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The Povoação Ignimbrite, Furnas Volcano, São Miguel, Azores
Journal of Volcanology and Geothermal Research 92 Ž1999. 55–65 www.elsevier.comrlocaterjvolgeores
The Povoac¸ao ˜ Ignimbrite, Furnas Volcano, Sao ˜ Miguel, Azores A.M. Duncan a
a,)
, G. Queiroz b, J.E. Guest c , P.D. Cole a , N. Wallenstein b, J.M. Pacheco b
Centre for Volcanic Studies, Faculty of Science, Technology and Design, UniÕersity of Luton, Luton LU1 3JU, UK b Departimento de Geociencias, UniÕersidade dos Ac¸ores, Ponta Delgada, Portugal c Planetary Image Centre, UniÕersity College London, London NW7 4SD, UK Accepted 20 April 1999
Abstract The Povoac¸ao ˜ Ignimbrite Formation ŽPIF. 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 Povoac¸ao ˜ Ignimbrite is the most extensive. The PIF may correlate with the formation of the main caldera of Furnas. In the Povoac¸ao ˜ 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 denser down valleys favouring the emplacement of thicker welded units. q 1999 Elsevier Science B.V. All rights reserved. Keywords: explosive volcanism; welded tuffs; ignimbrite emplacement; Furnas; Azores
1. Introduction There are three active trachytic composite volcanoes on the island of Sao ˜ Miguel in the Azores: Sete Cidades, Fogo ŽAgua de Pau. and Furnas ŽBooth et al., 1978; Moore, 1990, 1991.. In the areas between these central volcanoes, monogenetic basaltic eruptions 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 Povoac¸aorNordeste volcanic construct. The ˜ caldera complex of Furnas is 8 = 5 km in diameter and formed as the result of at least two distinct collapses ŽGuest et al., 1999-this issue.. Over the last 5000 years, volcanic activity of Furnas has been characterised by sub-plinian eruptions ŽCole et al., 1995., but prior to this there were a number of major eruptions, some of which were ignimbrite-forming events associated with caldera collapse ŽGuest 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. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 6 7 - 0
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plex history of caldera formation at Furnas involving a number of major eruptions. One of these major trachytic eruptions of Furnas Volcano produced the Povoac¸ao ˜ Ignimbrite, which is the topic of this study ŽSchmincke and Weibel, 1972; Booth et al., 1978.. The Povoac¸ao ˜ Ignimbrite is well exposed in the coastal cliffs south of Furnas and in the older Povoac¸ao ˜ caldera to the east of Furnas ŽMoore, 1991., and displays a variety of textural facies including substantial development of a welded fabric. Ignimbrites with welded facies have also been described from other Azorean volcanoes such as Fogo volcano ŽMoore, 1991. and on the island of Terceira ŽSelf, 1971, 1976.. It is the purpose of this paper to provide a description of the Povoac¸ao ˜ 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 southern coast are cut perpendicular to palaeovalleys down which the pyroclastic flows, that gave rise to the Povoac¸ao ˜ Ignimbrite, were channelled. This gives a good opportunity to investigate the relationships between emplacement and topography. Moore Ž1990, 1991. identified an ignimbrite with a distinctive welded facies on Furnas, which he equates with the Povoac¸ao ˜ Ignimbrite, as the caldera-outflow ignimbrite and associates it with the formation of the caldera. On the basis of a radiocarbon age of 11,230 " 100 BP for a soil directly separating a welded ignimbrite exposed near the top of the caldera-fill within the main Furnas caldera and the overlying Pico de Ferro trachyte domes, Moore Ž1991. argues that the caldera-outflow ignimbrite has an age of around 12,000 years. However, radiocarbon dating of soils immediately underlying the Povoac¸ao ˜ Ignimbrite from the southern coastal exposures and from Povoac¸ao ˜ provide dates of between 30,000 and 35,000 BP ŽGuest et al., 1999. and this suggests an age of around 30,000 BP for the Povoac¸ao ˜ Ignimbrite. The Povoac¸ao ˜ 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 associated with Furnas Volcano ŽGuest et al., 1999. and we consider, therefore, that there is not a single caldera-outflow ignimbrite as proposed by Moore Ž1990, 1991..
