1,720,990 research outputs found
Facies analysis of a Pliocene river-dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels
Full text linkThe recent recognition and description of terminal distributary channels in ancient delta-front deposits have led to important progress when it comes to understanding: delta construction processes; the plan-view shapes of both modern and ancient river-dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river-related processes. The studied deltaic succession is composed of vertically stacked parasequence-like units, which are invariably characterized by the lack of coarse-grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep-water turbidites. This study provides new elements for the correct distinction between hyperpycnal-dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement
Shoal-water deltas in high-accommodation settings: Insights from the lacustrine Valimi Formation (Gulf of Corinth, Greece)
Full text linkThe stratigraphic architecture of shoal-water deltaic systems developed in low-accommodation settings is relatively well-known. In contrast, the features of shoal-water deltas developed in high-accommodation settings remain relatively poorly documented, especially when compared with the available data sets for Gilbert-type deltaic systems developed in the same settings. The lacustrine Valimi Formation (Gulf of Corinth, Greece) provides an opportunity to investigate the facies assemblage and architectural style of shoal-water deltaic systems developed in high-accommodation settings. The studied interval accumulated during the Pliocene and Pleistocene and represents part of the early syn-rift Gulf of Corinth succession. Six facies associations, each described in terms of depositional processes and geometries, have been identified and interpreted to represent a range of proximal to distal deltaic sub-environments: delta plain, distributary channel, mouth-bar, delta front, prodelta and open lake. The facies associations and their architectural elements reveal characteristics which are not common in traditionally described shoal-water deltas. Of note, different facies arrangements are observed in the distributary channels in different sectors of the delta, passing from thick single-storey channel fills embedded within delta-plain fines in landward positions, to thin, amalgamated and multi-storey channels closer to the river mouth. This study proposes a new depositional model for shoal-water deltas in high-accommodation settings documenting, for the first time, that shoal-water delta deposits can form a substantial part of stratigraphic successions that accumulate in these settings. The proposed depositional model provides new criteria for the recognition and interpretation of these deposits; the results of this study have applied significance for reservoir characterization
The role of sediment supply in large-scale stratigraphic architecture of ancient Gilbert-type deltas (Pliocene Siena-Radicofani Basin, Italy)
Full text linkAggradation, progradation and retrogradation are the main patterns that define the large-scale architecture of Gilbert-type deltas. These patterns are governed by the ratio between the variation in accommodation space and sediment supply experienced during delta growth. Sediment supply variations are difficult to estimate in ancient settings; hence, it is rarely possible to assess its significance in the large-scale stratigraphic architecture of Gilbert-type deltas. This paper presents a stratigraphic analysis of a Pliocene deltaic complex composed of two coeval and narrowly spaced deltaic branches. The two branches recorded the same tectonic- and climate induced accommodation space variations. As a result, this deltaic complex represents a natural laboratory for testing the effects of sediment supply variations on the stratigraphic architecture of Gilbert-type deltas. The field data suggest that a sediment supply which is able to counteract the accommodation generated over time promotes the aggradational/progradational attitude of Gilbert-type deltas, as well as the development of thick foreset deposits. By contrast, if the sediment supply is not sufficient for counterbalancing the generated accommodation, an aggradational/retrogradational stratigraphic architecture is promoted. In this case, the deltaic system is forced to withdraw during the different phases of generation of accommodation, with the subsequent flooding of previously deposited sub-horizontal topset deposits (i.e., the delta plain). The subsequent deltaic progradation occurs above these deposits and, consequently, the available space for foresets growth is limited to the water depth between the base-level and the older delta plain.This leads to the vertical stacking of relatively thin deltaic deposits with an overall aggradatational/retrogradational attitude
A new Janulus species (Gastropoda Pulmonata Gastrodontidae) from the Zanclean (early Pliocene) of Tuscany (central Italy)
No full textJanulus spadinii n. sp. (Gastropoda, Pulmonata, Gastrodontidae) is described herein from the Zanclean (early Pliocene) of Baize di Caspreno (Tuscany). It belongs to a Macaronesian genus occasionally reported from the Tertiary of Europe. The new species is very similar to the Recent Madeiran Janulus stephanophorus by virtue of its shape, dorsal sculpture and radial rows of palatal teeth. It is also apparently similar to some fossil European species, however, due to insufficient information regarding the latter, no well supported distinction may be proposed at present. A brief survey of all the nominal taxa of the species-group assigned to Janulus is also provided
Magnetic susceptibility analysis of the archeological clastic succession at Grotta dei Santi (Grosseto, Italy): a palaeoclimatic and palaeoenvironmental perspective
No full textThis work presents the preliminary results of a high-resolution magnetic
susceptibility analysis performed in the clastic sequence exposed at Grotta dei Santi, one important Mousterian site in Italy. Magnetic susceptibility (χ) measures how much material is magnetised in an applied magnetic field. In clastic sequences in caves, magnetic susceptibility has been considered a good palaeoclimate proxy because χ fluctuations are also influenced by pedogenesis, which often is caves’ primary source of sediments.
The results have been combined with other independent proxies, such as faunal remains and radiocarbon dates. The data highlight that the lower portion of the succession is characterised by χmass
fluctuations that fit well with supposed climate changes recorded during sediment accumulation. Contrariwise, the upper part of the succession is characterised by extremely high χmass values due to diffuse
contamination by coarse-grained magnetite, which obliterates the palaeoclimate signal. Such granules are incompatible with infiltration processes, and regional considerations allow us to hypothesize their origin as connected with the erosion and re-deposition within the cave of nearshore coastal deposits
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
- …
