Geological Mapping of the Central Area of Terceira Island

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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

Abstract: The detailed geological mapping of the Basaltic Fissural Zone in the central area of Terceira Island, the field stratigraphical 1Universidade dos Açores, Departamento de Geociências, Apartado 1422, relationships observed and published and new isotopic ages now 9501-801 Ponta Delgada, Açores, Portugal. 2 provided allowed to correlate the 26 volcanic caves existing in the GESPEA – “Grupo de Estudo do Património Espeleológico dos Açores”, mapped area (between Pico da Bagacina area and the north coast) Edifício Matos Souto, Piedade, 9930-210 Lajes do Pico, Açores, Portugal. 3 with the correlated volcanic episodes/eruptions. United States Geological Survey, 345 Middlefield Road MS 977, Menlo Park, The main conclusions obtained include: i) the association of a 10 California 94025, USA. 4 volcanic caves system (including the lava-tube caves of Malha, Associação “Os Montanheiros”, Rua da Rocha, nº 8, 9700-169 Angra do Balcões and Chamusca) with Holocene (potentially younger than Heroísmo, Açores, Portugal. 5 6,600 years BP) pahoehoe lava flows extruded from Pico Galiarte Departamento de Geologia e Centro de Geologia, FCUL, Universidade de scoria cone; ii) the association of 8 volcanic caves (namely Natal Lisboa, Edifício C6, Campo Grande, 1749-016 Lisboa, Portugal. * cave) with the eruptive episode associated with the “Pico do Gaspar Corresponding author / Autor correspondente: [email protected] 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 1. Introduction the late eruption In the mapped area. The Azores Archipelago is located at the triple junction Keywords: Geological mapping, Volcanic caves, Azores islands, between the North American, Eurasian and African plates. Ages, Eruptive history. The first two are separated by the Mid-Atlantic Ridge Resumo: A cartografia geológica de detalhe da região central da ilha located between Flores and Faial islands, while Eurasian Terceira integrada na Zona Basáltica Fissural, as relações and African plates are separated by a complex tectonic estratigráficas observadas in situ, idades isotópicas publicadas e structure, the Azores-Gibraltar Plate Boundary, that in the novas datações agora apresentadas, permitiram correlacionar as 26 area of the archipelago comprises a wide shear zone (e.g. the cavidades vulcânicas existentes nesta região (e.g. entre a zona do Azores block, Luis et al., 1994), with a rough elongated Pico da Bagacina e o litoral norte) com os episódios/erupções vulcânicas que estiveram na sua génese. triangular shape. The northern edge of this shear zone is De entre os principais resultados merece especial destaque: i) a established by the Terceira Rift, a WNW-ESE hyper-slow associação de um sistema de 10 cavidades vulcânicas (onde se spreading centre (Vogt & Jung, 2004) incorporating incluem as grutas da Malha, dos Balcões e da Chamusca) a escoadas Graciosa, Terceira and S. Miguel islands. lávicas pahoehoe emitidas pelo Pico Galiarte, de idade holocénica Terceira Island, together with Graciosa, São Jorge, Pico (potencialmente de idade inferior a 6.600 anos BP); ii) a associação and Faial islands form the Central Group of the de 8 cavidades (de que faz parte a Gruta do Natal) ao episódio eruptivo associado ao “sistema fissural do Pico Gaspar”, de idade Archipelago. The island presents active volcanism (with a inferior a 12.100 anos BP; iii) a cartografia das escoadas lávicas wide variety of rocks compositions and structures) and an associadas à erupção do cone de escórias do Pico Gordo (que important seismic activity. Terceira Island includes four originou 4 cavidades vulcânicas), datada de há cerca de 4.480 anos central polygenetic volcanoes with caldera (Cinco Picos- BP, a qual antecede a erupção histórica do ano de 1761 A.D., que Serra do Cume, Guilherme Moniz, Pico Alto and Santa corresponde à erupção mais recente ocorrida na zona cartografada. Bárbara, from east to west) and a Basaltic Fissural Zone Palavras-chave: Cartografia geológica, Cavidades vulcânicas, Ilhas dos Açores, Idades, História eruptiva. 284 J. C. Nunes et al. / Comunicações Geológicas (2014) 101, Especial I, 283-288

