lava debris, have different rheologic properties and dynamic behavior. Flow morphology depends on the relationship between the amount and distribution of the lava and the debris, and to a lesser extent the ground morphology.Two main units characterize the flows: the channel zone and the frontal zone. The channel zone consists of two different units, the levées and the channel proper. A velocity profile in the channel shows a maximum value at the plug where the rate of shear is zero, and a velocity gradient increasing outward until, at the levées, the velocity becomes zero. Cooling produces a marked temperature gradient in the flow, leading to the formation of debris by brittle fracture when a critical value of shear rate to viscosity is reached. When the lava supply ceases, much of this debris and part of the lava is left behind after the flow nucleus drains out, forming a collapsed channel.Processes at the frontal zone include levée formation, debris formation, the change in shape of the front, and the choice of the flow path. These processes are controlled primarily by the rheological properties of the lava.Frontal zone dynamics can be understood by fixing the flow front as the point of reference. The lava flows through the channel into the front where it flows out into the levées, thereby increasing the length of the channel and permitting the front to advance. The front shows a relationship of critical height to the yield strength (τ₀) surface tension, and slope; its continued movement is activated by the pressure of the advancing lava in the channel behind. For an ideal flow (isothermal, homogeneous, and isotropic) the ratio of the section of channel proper to the section of levées is calculated and the distance the front will have moved at any time t_x can be determined once the amount of lava available to the front is known. Assuming that the velocity function of the front {G(t)} during the collapsing stage is proportional to the entrance pressure of the lava at the channel-front boundary, an exponential decrease of velocity through time is predicted, which shows good agreement with actual frontal velocity measurements taken on two flows. Local variations in slope have a secondary effect on frontal velocities.Under conditions of constant volume the frontal zone can be considered as a machine that consumes energy brought in by the lava to perform work (front advancement). While the front will use its potential energy to run the process, the velocity at which it occurs is controlled by the activation energy that enters the system as the kinetic energy of the lava flowing into the front. A relation for the energy contribution due to frontal acceleration is also derived. Finally the entrance pressure, that permits the front to deform, is calculated. Its small value confirms that the lava behaves very much like a Bingham plastic.  相似文献   

Improved magnetic anomalies of the Antarctic lithosphere from satellite and near-surface data     

Hyung Rae Kim  Ralph R. B. von Frese  Patrick T. Taylor  Alexander V. Golynsky  Luis R. Gaya-Piqué  Fausto Ferraccioli 《Geophysical Journal International》2007,171(1):119-126

The Antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60°S through 1999 and used Magsat data to help fill in the regional gaps between the surveys. Ørsted and CHAMP satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the Antarctic, have now supplanted the Magsat data. We combined the new satellite observations with the near-surface survey data for an improved magnetic anomaly map of the Antarctic lithosphere. Specifically, we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite-derived free-air gravity observations. Regional gaps in the near-surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near-surface and satellite crustal anomalies. Comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models. We also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near-surface crustal anomaly estimates.  相似文献   

Dynamic of the freshwater–saltwater interface in a karstic aquifer under extraordinary recharge action: the Merida Yucatan case study     

Oscar Escolero  Luis E. Marin  Eloisa Domínguez-Mariani  Sandra Torres-Onofre 《Environmental Geology》2007,51(5):719-723

A hydraulic analysis of the interface between freshwater–saltwater behavior was done in the Merida Yucatan zone, two machines that constantly register the groundwater levels were installed, and three electric conductivity logs were taken from wells. When comparing the measured results with the ones obtained using theoretical equations developed to calculate the freshwater–saltwater interface position, it was proved that in some cases these equations can be applied, and in others not. Two effects that rule the behavior of karst aquifers in extraordinary conditions were found.  相似文献   

Spatial and temporal scales of transport during the cooling phase of the ice-free period in a small high-mountain lake     

Francisco Rueda  Enrique Moreno-Ostos  Luis Cruz-Pizarro 《Aquatic Sciences - Research Across Boundaries》2007,69(1):115-128

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The Middle Jurassic radiolarites and pelagic limestones of the Nieves unit (Rondaide Complex, Betic Cordillera): basin starvation in a rifted marginal slope of the western Tethys     

Luis O'Dogherty  Agustín Martín-Algarra  Hans-Jürgen Gursky  Roque Aguado 《International Journal of Earth Sciences》2001,90(4):831-846

Middle Jurassic radiolarites and associated pelagic limestones occur in the Rondaide Nieves unit of the Betic Cordillera, southern Spain. The Rondaide Mesozoic includes: (a) a thick succession of Triassic platform carbonates, comparable to the Alpine Hauptdolomit and Kössen facies; (b) Lower Jurassic pelagic limestones comparable to the Alpine Hierlatz and Adnet facies; (c) the Middle Jurassic Parauta Radiolarite Formation, described herein; and (d) a thin Upper Jurassic-Cretaceous condensed limestone succession. The Parauta Radiolarite Formation and associated limestones were studied with respect to stratigraphy, petrography, micropalaeontology (radiolarians, calcareous nanno- and microfossils) and facies. Radiolarite sedimentation occurred in the Middle Bathonian in a restricted and dysoxic deep Nieves basin, perched in the distal zone of a continental margin fringing the Tethyan ocean. This margin was adjacent to a young narrow oceanic basin between the South-Iberian margin and a continental block called Mesomediterranean Terrane. The Nieves basin was part of a marine corridor between the Proto-Atlantic and Piedmont-Ligurian basins of the Alpine Tethys. The regional tectonic position, the stratigraphical evolution since the Triassic, the age and the nature of the Mesozoic facies and the palaeogeographic relations to adjacent domains show striking analogies between the Betic Rondaide margin and coeval units of the Alps.  相似文献   