2. The Povoac¸ao ˜ Ignimbrite Formation The Povoac¸ao ˜ Ignimbrite Formation ŽPIF. is made up of a number of different lithologies including lapilli-fall beds, thick surge deposits, massive nonwelded ignimbrite units as well as distinctive densely welded ignimbrite horizons. The ignimbrite can be readily traced from the town of Povoac¸ao ˜ to just west of Ribeira Quente ŽFig. 1.. Erosion of deposits and incision of steep valleys has made it difficult to directly correlate the deposits of the Povoac¸ao ˜ Ignimbrite in the Ribeira Quente area with the ignimbrite which is considered by Guest et al. Ž1999-this issue. to be the Povoac¸ao ˜ Ignimbrite in the Amoras area ŽFig. 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 supports a correlation with the Povoac¸ao ˜ 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 effective estimate of the volume though Moore Ž1990. suggests that the subaerial volume is of the order 7 km3 Žabout 2 km3 dense rock equivalent.. Informative exposures of the PIF occur in three distinct areas along the coast between Ponta Garc¸a and Povoac¸ao ˜ ŽFig. 1. which from west to east are: the Amoras area ŽSites 81, 79, 216., the Ribeira Quente area ŽSites 61, 219, 167. and the Povoac¸ao ˜ caldera ŽSites 205, 62.. Two main facies can be recognised: Ž1. a Valley Fill Facies and Ž2. 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 depressions. In these palaeovalleys, the deposits of the Povoac¸ao ˜ Ignimbrite thicken considerably; in the main valley north of Povoac¸ao ˜ there are more than 4 m of massive columnar jointed ignimbrite. On the edge of the town of Povoac¸ao ˜ the relationships between the Povoac¸ao ˜ Ignimbrite and the palaeotopography 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-
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Fig. 1. Location map of Furnas volcanoes and principal sites of the Povoac¸ao ˜ Ignimbrite.
verge on Povoac¸ao ˜ village densely welded ignimbrite is more than 20 m in thickness and in the quarry in the Ribeira Quente valley welded ignimbrite occurs up to 40 m in thickness. At Site 62, in the cliff section on the shore immediately west of the village of Povoac¸ao, ˜ 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. The headland ŽSite 79. to the east of Amoras ŽFig. . 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 ignimbrites and thin Ž- 2 m. pyroclastic surge sandwave beds Žsee Site 79 in Fig. 2.. Some of these non-welded pumiceous ignimbrite horizons show many of the features of the ‘classic’ ignimbrite sequence of Sparks et al. Ž1973. 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 interbedded with massive non-welded ignimbrite. In the upper part of the sequence, in the axis of the palaeovalley, there is up to 35 m of massive non-welded
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Fig. 2. Documented sections of the Povoac¸ao ˜ 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 deposits. 2.2. Valley Margin Facies To the east of Ribeira Quente, the PIF is exposed on higher ground between palaeovalleys ŽSites 219 and 167 in Fig. 3.. Here, the welded horizons are
much thinner Ž- 1 m. and interbedded with nonwelded cross-bedded surge deposits. In sections on the slopes Že.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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Fig. 3. Documented sections of the Povoac¸ao ˜ Ignimbrite Žlocalities shown in Fig. 1. — key to ornamentation provided in Fig. 2.
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The coastal path which runs along the cliff westwards from Ribeira Quente provides a good section through the PIF ŽSite 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 interpret 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 pumice concentration 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 composition. 2.3. Relationship between the Valley Fill Facies and the Valley Margin Facies In sections where it is possible to examine the transition Žperpendicular 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 nonwelded 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 showing the same preferred orientation of long axes ŽFig. 4..
