QUATERNARIA NOVA, VIII, 2004, 263-275
F. ANTONIOU*, G.B. CARULU""\ S. FURLANI**"', R. AURIEMMAo,
R. MAROCCO**
THE ENIGMA OF SUBMERGED MARINE NOTCHES
IN NORTHERN ADRIATIC SEA
keywords: tidal notches, Holocene, Tectonic, North Adriatic
1. INTRODUCTION AND PREVIOUSRESEARCH
The aim of this study is to identify e!ements for the understanding of the mechanisms that controlled the recent vertical movements of the northern Adriatic
carbonatic coastline. On the basis of the peculiar morphology of the Istria
peninsula, various Authors assumed that the area underwent active tectonic
subsidence movements. One of the factors that led to these hypotheses is the
absence of marine deposits at altitudes higher than the Present sea leve!. This
contrasts with most of the Italian coasts, where MIS 5.5 deposits (Tyrrhenian,
125 ky BP) were found in the stable coastal areas, at 5-9 meters above sea leve!.
Moreover, various Roman Age ruins were observed at lower altitudes than predicted by the Lambeck mode! (Lambeck et al. 2004). One of the first Authors
who integrated this hypothesis with geomorphological data is MarkoviéMarjanovié (1971), who summarized the situation quoting various Yugoslavian
Authors: the geomorphological research carried out along the coasts of Istria
reveals the absence of emerged marine terraces. On the opposite, the whole
coast presents various submerged terraces at depths comprised between -20 and
-123 m.
In his work, Degrassi (1957) quotes Gnirs (1908), who described the disco-
* ENEA, Rome, Italy - [email protected]
** Dipartimento di Se. Geologiche, Ambientali e Marine University of Trieste
[email protected];marocco@ univotrieste.it
*** Società di Studi Nettuno, Trieste, Italy - [email protected]
o
Dept. of Archaeology, University of Lecce
- [email protected]
263
very of the first submerged docking structures and hypothesized a sea level rise
of ca. 1.5 m. Later, D'Ambrosi (1958) assumed that, in the last two thousand
years, the sea level varied differently along the Istrian coast, i.e. with lower
rates in the south, where he recorded a variation of -1.6 m in Pola and up to 4.0 m in Trieste. In Muggia, Auriemma et al. (2003) observed that the top portions of the submerged docking structures pointed out by Gobet (Fig. 1) and
Frenopoulos at Punta Sottile are located at ca. -1.2 \ -1.5 m below sea level.
Even considering the removal of the upper rows of blocks in these litde piers
and with a failing estimate, a sea level change less than 1 m couldn't be considered, since the surface had to be at least 30\50 cm over sea leve!. The bed of
blocks juxtaposed ("platea"), from which the Punta Sottile SW pier juts out, is
stili more significant: in this case the visible surface is the original, a removal of
material is not likely; the depth is nowadays at -1.2 m, but to estimate the sea
level change one must consider that this "platea" had to rise enough to be protected from the waves and the high tide.
Fig. 1: Muggia, Punta Sottile SW: pier of Roman age (photo: Società di Studi Nettuno).
Along the Croatian Coast, Fouache et al. (2000) recently carried out a series
of divings in order to look for geomorphological and archaeological indicators
of ancient sea-Ievels. "A submerged notch, corresponding to a sea level that is
lower than the present one by about 50 cm, can be observed in several sites along
the Croatian Coast between Porec and Zadar. A number of submerged archaeological remains like Roman quarries, fish-ponds and cisterns prove that the
264
above mentioned notch corresponds to the sea leve! of Roman antiquity, i.e.
2000 years. South of this area, from Zadar to Split, Roman submerged archaeological remains give evidence of a sea leve! that is lower than the present one
byabout 1.50 m. We are presenting our observations along the Croatian coastline, from Porec to Split, to discuss the possibility of regional neotectonic
effects in the last 2000 years".
