A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio, Cape Verde
Abstract
In the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large-scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high-confidence U/Th age of 78.8 +- 0.9 ka, which overlaps within error with the 73 +- 7 ka age proposed for Fogo volcano’s flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea-level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower-confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well-preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly-active volcanoes such as in Cape Verde.
Full text
A geological eco d o mul iple Pleis ocene sunami inunda ions
in an oceanic island: The case o Maio, Cape Ve de
JOS
EMADEIRA*†, RICARDO S. RAMALHO*†‡§, DIRK L. HOFFMANN¶,
JO
~
AO MATA*†and M
ARIO MOREIRA***
*Ins i u o Dom Luiz, Faculdade de Ci^
encias, Uni e sidade de Lisboa, Edi
ıcio C1, Campo G ande,
1749-016 Lisboa, Po ugal (E-mail: [email p o ec ed])
†Depa amen o de Geologia, Faculdade de Ci^
encias, Uni e sidade de Lisboa, Edi
ıcio C6, Campo
G ande, Lisboa 1749-016, Po ugal
‡School o Ea h Sciences, Uni e si y o B is ol, Wills Memo ial Building, Queen’s Road, B is ol BS8 1RJ, UK
§Lamon -Dohe y Ea h Obse a o y, Columbia Uni e si y, Come Geochemis y Building, PO Box 1000,
Palisades, NY 10964-8000, USA
¶Depa men o Human E olu ion, Max Planck Ins i u e o E olu iona y An h opology, Deu sche Pla z 6,
Leipzig 04103, Ge many
**Ins i u o Supe io de Engenha ia de Lisboa, R. Conselhei o Em
ıdio Na a o 1, Lisboa 1959-007, Po ugal
Associa e Edi o – Ped o Cos a
ABSTRACT
In he Cen al A lan ic a chipelagos – he Cana ies, Cape Ve de, Madei a and he
Azo es – sunamihaza diso en ega dedaslow,whencompa edwi ho he
ex eme wa e e en s such as hu icanes and s o ms. The geological eco d o
many o hese islands, howe e , sugges s ha sunami haza d may be unde es i-
ma ed, no wi hs anding being lowe han in a eas adjacen o subduc ion zones,
such as he ma gins o he Paci ic and Indian oceans. Mo eo e , sunamis in
oceanic islands a e gene ally igge ed by local la ge-scale olcanic lank col-
lapses, o which li le is known abou hei equency, making i di icul o es i-
ma e he p obabili y o a new occu ence. Pa o he p oblem lies in he ac ha
sunami deposi s a e usually di icul o da e, and ew islands in he wo ld exhi-
bi e idence o epea ed sunami inunda ion on a p o ac ed imescale. This
s udy epo s on he p esence o abundan sunami deposi s (conglome a es and
sands ones) on Maio Island (Cape Ve de) and discusses hei s a ig aphy, sedi-
men ological cha ac e is ics, p obable age and sunamigenic sou ce. Obse a-
ions indica e ha ou dis inc inunda ion e en s o a iable magni ude ook
place du ing he Pleis ocene. One o he sunami deposi s yielded a high-con i-
dence U/Th age o 78809 ka, which o e laps wi hin e o wi h he
73 7 ka age p oposed o Fogo olcano’s lank collapse, an e en known o
ha e had a signi ican sunami impac on nea by San iago Island. This shows
ha he Fogo sunami also impac ed Maio, esul ing in unups in excess o 60 m
abo e coe al sea-le el a ca 120 km om he sou ce. Two olde deposi s, possi-
bly linked o ecu en lank collapses o he Tope de Co oa olcano in San o
An ~
ao Island, yielded lowe -con idence ages o 479 o 390 ka and 360 o 304 ka.
A younge deposi (<78 ka) emains unda ed. In summa y, he geological eco d
o Maio exhibi s well-p ese ed e idence o epea ed sunami inunda ion, ein-
o cing he no ion ha sunami haza d is no so low a olcanic a chipelagos ea-
u ing p ominen and highly-ac i e olcanoes such as in Cape Ve de.
Keywo ds Cape Ve de, equency, ocean islands, sedimen s, sunami.
1
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s
Sedimen ology (2019) doi: 10.1111/sed.12612
INTRODUCTION
Oceanic islands a e exposed o a ious na u al
haza ds depending on hei geodynamic se ing
and geog aphical loca ion; hese include ol-
canic e up ions, ea hquakes, loods, d ough s,
s o ms, landslides and sunamis. Island popula-
ion cen es a e usually loca ed a he coas and
o en a low ele a ions, and a e hus highly ul-
ne able o ma ine inunda ions esul ing om
s o m wa es and su ges o om sunami impac .
In he Cen al A lan ic a chipelagos, sunami
haza d is o en ega ded as low, bu he geologi-
cal eco d sugges s ha his haza d has been
conside ably unde es ima ed. Despi e being
much lowe han on coas lines o oceans su -
ounded by ac i e subduc ion zones capable o
p oducing high-magni ude subma ine ea h-
quakes, examples om Maca onesia (a biogeo-
g aphic egion including he Cen al A lan ic
a chipelagos o he Azo es, Madei a, Sel agens,
Cana ies and Cape Ve de; Fig. 1A) show ha he
isk imposed o popula ions by his ype o na u-
al haza d may, in ac , be much highe han is
usually conside ed. Accoun s o his o ical
e en s and geological e idence o pas sunami
inunda ions a ec ing he islands, igge ed by
bo h local and dis al sou ces, exis in all o Mac-
a onesia (Wa d & Day, 2001; Pa a as-Ca ayannis,
2002; And ade e al., 2006; Gisle e al., 2006;
Pe ez-To ado e al., 2006; Giache i e al., 2011;
Pa is e al., 2011, 2017, 2018; Fe e e al., 2013;
Hun e al., 2013; Ramalho e al., 2015; Madei a
e al., 2016; Omi a e al., 2016). Fo example,
chao ic conglome a es ea u ing ma ine ossils
occu dispe sed along he slopes o Aga€
e e al-
ley in G an Cana ia, om 41 o 188 m in ele a-
ion, a es ing o signi ican sunami inunda ion
esul ing om he G€
u
ıma olcanic lank col-
lapse(s) in he nea by island o Tene i e, which
ook place some ime be o e 083 Ma (Pe ez-To -
ado e al., 2006; Giache i e al., 2011; Pa is
e al., 2018). Ano he example in he Cana y
Islands conce ns he Isla Baja and Teno Bajo
deposi s in no h-wes Tene i e, ound up o
132 m in ele a ion, which a es o he succes-
si e impac o wo consecu i e sunamis ig-
ge ed espec i ely by he Icod lank collapse and
Ab igo ignimb i e e up ion ca 170 ka; hese
deposi s indica e a scena io in ol ing a coupled
lank collapse and majo explosi e e up ion,
esul ing in ex eme sunami inunda ion o adja-
cen coas lines (Pa is e al., 2017, 2018). The
collapse o Fogo olcano, in Cape Ve de, is ye
ano he example o a sunamigenic e en ha
esul ed in a de as a ing impac o adjacen
coas lines. E ec i ely, ields o s anded mega-
clas s (up o 243 m
3
) and chao ic ma ine con-
glome a es occu in he adjacen island o
San iago, up o 220 m o ele a ion, a es ing o
he impac o a sunami igge ed by his col-
lapse a ca 73 ka (Pa is e al., 2011, 2018;
Ramalho e al., 2015; Ma
ınez-Mo eno e al.,
2018). These deposi s imply sunami unups
exceeding 270 m along he no he n sho e o
San iago, once again con i ming he haza d
po en ial o olcanic lank collapses as nea - ield
sou ces o highly de as a ing sunamis (Ramalho
e al., 2015; Pa is e al., 2018).
