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Stratal patterns and sedimentary facies in tidal point bars
Get PDFTidal channels play a key role in the evolution of coastal environments and commonly dominate tidal landscapes. These channels commonly show a clear meandering pattern which shapes the architectural geometries of sedimentary successions accumulated in tidal coastal realms. Nevertheless, a limited number of studies analysed the morphodynamic evolution of tidal meanders and the related point bar sedimentary bodies, whose internal architecture and sedimentary facies distribution remain still poorly explored, as highlighted by their scarce documentation in the rock record. The capability of reading the signature of tidal processes in sedimentary successions represents a powerful tool for paleo-environmental reconstructions. Detecting tidal channel deposits will contribute to studies on Holocene deposits and ancient rocks, with implications spanning from adaptation of natural systems to anthropic pressure to subsurface exploration for georesources. The present work aims at improving our understanding of the relationship between the evolution of tidal meander bends and the related sedimentary products, through the investigation of different research issues (RI), including: i) the influence of vertical aggradation and substrate compaction in shaping geometries of tidal point bars (RI1); ii) the reconstruction of three-dimensional architecture of tidal point bars, through a numerical modelling approach, developed under different aggradational conditions and planform transformation styles (RI2); iii) the relationship between mechanisms of bar growth and internal facies arrangements (RI3); and iv) the analysis of piracy-controlled geometries of point bar bodies (RI4). These research issues have been carried out in different tidal settings, including microtidal (Venice Lagoon, Italy – RI1, 2, and 4) and macrotidal (Bay of Mont Saint Michel, France – RI3) environments; both in modern (RI1, 2, 3, and 4) and ancient record (Eocene deposits of the Tremp-Graus Basin, Spain – RI4). A multidisciplinary approach has been adopted for modern examples, with different methodologies encompassing remote sensing techniques (i.e. historical aerial photos and LiDAR topographic time-series analysis), geomorphological field observations, sedimentary core analysis, and 3D forward stratigraphic modelling. The analysis of the ancient case study examples relies on classical outcrop sedimentology approach, which included line-drawing of photomosaics, bed-by-bed logging, and collection of paleo-flow measurements.
The main results from this work highlighted that:
I) geometries of tidal point bars can vary following aggradational conditions of surrounding overbank areas, along with changes in local accommodation space and channel discharge. These are, in turn, influenced by differential substrate compaction and channel network evolution;
II) 3D geometries of point bars developed by slowly migrating tidal channels under aggradational conditions, differ from tabular bodies envisaged by traditional point bar models. Indeed, their geometries are shaped by the planform and vertical shift of channel bends;
III) tidal meanders can expand alternating accretionary stages along seaward and landward side of point bars. In this contest, large amounts of mud-rich deposits are stored in their axial zone, where rhythmic tidal deposition is better recorded;
IV) occurrence of densely-drained tidal networks prevents channel bends to freely meander and causes interaction with adjacent channels triggering piracies. These piracies cause premature deactivation of sinuous channels and hinder the development of laterally extensive point bar bodies
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)
Get PDFAlthough in tidal point bars, the point-bar brink (i.e. the break-in-slope between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) are thought to shift horizontally toward the outer bank, the occurrence of stable to slow-migrating channels in high-aggradational settings, e.g. salt marshes, is likely to promote a mixed, latero-vertical shift of meandering channel systems. The present study focuses on the trajectories of point-bar brink and channel thalweg of eight point bars located in the salt marshes of the Venice Lagoon, in order to understand how vertical aggradation can interact with lateral migration to shape their stratal geometries. High resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis, allowed us to define specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All the analyzed brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area
Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks
No full textEstablishing critical comparisons between fluvial- and tidal-channel morphodynamics is a major goal in the study of coastal landscapes. Freely migrating meandering rivers are known to produce laterally extensive, sand-prone point-bar bodies which commonly exhibit width:thickness ratios up to 250. Meandering channels are widespread in tidal channel networks draining intertidal plains, where they exhibit planform dynamics similar to their fluvial analogues. However, tidal networks are characterized by a high channel density that likely hinders tidal meanders from migrating laterally for long distances without interacting with other channels. In order to better understand how the interaction between adjacent meandering channels controls the development of tide-dominated point bars, two point-bar bodies from the Eocene Castigaleu Formation (Spain) are investigated and compared with the geometry of a modern tidal point bar of the northern Venice Lagoon (Italy). The Eocene bars are characterized by a low width:thickness ratio (<30), interpreted as the result of premature deactivation of their parent-channel migration. Such deactivations were likely related to avulsive piracy operated by adjoining tide-dominated channels, and likely prevented the parent channel bend from depositing a laterally extensive, tabular sand body along its inner bank. This analysis is corroborated by direct time-lapse observations of similar dynamics in modern tidal networks, and from morphodynamic and sedimentological evidence from recent deposits of the Venice Lagoon. We conclude that, in densely drained tidal networks, meander bends often interact with adjacent channels, thereby triggering piracy events and premature channel abandonments that generate point-bar architectures distinct from those produced in fluvial meandering rivers
Disentangling hydrodynamic drivers of the Southern Venice (Italy) coastal aquifer via frequency decomposition analysis: Insights, challenges, and limitations
No full textStudy region: Shallow coastal aquifer located in the southern part of the Venice lagoon (Italy). Study focus: This study aims to improve the understanding of coastal aquifers’ hydrodynamics by implementing systematic time-series analyses of data. A collection of non-intrinsically consistent time series from hydrological (surface water and groundwater) and meteo-mareographic monitoring networks was obtained from different institutions. Each signal was broken down through a frequency decomposition analysis, isolating the main driving forces to focus on phenomena that occur at different time and spatial scales. New hydrological insights for the region: Results highlighted that the aquifer is highly connected with the Venice lagoon, with a clear fluctuation of piezometric heads induced by tidal major constituents, decreasing landward. Besides, the effects exerted by reclamation canals and pumping stations were also determined and found to increase landward. Despite the relatively simple behaviour of piezometric heads, the groundwater salinity is influenced by additional local factors, like probe depth, wells’ screen length, and vertical salinity distribution along the aquifer. These findings suggested how to make use of limited and sparse data to enhance the conceptual model of coastal aquifer hydrodynamics, while highlighting the limitations of existing monitoring networks. This outcome justified the need for an intrinsically-consistent network of dedicated multi-level samplers to avoid intra-borehole mixing and reliably characterize the groundwater salinity distributio
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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