2. COASTAL AREASTUDIED
The study area is located on the north eastern end of the Adriatic Sea and it
is comprised between the coastal sectors of the so called ClassicCarso to the
North and the Istrian Peninsula to the South. The sectors stretch out for over
two hundred kilometres, from the cliffs of Duino (province of Trieste) to the
NW, down to the coast of Opatija (Abbazia) in Croatia to the SE (Fig. 2). The
CARSO
1"'
"GeomOrphological
and
archaeological
evidences
,:,.CLASSICO
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,sea
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of
level variations
LEGEND
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Archeologicalremainssurvayad
by authors
Notches surveyed byauthors
Archeological remains surveyed
by Fouacha at al. (2000)
Notchassurveyedby Fouache
al al. (2000)
ISTRIA
Fig. 2: Study area with loeation of the mentioned sites.
265
coast, whieh is mostly calcareous, preserved some interesting evidence of
ancient sea IevelsIocated at altitudes that are Iower than the present one and in
sites that are lithologically,morphologically and climatically different.
The Classie Carso is made up of a strong carbonatie succession; in the
Trieste sector, the succession goes from Aptian p.p. to Lower Eocene p.p. and
it was subdivided by Cucchi et alii (1989) into various informal Members. The
most recent of these is followed by the Lower-Mid Eocene torbiditie deposits
(Flysch). The succession is structured as an asymmetrieal antidinal whose axis
bears dinaric direction and whose south western side appears from very steep
to toppled; from a tectonie point of view, the succession tends to thrust southwestwards over the Flysch through a Iow-angled fault.
The Istrian stratigraphie succession, characterized by a mainly carbonatic
component, is instead comprised between Batonian and Mid Eocene (Velié et
al. 2000) and it is followed by a torbiditie unit of Mid-Upper Eocene age. The
Istrian peninsula was subdivided by various Authors into three sectors, using
the names given by the ancient populations: Red [stria, whieh constitutes the
southern and western portions of the peninsula; this portion, whieh is characterized by wide outcroppings of red soils, consists of the carbonatie jurassie-cretacic-paleogenie platform; Green or Grey [stria,whieh occupies the eastern and
north-eastern sector and is characterized by SW-directed thrusts; this part consists of an area with dastie-carbonatie sedimentation of cretaceous-paleogenie
age; White [stria, représented by the central paleogeriie basin, composed of
strongly folded Eocene limestones thrusting southwards over Green Istria.
The Duino-Sistiana coastal sector, whieh is the northernmost of the studied
sectors (Fig. 2), is characterized by the in/ormaI Member of Borgo Grotta
Gigante (Cucchi et al. 1989). The limestones are light coloured, with strong
stratifieation, decimetrie to metrie period and strong abundance of radioliths
and hippurites (Upper Cenomanian\Maastriehtian). In the portion considered,
the strata dip in the opposite direction of the face slope and they present a very
high slope, from sub-vertieal to, in same cases, overturned. This structural setup strongIy favours the development of tidal notches, as the physieal and chemieal action of sea waters acts on a surface (the stratum surface) that, unlike the
joints, is structurally and compositionally homogeneous. Moving eastwards, the
nearby stretch of Sistiana Bay-Costadei Barbari presents the thrust that brought
the Cretaceous limestones over the Holocene Flysch (Carulli, Cucchi 1991),
whieh therefore constitutes the foot of these carbonatie diffs. This is characterized by strong verticalIayers of sandstone. The base of the foot shows a small
beach placed on a not very wide coastal pIatform. Such a morphologieal and
structural pattern repeats itself, with different magnitudes, up to the Gulf of
Pirano (Slovenijaand Croatia), where the Flysch is replaced by the Istrian limestone formations. The high erodibility of the Flysch section does not present any
trace of the ancient tidal notches, whieh suggests a Iack of notches in the stretch
of coast comprised between Costa dei Barbari-Gulf of Piran: nevertheless, in
Canovella de' Zoppoli, Forti (1985) observed a notch on a series of blocks of
limestone breccias that collapsed over the Flysch of the over1yingdiff.
