MORPHOLOGICAL CONTROL ON SEDIMENT DISPERSAL ALONG

Geologica Romana 37 (2003-2004), 113-121
MORPHOLOGICAL CONTROL ON SEDIMENT DISPERSAL ALONG
THE SOUTHERN TYRRHENIAN COASTAL ZONES (ITALY)
Tommaso De Pippo, Carlo Donadio & Micla Pennetta
Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II
e-mail: [email protected]
ABSTRACT - The study shows the relationship between the coastal zone, the continental shelf-slope system
morphology and the sediment dispersal with respect to the coastal dynamics.
Three areas along the Tyrrhenian margin are studied: the Gulf of Policastro, the Gulf of Gaeta and the Volturno
River mouth.
A sedimentological analysis, using also quantitative techniques for sediment distribution and classification, was
carried out on bottom samples collected from the Gulf of Policastro in order to identify the morphological control
on recent and present-day sediment dispersal.
The sediment distribution which derives from cluster analysis shows that the morphological characteristics are
related to sedimentary trends. In fact, the groups identified by multivariate statistical analyses evince that the distribution and deposition of sediments are strongly conditioned by the morphology of the area as well as by morphodynamic evolution of the main structures.
The study of sediment properties, collected from the shelf and the slope of the Gulf of Gaeta, as well as the
hydrologic data registered during oceanographic surveys in the area, show that they are related both to the emerged and submerged coastal morphological features.
The water circulation in this littoral, which mainly determines sediment distribution, shows two separated regimes close to the shelf break: a coastal regime, characterised by the formation of secondary cells, and an offshore
regime, dominated by the cyclonic gyre of the Tyrrhenian Sea.
This differentiation is related to the features of the coastal zone, where three morphological elements affect the
circulation: the headland of Gaeta, the structural high to the S of the headland and the area near the shelf break.
The modal analysis carried out on the samples collected on the sea bottom in front of Volturno delta complex
enabled to identify the presence of 5 granulometric subpopulations and to define their participation degree in the
coastal dynamics, as well as to identify the sedimentary transit axes along the entire littoral. The research carried
out in front of the Volturno River mouth, in accordance with the analysis of modal isodensity curves, has demonstrated that the sediments move along directions defined by the sea bottom morphological trend.
KEY WORDS: Coastal geomorphology, submarine geomorphology, sediment dispersal, seawater dynamics, eastern Tyrrhenian
margin, southern Italy.
INTRODUCTION
Studies on sediment deposition and dispersal rarely
take into account the influence of coastal and gulf morphology. Therefore, this research carried out in three
areas along the Tyrrhenian margin (the Gulf of
Policastro, the Gulf of Gaeta and the Volturno River
delta) intended to show this important control operated
by the coast, the continental shelf and the slope, underlining the relationship between the coastal zone, the continental shelf-slope system morphology and the sediment dispersal, also related to the seawater dynamics.
The southernmost area studied is the Gulf of Policastro,
located between the southern Apennine chain and the
Tyrrhenian basin.
The Plio-Quaternary uplift, the intense tectonic activity of the Calabrian Arc and the synchronous subsidence
of the Tyrrhenian basin governed the complex and varied morphology of the Tyrrhenian margin of this area.
The physiographic setting of the seabed is conditioned
by morphostructural highs, basins and channels which
resulted from extensional tectonic activity producing
step-like blocks sloping down northward and seaward.
The second and northernmost area is the Gulf of Gaeta,
furnished by sediment supply of Garigliano and partial-
ly of Volturno rivers. The gulf represents the northern
submerged sector of the Campania Plain, a wide coastal
tectonic depression delimited to the E by the Apennine
chain, originated during the lower Pleistocene.
This subsiding coastal graben is filled with thick continental, transitional and marine deposits of Quaternary
age, interbedded with volcanic sediments from Phlegrean
Fields, Somma-Vesuvius and Roccamonfina vents.
Finally, the third area is the Volturno River mouth, to
the S of the Gulf of Gaeta, where the same gently sloping continental shelf is characterised by two orders of
sand bars close to the coastline and by the submerged
river mouth complex down to - 120 m depth, which is
related to the relict morphology of the late würmian river
mouth.
GULF OF POLICASTRO
A sedimentological analysis, using also quantitative
techniques for sediment distribution and classification,
was carried out on bottom samples collected from this
gulf in order to identify the morphological control on
recent and present-day sediment dispersal.
This area (Fig. 1) is characterised by sectors with a
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DE PIPPO et al.
