The Geological Society of AmericaSpecial Paper 464
2010
Ignimbrite stratigraphy and chronology on Terceira Island, Azores
Ralf Gertisser*Earth Sciences and Geography, School of Physical and Geographical Sciences, Keele University, Keele, ST5 5BG, UK
Stephen SelfDepartment of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
João L. GasparCentro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, 9501-801 Ponta Delgada, Açores, Portugal
Simon P. KelleyDepartment of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Adriano PimentelCentro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, 9501-801 Ponta Delgada, Açores, Portugal
Jost EikenbergDivision for Radiation Protection and Safety, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Tiffany L. BarryDepartment of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
José M. PachecoGabriela Queiroz
Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, 9501-801 Ponta Delgada, Açores, Portugal
Marika VespaMineralogy and Environments, LGCA, Université Joseph Fourier and Centre National de la Recherche Scientifi que,
38041 Grenoble Cedex 9, France
ABSTRACT
Ignimbrites are relatively rare on mid-oceanic volcanic islands, but they occur on at least three of the nine Azorean islands—São Miguel, Faial, and Terceira. Ignim-brites on Terceira, which are mostly comenditic trachyte in composition, constitute a signifi cant portion of the island’s volcanic stratigraphy exposed primarily in cliff sec-tions along the southern and northern coastlines. The base of the most recent group of deposits, called the Upper Terceira Group, is marked by the Lajes-Angra Ignimbrite,
Ignimbrites constitute signifi cant products of the activity of volcanic islands related to subduction zones, but they are com-parably rare on volcanic islands in mid-oceanic intraplate and rift settings. On Atlantic island volcanoes, ignimbrites are not uncommon, probably due the longevity of the volcanic centers and the consequent production of felsic magmas in shallow-level magma chambers. Ignimbrites have been reported from Iceland, the Canary Islands, and the Cape Verde Islands, amongst oth-ers. In the Azores archipelago (Fig. 1), ignimbrites occur on at least three of the nine volcanic islands—Terceira, Faial, and São Miguel (Self, 1974, 1976; Duncan et al., 1999; Pacheco, 2001). Pyroclastic density current deposits are also reported from the island of Graciosa (Gaspar, 1996). While relatively minor in volume on São Miguel and Faial compared with other erup-tion products, ignimbrites make up a signifi cant portion of the exposed volcanic products of the neighboring island of Terceira. The island could be especially prone to future ignimbrite- forming eruptions, which might pose a major hazard to its infrastructure and population because of its limited surface area (400 km2).
On Terceira, ignimbrites are known to occur at various stratigraphic horizons (Self, 1974, 1976). The Lajes-Angra Ignimbrite is the most recent deposit in a history of ignimbrite-forming eruptive episodes. This event produced a widespread, low-aspect-ratio ignimbrite that covers all but the western part of the island (Self, 1971). Deposits of older ignimbrite- and other pyroclastic density current–forming eruptions, such as a spat-ter fl ow and a block-and-ash fl ow, have been identifi ed in the sequences of deposits exposed almost exclusively in cliff sec-tions on the south and north coasts of Terceira, and in some iso-lated inland exposures.
Establishing a precise history of ignimbrite-forming erup-tions on Terceira has been challenging. Complicating factors are (1) the widely scattered nature of the exposures of the older ignimbrites, (2) rapid facies changes in the deposits in response
to deposition on a highly undulating topography, (3) the rela-tively small on-land volume and often limited lateral extent of the outcrops combined with subsequent erosion and burial, and (4) the lack of deep dissection by streams across the island. Con-sequently, correlating the various ignimbrites inland from coastal sections has been diffi cult. Similar lithologies, petrography, and chemical composition from one ignimbrite to the next further compound the uncertainties of correlation. Determining absolute eruption ages has also proven to be challenging due to the rela-tively young ages of the ignimbrites, which extend beyond the ca. 50 ka limit of reliable radiocarbon dating. We addressed this by using 40Ar/39Ar age determinations on anorthoclase crystals separated from pumice clasts within the ignimbrites.
The main purpose of this study is to distinguish between the various ignimbrites found on Terceira, establish a stratigraphy and chronology of ignimbrite-forming eruptions, and use this information to assess the potential for future ignimbrite erup-tions on the island. To make the correlations both within and between composite stratigraphic sections, we used stratigraphic position, physical characteristics of ignimbrite units, and dis-tinctive marker horizons, as well as the sequence of deposits within and between ignimbrite formations. Failing these meth-ods, geochemical compositions of juvenile clasts from the ignimbrites, and radiocarbon and radiometric (40Ar/39Ar) dating methods were used to aid correlations. This information also allows us to place time constraints on the stratigraphic frame-work for pyroclastic fl ow–forming eruptions and, indirectly, other eruptive activity on Terceira.
TECTONIC SETTING
The Azores archipelago is formed by nine inhabited vol-canic islands that emerge from a shallow oceanic platform, the Azores Plateau (Needham and Francheteau, 1974), in the cen-tral North Atlantic between latitude 37°N and 40°N and longi-tude 25°W and 32°W (Fig. 1). Its complex geological setting is
an extensive ignimbrite sheet produced during the latest ignimbrite-forming episode ~20–23 k.y. ago. The Lower Terceira Group consists of ignimbrites and interstratifi ed pyroclastic fall deposits and lava fl ows predating the Lajes-Angra Ignimbrite. Includ-ing the Lajes-Angra Ignimbrite, the exposed volcanic succession on Terceira includes at least seven formations containing ignimbrites plus two isolated exposures of other pyroclastic density current deposits (a spatter fl ow and a block-and- ash-fl ow–like deposit). Most of these are from Pico Alto or, possibly, Guilherme Moniz, the middle pair of central volcanoes of the four that form Terceira. Radiocarbon ages comple-mented by 40Ar/39Ar age determinations on anorthoclase crystals from several ignim-brites suggest a narrow period of ignimbrite-forming volcanism on Terceira dating from ca. 86 ka to ca. 20–23 ka, which followed a period of predominantly effusive basaltic to trachytic/rhyolitic activity. However, a recurrence of ignimbrite volcanism on the island cannot be completely discounted. The results from this study also indicate that the 40Ar/39Ar method can be successfully applied to date anorthoclase crystals in volcanic rocks younger than 100 ka from Terceira, although it does not provide the same precision as radiocarbon dating at the younger age range of these volcanic rocks.
Ignimbrite stratigraphy and chronology on Terceira Island 135
characterized by a mantle plume at the junction of the American, Eurasian, and African plates (White et al., 1976; Searle, 1980; Madeira and Ribeiro, 1990; McKenzie and O’Nions, 1995). The three main tectonic features in the area are (1) the Mid-Atlantic Ridge, which crosses the archipelago between the islands of the Western and Central Groups, (2) the East Azores fracture zone, which extends broadly E-W from the Mid-Atlantic Ridge to the
Strait of Gibraltar, and (3) the Terceira Rift, which trends NW-SE from the Mid-Atlantic Ridge through the northern islands of the Central Group (Graciosa, Terceira) to São Miguel in the Eastern Group (Machado, 1959; Krause and Watkins, 1970). Fracture systems of the islands of São Jorge, Faial, and Pico in the Cen-tral Group have a general WNW–ESE trend (Agostinho, 1931) and may be related to the slow-spreading Terceira Rift (Vogt and
600 km
Azores
Madeira
Canary Islands
SPAINP
OR
TU
GA
L
FRANCE
ALGERIAMOROCCO
North AtlanticOcean
30°W 28° 26°
40°N
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S. Jorge
Graciosa
Terceira
S. Miguel
Santa Maria
Corvo
FloresWestern Group
Central Group
Eastern Group
Mid
-Atla
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EURASIAN PLATE
AMERICANPLATE
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5 km
38° 45'N38° 40'N
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Pico Alto
G. MonizSanta Bárbara
Biscoitos
Q. Ribeiras
Vila Nova
Praia da Vitória
Porto JudeuAngra do Heroísmo
São Mateus
Lajes
Serreta
Caldeira
Rift zone
Lajes graben
Cinco Picos
A
C
B
Terceira Rift
S1
S2 S3S4
S5-6
S7
N1N2 N3
N4N7N5
N6
N8-9N10
Figure 1. (A) Location map of the Azores archipelago in the central North Atlantic Ocean. (B) Tectonic setting and main tectonic features of the Azores at the triple junction between the American, Eurasian, and African lithospheric plates. (C) Shaded relief map of Terceira showing the four central volcanoes Cinco Picos, Guilherme Moniz, Pico Alto and Santa Bárbara truncated by calderas, the rift zone that transects the island from NW to SE (dashed line), the Lajes graben, as well as major towns and villages (white squares). Locations of stratigraphic sections along the south coast (S1–7) and north coast (N1–10) discussed in this paper are also indicated.
136 Gertisser et al.
Jung, 2004), along which slow but persistent rates of magma pro-duction have occurred (Fig. 1).
The nine Azores islands are all of volcanic origin and are predominantly composed of alkali basalts and more differenti-ated products, which, on Terceira, include comenditic and pan-telleritic lava domes and fl ows as well as pyroclastic deposits (e.g., Schmincke and Weibel, 1972; Self and Gunn, 1976; White et al., 1979; Mungall and Martin, 1995). Since settlement in the fi fteenth century, about 30 eruptions have taken place, the most recent of which was a submarine eruption that occurred off the western coast of Terceira in 1998–2001 (Gaspar et al., 2003).
GEOLOGIC EVOLUTION AND VOLCANIC HISTORY OF TERCEIRA
The island of Terceira consists of four large central volca-noes (Pico Alto, Santa Bárbara, Guilherme Moniz, and Cinco Picos) grouped astride a basaltic rift zone that transects the island from NW to SE (Fig. 1). The base of the most recent volcanic group, called the Upper Terceira Group (Self, 1976), is marked by an extensive ignimbrite, herein referred to as the Lajes-Angra Ignimbrite Formation, which provides a convenient stratigraphic reference horizon previously dated at between 18,600 ± 650 and 23,100 ± 350 14C yr B.P.
The Upper Terceira Group consists of the products of at least 116 separate eruptions from the active Santa Bárbara and Pico Alto volcanoes with contemporaneous volcanic activity along the rift zone (Self, 1974, 1976). Santa Bárbara volcano has the most youthful morphology and occupies the western part of the island. It appears to be younger than the volcanoes to the SE, but deposits of its post–20 ka activity are interstrati-fi ed with those of the other active volcano, Pico Alto, which lies immediately north of the rift zone in the central part of the island. Pico Alto is a chaotic assemblage of comenditic and pantelleritic lava domes and coulées largely infi lling a young and hitherto undated caldera.
The rift zone is a 2-km-wide concentration of scoria cones, spatter rings, collapse pits, and lava fl ow vents extending from Serreta in the NW part of Terceira, through Santa Bárbara, Guil-herme Moniz, and Cinco Picos volcanoes, to the SE part of the island east of Porto Judeu (Fig. 1). Most basaltic eruptions, including the island’s most recent lava extrusion in 1761 and the 1998–2001 submarine ridge eruption 10 km W of Terceira, have occurred along the rift zone (Zbyszewski, 1966; Rosenbaum, 1974; Self, 1976; Gaspar et al., 2003). The two extinct volcanoes have concealed and partially destroyed caldera rims: the younger Guilherme Moniz caldera, which may be the older part of the same volcanic system as Pico Alto, exposes comendite domes and coulées up to 150 m thick in its walls, and the caldera fl oor is covered by young basalts extruded along the adjacent rift zone. The older caldera, Cinco Picos, is the largest in the Azores, with a topographic diameter (rim-to-rim) of ~7 km, but this may have been widened by rifting. Exposures on the caldera rim are mainly hawaiite and mugearite with minor fl ows of comenditic trachyte.