266
Moving southwards, after the Gulf of Pirano, limestone is the only existing
lithology unti! Opatija (Abbazia). Within the limestones, Veliéet al. (2000) identified four mega-sequences: the most ancient one, from Batonian to Lower Kimmerigian, involves the area around Rovinj (Rovigno). The second mega-sequence outcrops to the north and to the south with Upper Titonian to Upper
Altonian strata, which, given the structural pattern of the area -that is characterised by a wide anticlinal- includes the area of Porec (Parenzo) and, southwards, some km of coast from Rovinj (Rovigno) to Kolone (Porto Colonne).
The third megasequence, from the Upper Aptian to Lower Campanian involves
a large portion of the remaining Istrian coasts, in particular from Porec (Parenzo) to Secovlije (Sicciole) and from Kolone (Porto Colonne) to the coast of
Opatija (Abbazia). The morphological and structural characteristics with which
these limestones outcrop in the north western part do not allow the development of tidal notches as the coast is low and because of the sub-horizontality of
the strata which are very lightly tectonised; to the south, in the Pula (Pola) area
and on the Eastern Istria high cliffs dipping into the sea, the structural and
morphological conditions favour instead the development of notches.
3. DATA
For the present study, coastal and submarine research was carried out in the
Trieste coast and in the Istrian peninsula down to the Krk Island (Fig. 2).
Remains of marine fossils were searched in the Italian, Croatian and Slovenian
territories and inside the coastal marine caves, but nothing was found above sea
leve!. A continuous submerged marine notch was measured (04.2002) on the
limestone cliffs between Sistiana and Duino (Gulf of Trieste) at the tide corrected altitude of -1.7 :t 0.2 m (Figs. 3 and 4). The notch is continuous but it
presents different morphologies and concavities depending on the exposure of
the spot it occupies and on differences of lithology. Recent research in the
Miramare Park pointed out the existence of a notch at ca. - 0.7 m (11.2003), in
correspondence of the huge Eocene limestone olistoliths which, enveloped in
the Flysch, constitute the homonymous promontory. Along the Western Istrian
Coast, a submerged tidal notch was observed at depths similar to those observed by Fouache et al. (2000) (see the points in Fig. 2). In particular, a notch was
found and measured (10.2002)at - 0.8 m (tide corrected) within the Limski Kanal
(Fig. 5). Along the Eastern Istrian Coast, the tidal notch was observed at depths
similar to the Gulf of Trieste (-1.8 :t 0.2 m, measured on 07.2003). The complete
lack of present day notches (or other coastal morphologies) was verified in the
area comprised between Sistiana and Duino (Fig. 6). Some areas by Savudrija
show sea level scarps scarps whose formation is due to particular lithological
conditions whose formation is due to particular lithological conditions (Fig. 7).
On the opposite, in the Istrian area up to the Island of Krk, the present day
notch seems to be carved alI the way through, though its morphological evidence is not very strong if compared to the Tyrrhenian notches (Fig. 8).
267
Fig. 3: Submerged notch at -1.7 m by Duino.
Fig. 4: Submerged notch at -1.7 m by Sisriana.
Fig 5: The submerged ridal notch measured in the Limski Kanal (Croatia) at -0.8 m.
268
Fig. 6: The carbonatic coast between Duino and Sistiana (TS, Italy). There is no sign of the Present
day tidal notch.
Fig 7: The coast by Savudrija (TS, Italy). The particular morphology seems to show an ancient quarry
at about -0.5 m m.s.1.
269
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Fig 8: The carbonatic coast of the Limnski Kanal. Above the notch, at sea leve!, it is possible to observe a scarp with a small terrace.
4. DISCUSSION AND CONCLUSION
Sea levd change along the Italian coast is the sum of eustatic, glacio-hydroisostatic, and tectonic factors (Lambeck et al. 2004). Marine notches are frequent in limestone lithologies and a marine notch is normally carved in 1-4 centuries depending on the locallithology. The use of tidal notches as sea levd
markers is very important for sea levd change study, especially in low-tide seas
such as the Mediterranean. In some Italian tectonically active coastal areas (for
example eastern Sicily)there is a lack of Present day marine notches as tectonic rates are faster than the carving rates; but there may be other factors, that are
not necessarily rdated to tectonics, which may restÙt in a lack of the Present
day marine notch (see also Kershaw, Antonioli 2004).
