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The Povoação Ignimbrite, Furnas Volcano,São Miguel, Azores

Article in Journal of Volcanology and Geothermal Research · September 1999

Impact Factor: 2.54 · DOI: 10.1016/S0377-0273(99)00067-0

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Angus M. Duncan

University of Liverpool

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University of the Azores

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Ž .Journal of Volcanology and Geothermal Research 92 1999 55–65

www.elsevier.comrlocater jvolgeores

The Povoaçao Ignimbrite, Furnas Volcano, Sao Miguel, Azores˜ ˜

A.M. Duncan a,), G. Queiroz b, J.E. Guest c, P.D. Cole a, N. Wallenstein b,J.M. Pacheco b

aCentre for Volcanic Studies, Faculty of Science, Technology and Design, UniÕersity of Luton, Luton LU1 3JU, UK

b Departimento de Geociencias, UniÕersidade dos Açores, Ponta Delgada, Portugal

cPlanetary Image Centre, UniÕersity College London, London NW7 4SD, UK

Accepted 20 April 1999

Abstract

Ž .The Povoaçao Ignimbrite Formation PIF was emplaced by one of the larger explosive trachytic eruptions of Furnas˜Volcano, Sao Miguel, Azores. Trachytic ignimbrites are common in the products of Furnas Volcano and examples of ˜welding occur in at least three ignimbrites of which the Povoaçao Ignimbrite is the most extensive. The PIF may correlate˜with the formation of the main caldera of Furnas. In the Povoaçao Ignimbrite, the welded horizons thicken, without evidence˜of rheomorphism, into palaeovalleys and can be seen to thin and in some places become completely attenuated over old

ridges. The welded horizons are intimately associated with non-welded ignimbrites and in some places there is an alternation

between welded and non-welded horizons. On interfluves, the ignimbrite is stratified and some of the welded horizons show

pinch and swell and occasional cross-bedding. The welding is interpreted as a primary depositional feature with the clasts

sintering on emplacement. It is argued that this ignimbrite was emplaced from a turbulent pulsatory pyroclastic flow. Some

pulses were hotter which enabled more extensive development of welding. The flows became more concentrated and denserdown valleys favouring the emplacement of thicker welded units. q1999 Elsevier Science B.V. All rights reserved.

Keywords: explosive volcanism; welded tuffs; ignimbrite emplacement; Furnas; Azores

1. Introduction

There are three active trachytic composite volca-

noes on the island of Sao Miguel in the Azores: Sete˜Ž . ŽCidades, Fogo Agua de Pau and Furnas Booth et

.al., 1978; Moore, 1990, 1991 . In the areas between

these central volcanoes, monogenetic basaltic erup-

tions have occurred typically forming cinder cones

and aa lava flows. The last such eruption took place

in 1652 AD on the rift zone between Fogo and Sete

Cidades.

)

Corresponding author. E-mail: [email protected]

Furnas Volcano, the easternmost of the composite

volcanoes, does not form a substantive construct like

Sete Cidades and Fogo but is characterised by a

large caldera complex situated on the outer flanks of

the older PovoaçaorNordeste volcanic construct. The˜caldera complex of Furnas is 8=5 km in diameter

and formed as the result of at least two distinctŽ .collapses Guest et al., 1999-this issue . Over the last

5000 years, volcanic activity of Furnas has beenŽcharacterised by sub-plinian eruptions Cole et al.,

.1995 , but prior to this there were a number of major

eruptions, some of which were ignimbrite-formingŽevents associated with caldera collapse Guest et al.,

.1999-this issue . There has, therefore, been a com-

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .P I I : S 0 3 7 7 - 0 2 7 3 9 9 0 0 0 6 7 - 0

https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/226226537_Volcanic_geology_and_eruption_frequency_So_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/296516806_Geology_of_three_late_Quaternary_stratovolcanoes_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/296516806_Geology_of_three_late_Quaternary_stratovolcanoes_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/226226537_Volcanic_geology_and_eruption_frequency_So_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==


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( ) A.M. Duncan et al.r Journal of Volcanology and Geothermal Research 92 1999 55– 6556

plex history of caldera formation at Furnas involving

a number of major eruptions. One of these major

trachytic eruptions of Furnas Volcano produced the

Povoaçao Ignimbrite, which is the topic of this study˜Ž .Schmincke and Weibel, 1972; Booth et al., 1978 .