Despi e he exis ence o ample e idence o su-
nami impac on se e al olcanic a chipelagos, a
key ques ion in he deba e abou he haza d and
isk posed by sunami inunda ion along he
coas lines o hese a chipelagos –by ei he
locally-sou ced collapse- igge ed sunamis o
emo ely-gene a ed sunamis –conce ns he e-
quency o such e en s. The es ima ion o i s e-
quency is c ucial o assess he suscep ibili y/
ulne abili y o coas al popula ions o sunami
inunda ion. A lack o a easonable knowledge
conce ning he equency o such e en s is
de i ed om se e al ac o s, namely he poo
p ese a ion o many o he deposi s (pa icula ly
deposi s om smalle -magni ude e en s), di i-
cul y in da ing se e al o hose deposi s (a p ob-
lem exace ba ed by he in insic ex eme la e al
a ia ion o acies), a iable coas al mo phology
p o iding di e en sedimen accommoda ion
space and, pe haps as he esul o he p e ious
ac o s, he a i y o ou c ops ha clea ly a es o
mul iple sunami inunda ion o he same coas -
line on a p o ac ed imescale. So a , Aga€
e e al-
ley (G an Cana ia) cons i u es one o he ew
loca ions a oceanic islands wo ldwide wi h e i-
dence o mul iple ex eme sunami inunda ions
o e a pe iod ha was long enough o allow he
deposi ion o allu ial conglome a es and he o -
ma ion o palaeosols in be ween e en s (Madei a
e al., 2011; Pa is e al., 2018). Success ul da ing
o indi idual deposi s a Aga€
e e alley, howe e ,
has p o ed elusi e, mos ly owing o he lack o
da eable ma e ial o o he poo p ese a ion o
he deposi s hemsel es, a ac ha has so a
hampe ed a solid es ima ion o he equency o
sunami inunda ion a a single island o island
g oup (Pe ez-To ado e al., 2006; Madei a e al.,
2011).
The geological eco d o Maio in he Cape
Ve des, howe e , may p o ide he pe ec place
o s udy he equency o sunami inunda ion a
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
2J. Madei a e al.
a single island. Owing o i s a id clima e, low
coas al mo phology and e y slow upli a e,
Maio o e s he pe ec condi ions o hos and
p ese e sunami deposi s, allowing also o
mo e ho ough compa a i e s udies be ween
sunamigenic and ma ine e ace deposi s, gi en
ha he la e a e also well-p ese ed on he
island. Acco dingly, his s udy analyzes he geo-
logical eco d o Maio and demons a es ha he
island exhibi s well-p ese ed e idence o mul-
iple sunami inunda ion on a p o ac ed ime-
scale, possibly making i a e e ence locali y
amongs oceanic islands o in es iga e he e-
quency o such ex eme e en s. As he cu en
Fig. 1. Loca ion, mo phology and geology o Maio. (A) Loca ion o he Maca onesian a chipelagos, ba hyme y/
al ime y om ETOPO1 Global Relie Model (Na ional Geophysical Da a Cen e /NESDIS/NOAA/U.S. Depa men
o Comme ce, 2011). (B) The loca ion o Maio Island wi hin Cape Ve de A chipelago, ba hyme y om Gene al
Ba hyme ic Cha o he Oceans (GEBCO, 2014) and al ime y om Shu le Rada Topog aphy Mission, 1 a c sec
scene (SRTM3). (C) Hillshade om a digi al ele a ion model (DEM) o Maio Island (o iginal da ase a 1 : 5000
scale; MAHOT, 2010), wi h main oponymy. (D) Geological map o Maio (adap ed om Se alhei o, 1970, and
S illman e al., 1982) o e hillshade om a DEM (MAHOT, 2010).
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 3
s udy demons a es, he s a ig aphic ela ions
be ween he abundan ou c ops o sunamigenic
sedimen s (conglome a es and sands ones) in
his island indica e he occu ence o ou dis-
inc inunda ion e en s o a iable magni ude,
which ook place du ing he Pleis ocene. This
s udy documen s hese deposi s including hei
s a ig aphic ela ions, ex u al cha ac e is ics,
basal ea u es and palaeocu en ma ke s. The
age o he deposi s is add essed by U/Th da ing
o co als sampled om hese deposi s. Possible
sou ces o hese sunamis a e also discussed
wi hin he con ex o Cape Ve de.
MORPHOLOGY AND GEOLOGICAL
SETTING OF MAIO
The island o Maio is one o he oldes in he
Cape Ve de A chipelago (Fig. 1A and B), possi-
bly ha ing eme ged abo e sea-le el du ing he
ea ly Miocene (Se alhei o, 1970; Mi chell e al.,
1983; Ramalho e al., 2010a). I belongs o he
Leewa d o Sou he n G oup o he Cape Ve des
and is loca ed be ween he islands o Boa is a
and San iago (Fig. 1B). Maio is a ela i ely small
(269 km
2
), ellip ically-shaped island, 25 km
long in he no h o sou h di ec ion by 15 km
eas o wes . The island, howe e , has been sig-
ni ican ly educed by ma ine ab asion and is
p esen ly app oxima ely hal o i s o iginal a ea.
The ex ension o he island’s no h shel indi-
ca es an o iginal no h–sou h dimension o ca
44 km. P esen ly, he island edi ice exhibi s a
azed mo phology cha ac e ized by low-lying li -
o al pla o ms, su ounding esidual elie s in
he cen e o he island (Fig. 1C). The li o al
pla o ms exhibi a s ai case mo phology ha
co esponds o Holocene li o al sal pans and a
se o Pleis ocene aised beaches a ele a ions o
2 o6m,8 o12m,15 o20m,30 o40m,55
o 65 m, 65 o 75 m and 80 o 100 m abo e p e-
sen sea-le el (apsl). The ma ine e aces a e
in e p e ed as open-shel o wa e-domina ed
beach deposi s (Ramalho e al., 2010a), in e e y
way simila o he p esen -day beaches ha su -
ound he island, bu which we e consolida ed
and aised by upli , which di e en ially
a ec ed mos Cape Ve dean Islands (Ramalho
e al., 2010b). The esidual elie s ise o low
ele a ions, o example Mon e Penoso wi h
430 m, he maximum ele a ion in he island,
Mon e Ba alha wi h 294 m and Mon e B anco
wi h 253 m. Coas lines a e usually sandy, com-
p ising ex ensi e beaches la e ally con ined by
low ocky blu s composed o ei he basal (s.l.),
limes ones, o consolida ed Pleis ocene beach
and dune deposi s, al e na ing wi h s e ches o
ocky coas line (pa icula ly on he eas e n
sho e), cha ac e ized by low cli s (<5 m). Beach
sand composi ion is highly bioclas ic, composed
mos ly o shell and hodoli h deb is, o en wi h
<10% o li hoclas s/mine oclas s in he mix u e
(Johnson e al., 2013; Ramalho e al., 2013).
Geologically (Fig. 1D), he island exhibi s a
basemen o med by an upli ed Ju assic–C e ac-
eous ocean loo sequence comp ising upli ed
No mal Mid Ocean Ridge Basal s (N-MORB) pil-
low la as o he Ba alha Fo ma ion, in ensely
in uded by dykes, o e lain by limes ones o he
Mo o Fo ma ion and shales o he Ca queijo
Fo ma ion (Se alhei o, 1970; De Paepe e al.,
1974; Be na d-G i i hs e al., 1975; S illman
e al., 1982; Az
ema e al., 1990). Conglome a es
(Co uja Fo ma ion) ep esen ing shoaling o he
island o e lie he sea loo sequence. This base-
men was in uded and de o med ( olded and
aul ed) du ing he Palaeogene by he successi e
in usion o plu ons, o ming he Cen al
Igneous Complex (Rep esas e al., 2012), com-
p ising gabb os (essexi es and py oxeni es) and
syeni es. P obably o he same age a e some a e
ca bona i e dykes. A basal ic (s.l.) olcanic
sequence o Miocene age, comp ising hyalo-
clas i es and pillow la as (Casas Velhas Fo ma-
ion) ep esen ing he seamoun and eme gen
s ages, subae ial conglome a es (Ped o Vaz Fo -
ma ion) and la a lows (Malhada Ped a and
Mon e Penoso o ma ions), uncon o mably o e -
lies he Cen al Igneous Complex and he Meso-
zoic sea loo sequence. The lowlands a ound
he esidual elie s a e ex ensi ely co e ed by
calca eni es and conglome a es om he aised
Pleis ocene beaches, aeolian sands ones (consol-
ida ed dune ields), allu ial ans, ac i e sand
dunes and sal la s (sabkha).