Fig. 1 - The Gulf of Policastro, located in front of the Calabrian Arc and tectonically still active. The gulf is characterised by two different submarine morphological sectors: the first with a narrow continental shelf and a -90 m deep shelf break, the second with a wide platform and a -130 to 140 m deep shelf break. The inherited and present-day morphologies identified along the continental shelf and the slope both control and are reciprocally influenced by the sediment dispersal. A, B, C, D, E and F represent different cluster groups of sea bottom sediments.
narrow continental shelf (less than 3 km), with a -90 m
deep shelf break, and sectors where the continental shelf
is 8 km wide with a shelf break -130 to -140 m deep (De
Pippo & Pennetta, 2000).
Beyond the shelf break, on the upper slope, variations
of inclination due to the presence of marine terraces at
different depths were observed. The upper slope develops with very steep stretches where the shelf is narrow-
MORPHOLOGICAL CONTROL ON SEDIMENT ...
er, whereas it declines with a gentle slope in correspondence to the areas where the rivers to the S of the gulf
debouch. In the latter, the instability phenomena (creep,
landslides) are less frequent than in other areas where
they contribute to shelf break retreat.
The morphology of the upper slope, down to the depth
of -1100 m, is characterised by slope ridges, basins and
canyons (Selli, 1970; Selli & Fabbri, 1971; Fabbri et al.,
1981). The ridges, that reach -280 m depth, are isolated
and/or aligned according to the structural trend of the
Tyrrhenian margin. At the same time they are cut
through by tectonic lines in which the greater canyons
have been scoured.
The Basin of Sapri is located in the northern sector of
the study area; it is partially limited seaward by the
ridges and landward by a steep slope dissected by many
canyons. The latter, as evidenced by the contour lines
and the bottom morphology, would constitute the tributaries of the basin, while the effluent would open westward between the ridges reaching the base of the slope
(Cocco & De Pippo, 1988).
The sedimentological analysis was carried out on 85
bottom samples collected from the continental shelf and
the slope (Fig. 1). The data elaboration was developed
according to statistical methods (Davis, 1973; Poluzzi et
al., 1985) of multivariate analyses (cluster analysis).
Cluster analysis enabled to identify 6 sediment groups:
Cluster A: this group is present down to the depth of 50 m. Textural parameters may reflect a deltaic supply or
a mixing of deltaic and marine sands, moderately to
poorly sorted and strongly fine-skewed;
Cluster B: this group is present where the continental
shelf is wider. Microscopic analysis on calcarenite fragments (content of carbonate >75%) indicates that they
are related to autochthonous deposits (skeletal grains) of
continental shelf;
Cluster C: this group is found in the narrower portions
of the continental shelf and rarely in the outer portion
(>50 m depth) of the wider shelf. The textural characteristics indicate that the sediments grouped in this cluster
generally were deposited in areas dominated by highenergy processes such as waves or currents. Mixing with
relict sediments is suggested by the presence of well
rounded grains;
Cluster D: these sediments, well sorted and negativeskewed, were collected from the shelf break and from
the top of the submarine ridges. Their presence might
indicate a distal high-energy environment subject to
rapid morphological changes;
Cluster E: this group, represented by silty samples, is
found mainly on gentle slopes and in correspondence to
the wider continental shelf characterised by slow deposition. Mixing with biogenic and prodelta sediments
indicates that mass flow deposition occurred;
Geologica Romana 37 (2003-2004), 113-121
115
Cluster F: it includes silty-clayey sediments collected
at the base of steeper slopes in correspondence to the
narrower shelf. Their textural characteristics suggest
massive transport (turbidites, creep, etc.).
The sediment distribution which derives from cluster
analysis shows that the morphological characteristics are
related to sedimentary trends. In fact, the groups identified by multivariate statistical analyses evince that the
distribution and deposition of sediments are strongly
conditioned by the morphology of the area as well as by
morphodynamic evolution of the main structures (De
Pippo & Pennetta, 2000).
Coarser sediments of cluster A and B are present in the
inner portion (<-50 m) of the southern larger continental
shelf. More specifically, the sediments of cluster A have
a parallel distribution to the coastline with a convexity in
correspondence to the river mouths, confirming a clear
connection with the fluvial supply (Fig. 1).
Whereas, sediments of cluster B, showing a scattered
distribution, are autochthonous sediments deriving probably from bioclasts, found to the S of the study area.
