Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial I, 283-288 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

Geological mapping of the central area of Terceira Island (Azores, Portugal): associated volcanostratigraphy, ages and genetic implications on the Malha-Balcões-Chamusca lava caves system Cartografia geológica da região central da ilha Terceira (Açores, Portugal): vulcanoestratigrafia, idades associadas e implicações genéticas no Sistema Cavernícola Malha-Balcões-Chamusca J. C. Nunes1,2*, A. Calvert3, S. Medeiros1, E. A. Lima1, F. Pereira2,4, M. P. Costa2,4, P. Barcelos2,4, M. R. Carvalho5

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: The detailed geological mapping of the Basaltic Fissural Zone in the central area of Terceira Island, the field stratigraphical relationships observed and published and new isotopic ages now provided allowed to correlate the 26 volcanic caves existing in the mapped area (between Pico da Bagacina area and the north coast) with the correlated volcanic episodes/eruptions. The main conclusions obtained include: i) the association of a 10 volcanic caves system (including the lava-tube caves of Malha, Balcões and Chamusca) with Holocene (potentially younger than 6,600 years BP) pahoehoe lava flows extruded from Pico Galiarte scoria cone; ii) the association of 8 volcanic caves (namely Natal cave) with the eruptive episode associated with the “Pico do Gaspar fissural system”, with an age younger than 12,100 years BP; iii) the mapping of the lava flows associated with the Pico Gordo scoria cone (that formed 4 volcanic caves), dated of about 4,480 years BP and that preceded the 1761 A.D. historical eruption, which corresponds to the late eruption In the mapped area.

Keywords: Geological mapping, Volcanic caves, Azores islands, Ages, Eruptive history. Resumo: A cartografia geológica de detalhe da região central da ilha Terceira integrada na Zona Basáltica Fissural, as relações estratigráficas observadas in situ, idades isotópicas publicadas e novas datações agora apresentadas, permitiram correlacionar as 26 cavidades vulcânicas existentes nesta região (e.g. entre a zona do Pico da Bagacina e o litoral norte) com os episódios/erupções vulcânicas que estiveram na sua génese. De entre os principais resultados merece especial destaque: i) a associação de um sistema de 10 cavidades vulcânicas (onde se incluem as grutas da Malha, dos Balcões e da Chamusca) a escoadas lávicas pahoehoe emitidas pelo Pico Galiarte, de idade holocénica (potencialmente de idade inferior a 6.600 anos BP); ii) a associação de 8 cavidades (de que faz parte a Gruta do Natal) ao episódio eruptivo associado ao “sistema fissural do Pico Gaspar”, de idade inferior a 12.100 anos BP; iii) a cartografia das escoadas lávicas associadas à erupção do cone de escórias do Pico Gordo (que originou 4 cavidades vulcânicas), datada de há cerca de 4.480 anos BP, a qual antecede a erupção histórica do ano de 1761 A.D., que corresponde à erupção mais recente ocorrida na zona cartografada.

Palavras-chave: Cartografia geológica, Cavidades vulcânicas, Ilhas dos Açores, Idades, História eruptiva.

1Universidade dos Açores, Departamento de Geociências, Apartado 1422, 9501-801 Ponta Delgada, Açores, Portugal. 2GESPEA – “Grupo de Estudo do Património Espeleológico dos Açores”, Edifício Matos Souto, Piedade, 9930-210 Lajes do Pico, Açores, Portugal. 3United States Geological Survey, 345 Middlefield Road MS 977, Menlo Park, California 94025, USA. 4Associação “Os Montanheiros”, Rua da Rocha, nº 8, 9700-169 Angra do Heroísmo, Açores, Portugal. 5Departamento de Geologia e Centro de Geologia, FCUL, Universidade de Lisboa, Edifício C6, Campo Grande, 1749-016 Lisboa, Portugal. *Corresponding author / Autor correspondente: jcnunes@uac.pt

1. Introduction

The Azores Archipelago is located at the triple junction between the North American, Eurasian and African plates. The first two are separated by the Mid-Atlantic Ridge located between Flores and Faial islands, while Eurasian and African plates are separated by a complex tectonic structure, the Azores-Gibraltar Plate Boundary, that in the area of the archipelago comprises a wide shear zone (e.g. the Azores block, Luis et al., 1994), with a rough elongated triangular shape. The northern edge of this shear zone is established by the Terceira Rift, a WNW-ESE hyper-slow spreading centre (Vogt & Jung, 2004) incorporating Graciosa, Terceira and S. Miguel islands.

