Radiochemical,scanning electron microscope (SEM) and X-ray dispersive energy (EDAX) studies of a FAMOUS hydrothermal deposit     

Claude Lalou  Evelyne Brichet  Teh Lung Ku  Celestine Jehanno 《Marine Geology》1977,24(4):245-258

During the French-American Mid-Oceanic Undersea Survey (FAMOUS) the French team participating in the mission explored the active A transform fault with the diving saucer CYANA, and discovered a characteristic hydrothermal deposit. The results presented here concern the structural and radiochemical study of three samples: two metalenriched samples and one sample of consolidated calcareous sediment recovered from the area.It is shown that there is a chemical segregation between silica and iron on one hand, and manganese in the other hand, and that on such deposits the layer-by-layer radio-chemical analysis of the uranium series is not applicable.Three different age estimations may be made:In the managanese encrustation, thorium 230 is depleted towards equilibrium with its parent uranium 234, suggesting a maximum age of 45,000 years to the deposit.On the contrary, the outermost layer, 0.1 cm thick, formed essentially with loose sediment stained with oxides, presents an excess of thorium 230. The Σ²³⁰Th_(excess)/cm² may be used to calculate a minimum age of 20 years.Such a sediment thickness, in this area, corresponds to a time span of 50 years.  相似文献   

An absolute calibration site for radar altimeters in the continental domain: Lake Issykkul in Central Asia     

Jean-François Crétaux  Stephane Calmant  Vladimir Romanovski  Anton Shabunin  Florent Lyard  Muriel Bergé-Nguyen  Anny Cazenave  Fabrice Hernandez  Felix Perosanz 《Journal of Geodesy》2009,83(8):723-735

Altimetry missions such as Topex/Poseidon, Jason-1, GFO and ENVISAT have been widely used in the continental domain over lakes, rivers and wetland although they were mostly dedicated to oceanic studies. Knowledge of the instrumental biases is a key issue. Numerous sites have been dedicated to calibration purposes, either in the oceanic domain (Harvest offshore platform in California, Corsica, Bass Strait in Australia) or over lakes (Lake Erie in United States). A new site (Lake Issykkul in Kirghizstan) is proposed for calibration in the continental domain. This lake is covered by past (T/P) and current radar altimetry satellites (Jason-1, T/P, GFO, and ENVISAT). Several in situ water levels and local meteorological variables are available at the site. Located in a mountainous area, it offers an opportunity for calibration far away from all other existing sites and very different environment contexts. Two GPS campaigns have been conducted on the lake in 2004 and in 2005. They consisted of cruises with stations installed onboard a boat following the satellite ground tracks, and onshore settings. This enabled estimating a bias for each altimeter and each tracking algorithm available. Biases obtained for Envisat, GFO, T/P and Jason-1 using the default ocean tracker (respectively, 48.1 ± 6.6, 7.5 ± 4.0, 0 ± 4.3 and 7.0 ± 5.5 cm) agree with biases published at the other calibration sites. For Jason-1, there is a significant disagreement with results obtained in the ocean field (7 cm instead of 13 cm) but is coherent with bias obtained on the Lake Erie site. Erroneous estimates of the sea state bias correction from non-oceanic-like waveforms is discussed as a possible explanation. Errors in the ionospheric, wet and dry tropospheric corrections for the continental domain are also highlighted and quantified.  相似文献   

3D gravimetric investigation of the Cerro do Jarau structure,Rio Grande do Sul,Brazil          下载免费PDF全文

Bruno B. Giacomini  Emilson P. Leite  Alvaro P. Crósta 《Meteoritics & planetary science》2017,52(4):565-583

The Cerro do Jarau structure is possibly the third Brazilian basaltic crater formed in continental flood basalt of the Serra Geral Formation, Paraná Basin, a large igneous province (LIP) in southern Brazil. It is a nearly circular landform with a diameter of approximately 13 km that rises 200 m above the plains of the “pampas” in southern Brazil. In this work, Bouguer anomalies were calculated from gravity accelerations measured on the area of this structure. The residual Bouguer map shows a strong positive anomaly trending NE‐SW, located in the northeastern part of the structure, a feature not commonly associated with impact structures. However, the negative anomaly present in its center and the circular positive anomaly surrounding the central portion are typical of impact structures. The residual Bouguer anomaly varies from ?2 mGal to 8 mGal. The positive circular anomaly is not spatially coincident with the rim of the structure. Based on the interpretation of our gravimetric data, the estimated diameter of the structure is 12 km and the central portion has a diameter of approximately 5 km, both slightly smaller than previously suggested. The Bouguer anomaly map was inverted into a 3D density model using a constrained inversion method with a maximum density contrast of 0.5 g cm^?3. This model was interpreted to associate densities with rock types, resulting in a geological model. This geological model is in accordance with the meteorite impact nature of Cerro do Jarau.  相似文献   

Revisiting an old profile: an updated geoarchaeological study at Nasera Rockshelter (Tanzania)     

David Manuel MartÍn-Perea  JosÉ-Manuel MaÍllo-FernÁndez  Alicia Medialdea  Juan MarÍn  Irene Solano-MegÍas  Agness Gidna  Audax Mabulla 《第四纪科学杂志》2020,35(7):951-960

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Downslope‐migrating sandwaves and platform‐margin clinoforms in a current‐dominated,distally steepened temperate‐carbonate ramp (Guadix Basin,Southern Spain)     