Also cluster C gathers samples collected from the continental shelf, mainly in the narrower and steeper northern areas, secondarily from the wider shelf deeper than 50 m. The textural characteristics and the sediment distribution observed along this border, indicate high energy processes related to waves and currents. These
processes act on the finest fraction of the samples scattering it toward the continental shelf. At the same time,
the sediments could be associated to relict deposits.
Other studies underlined similar sedimentary characteristics at same continental shelf depths (Aguado, 1973;
Fabbri et al., 1981; Poluzzi et al., 1985; Argnani et al.,
1989; De Pippo & Pennetta, 2000).
The samples of cluster D represent the complex
dynamics of the shelf break dominated by strong turbulence and/or intense currents able to hold the mud in suspension.
The sediments of cluster E indicate a biogenic and
clastic deposition. They are more frequent in the zones
where the continental shelf is wider and along the gentler slopes, where the currents deposit the finest fraction.
Finally, the characteristics of cluster F, composed of
very fine sediments, suggest mass flow processes. The
samples are located mainly at the continental shelf and
of the submerged ridges.
GULF OF GAETA
The coast between Gaeta and Cuma (Fig. 2), located in
correspondence to Garigliano and Volturno coastal
plains, is 65 km long and presents a NW-SE direction.
Along the coast, sandy beaches alternate with tracts of
rocky coast. Quaternary marine terraces are present in
the continental area between +230 and +30 m and notches at different heights (between +5.3 and +1.3 m) on the
cliffs testify to ancient sea-levels (Ozer, 1987; Antonioli,
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Geologica Romana 37 (2003-2004), 113-121
DE PIPPO et al.
Fig. 2 - The Gulf of Gaeta, a wide depression originated during the lower Pleistocene tectonic events linked to the final uplift of the Apennines chain.
Sea bottom sampling points are indicated: the sediments distributed along the inner shelf are mainly provided by the Garigliano River and subordinately by the Volturno River scarce supply. The coastal physiography and three main submarine morphologies strictly influence the present-day
marine sedimentary processes of the gulf: the headland of Gaeta, the structural high to the S of the headland and the shelf break area.
1991; Brancaccio et al., 1991).
The continental shelf, gently tilted seaward, shows the
maximum of width in front of the Garigliano River
mouth (20 km) and the minimum one in correspondence
to Cuma (~10 km); the shelf break is located at -120/130 m depth. In correspondence to the present-day
prominent Volturno delta it is possible to identify a pronounced seaward convexity of the isobaths related to the
ancient mouth (Pennetta et al., 1998). The latter could be
attributed to the maximum low-stand of the Würm. The
marine terrace visible between -100 and -125 m depth to
the S of the Gaeta Promontory could be related to several remodelling phases before and during the same glacial
period (Brancaccio, 1990).
The upper slope is affected by many canyons; the
Cuma canyon, probably corresponding to a fault, shows
headward erosion due to gravity-collapse and mass
flows. A submerged morphological headland is present
to the S of the Gaeta Promontory in correspondence to a
structural high (Bartole, 1984).
The circulation pattern of the Tyrrhenian Sea (Fig. 3),
which influences the circulation in the study area, shows
the presence of a cyclonic vortex that interests both the
superficial (down to -10 m depth) and the intermediate
(between -10 and -100 m depth) layers. This circulation
is more intense during winter, whereas in summer, it is
MORPHOLOGICAL CONTROL ON SEDIMENT ...
Fig. 3 - Circulation patterns of the Tyrrhenian Sea and their influence on
the circulation in the Gulf of Gaeta. A cyclonic vortex interests both the
superficial (MAW) and intermediate (LIW) layers. In winter (a) this
type of circulation is more frequent, mainly with a NW seawater flow
direction; in summer (b) the circulation preserves his cyclonic character
but is interested by smaller cells and reduced dynamics, with S and SEward seawater movements.
characterised by a reduced dynamics, although preserving his cyclonic character. The agreement between
geostrophic and wind-related circulation documents the
importance of the eolic vector for the determination of
the dynamics of the superficial layers (Hopkins, 1988).
In the Tyrrhenian Sea, based on its thermohaline characteristics, three main masses of water can be recognised: the Modified Atlantic Water (MAW), the
Levantine Intermediate Water (LIW) and the Tyrrhenian
Deep Water (TDW). The MAW occupies the superficial
layer of the water column and is characterised by low
salinity. This fact is due to the penetration and modification of the Atlantic water through the Strait of Gibraltar.