Numerous fl ows and domes of more viscous lavas (mugearites to pantellerites) are scattered on the fl anks of all the central vol-canoes.
Despite a number of radiocarbon and radiometric age dates (Zbyszewski et al., 1971; Self, 1976; White et al., 1976; Féraud et al., 1980; Lloyd and Collis, 1981; Calvert et al., 2006), the exact age and timing of volcanism on Terceira and the chronology of ignimbrite-forming eruptions prior to Lajes-Angra Ignimbrite are still poorly constrained. Recently published 40Ar/39Ar ages sug-gest that Cinco Picos caldera, the oldest volcanic structure on Terceira, formed prior to ca. 370–388 ka (Calvert et al., 2006). These ages are in broad agreement with K/Ar ages determined by Féraud et al. (1980), but they contrast with earlier ages that ranged from 0.75 to 3.50 Ma (White et al., 1976).
STRATIGRAPHY AND CHRONOLOGY OF IGNIMBRITES ON TERCEIRA
The Lajes-Angra Ignimbrite is the latest in a history of pyro-clastic fl ow–forming eruptions from Pico Alto volcano, and, pos-sibly, from the extinct volcanoes Guilherme Moniz and Cinco Picos. Including the Lajes-Angra Ignimbrite, we identifi ed seven distinct ignimbrite formations and two other pyroclastic density current deposits on Terceira, and next we discuss the salient fea-tures of each. The pattern of ignimbrite-forming activity appears to be spasmodic, with occasional eruptive episodes that produce more than one ignimbrite unit within a short time interval inter-spersed by longer intervals during which pyroclastic fall deposits dominate, as discussed later.
The ignimbrites are mainly found along the sea cliffs on the south and north coasts of Terceira and inland within 2–3 km of the coast (Figs. 2 and 3). We derive most of our information for the ignimbrite stratigraphy from these cliff sections, which pro-vide well-defi ned, but discontinuous sequences of ignimbrites and other pyroclastic deposits that alternate with lava fl ows and reworked deposits (Figs. 4 and 5). Only a few isolated exposures of ignimbrites occur in the interior of the island where younger units of the Upper Terceira Group and thick vegetation cover them. Several important exposures for the pre–Lajes-Angra Ignimbrite volcanic history of the island, located around the har-bor area of the main town of Angra do Heroísmo (Fig. 1), have been progressively destroyed or hidden over the past 30 years by harbor (and other) development. The latest development in 2004 involved the complete cementing-over of a 25-m-high by 200-m-long exposure containing all but one of the ignimbrites exposed along the south coast, the only exposure displaying such a complete succession of the island’s volcanic history (UTM: 481267E, 4278298N). Thus, in this paper, when some ignim-brites are described as exposed, we refer, unfortunately, to past exposures that no longer exist.
All ignimbrites predating the Lajes-Angra Ignimbrite are here informally named, broadly following (as far as new devel-opments permit) the names used by Self (1974). Each ignimbrite is bounded by unconformities and thus has formation status.
Ignimbrite stratigraphy and chronology on Terceira Island 137
E
C
A B
D
F
LAI (Lajes)
Weldedbasal layer
CCI
Monte Brasil Tuff
VFI
LAI (Angra)
Monte Brasil Tuff
VFI
CCI
LAI (Angra)
GVI
LMI (Linhares)
Tuffs/soils (undiff.)
Pre-LAI ignimbrites(VFI, CCI)
Basalt lava flow
Basalt lava flow
Trachyte lava flow
Trachyte lava flow
Tuffs/soils (undiff.)
Tuffs/soils (undiff.)
Monte Brasil Tuff
Trachyte lava flow
1 m
5 m
5 m
10 m
5 m
Weldedbasal layer
Figure 2. Field characteristics of ignimbrites exposed along cliff sections and near the south coast of Terceira. (A) The Lajes member of the Lajes-Angra Ignimbrite (LAI) at São Mateus. Note welded basal layer and underlying basalt lava fl ow. (B) Baía do Fanal west of Angra do Heroísmo, showing superposition of the Caldeira-Castelinho Ignimbrite (CCI) and the Vila Nova–Fanal Ignimbrite (VFI). (C) Section at Angra Harbor (now cemented over) showing Lajes-Angra Ignimbrite, Vila Nova–Fanal Ignimbrite, and Caldeira-Castelinho Ignimbrite be-low the Monte Brasil Tuff. (D) Angra part of the Lajes-Angra Ignimbrite in a cliff section east of Angra do Heroísmo. (E) Grota do Vale Ig-nimbrite (GVI) at the base of the cliffs east of Angra do Heroísmo. Person for scale. (F) Linhares member of the Linhares-Matela Ignimbrite (LMI) at Linhares quarry north of Angra do Heroísmo. Tuffs/soils (undiff.)—sequences of undifferentiated ash deposits (see also Fig. 3).
138 Gertisser et al.A B
E F G
C D
LAI (Lajes)
LAI (Lajes)
VFI
CCI
Ig-i
CCI
LAI (Lajes)
LAI (Lajes)
VFI
CCI
QR
VFI(pyr. sequence)
PNI
CCI(debris-flow deposit)
CCI
Tuffs/soils (undiff.)
Basalt lava flow
Trachyte lava flow
Basalt lava flow
Basalt lava flow
Tuffs/soils (undiff.)
Tuffs/soils (undiff.)
Basaltlava flow
Tuffs/soils (undiff.)
LAI
VFI(pyr. sequence)
5 m
10 m 2.5 m
2 m5 m
1 m
Figure 3. Field characteristics of ignimbrites exposed along coastal cliffs and sections near the north coast of Terceira. (A) The Lajes part of the Lajes-Angra Ignimbrite (LAI) with its characteristic two depositional units (cf. inset) near the Lajes Air Base. Hammer (inset) for scale. (B) Superposition of the Lajes part of the Lajes-Angra Ignimbrite above the Caldeira-Castelinho Ignimbrite (CCI), and Ignimbrite i (Ig-i) west of Caldeira. (C) Cliff section at Porto da Vila Nova showing superposition of Caldeira-Castelinho Ignimbrite, Vila Nova part of the Vila Nova–Fanal Ignimbrite (VFI) and Lajes-Angra Ignimbrite. (D) Cliff section at Escaleiras showing two fl ow units of the Caldeira-Castelinho Ignimbrite overlain by the Vila Nova–Fanal Ignimbrite and erosional remnants of the Lajes-Angra Ignimbrite. (E) Debris-fl ow deposit within the Caldeira-Castelinho Ignimbrite formation in a cliff section west of Escaleiras. (F) Cliff section east of Alagoa exposing the Pedras Negras Ignimbrite (PNI) at the base of the cliff and overlying pyroclastic sequence of the Vila Nova–Fanal Ignimbrite Formation. (G) Quatro Ribeiras pyroclastic fl ow deposit (QR) near the north coast east of Quatro Ribeiras.
Ignimbrite stratigraphy and chronology on Terceira Island 139
Section S4Old cliffs at Angra Harbor
(now cemented over)UTM: 481267 E / 4278298 N
Section S5Coast at Vale do Linhares
UTM: 482001 E / 4278707 N
Section S6Grota do Vale
UTM: 482411 E / 4278668 N
Section S7Porto Judeu
UTM: 490032 E / 4277872 N
Section S1E of São Bartolomeu
UTM: 474429 E / 4279085 N
Section S2São Mateus
UTM: 476133 E / 4278534 N
Section S3Baía do Fanal
UTM: 480213 E / 4278717 N
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o o o o o o o o o o o o o o o o o o o o o o o o o o oo o oo o o o o o o
Figure 4. Stratigraphic sections made from cliff exposures along the south coast of Terceira, showing ignimbrites and other volcanic units. Ab-breviations: LAI—Lajes-Angra Ignimbrite; LMI—Linhares-Matela Ignimbrite; VFI—Vila Nova–Fanal Ignimbrite; CCI—Caldeira-Castelinho Ignimbrite; GVI—Grota do Vale Ignimbrite. Photos of sections (Fig. 2): A (S2), B (S3), C (S4), D (S5), E (S6).
140 Gertisser et al.
0
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ers
Section N1Main road, E of Biscoitos
UTM: 478300 E / 4293979 N
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T T T TT TT T
T T TT TT T
e
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o o o o o o o o o o o o oo o o o o o o o o o o o o
Figure 5. Stratigraphic sections along the north coast of Terceira. Abbreviations: LAI—Lajes-Angra Ignimbrite; LMI—Linhares-Matela Ignimbrite; VFI—Vila Nova–Fanal Ignimbrite; CCI—Caldeira-Castelinho Ignimbrite; QR—Quatro Ribeiras pyroclastic fl ow deposit; Ig-i—Ignimbrite i; PNI—Pedras Negras Ignimbrite; s.l.—sea level. Other details are as for Figure 4. Photos of sections (Fig. 3): A (near N10, LAI only shown), B (N9), C (N6), D (N7), F (west of N3), G (N2, QR only shown).
Ignimbrite stratigraphy and chronology on Terceira Island 141
Together with formations dominated by pyroclastic fall deposits or lava fl ows, these ignimbrite formations are defi ned as the Lower Terceira Group. As mentioned already, each of these ignimbrite formations is either the product of a single eruption, or, more often, the result of a few events closely spaced in time, in which case, they consist of several members. Some of the formations dominated by ignimbrites also contain minor pyroclastic fall unit and surge deposit members. Like many pyroclastic successions, the formations on Terceira are bounded by unconformities, usu-ally disconformities, testifying to periods of intense erosion fol-lowing major eruptive events or periods of pyroclastic fall or lava fl ow activity. In this regard, these boundaries are equivalent to the unconformity-bounded stratigraphic units (UBSU) used to group stratigraphic units in other volcanic regions.
The south and north coast volcanic sequences provide a detailed picture of the pyroclastic stratigraphy on Terceira, while radiocarbon ages (Table 1) and 40Ar/39Ar ages obtained on anorthoclase crystals separated from pumice clasts within the ignimbrites (Table 2) constrain the timing of ignimbrite-forming volcanism on Terceira. The new data suggest that ignimbrite-forming eruptions on Terceira occurred both within
the age limits of the radiocarbon method and extend beyond its oldest limit of ca. 48–53 14C yr B.P. (Beta Analytic Inc, 2004, written commun.). Additionally, new 40Ar/39Ar ages of precal-dera lavas from Pico Alto and Guilherme Moniz (Table 3) give further indications of the age range of these volcanoes given by Calvert et al. (2006).