The Povoaçao Ignimbrite is well exposed in the˜coastal cliffs south of Furnas and in the older

ŽPovoaçao caldera to the east of Furnas Moore,˜.1991 , and displays a variety of textural facies in-

cluding substantial development of a welded fabric.

Ignimbrites with welded facies have also been de-

scribed from other Azorean volcanoes such as FogoŽ .volcano Moore, 1991 and on the island of Terceira

Ž .Self, 1971, 1976 . It is the purpose of this paper to

provide a description of the Povoaçao Ignimbrite˜with particular emphasis on the welded facies and to

discuss the mechanisms involved in its eruption and

emplacement. Many of the sections along the south-

ern coast are cut perpendicular to palaeovalleys downwhich the pyroclastic flows, that gave rise to the

Povoaçao Ignimbrite, were channelled. This gives a˜good opportunity to investigate the relationships be-

tween emplacement and topography.Ž .Moore 1990, 1991 identified an ignimbrite with

a distinctive welded facies on Furnas, which he

equates with the Povoaçao Ignimbrite, as the˜caldera-outflow ignimbrite and associates it with the

formation of the caldera. On the basis of a radiocar-

bon age of 11,230"100 BP for a soil directly

separating a welded ignimbrite exposed near the topof the caldera-fill within the main Furnas caldera and

the overlying Pico de Ferro trachyte domes, MooreŽ .1991 argues that the caldera-outflow ignimbrite has

an age of around 12,000 years. However, radiocar-

bon dating of soils immediately underlying the

Povoaçao Ignimbrite from the southern coastal expo-˜sures and from Povoaçao provide dates of between˜

Ž .30,000 and 35,000 BP Guest et al., 1999 and this

suggests an age of around 30,000 BP for the

Povoaçao Ignimbrite. The Povoaçao Ignimbrite is,˜ ˜therefore, clearly older than the welded ignimbrite

that occurs near the top of the Furnas caldera-fill.

Indeed, detailed field investigations have revealed at

least three ignimbrites displaying welded facies asso-Ž .ciated with Furnas Volcano Guest et al., 1999 and

we consider, therefore, that there is not a single

caldera-outflow ignimbrite as proposed by MooreŽ .1990, 1991 .

2. The Povoaçao Ignimbrite Formation˜

Ž .The Povoaçao Ignimbrite Formation PIF is made˜up of a number of different lithologies including

lapilli-fall beds, thick surge deposits, massive non-

welded ignimbrite units as well as distinctive densely

welded ignimbrite horizons. The ignimbrite can bereadily traced from the town of Povoaçao to just˜Ž .west of Ribeira Quente Fig. 1 . Erosion of deposits

and incision of steep valleys has made it difficult to

directly correlate the deposits of the Povoaçao Ign-˜imbrite in the Ribeira Quente area with the ign-

Žimbrite which is considered by Guest et al. 1999-this.issue to be the Povoaçao Ignimbrite in the Amoras˜Ž .area Fig. 1 . However, the fact that this ignimbrite

in the Amoras area is overlain by the Tufo Member,

which is dated at 27,000 BP, placing it in the correct

age range and the similarity in facies strongly sup-

ports a correlation with the Povoaçao Ignimbrite.˜Only proximal exposures of the PIF are preserved;

most of the material must have been deposited out to

sea. Therefore, it is not possible to provide an effec-Ž .tive estimate of the volume though Moore 1990

suggests that the subaerial volume is of the order 73 Ž 3 .km about 2 km dense rock equivalent .

