Neoproterozoic glacial dynamics revealed by provenance of diamictites of the Bebedouro Formation, São Francisco Craton, Central Eastern Brazil     

Felipe Torres Figueiredo  Renato Paes de Almeida  Eric Tohver  Marly Babinski  Dunyi Liu  Christopher Mark Fanning 《地学学报》2009,21(5):375-385

One of the main questions on Neoproterozoic geology regards the extent and dynamics of the glacial systems that are recorded in all continents. We present evidence for short transport distances and localized sediment sources for the Bebedouro Formation, which records Neoproterozoic glaciomarine sedimentation in the central-eastern São Francisco Craton (SFC), Brazil. New data are presented on clast composition, based on point counting in thin section and SHRIMP dating of pebbles and detrital zircon. Cluster analysis of clast compositional data revealed a pronounced spatial variability of clast composition on diamictite indicating the presence of individual glaciers or ice streams feeding the basin. Detrital zircon ages reveal distinct populations of Archean and Palaeoproterozoic age. The youngest detrital zircon dated at 874 ± 9 Ma constrains the maximum depositional age of these diamictites. We interpret the provenance of the glacial diamictites to be restricted to sources inside the SFC, suggesting deposition in an environment similar to ice streams from modern, high latitude glaciers.  相似文献   

Optically stimulated luminescence (OSL) dating of quartzite cobbles from the Tapada do Montinho archaeological site (east‐central Portugal)     

Reza Sohbati  Andrew S. Murray  Jan‐Pieter Buylaert  Nelson A. C. Almeida  Pedro P. Cunha 《Boreas: An International Journal of Quaternary Research》2012,41(3):452-462

The burial age of an alluvially deposited cobble pavement at the Tapada do Montinho archaeological site (east‐central Portugal) is investigated using optically stimulated luminescence (OSL) dating. Measurements on the cobbles (quartzite clasts) were carried out on intact slices and large aliquots (～8 mm) of quartz grains (63–300 μm), both recovered from the outer 1.5‐mm surface of the cobbles. The recycling ratio, recuperation and dose‐recovery tests show that the single‐aliquot regenerative‐dose (SAR) protocol is applicable to both rock slices and quartz grains; both have similar luminescence characteristics. The variation in the natural OSL signal with depth below the cobble surface using intact slices from two different cobbles shows that both were bleached to a depth of at least ～2 mm before deposition. A model of the variation of dose with depth fitted to data from one of the cobbles gives a burial age of ～19 ka and also predicts the dose‐depth variation at the time of deposition. Ages based on rock slices suggest that one cobble surface, and the inner parts of two other cobbles experienced a resetting event at ～45 ka, consistent with the age control. However, the surfaces of the other cobbles all record light‐exposure events in the range 26 to 14 ka, suggesting that some of the cobbles were exposed to daylight perhaps more than once in this period. Given the shallow burial depth and unexpectedly young ages of the surrounding and overlying finer‐grained sediment, it is suggested that phases of light exposure following surficial erosion are probably responsible for this underestimate. Nevertheless, it is remarkable that we can identify and quantify four events (two light exposures of different durations and two sequential burial periods) in the dose record contained within a single clast, and this suggests that the luminescence dating of rock surfaces may prove, in the future, to be at least as important as sand/silt sediment dating.  相似文献   

Characterization and origin of spongillite-hosting sediment from João Pinheiro,Minas Gerais,Brazil     

A.C.S. Almeida  A.F.D.C. Varajão  N.S. Gomes  C.A.C. Varajão  C. Volkmer-Ribeiro 《Journal of South American Earth Sciences》2010,29(2):439-453

Spongillite from João Pinheiro, Minas Gerais, Brazil is mainly known for its use in brick production and in the refractory industry. Very few studies have focused on its geological context. Spongillite-rich deposits occur in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic São Francisco Supergroup. Cenozoic siliciclastic sediments are related to this surface. A field study of these deposits and analysis of multispectral images showed a SE–NW preferential drainage system at SE, suggesting that Mesozoic Areado Group sandstones were the source area of the spongillite-hosting sediments. Mineralogical and textural characterization by optical microscopic analysis, X-ray diffraction (XRD), differential and gravimetric thermal analysis (DTA-GTA), infrared spectroscopy (IR) and scanning electron microscopy (SEM) of seven open-pit spongillite-rich deposits (Avião, Carvoeiro, Vânio, Preguiça, Divisa, Severino, Feijão) showed a sedimentological similarity between the deposits. They are lens-shaped and are characterized at the bottom by sand facies, in the middle by spicules-rich muddy-sand facies and at the top by organic matter-rich muddy-sand facies.Petrographically, the spongillite-hosting sediments and the siliclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of well-sorted quartz grains and surface features of abrasion typical of aeolian reworking that occurred in the depositional environment in which the sandstones of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as kaolinite, low amounts of illite, scarce chlorite and mixed-layer chlorite/smectite and illite/smectite occur in the spongillite-hosting sediments and in sandstones from the Areado Group. In both formations, staurolite has similar chemical composition. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite.  相似文献   