3. The nature of welding in the Povoac¸ao ˜ Ignimbrite The range of welding in the Povoac¸ao ˜ Ignimbrite is similar to that described by Smith Ž1960. in his classic paper on welded tuffs. At the minimum end of 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 palaeovalleys, the glassy clasts have been markedly deformed and pore space has been largely eliminated. The welded zones show a fabric produced by flattening 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. Povoac¸ao ˜ 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 associated with: Ž1. a variation in fiamme size and Ž2. a variation in the intensity of welding over intervals of 20–30 cm, both developed perpendicular to the depositional surface ŽFig. 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 transitions from non-welded to welded ignimbrite ŽFig. 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,
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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 deformed to form fiamme developing a eutaxitic texture. In some places, some of the fiamme show crude imbrication. This is well displayed in one of the coarser layers at Site 61 Žsee 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 average 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 specimen. The rock does show a well-developed platey fabric, however, and fiamme are clearly visible in thin section.
Fig. 5. Stratification in the Povoac¸ao ˜ Ignimbrite defined by a variation in clast size in the welded horizon at Site 61.
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Fig. 6. Exposure of the PIF on southern coast of Furnas Volcano ŽSite 216. showing vertical variation in lithofacies. Three separate distinct welded layers ŽW. separated by unwelded tuff, ranging from massive ŽM. to stratified ŽS. 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 the fabric ŽSite 61, Fig. 2.. Where these stringers occur,
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Fig. 7. Imbrication of clasts in the welded horizon of the Povoac¸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 inhibited as a result of the cooling influence of the lithic clasts.
4. Emplacement mechanisms for the ignimbrites There has been much discussion in the literature ŽMahood, 1984. as to whether stratified welded pyroclastic deposits such as these have a fall or a flow origin. The Thera Welded Tuff of Santorini, Greece, which mantles the topography with uniform thickness is interpreted by Sparks and Wright Ž1979. as being of fall origin. Wright Ž1980., working on the
welded green tuff of Pantelleria, Italy, considered that deposition on slopes in excess of 208 and stratification of the deposits are evidence of a fall origin. Mahood Ž1984., however, reconsidered the origin of some of the welded tuffs on Pantelleria and argued that mantling tuffs could have been emplaced by pyroclastic flows. Branney and Kokelaar Ž1992. argue that welded tuffs of pyroclastic flow origin can show stratification. The welded facies of the Povoac¸ao ˜ Ignimbrite show imbrication of fiamme ŽFig. 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-
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nism, the welded zones would not show thickening towards the axis of a valley unless rheomorphism had taken place. There is little evidence for rheomorphism 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-depositional in character — that these are agglutinates as defined by Branney and Kokelaar Ž1992.. It is our judgement that the welded zones accumulated incrementally and evidence for an aggradational mode of emplacement is provided by: Ž1. the imbrication of fiamme, see discussion in the works of Branney and Kokelaar Ž1994. and Wolff and Turbeville Ž1994.. There are no deformation features which would support an explanation that these imbricate textures formed as a result of rheomorphic flowage. Ž2. The stratification of the deposits with discrete beds of widely differing fiamme sizes and degrees of welding is further evidence for progressive aggradation with layer by layer incremental deposition and variation in the nature of the material supplied. This agrees with Branney and Kokelaar Ž1992. who argue that a slope-parallel, complex, vertical variation in the degree of welding supports an aggradational mode of emplacement. On the interfluves, the stratification and alternation of welded with non-welded deposits would support deposition occurring directly from pulses of turbulent flow of varying temperature. Pulses which were hotter deposited the welded layers. The thickening of the welded facies within the palaeovalleys suggests that the particle concentration of flows became greater down valleys. In a study of the Valley of Ten Thousand Smokes deposits of the eruption of Katmai in 1912, Fierstein and Hildreth Ž1992. also recognise two different facies: one associated with infilling the valleys, Valley-Filling Ignimbrite, and the other stratified, lenticular pyroclastic flow deposits on ridge crests, High-Energy Proximal Ignimbrite. Fierstein and Hildreth propose that these
proximal ignimbrites were emplaced from the upper zones of turbulent, but still concentrated, density stratified flows as defined by Valentine Ž1987.. A similar process seems likely to have operated for the emplacement of the Povoac¸ao ˜ 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 displayed in these ignimbrites are indicative of rapid transformations in the nature of deposition and eruptive style during emplacement which are occurring in proximal locations close to the caldera rim Žsee Walker, 1985.. Sharp variations in facies of pyroclastic flow deposits have been observed in proximal situations at other volcanoes such as Santorini ŽDruitt and Sparks, 1982; Mellors and Sparks, 1991., Vulsini ŽTurbeville, 1992.; Campi Flegrei ŽPerrotta and Scarpati, 1994., Roccamonfina ŽCole et al., 1993. and Menengai ŽMacdonald 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 available exposures 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 production of pyroclastic flows Žsensu lato.. These flows were 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 explosive eruption relates to the formation of the older caldera of Furnas.
