Evinosponges in the Triassic Esino Limestone (Southern Alps): documentation of early lithification and late diagenetic overprint     

SILVIA FRISIA-BRUNI  FLAVIO JADOUL  HELMUT WEISSERT 《Sedimentology》1989,36(4):685-699

A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ¹⁸O(PDB)=?5‰ to ?12‰ and from δ¹³O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny.  相似文献   

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)     

PAOLA RONCHI  FLAVIO JADOUL  ANDREA CERIANI  ANDREA DI GIULIO  PAOLO SCOTTI  ANDREA ORTENZI  ELISABETTA PREVIDE MASSARA 《Sedimentology》2011,58(2):532-565

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.  相似文献   

Compositional changes in sigmoidal carbonate clinoforms (Late Tithonian,eastern Sardinia,Italy): insights from quantitative microfacies analyses     

ALESSANDRO LANFRANCHI  FABRIZIO BERRA  FLAVIO JADOUL 《Sedimentology》2011,58(7):2039-2060

Quantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings.  相似文献   

Seismic evidence of up to 200 m lake‐level change in Southern Patagonia since Marine Isotope Stage 4     

ANDREA CATALINA GEBHARDT  CHRISTIAN OHLENDORF  FRANK NIESSEN  MARC DE BATIST  FLAVIO S. ANSELMETTI  DANIEL ARIZTEGUI  PIERRE KLIEM  STEFAN WASTEGÅRD  BERND ZOLITSCHKA 《Sedimentology》2012,59(3):1087-1100

Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south‐eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo‐shorelines indicating a stepwise water‐level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake‐level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present‐day lake level point towards a long period of lake‐level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp . Dryer conditions during the Mid‐Holocene are evidenced by a dropping lake level, resulting in a basin‐wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo‐shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.  相似文献   

Environmental history of southern Patagonia unravelled by the seismic stratigraphy of Laguna Potrok Aike     

FLAVIO S. ANSELMETTI  DANIEL ARIZTEGUI†  MARC DE BATIST‡  A. CATALINA GEBHARDT§  TORSTEN HABERZETTL¶  FRANK NIESSEN§  CHRISTIAN OHLENDORF  BERND ZOLITSCHKA 《Sedimentology》2009,56(4):873-892

Laguna Potrok Aike, located in southernmost Patagonia (Argentina, 52°S) is a 100 m deep hydrologically closed lake that probably provides the only continental southern Patagonian archive covering a long and continuous interval of several glacial to interglacial cycles. In the context of the planned ‘International Continental Scientific Drilling Program’ initiative ‘Potrok Aike Maar Lake Sediment Archive Drilling Project’, several seismic site surveys that characterize in detail the sedimentary subsurface of the lake have been undertaken. Long sediment cores recovered the material to date and calibrate these seismic data. Laguna Potrok Aike is rimmed steeply, circular in shape with a diameter of ∼3·5 km and is surrounded by a series of subaerial palaeoshorelines, reflecting varying lake-level highstands and lowstands. Seismic data indicate a basinwide erosional unconformity that occurs consistently on the shoulder of the lake down to a depth of −33 m (below 2003 ad lake level), marking the lowest lake level during Late Glacial to Holocene times. Cores that penetrate this unconformity comprise Marine Isotope Stage 3-dated sediments (45 kyr bp ) ∼3·5 m below, and post-6800 cal yr bp transgressional sediments above the unconformity. This Middle Holocene transgression following an unprecedented lake-level lowstand marks the onset of a stepwise change in moisture, as shown by a series of up to 11 buried palaeoshorelines that were formed during lake-level stillstands at depths between −30 and −12 m. Two series of regressive shorelines between ∼5800 to 5400 and ∼4700 to 4000 cal yr bp interrupt the overall transgressional trend. In the basin, mound-like drift sediments occur after ∼6000 cal yr bp, documenting the onset of lake currents triggered by a latitudinal shift or an increase in wind intensity of the Southern Hemispheric Westerlies over Laguna Potrok Aike at that time. Furthermore, several well-defined lateral slides can be recognized. The majority of these slides occurred during the mid-Holocene lake-level lowering when the slopes became rapidly sediment-charged because of erosion from the exposed shoulder sediments. Around 7800 and 4900 cal yr bp , several slides went down simultaneously, probably triggered by seismic shaking.  相似文献   

Late Quaternary deposition and facies model for karstic Lake Estanya (North-eastern Spain)     

MARIO MORELLÓN  BLAS VALERO-GARCÉS  FLAVIO ANSELMETTI†  DANIEL ARIZTEGUI‡  MICHAEL SCHNELLMANN§  ANA MORENO¶  PILAR MATA  MAYTE RICO  JUAN PABLO CORELLA 《Sedimentology》2009,56(5):1505-1534

Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20 m, located in the Pre‐Pyrenean Ranges (North‐eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub‐environments can be recognized as: (i) a carbonate‐producing ‘littoral platform’; (ii) a steep ‘talus’ dominated by reworking of littoral sediments and mass‐wasting processes; and (iii) an ‘offshore, distal area’, seasonally affected by anoxia with fine‐grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a ‘basal unit’, seismically transparent and restricted to the depocentres of both sub‐basins; (ii) an ‘intermediate unit’ characterized by continuous high‐amplitude reflections; and (iii) an ‘upper unit’ with strong parallel reflectors. Several mass‐wasting deposits occur in both sub‐basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry ¹⁴C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite‐producing lake during full glacial times (21 to 17·3 kyr bp ); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp ); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp ); (iv) a saline lake with gypsum‐rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic‐dominated deposition during the Holocene (9·4 to 0·8 kyr bp ); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800 years. Climate‐driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake‐level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass‐flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations.  相似文献   