Terceira Island, together with Graciosa, São Jorge, Pico and Faial islands form the Central Group of the Archipelago. The island presents active volcanism (with a wide variety of rocks compositions and structures) and an important seismic activity. Terceira Island includes four central polygenetic volcanoes with caldera (Cinco Picos-Serra do Cume, Guilherme Moniz, Pico Alto and Santa Bárbara, from east to west) and a Basaltic Fissural Zone

Artigo Curto Short Article

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(Nunes, 2000), mainly in the central and SE parts of the island (Fig. 1).

Volcanic products extruded from these central volcanoes, mostly in Quaternary times include ignimbrites, pumice fall deposits, strongly evolved silicic lava flows (as domes and coulées of comendite to pantellerite

composition), scoria and spatter deposits, surtseyan tuffs and basaltic lava flows. Unlike other islands of the archipelago, Terceira volcanism main feature is the presence of extensive effusive episodes of silicic nature (domes and coulées, 40-90 m thick) which represent about 80% of the erupted material (Self, 1976).

Fig. 1. Volcano-tectonic sketch of Terceira Island: 1- Cinco Picos-Serra do Cume Volcano; 2- Guilherme Moniz Volcano; 3- Santa Bárbara Volcano; 4- Pico Alto Volcano; 5- Basaltic Fissural Zone, including the 1761 A.D. and the Algar do Carvão lava flow, in yellow. Dotted areas include trachyte s.l. domes and coulées. a- volcanic and tectonic lineament; b- fault scarp; c- crater rim; d- caldera rim; e- basaltic s.l. eruptive centre (scoria cone/tuff cone); f- silicic eruptive centre (dome/coulée); g- eruptive fissure; h- fumaroles; i- thermal waters spring. Stars point out locations of 14C ages of the Lajes and Angra ignimbrites. UTM coordinates in metres (Nunes, 2000). Rectangle indicates mapped area of figures 2 and 3. Fig. 1. Esboço vulcano-tectónico da Ilha Terceira.

After the settlement of the island (in the fifteenth century) three volcanic eruptions took place: the first one in 1761, the second in 1867 and the last one on the period 1998-2001. The 1761 eruption occurred on-land, east of Santa Bárbara Volcano (with two distinct episodes and locations), while the later two were located at sea, NW of the island (about 2.5 km and 8.5 km offshore, respectively).

The Basaltic Fissural Zone is developed on two main sectors of the Terceira Island: i) in the central part of the island, namely along the Pico da Bagacina-Pico do Gaspar volcano-tectonic lineament, and, ii) in the SE part of the island including the S. Sebastião-Porto Martins and the Cinco Picos areas (Fig. 1). The associated volcanism is strongly controlled by NW-SE to W-E fractures, is almost exclusively of the Hawaiian to Strombolian style, and was responsible for the building up of about 75 monogenetic volcanoes, mostly scoria cones and eruptive fissures, and emission of aa and pahoehoe lava flows.

Given its volcanic origin and the dominant basaltic nature of its eruptions, Azores archipelago presents a wide diversity of volcanic caves in all but Corvo Island. Presently, 272 natural caves are known in the Azores Islands, corresponding to about 32 kilometres of underground paths, with peculiar geologic features and live forms (Costa et al., 2008). These caves are of different types: lava-tube caves, volcanic pits, fractures and erosion caves, including the combination of these structures. Most caves are located in the islands of Pico with 129, Terceira and S. Miguel islands, with 69 and 28 caves respectively.