(Nunes, 2000), mainly in the central and SE parts of the composition), scoria and spatter deposits, surtseyan tuffs island (Fig. 1). and basaltic lava flows. Unlike other islands of the Volcanic products extruded from these central archipelago, Terceira volcanism main feature is the volcanoes, mostly in Quaternary times include ignimbrites, presence of extensive effusive episodes of silicic nature pumice fall deposits, strongly evolved silicic lava flows (as (domes and coulées, 40-90 m thick) which represent about domes and coulées of comendite to pantellerite 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 Given its volcanic origin and the dominant basaltic century) three volcanic eruptions took place: the first one in nature of its eruptions, Azores archipelago presents a wide 1761, the second in 1867 and the last one on the period diversity of volcanic caves in all but Corvo Island. 1998-2001. The 1761 eruption occurred on-land, east of Presently, 272 natural caves are known in the Azores Santa Bárbara Volcano (with two distinct episodes and Islands, corresponding to about 32 kilometres of locations), while the later two were located at sea, NW of underground paths, with peculiar geologic features and live the island (about 2.5 km and 8.5 km offshore, respectively). forms (Costa et al., 2008). These caves are of different The Basaltic Fissural Zone is developed on two main types: lava-tube caves, volcanic pits, fractures and erosion sectors of the Terceira Island: i) in the central part of the caves, including the combination of these structures. Most island, namely along the Pico da Bagacina-Pico do Gaspar caves are located in the islands of Pico with 129, Terceira volcano-tectonic lineament, and, ii) in the SE part of the and S. Miguel islands, with 69 and 28 caves respectively. island including the S. Sebastião-Porto Martins and the About 70% of the Terceira island caves are located in Cinco Picos areas (Fig. 1). The associated volcanism is the Basaltic Fissural Zone, thus the importance to map the strongly controlled by NW-SE to W-E fractures, is almost geological formations with which the caves are associated. exclusively of the Hawaiian to Strombolian style, and was Since the area of São Sebastião-Porto Martins was already responsible for the building up of about 75 monogenetic mapped (cf. Montesinos et al., 2003) – where caves such as volcanoes, mostly scoria cones and eruptive fissures, and Gruta da Madre de Deus, Gruta de Santo António and emission of aa and pahoehoe lava flows. 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  the eastern sector includes the scoria cones of Pico da present work were to obtain a detailed geological map of the Bagacina (hawaiite), Criação do Filipe/Achadas central sector of the Basaltic Fissural Zone and establish the (mugearite) and “536”cone/Biscoito da Atalhada (basalt), local volcanostratigraphy, allowing correlations with the whose aa-type lava flows where extruded to the south. volcanic caves emplaced in that area. To the north it is worth mentioning three important episodes/eruptions: 2. Geological mapping and volcanostratigraphy a) the extensive pahoehoe basaltic lava field emitted from Pico Galiarte (also known as “Pico dos Pedreiros”), Detailed geological mapping was carried out on the north that flooded the flat areas near the axial part of the part of central sector of the Basaltic Fissural Zone (Fig. 1) BFZ, but, mostly, moved towards the north up to since the known volcanic caves are located in this area. Biscoitos village (Fig. 3) and probably reached the sea The field work was done in the period 2009-2011 and (at the time the coast line was about 0.5 km further included the establishment of the detailed south in the area of Biscoitos); this lava field includes volcanostratigraphy of the area, the nature of its many lava caves (Fig. 3 and Table 1), such as the volcanism, the eruptive styles and the mapping of the Malha and the Chamusca lava caves, the most southern major tectonic lineaments and fractures. Geochemical and northern caves in this lava field. Since this unit analyses of collected samples were performed at also includes the Balcões lava cave, the most important Activation Laboratories, Ltd., in Canada. one in Terceira island (4,421 m total length – Costa et The obtained geologic map is presented in figure 2, al., 2008), the volcanic unit composed by the Pico which includes the location of the main entrance of the 26 Galiarte scoria cone and the extensive pahoehoe volcanic caves and pits existing in the area. The mapped basaltic lava field associated was named as “Malha- area is limited to the W by the volcanism associated with Balcões-Chamusca lava caves system”; 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 Fig. 2. Geological map of the Basaltic Fissural Zone (BFZ) in the central different sectors: area of Terceira Island (north sector) and location of the main entrances  the western sector includes the “Pico do Gaspar fissural (black circles) of the 26 associated caves. See figure 3 for legend. system”, with the Pico do Gaspar and Lombinhas spatter Fig. 2. Carta geológica da Zona Basáltica Fissural, na região central da cones and the Arrochinha scoria cone; the former fed a ilha Terceira (sector norte) e localização da abertura principal (círculo a pahoehoe-type basaltic lava flow that moved both to the negro) das 26 cavidades vulcânicas associadas Legenda: ver figura3. north and south; it includes the Gruta do Natal lava cave among others (Fig. 3 and Table 1); 286 J. C. Nunes et al. / Comunicações Geológicas (2014) 101, Especial I, 283-288