The pattern of the isohaline lines, mainly parallel to
the coastline, suggests that the MAW is not affected by
important mixing phenomena running northward. In
summer, the situation is substantially different both due
to the numerous vortexes that characterise all the basin
Geologica Romana 37 (2003-2004), 113-121
117
and to the presence of a front that separates the southeastern zone (low salinity) from the northwestern one
(high salinity) (Brown et al., 1979). At intermediate
depth, the maximum of salinity is related to the presence
of the LIW, coming from the eastern Mediterranean Sea,
through the Sicily Channel. Recent studies have shown
that the LIW in the Tyrrhenian Sea results conditioned
mainly by the bathymetry and it runs towards the
Campania Region coasts (Hopkins, 1988).
During oceanographic cruises, bottom sampling and
hydrologic surveys were carried out. The samples are
located along three profiles: one parallel to the coastline,
between -10 and -50 m depth; the other two transverse to
SW of the Garigliano River, between -26 and -272 m
depth. Table I reports sedimentological (Folk & Ward,
1957) and physical characteristics of the studied samples.
During the hydrologic surveys, meteorological data
and measurements of surface currents were registered.
The results of this research suggest that the dynamics
known represents a reliable indication of the present-day
dynamics for major depths (Budillon & Moretti, 1994).
Sediment distribution along the parallel profile shows
a great fluvial control close to the mouth of the Volturno
River and a decreasing influence in the adjacent portions
due to coastal dynamics (waves and currents).
For the sediments collected along the profiles perpendicular to the coast it is possible to distinguish three
areas: the first area is characterised by sediments of continental shelf (<-50 m), composed essentially of coarsemedium silt with a sand fraction between 2.5% and 14%
(Tab. I); the second one, with samples collected at -120
m depth (shelf break), shows a prevalence of fine and
clayey silt with an anomalous percentage of sand
(between 5% and 40%) and coarse silt (between 56%
and 90%) sediments. This latter might be attributed to
relict sediments locally not covered by recent mud, probably due to erosive effects of the currents. Finally, the
third area is characterised by slope samples that show
equal percentage of coarse sandy-silty and fine siltyclayey sediments (Tab. I). These characteristics could be
explained by the deposition of erosional relict sediments
(coarser fractions) during a normal mud sedimentation
along the slope.
The hydrodynamic processes recognised in this shelf
break area are complex. Under the hydrodynamic point
of view, the oceanographic studies enabled to define two
zones with two different circulation regimes in the Gulf
of Gaeta: a coastal zone and an offshore zone (Fig. 3).
The first is mainly characterised by closed cyclonic
and anticyclonic circulations and is separated, in correspondence to the shelf break, from the latter, mainly
interested by a northern flow.
In winter (Fig. 3a), the main water flow is from the S
and it moves along the shelf break forming secondary
cells in correspondence to the main rivers due to the
obstacle created by the Ischia Island. On the contrary, in
summer (Fig. 3b), when flows are from the N, the water
circulation is influenced by the coastal morphology and
by the presence of the submerged morphostructural
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DE PIPPO et al.
Geologica Romana 37 (2003-2004), 113-121
sample (n°)
sand (%)
silt (%)
clay (%)
FOLK’s definition
s I (f)
KG
SK I
CaCO3 (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1.28
1.34
0.72
0.9
2.54
5.13
6.5
2
40
27
1.97
8.08
1.26
7
8.18
8.6
9
1.73
5.42
2.53
6.48
4.6
13.58
96.1
94.03
92.73
94.1
89.95
90.32
79
88
56
70
84.99
86.19
97.55
92.48
88.34
88.65
90.53
93.8
93.89
89.92
91.61
90.78
82.02
2.1
4.53
6.17
5
5.82
4.08
14.5
10
4
3
12.81
5.59
0.57
0.5
1.98
2.69
0.41
3.23
0.69
5.44
1.36
2.76
2.79
medium silt
medium silt
fine silt
fine silt
fine silt
medium silt
fine silt
fine silt
medium silt
coarse silt
medium silt
medium silt
medium silt
fine silt
coarse silt
medium silt
medium silt
medium silt
medium silt
medium silt
coarse silt
medium silt
coarse silt
0.682
0.823
0.738
0.811
1.048
1.13
1.608
0.789
2.323
1.578
1.305
1.331
0.653
0.881
0.823
0.844
0.859
0.631
0.873
1.097
0.781
0.985
1.047
1.279
1.623
2.72
1.906
1.663
1.25
0.786
2.943
0.673
1
1.078
1.68
1.136
2.957
1.386
1.309
1.132
2.357
0.81
1.864
1.502
1.882
1.701
0.101
0.242
-0.013
-0.489
-0.137
-0.363
-0.366
-0.118
-0.476
-0.538
-0.146
-0.447
0.019
-0.264
0.196
0.056
-0.111
0.165
-0.305
0.313
-0.212
0.158
0.264
45.773
58.662
53.816
53.704
64.675
55.55
54.727
57.377
47.243
39.5
54.078
58.676
61.202
59.817
32.335
30.153
39.97
35.65
46.121
46.837
40.699
36.764
22.664
Tab. 1 - Textural and physical characteristics of bottom sediments collected from the Gulf of Gaeta; SI: standard deviation; KG: kurtosis; SKI: skewness.
