Две статьи по вулканической стратиграфии


Автор сообщения: gorm
Дата и время сообщения: 29 December 2008 at 12:45:20:

В ответ на сообщение: Re: Фальшивый Геркуланум2

В Journal of Volcanology and Geothermal Research недавно опубликованы две, судя по всему, интересные для нас статьи, которые выложил у себя. Из обоих статей видно, сколь незначительно было извержение 1631 года по сравнению с извержением 79 года, насколько они отличаются по объему и составу отложений и насколько легко они идентифицируются. Кроме того, в обоих статьях приводятся корреляции радиоуглеродных дат и стратиграфических слоев.

Разыскания elcano в области истории раскопок иногда занятны, но они, конечно, не имеют ничего общего с хронологией.


Age and whole rock–glass compositions of proximal pyroclastics from the major explosive eruptions of Somma-Vesuvius: A review as a tool for distal tephrostratigraphy, p.1-18


A review of compositional data of the major explosive eruptions of Vesuvius is presented, comparing compositions (major elements) of whole rock with glass shards from the proximal deposits, hopefully useful for long-distance correlation. A critical review of published and new geochronological data is also provided. All available 14C ages are calibrated to give calendar ages useful for the reconstruction of the volcanological evolution of the volcanic complex. The pyroclastic deposits of the four major Plinian eruptions (22,000 yr cal BP “Pomici di Base”, 8900 yr cal BP “Mercato Pumice”, 4300 yr cal BP “Avellino Pumice”, and A.D. 79 “Pompeii Pumice”) are widely dispersed and allow a four-folded, Plinian to Plinian, stratigraphic division: 1. B–M (between Pomici di Base and Mercato); 2. M–A (between Mercato and Avellino); 3. A–P (between Avellino and Pompeii); 4. P–XX (from the Pompeii Pumice to the last erupted products of the XXth century). Within each interval, the age, lithologic and compositional features of pyroclastic deposits of major eruptions, potentially useful for tephrostratigraphic purposes on distal areas, are briefly discussed. The Vesuvius rocks are mostly high Potassic products, widely variable in terms of their silica saturation. They form three groups, different for both composition and age: 1. slightly undersaturated, older than Mercato eruption; 2. mildly undersaturated, from Mercato to Pompeii eruptions; 3. highly undersaturated, younger than Pompeii eruption. For whole rock analyses, the peculiar variations in contents of some major and trace elements as well as different trends in element/element ratios, allow a clear, unequivocal, easy diagnosis of the group they belong. Glass analyses show large compositional overlap between different groups, but selected element vs. element plots are distinctive for the three groups. The comparative analysis of glass and whole rock major element compositions provides reliable geochemical criteria helping in the recognition, frequently not obvious, of distal products from the different single eruptions.
Article Outline

1. Introduction
2. Stratigraphy and chronology

2.1. Explosive activity older than ca 22 ka (of uncertain source)
2.2. Explosive activity from 22 ka to 1631

2.2.1. The Pomici di Base Plinian eruption and the B–M interval
2.2.2. The Mercato Plinian eruption and the M–A interval
2.2.3. The Avellino Plinian eruption and the A–P interplinian interval
2.2.4. The Pompeii Plinian eruption and the P–XX interval

3. Whole rock and glass compositions

3.1. Strongly silica—undersaturated magmas

3.1.1. A.D. 472 “Pollena”
3.1.2. A.D. 512
3.1.3. A.D. 1631

3.2. Mildly silica—undersaturated magmas

3.2.1. A.D. 79 “Pompeii”
3.2.2. AP eruptions
3.2.3. 4300 cal yr BP “Avellino”
3.2.4. 8900 cal yr BP “Mercato”

3.3. Slightly silica—undersaturated (to saturated) magmas

3.3.1. 19,000 cal yr BP “Greenish Pumice”
3.3.2. 22,000 cal yr BP “Pomici di Base”
3.3.3. The older trachytic eruptions

4. Summary and conclusions
Acknowledgements
Appendix A. Analytical methods
Appendix B. Supplementary data
References


The proximal marine record of Somma–Vesuvius volcanic activity in the Naples and Salerno bays, Eastern Tyrrhenian Sea, during the last 3 kyrs
Pages
p.170-186


Abstract
The tephrostratigraphic analysis of nine gravity cores acquired on the continental shelf of the Naples and Salerno bays documents the proximal record of Somma–Vesuvius volcanic activity during the latest Holocene (last 3 kyrs). Five tephra layers from southern Naples Bay and three tephra layers from northern Salerno Bay were recognised in cores and their sedimentologic structures, textural parameters, stratigraphy and major and trace-element composition described.

Proximal products of the well known 79 A.D. eruption along with those of the interplinian activity at 2.7 ka B.P., were identified both in the southern Naples Bay and in the northern Salerno Bay. They represent excellent marker horizons interbedded within the Late Holocene marine record. In addition, several radiocarbon datings provided age constraints on a number of tephras overlaying the 79 A.D. deposits and allowed a detailed stratigraphic reconstruction of medieval (1137 A.D.–685 A.D.) and more recent eruptions as recorded for the first time at sea.

These results integrate previous research carried out on subaerial deposits providing new insights on the dispersion, deposition and grain-size distributions, along the northern Campania coastal zone, of recent Somma–Vesuvius products, some of which are still poorly known. The tephrostratigraphic approach to the study of marine sequences of proximal and adjacent areas can thus provide a more detailed description of physical characteristics of pyroclastic deposits, necessary for the correct assessment of volcanic hazard in the area.
Article Outline

1. Introduction
2. Geological outline

2.1. Physiography of the Naples Bay
2.2. Physiography of the Salerno Bay
2.3. The Somma–Vesuvius activity in the last 3 kyrs

3. Materials and methods

3.1. Sedimentological analyses
3.2. Chemical composition
3.3. 14C dating

4. Results

4.1. Stratigraphy of cores
4.2. Sedimentology and chemical classification of the studied tephra

4.2.1. Tephra layers of the Naples Bay
4.2.2. Tephra layers of the Salerno Bay

4.3. 14C ages

5. Discussion

5.1. Correlation of the marine tephra in Naples and Salerno bays with the Somma–Vesuvius events of the last 3 kyrs

5.1.1. Medieval and younger deposits
5.1.2. The 79 A.D. tephra
5.1.3. The AP interplinian activity

6. Conclusion
Acknowledgements
References


2877. Фальшивый Геркуланум. - elcano 21:36 25.12.08 (40)
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