Table 1. Location of the entrances of the volcanic caves and associated coulées of “Mistérios Negros” (“Va” in Fig. 3). It volcanic episode/eruption at the mapped area. should be mentioned that historical records and

Tabela 1. Localização da entrada das cavidades vulcânicas existentes na descriptions of the 1761 A.D. eruption clear identify área cartografada e episódio/erupção vulcânica associada. this two phases, either in time and geographical terms, either with detailed descriptions of the eruption characteristics (see for example Drummond, 1981).

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).

Fig. 3. Geological map and volcanostratigraphy of the mapped area (see also table 1). 1-gravel and pebble beach deposits; 2- undifferentiated b) the basaltic lava flows extruded from Pico Gordo lapilli mantling; 3- old BFZ scoria cone (dotted); 4- old BFZ lava flows; scoria cone, that flowed towards the north for about 7 5- Achadas eruption (scoria deposits dotted); 6- Biscoito da Atalhada km, reaching the sea on the Biscoitos village area; this eruption (scoria deposits dotted); 7- caldera rim; I, II, III and IV- is the eruption that preceded the 1761 A.D. event in intermediate to young BFZ volcanic episodes; V- 1761 A.D. eruption: Va- Mistérios Negros trachyte domes and coulées; Vb- Mistério Velho this sector of the Basaltic Fissural Zone; charcoal dos Biscoitos hawaiite eruption; PA pp- Pico Alto Volcano pumice collected beneath this flow, near Biscoitos, yielded a sequences, including ignimbrites; PA T- Pico Alto Volcano trachyte s.l. 14C age of 4,480±40 y BP (Calvert et al., 2006), domes and coulées; PA b&a- Pico Alto Volcano block and ash flow consistent with the local volcanostratigraphy and deposit; SB pp- Santa Bárbara Volcano pumice sequences; SB B- Santa Barbara Volcano basaltic s.l. lava flows; SB T- Santa Barbara Volcano mapped outcrops; trachyte s.l. domes and coulées; GM- Guilherme Moniz Volcano pumice c) the hawaiite lava flow erupted in 1761 A.D. – the sequences. Main entrances of volcanic caves marked as black circles and “Mistério Velho dos Biscoitos” – as an aa-type lava other entrances marked as red circles. flow with an average thickness of about 3 meters, Fig. 3. Carta geológica e vulcano-estratigrafia da zona cartografada (ver strongly controlled by the paleomorphology of the também Tabela 1). Indicada a abertura principal (círculo a negro) e outras area; the first phase of this eruption, of silicic nature, aberturas (círculo a vermelho) das cavidades vulcânicas. was also mapped, as the trachyte s.l. domes and Geological mapping of the central area of Terceira Island (Azores) 287

Besides these broadly speaking Holocene age lava field extends westwards up to Altares village and episodes/eruptions, other basaltic s.l. eruptive centers were Matias Simão spatter cone, forming the sea-cliffs up to identified and mapped as “old BFZ” scoria deposits/lava Biscoitos harbor. This lava flow is attributed to Santa flows (Fig. 3 and Table 1) – like the Pico Viana, Pico das Barbara Volcano and is locally covered by the Pico Caldeirinhas, “592/Pico Gordo 2” and the “565” scoria Gordo lava flows or mantled by a widespread lapilli fall cones – whose lava flows were buried by the younger deposit (see Fig. 3). Up-hill, closer to the alignment of pyroclasts and lava flows mentioned above. domes and coulées from the northeastern flank of Santa As a final remark, it should be mentioned that at Alto da Bárbara Volcano, this flow is also buried by Santa Caparica/Cancela area an old trachyte flow outcrop (“PA T” Bárbara pumice deposits. in Fig. 3) surrounded by the pahoehoe lava flows of the As an attempt to better constrain the ages of the mapped Malha-Balcões-Chamusca lava caves system, extruded from volcanic episodes/eruptions, two samples from the areas the Pico do Galiarte scoria cone. This kipuka, previously close to Gruta dos Buracos (sample 06AZT15) and Gruta mapped by Lloyd & Collis (1981) as the “Biscoitos do Natal caves (sample 06AZT16) were analyzed for Formation”, belongs to the Pico Alto polygenetic silicic Ar/Ar dating, following the methodology and volcano and represents the older formation cropping out in specifications indicated in Calvert et al., 2006. Although well the area, presently buried by the BFZ products. 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