Most ignimbrites on Terceira have similar petrographic and geochemical characteristics. The dominant juvenile component in all of them is anorthoclase-rich (up to ~15 modal percent) pumice, ranging in color from light gray to black depending on the degree of microcrystallinity. In terms of their geochemistry (Table 4), all ignimbrites as well as the other pyroclastic den-sity current deposits are classifi ed as trachyte, according to the total alkali-silica diagram (Le Bas et al., 1986) (Fig. 6A). They span a compositional range from metaluminous to mildly per-alkaline and have agpaitic (peralkalinity) indices ranging from 0.92 to 1.30 (Table 4). The mildly peralkaline rocks are all classi-fi ed as comenditic trachyte in the Al
2O
3 versus FeO
t classifi cation
grid for peralkaline silicic rocks (MacDonald, 1974) (Fig. 6B). Incompatible trace-element concentrations, such as Zr and Nb (Table 4; Fig. 6C), indicate different degrees of differentiation for
TABLE 1. CONVENTIONAL RADIOCARBON AGES (1σ ERROR) FOR TERCEIRA IGNIMBRITES
Key (Fig. 7)
Ignimbrite formation*
Sample no. Technique Radiocarbon age (yr B.P.)
Description Sample location
Easting (UTM)
Northing (UTM)
1 LAI TER03-1 (A)*
Radiometric 20,110 ± 470 Charcoal from soil below Lajes-Angra Ignimbrite
São Mateus 476133 4278534
2 LAI TER03-1 (B) Radiometric 21,110 ± 110 Charcoal from soil below Lajes-Angra Ignimbrite
São Mateus 476133 4278534
3 LAI TER47-1 (A)*
Radiometric 20,500 ± 500 Charcoal from within Lajes-Angra Ignimbrite
Outeiros 484657 4292979
4 LAI TER47-1 (B) Radiometric 21,030 ± 120 Charcoal from within Lajes-Angra Ignimbrite
Outeiros 484657 4292979
5 LAI TER50-1 AMS 21,220 ± 120 Charcoal from within Lajes-Angra Ignimbrite
Angra ring road
481592 4280180
6 LAI TER26-1 AMS 21,300 ± 110 Charcoal from lowermost part of Lajes-Angra Ignimbrite
Caldeira 489176 4292535
7 LAI TER07-1* Radiometric 22,310 ± 800 Charcoal from stratified basal layer of Lajes-Angra Ignimbrite
Linhares quarry
481393 4280204
8 LAI TER11-3 (A)*
Radiometric 22,310 ± 800 Charcoal from lower part of Lajes-Angra Ignimbrite
Porto da Vila Nova
487685 4292725
9 LAI TER11-3 (B) Radiometric 23,150 ± 730 Charcoal from lower part of Lajes-Angra Ignimbrite
Porto da Vila Nova
487685 4292725
10 LMI TER17A-1 (A)*
Radiometric 34,600 ± 3910 Charcoal from central part of Linhares-Matela Ignimbrite
Matela 478376 4282746
11 LMI TER17A-1 (B)*
Radiometric 37,320 ± 4960 Charcoal from within Linhares-Matela Ignimbrite
Matela 478376 4282746
12 LMI TER05-1* Radiometric 34,690 ± 7500 Charcoal from within oldest ignimbrite exposed in Linhares quarry
Linhares quarry
481607 4280134
13 VFI TER02-2 AMS 39,880 ± 800 Charcoal from within prominent pumiceous ignimbrite predating the Lajes-Angra Ignimbrite
Angra Harbor 481267 4278298
14 VFI TER51-2 (1) AMS >46,700 Charcoal from within uppermost of two ignimbrites exposed at Baía do Fanal
Baía do Fanal 480213 4278717
Note: All samples were analyzed by Beta Analytic Inc., USA, except for those marked with an asterisk, which were analyzed by Jost Eikenberg, Paul Scherrer Institute, Switzerland. AMS—accelerator mass spectrometry. “A” and “B” denote measurements of the same sample. *LAI—Lajes-Angra Ignimbrite; LMI—Linhares-Matela Ignimbrite; VFI—Vila Nova–Fanal Ignimbrite.
142 Gertisser et al.
the various ignimbrite units, and these are somewhat diagnostic in separating one ignimbrite from another.
In the next sections, we describe the occurrence, fi eld char-acteristics, geochemical composition, stratigraphy, and age dates of the Terceira ignimbrites, starting with the youngest ignimbrite formation. Analytical techniques and data for the 40Ar/39Ar age determinations are given in the Appendix.
Lajes-Angra Ignimbrite
The Lajes-Angra Ignimbrite consists of widespread depos-its at the same stratigraphic level that extend to the southern and northern parts of the island (Fig. 7A). It is characterized by coarsely porphyritic comenditic trachyte pumice and its stratigraphic position as the topmost ignimbrite. Juvenile clasts include a complete density range from highly vesicular pumice to dense vitrophyric clasts, varying from light gray to black in color, respectively. The ignimbrite matrix can be nonwelded and light gray to welded and dark gray, almost forming a black vitrophyre. The Lajes-Angra Ignimbrite varies in thickness between 25 m and less than 1 m, and is strongly topographically controlled. Where it is a veneer deposit <1 m thick, the Lajes-Angra Ignim-brite forms a fi ne-grained, anorthoclase crystal–rich weathering horizon that drapes the landscape up to at least 300 m above sea level (Self, 1971, 1976). Thus, the pyroclastic fl ows that depos-ited this ignimbrite and its various facies covered the topography up to all but the highest massifs.
The Lajes member of the Lajes-Angra Ignimbrite fl oors the Lajes graben in the NE of the island (Fig. 1) and is found exten-sively along the north coast as sheets and valley-ponds (cf. Wil-son and Walker, 1985), sometimes thinning against topography and in places completely removed by erosion (Figs. 3B and 5). Westward from the main outcrop area, it extends as far as Biscoi-tos in small valley-ponds connected by an occasionally preserved veneer deposit. There are usually two layers in the ignimbrite, suggesting two pulses of material, defi ned by a strong pumice concentration zone at the top of each layer (Fig. 3A). The lower layer is generally poorer in coarse juvenile clasts. It tends to be more welded than the upper layer, is gray-brown in color, and sometimes displays a well-developed eutaxitic texture. The lower depositional layer often contains surface-derived lithic clasts, which are mostly basaltic lavas, as well as an assemblage of vent-derived lithics that are dominated by trachytic to comenditic compositions. Very rare syenite cognate xenoliths are occasion-ally found.
The upper Lajes depositional layer is usually thicker than the lower one and dominates coastal exposures, where it shows spectacular size-density stratifi cation of coarse pumice clasts as layer 2bP (Sparks et al., 1973). Juvenile clasts reach up to 1.5 m in maximum diameter. The upper depositional layer is usually welded at the bottom and becomes less welded upward; the change in welding grade depends on thickness. The maximum exposed thickness is ~25 m in a valley-pond near Vila Nova; however, greater thicknesses may be present where the deposit
TABLE 2. 40Ar/39Ar AGES (1σ ERROR) FOR TERCEIRA IGNIMBRITES
*LAI—Lajes-Angra Ignimbrite; VFI—Vila Nova–Fanal Ignimbrite; CCI—Caldeira-Castelinho Ignimbrite; Ig-i—Ignimbrite i.
TABLE 3. 40Ar/39Ar AGES (1σ ERROR) FOR LAVAS FROM PICO ALTO AND GUILHERME MONIZ CALDERAS
)MTU( gnihtroN)MTU( gnitsaEnoitacol elpmaStinu cihpargitartS )ak( ega rA-rA .on elpmaSTPA1 141 ± 27 Anorthoclase crystals from precaldera lava
flow of Pico Alto volcano
Inner wall of Pico Alto caldera at Lavaçal
479693 4288927
TGM1 181 ± 14 Anorthoclase crystals from precaldera lava flow near top of Guilherme Moniz volcano
Near Guilherme Moniz caldera wall
480766 4283587
Ignimbrite stratigraphy and chronology on Terceira Island 143
TA
BLE
4. S
ELE
CT
ED
WH
OLE
-RO
CK
MA
JOR
- (W
T%
) A
ND
TR
AC
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LEM
EN
T (
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M)
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ALY
SE
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ER
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IRA
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S
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e no
. T
ER
10-3
T
ER
1-1
TE
R5-
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ER
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A
TE
R2-
2 T
ER
11-2
B
TE
R35
-2
TE
R2-
1 T
ER
56-1
T
ER
10-2
T
ER
13-1
T
ER
46-1
T
ER
14-1
Igni
mbr
ite
LAI
LAI
LMI
LMI
VF
IV
FI
CC
IC
CI
PN
I Ig
-iG
VI
Oth
er P
FD
Oth
er P
FD
bran
ch/
Dep
osit
Laje
s A
ngra
Li
nhar
es
Mat
ela
Fan
al
Vila
Nov
a C
alde
ira
Cas
telin
ho
As
abov
e A
s ab
ove
As
abov
e Q
R
PS
SiO
2 64
.80
63.7
4 62
.64
60.7
261
.22
63.1
061
.69
61.8
761
.11
62.9
060
.52
62.9
664
.68
TiO
2 0.
61
0.60
0.
79
0.64
0.57
0.56
0.43
0.45
0.78
0.
560.
760.
570.
45A
l 2O3
15.6
5 15
.04
15.9
1 17
.10
14.7
9 15
.31
14.4
1 14
.89
16.1
9 15
.07
17.5
5 15
.56
15.8
1 F
e 2O3
4.99
5.
00
5.79
6.
31
5.66
5.
67
5.28
5.
12
6.10
5.
84
4.89
6.
27
5.06
M
nO
0.20
0.
20
0.20
0.
204
0.21
0.
21
0.21
0.
20
0.22
0.
22
0.21
0.
19
0.19
M
gO
0.40
0.
40
0.86
0.
22
0.53
0.
35
0.40
0.
38
0.81
0.
36
0.65
0.
43
0.25
C
aO
0.75
0.
73
1.69
0.
49
1.06
1.
00
0.67
0.
74
1.56
0.
95
0.77
0.
52
0.25
N
a 2O
7.32
7.
30
7.02
6.
41
7.31
7.
25
8.28
7.
03
6.92
7.
24
7.30
7.
22
7.14
K
2O
5.05
4.
92
4.30
4.
834.
644.
784.
724.
834.
05
4.80
4.53
4.80
4.99
P2O
5 0.
10
0.09
0.
15
0.12
0.09
0.09
0.05
0.06
0.14
0.
080.
110.
050.
04LO
I 0.
28
1.58
0.
30
1.59
3.25
1.38
2.74
4.00
1.67
1.
252.
560.
610.
84S
um
100.
15
99.6
0 99
.65
98.6
3 99
.33
99.7
0 98
.88
99.5
7 99
.55
99.2
7 99
.85
99.1
8 99
.70
A.I.
* 1.
12
1.15
1.
02
0.92
1.
15
1.12
1.
30
1.13
0.
97
1.13
0.
96
1.10
1.
09
Rb
97
103
109
91
96
100
154
136
112
102
118
62
155
Sr
10
10
110
26
36
29
8 11
86
21
24
2
4 Y
59
.2
62.2
70
.5
40.1
63
.2
66.9
99
.1
85.6
92
.5
67.9
67
.0
22.3
71
.2
Zr
649
677
845
854
719
765
1293
1059
973
779
952
1029
1217
Nb
121.