headland located to the S of the Gaeta Promontory.
However, currents in coastal areas are less intense than
in the offshore when they are interested by the northern
branch of the southern Tyrrhenian Sea circulation.
Secondary cell circulations form on the continental
shelf, whereas the flow of the Tyrrhenian cyclonic vortex mainly acts on the slope. The latter changes direction
near the terraced surface to the S of the promontory,
forming a secondary cell. This circulation amplifies the
erosive processes near the shelf break, mainly on the
northernmost transverse profiles. The sediments show
characteristics related to sandy deposits (high percentages of sand and CaCO3), in accordance with literature.
The present-day reduced thickness of the mud in the
area near the shelf break favoured the partial removal
and reworking of the relict transgressive sediments
caused by the hydrodynamic forces. At greater depth, the
offshore flow contributes to the shifting of finest fraction
from the slope sediments, negative-skewed.
The study of the variations in textural characteristics
enabled to individualise a proximal sector, -10/-50 m
deep, and a distal one at a major depth (>-50 m), both
dominated by coarser sediments. Between the two,
another sector dominated by the finest fractions with
transitional features is present (Pennetta et al., 1998).
The dynamics down to -50 m depth is related to the
flat or gently inclined morphology of the continental
shelf and to the secondary cell circulations. It shows a
sediment distribution with finest fractions seaward, in a
sector of the continental shelf bounded by the Gaeta
Promontory. This dynamics is controlled by the interaction of the fluvial processes and the swell. Moreover, it
is also influenced by the effect of the Gulf of Gaeta and
the protection operated by the Gaeta Promontory.
VOLTURNO RIVER MOUTH
A morpho-sedimentary study was carried out in correspondence to the Volturno River delta by bathymetric
survey and sampling of bottom sediments. The morphological trend of the sea bottom (Fig. 4) indicates the
presence of two orders of submarine sand bars in the
northern area of the mouth. A -5 m deep trough divides
the outer bars, situated parallel to the coastline, from the
more irregular inner ones, located transversally to the
shoreline (Cocco & De Pippo, 1988). Only a single
order of bars is present to the S of the mouth, extending
with a regular trend in an arrangement parallel to the
shore, with a ridge generally located at -3 m depth and
separated from the shoreline by a trough about -5 m
deep.
In correspondence to the mouth there is a very wide
bar that could be linked to the outer bars, although its
section presents an asymmetry of the flanks much less
marked than the other bars. In addition, the irregular
trend of the bathymetric lines indicates a rather distinct
submerged river mouth complex, with some weak incisions in the flanks, located in correspondence to the
slight depression of the outer bar ridges and in axis with
the greatest depth points of the troughs.
The right bank of the mouth is characterised by the
presence of a deep channel longitudinally arranged to
the river, while the left bank by a bar at -1.5 m depth. In
accordance with the technique tested by Aguado (1973),
MORPHOLOGICAL CONTROL ON SEDIMENT ...
Geologica Romana 37 (2003-2004), 113-121
119
Fig. 4 - The Volturno River mouth, located to the S of the Gulf of Gaeta. The sea bottom, in front of the present-day delta, is characterised by two orders
of sand bars along the inner and flat platform dominated by waves, longshore and rip currents. The bars are parallel to the coastline and cut transversally by channels.
Barusseau (1973), Pauc (1973) and Long (1975), the
modal analysis carried out on the samples collected on
the sea bottom in front of Volturno delta complex
enabled to identify the presence of 5 granulometric subpopulations and to define their participation degree in
the coastal dynamics, as well as to identify the sedimentary transit axes along the entire littoral. Only 4 of the 5
subpopulations are important in littoral dynamics: two
of medium sands, one of fine sands and a last one of
very fine sands. Whereas the fifth, being quite absent, is
not significant.
