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Reconstructing fluvial bar surfaces from compound cross‐strata and the interpretation of bar accretion direction in large river deposits 
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下载免费PDF全文 Renato P. Almeida Bernardo T. Freitas Bruno B. Turra Felipe T. Figueiredo André Marconato Liliane Janikian 《Sedimentology》2016,63(3):609-628
The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces. 相似文献
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A high-resolution, well-dated dinoflagellate cyst record from a lagoon of the southeastern Swedish Baltic Sea reveals climate and hydrological changes during the Holocene. Marine dinoflagellate cysts occurred initially at about 8600 cal yr BP, indicating the onset of the Littorina transgression in the southeastern Swedish lowland associated with global sea level rise, and thus the opening of the Danish straits. Both the species diversity and the total accumulation rates of dinoflagellate cysts continued to increase by 7000 cal yr BP and then decreased progressively. This pattern reveals the first-order change in local sea level as a function of ice-volume-equivalent sea level rise versus isostatic land uplift. Superimposed upon this local sea level trend, well-defined fluctuations of the total accumulation rates of dinoflagellate cysts occurred on quasi-1000- and 500-yr frequency bands particularly between 7500 and 4000 cal yr BP, when the connection between the Baltic basin and the North Atlantic was broader. A close correlation of the total accumulation rates of dinoflagellate cysts with GISP2 ice core sea-salt ions suggests that fluctuations of Baltic surface conditions during the middle Holocene might have been regulated by quasi-periodic variations of the prevailing southwesterly winds, most likely through a system similar to the dipole oscillation of the modern North Atlantic atmosphere. 相似文献
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The ML 5.3 Épagny (French Alps) earthquake of 1996 July 15: a long-awaited event on the Vuache Fault
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The compositions of arsenopyrite and sphalerite from five Swedish metamorphosed Cu, Zn-sulfide deposits are related to T and P, according to the methods described by Kretschmar and Scott (1976) and Scott (1973). The As/S ratio of arsenopyrite indicates an equilibrium temperature around 400°C for all the deposits studied, whereas the sphalerite barometer shows pressures between 5 and 7 kb. The mineral assemblages of the bedrocks indicate a similar temperature but a lower pressure. A constant fs2 is probably only effective over distances of millimetres or a few centimetres in the samples studied. The fs2 has always been lower in the surrounding rocks than in the ores as indicated by a higher As/S in arsenopyrite, more FeS in sphalerites and the absence of pyrite. 相似文献
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Liliane Janikian Renato Paes De Almeida Ricardo Ivan Ferreira Da Trindade Antonio Romalino Santos Fragoso‐Cesar Manoel Souza D′Agrella‐Filho Elton Luis Dantas Eric Tohver 《地学学报》2008,20(4):259-266
The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft‐bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaquã Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar‐Ar and U‐Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic‐type glacial context during the last Neoproterozoic glacial event and presents the Picada das Graças Formation (580 ± 3.6 Ma) as the first dated non‐glacial unit coeval to the Gaskiers Formation. 相似文献
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Identifying local spatial association in flow data 总被引：5，自引：1，他引：4
In this paper we develop a spatial association statistic for flow data by generalizing the statistic of Getis-Ord, G
i (and G
i
*). This local measure of spatial association, G
ij, is associated with each origin-destination pair. We define spatial weight matrices with different metrics in flow space.
These spatial weight matrices focus on different aspects of local spatial association. We also define measures which control
for generation or attraction nonstationarity. The measures are implemented to examine the spatial association of residuals
from two different models. Using the permutation approach, significance bounds are computed for each statistic. In contrast
to the G
i statistic, the normal approximation is often appropriate, but the statistics are still correlated. Small sample properties
are also briefly discussed.
Received: 18 February 1998/Accepted: 29 September 1998 相似文献
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J. Berglund J. Mattila O. Rnnberg J. Heikkil E. Bonsdorff 《Estuarine, Coastal and Shelf Science》2003,56(5-6):1167-1175
Submerged rooted macrophytes and drift algae were studied in shallow (0–1 m) brackish soft-bottom bays in the Åland Islands, N Baltic Sea, in 1997–2000. The study was performed by aerial photography and ground-truth sampling and the compatibility of the methods was evaluated. The study provided quantitative results on seasonal and inter-annual variation in growth, distribution and biomass of submerged macrophytes and drift algae.On an average, 18 submerged macrophyte species occurred in the studied bays. The most common species, by weight and occurrence, were Chara aspera, Cladophora glomerata, Pilayella littoralis and Potamogeton pectinatus. Filamentous green algae constituted 45–70% of the biomass, charophytes 25–40% and vascular plants 3–18%. A seasonal pattern with a peak in biomass in July–August was found and the mean biomass was negatively correlated with exposure. There were statistically significant differences in coverage among years, and among levels of exposure. The coverage was highest when exposure was low. Both sheltered and exposed bays were influenced by drift algae (30 and 60% occurrence in July–August) and there was a positive correlation between exposure and occurrence of algal accumulations. At exposed sites, most of the algae had drifted in from other areas, while at sheltered ones they were mainly of local origin.Data obtained by aerial photography and ground-truth sampling showed a high concordance, but aerial photography gave a 9% higher estimate than the ground-truth samples. The results can be applied in planning of monitoring and management strategies for shallow soft-bottom areas under potential threat of drift algae. 相似文献
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