1 12
5.8
159.
3 13
9.5
130.
513
9.5
225.
919
7.3
171.
8 14
1.8
175.
017
7.1
219.
5B
a 40
9 31
3 65
3 73
252
874
434
4852
4 59
736
692
111
Pb
4 4
6 4
7 3
10
9 8
8 11
8
8 T
h 11
10
14
14
9
12
21
19
17
13
17
17
25
U
2 3
5 5
5 5
5 4
5 4
5 2
6 S
c 2
3 6
6 1
4 3
1 3
2 3
8 3
V
1 1
18
7 5
8 4
4 22
0
0 4
0 C
r 6
5 14
7
64
34
14
57
35
Co
0 0
2 2
01
11
5 1
00
2N
i 0
3 10
7
43
54
11
42
32
Cu
3 2
6 6
5 4
7 7
7 6
7 3
4 Z
n 15
2 15
9 17
4 16
5 16
3 17
1 21
4 19
4 19
9 17
1 16
6 93
18
5 G
a 31
31
29
33
28
31
36
32
32
30
29
38
33
*A.I.
(A
gpai
tic in
dex)
= m
olar
(N
a 2O +
K2O
)/A
l 2O3.
Oth
er a
bbre
viat
ions
use
d: L
AI—
Laje
s-A
ngra
Igni
mbr
ite; L
MI —
Linh
ares
-Mat
ela
Igni
mbr
ite; V
FI—
Vila
Nov
a–F
anal
Igni
mbr
ite;
CC
I—C
alde
ira-C
aste
linho
Igni
mbr
ite; P
NI—
Ped
ras
Neg
ras
Igni
mbr
ite; I
g-i—
Igni
mbr
ite i;
GV
I—G
rota
do
Val
e Ig
nim
brite
; Oth
er P
FD
—ot
her
pyro
clas
tic fl
ow d
epos
its: Q
R—
Qua
tro
Rib
eira
s py
rocl
astic
flow
dep
osit;
PS
—P
osto
San
to s
patte
r-flo
w d
epos
it; L
OI—
loss
on
igni
tion.
144 Gertisser et al.
ponded in the Lajes graben, but where the base is not exposed. In places, the entire deposit may be densely welded even when only 1–2 m thick.
The Angra member of the Lajes-Angra Ignimbrite in the southern part of Terceira (Fig. 7A) has many of the same fea-tures as the Lajes member, including a lower gray-weathering depositional layer that is coarse clast–depleted and tends to be welded. Above this, there is usually a single unit of nonwelded, coarse clast–bearing ignimbrite, with, in places, a second local fl ow unit developed. The deposit covers a small area topographi-cally above and north of Angra do Heroísmo (Fig. 2D), where the top once formed a small plain now largely destroyed by quarry-ing, farming, and urban development. It is up to 20 m thick at the south coast east of Angra, where it is, again, strongly topographi-cally controlled.
Two other remnants of the Lajes-Angra Ignimbrite (Fig. 7A) are preserved along the south coast; these must have been connected to the main area of deposition by veneers, and they imply somewhat sinuous pyroclastic fl ow paths. They are exposed near São Mateus (Figs. 1, 2A, and 4) and at Porto Judeu (Figs. 1 and 4). These thinner remnants (2–4 m) have the same general features as the main parts of the deposit, and the lower welded layer at São Mateus has a well-developed eutaxitic texture despite a thickness of only ~1 m (Fig. 2A). The São Mateus fl ow path can be traced by one 3-m-high exposure of densely welded Lajes-Angra Ignimbrite toward the middle of the island near the old comenditic Matela lava dome (UTM: 478129E, 4284045N). The Angra and also, pos-sibly, the Porto Judeu fl ow paths have one good exposure in a disused, small quarry (UTM: 484266E, 4284932N) near Furna d’Agua, where the ignimbrite is also welded with a poorly developed eutaxitic texture, despite being only a maximum of 1.1 m thick. In no place can the Lajes and Angra members now be found together in the same outcrop (cf. Self, 1971), and, due to the lack of other discriminatory criteria, they are assumed to be the result of the same eruption sequence. This conclusion is corroborated by nine new radiocarbon ages obtained for the Lajes-Angra Ignimbrite at various locations. These bracket both the Lajes and Angra members of the Lajes-Angra Ignimbrite between 20,110 ± 470 and 23,150 ± 730 yr B.P. (Table 1), eliminating the time gap between the Lajes and the Angra ignimbrite inferred from previously available radio-carbon ages (Self, 1971, 1976).
Linhares-Matela Ignimbrite
In a quarry to the north of Angra (UTM: 481746E, 4280372N) under the Lajes-Angra Ignimbrite and separated from it by ~10 m of fall deposits and a lava fl ow, a dark-gray, nonwelded ignimbrite occurs, which is herein referred to as the Linhares member of the Linhares-Matela Ignimbrite after the valley (Vale dos Linhares) in which the quarry (and ignim-brite) occurs (Fig. 2F). It has uniformly dark, anorthoclase- and plagioclase-bearing pumice clasts up to 1 m in diameter,
Figure 6. (A) Total alkali-silica (TAS) diagram (Le Bas et al., 1986), (B) classifi cation grid for peralkaline silicic rocks (MacDonald, 1974), and (C) Nb versus Zr plot for ignimbrites and other pyroclas-tic fl ow deposits on Terceira.
Ignimbrite stratigraphy and chronology on Terceira Island 145
Lajes - Angra (LAI)
Vila Nova - Fanal (VFI)
Linhares - Matela (LMI)
Caldeira - Castelinho (CCI)
Caldeira
Castelinho
Pre-Caldeira - Castelinho (CCI)
Other pyroclastic flow depoits
Q. Ribeiras (QR)
P. Santo (PS)
Grota do Vale (GVI)
(1,2)
(6)
(5,7)
(8,9)(3,4)
(10,11)
(12)
(14) (13,17)
(15) (18)
(16)
(19)
(20)
A
B
C
D
E
F
27° 20'W
38° 40'N38° 45'
38° 40'N38° 45'
38° 40'N38° 45'
38° 40'N38° 45'
38° 40'N38° 45'
38° 40'N38° 45'
27° 10' 27° 20'W 27° 10'
27° 20'W 27° 10' 27° 20'W 27° 10'
27° 20'W 27° 10'27° 20'W 27° 10'
Angra
Lajes
Matela
Linhares
Vila Nova
Fanal
Ignimbrite i
P. Negras (PNI)
Figure 7. Distribution of ignimbrites and other pyroclastic density current deposits on Terceira. Ignimbrite fl ow paths and geochronol-ogy sample locations (numbers in parentheses), keyed to Tables 1 and 2, are indicated on the maps. (A) Lajes-Angra Ignimbrite (LAI); (B) Linhares-Matela Ignimbrite (LMI); (C) Vila Nova–Fanal Ignimbrite (VFI); (D) Caldeira-Castelinho Ignimbrite (CCI); (E) ignimbrites predating the Caldeira-Castelinho Ignimbrite (Pedras Negras Ignimbrite, Ig-i, Grota do Vale Ignimbrite); and (F) other pyroclastic fl ow deposits on Terceira (QR, PS). Places are named in Figure 1C. Abbreviations are as in Figures 2–5.
146 Gertisser et al.
which are slightly less SiO2-rich than those in the Lajes-Angra
Ignimbrite (Table 4). Carbonized wood yielded an age date of 34,690 ± 7500 yr B.P. (Table 1), which, despite the large ana-lytical uncertainty, is in agreement with ages of 34,600 ± 3910 and 37,320 ± 4960 yr B.P. (Table 1) obtained from carbonized wood in another exposure of ignimbrite in the interior of the island (UTM: 478376E, 4282746N), which is also located near the Matela lava dome. In this exposure, the ignimbrite, which is herein referred to as the Matela member of the Linhares-Matela Ignimbrite, is intensely weathered, but some characteristic large pumices yielded a reliable geochemical composition, which, based on the metaluminous character, suggests that the ignim-brites at Matela and Linhares represent distinct units erupted approximately contemporaneously (Table 4). Both members of the Linhares-Matela Ignimbrite appear to be restricted to the southern part of the island (Fig. 7B), and apparently are now not exposed on the south coast apart from a possible bed identifi ed by Self (1974) as a thin “basaltic” ignimbrite, based on the dark color of the ash matrix and small weathered pum-ices, in the now covered Angra Harbor section (Fig. 2C). This deposit occurred in the correct stratigraphic position (Fig. 4, S4), and was exposed along the extension of the inferred fl ow path (Fig. 7B), to be considered an equivalent of the Linhares member of the Linhares-Matela Ignimbrite.
Vila Nova–Fanal Ignimbrite
Named for exposures at Porto da Vila Nova and Baía do Fanal on the north and south coasts, respectively, the Vila Nova–Fanal Ignimbrite Formation appears to be a group of closely related pyroclastic fl ow units, with some associated fall layers. They all share similar compositional and physical char-acteristics between the two sets of coastal localities. Further, the deposits occur at approximately the same stratigraphic level on both coasts, and radiocarbon and 40Ar/39Ar age determinations of ignimbrites at Baía do Fanal (Fig. 2B), and Porto da Vila Nova (Fig. 3D) suggest ages of older than 46,700 and 58,000 ± 20,000 yr B.P., respectively (Tables 1 and 2). While only being seen in the central south coast portion around Angra do Heroísmo (Figs. 4 and 7C), the Vila Nova–Fanal Ignimbrite is more widespread along the north coast, where it extends from the NW end of the Lajes graben, under which it may extend to the SE, to the west of Vila Nova (Fig. 5). Related pyroclastic deposits in the same stratigraphic position are dispersed further west over the Quatro Ribeiras area (Figs. 1, 3F, and 7C).
The Vila Nova–Fanal Ignimbrite is composed of up to eight fl ow units (Fig. 3C), and it is always nonwelded and of a light cream to gray color. The number of fl ow units varies from exposure to exposure, and their variability is thought to have mainly developed in response to local fl ow and depositional conditions. Any possible correlation between individual units is confounded by the incomplete preservation of the deposit in all exposures. The top is strongly eroded, and the maximum remnant thickness is ~22 m. The deposit has abundant pumice
clasts, the concentrations of which defi ne the tops of individual depositional units (cf. Sparks et al., 1973, their fi gure 2), but they are never as coarse as the large, meter-sized clasts seen in the Lajes-Angra Ignimbrite and Linhares-Matela Ignimbrite. One of the thicker Vila Nova–Fanal Ignimbrite depositional units in the now-concealed Angra Harbor section (Fig. 2C) has well-developed fossil fumarole pipes, some leading off carbon-ized wood remains, from which a radiocarbon age of 39,880 ± 800 yr B.P. was obtained (Table 1).
At the base and in the middle parts of the Vila Nova–Fanal Ignimbrite, exposures show fi ne ash-dominated layers, which can be correlated between some outcrops on the north coast and in one place on the south coast, where they contain poorly formed accretionary lapilli. At the very base along north coast exposures, a pumice lapilli-fall layer up to 50 cm thick is occa-sionally preserved. Also, at the top of the sequence, two slightly later, decimeter-thick pumice lapilli fall deposits, each consist-ing of several (presumed) sub-Plinian fall units and separated by a poorly developed soil horizon, are seen. Compositionally, the pumices from both pyroclastic fall and fl ow deposits are all very similar comenditic trachyte (Table 4).