Informative exposures of the PIF occur in three

distinct areas along the coast between Ponta GarçaŽ .and Povoaçao Fig. 1 which from west to east are:˜

Ž .the Amoras area Sites 81, 79, 216 , the Ribeira

Ž .Quente area Sites 61, 219, 167 and the Povoaçao˜Ž .caldera Sites 205, 62 . Two main facies can be

Ž . Ž .recognised: 1 a Valley Fill Facies and 2 a Valley

Margin Facies, and these are described below.

2.1. The Valley Fill Facies

This facies is developed in palaeovalleys where

the ignimbrite was emplaced in topographic depres-

sions. In these palaeovalleys, the deposits of the

Povoac¸

ao Ignimbrite thicken considerably; in the˜main valley north of Povoaçao there are more than 4˜

m of massive columnar jointed ignimbrite. On the

edge of the town of Povoaçao the relationships be-˜tween the Povoaçao Ignimbrite and the palaeotopog-˜raphy are clearly demonstrated and the welded units

can be observed to thicken markedly towards the

valley centre. In the axis of the valleys which con-

https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/286676581_The_Lajes_ignimbrite_Ilha_Teceira_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/286676581_The_Lajes_ignimbrite_Ilha_Teceira_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/249545193_The_recent_volcanology_of_Terceira_Azores_J_Geol_Soc_London?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/286676581_The_Lajes_ignimbrite_Ilha_Teceira_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/235412564_A_Quantitative_Study_of_Five_Thousand_Years_of_Volcanism_on_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/222315683_Volcanic_geology_of_Furnas_Volcano_Sao_Miguel_Azores?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/249545193_The_recent_volcanology_of_Terceira_Azores_J_Geol_Soc_London?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==


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( ) A.M. Duncan et al.r Journal of Volcanology and Geothermal Research 92 1999 55– 65 57

Fig. 1. Location map of Furnas volcanoes and principal sites of the Povoaçao Ignimbrite.˜

verge on Povoaçao village densely welded ign-˜imbrite is more than 20 m in thickness and in the

quarry in the Ribeira Quente valley welded ign-

imbrite occurs up to 40 m in thickness. At Site 62, in

the cliff section on the shore immediately west of the

village of Povoaçao, some 8 m of welded ignimbrite˜occupies the axis of a palaeovalley with the welded

ignimbrite resting directly on fluvial gravels. Layers

of rounded lithic clasts occur within the ignimbrite

and it is probable that these clasts were entrained by

the pyroclastic flow from these underlying gravels

during passage down the valley.Ž . ŽThe headland Site 79 to the east of Amoras Fig.

.1 is another palaeovalley which has been filled by

the PIF. At the base of the sequence there is more

than 10 m of massive lapilli beds, some of which are

lapilli-fall, massive non-welded pumice-rich ign-Ž .imbrites and thin -2 m pyroclastic surge sand-

Ž .wave beds see Site 79 in Fig. 2 . Some of these

non-welded pumiceous ignimbrite horizons show

many of the features of the ‘classic’ ignimbrite se-Ž .quence of Sparks et al. 1973 with fine grained

basal layers, layer 2a, and massive main body with

matrix supported pumice clasts, layer 2b. Overlying

this are two welded horizons; the lower is less than 1

m in thickness, whereas the upper horizon occurs up

to 8 m in thickness. These two horizons are interbed-

ded with massive non-welded ignimbrite. In the up-

per part of the sequence, in the axis of the palaeoval-

ley, there is up to 35 m of massive non-welded

https://www.researchgate.net/publication/238422045_Products_of_Ignimbrite_Eruptions?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/238422045_Products_of_Ignimbrite_Eruptions?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==https://www.researchgate.net/publication/238422045_Products_of_Ignimbrite_Eruptions?el=1_x_8&enrichId=rgreq-7cc1018c-07f7-41c6-bae8-f04c59956023&enrichSource=Y292ZXJQYWdlOzI0ODI1NjgwNTtBUzoyMjY3OTM3NDAwODMyMDBAMTQzMTA4MzI0OTMwOA==


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Fig. 2. Documented sections of the Povoaçao Ignimbrite exposed on the southern coast. Note the vertical variations in welding and clast size˜in Site 61. More than one welded horizon occurs in Site 79.

ignimbrite interbedded with cross-bedded surge de-

posits.