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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.
References Booth, B., Walker, G.P.L., Croasdale, R., 1978. A quantitative study of five thousand years of volcanism on Sao ˜ Miguel, Azores. Philos. Trans. R. Soc. London 228A, 271–319. Branney, M.J., Kokelaar, P., 1992. A reappraisal of ignimbrite emplacement: progressive aggradation and particulate to nonparticulate flow transitions during emplacement of high-grade ignimbrite. Bull. Volcanol. 54, 504–520. Branney, M.J., Kokelaar, P., 1994. Reply to Wolff and Turbeville’s comment on ‘A reappraisal of ignimbrite emplacement: progressive aggradation and particulate to non-particulate flow transitions during emplacement of high-grade ignimbrite’. Bull. Volcanol. 56, 138–143. Cole, P.D., Guest, J.E., Duncan, A.M., 1993. The emplacement of intermediate volume ignimbrites: a case study from Roccamonfina Volcano, Southern Italy. Bull. Volcanol. 55, 467–480. Cole, P.D., Queiroz, G., Wallenstein, N., Gaspar, J.L., Duncan, A.M., Guest, J.E., 1995. An historic sub-plinianrphreatomagmatic eruption: 1630 AD eruption of Furnas Volcano, Sao Miguel, Azores. J. Volcanol. Geotherm. Res. 69, 117–135. Druitt, T.H., Sparks, R.S.J., 1982. A proximal ignimbrite facies on Santorini, Greece. J. Volcanol. Geotherm. Res. 13, 147–171. Fierstein, J., Hildreth, W., 1992. The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska. Bull. Volcanol. 54, 646–684. Guest, J.E., Gaspar, J.L., Cole, P.D., Queiroz, G., Duncan, A.M., Wallenstein, N., Ferrara, T.N., Pacheco, J.-M., 1999. Volcanic geology of Furnas Volcano, Sao ˜ Miguel, Azores. J. Volcanol. Geotherm. Res. 92, 1–29. Macdonald, R., Navarro, J.M., Upton, B.G.J., Davies, G.R., 1994.
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Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. J. Volcanol. Geotherm. Res. 60, 301–325. Mahood, G.A., 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. J. Geophys. Res. 89, 8540–8552. Mellors, R.A., Sparks, R.S.J., 1991. Spatter-rich pyroclastic flow deposits on Santorini, Greece. Bull. Volcanol. 53, 327–342. Moore, R.B., 1990. Volcanic geology and eruption frequency, Sao ˜ Miguel, Azores. Bull. Volcanol. 52, 602–614. Moore, R.B., 1991. Geology of three late quaternary stratovolcanoes on Sao ˜ Miguel, Azores. U.S. Geol. Surv. Bull. 1900. Perrotta, A., Scarpati, C., 1994. The dynamics of the Breccia Museo eruption ŽCampi Flegrei, Italy. and the significance of spatter clasts associated with lithic breccias. J. Volcanol. Geotherm. Res. 59, 355–357. Schmincke, H.-U., Weibel, M., 1972. Chemical study of rocks from Madeira, Porto Santo, and Sao ˜ Miguel and Terceira ŽAzores.. Neues Jahrbuch fur Mineralogie 117, 253–281. Self, S., 1971. The Lajes ignimbrite, Ilha Terceira, Ac¸ores. Com. Serv. Geol. Portugal 55, 165–180. Self, S., 1976. The recent volcanology of Terceira, Azores. J. Geol. Soc. London 132, 645–666. Smith, R.L., 1960. Ash flows. Bull. Geol. Soc. Am. 71, 795–842. Sparks, R.S., Wright, J.V., 1979. Welded air-fall tuffs. Geol. Soc. Am., Spec. Pap. 180, 155–166. Sparks, R.S.J., Self, S., Walker, G.P.L., 1973. The products of ignimbrite eruptions. Geology 1, 115–118. Turbeville, B.N., 1992. Tephra fountaining, rheomorphism, and spatter flow during emplacement of the Pitigliano Tuffs, Latera Caldera, Italy. J. Volcanol. Geotherm. Res. 53, 309–327. Valentine, G.A., 1987. Stratified flow in pyroclastic surges. Bull. Volcanol. 49, 616–630. Walker, G.P.L., 1985. Origin of coarse lithic breccias near ignimbrite source vents. J. Volcanol. Geotherm. Res. 25, 157–171. Wolff, J.A., Turbeville, B.N., 1994. Comment on ‘A reappraisal of ignimbrite emplacement: progressive aggradation and particulate to non-particulate flow transitions during emplacement of high-grade ignimbrite’ by MJ Branney and P Kokelaar. Bull. Volcanol. 56, 134–137. Wright, J.V., 1980. Stratigraphy and geology of the welded air-fall tuffs of Pantelleria, Italy. Geol. Rundsch. 69, 263–291.