Initial results in the development of a reporter cell line for toxicology studies at gene expression level: activation of the electrophile-responsive element by copper and methyl parathion     

Almeida DV  Trindade GS  Geracitano LA  Barros DM  Monserrat JM  Marins LF 《Marine environmental research》2008,66(1):158-160

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Facies and architectural variability of sub-seismic slope-channel fills in prograding clinoforms,Mid-Jurassic Neuquén Basin,Argentina     

Yuqian Gan  Ronald J. Steel  Cornel Olariu  Flávio De Almeida Jr 《Basin Research》2020,32(2):348-362

Most slope-channel outcrop studies have been conducted at continental margin-scale on seismic data. However, in foreland and back-arc deepwater settings, sub-seismic scale slope channels hold equally important information on deepwater sediment delivery, often in hydrocarbon-bearing provinces. One such slope-channel system is examined in Lower Jurassic prograding shelf-margin clinoforms in Bey Malec Estancia, La Jardinera area, southern Neuquén Basin, Argentina. In a 4 km wide, 300 m tall, slightly oblique- to depositional-dip section of Jurassic Los Molles Formation deepwater slope deposits, seven clinoform timelines were identified by isolated slope-channel fills with thicknesses less than 50 m. Sedimentary logs, satellite images, a digital elevation model and drone photogrammetry were used to map variations in downslope channel geometry and infill facies. The slope channels are filled with sediment density flow deposits: poorly sorted conglomeratic debrites, structureless sandy high-density turbidites and well-sorted, fine-grained, graded low-density turbidites. The debrite portion decreases downslope, whereas high- and low-density turbidites increase. A grain-size analysis reveals a broad downslope fining trend of turbidite and debrite beds within slope channels with increasing water depth, and some notable bypass of conglomeratic facies to the lowermost slope channels and basin floor fans. The architecture of the slope channels changes from lateral to aggradational infill downstream. The Bey Malec clinoforms and its slope channels add new knowledge on downslope changes for sediment delivery in relatively shallow (<500 m water depth), prograding-dominant deepwater basins. They also highlight one of very few outcropping examples of oblique-type clinoforms.  相似文献   

Effects of pre-orogenic tectonic structures on the Cenozoic evolution of Andean deformed belts: Evidence from the Salar de Punta Negra Basin in the Central Andes of Northern Chile     

Fernando Martínez  Cristopher López  Mauricio Parra 《Basin Research》2020,32(6):1441-1462

We integrated new field observations, two-dimensional (2-D) seismic profiles and new and previously reported chronological data to understand the effects of pre-orogenic structures on the tectonic evolution of the Salar de Punta Negra in the Central Andes. For first time a series of restored geological cross-sections are presented, thus showing the pre-orogenic tectonic architecture of the region and new ideas about the tectonic evolution of the inner forearc of the Central Andes. Our results show a series of east-dipping normal faults as the main pre-orogenic structures in the region, which resulted from lithospheric stretching of the western continental margin during the Paleozoic to Mesozoic (Triassic–Jurassic). These were later incorporated into the Andean orogen by tectonic inversion, forming west-verging inversion anticlines. The beginning of the tectonic inversion is constrained by the first on-lap of the Upper Cretaceous-Palaeocene syn-kinematic deposits on the top of the Mesozoic syn-rift successions, highlighting that inversion occurred during this period. These syn-kinematic deposits display zircons with older age peaks between ca. 200 and 300 Ma, thus indicating that some Carboniferous to Triassic sources of sediments were eroded during the uplift of the orogen. Other basement reverse faults affect the footwalls of normal inverted faults and the shoulders of ancient half-graben structures. These truncate and decapitate previous inverted faults and completely cut the infill of the basin, leading to exhumation of the pre-rift basement rocks. We propose that the propagation of these structures was favoured by the modified thermal-tectonic state of the lithosphere from the eastward migration of the volcanic arc, and not by the previous pre-orogenic structures. The structural and stratigraphic relationships recognized both in the field and 2-D seismic profiles indicate that many reverse faults originated after the initial tectonic inversion and continued to be active from the Eocene until the Pleistocene period.  相似文献   

Entropy theory for analysing water resources in northeastern region of Brazil   总被引：1，自引：1，他引：0  

Vicente de P. Rodrigues da Silva  Adelgicio F. Belo Filho  Vijay P. Singh  Rafaela S. Rodrigues Almeida  Bernardo B. da Silva  Inajá F. de Sousa 《水文科学杂志》2017,62(7):1029-1038