The deposi s epo ed in his s udy ha e been
o me ly classi ied and mapped as aised Pleis-
ocene beach deposi s by p e ious au ho s (e.g.
Se alhei o, 1970; S illman e al., 1982; Ramalho
e al., 2010a), no wi hs anding hei di e en and
dis inc i e ex u al and s a ig aphic cha ac e is-
ics. On close inspec ion, howe e , hese cha ac-
e is ics se hem apa om a ypical ‘no mal’
beach deposi . This wo k demons a es he
sunamigenic na u e o hese deposi s, p esen ing
also e idence o a mul is age o igin (based on a
ex u al and s a ig aphical analysis), and docu-
men ing he ela ionship be ween hese and he
adjacen ‘no mal’ aised beach deposi s.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
4J. Madei a e al.
METHODS
Field obse a ions
The sedimen s we e su eyed h ough de ailed
ieldwo k and mapped a he 1 : 25 000 scale.
The ex u e, s uc u e, s a ig aphy and hickness
o he deposi s we e desc ibed and measu ed,
and he na u e o hei indi idual componen s
analyzed. S a ig aphic columns we e p oduced
o selec ed ou c ops, using s anda d me hods.
Whene e palaeocu en ma ke s we e obse ed,
hese we e measu ed o in e anspo di ec ion.
A en ion was also dedica ed o he basal ea u es
displayed by he ou c ops, i.e. he geome y and
na u e o he s a ig aphic con ac be ween he
deposi s and he unde lying subs a e, as well as
dis inc i e ex u es a he base ( o example,
inclusion o ‘ ip-up’ clas s o he subs a um
wi hin he sunami deposi ). The deposi s we e
sampled o palaeon ological con en , bu i s
s udy will no be add essed he e.
Sedimen ological c i e ia used o iden i y
g a elly sunami deposi s
A de ailed analysis o he sedimen ological c i e-
ia used o iden i ying sunami conglome a es
and g a el beds on oceanic islands can be ound
in he ecen e iew by Pa is e al. (2018). In
gene al, sunami deposi s ypically man le he
opog aphy (e en on s eep slopes) and a e usu-
ally p ese ed as pa ches (len icula geome y)
a di e en ele a ions (Moo e & Moo e, 1984;
Moo e, 2000; McMu y e al., 2004a,b; Pe ez-
To ado e al., 2006; Pa is e al., 2011, 2017,
2018; Ramalho e al., 2015). Addi ionally, su-
nami conglome a es and g a el beds almos
in a iably exhibi chao ic ex u es, being ei he
clas o ma ix-suppo ed bu gene ally ea u ing
a mixing o sedimen s om di e en sou ces
edis ibu ed bo h inland and o sho e (i.e. e -
es ial and ma ine) (Pe ez-To ado e al., 2006;
Ramalho e al., 2015; Pa is e al., 2017, 2018).
G ain-size dis ibu ion is commonly highly
he e ogeneous, o en including la ge boulde s
‘ loa ing’ in ine conglome a ic o sandy
ma ixes and equen ly displaying a mix u e o
bo h well- ounded and angula boulde s/pebbles
(Moo e & Moo e, 1984; Moo e, 2000; McMu y
e al., 2004a,b; Pe ez-To ado e al., 2006; Pa is
e al., 2011, 2017, 2018). Deposi s a e o en o ga-
nized in subuni s sepa a ed by e osional uncon-
o mi ies o by di use bu ab up ansi ions;
ex eme la e al a ia ion o acies may be
p esen (Pe ez-To ado e al., 2006; Ramalho
e al., 2015; Pa is e al., 2017, 2018). Landwa d
ining and hinning o sedimen is a key ea u e
o sunami deposi s bu is no always easy o
iden i y gi en he deposi s’ ex eme a ia ion in
g ain sizes. Tsunami deposi s usually es o e
i egula e osi e basal con ac s and may exhibi
clas ic dykes injec ed downwa d in o he sub-
s a um (Pe ez-To ado e al., 2006; Pa is e al.,
2017, 2018). The p ese a ion o iable subae ial
subs a es benea h sunami deposi s and he
p esence o ip-up clas s o his subs a um, as
well as e e se g aded ac ion ca pe s, a e pa -
icula ly dis inc i e ea u es linked o sunami
inunda ion (Pa is e al., 2018). In e ms o
palaeon ological con en , sunami deposi s also
gene ally exhibi a mix u e o axa wi h di e en
ba hyme ical ecological zona ion, di e en
habi a s and di e en ypes o subs a e (e.g.
Massa i e al., 2009; Coello B a o e al., 2014;
Pa is e al., 2017), o en esul ing in highe bio-
di e si y indexes (Pa is e al., 2018). The only
bio u ba ion ound in sunami sedimen s depos-
i ed onsho e co esponds o oo conc e ions,
o med by subsequen plan coloniza ion a he
su ace o he deposi s.
In con as o he a o emen ioned cha ac e is-
ics, aised beach deposi s ypically show g ea
la e al con inui y alongsho e, es ing on wa e-cu
low-angle sho e pla o ms, o en close o o e en
abu ing agains he o me sho e angle ha ma ks
he maximum each o he high/s ill sea-le el
s and. Basal sho e pla o ms almos always co e-
spond o ha d subs a es, gi en ha so sub-
s a es a e a ely p ese ed by e e yday p ocesses
o wa e-cu ing (Ramalho e al., 2013; Pa is e al.,
2018). Sedimen a y sequences ypically exhibi
no mal g ading and, despi e he p esence o con-
glome a ic basal laye s in e p e ed as ‘ ansg es-
si e lag’, a ely exhibi boulde s o pebble-sized
sedimen s excep in well-s a i ied beds o well-
ounded boulde s/pebbles associa ed wi h clea
palaeo-channels o seawa d-di ec ed e igenous
deb is lows. Sedimen a y s uc u es a e hose
ypical o wa e-bea en sandy beaches, exhibi ing
e en and c oss-bedding and lamina ion (depend-
ing on which zone o he beach i co esponds
o), o swaley and/o hummocky c oss-lamina ion
in p ese ed s o m/highe ene gy sedimen s
deposi ed u he o sho e (Ramalho, 2011;
Mayo al e al., 2013). T ansi ion o aeolian s uc-
u es is equen ly ound in he uppe pa o
aised e aces, and i is o en possible o ollow
he en i e ypical beach p o ile along con inuous
exposu es ups eam p esen -day d ainage
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 5
channels (Ramalho, 2011). Bio u ba ion is pa a-
moun in p ese ed aised beach p o iles, as
desc ibed in de ail in Mayo al e al. (2013).
As shall be demons a ed in he ollowing sec-
ions, he s udied deposi s o Maio Island exhi-
bi cha ac e is ics ha con o m wi h hose
e e ed abo e as ypical o sunamii es and a e
a odds wi h hose o beach deposi s. Key sedi-
men ological c i e ia used o iden i y hese
deposi s as sunamii es a e also he e illus a ed
h ough ield pho og aphs, palaeocu en in o -
ma ion and s a ig aphic sec ions.