The Povoac¸ao ˜ Ignimbrite, Furnas Volcano, Sao ˜ Miguel, Azores A.M. Duncan a
a,)
, G. Queiroz b, J.E. Guest c , P.D. Cole a , N. Wallenstein b, J.M. Pacheco b
Centre for Volcanic Studies, Faculty of Science, Technology and Design, UniÕersity of Luton, Luton LU1 3JU, UK b Departimento de Geociencias, UniÕersidade dos Ac¸ores, Ponta Delgada, Portugal c Planetary Image Centre, UniÕersity College London, London NW7 4SD, UK Accepted 20 April 1999
Abstract The Povoac¸ao ˜ Ignimbrite Formation ŽPIF. 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 Povoac¸ao ˜ Ignimbrite is the most extensive. The PIF may correlate with the formation of the main caldera of Furnas. In the Povoac¸ao ˜ 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 denser down valleys favouring the emplacement of thicker welded units. q 1999 Elsevier Science B.V. All rights reserved. Keywords: explosive volcanism; welded tuffs; ignimbrite emplacement; Furnas; Azores
1. Introduction There are three active trachytic composite volcanoes on the island of Sao ˜ Miguel in the Azores: Sete Cidades, Fogo ŽAgua de Pau. and Furnas ŽBooth et al., 1978; Moore, 1990, 1991.. In the areas between these central volcanoes, monogenetic basaltic eruptions 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 Povoac¸aorNordeste volcanic construct. The ˜ caldera complex of Furnas is 8 = 5 km in diameter and formed as the result of at least two distinct collapses ŽGuest et al., 1999-this issue.. Over the last 5000 years, volcanic activity of Furnas has been characterised by sub-plinian eruptions ŽCole et al., 1995., but prior to this there were a number of major eruptions, some of which were ignimbrite-forming events associated with caldera collapse ŽGuest 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. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 6 7 - 0
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plex history of caldera formation at Furnas involving a number of major eruptions. One of these major trachytic eruptions of Furnas Volcano produced the Povoac¸ao ˜ Ignimbrite, which is the topic of this study ŽSchmincke and Weibel, 1972; Booth et al., 1978.. The Povoac¸ao ˜ Ignimbrite is well exposed in the coastal cliffs south of Furnas and in the older Povoac¸ao ˜ caldera to the east of Furnas ŽMoore, 1991., and displays a variety of textural facies including substantial development of a welded fabric. Ignimbrites with welded facies have also been described from other Azorean volcanoes such as Fogo volcano ŽMoore, 1991. and on the island of Terceira ŽSelf, 1971, 1976.. It is the purpose of this paper to provide a description of the Povoac¸ao ˜ 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 southern coast are cut perpendicular to palaeovalleys down which the pyroclastic flows, that gave rise to the Povoac¸ao ˜ Ignimbrite, were channelled. This gives a good opportunity to investigate the relationships between emplacement and topography. Moore Ž1990, 1991. identified an ignimbrite with a distinctive welded facies on Furnas, which he equates with the Povoac¸ao ˜ Ignimbrite, as the caldera-outflow ignimbrite and associates it with the formation of the caldera. On the basis of a radiocarbon age of 11,230 " 100 BP for a soil directly separating a welded ignimbrite exposed near the top of the caldera-fill within the main Furnas caldera and the overlying Pico de Ferro trachyte domes, Moore Ž1991. argues that the caldera-outflow ignimbrite has an age of around 12,000 years. However, radiocarbon dating of soils immediately underlying the Povoac¸ao ˜ Ignimbrite from the southern coastal exposures and from Povoac¸ao ˜ provide dates of between 30,000 and 35,000 BP ŽGuest et al., 1999. and this suggests an age of around 30,000 BP for the Povoac¸ao ˜ Ignimbrite. The Povoac¸ao ˜ 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 associated with Furnas Volcano ŽGuest et al., 1999. and we consider, therefore, that there is not a single caldera-outflow ignimbrite as proposed by Moore Ž1990, 1991..