Radioactive Isotopes in Hoba West and Other Iron Meteorites     

R. H. McCORKELL  E. L. FIREMAN  J. D'AMICO  S. O. THOMPSON 《Meteoritics & planetary science》1968,4(2):113-122

Abstract The radioactivities Be¹⁰, Al²6, Cl³⁶, Mn⁵³, Ni⁵⁹, and Co⁶⁰ and the rare gas isotopes were measured in a sample taken from the surface of the Hoba meteorite. The spallation-produced radioactivities indicate that the sample was at a depth of 35 to 40 cm when the body was in space. The Ni⁵⁹ activity indicates that the terrestrial age of Hoba is less than 80,000 years. Its Cl³⁶-Ar³⁶ exposure age is 263±40 million years. The Cl³⁶ and rare-gas isotopes were also measured in the Deelfontein meteorite; its Cl³⁶-Ar³⁶ exposure age is 400±40 million years. The Cl³⁶ and the Ni⁵⁹ were also measured in a sample of Sikhote-Alin. In both Hoba and Sikhote-Alin the Ni⁵⁹, which is a neutron-produced isotope, is higher than would be expected from the Cl³⁶ content. This indicates that solar flares contributed to the neutron-produced isotopes in these iron meteorites.  相似文献   

Development of the hard X-ray imaging experiment timax: Laboratory images and first balloon flight     

JOÃO BRAGA  FLAVIO D'AMICO  THYRSO VILLELA 《Experimental Astronomy》1994,5(3-4):269-278

We present calibration results and laboratory images produced by the balloon-borne hard X-ray imaging telescope TIMAX. The images were produced with an²⁴¹Am radioactive source placed 45 m away from the detector plane, in the center of the field of view. It is shown that the mask 3-antimask imaging reconstruction process, when combined with flat-fielding techniques, is very effective at recovering signal-to-noise ratio lost due to systematic non-uniformity in the background measured by the 35 detectors. The experiment was launched in June 8th, 1993 from Birigüi, SP, Brazil, onboard a 186,000 m³ stratospheric balloon, and remained at an atmospheric depth of 2 g cm^–2su for 8 hours. Even though no scientific data were gathered in this first flight, we obtained valuable engineering data and could also calculate the sensitivity of the experiment based on the instrumental background spectrum at balloon altitudes. In the 60–70 keV energy band, the experiment can detect 3 sources at a level of 1.2 x 10^–4 photons cm^–2 s^–1 keV^–1 for an integration of 6 hours at 2.1 g cm^–2.  相似文献   

EARLINET observations of the 14–22-May long-range dust transport event during SAMUM 2006: validation of results from dust transport modelling     

By D. MÜLLER  B. HEINOLD  M. TESCHE  I. TEGEN  D. ALTHAUSEN  L. ALADOS ARBOLEDAS  V. AMIRIDIS  A. AMODEO  A. ANSMANN  D. BALIS  A. COMERON  G. D'AMICO  E. GERASOPOULOS  J. L. GUERRERO-RASCADO  V. FREUDENTHALER  E. GIANNAKAKI  B. HEESE  M. IARLORI  P. KNIPPERTZ  R. E. MAMOURI  L. MONA  A. PAPAYANNIS  G. PAPPALARDO  R.-M. PERRONE  G. PISANI  V. RIZI  M. SICARD  N. SPINELLI  A. TAFURO  M. WIEGNER 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):325-339

We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ. Ångström exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical properties of the dust plume with a dust transport model. The model results and the experimental data show similar times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one reason for these deviations.  相似文献   

Mass movement-induced fold-and-thrust belt structures in unconsolidated sediments in Lake Lucerne (Switzerland)   总被引：2，自引：0，他引：2  

MICHAEL SCHNELLMANN  FLAVIO S. ANSELMETTI  DOMENICO GIARDINI†  JUDITH A. McKENZIE 《Sedimentology》2005,52(2):271-289

High-resolution seismic imaging and piston coring in Lake Lucerne, Switzerland, have revealed surprising deformation structures in flat-lying, unconsolidated sediment at the foot of subaqueous slopes. These deformation structures appear beneath wedges of massflow deposits and resemble fold-and-thrust belts with basal décollement surfaces. The deformation is interpreted as the result of gravity spreading induced by loading of the slope-adjacent lake floor during massflow deposition. This study investigated four earthquake-triggered lateral mass-movement deposits in Lake Lucerne affecting four sections of the lake floor with areas ranging from 0·25 to 6·5 km² in area. Up to 6 m thick sediment packages draping the subaqueous slopes slid along the acoustic basement. The resulting failure scars typically lie in water depths of >30 m on slopes characterized by downward steepening and inclinations of >10°. From the base-of-slope to several hundred metres out onto the flat plains, the wedges of massflow deposits overlie deeply (10–20 m) deformed basin-plain sediment characterized by soft sediment fold-and-thrust belts with arcuate strikes and pronounced frontal thrusts. The intensity of deformation decreases towards the more external parts of the massflow wedges. Beyond the frontal thrust, the overridden lake floor remains mostly undisturbed. Geometrical relationships between massflow deposits and the deformed basin-plain sediment indicate that deformation occurred mainly during massflow deposition. Gravity spreading induced by the successive collapse of the growing slope-adjacent massflow wedge is proposed as the driving mechanism for the deformation. The geometry of fjord-type lakes with sharp lower slope breaks favours the deposition of thick, basin-marginal massflow wedges, that effectively load and deform the underlying sediment. In the centre of the basins, the two largest massflow deposits described are directly overlain by thick contained (mega-)turbidites, interpreted as combined products of the suspension clouds set up by subaqueous mass movements and related tsunami and seiche waves.  相似文献