Fig. 4. Classification of the samples on the TAS diagram, Na2O + K2O than 6,600 y BP) can be assigned for the Malha- versus SiO2, expressed in %, after Le Bas et al. (1986). Balcões-Chamusca lava caves system (Pico Galiarte eruption) and an age of “younger than Fig. 4. Classificação das amostras com base no diagrama de TAS, Na2O 12,100 y BP” can be assigned for the “Pico do + K2O versus SiO2, expressa em percentagem, segundo Le Bas et al. (1986). Gaspar fissural system”, the later responsible for the formations of Gruta do Natal lava cave. Those Unlike the silicic formations related with Pico Alto assigned Ar/Ar ages are consistent and in good Volcano, to the West of the Pico Gordo lava flows there agreement with field data gathered from is an extensive lava field of a hawaiitic compound stratigraphical relationships among the geological pahoehoe lava flow, which is very distinctive, since it is formations and the detailed mapping of the highly porphyritic and containing abundant acicular outcrops. phenocrystals of plagioclase (often 5-10 mm long). This 288 J. C. Nunes et al. / Comunicações Geológicas (2014) 101, Especial I, 283-288

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 Research, 156, 103-115. Costa, M.P., Nunes, J.C., Constância, J.P., Borges, P.A.V., Geological mapping in the central northern area of Terceira Barcelos, P., Pereira, F., Farinha, N., Góis, J., 2008. Cavidades Island revealed that: Vulcânicas dos Açores. Amigos dos Açores/Os 1) Most volcanic caves in the area are associated with Montanheiros/GESPEA (Ed.), 48 p. ISBN: 978-989-95432-2- the Malha-Balcões-Chamusca lava caves system, related with 5. the lava flows of Pico Galiarte eruption; Drummond, F.F., 1981. Anais da Ilha Terceira. Secretaria 2) The 1761 A.D. hawaiite lava flow does not Regional da Educação e Cultura (Ed.), Angra do Heroísmo, vol. II, 288-290. incorporate any volcanic cave; lava-tube caves mapped in its Le Bas, M., Le Maitre, R., Streckeisen, A., Zanettin, B., 1986. A vicinity are related to underlying lavas and thus, occasionally chemical classification of volcanic rocks based on the total have some mapped branches underneath the 1761 A.D. alkali-silica diagram. Journal of Petrology, 27, 745-750. hawaiite lava flow. Lloyd, E.F., Collis, S.K., 1981. Geothermal prospection – Ilha The Basaltic Fissural Zone in the central area of Terceira Terceira, Açores. Geological report, SRCI-LGT Report, Ponta is a relatively thin volcanic unit (probably just of a few tens of Delgada, 96 p. meters thick), whose lava flows occupied a topographically Luis, J.F., Miranda, J.M., Galdeano, A., Patriat, P., Rossignol, depressed area with a general N-S trend, at the intersection of J.C., Victor, L.A.M., 1994. The Azores triple junction since 10 Santa Bárbara Volcano, to the West, and Pico Alto and Ma from an aeromagnetic survey of the Mid-Atlantic Ridge, Earth and Planetary Science Letters, 125, 439-459. Guilherme Moniz polygenetic volcanoes, to the East. This can Montesinos, F.G., Camacho, A.G., Nunes, J.C., Oliveira, C.S., be inferred from the field relations with widely dispersed Vieira, R., 2003. A 3-D gravity model for a volcanic crater in hawaiite lava flows from Santa Barbara Volcano, cropping Terceira Island (Azores). Geophysical Journal International, out to the west of Biscoitos harbor, and with the Alto da 154(2), 393-406. Caparica trachyte kipuka, presently surrounded by the Pico Nunes, J.C., 2000. Notas sobre a geologia da Ilha Terceira Galiarte pahoehoe lava flows. (Açores). Açoreana, 9(2), 205-215. Self, S., 1976. The recent volcanology of Terceira, Azores. References Journal of the Geological Society of London, 132, 645-666. Vogt, P.R., Jung, W.Y., 2004. The Terceira Rift as hyper-slow, Calvert, T.A., Moore, R.B., Mcgeehin, J.P., da Silva, A.M.R., 2006. 40 39 14 hotspot-dominated oblique spreading axis: A comparison with Volcanic history and Ar/ Ar and C geochronology of Terceira other slow spreading plate boundaries. Earth and Planetary Island, Azores, Portugal. Journal of Volcanology and Geothermal Science Letters, 218, 77-90.