Using the Shannon entropy, the space–time variability of rainfall and streamflow was assessed for daily rainfall and streamflow data for a 10-year period from 189 stations in the northeastern region of Brazil. Mean values of marginal entropy were computed for all observation stations and entropy maps were then constructed for delineating annual and seasonal characteristics of rainfall and streamflow. The Mann-Kendall test was used to evaluate the long-term trend in marginal entropy as well as relative entropy for two sample stations. The marginal entropy values of rainfall and streamflow were higher for locations and periods with the highest amounts of rainfall. The entropy values were higher where rainfall was higher. This was because the probability distributions of rainfall and the resulting streamflow were more uniform and less skewed. The Shannon entropy produced spatial patterns which led to a better understanding of rainfall and streamflow characteristics throughout the northeastern region of Brazil. The total relative entropy indicated that rainfall and streamflow carried the same information content at annual and rainy season time scales.  相似文献   

Evidence of shock‐induced vaporization of matrix to form porosity in Baszkówka,a porous L5 chondrite     

Agata M. Krzesi&#x;ska  Natasha V. Almeida 《Meteoritics & planetary science》2019,54(1):54-71

Baszkówka is an equilibrated, apparently low‐shock, unusually porous chondrite. Some earlier studies were undertaken to understand whether the porosity in Baszkówka, and similar porous chondrites, is a relic of a primordial feature or rather the effect of atypical reprocessing on the parent body. Neither of the studies reconstructed the accurate thermal and deformational evolution of chondrites, however, while it is known that shock‐induced compaction is the main means to affect chondritic porosity. Here we use a combination of 3‐D and 2‐D petrographic examination to understand how the evolution of pores correlates with thermal and shock history recorded in the Baszkówka chondrite. The grain framework silicates in Baszkówka contain healed shock fractures—a clear recorder of significant shock process and postshock annealing. Simultaneously, metal grains do not exhibit any preferred orientation or fabric, which would be expected to develop in response to the deformation as recorded by silicates. We interpret this as evidence for re‐agglomeration and annealing of shocked material. Pore spaces in Baszkówka are connected and decorated by fine‐grained plagioclase‐dominated mass and bulky euhedral olivine crystals, which exhibit growth steps on crystal surfaces. The euhedral olivine must have formed owing to the condensation of a vapor, while plagioclase most likely crystallized from melted chondritic matrix. During the shock event, fine‐grained matrix in Baszkówka was melted and vaporized. Vapor expansion added to ballistic velocity led to ejection and opening of the pore spaces. After re‐agglomeration in a hot ejecta blanket the rock was annealed, melted material circulated in created pore spaces and vapor condensed.  相似文献   

Widely linear denoising of multicomponent seismic data     

Breno Bahia  Mauricio D. Sacchi 《Geophysical Prospecting》2020,68(2):431-445

Seismic data processing is a challenging task, especially when dealing with vector-valued datasets. These data are characterized by correlated components, where different levels of uncorrelated random noise corrupt each one of the components. Mitigating such noise while preserving the signal of interest is a primary goal in the seismic-processing workflow. The frequency-space deconvolution is a well-known linear prediction technique, which is commonly used for random noise suppression. This paper represents vector-field seismic data through quaternion arrays and shows how to mitigate random noise by proposing the extension of the frequency-space deconvolution to its hypercomplex version, the quaternion frequency-space deconvolution. It also shows how a widely linear prediction model exploits the correlation between data components of improper signals. The widely linear scheme, named widely-linear quaternion frequency-space deconvolution, produces longer prediction filters, which have enhanced signal preservation capabilities shown through synthetic and field vector-valued data examples.  相似文献   

Winchcombe: An example of rapid terrestrial alteration of a CM chondrite     

Laura E. Jenkins  Martin R. Lee  Luke Daly  Ashley J. King  Cameron J. Floyd  Pierre-Etienne Martin  Natasha V. Almeida  Matthew J. Genge 《Meteoritics & planetary science》2024,59(5):988-1005

Winchcombe is a CM chondrite that fell in England on February 28, 2021. Its rapid retrieval was well characterized. Within two polished sections of Winchcombe, terrestrial phases were observed. Calcite and calcium sulfates were found in a sample recovered from a field on March 6, 2021, and halite was observed on a sample months after its recovery from a driveway on March 2, 2021. These terrestrial phases were characterized by scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Calcite veins crosscut the fusion crust and therefore postdate it. The calcite likely precipitated in the damp environment (sheep field) where the meteorite lay for six days prior to its retrieval. The sulfates occur on the edges of the sample and were identified as three minerals: gypsum, bassanite, and anhydrite. Given that the sulfates occur only on the sample's edges, including on top of the fusion crust, they formed after Winchcombe fell. Sulfate precipitation is attributed to the damp fall environment, likely resulted from sulfide-derived H₂S reacting with calcite within the meteorite. Halite occurs as euhedral crystals only on the surface of a polished section and exclusively in areas relatively enriched in sodium. It was likely produced by the interaction of the polished rock slice with the humid laboratory air over a period of months. The sulfates, fusion crust calcite, and halite all post-date Winchcombe's entry into the Earth's atmosphere and showcase how rapidly meteorite falls can be terrestrially altered.  相似文献