2.2. Valley Margin Facies

To the east of Ribeira Quente, the PIF is exposedŽon higher ground between palaeovalleys Sites 219

.and 167 in Fig. 3 . Here, the welded horizons are

Ž .much thinner -1 m and interbedded with non-

welded cross-bedded surge deposits. In sections onŽ .the slopes e.g., Site 205, Fig. 3 , the deposits are

markedly stratified, typically with several distinct

welded horizons developed. The welded horizons at

Site 205 are stratified and individual layers show

development of pinch and swell structures indicative

of emplacement from turbulent flow.


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Ž .Fig. 3. Documented sections of the Povoaçao Ignimbrite localities shown in Fig. 1 — key to ornamentation provided in Fig. 2.˜


7/12


The coastal path which runs along the cliff west-

wards from Ribeira Quente provides a good sectionŽ .through the PIF Site 61, Fig. 2 on the side of a

palaeovalley. The base of the unit consists of 4 m of

alternating thin ash and lapilli beds that mantle the

slopes of a depression which is infilled by 6 m of

thinly bedded lithic-rich deposits. These deposits are

overlain by up to 11 m of pumice-rich discontinuous

massive beds with lithic-rich lenses, with occasional

bomb sags and local cross-bedding which we inter-

pret as a surge deposit. This grades up into 4 m of

lithic and pumice-rich, fines-poor, crudely bedded

material which itself grades up into 1 m of massive

non-welded pumice rich ignimbrite. Overlying this

with a distinct but gradational boundary is ;5 m of

densely welded ignimbrite. This is followed by a

further 5.3 m of massive non-welded ignimbrite with

abundant matrix-supported dark grey juvenile clasts

up to 15 cm in size showing a well-developed pumiceconcentration zone within the upper part. Chemical

analysis shows that the fiamme in the intensely

welded zone and the dark pumice in the non-welded

horizon above are all of a similar trachyte composi-

tion.

2.3. Relationship between the Valley Fill Facies and

the Valley Margin Facies

In sections where it is possible to examine the

Ž .transition perpendicular to the valley axis from

deposits on the valley floor up onto the materials

which rest on the valley margins, it can be seen that

the welded ignimbrite horizons either thin or become

completely attenuated grading laterally into non-

welded ignimbrite. This is clearly shown at Site 62,

where following the ignimbrite up the valley side,

parallel to the palaeo-surface, the welded horizon

becomes progressively thinner until it occurs as a

few clasts which are weakly sintered; this then grades

into non-welded material with similar clasts showingŽ .the same preferred orientation of long axes Fig. 4 .

3. The nature of welding in the Povoaçao Ign-˜imbrite

The range of welding in the Povoaçao Ignimbrite˜Ž .is similar to that described by Smith 1960 in his

classic paper on welded tuffs. At the minimum endof the spectrum, welding involves glassy clasts being

weakly sintered together and this typically occurs

where the welded zone becomes attenuated up the

side of a palaeovalley. In the more intensely welded

zones, which occur in the axial regions of the palaeo-

valleys, the glassy clasts have been markedly de-

formed and pore space has been largely eliminated.

The welded zones show a fabric produced by flatten-

ing of the fiamme broadly parallel to the substrate. In

addition, the fabric is parallel to a crude stratification

which is commonly developed. This distinctive strat-

Fig. 4. Povoaçao Ignimbrite at Site 62 showing thickening of welded facies into the axis of the palaeovalley. The lower unit of the welded˜facies can be seen to completely pinch out to the left up the side of the old valley.


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ification within single cooling units is associatedŽ . Ž .with: 1 a variation in fiamme size and 2 a varia-

tion in the intensity of welding over intervals of

20–30 cm, both developed perpendicular to the de-Ž .positional surface Fig. 5 . These welded beds were

emplaced directly on slopes dipping at angles of up

to 348 yet show no evidence of rheomorphism.

