Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
TSUNAMIGENIC DEPOSITS ALONG THE SOUTHERN GULF OF CÁDIZ
(SOUTHWESTERN SPAIN) CAUSED BY TSUNAMI IN 1755?
B. Koster (1), D. Vonberg (1) and K. Reicherter (1)
(1) RWTH Aachen University, Neotectonics and Natural Hazards, Lochnerstr. 4‐20, D‐52056 Aachen, GERMANY.
Benjamin.Koster@rwth‐aachen.de, David.Vonberg@rwth‐aachen.de
Abstract: Shallow drilling in coastal areas proved sedimentary evidence for palaeo‐tsunamis along a 50 km long segment of the Atlantic coast
of southern Spain. The study area was the coast between Barbate and Zahara de los Atunes, both on top of rocky cliffs as well as in lagunas
and along sedimentary beaches. In our studies we focused on drill cores, on which we did sedimentary analysis (sieve curves), as well as
magnetic susceptibility and foraminifera identification. They give us clues to one or more tsunami events in this region. Different
characteristics have been detected and evaluated, such as “fining‐up” sequences with coarse shell debris and rising magnetic susceptibility as
well as outcropped “backwash” sediments at Barbate beach. Geomorphologic clues by former publications can be validated like the bridge at
Los Lances Bay which acted like a huge flute mark.
Key words: 1755 Lisbon tsunami, tsunamites, back flow sediments, Gulf of Cádiz
TSUNAMIGENIC DEPOSITS ALONG THE COAST BETWEEN
BARBATE AND TARIFA
Outcrop evidence and shallow percussion drilling in
coastal areas proved sedimentary evidence for
palaeotsunamis along a 50 km long segment of the
Atlantic coast of southern Spain. We studied the coast
between Barbate and Tarifa (Fig. 1 and Fig. 2), situated on
top of rocky cliffs as well as in lagunas and along
sedimentary beaches (Marismas de Barbate and Zahara
de los Atunes, Los Lances N of Tarifa). Also, we focused on
bays (Bolonia, Valdevaqueros), which are most probably
sheltered from direct tsunami wave action. However,
reflections of the waves may occur and may hit these
bays. In these bays, the Roman villages of Baelo Claudia
and Mellaria, respectively, have been situated (e.g. Silva
et al., 2006, 2009).
Fig. 1: Map of the study area between Barbate and Tarifa; every
dot indicate a location of shallow drilling to detect tsunamite
layers.
Following, we will describe the different situations at
several locations in the study area.
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Marismas de Barbate
The marshlands of Barbate have only little elevation of
about 0 to 1.0 m above mean sea level and are flooded
permanently by the tide. This ideal tsunamite reservoir
was probed with percussion coring with an open window
sampler. We reached depths of about 5.0 meters. After
core description the same site was drilled with a sampler
with PVC liners for lab analyses. Cores provided evidence
for tsunamigenic layers. Laboratory analyses validated the
clues of the field‐work.
We found "fining‐up" sequences, typical for tsunamites
(Bryant, 2007), with coarse grain size at the bottom up to
fine clays at the top of these layers, as well as shell debris.
The sequences include foraminifera, such as sponge
spiculae sp., elphidium crispum and globigerina sp. They
are indicator for tsunamigenic deposits, in fact their
habitat lies in deep water regions. Deep water
foraminifera can only deposit at landside by an event,
which affects the whole water head (e.g. Nott, 2006;
Bryant, 2007).
Fig. 2: Map of theBarbate and Zahara de los Atunes area with
possible tsunami action (green arrows); box at the beach shows
place of outcrop evidence near to Barbate.