North coast exposures of thick Vila Nova–Fanal Ignim-brite die out west of Vila Nova (Fig. 1), but, at the same strati-graphic position, a pyroclastic succession consisting of sev-eral distinct, thinner pyroclastic fl ow layers, separated by the aforementioned sub-Plinian fall deposits, persists westwards (Fig. 3F). The fall units thicken in this direction, suggesting dispersal axes to the NW from a presumed source on Pico Alto. These are found below a localized unit, here termed the Quatro Ribeiras pyroclastic fl ow deposit (Fig. 3G), which will be described later.
Caldeira-Castelinho Ignimbrite
The Caldeira-Castelinho Ignimbrite Formation is a group of closely related pyroclastic fl ow deposits that occur strati-graphically below the Vila Nova–Fanal Ignimbrite and share compositional and physical characteristics between the two sets of coastal localities (Figs. 4 and 5). It is named for expo-sures west of Caldeira (UTM: 489176E, 4292535N; Fig. 3B) and in now-obscured exposures at Angra Harbor (Fig. 2C) on the north and south coasts, respectively. On both coasts, there are only erosional remnants preserved, and any one exposure rarely exceeds a length of a few 100 m. Both its lower strati-graphic position and several characteristics allow this ignim-brite to be correlated between different localities and distin-guished from the others.
On the north coast, the Caldeira member of the Caldeira-Castelinho Ignimbrite (Fig. 7D) can be distinguished by sev-eral features. It is the only ignimbrite on Terceira to contain abundant (but still <1 vol%) syenite xenoliths up to 25 cm in diameter. It has very large juvenile clasts up to 2.5 m in diame-ter that in places consist of dark-gray to black pumices as well as denser glassy clasts, and it contains abundant lithic clasts.
Ignimbrite stratigraphy and chronology on Terceira Island 147
Moreover, it is the only ignimbrite on Terceira that has a well-developed, cross-bedded ground-surge layer underlying it, which contrasts with the lithic-rich ground layer sometimes observed at the base of the Lajes-Angra Ignimbrite. Also, the Caldeira member of the Caldeira-Castelinho Ignimbrite has a relatively thin, but consistently exposed pumice-fall deposit at its base.
On both coasts, exposures of the Caldeira-Castelinho Ignim-brite include an interbedded, polymict debris-fl ow deposit. In some locations, this debris-fl ow deposit occurs between ignim-brite units; in others, it crops out above them or is capped by a lahar-like deposit rich in rounded lapilli-size pumice clasts (Fig. 3E). While the Caldeira-Castelinho Ignimbrite is usually nonwelded, the debris-fl ow deposit contains, amongst a wide range of clast types, large blocks (up to 5 m in diameter) of coarse-grained welded ignimbrite and welded pyroclastic fall material, both also containing abundant syenite clasts. The com-positional similarity between the juvenile clasts in these blocks and pumices in the Caldeira-Castelinho Ignimbrite suggests that the former is a proximal facies of the same ignimbrite rapidly reworked during, or in a lull in, the eruption. The abundant sur-face water implied by the debris-fl ow deposit, and even more so by the overlying, hyperconcentrated, pumice-rich fl ood fl ow or lahar deposit, suggests, perhaps, break out of a caldera lake dur-ing this event or, at the very least, intense rainstorms during erup-tion and deposition.
On the south coast at Angra Harbor, the Castelinho mem-ber of the Caldeira-Castelinho Ignimbrite (Fig. 7D) consists of a gray, pumice-clast–rich and nonwelded ignimbrite fl ow unit overlying a debris-fl ow deposit, which, together with the com-positional similarity of juvenile clasts, strongly suggests cor-relation between the north and south coast sequences.
Another ignimbrite unit occurs stratigraphically below the debris-fl ow deposit at Angra Harbor. This formerly poorly exposed ignimbrite, named Porto das Pipas Ignimbrite by Self (1974), may now have no exposure since the cementing-over of the Angra Harbor walls, part of which is locally known as Porto das Pipas. It is a single depositional unit of a light-gray, nonwelded, and pumice-clast–rich ignimbrite. This ignimbrite unit was exposed near the base of the Angra Harbor pyroclas-tic succession, and it rests on an older basaltic lava fl ow from Guilherme Moniz volcano (Nunes et al., 2001). Its maximum exposed thickness was ~4 m, although its top was truncated by erosion. It is the oldest ignimbrite that occurs at Angra Harbor where its stratigraphic position is constrained by the overlying debris fl ow and the upper ignimbrite fl ow unit of the Caldeira-Castelinho Ignimbrite. This stratigraphic relationship places the Porto das Pipas Ignimbrite of Self (1974) in the same relative position as the lowermost ignimbrite fl ow unit of the Caldeira-Castelinho Ignimbrite within Terceira’s north coast pyroclastic sequence.
Two 40Ar/39Ar ages of 71 ± 4 ka and 83 ± 18 ka obtained on anorthoclase crystals from south and north coast expo-sures of the Caldeira-Castelinho Ignimbrite suggest that this
ignimbrite-forming episode is signifi cantly older than the over-lying Vila Nova–Fanal Ignimbrite (Table 2). A 40Ar/39Ar age of 116.3 ± 1.6 ka (Calvert et al., 2006) for a lava fl ow below the Caldeira-Castelinho Ignimbrite at Ponta das Escaleiras (Fig. 5, section N7) provides an older age limit for this ignimbrite.
Ignimbrites Predating the Caldeira-Castelinho Ignimbrite
Three petrographically and geochemically distinct ignim-brites, namely the Pedras Negras Ignimbrite, Ignimbrite i, and the Grota do Vale Ignimbrite occur in roughly the same stratigraphic position below the Caldeira-Castelinho Ignim-brite along both Terceira’s northern and southern coastlines (Fig. 7E). Because of the widely scattered nature of the expo-sures and the lack of superposition, the stratigraphic relation-ship between these ignimbrites remains uncertain. However, it appears that they are the oldest ignimbrites exposed on Ter-ceira (Figs. 4 and 5), which is supported by a 40Ar/39Ar age of 86 ± 9 ka obtained for Ignimbrite i (Table 2).
Exposed in north coast cliffs between Vila Nova and Quatro Ribeiras, near where the Ribeira das Pedras stream enters the sea (UTM: 486973E, 4293265N), there are sev-eral erosional remnants of a dark weathering ignimbrite that is referred to as the Pedras Negras Ignimbrite (Fig. 3F). It underlies the Caldeira-Castelinho Ignimbrite and, together with a small remnant of Ignimbrite i (Ig-i), exposed at the base of a cliff section near Ponta de José Vieira west of Cal-deira (Fig. 3B), appears to be the oldest ignimbrite exposed on Terceira’s north coast (Fig. 5). The Pedras Negras Ignimbrite is up to 8 m thick and contains fairly crystal-poor, dark, and relatively dense pumiceous clasts of similar composition to the other ignimbrites (Table 4). Isolated exposures of the Pedras Negras Ignimbrite can be traced westward until just east of Alagoa (Fig. 3F), around where almost continuous outcrops of this ignimbrite occur near or at the base of the cliffs. Here, the Pedras Negras Ignimbrite has a welded basal layer, yellowish in color, that contains large black pumice clasts and fi amme, as well as basaltic lithic clasts incorporated into the pyroclas-tic fl ow from the underlying basaltic lavas. The overlying fl ow unit is non- to poorly welded and characterized by abundant dark and dense pumiceous clasts.
Neither the Pedras Negras Ignimbrite nor Ignimbrite i cor-relates with the lowest ignimbrite in the stratigraphy on the south coast because that deposit, herein referred to as the Grota do Vale Ignimbrite, contains biotite, a rather unusual mineral in Terceira ignimbrites (Fig. 4). This remnant has been found in only one 100-m-long exposure along the south coast (UTM: 482411E, 4278668N), near where the Grota do Vale stream enters the sea (Fig. 2E). It consists of two ignimbrite fl ow units and, as mentioned already, is notable for containing biotite-bearing juvenile clasts, mainly in the form of small fi amme, which impart a eutaxitic texture. It is thin (only up to 1.5 m exposed) but has a truncated top, and it has a highly altered matrix such that other features are masked.
148 Gertisser et al.
Other Pyroclastic Density Current Deposits on Terceira
Two previously unknown pyroclastic density current depos-its have been found in the southern part and on the north coast of Terceira (Fig. 7F). Over an area of ~5 km2 around Quatro Ribeiras (Fig. 1), in a series of exposures extending from the main circumisland road to the sea cliffs, a coarse clast–supported pyroclastic fl ow deposit is found, herein referred to as the Quatro Ribeiras pyroclastic fl ow deposit (Fig. 3G). It falls stratigraphi-cally below the Lajes-Angra Ignimbrite and above the series of deposits associated with the Vila Nova–Fanal Ignimbrite (Fig. 5). Juvenile clasts are rounded and very poorly vesicular, and the matrix appears to have been generated by attrition of the coarser clasts. Compositionally, the deposit is comenditic trachyte simi-lar to the ignimbrites, although the trace-element composition sets it apart from both the Vila Nova–Fanal Ignimbrite and Lajes-Angra Ignimbrite (Table 4). It is most logically associated with one of the pre–Lajes-Angra Ignimbrite lava dome–forming erup-tions from the upper slopes of Pico Alto volcano that was able to spread widely over the steep topography because of its high originating altitude.
In a single road cut near Posto Santo (UTM: 480186E, 4282455N), there is a 5-m-high exposure of a poorly sorted, weathered deposit with unmistakable abundant, deformed spatter clasts. We refer to this unit as the Posto Santo spatter-fl ow deposit, although the origin, and even the exact strati-graphic position of the deposit remain uncertain. It overlies lava fl ows from adjacent Guilherme Moniz caldera that are locally deeply buried by pyroclastic fall deposits. Composi-tionally, the deposit is evolved comenditic trachyte, with trace-element concentrations distinct from ignimbrites such as the Linhares-Matela Ignimbrite, which could share a common
source (Table 4). This interesting deposit has not been found at other locations as yet.
DISCUSSION
Pyroclastic Flow–Forming Eruptions and Ignimbrite Formations on Terceira
The distinction and correlation of the various ignimbrites found in the volcanic successions primarily along the south and north coasts of Terceira play a crucial role in establishing the number of ignimbrite-forming and other pyroclastic den-sity current–forming eruptions that have occurred on Terceira. Stratigraphic position, physical characteristics, differences in geochemical composition of ignimbrite units, distinctive marker horizons, sequence of deposits within and between ignimbrite formations, as well as radiocarbon and 40Ar/39Ar age determina-tions form the bases for correlations, both within and between composite stratigraphic sections.
