MORPHOBATHYMETRY OF SMALL-SCALE MUD RELIEFS ON THE
ADRIATIC SHELF
A. CATTANEO, A. CORREGGIARI, D. PENITENTI, F. TRINCARDI
Istituto di Geologia Marina (CNR), v. Gobetti 101, 40129 Bologna, Italy
T. MARSSET
IFREMER, BP 70, 29280 Plouzané Cédex, France
Abstract
Morpho-bathymetric and stratigraphic data reveal small-scale mud reliefs in the toe
region of the late-Holocene mud wedge on the Adriatic shelf. The reliefs are elongate
features with acoustically-transparent cores. They are present in two geologic settings:
seaward of shore-parallel undulations within a thick mud wedge and seaward of
basement highs where the mud wedge is thin. In both settings, the reliefs define clusters
sub-perpendicular to the regional contours, possibly indicating an origin related to
escape of fluids from an impregnated unit at the base of the late-Holocene wedge. Shoreparallel bottom-hugging currents appear to modify the reliefs following their episodic
growth.
Keywords: late-Holocene, swath bathymetry, seismicity, weak layer, fluid escape
1. Introduction
On the Adriatic continental shelf the late Holocene mud wedge accumulated as part of
the highstand systems tract (HST) after the attainment of the present sea-level highstand,
about 5.5 cal kyr BP (Correggiari et al., 2001). High sediment accumulation rates (up to
> 1.5 cm yr-1) reflect the combined supply of the Po and the Apennine rivers resulting in
a total thickness of the HST up to 35 m in a shore-parallel trend (Fig. 1). Geochronological data indicate that a basal HST unit above the maximum flooding surface
(mfs) marks an interval of condensed deposition between 5.5 and ca. 3.7 kyr BP
(Oldfield et al., 2003; Cattaneo et al., in press). The late Holocene mud wedge has a
subaqueous offlap break in water depths of about 25 m and a laterally continuous
prodelta slope of about 0.5o. Gas is common at very shallow levels (a few metres below
the seafloor) in the topset region; gas venting and gas-charged sediments have been
reported from other shallow stratigraphic units beneath the late-Holocene HST (Conti et
al., 2002). The study area lies within the Adriatic foreland basin, an area characterized
by intense shallow seismicity (Fig. 1) and historical tsunamis (Tinti et al., 1995).
In water depths greater than 70 m, the toe of the late-Holocene mud wedge contains
small-scale mud reliefs characterized by an acoustically transparent core in seismic lines
above a basal discontinuous unit of high-amplitude reflectors. The reliefs are rooted on
the basal HST unit and protrude 4-8 m above the mfs (or 2-4 m above the adjacent
seafloor), and are several tens of m in cross section. The reliefs occur in two different
geologic settings: 1) where the late-Holocene HST reaches its maximum thickness on a
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virtually horizontal basal surface (Fig. 1A), and 2) where the late Holocene mud wedge
is characterized by reduced thickness and a more uniform, draped, geometry seaward of
basement highs (Fig. 1B). In the latter case the reliefs are smaller and affect the seafloor
where the basal mfs flattens seaward.
These mud reliefs in the central Adriatic were first described and initially interpreted as
mud diapirs, based on the presence of their acoustically-transparent core (Hovland and
Curzi, 1989) or as fluid escape features, based on the evidence that core stratigraphy can
be precisely correlated from crest to the flanks of the reliefs implying that deformation is
not pervasive but confined to narrow fissured areas (Trincardi et al., 2000). In this paper
we review morpho-bathymetric, seismic-stratigraphic and core data from two areas were
mud reliefs form in contrasting geologic settings: the Ortona offshore, where buried and
exposed reliefs are in association with a thick late-Holocene HST affected by shoreparallel undulations (Fig. 1A), and the Vieste offshore, an area of reduced HST
deposition and basement high NE of the Gargano promontory (Fig. 1B).
Figure 1. Shore-parallel thickness distribution (in msec TWTT) of late-Holocene HST mud wedge. The
seismicity of the area is expressed by the epicenter distribution in historical times (1000-2002 AD) from
Camassi and Stucchi (1997) at http://emidius.mi.ingv.it/NT/, and IRIS database (Incorporated Research
Institutions for Seismology) at http://www.iris.washington.edu/FORMS/event.search.form.htm. Insets A and B
depict the contrasting geologic setting of the two areas where mud reliefs form at the base the HST.