2. The Povoac¸ao ˜ Ignimbrite Formation The Povoac¸ao ˜ Ignimbrite Formation ŽPIF. is made up of a number of different lithologies including lapilli-fall beds, thick surge deposits, massive nonwelded ignimbrite units as well as distinctive densely welded ignimbrite horizons. The ignimbrite can be readily traced from the town of Povoac¸ao ˜ to just west of Ribeira Quente ŽFig. 1.. Erosion of deposits and incision of steep valleys has made it difficult to directly correlate the deposits of the Povoac¸ao ˜ Ignimbrite in the Ribeira Quente area with the ignimbrite which is considered by Guest et al. Ž1999-this issue. to be the Povoac¸ao ˜ Ignimbrite in the Amoras area ŽFig. 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 supports a correlation with the Povoac¸ao ˜ 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 effective estimate of the volume though Moore Ž1990. suggests that the subaerial volume is of the order 7 km3 Žabout 2 km3 dense rock equivalent.. Informative exposures of the PIF occur in three distinct areas along the coast between Ponta Garc¸a and Povoac¸ao ˜ ŽFig. 1. which from west to east are: the Amoras area ŽSites 81, 79, 216., the Ribeira Quente area ŽSites 61, 219, 167. and the Povoac¸ao ˜ caldera ŽSites 205, 62.. Two main facies can be recognised: Ž1. a Valley Fill Facies and Ž2. 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 depressions. In these palaeovalleys, the deposits of the Povoac¸ao ˜ Ignimbrite thicken considerably; in the main valley north of Povoac¸ao ˜ there are more than 4 m of massive columnar jointed ignimbrite. On the edge of the town of Povoac¸ao ˜ the relationships between the Povoac¸ao ˜ Ignimbrite and the palaeotopography 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-
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Fig. 1. Location map of Furnas volcanoes and principal sites of the Povoac¸ao ˜ Ignimbrite.
verge on Povoac¸ao ˜ village densely welded ignimbrite is more than 20 m in thickness and in the quarry in the Ribeira Quente valley welded ignimbrite occurs up to 40 m in thickness. At Site 62, in the cliff section on the shore immediately west of the village of Povoac¸ao, ˜ 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. The headland ŽSite 79. to the east of Amoras ŽFig. . 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 ignimbrites and thin Ž- 2 m. pyroclastic surge sandwave beds Žsee Site 79 in Fig. 2.. Some of these non-welded pumiceous ignimbrite horizons show many of the features of the ‘classic’ ignimbrite sequence of Sparks et al. Ž1973. 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 interbedded with massive non-welded ignimbrite. In the upper part of the sequence, in the axis of the palaeovalley, there is up to 35 m of massive non-welded
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Fig. 2. Documented sections of the Povoac¸ao ˜ 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 deposits. 2.2. Valley Margin Facies To the east of Ribeira Quente, the PIF is exposed on higher ground between palaeovalleys ŽSites 219 and 167 in Fig. 3.. Here, the welded horizons are
much thinner Ž- 1 m. and interbedded with nonwelded cross-bedded surge deposits. In sections on the slopes Že.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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Fig. 3. Documented sections of the Povoac¸ao ˜ Ignimbrite Žlocalities shown in Fig. 1. — key to ornamentation provided in Fig. 2.