U anium/ ho ium (U/Th) da ing
Well-p ese ed co als we e chosen o da ing
pu poses and small sec ions we e cu and pol-
ished o isual inspec ion. The chosen p is ine
co al samples showed no isible indica ion o
ec ys alliza ion.
Subsamples o U/Th age de e mina ions we e
milled om p is ine, dense sec ions o he co als
using a hand-held den al d ill. The masses o
indi idual subsamples we e in he ange
be ween 2 mg and 5 mg. Chemical p epa a ion
ollowed a s anda d p o ocol (Ho mann, 2008)
and U/Th measu emen s we e pe o med on a
mul icollec o induc i ely-coupled-plasma mass-
spec ome e (MC–ICP–MS; Nep une; The mo
Fishe Scien i ic, Wal ham, MA, USA) a he
CENIEH (Cen o Nacional de In es igaci
on sob e
la E oluci
on Humana, Bu gos, Spain) ollowing
p ocedu es ou lined in Ho mann e al. (2007).
All ages we e calcula ed using he hal -li es
epo ed in Cheng e al. (2000) and a e he e
epo ed wi hin 2 o unce ain y.
RESULTS
Geog aphical dis ibu ion o he sunami
deposi s
Discon inuous sunami deposi s we e iden i ied
along mos o Maio’s li o al coas line and up o
7 km inland om he p esen -day coas line
(Fig. 2). Ou c op ele a ions (ob ained om
1 : 25 000 scale opog aphic maps and con-
i med wi h an al ime e ) ange om p esen -day
sea-le el up o 70 m apsl, as a es ed by a su-
nami deposi exposed in a qua y loca ed sou h
o he main oad 44 km o he ENE o Vila de
Po o Ingl^
es (si e 1 in Fig. 2; a N15.15515°,
W23.17500°). The sunami deposi loca ed u -
hes inland (7 km) is exposed in he uppe
eaches o Ribei a do Luga (si e 2 in Fig. 2; a
N15.25953°, W23.14935°), a ca 40 m in ele a-
ion. Local discon inui y o he ou c ops is in e -
p e ed o be ei he he esul o e osion o o
co e age by younge sedimen s. This is he case
along he sou h-wes coas , he longes s e ch
wi hou isible ou c ops, owing o he exis ence
o ex ensi e sal la s and a con inuous and wide
sandy beach wi hou ocky exposu es.
Al hough he base o he sunami deposi s is
o en sub-ho izon al, in some places hey co e
li o al and lu ial alley slopes (Fig. 3A o D).
Gene ally, he deposi s can only be ecognized
in c oss-sec ion exposu es (a coas al cli aces,
s eam banks, oad cu s and qua ies). A su ace
exposu es, a pe asi e ca bona ed pedogenic
al e a ion (calc e e) su ace makes i impossible
o dis inguish sunami om o he ca bona e
deposi s such as aised beach and consolida ed
aeolian sands ones (Fig. 3E).
S a ig aphy and s uc u e o he deposi s
The sunami deposi s a e amongs he mos ecen
geological uni s in he island and a e only co -
e ed by Holocene o Uppe Pleis ocene o ma-
ions (ac i e o consolida ed dune ields, allu ial
and sal - la deposi s, and beach sands in he case
o he olde sunami deposi ; Fig. 4A and B).
They o e lie all o he geological o ma ions
(Ju assic–C e aceous sea loo sequence, Miocene
subae ial and subma ine la a lows, Pleis ocene
aised beaches, consolida ed dunes, lagoon
sequences, palaeosols and allu ial an conglom-
e a es; Fig. 4C o G). The indi idual sunami
deposi s consis o one o h ee laye s
ep esen ing successi e low uni s, and ypically
s a wi h ma ix-suppo ed chao ic conglome -
a es (Fig. 4G). The hickness o indi idual uni s
anges om a ew decime es up o se e al (4 o 6)
me es. Fou di e en deposi s we e ecognized
in he island. In one key coas al sec ion no h-eas
o he Ba ei o i e mou h (si e 3 in Fig. 2; a
N15.12817°, W23.14037°), h ee sunami deposi s
a e supe posed in a con inuous ou c op (Fig. 5A
and B). All h ee deposi s exhibi e osional bases.
The lowe sunami deposi ( 1 in Fig. 5A) is dis-
con inuous and is ep esen ed by wo len icula
bodies illing palaeochannels on a basemen o
subae ial basal s om he Casas Velhas Fo ma-
ion; i is a ma ix-suppo ed, ining-upwa d, con-
glome a e composed o angula basal clas s (1 o
30 cm in diame e ), chao ically se in a bioclas ic
sandy-sil y ma ix, and exhibi ing ma ine ossils.
The e a e emains o a palaeosol de eloped on
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
6J. Madei a e al.
op o his deposi . The lowe and middle ( 1 and
2 in Fig. 5A and B) sunami deposi s a e sepa-
a ed by a 06 o09 m hick ed allu ial an
deposi , which o e lies a 08 o16 m hick beach
sands one laye . The middle sunami deposi ( 2)
is composed o wo laye s (laye s 4a and 4b in
Fig. 5A). The base is a discon inuous len icula
laye uncon o mably o e lying he allu ial an
and palaeosol, o es ing di ec ly on he beach
sands one, whe e he allu ial deposi was e oded.
I is a ossili e ous ( hodoli hs and molluscs)
ma ix-suppo ed o clas -suppo ed conglome -
a e made o angula basal clas s chao ically se in
a clay o sand ma ix (p obably he esul o mix-
ing be ween he allu ial an sedimen s and ma -
ine sands) and con ains ip-up clas s o he beach
sands one. I is less consolida ed han he uppe
laye , which s ands ou as an e osion- esis an
bed. The uppe laye o he middle sunami (uni
4b o 2 in Fig. 5) is a ossili e ous upwa d- ining
ma ix-suppo ed conglome a e composed o
angula basal agmen s (<20 cm) chao ically se
in a bioclas ic sand ma ix. Locally i displays a
palaeosol de eloped on op. This deposi is co -
e ed by a 07 m hick ed allu ial an deposi ,
which in u n is o e lain by he uppe sunami
deposi ( 3 in Fig. 5A and B). This sunami
deposi p esen s a wa y base ha co esponds o
palaeochannels ca ed on he unde lying allu ial
sedimen s. I is a single ossili e ous conglome -
a e laye wi h angula basal clas s suppo ed by a
bioclas ic sand ma ix. This deposi is iche in
ossils and sligh ly less consolida ed han he
lowe and middle sunami deposi s. The
sequence is co e ed by mode n ed allu ial an
conglome a es.
A ou h, younge sunami deposi ( 4)
occu s a he coas jus no h o he mou h o
Ribei a Funda de Cima (si e 4 in Fig. 2; a
N15.15102°, W23.10458°). This deposi co e s
a slope ha ex ends om he base o he sea
cli a 1 o 2 m up o 9 o 10 m apsl and cu s
he pla o m (a an ele a ion o 9 o 13 m) ha
suppo s one o he olde sunami deposi s
Fig. 2. Simpli ied map o he
sunami deposi s ecognized in he
Island o Maio (shaded in yellow:
da ke a eas –obse ed deposi s;
ligh e a eas –in e ed deposi s)
and main oponymy, o e hillshade
om a digi al ele a ion model
(MAHOT, 2010), numbe s ma k he
si es e e ed o in he ex and in
Table 1. U/Th ages o da ed co als
a e assigned o he espec i e
ou c ops.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 7
(p obably 1); i is a ma ix-suppo ed chao ic
conglome a e wi h angula clas s o MORB
basal and aeolian sands one se in a weakly
consolida ed bioclas ic sand ma ix (Fig. 3C).
This deposi is e y ossili e ous and some o
he gas opod shells s ill display colou . The
geome ic ela ion wi h he sunami deposi
ha c ops ou on he 9 o 13 m apsl pla o m,
he weak deg ee o consolida ion and he good
p ese a ion o he ossil con en sugges a
younge age when compa ed wi h he h ee
olde sunami deposi s.