A continuous submerged marine notch was observed on the limestone cliff
between Sistiana and Duino (Gulf of Trieste, north Adriatic sea) and in Brsec
at the tide corrected altitude of about .1.7 m; a similar notch was measured by
Fouache et al. (2000) along the Croatian coast; the notch was well carved and
located at about -0.5 m, Le. at a lower depth than the Duino notch. In Trieste
instead, the Present day notch is missing. These differences between the various notches may be the restÙt of a different equilibriums between eustatic sea
levd, hydro-glacio isostasy and tectonics in the three areas.
270
The deeper submerged marine notch on the limestones of Trieste, together
with the absence of the Present day marine notch could be explained as resulting from a Plio-quaternary tilting of the Carso plateau, which gently dips to
NW, as illustrated by other observations from Carulli et al. (1980). This may
also be related to the increasing weight of the external fronts of the Southalpine and of External Dinarides mountain ranges. Both chains are characterized by strong seismic activity with consequent neotectonics effects far from
Trieste.
.
The following points give further evidence of a continuous NW-oriented
tectonic tilting:
1) the altitude of the submerged marine notch is about -1.7 m at Trieste, and
between -0.5 and -0.8 m in Istria (Croatian coast);
2) the Trieste area lacks a Present day marine notch in Trieste. This is possibly
due to higher subsidence in the Trieste area; the nearby Istria Peninsula is
instead characterized by a well carved Present day notch;
3) the MIS 5.5 highstand is located at different altitudes; Amorosi et al. (1999)
observed that the highstand is located at -120 m in the delta Po river (about
-lmm/yr); Kent et al. (2002) observed the highstand at -79 m (about -0.7
mm/yr). These factors, together with the absence of MIS 5.5 deposits outcropping in the Istria and Trieste area show a clear E-W oriented gradient;
4) when compared with the predicted sea level change curve by Lambeck et al.
(2004), the altitude of the Holocene lagoonal deposits of Grado (Marocco's
papers) indicates a subsidence rate of about -0.3 mm/yr, which fits the MIS
5.5 data;
5) in the Gulf ofTrieste, Albrecht and Mosetti (1987)found a karsticcave(seemap)
at -100 m and a submerged series of well carved marine terraces, the last one
of which, placed at an altitude of -35 m is presumably related to MIS 5.5;
6) the presence of a topographic paleo surface (Fig. 9) separated by a visible
scarp due to a lithological discontinuity (Flysch limestone) highlights the
lowering of the area by ca. 60-70 m in 50 km in SE-NW direction.
A few questions must still be answered: why was a late Holocene marine
notch carved in a tectonically subsiding area like the northern Adriatic? Marine
notches are in fact usuallyexpected in a stillstand (land and sea) area, or in coastal
areas showing the same sea level uplift rates (like in Taormina, Kersaw and
Antonioli, this volume). However, Lambeck's model excludes positive isostatic
movements for all Italian coasts. A solution to this "enigma" could be a late
Holocene coseismic event or an unexpected isostatic movement of opposite
direction, such as a relative sea levellowering which lasted for a period of time
that allowed the formation of the notch. This may have occurred because of an
extraordinary positive isostatic behaviour originating from the Alps, as recently predicted by Carminati et al. 2003. Doubts occur when the current tectonic
subsidence calculated from the archaeological data of the last 2 ka (ca 1 meter
in 2000 years, therefore 0.5 mm/year) are compared to 0.3 mm/year calculated
271
J
km
---
1
2
Fig 9: The DEM of the Trieste area;it is possible to observe the topographic surface (bordered by
scarps seawards)which lowers in SElNW direction; the numbers indicate meters above sea leve!.
by Lambeck et al. (2004) for the Trieste area, or to the 1.3 mm/year calculated
on the basis of the observations from the Trieste tide gauge in 110 years that are
similar from Istria and Dalmazia tide gauge (0.9 mm/year at Rovinj and 1.1
mm/year at Bakar (Emery et al. 1988).