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The coastal path which runs along the cliff westwards from Ribeira Quente provides a good section through the PIF ŽSite 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 interpret 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 pumice concentration 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 composition. 2.3. Relationship between the Valley Fill Facies and the Valley Margin Facies In sections where it is possible to examine the transition Žperpendicular 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 nonwelded 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 showing the same preferred orientation of long axes ŽFig. 4..
3. The nature of welding in the Povoac¸ao ˜ Ignimbrite The range of welding in the Povoac¸ao ˜ Ignimbrite is similar to that described by Smith Ž1960. in his classic paper on welded tuffs. At the minimum end of 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 palaeovalleys, the glassy clasts have been markedly deformed and pore space has been largely eliminated. The welded zones show a fabric produced by flattening 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. Povoac¸ao ˜ 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 associated with: Ž1. a variation in fiamme size and Ž2. a variation in the intensity of welding over intervals of 20–30 cm, both developed perpendicular to the depositional surface ŽFig. 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 transitions from non-welded to welded ignimbrite ŽFig. 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,
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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 deformed to form fiamme developing a eutaxitic texture. In some places, some of the fiamme show crude imbrication. This is well displayed in one of the coarser layers at Site 61 Žsee 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 average 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 specimen. The rock does show a well-developed platey fabric, however, and fiamme are clearly visible in thin section.
Fig. 5. Stratification in the Povoac¸ao ˜ Ignimbrite defined by a variation in clast size in the welded horizon at Site 61.
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Fig. 6. Exposure of the PIF on southern coast of Furnas Volcano ŽSite 216. showing vertical variation in lithofacies. Three separate distinct welded layers ŽW. separated by unwelded tuff, ranging from massive ŽM. to stratified ŽS. 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 the fabric ŽSite 61, Fig. 2.. Where these stringers occur,
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Fig. 7. Imbrication of clasts in the welded horizon of the Povoac¸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 inhibited as a result of the cooling influence of the lithic clasts.
4. Emplacement mechanisms for the ignimbrites There has been much discussion in the literature ŽMahood, 1984. as to whether stratified welded pyroclastic deposits such as these have a fall or a flow origin. The Thera Welded Tuff of Santorini, Greece, which mantles the topography with uniform thickness is interpreted by Sparks and Wright Ž1979. as being of fall origin. Wright Ž1980., working on the
welded green tuff of Pantelleria, Italy, considered that deposition on slopes in excess of 208 and stratification of the deposits are evidence of a fall origin. Mahood Ž1984., however, reconsidered the origin of some of the welded tuffs on Pantelleria and argued that mantling tuffs could have been emplaced by pyroclastic flows. Branney and Kokelaar Ž1992. argue that welded tuffs of pyroclastic flow origin can show stratification. The welded facies of the Povoac¸ao ˜ Ignimbrite show imbrication of fiamme ŽFig. 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-
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nism, the welded zones would not show thickening towards the axis of a valley unless rheomorphism had taken place. There is little evidence for rheomorphism 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-depositional in character — that these are agglutinates as defined by Branney and Kokelaar Ž1992.. It is our judgement that the welded zones accumulated incrementally and evidence for an aggradational mode of emplacement is provided by: Ž1. the imbrication of fiamme, see discussion in the works of Branney and Kokelaar Ž1994. and Wolff and Turbeville Ž1994.. There are no deformation features which would support an explanation that these imbricate textures formed as a result of rheomorphic flowage. Ž2. The stratification of the deposits with