About 70% of the Terceira island caves are located in the Basaltic Fissural Zone, thus the importance to map the geological formations with which the caves are associated. Since the area of São Sebastião-Porto Martins was already mapped (cf. Montesinos et al., 2003) – where caves such as Gruta da Madre de Deus, Gruta de Santo António and Galeria do Sequeira were associated to the 2,000-3,000

Geological mapping of the central area of Terceira Island (Azores) 285

years old Fonte Bastardo eruption – the main goals of the present work were to obtain a detailed geological map of the central sector of the Basaltic Fissural Zone and establish the local volcanostratigraphy, allowing correlations with the volcanic caves emplaced in that area.

2. Geological mapping and volcanostratigraphy

Detailed geological mapping was carried out on the north part of central sector of the Basaltic Fissural Zone (Fig. 1) since the known volcanic caves are located in this area. The field work was done in the period 2009-2011 and included the establishment of the detailed volcanostratigraphy of the area, the nature of its volcanism, the eruptive styles and the mapping of the major tectonic lineaments and fractures. Geochemical analyses of collected samples were performed at Activation Laboratories, Ltd., in Canada.

The obtained geologic map is presented in figure 2, which includes the location of the main entrance of the 26 volcanic caves and pits existing in the area. The mapped area is limited to the W by the volcanism associated with the Santa Bárbara Volcano, while to the NE and SE, respectively, the Pico Alto and the Guilherme Moniz polygenetic silicic volcanoes formations crop out.

Geological mapping and local volcanostratigraphy was also supported on isotope age determinations available for the area, namely 14C and Ar/Ar ages reported in Calvert et al. (2006). Additionally, the present work includes unpublished Ar/Ar ages of samples collected in 2006 by A. Calvert at Gruta dos Balcões and Gruta do Natal lava caves. Compiled age information relevant for the present work and its relations with the geological map and volcanostratigraphy of the area are presented in table 1 and in figure 3. Table 2 presents the geochemical composition of the samples analyzed and the correspondent rock designation in accordance to the Le Bas et al. (1986) diagrams presented in figure 4.

All the gathered information allows new insights on the volcanism of Terceira Island and especially on the eruptive history of this sector of the Basaltic Fissural Zone (BFZ), as presented below.

3. Discussion

The axial part of the mapped area is defined by the volcano-tectonic lineament of Pico do Gaspar – Arrochinha (“573” cone) – Pico da Bagacina – Criação do Filipe/Achadas (see also Fig. 1). This WNW-ESE alignment of spatter and scoria cones defines a narrow and small ridge, from which altitudes decrease both to north and south coasts. This alignment includes two different sectors: the western sector includes the “Pico do Gaspar fissural

system”, with the Pico do Gaspar and Lombinhas spatter cones and the Arrochinha scoria cone; the former fed a pahoehoe-type basaltic lava flow that moved both to the north and south; it includes the Gruta do Natal lava cave among others (Fig. 3 and Table 1);

the eastern sector includes the scoria cones of Pico da Bagacina (hawaiite), Criação do Filipe/Achadas (mugearite) and “536”cone/Biscoito da Atalhada (basalt), whose aa-type lava flows where extruded to the south. To the north it is worth mentioning three important

episodes/eruptions: a) the extensive pahoehoe basaltic lava field emitted from

Pico Galiarte (also known as “Pico dos Pedreiros”), that flooded the flat areas near the axial part of the BFZ, but, mostly, moved towards the north up to Biscoitos village (Fig. 3) and probably reached the sea (at the time the coast line was about 0.5 km further south in the area of Biscoitos); this lava field includes many lava caves (Fig. 3 and Table 1), such as the Malha and the Chamusca lava caves, the most southern and northern caves in this lava field. Since this unit also includes the Balcões lava cave, the most important one in Terceira island (4,421 m total length – Costa et al., 2008), the volcanic unit composed by the Pico Galiarte scoria cone and the extensive pahoehoe basaltic lava field associated was named as “Malha-Balcões-Chamusca lava caves system”;

Fig. 2. Geological map of the Basaltic Fissural Zone (BFZ) in the central area of Terceira Island (north sector) and location of the main entrances (black circles) of the 26 associated caves. See figure 3 for legend. Fig. 2. Carta geológica da Zona Basáltica Fissural, na região central da ilha Terceira (sector norte) e localização da abertura principal (círculo a negro) das 26 cavidades vulcânicas associadas Legenda: ver figura3.