A
BC
DE
Fig. 3. Field pho og aphs o sunami deposi s and hei ela ionship wi h he opog aphy. Map shows he loca ion
o he depic ed ou c ops. (A) Tsunami deposi o e lapping a slope a he igh bank o Casas Velhas c eek (base
ma ked by do ed line), in he a ield he ho izon al whi e deposi is a Pleis ocene aised beach (base ma ked by
dashed line). Leng h o ma ked sunami deposi is ca 80 m. (B) Tsunami deposi o e lapping a slope a he igh
bank o a c eek on he eas coas (base ma ked by do ed line, 14 m walking s ick o scale). (C) Pho og aph looking
upslope o he younge sunami ( 4) deposi (base ma ked by do ed line) o e lapping a coas al slope on he eas
coas , he la su ace in he backg ound is co e ed wi h an olde sunami (p obably 1; base ma ked by dashed line)
and he whi e cli behind is cu on a Pleis ocene aised beach deposi ( he isible pa o walking s ick is 12m).
(D) Geological ske ch showing he ela ion be ween sunamis 1 and 4. (E) Tsunami conglome a e exposed a he
coas sou h o Calhe a, wi h a 20 cm hick calc e e de eloped on op, he deposi is co e ed by a sandy collu ium.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
8J. Madei a e al.
A ano he loca ion in he sou h-eas coas
(si e 5 in Fig. 2; a N15.13639°, W23.12778°), an
ou c op a he sea cli exposes wo supe posed
sunami deposi s (Fig. 5C). The lowe sunami
sequence is o med by ou laye s (uni s 1a o
1d in Fig. 5C) dipping 16° o he no h (S
0
–
N80W,16N), i.e. landwa d, es ing on an e osion
su ace cu on MORB pillows, in uded by
dykes, om he Ba alha Fo ma ion. The land-
wa d dip o he lowe sunami sequence indi-
ca es ha i ossilized a no hwa d-dipping
palaeo opog aphy, a ea u e ha is ne e
obse ed in he case o ma ine e aces, which
in a iably es on gen le seawa d-dipping sho e
pla o ms. This sunami deposi s a s wi h a
chao ic coa se conglome a e, mos ly clas -sup-
po ed, comp ising angula basal clas s se in a
bioclas ic sand ma ix con aining hodoli hs and
oys e s; i is ollowed by h ee ining-upwa d
laye s o ossili e ous conglome a ic sil y-sands.
The uppe sunami deposi s ands di ec ly,
h ough an e osi e undula ed su ace, on he
lowe sunami deposi . This deposi consis s o
wo laye s (uni s 2a and 2b in Fig. 5C). The
basal laye is a ossili e ous ining-upwa d
ma ix-suppo ed conglome a e wi h a bioclas ic
sandy ma ix. Mos o he la ge clas s a e a he
base illing he e osi e palaeochannels, bu some
a e loa ing in he sandy ma ix, in chao ic posi-
ions. The la ge clas s consis o igneous ocks,
aeolian sands one, and ip-up clas s om he
unde lying sunami conglome a e. The base o
uppe laye is also e osi e, and he conglome a e
displays e e se g ading wi h he la ge blocks
on op. Igneous clas s a e dominan , and he ex-
u e is mos ly ma ix-suppo ed.
In all o he loca ions only one sunami deposi
is p esen , making i di icul o de e mine o
which e en i co esponds. The e a e, none he-
less, some indica o s such as he deg ee o li hi-
ica ion, dissolu ion o mollusc shells, o
co e age by consolida ed aeolian sands all
poin ing o an olde age o he deposi s. The
a ibu ion o he ou c ops o a de e mined su-
nami e en is in mos cases dependen on u -
he iso opic age de e mina ions.
The base o he deposi s is e osional in mos
cases (Fig. 6A o D), al hough la e ally he con-
ac may be appa en ly con o mable wi h he
unde lying uni s. The e osional ea u es include
palaeochannels and enches unca ing he
basemen o ma ions in bo h so and ha d ock,
o downwa d injec ed sedimen dykes. A Pon a
P e a beach, he p ocess o qua ying he sub-
s a e by he sunami was p ese ed in he
ex u e o he conglome a e, wi h ip-up clas s
o he unde lying beds loa ing in he conglom-
e a ic ma ix in he icini ies o he e osi e s ep
(Fig. 6D). The e, he sunami deposi o e lies a
25 o28 m hick lagoon sequence consis ing o
e igenous sands ones and consolida ed mud-
s one laye s exhibi ing bio u ba ion by ine
oo s. The sunami sedimen can be ound
injec ed in o he laye s o he lagoon sequence
along mechanical discon inui ies (sub e ical
join s and s a i ica ion su aces). These injec-
ions ac as wedges ha plucked blocks om he
unde lying laye . Blocks o hese cha ac e is ic
sedimen a y ocks a e ound inco po a ed
wi hin he o e lying sunami conglome a e
(Fig. 6D). So in aclas s o palaeosol, olcanic
u s, aeolian sands one and wea he ed la a a e
occasionally obse ed bu a e no abundan
(Fig. 6E).
Tex u e and composi ion o he deposi s
The sunamigenic sedimen s exhibi a bimodal
g anulome y co esponding o e y coa se con-
glome a es and sands ones con aining loa ing
pebbles, boulde s and blocks (Fig. 7A o G).
Some sands ones exhibi well-de eloped undu-
la ing hin lamina ion and include loa ing (i.e.
ma ix-suppo ed) boulde s (Fig 7D). The ex-
u e o he conglome a es can be ei he clas -
suppo ed o ma ix-suppo ed depending on
he amoun o ma ix. The ma ix o bo h he
conglome a es and sands ones is a medium o
coa se biogenic sand (Fig. 7C) dominan ly com-
posed o agmen s o calca eous algae (bu
also con aining agmen s o echinode m spi-
cules, mollusc shells and occasional o amini-
e a es s) wi h a mino ac ion o li hoclas s
(basal and calca eni e) and mine oclas s
(mos ly agmen s o py oxene c ys als). The
na u e o he boulde s and blocks is a iable
and depends on he geological uni s ha c op
ou in he su ounding a eas. These may
include blocks o olcanic ocks (subae ial and
subma ine basal s and dykes), a ious ypes o
sedimen a y ocks (lagoon muds ones and
sands ones, calca eni es om aised beaches,
consolida ed aeolian sands ones, o Mesozoic
limes ones) and plu onic ocks (gabb o). The
coa se ac ion includes bo h well- ounded
beach pebbles and boulde s and angula ock
agmen s (Fig. 7B and D). Some o he
ounded boulde s we e anspo ed inland om
he li o al as e idenced by he p esence o
a ached oys e s and bioe osion ea u es
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 9
A
BC
DE
FG
Fig. 8. Examples o he ossili e ous con en o he sunami deposi s. Map shows he loca ion o he depic ed ou c ops. (A)
La ge sha e ed block o ee co al (whi e do ed con ou ) a Ribei a do Luga c eek (geological hamme as scale). (B) Small
co al head a sunami ou c op a he eas coas (co al is 10 cm in diame e ). (C) and (D) Examples o ossil gas opods: ossil
(C) is 25 cm in diame e ; he limpe in (D) belongs o genus Fissu ella and is 35 cmindiame e .(E)Fossilo anechinode m
spicule (3 cm long). (F) and (G) Two di e en species o bi al es p esen ing a icula ed, closed al es. (F) Sample om Pon a
Pede nau on he no h coas (23 cm in diame e ). (G) Sample om he wes li o al sou h o Calhe a (7 cm in diame e ).
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
16 J. Madei a e al.
deposi s indica es ha he wa es o he sunami
e en s we e e ac ed a ound he island and ha
he inunda ions pene a ed signi ican ly inland
ac oss la land and climbing low-g adien slopes.