discrete beds of widely differing fiamme sizes and degrees of welding is further evidence for progressive aggradation with layer by layer incremental deposition and variation in the nature of the material supplied. This agrees with Branney and Kokelaar Ž1992. who argue that a slope-parallel, complex, vertical variation in the degree of welding supports an aggradational mode of emplacement. On the interfluves, the stratification and alternation of welded with non-welded deposits would support deposition occurring directly from pulses of turbulent flow of varying temperature. Pulses which were hotter deposited the welded layers. The thickening of the welded facies within the palaeovalleys suggests that the particle concentration of flows became greater down valleys. In a study of the Valley of Ten Thousand Smokes deposits of the eruption of Katmai in 1912, Fierstein and Hildreth Ž1992. also recognise two different facies: one associated with infilling the valleys, Valley-Filling Ignimbrite, and the other stratified, lenticular pyroclastic flow deposits on ridge crests, High-Energy Proximal Ignimbrite. Fierstein and Hildreth propose that these
proximal ignimbrites were emplaced from the upper zones of turbulent, but still concentrated, density stratified flows as defined by Valentine Ž1987.. A similar process seems likely to have operated for the emplacement of the Povoac¸ao ˜ 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 displayed in these ignimbrites are indicative of rapid transformations in the nature of deposition and eruptive style during emplacement which are occurring in proximal locations close to the caldera rim Žsee Walker, 1985.. Sharp variations in facies of pyroclastic flow deposits have been observed in proximal situations at other volcanoes such as Santorini ŽDruitt and Sparks, 1982; Mellors and Sparks, 1991., Vulsini ŽTurbeville, 1992.; Campi Flegrei ŽPerrotta and Scarpati, 1994., Roccamonfina ŽCole et al., 1993. and Menengai ŽMacdonald 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 available exposures 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 production of pyroclastic flows Žsensu lato.. These flows were 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 explosive eruption relates to the formation of the older caldera of Furnas.
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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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Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley. J. Volcanol. Geotherm. Res. 60, 301–325. Mahood, G.A., 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. J. Geophys. Res. 89, 8540–8552. Mellors, R.A., Sparks, R.S.J., 1991. Spatter-rich pyroclastic flow deposits on Santorini, Greece. Bull. Volcanol. 53, 327–342. Moore, R.B., 1990. Volcanic geology and eruption frequency, Sao ˜ Miguel, Azores. Bull. Volcanol. 52, 602–614. Moore, R.B., 1991. Geology of three late quaternary stratovolcanoes on Sao ˜ Miguel, Azores. U.S. Geol. Surv. Bull. 1900. Perrotta, A., Scarpati, C., 1994. The dynamics of the Breccia Museo eruption ŽCampi Flegrei, Italy. and the significance of spatter clasts associated with lithic breccias. J. Volcanol. Geotherm. Res. 59, 355–357. Schmincke, H.-U., Weibel, M., 1972. Chemical study of rocks from Madeira, Porto Santo, and Sao ˜ Miguel and Terceira ŽAzores.. Neues Jahrbuch fur Mineralogie 117, 253–281. Self, S., 1971. The Lajes ignimbrite, Ilha Terceira, Ac¸ores. Com. Serv. Geol. Portugal 55, 165–180. Self, S., 1976. The recent volcanology of Terceira, Azores. J. Geol. Soc. London 132, 645–666. Smith, R.L., 1960. Ash flows. Bull. Geol. Soc. Am. 71, 795–842. Sparks, R.S., Wright, J.V., 1979. Welded air-fall tuffs. Geol. Soc. Am., Spec. Pap. 180, 155–166. Sparks, R.S.J., Self, S., Walker, G.P.L., 1973. The products of ignimbrite eruptions. Geology 1, 115–118. Turbeville, B.N., 1992. Tephra fountaining, rheomorphism, and spatter flow during emplacement of the Pitigliano Tuffs, Latera Caldera, Italy. J. Volcanol. Geotherm. Res. 53, 309–327. Valentine, G.A., 1987. Stratified flow in pyroclastic surges. Bull. Volcanol. 49, 616–630. Walker, G.P.L., 1985. Origin of coarse lithic breccias near ignimbrite source vents. J. Volcanol. Geotherm. Res. 25, 157–171. Wolff, J.A., Turbeville, B.N., 1994. Comment on ‘A reappraisal of ignimbrite emplacement: progressive aggradation and particulate to non-particulate flow transitions during emplacement of high-grade ignimbrite’ by MJ Branney and P Kokelaar. Bull. Volcanol. 56, 134–137. Wright, J.V., 1980. Stratigraphy and geology of the welded air-fall tuffs of Pantelleria, Italy. Geol. Rundsch. 69, 263–291.