Archaeological remains along Istrian coast, in part located by A. Degrassi 50
years ago (1957), and recently re-examined like markers ofHolocenic
sea-level
changes by Fouache et al. (2000), deserve a deeper analysis. Unpublished
researches - to be completed - about the pier of U. Busuja (Porto Bussolo) bay
indicate that the top of the structure is now at - 1.2/1.5 m (courtesy of M. B.
Carre and V. Kovacié). Also the two breakwaters ofRoman age that protect the
Ivan Bay (S. Giovanni della Cometa) have the top at -1 m below sea level; as
we know by the ancient sources, the breakwaters had to emerge, to be not a
danger
for sailing.
Even presuming a partial dismantlement
of the structures,
caused by marine activity, the sealeve! in the beginning of the Imperial age, when
those were built, had to be at least 1 m lower. Specific researches are scheduled
on the presumed fish-ponds of Zaliv Sv. Jemej (S. Bartolomeo, Slovenija), in
collaboration between the Trieste University and the Piran Museum, and on
those of Kupanja, N of Porec (Parenzo) to be done under the direction of the
Porec Museum (V. Kovacié), with the collaboration of CNRS (Centre Camille
Julien, M. B. Carre). The situation of these presumed vivaria suggests, only on
the ground of preliminary
272
surveys, a sea level change of about 1 m.
-
-
._~-
Specific researches are schedtÙed on the presumed fish-ponds of Jernejeva
Draga (S.Bartolomeo, Slovenija),in collaboration between the Trieste University
and the Piran Museum, and on those of Kupanja, N of Porec (Parenzo) to be
done under the direction of the Porec Museum (V.Kovacié), with the collaboration of Centre Ausonius (F.Tassaux,Université Bordeaux 3) and M.E. Carre
(UMR 6375 CNRS).
The enigma on the submerged notch of the Adriatic sea has not been solved
yet; however, we believe that we have found the elements for the identification
of the vertical movements in progress, although we stili must find a way to date
the notch.
ACKNOWLEDGEMENTS
We wotÙd like to thank ing. Maurizio Sbogar and Marco Rizzo for the kind
assistance and improvement for this work
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--
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--
ABSTRACT
The analysisof tidal marine notches on the northern Adriatic coast allowed to identify useful elements for the understanding of coastal tectonic mechanisms. The depth
of the submerged marine tidal notches (ranging between -0.5 m and -1.7) and the lack
of a Present day tidal notch on the Trieste coast, together with other stratigraphic and
structural observations are elements that lead to the hypothesis that a SE-NW subsidence tilting is in progress in the whole study area.
RIASSUNTO
La ricerca ha avuto come oggetto l'individuazione di elementi stratigrafici e morfologici in grado di permettere la valutazione di recenti meccanismi tettonici in atto nella
zona di Trieste e penisola Dalmata. Numerosi Autori riportano, sulla base di osservazioni morfologiche, che l'intera area sembra avere subito un costante recente abbassamento. Nell' area non sono presenti forme o depositi Tirreniani tipici invece della maggior parte delle coste stabili o in sollevamento Italiane. Nella zona tra Duino e Sistiana
sono stati individuati e misurati solchi marini mareali sommersi alle quote medie di
circa -1,7 e constatata l'assenza del solco attuale. Tali solchi sono stati misurati a circa 0,7 poco ad est, nei pressi di Trieste. In tutta la costiera Istriana invece Fouache et al.
(2000) hanno misurato tali solchi alla profondità di circa -0,5 e li hanno attribuiti all'epoca Romana, in questa zona costiera inoltre è presente il solco attuale. Più a sud sul versante orientale dell'Istria sono stati misurati a -1,8 m. Ricerche archeologiche hanno rinvenuto e misurato dei muri portuali nella zona di punta Sottile ad una profondità di
-1,2\1,5 m.
Alla luce di dati relativi a depositi quaternari più antichi, indagini geofisiche, dati
provenienti da sondaggi, si rileva un tilting di subsidenza tettonica che si abbassa progressivamente da SE verso NW.
Rimane aperto il problema di quale origine abbia il solco di battente sommerso, nel
lavoro vengono fatte alcune ipotesi compresa quella di una isostasia anomala dovuta
allo scioglimento dei ghiacci delle Alpi.
275