Sedimen sou ces and sedimen anspo
along he sho e
When he geology o he coas line a ies apidly,
he sunami deposi s may con ain boulde s o
li hologies ha do no c op ou in ha speci ic
locali y, implying along-sho e, landwa d o sea-
wa d anspo . Along he Ribei a P e a alley
(si e 8 in Fig. 2; a N15.14503°, W23.19338°) he
sunami deposi consis s o wo laye s: a basal
conglome a e up o 14 m hick, o e lain by a 4
o 6 m hick massi e sands one con aining big
blocks o columna -join ed subma ine basal ic
shee lows. The dimensions o 41 pa allel-
epipedal blocks we e measu ed, wi h majo axes
anging om 07 o46 m. In mos cases he
longe axis is pa allel o he join ing indica ing
he minimum hickness o he la a low om
which hey we e e oded. The sandy ma ix is
bioclas ic (ca bona ed), and hus o ma ine o i-
gin. The basemen he e co esponds o he Casas
Velhas Fo ma ion, which consis s o hyalo-
clas i es and isola ed pillow la as and does no
con ain shee lows. This ype o la a p esen ly
does no c op ou anywhe e onsho e, so he
blocks mus ha e been anspo ed no hwa d
along he alley om ou c ops p esen ly below
sea-le el, o om below he deposi i sel . This
assump ion is consis en wi h he low di ec-
ions obse ed a he coas .
An addi ional indica o o he inunda ion
di ec ion is gi en by he na u e o boulde s
included in he sunami conglome a es along
he eas sho eline. In ac , on h ee loca ions he
dominan na u e o he boulde s does no co e-
spond o he local basemen li hologies. Con-
e sely, he dominan composi ion o he clas s
ep esen s li hologies ha consis en ly only c op
ou u he sou h along he coas , implying a
no hwa d anspo , sligh ly oblique o he
coas . E ec i ely, inland om he dune o Pon a
do Mo o da A eia (si e 14 in Fig. 2; a
N15.15417°, W23.10167°), he sunami deposi
s ands on limes ones o Mo o Fo ma ion, whils
he dominan clas s in he sunami conglome a e
a e basal s om he Ba alha Fo ma ion, which
only c ops ou u he sou h. On ha same ou -
c op he sunami deposi injec s he basemen
limes ones along a na ow c ack ha dips 30° o
he NNE, a ea u e consis en wi h he deduced
inunda ion di ec ion. Fu he no h o Pon a do
Mo o da A eia (si e 15 in Fig. 2; a N15.15639°,
W23.09972°), he ela ion is he same; in depos-
i s o e lying limes ones o he Mo o Fo ma ion,
he dominan conglome a e boulde s a e basal-
ic. On he no h bank o Cimido lagoon (si e
16 in Fig. 2; a N15.16861°, W23.09778°), he
sunami deposi o e lies aeolian sands ones ha
co e a basemen o shales belonging o he Ca -
queijo Fo ma ion. A his loca ion, abou 50% o
he conglome a e boulde s a e limes one blocks
om he Mo o Fo ma ion, which only c ops ou
sou hwa d o his loca ion. These h ee ou c ops
he e o e exhibi e idence o alongsho e sedi-
men anspo om sou h o no h, compa ible
wi h he o ien a ion o he conglome a e boul-
de s a Ribei a de San a Cla a. This means ha
along he sou he n hal o he eas sho e o
Maio, he sunami inunda ed he island owa ds
a no hwa d di ec ion.
On he no h coas , he sunami deposi is
composed o ma ine bioclas ic sand (wi h abun-
dan ma ine ossils) and a coa se ac ion mos ly
composed o angula basal ic boulde s om he
Mon e Penoso Fo ma ion. Howe e , a ew blocks
o basal ic subma ine shee lows also occu in
he conglome a e. This li hology does no p e-
sen ly c op ou in he egion, so hese blocks o
subma ine la as mus ha e been picked up by
he sunami om a loca ion below p esen -day
sea-le el. The sand and ossils we e, hus, ans-
po ed inland om a sandy sho e, and he angu-
la boulde s we e picked up om he inunda ed
subae ial opog aphic su ace whe e hose
basal s c op ou .
Tsunami sou ces
The mos p obable sou ce o he sunami
deposi yielding an age o ca 79 ka is he lank
collapse o Fogo olcano (Pa is e al., 2011;
Ramalho e al., 2015). This age o 78809is
well wi hin he 65 o 84 ka age in e al and
compa ible, wi hin e o , wi h he mean age o
73368 ka epo ed by Ramalho e al. (2015)
on he basis o cosmogenic exposu e da ing o
he ields o megaclas s o no he n San iago,
which a e a ibu ed o he impac o a mega-
sunami esul ing om Fogo’s lank collapse.
Because he island o San iago s ands be ween
Fogo and Maio, he sunami wa es had o be
e ac ed a ound he conside able obs acle
o med by he island o San iago o hi Maio and
hen a ound Maio o lea e deposi s a he eas
coas o he island. Gi en ha sea-le el du ing
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 17
65 o 84 ka ( he age in e al epo ed o his
sunami; Ramalho e al., 2015) was 50 o 60 m
below p esen -day le el, he ele a ion (a leas
8 m) o his sunami deposi implies a minimum
unup in excess o 60 m abo e coe al sea-le el,
a a loca ion ha is a a dis ance o ca 120 km
A
BC
DE
FG
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
18 J. Madei a e al.
(as he c ow lies) om he sou ce o his su-
nami. The same sunami p oduced a minimum
unup o 270 m a he no h coas o San iago, a
a dis ance o 70 km om Fogo.
The e a e no con i med sou ces o he h ee
emaining sunami deposi s ( 1, 2 and 4) ound
on Maio. Howe e , onsho e and o sho e e i-
dence o la ge lank collapses abound in he
a chipelago, namely a San o An ~
ao, S~
ao Vicen e
and S~
ao Nicolau Islands (Le Bas e al., 2007; Mas-
son e al., 2008; Ancochea e al., 2010; Eisele
e al., 2015). Acco dingly, he wo olde sunami
deposi s could be associa ed wi h he wo known
lank collapses o he Tope de Co oa olcano in
sou h-wes San o An ~
ao, which may ha e aken
place a a ound 200 o 400 ka and >500 ka (Mas-
son e al., 2008). Bea ing in mind he p obable
ages epo ed by Masson e al. (2008), he o he
majo landslides in he Cape Ve de Islands (no h
and no heas San o An ~
ao, no h and sou h S~
ao
Vicen e, and no h and sou h S~
ao Nicolau) a e oo
old o accoun o he wo olde sunami deposi s
documen ed on Maio, assuming a maximum age
o 479 ka o hese deposi s. The ages ob ained o
Maio’s sunami e en 2 is compa ible wi h he
younge collapse o San o An ~
ao (Tope de Co oa
1), en a i ely da ed a 200 o 400 ka by Masson
e al. (2008). Mo eo e , he age ange displayed
by he co al samples collec ed on his deposi
(Fig. 11), oughly coincides wi h ma ine iso ope
s age 9 (MIS 9; 337 o 300 ka; Lisiecki & Raymo,
2005). This sugges s ha his sunami occu ed
du ing o soon a e his in e glacial. In a simila
ashion, he age o he olde sunami pa ially
o e laps wi h MIS 11c ( h ee peaks o empe a-
u e maxima a 425 ka, 420 o 412 ka and 407 ka;
Candy e al., 2014). This in e glacial co esponds
o a long (20 o 40 ka) wa m pe iod du ing which
he sea was close o Holocene le els (Candy e al.,
2014; Pas In e glacials Wo king G oup o
PAGES, 2016). The ac ha he clima e was
wa m, and he in e glacial pe iod long, may
accoun o he de elopmen o co al ee s on he
Maio coas . This could explain he la ge (me e-
sized) blocks o co al included in he Ribei a do
Luga sunami deposi s. P esen ly no co al ee s
exis in Maio and he ocky ou c ops in he coas
only display small enc us ing and occasional
b anching co als. Ano he signi ican aspec
obse ed in one o he Ribei a do Luga ou c ops
is ha he sunami deposi seems o co e a ocky
ab asion pla o m. This is sugges ed by a dyke
p o uding om ha su ace ha displays bioe o-
sion ea u es, blocks o which we e inco po a ed
in he sunami deposi and anspo ed eas wa d.