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Table 1. Location of the entrances of the volcanic caves and associated volcanic episode/eruption at the mapped area.

Tabela 1. Localização da entrada das cavidades vulcânicas existentes na

área cartografada e episódio/erupção vulcânica associada.

14C age of Pico Gordo by Calvert et al. (2006) and new Ar/Ar ages (this study)

Table 2. Geochemical composition of the samples, rock designation and associated volcanic episode/eruption. Analysis by Activation Lab., Ltd.

(Canada).

Tabela 2. Composição geoquímica das amostras, classificação da rocha e episódio/erupção vulcânica associada. Análises realizadas por Activation

Lab., Ltd. (Canadá).

b) the basaltic lava flows extruded from Pico Gordo scoria cone, that flowed towards the north for about 7 km, reaching the sea on the Biscoitos village area; this is the eruption that preceded the 1761 A.D. event in this sector of the Basaltic Fissural Zone; charcoal collected beneath this flow, near Biscoitos, yielded a 14C age of 4,480±40 y BP (Calvert et al., 2006), consistent with the local volcanostratigraphy and mapped outcrops;

c) the hawaiite lava flow erupted in 1761 A.D. – the “Mistério Velho dos Biscoitos” – as an aa-type lava flow with an average thickness of about 3 meters, strongly controlled by the paleomorphology of the area; the first phase of this eruption, of silicic nature, was also mapped, as the trachyte s.l. domes and

coulées of “Mistérios Negros” (“Va” in Fig. 3). It should be mentioned that historical records and descriptions of the 1761 A.D. eruption clear identify this two phases, either in time and geographical terms, either with detailed descriptions of the eruption characteristics (see for example Drummond, 1981).

Fig. 3. Geological map and volcanostratigraphy of the mapped area (see also table 1). 1-gravel and pebble beach deposits; 2- undifferentiated lapilli mantling; 3- old BFZ scoria cone (dotted); 4- old BFZ lava flows; 5- Achadas eruption (scoria deposits dotted); 6- Biscoito da Atalhada eruption (scoria deposits dotted); 7- caldera rim; I, II, III and IV- intermediate to young BFZ volcanic episodes; V- 1761 A.D. eruption: Va- Mistérios Negros trachyte domes and coulées; Vb- Mistério Velho dos Biscoitos hawaiite eruption; PA pp- Pico Alto Volcano pumice sequences, including ignimbrites; PA T- Pico Alto Volcano trachyte s.l. domes and coulées; PA b&a- Pico Alto Volcano block and ash flow deposit; SB pp- Santa Bárbara Volcano pumice sequences; SB B- Santa Barbara Volcano basaltic s.l. lava flows; SB T- Santa Barbara Volcano trachyte s.l. domes and coulées; GM- Guilherme Moniz Volcano pumice sequences. Main entrances of volcanic caves marked as black circles and other entrances marked as red circles. Fig. 3. Carta geológica e vulcano-estratigrafia da zona cartografada (ver também Tabela 1). Indicada a abertura principal (círculo a negro) e outras aberturas (círculo a vermelho) das cavidades vulcânicas.

Geological mapping of the central area of Terceira Island (Azores) 287

Besides these broadly speaking Holocene age episodes/eruptions, other basaltic s.l. eruptive centers were identified and mapped as “old BFZ” scoria deposits/lava flows (Fig. 3 and Table 1) – like the Pico Viana, Pico das Caldeirinhas, “592/Pico Gordo 2” and the “565” scoria cones – whose lava flows were buried by the younger pyroclasts and lava flows mentioned above.

As a final remark, it should be mentioned that at Alto da Caparica/Cancela area an old trachyte flow outcrop (“PA T” in Fig. 3) surrounded by the pahoehoe lava flows of the Malha-Balcões-Chamusca lava caves system, extruded from the Pico do Galiarte scoria cone. This kipuka, previously mapped by Lloyd & Collis (1981) as the “Biscoitos Formation”, belongs to the Pico Alto polygenetic silicic volcano and represents the older formation cropping out in the area, presently buried by the BFZ products.