Our results suggest that there have been at least fi ve ignimbrite-forming episodes that have left widespread deposits either at both coasts or, at least, have been distributed quite widely along one coastline. There are two others that are seen in isolated exposures on either the south or north coast only, and at least two other events that have formed pyroclastic density current deposits on Terceira (Figs. 7 and 8). Some of the ignimbrite-dominated sequences were probably the products of more than one eruption within a short time interval and are grouped as a single formation. Together with intercalated lava fl ows and pyroclastic deposits, all ignimbrite formations predating the Lajes-Angra Ignimbrite are defi ned as the Lower Terceira Group (Fig. 9). Some ignim-brite formations have several superposed pyroclastic fl ow units
South coast Interior North coast
LAI (Lajes)
? GVI
LAI (Angra)
QR
? PS
Tim
e (k
a)
0
10
20
30
40
60
50
70
80
90
100
LAI (Lajes)
VFI (Fanal)
CCI (Castelinho)
LMI (Matela)
VFI (Vila Nova)
CCI (Caldeira)
PNIIg-i ?
LMI (Linhares)
Figure 8. Ignimbrite correlation and age chart for Terceira, showing average ages and maximum error margins for ignim-brites exposed on the south and north coasts and in the interior of Terceira (cf. Tables 1 and 2). Ignimbrite members are given in brackets. See text for discus-sion. Abbreviations: LAI— Lajes-Angra Ignimbrite; LMI—Linhares- Matela Ig-nimbrite; VFI—Vila Nova–Fanal Ignim-brite; CCI— Caldeira-Castelinho Ignim-brite; GVI—Grota do Vale Ignimbrite; PS—Posto Santo spatter-fl ow deposit; QR—Quatro Ribeiras pyroclastic fl ow deposit; Ig-i—Ignimbrite i; PNI— Pedras Negras Ignimbrite.
Ignimbrite stratigraphy and chronology on Terceira Island 149
~ ~~
~
~
~~
~~
~
~~
oo
oo
oo
oo
oo
o
oo
oo
oo
o
oo
oo
oo
o
TT
TT
TT
vv
vv
v
s.l.
CC
I
PN
IIg
-iG
VI
TT
TT
TT
TT
TT
TT
TT
T
VF
I
QR
LMI
TT
TT
vv
vv
LAI
T
vv
Gro
upG
ener
aliz
edsu
cces
sion
For
mat
ion
Fie
ld c
hara
cter
istic
sR
ock
type
Cry
stal
con
tent
vv
vv
TT
TT
vv
Upper Terceira Group Lower Terceira Group
TT
Unn
amed
Unn
amed
Unn
amed
Unn
amed
Unn
amed
Unn
amed
Gra
y, w
elde
d to
non
wel
ded,
mas
sive
low
-asp
ect-
ratio
ign
imbr
ite w
ith l
arge
gr
ay
to
blac
k pu
mic
e cl
asts
; 1
m
- (e
xcep
tiona
lly)
25
m
thic
k;
low
er
depo
sitio
nal u
nit
of L
ajes
mem
ber
of t
he L
AI
can
disp
lay
a w
ell-d
evel
oped
eu
taxi
tic te
xtur
e; lo
wes
t par
t of A
ngra
mem
ber
also
occ
asio
nally
wel
ded
Bas
altic
to
trac
hytic
/rhy
oliti
c la
va f
low
s an
d py
rocl
astic
dep
osits
of
over
100
er
uptio
ns fr
om S
anta
Bár
bara
, Pic
o A
lto, a
nd th
e rif
t zon
e (c
f. S
elf,
1976
)
Var
ious
bas
altic
to tr
achy
tic/r
hyol
itic
lava
flow
s an
d py
rocl
astic
dep
osits
Var
ious
bas
altic
to tr
achy
tic/r
hyol
itic
lava
flow
s an
d py
rocl
astic
dep
osits
Dar
k-gr
ay,
nonw
elde
d to
slig
htly
wel
ded
igni
mbr
ite w
ith d
ark,
ano
rtho
clas
e-
and
plag
iocl
ase-
bear
ing
pum
ice
clas
ts u
p to
1 m
in
diam
eter
; M
atel
a m
embe
r of
the
LMI i
s in
tens
ely
wea
ther
ed w
here
exp
osed
Cre
am t
o gr
ay-c
olor
ed g
roup
of
up t
o ei
ght,
clos
ely
rela
ted,
non
wel
ded
pyro
clas
tic f
low
uni
ts a
nd a
ssoc
iate
d fa
ll la
yers
; al
ong
the
N c
oast
, a
pum
ice-
fall
laye
r up
to 5
0 cm
thic
k is
som
etim
es p
rese
rved
at t
he b
ase
QR
: Coa
rse-
clas
t-su
ppor
ted
pyro
clas
tic f
low
dep
osit
with
ligh
t-co
lore
d, r
ound
ed a
nd p
oorly
ves
icul
ar
juve
nile
cla
sts
PS
: Up
to 5
m t
hick
, po
orly
sor
ted
and
wea
ther
ed p
yroc
last
ic d
ensi
ty c
urre
nt d
epos
it w
ith a
bund
ant,
defo
rmed
spa
tter
clas
ts (
stra
tigra
phic
pos
ition
pre
sent
ly u
nkno
wn)
Gra
y, u
sual
ly n
onw
elde
d, p
umic
e-cl
ast-
rich
(juve
nile
cla
sts
up t
o 2.
5 m
in
diam
eter
) ig
nim
brite
with
abu
ndan
t sy
enite
nod
ules
in
N c
oast
exp
osur
es;
inte
rbed
ded
debr
is-f
low
dep
osit;
cro
ss-b
edde
d gr
ound
-sur
ge la
yer
at b
ase
PN
I: D
ark
wea
ther
ed,
up t
o 8-
m-t
hick
igni
mbr
ite w
ith d
ark,
cry
stal
-poo
r an
d re
lativ
ely
dens
e pu
mic
eous
cl
asts
; a
yello
wis
h,
wel
ded
basa
l la
yer
cont
aini
ng la
rge
blac
k pu
mic
e cl
asts
and
fiam
me
may
be
pres
erve
dIg
-i: S
mal
l rem
nant
of
inci
pien
tly w
elde
d ig
nim
brite
exp
osed
at
the
base
of
a cl
iff s
ectio
n ne
ar P
onta
de
José
Vie
ira w
est o
f Cal
deira
GV
I: U
p to
1.5
-m-t
hick
ign
imbr
ite c
onsi
stin
g of
tw
o flo
w u
nits
; no
tabl
e fo
r co
ntai
ning
bi
otite
-bea
ring
ju
veni
le
clas
ts,
mai
nly
in
the
form
of
sm
all
fiam
me,
set
in a
hig
hly
alte
red
ash
mat
rix
? P
S
afsp
, ol,
cpx,
mt,
± il
m
afsp
, pla
g; o
l, cp
x, m
t
afsp
, ± o
l, cp
x, m
tQ
R: a
fsp,
cpx
, mt
PS
: afs
p, c
px, m
t
afsp
, cpx
, mt,
± il
m
PN
I: af
sp, c
px, m
t, ±
ilm
Ig-i:
afs
p, c
px, m
tG
VI:
afsp
, bio
; cpx
, mt
ct
b -
t / r
t / c
t
ct ct
PN
I: ct
Ig-i:
ct
GV
I:t /
ct
vv
vv
vv
vv
Var
ious
bas
altic
to tr
achy
tic/r
hyol
itic
lava
flow
s an
d py
rocl
astic
dep
osits
Var
ious
bas
altic
to tr
achy
tic/r
hyol
itic
lava
flow
s an
d py
rocl
astic
dep
osits
Bas
altic
(a
nd
min
or
trac
hytic
/rhy
oliti
c)
lava
flo
ws
and
pyro
clas
tic
depo
sits
from
Gui
lher
me
Mon
iz a
nd C
inco
Pic
os v
olca
noes
TT
T
b -
t / r
b -
t / r
b -
t / r
b -
t / r
b -
t / r
vario
us
vario
us
vario
us
vario
us
vario
us
vario
us
Figu
re 9
. Gen
eral
ized
ver
tical
suc
cess
ion
(not
to s
cale
) of
the
volc
anic
dep
osits
of T
erce
ira
and
sum
mar
y de
scri
ptio
ns o
f th
e m
ain
fi eld
, pet
rogr
aphi
c, a
nd g
eoch
emic
al c
hara
cter
istic
s of
th
e ig
nim
brite
for
mat
ions
and
oth
er p
yroc
last
ic d
ensi
ty c
urre
nt d
epos
its. F
or a
bsol
ute
ages
, see
text
and
Tab
les
1 an
d 2.
Abb
revi
atio
ns:
Igni
mbr
ite f
orm
atio
ns: L
AI—
Laj
es-A
ngra
Ign
im-
brite
; LM
I—L
inha
res-
Mat
ela
Igni
mbr
ite; V
FI—
Vila
Nov
a–Fa
nal I
gnim
brite
; CC
I—C
alde
ira-
Cas
telin
ho Ig
nim
brite
; PN
I—Pe
dras
Neg
ras
Igni
mbr
ite; I
g-i—
Igni
mbr
ite i;
GV
I—G
rota
do
Val
e Ig
nim
brite
; QR
—Q
uatr
o R
ibei
ras
pyro
clas
tic fl
ow d
epos
it; P
S—Po
sto
Sant
o sp
atte
r-fl o
w d
epos
it. M
iner
als:
afs
p—al
kali
feld
spar
(an
orth
ocla
se);
ol—
oliv
ine;
cpx
— cl
inop
yrox
ene;
m
t—m
agne
tite;
ilm
—ilm
enite
; pla
g—pl
agio
clas
e; b
io—
biot
ite. R
ock
type
s: b
—ba
salt;
t—tr
achy
te; c
t—co
men
ditic
trac
hyte
; r—
pera
lkal
ine
rhyo
lite.
Sym
bols
as
in F
igur
es 4
and
5.
150 Gertisser et al.
together with other deposits, or signs of depositional hiatuses, between them (e.g., Caldeira-Castelinho Ignimbrite), plus some subtle geochemical differences between units. Others have dis-tinct ignimbrite units characterized by individual fl ow paths (e.g., Lajes-Angra Ignimbrite) in which the ignimbrites can be corre-lated by their geochemistry and similar ages to constitute indi-vidual formations. It must be noted that the analytical uncertainty of the age determinations permits these groupings to be made.
Timing of Ignimbrite-Forming Volcanism on Terceira
Our preliminary data support a rather narrow period of ignimbrite-forming volcanism from ca. 86 ka to the youngest at ca. 20–23 ka (Fig. 8). Although we were not able to defi ne ages for the Grota do Vale and Pedras Negras Ignimbrite Formations, it appears, on stratigraphic grounds, that their eruptions may also fi t into this time period. No ignimbrites have been found in the earlier evolutionary stages of Terceira, for instance, interbed-ded between the dominantly basaltic lava fl ows exposed in the coastal cliffs of the uplifted Lajes horst (e.g., Serra de Santiago–Santa Rita) or within the caldera wall successions of the extinct Cinco Picos volcano. The subaerial activity forming these lava fl ows may have begun between 500 ka and 400 ka (Calvert et al., 2006). This indicates that ignimbrite-forming eruptions solely occurred during the younger evolutionary stages of the island’s development (Fig. 9). However, it may be noteworthy that ignimbrites and lava fl ows and coulées of evolved compo-sitions extend right to sea level along both the south and north coasts in the central portion of Terceira, so the possibility of the occurrence of older ignimbrites, now beneath sea level, cannot be discounted.