The bioe osion bo ings in he basal ic dyke indi-
ca e ha he su ace on which he sunami
deposi s ands was a o sligh ly below sea-le el.
Since he ou c op is p esen ly a an ele a ion o
ca 40 m apsl, i indica es an a e age upli o he
island o 01 mm yea
1
du ing he las 400 ka,
which is a a e compa ible wi h he upli econ-
s uc ions o Maio (Ramalho e al., 2010a,b,c).
The igge o his sunami is s ill unce ain, bu
he olde lank collapse o Topo de Co oa (Tope
de Co oa 2 o Masson e al., 2008) is a good candi-
da e, gi en he p oximi y (bu no o e lap)
be ween he age in e al yielded by 2 (479 o
390 ka) and he age in e ed (>500 ka; Masson
e al., 2008) o his collapse, pa icula ly aking
in o accoun he la ge unce ain y in he age es i-
ma es epo ed o hese landslides.
The beach–lagoon– sunami sequence o Pon a
P e a beach (si es 6 and 7 in Fig. 2) sugges s
ei he a sea-le el d op o coas al p og ada ion.
The beach sands one c ops ou in he p esen -
day sandy beach and sea-cli om sea-le el up
o ca 10 m apsl, while he op o he lagoon
sequence is a ca 14 m apsl. Conside ing he
age o 3971283 ka yielded by he co al sam-
pled om he beach sands one, he sea-le el
d op ollowing he MIS 11c high-s and could
accoun o his eg essi e sedimen a y
sequence. This age also means ha he o e ly-
ing sunami deposi s a e ela ed ei he o e en s
2 o 3.
Fig. 9. Pho og aphs o he di e en aspec s o di ec ional s uc u es displayed by he sunami deposi s. Map
shows he loca ion o he depic ed ou c ops. (A) and (B) Imb ica ed clas s a he base o sunami conglome a es
indica ing in low (coas is o he le ) a he igh bank o Casas Velhas c eek –po ion o walking s ick isible in
(A) is 1 m long; geological hamme o scale in (B) is 33 cm long. (C) and (D) Imb ica ed la clas s in sunami con-
glome a es indica ing up ush (C) and backwash (D) a he le bank o Cumeazo c eek –coas o he igh in bo h
pho og aphs; yellow ule in (C) is 20 cm long; geological hamme o scale in (D) is 33 cm long. (E) O ien ed la
clas s in sunami conglome a e indica ing cu en s om he no h ( egion o Laje B anca Isle , geological hamme
o scale). (F) G oo es a he base o he sunami conglome a es o Pon a P e a beach indica ing a no h–sou h cu -
en (13 cm long pen o scale and o indica e cu en di ec ion). (G) Linea ions in lamina ed sands one laye om
he sunami deposi s o Casas Velhas c eek alley in e p e ed as low s uc u es (pen o scale and o indica e
cu en di ec ion). Whi e a ows in pho og aphs (A) o (E) indica e cu en di ec ion.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 19
Fig. 10. S e eog aphic plo s o he o ien ed and imb ica ed boulde s, g oo es and linea ions desc ibed in he ex
(and in Appendix S1) and espec i e loca ions o e he hillshade om a digi al ele a ion model o Maio
(MAHOT, 2010). (A) Pon a P e a beach da a wi h o ien ed clas s in blue (blue a ow indica es cu en sense and
associa ed azimu h) and g oo es and lu e cas s in ed (black a ow). (B) and (C) O ien ed clas s (B) and linea ions
(C) measu ed a Casas Velhas alley a he ou c op close o he sho e. (D) o (F) S e eog aphic plo o da a om
wo nea by ou c ops ups eam om (B) and (C) along Casas Velhas alley –(D) and (E) co espond o he lowe
and uppe pa o he same ou c op, showing in low and backwash. (G) Da a om o ien ed la clas s ( ed do s
and black a ow) and imb ica ed clas s (blue do s and blue a ow) om Cumeazo alley. (H) Da a om he coas
nea Pil~
ao C~
ao. (I) Da a om o ien ed la clas s om he coas eas o Laje B anca isle wi h he ose diag am
showing a dominan SSW–NNE di ec ion and a ain e NNW–SSE di ec ion. (J) Da a om la clas s a Pon a Ped-
e nau coas wi h he la ge dispe sion co esponding o in low om bo h he NNW and om he WSW ( ed do s)
and some blocks o e u ned by backwash (blue do s). Dip and dip di ec ion o la clas s, g oo es and linea ions
we e p ojec ed as lines ( ep esen ed as poin s in he plo s), lowe hemisphe e s e eog aphic p ojec ions p oduced
wi h GEORIENT, e sion 9.5.1; GEORIENT, 2015.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
20 J. Madei a e al.
Finally, he younges o he sunami deposi s
( 4) only c ops ou a one loca ion. The e o e, i
seems o be he esul o a e y localized e en
and may ha e been p oduced by a local sub-
ma ine landslide (on he sou h-eas subma ine
lank o Maio?), o by a smalle landslide a
Fogo, ollowing he main e en a 73 o 79 ka.
Final ema ks
No wi hs anding he s ill incomple e knowledge
o he likely sou ces and he need o mo e p ecise
ages o hese e en s, he geological eco d shows
ha he island o Maio was impac ed by ou su-
nami inunda ions in he las ( ou ?) hund ed hou-
sand yea s, h ee o which p oduced signi ican
inunda ion. This cons i u es e idence o mul iple
sunami inunda ions wi hin a p o ac ed ime-
scale, wi h a mean equency o one e en e e y
100 ka. Combined wi h examples om o he
a chipelagos (see Pa is e al., 2018, o a e iew),
he indings epo ed he e ein o ce he no ion
ha sunami haza d is high a mos olcanic a chi-
pelagos, including hose loca ed in oceans su -
ounded by dominan ly passi e ma gins. Indeed,
olcanic islands can g ow quickly p oducing
s eep mo phologies and ab up lanks, which a e
g a i a ionally uns able, being p one o ail. This
is especially ue o a chipelagos ha s ill ea u e
p ominen and highly ac i e olcanoes ha may
one day collapse (again) as is he case o Fogo and
Tope de Co oa in Cape Ve de, Tene i e, La Palma
and El Hie o in he Cana y Islands, o Pico in he
Azo es. Islands wi h ex ensi e coas al lowlands
such as Maio a e pa icula ly ulne able o
Table 1. Loca ion o he samples analyzed by U/Th.
Si e
No.
Field
Re e ence N La . W Long.
Ele a ion
(m)
1 CVP116 15.15528°23.17500°70
2 CVP71a 15.25917°23.14983°39
2 CVP100a 15.25944°23.14944°40
2 CVP142a 15.25944°23.14944°40
2 CVP142b 15.25944°23.14944°40
4 CVP128 15.15111°23.10472°4
7 CVP77 15.12608°23.20255°1
17 CVP135 15.30944°23.12888°1
18 CVP95 15.31222°23.12111°4
18 CVP138 15.31222°23.12111°6
19 CVP122 15.12250°23.14722°3
19 CVP123 15.12250°23.14722°4
20 CVP127 15.20500°23.09416°10
21 CVP129 15.21805°23.09444°3
Table 2. Resul s o he U/Th analyses o co als om Maio: n.d. = no da a.