ÁNGEL PUGA‐BERNABÉU  JOSÉ M. MARTÍN  JUAN C. BRAGA  ISABEL M. SÁNCHEZ‐ALMAZO 《Sedimentology》2010,57(2):293-311

During the Late Tortonian, platform‐margin‐prograding clinoforms developed at the south‐western margin of the Guadix Basin. Large‐scale wedge‐shaped deposits here comprise 26 rhythms of mixed carbonate–siliciclastic bedset packages and marl beds. These sediments were deposited on a shallow‐water, temperate‐carbonate distally steepened ramp. A downslope‐migrating sandwave field developed in this ramp, with sandwaves moving progressively down the ramp to the ramp‐slope, where they destabilized, folded and occasionally collapsed. Downslope sandwave migration was induced by currents flowing basinwards. During the Late Tortonian, the Guadix Basin was open north to the Atlantic Ocean via the Dehesas de Guadix Strait and connected east to the Mediterranean Sea through the Almanzora Corridor. According to the proposed current circulation model for the Guadix Basin for this time, surface marine currents from the Atlantic entered the basin from the northern seaway. These currents moved counter‐clockwise and shifted the sediment on the ramp, forming sandwaves that migrated downslope. The development of platform‐margin prograding clinoforms by the basinward sediment‐transport mechanisms inferred here is known relatively poorly in the ancient sedimentary record. Moreover, these wedge‐shaped geometries are similar to those found in some shelves in the Western Mediterranean Sea and could represent an outcrop analogue to (sub)‐recent, platform‐margin clinoforms revealed by high‐resolution seismic studies.  相似文献   

Fluvial and submarine morphodynamics of laminar and near‐laminar flows: a synthesis     

ERIC LAJEUNESSE  LUCE MALVERTI  PIERRE LANCIEN  LAWRENCE ARMSTRONG  FRANCOIS MÉTIVIER  STEPHEN COLEMAN  CHARLES E. SMITH  TIMOTHY DAVIES  ALESSANDRO CANTELLI  GARY PARKER 《Sedimentology》2010,57(1):1-26

The interaction of flow with an erodible bed in alluvial rivers and deep‐sea channels gives rise to a wide range of self‐formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple‐row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near‐laminar flow analogues of morphologies observed in the field. Laminar‐flow analogues of turbulent‐flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent‐flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.  相似文献   

The action of wind and water in a mid‐Cretaceous subtropical erg‐margin system close to the Variscan Iberian Massif,Spain     

JUAN PEDRO RODRÍGUEZ‐LÓPEZ  NIEVES MELÉNDEZ  POPPE L. De BOER  ANA ROSA SORIA 《Sedimentology》2010,57(5):1315-1356

Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid‐Cretaceous outer back‐erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain‐size distributions, mud playa, ephemeral floodplain, pebble‐sand and cobble‐sand bedload stream, pebble–cobble‐sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub‐environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind‐driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back‐erg depositional system comprises an outer back‐erg close to the Variscan highlands, and an inner back‐erg close to the central‐erg area. The inner back‐erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back‐erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud‐cracked surfaces, resulting in largely structureless sandstones with boulder‐size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser‐grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back‐erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back‐erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non‐exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti‐clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well‐preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre‐Cretaceous times.  相似文献   

Coexistence of lawsonite‐bearing eclogite and blueschist: phase equilibria modelling of Alpine Corsica metabasalts and petrological evolution of subducting slabs     

A. VITALE BROVARONE  C. GROPPO  G. HETÉNYI  R. COMPAGNONI  J. MALAVIEILLE 《Journal of Metamorphic Geology》2011,29(5):583-600

In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite‐bearing eclogite (Law‐Ecl), occurrence of which is restricted to ～10 localities worldwide. The Corsican Law‐Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occurs as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law‐Bs), and carbonate‐bearing eclogitic veins. Law‐Bs are found as variably deformed metabasaltic pillows locally cross‐cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law‐Ecl and Law‐Bs are stable at the metamorphic peak in the lawsonite‐eclogite stability field. Isochemical phase diagrams (pseudosections) calculated for representative Law‐Ecl and Law‐Bs samples indicate that both lithologies equilibrated at the same conditions of ～520 ± 20 °C and 2.3 ± 0.1 GPa. Therefore, the coexistence at the same peak metamorphic conditions of Law‐Ecl and Law‐Bs implies that different portions of deeply subducted oceanic crust may store significantly different H₂O contents, depending on bulk‐rock chemical composition. In addition, thermodynamic modelling of phase equilibria indicates that the occurring progressive dehydration reactions, which are significantly depending on bulk‐rock chemical composition, strongly influence rock densification and eclogite formation in subducting slabs.  相似文献   

Late Pleistocene biota from Pubenza,Colombia; turtles,mammals, birds,invertebrates and plant remains     

AndrÉs Alfonso-Rojas  Laura M. Herrera-Gutierrez  Catalina SuÁRez  MartÍN R. Ciancio  Jonathan S. Pelegrin  Edwin-Alberto Cadena 《第四纪科学杂志》2021,36(3):450-466

Pubenza is a remarkable palaeontological site of Colombia, and a place that could hold some of the potentially oldest evidence of humans in northern South America. Previous palaeontological research at this site has mainly focused on the megafauna. Here we describe and establish the systematic palaeontology for the small fauna that inhabited this ancient lacustrine ecosystem, including the first report of birds, tortoises and vipers for the Late Pleistocene in Colombia. Furthermore, exceptionally well preserved fossilised wasp nests are morphologically and elementally characterised, which is the first report of an ichnofossil of this kind in northern South America. In addition, new material of kinosternid turtles, armadillos and rodents is also described. Our results reveal that the Bogotá River Basin, where Pubenza is located, was a rich ecosystem during the Late Pleistocene and a region of great interest for future articulated palaeontological and archaeological studies.  相似文献   

Comment on “Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau”          下载免费PDF全文

Wolf Uwe Reimold  Alvaro P. Crósta  Christian Koeberl  Natalia Hauser 《地学学报》2017,29(6):409-410

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