Cretaceous stratigraphy and palaeoenvironments of the Southern Petén Basin, Guatemala     

Eric Fourcade  Luis Piccioni  Jos Escrib  Eduardo Rosselo 《Cretaceous Research》1999,20(6):793

The Cretaceous deposits of the Southern Petén Basin, an oil-producing province, are located to the south of the Yucatan Platform and to the east of the Chiapas Carbonate Platform of Mexico. The succession in the southern part of this basin has been studied both in wells and at outcrop by microfacies analysis. It is composed of 5000 m of shallow marine carbonates and evaporites with a few thin layers of pelagic limestones rich in organic carbon and planktic foraminifera deposited during peak transgressions or maximum flooding. The sedimentation of this thick succession was not continuous, and the section is punctuated by subaerial erosional bounding surfaces and a single hardground which marks the final drowning of the carbonate platform during the late Maastrichtian. New age determinations have been obtained on the basis of planktic and benthic foraminifera from the Aptian to Santonian Cobán Formation, D, C, B and A Members. Cobán C (Albian) and B (Cenomanian) Members are the present oil reservoirs. The rudist and alveolinid limestones of late Campanian and Maastrichtian age referred to the Campur Formation in the Southern Petén Basin are here reassigned to the Angostura Formation, as recognized in the Chiapas area. The Actela Formation is defined here to encompass the limestone breccia deposits that occur at the Cretaceous–Tertiary boundary and extend into the Parvularugoglobigerina eugubina Zone of early Danian age. The D, C, B and A Members of the Cobán Formation and the Angostura Formation represent second order transgressive and regressive trends in a passive margin area where the deposits indicate a succession of various environments, including fluviatile, salinas, shallow marine carbonate platform, outer shelf and intrashelf basin. High subsidence rates, sea-level changes and tectonic uplift controlled the sedimentation.

Abstract

Les formations crétacées du sud-est du Bassin du Petén, région pétrolière du nord du Guatemala, sont situées au Sud du Yucatan et à l'Est du Chiapas au Mexique. Ces sédiments ont été étudiés à la fois dans deux puits et à l'affleurement par l'analyse des microfaciès. Cet ensemble sédimentaire est composé de 5000 m de carbonates de plate-forme de faible profondeur et d'évaporites, dans lesquels s'intercalent de minces couches de calcaires pélagiques riches en matière organique et en foraminifères planctoniques. Ces calcaires pélagiques se sont déposés lors des maximums de transgressions. La sédimentation de cette épaisse série n'est pas continue et est ponctuée par quelques surfaces d'érosion sub-aérienne et une surface durcie qui souligne l'ennoyage définitif de la plate-forme carbonatée au Maastrichtien terminal. De nouvelles datations ont été obtenues sur la formation Cobán et ses membres D, C, B et A qui s'étagent de l'Aptien au Santonien. Les membres C (albien) et B (cénomanien) constituent les réservoirs productifs. Les calcaires à rudistes et à alveolinidés du Campanien supérieur–Maastrichtien ont été appelés à tort au Petén formation Campur. Ils doivent, selon nous, être attribués à la formation Angostura comme au Chiapas voisin, où affleurent des faciès comparables et de même âge. Une nouvelle formation est créée (formation Actela) pour les brèches calcaires de la limite Crétacé–Tertiaire et de la Zone à Eugubina du Danien inférieur. Les membres D, C, B, A de la formation Cobán et la formation Angostura représentent des cycles transgressifs–régressifs de deuxième ordre. Sur cette marge passive du Petén, les dépôts correspondent à des environnements variés comme des terres émergées soumises à une altération karstique et à des dépôts fluviatiles, des salinas, des plates-formes carbonatées marines, des shelf externes et des bassins intra plate-forme. La sédimentation a été, ici, contrôlée par une forte vitesse de subsidence, les variations du niveau marin et des soulèvements tectoniques.  相似文献   

Analyzing ice dynamics using Sentinel-1 data at the Solheimajoküll Glacier,Iceland     

Diego Gómez  Pablo Salvador  Julia Sanz  Mikhail Urbazaev  José Luis Casanova 《地理信息系统科学与遥感》2020,57(6):813-829

ABSTRACT

The climate in southern Iceland has warmed over the last 70 years, resulting in accelerated glacier dynamics at the Solheimajoküll glacier. In this study, we compare glacier terminus locations from 1973 to 2018, to changes in climate across the study area, and we derive ice-surface velocities (2015–2018) from satellite remote-sensing imagery (Sentinel-1) using the offset-tracking method. There have been two regional temperature trends in the study period: cooling (1973–1979) and warming (1980–2018). Our results indicate a time lag of about 20 years between the onset of glacier retreat (?53 m/year since 2000) and the inception of the warming period. Seasonally, the velocity time series suggest acceleration during the summer melt season since 2016, whereas glacier velocities during accumulation months were constant. The highest velocities were observed at high elevations where the ice-surface slope is the steepest. We tested several scenarios to assess the hydrological time response to glacier accelerations, with the highest correlations being found between one and 30 days after the velocity estimates. Monthly correlation analyses indicated inter-annual and intra-annual variability in the glacier dynamics. Additionally, we investigate the linkage between glacier velocities and meltwater outflow parameters as they provide useful information about internal processes in the glacier. Velocity estimates positively correlate with water level and negatively correlate with water conductivity between April and August. There is also a disruption in the correlation trend between water conductivity and ice velocity in June, potentially due to a seasonal release of geothermal water.  相似文献