The analysis of modal isodensity curves (Cortemiglia,
1978) related to the materials which actively take part in
the littoral dynamics demonstrated that the medium
sands are essentially shifted by beach drift (Fig. 5a)
and/or by longshore currents (Fig. 5b).
Fine sands are moved by longshore currents parallel
to the coastline, to greater depths than medium sands
and sometimes shifted offshore by rip-currents (Fig. 5c).
Whereas, very fine sands are exclusively carried offshore by rip-currents which move along directions
defined by the morphological trend of the sea bottom
(Fig. 5d). In fact, the shifting of very fine materials offshore occurs where troughs reach their greatest depth
due to the deepening of the bar ridges.
Considering that the load transport of the Volturno
River has been estimated nearly non-existent because of
an artificial dam (Traversa di Ponte Annibale) close to
the river mouth and that the littoral is in recession since
1909 at a medium rate of 19 m/year (Cocco & De Pippo,
1988), it is presumed that the materials involved in this
movement are eroded exclusively from the beaches and
the sea bottom near the river mouth.
DISCUSSION AND CONCLUSIONS
Morphological research and analysis of textural characteristics of bottom sediments carried out along three areas
of the Tyrrhenian coast enabled to define the present-day
processes and their evolution in time and in space.
The relationship between such processes and seabed
morphology were also identified, as well as the wave
formations which affect the coast and the complex system of sediment transport. The interaction of coastal and
submarine morphology with the hydrodynamic regimes
determines a control on coastal dynamic processes, conditioning the dispersal of sediments in the three selected
areas.
Generally the hydrodynamic processes, under similar
morphological and hydrological conditions, produce sea
bottom deposits with similar sedimentary features.
Whereas, in the three coastal zones under study, characterised by different sea bottom morphologies each one
controlled by a specific coastal physiography, a different
response to sedimentary processes is observed.
Consequently, the evolution of submarine morphologies like bars, structural highs, submarine ridges,
canyons and sea bottom depressions, is closely and
mutually controlled by the influence of these forms, both
inherited and of new genesis, on sediment dispersal.
In particular, within the submerged sector of the Gulf
of Policastro, still tectonically active and generally characterised by a narrow and steep continental shelf, there
are sediment groups strictly connected to the morphologies of the area, to the morphodynamic evolution of their
main submarine structures and to the presence of secondary cell circulations. The coarser fraction is distributed parallel to the coastline in the high-energy, flat zone
Geologica Romana 37 (2003-2004), 113-121
Fig. 5 - Shoreface sampling points in front of the Volturno River mouth and littoral. Sediment drift patterns along the shallow sea bottom are based on the analysis of modal isodensity curves. Medium sands (0.461-0.346
mm) are shifted mostly by beach drift (a) and/or (0.344-0.250 mm) by longshore currents (b). Fine sands (0.248-0.128 mm) are moved by longshore currents parallel to the coastline, to greater depths than medium sands
and sometimes carried offshore by rip-currents (c). Very fine sands (0.121-0.077 mm) are exclusively carried offshore by rip-currents moving along directions defined by the morphological trend of the sea bottom (d),
where troughs reach their greatest depth due to the deepening of the bar ridges.
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Geologica Romana 37 (2003-2004), 113-121
MORPHOLOGICAL CONTROL ON SEDIMENT ...
of the inner platform, resulting mainly linked to fluvial
supply mixed with marine sedimentation or occasionally to biogenic sediments. These deposits and the related
morphologies, sometimes associated to relict sediments,
are found both along the narrower and steeper zones and
the deeper and wider shelf areas, where high-energy
processes due to waves and currents are observed. The
fine fraction is frequently distributed both along the
wider continental shelf and gently sloping areas, dominated by currents, whereas the very fine sand fraction,
found along the continental shelf and the submerged
ridges, is related to massive flows.
The circulation in the Gulf of Gaeta, which controls
the sediment distribution, shows two different regimes
close to the shelf break. These are directly controlled
121
both by coastal physiography and three seabed morphologies along the continental shelf: the headland of
Gaeta, the structural high to the S of the headland and
the area near the shelf break.
Finally, Volturno River mouth, characterised by a
wide, shallow continental platform with a small presentday delta, a double sand bar strip and a reduced river
sediment supply, shows a littoral dynamics where medium sands are moved by beach drift as well as by longshore currents. The latter also shift fine and sometimes
medium sands parallel to the coastline, whereas rip-currents carry offshore exclusively very fine sands along
directions defined by the sea bottom morphological
trend (bar channels).
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Accettato per la stampa: Marzo 2003

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