With the distribution of ignimbrite ages, it is perhaps unwise to give an average time interval between ignimbrite-forming eruptive episodes on Terceira; instead it may be more useful to examine the time intervals between the major explo-sive eruptions from Pico Alto and Guilherme Moniz volcanoes. Some eruptions may have occurred in clusters, while others appear to be of the order of 15–30 k.y. apart. Thus, the ~20 k.y. time interval elapsed since the most recent ignimbrite-forming eruptive episode (Lajes-Angra Ignimbrite) cannot be taken to indicate that ignimbrite-forming volcanism on Terceira is dead. Santa Bárbara volcano, which has produced abundant young pyroclastic fall– and lava dome–producing eruptions, and has two small, nested calderas, inserts another question mark into the issue of whether there might be further ignimbrite- forming activity on Terceira. With the evidence at hand, we conclude that Terceira’s volcanoes could still produce another ignimbrite-forming eruption in the future, either from Pico Alto or, possibly, Santa Bárbara.
The new 40Ar/39Ar ages of 141 ± 27 and 181 ± 14 ka obtained on samples collected from on the caldera wall– forming lavas of Pico Alto and Guilherme Moniz volcanoes, respec-
tively (Table 3), can also be interpreted as upper age limits of ignimbrite-forming volcanism on Terceira, as they are from pre-caldera lavas. Thus, some of the exposed older ignimbrites, such as Ignimbrite i and the Grota do Vale Ignimbrite, and, possibly, the Posto Santo spatter-fl ow deposit, may well be derived from Guilherme Moniz, although there are no geochemical data pres-ently available to support this contention. We can draw no con-clusions from our work regarding the origin and status of Cinco Picos caldera, and whether it ever produced pyroclastic fl ow deposits or any extensive explosive activity during its history.
Ignimbrite-Forming Volcanism on Terceira and Potential Future Hazards
Ignimbrites on Terceira appear to be small in volume (~0.3 km3 dense-rock equivalent [DRE] volume for the Lajes-Angra Ignimbrite on the island; Self, 1976), but considerable proportions of the eruptive volumes must be missing. Almost all pyroclastic fl ows must have reached the shoreline and deposited ignimbrite into the sea. However, considering the relatively small volume of the calderas of the central volcanoes (~5–6 km3), and the evidence that they apparently formed incrementally from sev-eral eruptions, individual episodes of ignimbrite-forming erup-tions yielding ~1–2 km3 of magma are probably the maximum that would be generated from Terceira’s volcanoes.
Although not a major aspect of this paper, if, as discussed above, Terceira has a small probability of a future ignimbrite-forming event, it is worth briefl y discussing some of the hazard implications. Regarding the pyroclastic fl ow hazard on Ter-ceira, we consider Pico Alto volcano to be more likely to have a signifi cant explosive eruption than neighboring Santa Bárbara volcano, which, based on our fi eld studies, has not produced a pyroclastic fl ow–forming eruption, although Calvert et al. (2006) reported a lithic-rich pyroclastic fl ow deposit possibly related to the formation of the volcano’s outer summit caldera. Some ignimbrite-forming eruptions on Terceira apparently began without any opening Plinian phases, but two had early fall deposit–producing phases. Moreover, the welded, yet thin, nature of some of these peralkaline ignimbrites indicates very hot pyroclastic fl ows.
Pyroclastic fl ows that originated from Pico Alto and possi-bly Guilherme Moniz, in the central part of the island, generally followed topographic depressions toward the south and north coasts. They also spread over interfl uve surfaces where they left thin and inconspicuous veneer deposits, the signifi cance of which may be readily missed when assessing the past record of the island’s volcanic activity (Self, 1971, 1976). It is now rec-ognized that thin ignimbrite veneer deposits cover large areas, such as those along the central northern part of the island below Pico Alto volcano, which were previously thought to be free of hazards from pyroclastic fl ows. The well-populated coastal plain between Lajes and Praia da Vitória, which includes the island’s
Ignimbrite stratigraphy and chronology on Terceira Island 151
only airport and Lajes Air Base, may be especially susceptible to pyroclastic fl ows from Pico Alto.
CONCLUSIONS
The stratigraphy of Terceira reveals the occurrence of seven pyroclastic fl ow–forming eruptive episodes that produced ignimbrites now exposed primarily along cliff sections on the south and north coasts of the island. The formation containing the most recent of these, the widespread Lajes-Angra Ignim-brite, marks the base of the youngest volcanic group, called the Upper Terceira Group, and we propose here to include all ignimbrites, interstratifi ed pyroclastic fall deposits, and lava fl ows stratigraphically below the Lajes-Angra Ignimbrite, in the Lower Terceira Group. Two further isolated exposures of pyroclastic density current deposits also occur within this group (Fig. 9). As in many volcanostratigraphic sequences, the strati-graphic successions on Terceira are complex, and correlations are made diffi cult by compositional similarity and discontinu-ity of deposits due to strong topographic control and erosion of units. New radiocarbon ages obtained from charcoal found within ignimbrites and 40Ar/39Ar age determinations on anortho-clase crystals from pumice clasts from several Terceira ignim-brites support a rather narrow period of ignimbrite- forming volcanism on Terceira from ca. 86 ka to ca. 20–23 ka after a long period of predominantly effusive activity. Terceira has a low probability of a future ignimbrite-forming event, and future explosive activity leading to ignimbrite formation will probably occur from Pico Alto volcano, since Santa Bárbara volcano, the youngest on the island, has not produced a pyroclastic fl ow–forming eruption. Our results indicate that the 40Ar/39Ar method can be successfully applied to date anorthoclases in volcanic rocks younger than 100 ka from Terceira, and, potentially, in similar peralkaline volcanic suites elsewhere. The 40Ar/39Ar dat-ing method can extend the radiocarbon age range, although it does not provide the same precision as radiocarbon dating for the younger age range of these volcanic deposits.
ACKNOWLEDGMENTS
We thank Sarah Sherlock (The Open University) for help with the 40Ar/39Ar analytical work, John Watson (The Open Uni-versity) for performing the X-ray fl uorescence (XRF) analy-ses, John Taylor (The Open University) for drafting Figures 4 and 5, and Darden Hood (Beta Analytic Inc, USA) for pro-viding many of the radiocarbon ages reported in this paper. We thank John Wolff, João Carlos Nunes, and an anonymous reviewer for thorough and constructive reviews, and Gianluca Gropelli and Lothar Viereck-Götte for their careful editorial handling. Financial support by The Royal Society through a UK- Portugal Joint Project Grant to Stephen Self and João Gaspar is gratefully acknowledged.
APPENDIX. ANALYTICAL TECHNIQUES
Radiocarbon Dating
Radiocarbon age determinations were performed on charcoal from wood directly incorporated in the ignimbrites. Where available, small-diameter twigs and rootlets and the outer parts of larger branches and tree trunks were chosen for radiocarbon dating to obtain ages as close as possible to the eruption age. To minimize sample contamina-tion by modern and decomposed rootlets, effort was taken to selec-tively collect vitreous over more poorly developed charcoal and to remove visible modern rootlets.
Depending on sample size, radiocarbon age determinations were performed either by conventional radiometric analysis at the Paul Scherrer Institute, Switzerland, and Beta Analytic Inc., USA, or by accelerator mass spectrometry (AMS) at the latter laboratory. For enhanced precision, extended counting times were used for radiomet-ric analysis. The reported ages (Table 1) are expressed as radiocar-bon years before present (before A.D. 1950) using—by international convention—a half-life of 5568 yr (Stuiver and Pollach, 1977). Quoted errors represent 1 standard deviation statistics (68% probability) and are based on combined measurements of the sample, background, and modern reference standards.
40Ar/39Ar Dating
Anorthoclase grains were separated from pumice clasts from sev-eral ignimbrites as well as two lava fl ows from Pico Alto and Guil-herme Moniz volcanoes by gently crushing the rock samples and then sieving and fi nally handpicking of grain size fractions >0.25 mm under a binocular microscope. Crystal separates were cleaned alter-nately in deionized water and dilute acid in an ultrasonic bath. For these inferred young volcanic mineral (anorthoclase) separates, laser step-heating experiments were performed, where ~10 mineral grains were melted with increasing laser power using a focused infrared (IR) laser automated argon extraction system connected to a MAP 215-50 mass spectrometer at The Open University. The 40Ar/39Ar analytical data for the ignimbrites and two lavas are reported in Table A1, and the ages are listed in Tables 2 and 3, respectively. Details of the analyti-cal techniques used at The Open University’s Argon-Argon and Noble Gas Research Laboratory can be found at http://www3.open.ac.uk/earth-sciences/argonlab/index.shtml.
Whole-Rock Geochemical Analysis
Juvenile clasts from all pyroclastic density current deposits on Terceira were analyzed for major- and trace-element compositions (Table 4) at the X-Ray Fluorescence Laboratory, The Open Univer-sity, using an Applied Research Laboratories 8420 + dual goniometer wavelength-dispersive X-ray spectrometer. Before analysis, visible alteration surfaces were removed from the rock samples, which were subsequently processed in a jaw-crusher and ground in an agate mill. Whole-rock major element analysis was carried out on fused glass discs for Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe. For trace elements, pressed powder pellets were analyzed for Rb, Sr, Y, Zr, Nb, Ba, Pb, Th, U, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, among others. Reproducibility and accuracy for major elements were better than 1% relative, while those for trace elements were of the order of 5%. Accuracy and precision were monitored using several in-house rock standards. Loss on igni-tion (LOI) was determined by weight difference at heating ~1 g of rock powder to 1000 °C for 60 min. LOI, as listed in Table 4, is the total effect of dehydration (loss of weight) and oxidation (gain of weight).
152 Gertisser et al.
TA
BLE
A1.
40A
r/39
Ar
AN
ALY
TIC
AL
DA
TA
(1σ
ER
RO
R)
FO
R A
NO
RT
HO
CLA
SE
CR
YS
TA
LS F
RO
M S
ELE
CT
ED
TE
RC
EIR
A IG
NIM
BR
ITE
S
Sam
ple
Ste
p 40
Ar
± 39
Ar
± 38
Ar
± 37
Ar
± 36
Ar
± 40
Ar*
/39A
r±
Age
(M
a)±
TE
R01
-1
1 0.
0989
1 0.
0001
3 0.
0001
60.
0000
00.
0000
50.
0000
1–0
.000
070.
0000
7 0.
0002
50.
0000
416
1.48
782
78.6
0316
35.1
98
17.0
52
2 0.
4187
1 0.
0003
0 0.
0149
70.
0000
60.
0004
70.
0000
10.
0014
70.
0000
9 0.
0013
20.
0000
41.
9216
50.
8142
20.
423
0.18
0
3 0.
7824
2 0.
0016
8 0.
0779
80.
0001
50.
0016
30.
0000
20.
0037
50.
0000
7 0.
0025
60.
0000
50.
3364
60.
1907
00.
074
0.04
2
4 0.
3109
0 0.
0001
6 0.
1329
20.
0003
50.
0019
40.
0000
10.
0044
20.
0001
3 0.
0009
90.
0000
40.
1406
90.
0916
70.
031
0.02
0
5 0.
3064
6 0.
