Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Rising magnetic susceptibility within the "fining‐up"
sequences validated the tsunamigenic grain size
distribution.
Beach section between Barbate and Zahara de los Atunes
The 5 km long rock cliff has been mapped, leveled,
sampled (including sediment lacquer films; Fig. 3). We
have surprisingly found only one layer on top of the
basement at the cliff, in varying altitudes between 1.0 and
4.5 m above mean sea level. The basement is Cretaceous
to Eocene flysch deposits or OIS 5 terraces (Tyrrhenian) of
approximately 125 ky. The dark sandy layer of about 1.0
meter thickness constitutes a "fining‐up" sequence with a
coarse‐grained base with conglomerates, shell debris and
charcoal. These deposits are channeled and clasts are
imbricated, palaeo‐flow direction is towards the Atlantic.
As the beach sands are white to yellowish and the layer is
dark, organic‐ and clay mineral‐rich, but sandy, we
interpret this layer as a back flow. Palaeo‐current
directions endorse these observations and the evidence
that only one layer is preserved. Presumably, former
tsunamites were eroded due to multiple wave action and
the cliff was “cleaned” to the basement by the waves.
Fig. 3: Sediment lacquer film of the "back‐wash" sediment,
outcropped at the beach of Barbate; Bottom line is the boundary
of flysch deposits and “back‐wash” sediment (erosive base); Top
line is the boundary of “back‐wash” sediment and topsoil.
The dark clayey marshlands of Barbate have been
reworked and the tsunamite was deposited as a mixture
of beach sands, boulders and shell and the clayey marsh
deposits during the back flow of the last major wave. The
Flandrian transgression had a maximum at about 7.000 to
6000 years BP and reached altitudes of 3.0 to 4.0 m above
the present sea level and cannot account for these
deposits. Our interpretation of the principal sediment‐
depositing mechanisms effective in tsunami surges is
based on field observations of deposit geometry and
internal sedimentary characteristics, which are clearly not
related to a beach.
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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Shallow drilling at location B 7 give us further clues for a
tsunami event. Three “fining‐up” sequences on top of
each other show us that accordingly more than one wave
of the so‐called tsunami wave train is documented by
these deposits. Also “rip‐up” clasts ‐ mostly angular clasts
which are imbricated by clayey sediments ‐ are detected
in the sedimentary drilling core at this location.
Bolonia Bay
Here, we investigated the Roman ruins of Baelo Claudia.
As mentioned above, several indicators of tsunami
deposits have been published. We are currently
investigating deposits in the ruins, which may also contain
tsunami‐reworked “post‐Roman” colluvium (Silva et al.,
2009). Also, the deposits described by Becker‐Heidmann
et al. (2007) are under examination. The “block fields” of
Garcia et al. (2006) are related to small creek mouths into
the Atlantic, we regard these as storm deposits reworking
fluvial pebbles. They are only found near the creeks and in
heights of 2.0 meters to 3.0 meters above the mean sea
level along the beach.
Valdevaqueros Bay (Mellaria)
In this bay the rests of the Roman village of Mellaria are
partly exposed, they are not excavated and covered by
2.0 meters of sandy, clayey sediments. The laguna of
Valdevaqueros yields dark organic‐rich marshy sediments,
which may possibly be reworked during wave action. We
have drilled a profile perpendicular to the coast up to 4.0
m depth. Surprisingly, laboratory analysis gave no clues to
tsunami event in this area. The sedimentary cores of
Valdevaqueros do not contain any tsunamigenic features.
Either tsunami deposits are reworked and/or eroded by
the river nearby or the tsunami waves did not hit
Valdevaqueros frontally due to wave directions of the
1755 Lisbon event and bay protection. As well the locality
of Mellaria is not the one indicated by Gracia et al. (2006),
who put the village in the Los Lances bay. Mellaria was a
Roman fishery village directly at the coast.
Los Lances area (Tarifa)
The marshlands of the Río Jara north of Tarifa are called
Los Lances (Fig. 4). Here, wash‐over fans of the Lisbon
tsunami (without age constraints) (Gracia et al., 2006).
Fig. 4: Map of Los Lances Bay near Tarifa; washover fans caused
by the 1755 tsunami are also illustrated.
We have drilled a profile perpendicular and parallel to the
coast up to 4.0 m depth. We have found similar
intercalations of tsunamites downhole, which are