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Density- and viscosity-stratified gravity currents: Insight from laboratory experiments and implications for submarine flow deposits
No full textVertical stratification of particle concentration is a common if not ubiquitous feature of submarine particulate gravity flows. To investigate the control of stratification on current behaviour, analogue stratified flows were studied using laboratory experiments. Stratified density currents were generated by releasing two-layer glycerol solutions into a tank of water. Flows were sustained for periods of tens of seconds and their velocity and concentration measured. In a set of experiments the strength of the initial density and viscosity stratification was increased by progressively varying the lower-layer concentration, CL. Two types of current were observed indicating two regimes of behaviour. Currents with a faster-moving high-concentration basal region that outran the upper layer were produced if CL < 75%. Above this critical value of CL, currents were formed with a relatively slow, high-concentration base that lagged behind the flow front. The observed transition in behaviour is interpreted to indicate a change from inertia- to viscosity-dominated flow with increasing concentration. The reduction in lower-layer velocity at high concentrations is explained by enhanced drag at low Reynolds numbers. Results show that vertical stratification produces longitudinal stratification in the currents. Furthermore, different vertical and temporal velocity and concentration profiles characterise the observed flow types. Implications for the deposit character of particle-laden currents are discussed and illustrated using examples from ancient turbidite systems
Basin plain deposits of the Marnoso-Arenacea Formation, Italy
No full textThe Marnoso Arenacea turbidite system outcrops across a large area (125 × 35 km [78 × 22 mi]) of the northern Apennines in Italy (1). It records sedimentation within a Miocene foreland basin and has been an area of turbidite research for more than fifty years. Correlations of individual flow deposits (beds) within the Serravallian, ponded basin-plain sequence are some of the most extensive in any ancient turbidite system. Long-distance (>100 km [>62 mi]) bed correlation was first established by Ricci Lucchi and Valmori (1980) for a 200-m (656-ft)-thick stratigraphie interval, between 18 measured sections. Correlation is aided in this system by the presence of carbonate-rich marker beds which have a distinct provenance and composition. This contribution presents an enhanced correlation framework for a 30-m (98-ft)-thick sub-interval, positioned above the most prominent marker bed (the Contessa). This new correlation framework is based on more than 100 measured sections (Figures 1,2). The additional sections were identified using new 1:10,000 geological maps (produced by the Emilia-Romagna, Marche, Toscana, and Umbria Geological Surveys) that show the position of the Contessa marker bed
New insight into the evolution of large-volume turbidity currents: comparison of turbidite shape and previous modelling results
No full text.The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log-normal frequency distribution; a small number of flows dominated sediment supply to this non-channelized basin plain. Turbidite sandstone within small-volume (<0·7 km3) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger-volume (0·7–7 km3) beds have a broad thickness maximum in their proximal part. Grain-size trends within this broad thickness maximum indicate spatially near-uniform flow for distances of 30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea-floor gradient. Intra-bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large-volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea-floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments
No full textCo-genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud-poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud-poor sandstone, siltstone and mudstone show features indicating progressive layer-by-layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co-genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin-plain settings. Co-genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud-rich sea-floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 x 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run-out on low-gradient sea floor (< 0.1?) may simply be due to low yield strength (<<50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events
Abrupt transitions in gravity currents
No full textPyroclastic flows and snow avalanches sometimes exhibit a rapid deceleration of their dense flow fronts and detachment of their dilute clouds. This behavior is also inferred for submarine flows and could explain stepped thickness patterns in their deposits. A similar “abrupt transition” process occurs in particle-laden, lock release laboratory currents with relatively high concentrations. New experiments on nonparticulate, solute-driven density currents were run to investigate the cause of abrupt transitions. Abrupt transitions occur in laboratory currents with Reynolds numbers (Re) less than 1000 and are interpreted, supported by theoretical scaling analysis, to signify a change in dynamic regime. Currents with high Re, which do not show abrupt transitions, undergo a downstream change in dynamic regime from (1) inertial slumping to (2) inertial-buoyancy spreading to (3) viscous-buoyancy spreading. In low Re currents that undergo abrupt transitions, however, the duration of the second regime is very short, and hence they appear to pass directly from the quickly moving slumping phase into the slowly moving viscous phase. Scaling analysis indicates that an abrupt transition should occur in currents below a critical value of Re of ?10–5000 for currents with different initial aspect ratios. Given that natural flows typically have greater Reynolds numbers, we suggest that abrupt transitions in laboratory and natural currents are likely to be dynamically different. This work has important implications for the physical modeling of gravity flows
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The character and origin of thick base-of-slope sandstone units of the Peïra Cava outlier, SE France
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