Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

interpreted as either an expression of repeated
earthquake activity or tsunami‐like waves induced by
submarine slides. This sedimentary drilling core B 5 shows
three significant “fining‐up” sequences. The upper one
reaches from clayey sediments at the top to coarser
grained, unsorted pebbly layers containing shell debris
and a certain composition of microfossils. Both of the two
lower fining‐up sequences are clearly cut at their base
where the changeover to the next core meter is located,
indicating sedimentary fall while drilling. Every fining‐up
sequence contains a similar composition of microfossils as
well as shell debris in the lower parts near the base.
Mainly foraminifera such as Elphidium Crispum and
Quinqueloculina sp., both of which are found in 20 meters
to 30 meters dephts of water, were also found in
tsunamites at the south Portuguese coast assigned to the
1755 event (Bryant et al., 2007), once more confirming
the tsunamigenic origin of these units. Storm events as
potential cause of these deposits can be excluded
because at present they would not even reach further
inland than 250 meters to 280 meters (Gracia et al.,
2006). Considering the great depth of about 4.0 m, the
lower “fining‐up” sequences can be related to former
tsunami events, what has to be proved by dating of
certain sections in the near future.
Also the damage to the old bridge at Los Lances Bay (Fig.
5) is described by Gracia et al. (2006) as reason to the
tsunami in 1755.
Fig. 5: The old bridge at Los Lances Bay.
It was constructed by the beginning of the 18 th century
and partly destroyed by the 1755 tsunami. The
distribution of erosion and accumulation zones around
this structure suggests that the bridge acted as an
obstacle to the tsunami waves, and all the system could
be considered as a huge flute mark.
CONCLUSIONS
Combined, we describe new findings of tsunami deposits
along the coast between Barbate and Tarifa.
In conclusion, we have found several different distinctive
features of tsunamigenic deposits along the Spanish
Atlantic coast, which are characterized by:
Clast‐supported, polymodal, boulder‐bearing basal
deposits composed mostly of well rounded clasts
and fewer angular clasts, which are partly
imbricated.
Normal grading or crude normal grading. The lateral
changes in characteristics of depositional facies are
common and abrupt (channels)
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Clay to sand‐sized, bioclastic (and Roman ceramic)‐
rich matrix is poorly sorted, implying that soft
sediments eroded at the lower erosional surface
contributed to the tsunami deposit.
Mixed source of sediments (beach and marshes).
Deep water foraminifera deposited by high energy
event, such as a tsunami.
These features are interpreted as non‐cohesive and
sediment‐loaded subaquatic density flows and deposits of
successive waves in the tsunami wave train. The
incorporation of sediments derived from mixed sources
within the tsunami deposits, such as angular clasts from
nearby subaerial settings, rounded clasts reworked from
beach gravels, and shell debris and yellowish beach sands
eroded from older, and unconsolidated, shoreface
deposits are interpreted as back flow or back wash
deposits.
OUTLOOK
Further fieldwork in the study area near to Barbate beach
will concentrate on palaeo‐relief, which can be detected
by GPR, to evaluate the display of tsunamigenic deposits
along the beach and the inland.
Also 14 C dating of some samples within the PVC liners is in
progress, to ensure, that the event took place at 1755,
which is the final question of this project.
Acknowledgements: We would like to thank Spanish‐German
Acciones Integradas Program HA2004‐0098, by the Spanish
Research Projects CGL2005‐04655/BTE (USAL), CGL2005‐
01336/BTE (CSIC) and by Deutsche Forschungsgemeinschaft
Project Re 1361/9 for the contingency to realize our fieldwork
and studies.
References
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charcoal and sediment drilling cores as first evidence of
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