In most localities, there is more than one welded

layer developed and there are abrupt vertical transi-Ž .tions from non-welded to welded ignimbrite Fig. 6 .

Two welded horizons occur at Amoras. In the axial

region of the valley, the lower welded unit is less

than a metre thick; this is overlain by 2 m of poorly

sorted pumice–lapilli tuff with crude cross-bedding

and this then grades relatively abruptly into the

second, thicker, welded horizon. The thicker, upper

welded unit shows marked vertical variation in the

intensity of welding and size of clasts. Near the base,

three more densely welded layers a few cm thick can

be identified; above this the clasts become larger and

more clearly visible. These clasts are clearly de-

formed to form fiamme developing a eutaxitic tex-

ture. In some places, some of the fiamme show crude

imbrication. This is well displayed in one of theŽ .coarser layers at Site 61 see Fig. 7 . The larger

fiamme are typically 20=2 cm in size but there are

layers of different clast size and there is a horizon

with fiamme up to 60=5.5 cm. The flattening ratio

of the fiamme in the more intensely welded layers

ranges between 1:5 and 1:10 as opposed to an aver-

age of 1:5 in the less densely welded zones. The

upper 3 m of the welded unit is much finer grained

with individual fiamme rarely visible in hand speci-

men. The rock does show a well-developed platey

fabric, however, and fiamme are clearly visible in

thin section.

Fig. 5. Stratification in the Povoaçao Ignimbrite defined by a variation in clast size in the welded horizon at Site 61.˜


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Ž .Fig. 6. Exposure of the PIF on southern coast of Furnas Volcano Site 216 showing vertical variation in lithofacies. Three separate distinctŽ . Ž . Ž .welded layers W separated by unwelded tuff, ranging from massive M to stratified S in nature. Scale bar resting on lowermost welded

layer has 10 cm graduations.

Small syenite lithics, normally 1–2 cm but up to 9

cm in size, are scattered throughout the welded units

but typically form small stringers parallel to theŽ .fabric Site 61, Fig. 2 . Where these stringers occur,


10/12


Fig. 7. Imbrication of clasts in the welded horizon of the Povoaç ao Ignimbrite at Site 61. The scale is in 1 and 10 cm divisions.˜

a distinctive parting due to a reduction in welding is

often present, presumably because welding was in-

hibited as a result of the cooling influence of the

lithic clasts.

4. Emplacement mechanisms for the ignimbrites

There has been much discussion in the literatureŽ .Mahood, 1984 as to whether stratified welded py-

roclastic deposits such as these have a fall or a flow

origin. The Thera Welded Tuff of Santorini, Greece,

which mantles the topography with uniform thick-Ž .ness is interpreted by Sparks and Wright 1979 as

Ž .being of fall origin. Wright 1980 , working on the

welded green tuff of Pantelleria, Italy, considered

that deposition on slopes in excess of 208 and strati-

fication of the deposits are evidence of a fall origin.Ž .Mahood 1984 , however, reconsidered the origin of

some of the welded tuffs on Pantelleria and argued

that mantling tuffs could have been emplaced byŽ .pyroclastic flows. Branney and Kokelaar 1992 ar-

gue that welded tuffs of pyroclastic flow origin can

show stratification. The welded facies of the

Povoaçao Ignimbrite show imbrication of fiamme˜Ž .Fig. 7 suggesting that they were deposited by a

flow. In addition, the lateral variations in thickness

with pinch and swell shown by some of the welded

layers is indicative of emplacement from a flow. If

the deposits had been produced by a fall-out mecha-


11/12


nism, the welded zones would not show thickening

towards the axis of a valley unless rheomorphism

had taken place. There is little evidence for rheomor-

phism even on slopes of more than 308. We therefore

consider the thickening of the welded zones in the

axial regions of valleys to be a primary depositional

feature relating to the flow origin of these deposits.