0002
0 0.
1789
40.
0000
50.
0025
30.
0000
20.
0048
90.
0000
9 0.
0009
50.
0000
40.
1459
40.
0681
00.
032
0.01
5
6 0.
3099
5 0.
0006
4 0.
2865
80.
0003
60.
0040
20.
0000
30.
0082
80.
0000
9 0.
0007
70.
0000
40.
2898
30.
0413
10.
064
0.01
0
7 0.
2039
0 0.
0003
8 0.
1203
80.
0004
80.
0016
60.
0000
10.
0031
20.
0000
7 0.
0005
70.
0000
40.
2966
00.
0982
50.
065
0.02
2
TE
R11
-2
1 0.
5902
9 0.
0051
4 0.
0445
40.
0015
90.
0008
20.
0000
40.
0013
50.
0002
3 0.
0017
20.
0000
41.
8419
90.
2736
90.
194
0.02
9
2 0.
3058
0 0.
0023
7 0.
2718
20.
0016
10.
0023
80.
0000
60.
0066
00.
0002
6 0.
0005
80.
0000
20.
4944
70.
0236
10.
052
0.00
3
3 0.
5049
8 0.
0016
7 0.
1501
10.
0016
00.
0014
90.
0000
50.
0028
20.
0002
4 0.
0012
70.
0000
20.
8640
30.
0463
20.
091
0.00
5
4 2.
2540
4 0.
0104
9 0.
0448
50.
0016
00.
0015
60.
0000
50.
0014
10.
0002
3 0.
0066
00.
0000
36.
7731
30.
3839
70.
715
0.04
1
5 3.
2975
7 0.
0149
2 0.
5524
20.
0020
30.
0075
10.
0000
70.
0238
40.
0002
9 0.
0124
50.
0000
3–0
.690
43–0
.031
05–0
.073
0.
003
6
1.28
491
0.00
348
0.14
845
0.00
159
0.00
221
0.00
005
0.00
422
0.00
023
0.00
411
0.00
002
0.47
428
0.05
056
0.05
0 0.
005
TE
R02
-2
1 1.
9632
1 0.
0061
3 0.
0331
40.
0013
10.
0012
30.
0000
50.
0016
00.
0002
5 0.
0054
50.
0000
310
.642
900.
5318
21.
123
0.05
7
2 6.
6000
7 0.
0212
6 0.
4548
30.
0017
00.
0065
50.
0001
80.
0132
70.
0002
6 0.
0188
60.
0002
02.
2578
40.
1383
50.
238
0.01
5
3 0.
9017
5 0.
0040
5 0.
2983
20.
0023
30.
0031
00.
0000
70.
0045
90.
0002
7 0.
0023
70.
0000
10.
6751
80.
0202
10.
071
0.00
2
4 4.
4122
8 0.
0170
2 0.
1276
50.
0013
10.
0048
40.
0000
60.
0107
30.
0003
6 0.
0154
30.
0001
3–1
.153
80–0
.330
16–0
.122
0.
035
5
3.31
150
0.00
837
0.56
761
0.00
133
0.00
682
0.00
011
0.02
468
0.00
077
0.00
762
0.00
007
1.86
711
0.03
990
0.19
7 0.
005
6
2.87
751
0.00
486
0.47
986
0.00
304
0.00
568
0.00
009
0.01
288
0.00
031
0.00
666
0.00
013
1.89
540
0.08
181
0.20
0 0.
009
TE
R02
-1
1 0.
2406
7 0.
0005
6 0.
0034
80.
0002
30.
0001
70.
0000
1–0
.000
060.
0000
8 0.
0005
60.
0000
221
.611
382.
3761
62.
279
0.25
2
2 2.
0608
0 0.
0139
2 0.
6187
60.
0019
20.
0067
40.
0001
10.
0055
40.
0001
0 0.
0048
60.
0000
91.
0095
60.
0495
40.
107
0.00
5
3 0.
7221
2 0.
0025
4 0.
4317
20.
0004
10.
0037
70.
0000
40.
0047
70.
0000
9 0.
0016
40.
0000
30.
5501
20.
0202
40.
058
0.00
2
4 0.
3810
5 0.
0015
1 0.
0139
60.
0015
90.
0002
20.
0000
4–0
.000
080.
0002
3 0.
0008
90.
0000
28.
4539
91.
0775
00.
892
0.11
4
5 0.
5529
7 0.
0016
0 0.
3001
10.
0035
90.
0025
00.
0000
60.
0011
90.
0002
3 0.
0008
70.
0000
20.
9859
30.
0255
40.
104
0.00
3
6 12
.125
36
0.01
372
0.57
997
0.00
183
0.00
556
0.00
011
0.00
375
0.00
023
0.00
306
0.00
003
19.3
4766
0.06
706
2.04
1 0.
022
TE
R10
-1
1 0.
3194
2 0.
0013
8 0.
0252
60.
0002
50.
0003
60.
0000
10.
0006
70.
0000
8 0.
0004
90.
0000
26.
8410
30.
2727
10.
722
0.03
0
2 1.
8680
5 0.
0033
7 0.
3329
30.
0006
30.
0035
60.
0000
60.
0027
50.
0000
9 0.
0044
90.
0000
41.
6257
00.
0337
00.
172
0.00
4
3 0.
3148
4 0.
0010
7 0.
1664
20.
0004
60.
0013
40.
0000
10.
0010
10.
0000
8 0.
0006
30.
0000
20.
7731
70.
0361
60.
082
0.00
4
4 0.
2484
8 0.
0015
6 0.
0118
10.
0015
90.
0002
40.
0000
40.
0001
20.
0002
3 0.
0007
20.
0000
23.
0257
80.
7047
20.
319
0.07
4
5 0.
6541
7 0.
0015
5 0.
3434
70.
0016
20.
0022
90.
0000
40.
0018
40.
0002
3 0.
0012
40.
0000
30.
8377
80.
0250
60.
088
0.00
3
6 1.
0362
7 0.
0038
5 0.
3427
30.
0027
60.
0034
90.
0000
60.
0038
60.
0002
4 0.
0026
90.
0000
40.
7042
80.
0335
40.
074
0.00
4
TE
R10
-2
1 0.
4647
9 0.
0063
5 0.
0773
60.
0004
40.
0009
60.
0000
30.
0023
20.
0001
1 0.
0013
40.
0000
30.
8895
70.
1358
10.
094
0.01
4
2 1.
2505
5 0.
0043
0 0.
5919
60.
0020
80.
0052
40.
0000
70.
0097
50.
0001
3 0.
0025
80.
0000
50.
8246
60.
0280
00.
087
0.00
3
3 0.
5385
1 0.
0036
5 0.
2557
70.
0006
20.
0021
60.
0000
40.
0030
00.
0001
1 0.
0011
60.
0000
30.
7652
70.
0357
10.
081
0.00
4
4 0.
8125
9 0.
0101
2 0.
0830
00.
0001
50.
0011
40.
0000
30.
0027
60.
0001
2 0.
0022
50.
0000
31.
7797
60.
1581
30.
188
0.01
7
5 2.
5979
4 0.
0058
3 0.
6322
40.
0013
30.
0060
30.
0000
90.
0191
70.
0002
7 0.
0062
60.
0000
81.
1832
70.
0385
90.
125
0.00
4
6 0.
8383
7 0.
0032
8 0.
2380
20.
0014
30.
0027
90.
0000
70.
0047
30.
0002
5 0.
0021
90.
0002
00.
4557
80.
0285
20.
085
0.00
3
7 0.
1095
2 0.
0028
6 0.
0130
90.
0015
20.
0003
60.
0000
30.
0005
10.
0000
9 0.
0004
90.
0000
1–2
.695
16–0
.442
94–0
.284
0.
047
8
2.19
952
0.00
533
0.53
297
0.00
170
0.00
462
0.00
005
0.00
397
0.00
012
0.00
468
0.00
005
1.53
214
0.02
987
0.16
2 0.
004
9
2.49
772
0.00
552
0.67
114
0.00
237
0.00
607
0.00
009
0.00
372
0.00
010
0.00
560
0.00
002
1.25
596
0.01
284
0.13
3 0.
002
(Con
tinue
d)
Ignimbrite stratigraphy and chronology on Terceira Island 153
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TA
BLE
A1.
40A
r/39
Ar
AN
ALY
TIC
AL
DA
TA
(1σ
ER
RO
R)
FO
R A
NO
RT
HO
CLA
SE
CR
YS
TA
LS F
RO
M S
ELE
CT
ED
TE
RC
EIR
A IG
NIM
BR
ITE
S(C
ontin
ued)
Sam
ple
Ste
p 40
Ar
± 39
Ar
± 38
Ar
± 37
Ar
± 36
Ar
± 40
Ar*
/39A
r ±
Age
(M
a)±
TP
A1
1 0.
0050
4 0.
0001
1 0.
0006
10.
0000
10.
0000
20.
0000
00.
0003
40.
0000
7 0.
0000
00.
0000
48.
3123
119
.996
821.
829
4.39
8
2 0.
0379
4 0.
0001
7 0.
0151
50.
0000
20.
0004
00.
0000
10.
0049
90.
0001
0 0.
0004
40.
0000
4–6
.050
90–0
.804
19–1
.332
0.
189
3
0.13
255
0.00
015
0.06
953
0.00
004
0.00
095
0.00
001
0.00
353
0.00
007
0.00
030
0.00
004
0.63
540
0.17
525
0.14
0 0.
039
4
0.06
074
0.00
011
0.03
428
0.00
006
0.00
045
0.00
001
0.00
201
0.00
000
0.00
014
0.00
004
0.56
969
0.34
485
0.12
5 0.
076
5
0.04
265
0.00
008
0.06
390
0.00
002
0.00
078
0.00
001
0.00
277
0.00
007
0.00
000
0.00
004
0.67
094
0.18
499
0.14
8 0.
041
TG
M1
1 0.
1625
1 0.
0001
0 0.
0144
30.
0000
20.
0012
90.
0000
30.
0238
20.
0005
9 0.
0025
00.
0000
6–4
0.01
578
–1.1
5996
–8.8
29
0.50
5
2 0.
5391
8 0.
0003
3 0.
0625
40.
0000
50.
0011
90.
0000
10.
0054
60.
0001
3 0.
0016
40.
0000
40.
8791
90.
2113
70.
194
0.04
7
3 0.
2932
4 0.
0002
0 0.
1036
60.
0002
10.
0014
40.
0000
20.
0037
30.
0000
7 0.
0006
90.
0000
40.
8647
20.
1175
60.
190
0.02
8
4 0.
1493
9 0.
0001
6 0.
0890
10.
0000
70.
0010
80.
0000
10.
0028
00.
0000
7 0.
0002
40.
0000
40.
8840
80.
1328
00.
195
0.03
1
5 0.
1376
6 0.
0001
0 0.
1130
20.
0000
70.
0013
20.
0000
10.
0035
40.
0000
7 0.
0001
60.
0000
40.
8021
50.
1045
90.
177
0.02
5
6 0.
0725
0 0.
0001
3 0.
0571
70.
0001
10.
0007
10.
0000
10.
0024
70.
0000
7 0.
0001
30.
0000
40.
5996
00.
2067
60.
132
0.04
6
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