Lab ID Sample
238
U
(ng g
1
)
232
Th
(ng g
1
)
230
Th
(ng g
1
)
[
230
Th/
232
Th]
ac i i y a io
(
232
Th/
238
U)
ac i i y a io
(
230
Th/
238
U)
ac i i y a io
(
234
U/
238
U)
ac i i y a io
Age
(ka)
(
234
U/
238
U)
ini ial
ac i i y a io
UTO221 CVP129 2478013039489 0573 2353E-02 124E-03 4630441253E-03 1310E-05 5801E-01 4239E-03 1117E+00 2117E-03 7880911457 00026
UTO197 CVP123 2850316292790 0173 4789E-02 275E-03 320532753202E-04 3071E-06 1026E+00 6557E-03 1065E+00 2423E-03 316112311597 00062
UTO218 CVP138a 2747021421162 0179 4629E-02 362E-03 7436710431384E-04 8272E-06 1029E+00 4821E-03 1061E+00 2634E-03 332011611555 00057
UTO192 CVP127 2511421120096 0042 4298E-02 371E-03 83588951401251E-05 1967E-06 1046E+00 6203E-03 1071E+00 3125E-03 338515111848 00077
UTO219 CVP135 2705315900202 0064 4610E-02 268E-03 42579032762445E-05 4323E-06 1041E+00 5150E-03 1067E+00 2470E-03 339312511749 00062
UTO220 CVP95 23959179213241 0987 4130E-02 311E-03 5823441808E-03 1587E-05 1053E+00 5576E-03 1075E+00 2348E-03 344113511976 00071
UTO216 CVP122 3078911821806 0124 5234E-02 222E-03 541035721920E-04 5261E-06 1039E+00 4074E-03 1062E+00 1937E-03 348810911655 00050
UTO196 CVP77 165508807210 0412 2662E-02 131E-03 6893551425E-03 1198E-05 9825E-01 6145E-03 1007E+00 2487E-03 397128310204 00070
UTO195 CVP71a 233499690097 0015 3783E-02 180E-03 73075953401354E-05 9697E-07 9897E-01 5445E-03 1008E+00 2326E-03 422332010261 00068
UTO198 CVP142a 260267080172 0010 4227E-02 119E-03 45911543702161E-05 6896E-07 9921E-01 6255E-03 1009E+00 2403E-03 423736110314 00071
UTO217 CVP128 2674210881589 0096 4658E-02 176E-03 547496941944E-04 5047E-06 1064E+00 4662E-03 1063E+00 2061E-03 424724012085 00121
UTO194 CVP100a 2545515654274 0276 4188E-02 256E-03 182951405494E-04 4994E-06 1005E+00 4995E-03 1017E+00 2480E-03 442835910577 00073
UTO193 CVP116 241910732846 1444 4089E-03 184E-04 232024443E-02 3540E-04 1033E+00 7987E-03 1021E+00 5552E-03 Ou o ange
UTO222 CVP142b 277861005 n.d. 4665E-02 165E-03 1026E+00 3716E-03 1015E+00 1701E-03 Ou o ange
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
Mul iple sunami inunda ions in Maio Island 21
sunami inunda ion, which may a ec a eas up o
se e al kilome es inland. In Cape Ve de his is
he case o Boa Vis a, Sal and S~
ao Vicen e (in
addi ion o Maio i sel ), which a e islands wi h
g owing coas al occupa ion as a esul o a s eady
inc ease in ou ism.
CONCLUSIONS
Resea ch on sunami deposi s om he geologi-
cal eco d o olcanic oceanic islands, including
hei p obable age and sou ce, is c ucial o es i-
ma e he equency and magni ude o hese low-
p obabili y, high-impac e en s, and o assess
he h ea posed by such ex eme geohaza ds o
island popula ions. In his s udy, he occu ence
o abundan sunami deposi s on he geological
eco d o Maio Island is epo ed, wi h key ind-
ings ha a e summa ized he e:
1Sedimen a y deposi s assigned o sunami
deposi ion occu a mul iple places along he
coas o Maio island, being ound up o 70 m in
ele a ion, and up o ca 7 km inland om he
p esen -day coas line.
2The geological eco d o Maio a es s o he
p esence o possibly ou dis inc e en s o su-
nami inunda ion a a p o ac ed imescale, du -
ing he Pleis ocene.
3Based on U/Th da ing o co als sampled in
hese deposi s, one o he e en s yielded an age
o 78 09 ka, which o e laps wi hin e o wi h
he ca 73 7 ka mega sunami igge ed by he
lank collapse o Fogo olcano, an e en ha le
ample e idence o i s impac on he nea by
San iago Island (Pa is e al., 2011; Ramalho
e al., 2015). Two olde sunami e en s a e en-
a i ely da ed a 479 o 390 ka and 360 o
304 ka, da es ha a e compa ible wi h lank col-
lapses a Tope de Co oa in San o An ~
ao (Masson
e al., 2008), hus sugges ing ha his olcano is
a possible sou ce o hese e en s. A younge
(<79 ka), smalle e en emains unda ed.
4The p esence o sunami deposi s co ela i e
o he Fogo lank collapse implies a local unup
o his e en in excess o 60 m abo e coe al
sea-le el, a a dis ance o ca 120 km om he
sunamigenic sou ce.
5The geological eco d o Maio Island hus
exhibi s well-p ese ed e idence o epea ed su-
nami inunda ion, making i ano he e e ence
locali y amongs oceanic islands o in es iga e
he equency o such ex eme e en s. I also high-
ligh s ha low-lying islands a e pa icula ly ul-
ne able o sunami inunda ion, and how sunami
haza d assessmen s o olcanic a chipelagos
should conside such low-p obabili y, high-
impac e en s.
ACKNOWLEDGEMENTS
This wo k bene i ed om discussions wi h col-
leagues M
a io Cach~
ao, C
esa And ade and Ped o
Cos a. This is a con ibu ion om p ojec s CV-
PLUME (PTDC/CTE-GIN/64330/2006), MEGA
WAVE (IF/01641/2015) and UID/GEO/50019/
2013 o Ins i u o Dom Luiz, unded by FCT –
Fundacß
~
ao pa a a Ci^
encia e Tecnologia (Po ugal).
The suppo o colleagues om he Ins i u o
Nacional pa a a Ges ~
ao do Te i
o io (INGT) and
Ins i u o Nacional de Me eo ologia e Geo
ısica
(INMG) o Cabo Ve de, as well as he suppo o
colleague Jai Rod igues, is much app ecia ed.
We acknowledge he con ibu ions by he Associ-
a e Edi o Ped o Cos a, and F ancisco Pe ez-
Fig. 11. Plo o he se o U/Th co al da es by age and ele a ion; sampling si es a e iden i ied by colou s shown
in he inse legend, ho izon al ba s show he age unce ain y a he 2-sigma le el, ele a ion ange o he deposi s
is depic ed by e ical ba s and ci cles a e posi ioned a he sampling ele a ion. The age o si e 4 (ma ked wi h an
as e isk in he inse legend) ep esen s a co al sampled om sunami 4, which was p obably ewo ked om su-
nami 1 and he e o e is no indica i e o he ue age o deposi ion o 4.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
22 J. Madei a e al.
To ado, Raphael Pa is and an anonymous
e iewe o hei ca e ul e ision ha helped o
subs an ially imp o e he o iginal manusc ip .
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Manusc ip ecei ed 28 Sep embe 2018; e ision
accep ed 2 Ap il 2019
Suppo ing In o ma ion
Addi ional in o ma ion may be ound online in he
Suppo ing In o ma ion sec ion a he end o he
a icle:
Appendix S1. De ails o he di ec ional s uc u es
obse ed in Maio’s sunami deposi s.
©2019 The Au ho s. Sedimen ology ©2019 In e na ional Associa ion o Sedimen ologis s, Sedimen ology
24 J. Madei a e al.