Fig. 4. Classification of the samples on the TAS diagram, Na2O + K2O versus SiO2, expressed in %, after Le Bas et al. (1986). Fig. 4. Classificação das amostras com base no diagrama de TAS, Na2O + K2O versus SiO2, expressa em percentagem, segundo Le Bas et al. (1986).

Unlike the silicic formations related with Pico Alto Volcano, to the West of the Pico Gordo lava flows there is an extensive lava field of a hawaiitic compound pahoehoe lava flow, which is very distinctive, since it is highly porphyritic and containing abundant acicular phenocrystals of plagioclase (often 5-10 mm long). This

lava field extends westwards up to Altares village and Matias Simão spatter cone, forming the sea-cliffs up to Biscoitos harbor. This lava flow is attributed to Santa Barbara Volcano and is locally covered by the Pico Gordo lava flows or mantled by a widespread lapilli fall deposit (see Fig. 3). Up-hill, closer to the alignment of domes and coulées from the northeastern flank of Santa Bárbara Volcano, this flow is also buried by Santa Bárbara pumice deposits. As an attempt to better constrain the ages of the mapped volcanic episodes/eruptions, two samples from the areas close to Gruta dos Buracos (sample 06AZT15) and Gruta do Natal caves (sample 06AZT16) were analyzed for Ar/Ar dating, following the methodology and specifications indicated in Calvert et al., 2006. Although well constrained age determinations were not obtained for these samples, it seems reasonable to point out that (Fig. 5): Step heating of basalt sample 06AZT15 yielded a

plateau age of -2.4 ± 9.0 ka and an isochron age of -1.1 ± 18.9 ka (2σ); the sample is constrained by the limit of the plateau age as younger than 6,600 years and the limit of the isochron age as younger than 17,800 years;

Step heating of basalt sample 06AZT16 yielded a plateau age of -16.5 ± 15.4 ka and an isochron age of -11.8 ± 23.9 ka (2σ); the sample is constrained by the limit of the isochron age as younger than 12,100 years. In spite the 2σ errors associated with the obtained

ages and the limited isochron/plateau ages of the samples – in close relations with the youth of this lava flows – it seems reasonable to use these ages as indicative time intervals for the occurrence of the associated volcanic eruptions/episodes (see also Table 1).

Thus, an “Holocene age” (potentially younger than 6,600 y BP) can be assigned for the Malha-Balcões-Chamusca lava caves system (Pico Galiarte eruption) and an age of “younger than 12,100 y BP” can be assigned for the “Pico do Gaspar fissural system”, the later responsible for the formations of Gruta do Natal lava cave. Those assigned Ar/Ar ages are consistent and in good agreement with field data gathered from stratigraphical relationships among the geological formations and the detailed mapping of the outcrops.

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Fig. 5. Age spectra for samples 06AZT15 collected at Gruta dos Buracos (A) and 06AZT16 collected at Gruta do Natal (B) lava caves. Fig. 5. Espectro da idade das amostras 06AZT15, colhida na Gruta dos Buracos (A) e 06AZT16, colhida na Gruta do Natal (B).

4. Final notes

Geological mapping in the central northern area of Terceira Island revealed that:

1) Most volcanic caves in the area are associated with the Malha-Balcões-Chamusca lava caves system, related with the lava flows of Pico Galiarte eruption;

2) The 1761 A.D. hawaiite lava flow does not incorporate any volcanic cave; lava-tube caves mapped in its vicinity are related to underlying lavas and thus, occasionally have some mapped branches underneath the 1761 A.D. hawaiite lava flow.

The Basaltic Fissural Zone in the central area of Terceira is a relatively thin volcanic unit (probably just of a few tens of meters thick), whose lava flows occupied a topographically depressed area with a general N-S trend, at the intersection of Santa Bárbara Volcano, to the West, and Pico Alto and Guilherme Moniz polygenetic volcanoes, to the East. This can be inferred from the field relations with widely dispersed hawaiite lava flows from Santa Barbara Volcano, cropping out to the west of Biscoitos harbor, and with the Alto da Caparica trachyte kipuka, presently surrounded by the Pico Galiarte pahoehoe lava flows.

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