The welded zones of the PIF consist of flattened

clasts of juvenile material welded together. The crude

stratification with beds of differing grain size and

degree of welding, the orientation of fiamme broadly

parallel to the deposition surface, and the abrupt

transition from non-welded to welded layers lead us

to consider that the welding was primary and syn-de-

positional in character — that these are agglutinatesŽ .as defined by Branney and Kokelaar 1992 . It is our

judgement that the welded zones accumulated incre-

mentally and evidence for an aggradational mode of

Ž .emplacement is provided by: 1 the imbrication of fiamme, see discussion in the works of Branney and

Ž . Ž .Kokelaar 1994 and Wolff and Turbeville 1994 .

There are no deformation features which would sup-

port an explanation that these imbricate texturesŽ .formed as a result of rheomorphic flowage. 2 The

stratification of the deposits with discrete beds of

widely differing fiamme sizes and degrees of weld-

ing is further evidence for progressive aggradation

with layer by layer incremental deposition and varia-

tion in the nature of the material supplied. This

Ž .agrees with Branney and Kokelaar 1992 who arguethat a slope-parallel, complex, vertical variation in

the degree of welding supports an aggradational

mode of emplacement.

On the interfluves, the stratification and alterna-

tion of welded with non-welded deposits would sup-

port deposition occurring directly from pulses of

turbulent flow of varying temperature. Pulses which

were hotter deposited the welded layers. The thicken-

ing of the welded facies within the palaeovalleys

suggests that the particle concentration of flows be-

came greater down valleys. In a study of the Valley

of Ten Thousand Smokes deposits of the eruption of Ž .Katmai in 1912, Fierstein and Hildreth 1992 also

recognise two different facies: one associated with

infilling the valleys, Valley-Filling Ignimbrite, and

the other stratified, lenticular pyroclastic flow de-

posits on ridge crests, High-Energy Proximal Ign-

imbrite. Fierstein and Hildreth propose that these

proximal ignimbrites were emplaced from the upper

zones of turbulent, but still concentrated, densityŽ .stratified flows as defined by Valentine 1987 . A

similar process seems likely to have operated for the

emplacement of the Povoaçao Ignimbrite.˜The denser flows in the valleys would retain heat

more readily and this would favour deposition of

agglutinates. The field evidence suggests that the

flows on the margins of the valleys and over the

interfluves were less dense and more efficient at

entraining air which would accelerate cooling and

this would account for the thinning of the welded

zones and in some places, their complete attenuation

over interfluves.

The vertical textural and lithological changes dis-

played in these ignimbrites are indicative of rapid

transformations in the nature of deposition and erup-

tive style during emplacement which are occurring in

Žproximal locations close to the caldera rim see.Walker, 1985 . Sharp variations in facies of pyro-

clastic flow deposits have been observed in proximalŽsituations at other volcanoes such as Santorini Druitt

.and Sparks, 1982; Mellors and Sparks, 1991 , VulsiniŽ . ŽTurbeville, 1992 ; Campi Flegrei Perrotta and

. Ž .Scarpati, 1994 , Roccamonfina Cole et al., 1993Ž .and Menengai Macdonald et al., 1994 .

The eruption that formed the PIF with a thickness

of )50 m of pyroclastic material being deposited in

valleys on the south coast was obviously a major

event in the history of Furnas Volcano. The availableexposures are all in relatively proximal positions

within 5 km of the margin of the older caldera. Most

of the products of the eruption would have been

deposited in the sea and much material must have

been lost through erosion. Correlation between the

different sections suggests that the eruption began

with an eruptive column giving rise to lapilli-fall

followed by a phase of phreatomagmatic activity

generating pyroclastic surges that flowed south of

the caldera and that this was followed by the produc-Ž .

tion of pyroclastic flows sensu lato . These flowswere pulsatory and turbulent in nature giving rise to

distinctive stratified deposits with a high proportion

of surge material. Flows became concentrated and

more dense down valleys giving rise to thick welded

sequences. It is quite probable that this major explo-

sive eruption relates to the formation of the older

caldera of Furnas.


12/12


Acknowledgements

The manuscript was much improved following the

comments from the reviewers Steve Self and Steve

Sparks. The authors are pleased to acknowledge the

EC Framework III Environment Programme grant

which supported this research.

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