Morphobathymetry of small-scale mud reliefs on the Adriatic shelf
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A
B
Figure 2. Swath bathymetry from EM3000 surveys (locations in Fig. 1) offshore Ortona (A) and Vieste (B). In
both areas, relief size decreases seaward. A): preferred NE-SW orientation of relief clusters seaward of
contour-parallel undulations (low left corner) Square = Fig. 3; rectangle = 3D seismic survey (Marsset et al.,
this volume). B): preferred N-S orientation of relief clusters with contour-parallel crests becoming increasingly
evident towards the east around a basement high (low right corner). Rectangle = Fig. 4.
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2. Methods
This paper is based on the interpretation of 1) high-frequency multibeam surveys using a
300 kHz Simrad EM-3000 system with beamwidth of 1.5° (Figs. 2-4); 2) a dense grid of
Chirp-sonar profiles with a 150 m spacing between lines (Fig. 5); and 3) geochronological control by 14C dates, tephrochronology and magneto-stratigraphy not
discussed here (Oldfield et al., 2003; Cattaneo et al., in press). A VHR 3-D seismic
survey (Fig. 6) covers a portion of the Ortona area (Marsset et al., this vol.). D-GPS
navigation data refer to WGS84 datum.
3. Results
Morphology. Offshore Ortona, the prodelta slope is characterized by seafloor and
subsurface shore-parallel undulations; these undulations have decreasing continuity
moving from shallower into deeper waters (from 30 to ca 50 m water depth, Fig. 2a).
Seaward of about 70 m, small-scale reliefs (up to 4 m high and few hundred m long)
define clusters roughly normal to the shore-parallel undulations. The reliefs are
asymmetric with the steeper side facing SE with a parallel depression and a gentler side
facing NW (Fig. 3). The crests of the reliefs show small patches of high backscatter,
roughly parallel to the regional contour and to the undulations upslope. Offshore Vieste,
the reliefs are smaller: typically 2 m high, 20-50 m wide perpendicular to the regional
contours, and 50-200 m long parallel to contours. As offshore Ortona, the reliefs form
clusters normal to the regional contour but the individual crests are sub-parallel to the
contour. The contour-parallel trend becomes dominant, moving eastward along the
northward dipping basement high (Figs. 2b and 4).
Seismic-stratigraphy. In both Ortona and Vieste areas mud reliefs grow on a thin basal
unit that is affected by significant reflector discontinuities; in particular, discontinuities
appear related to triangles of high-amplitude reflectors and sub-circular patches of
enhanced seismic amplitude, respectively in vertical sections and in plan view (Fig. 5).
Deformation appears concentrated at the base of the reliefs. The triangle zones are
composed of mud and occur immediately on the basal mfs. The growth of mud reliefs
occurs only where the mfs shows small-scale deformation that affects only the late
Holocene mud wedge, even though seismic facies, reflector continuity and muddy
composition of the marine deposits below the mfs are very similar. Seismic profiles
show that the depressions observed on the SE-facing side of the reliefs correspond to
areas of non-deposition or localized erosion. In contrast, the NW-facing sides of the
reliefs are gentler because of preferential deposition subsequent to their formation (Figs.
3-5). Offshore Vieste, the basal unit is overlain by reflectors of enhanced amplitude. The
growth of the reliefs is confined to the lower part of the HST. deposition after their
formation is more uniform resulting in a drape with subtle reductions in thickness over
the reliefs (Fig. 4).
Horizontal slices from VHR 3D seismic data. Offshore Ortona, depth slices (Z =
constant, in metres) show that the mud reliefs appear as small-scale features
characterized by major changes in reflector geometry over short distances. Figure 6
shows examples of Z sections across the base of the late-Holocene wedge.
Morphobathymetry of small-scale mud reliefs on the Adriatic shelf
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Figure 3. Detail of bathymetry offshore Ortona showing elongated reliefs and location of cores. Deposition
following relief formation is from the NW (right). Whole-core magnetic susceptibility logs can be correlated
between sites within and outside the reliefs, indicating that no pervasive deformation took place.
Figure 4. Detailed bathymetry offshore Vieste. The crests of individual reliefs show more pronounced shoreparallel elongations, compared to Ortona area.
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Figure 5. Fence diagram Ortona (Chirp). Stars denote areas where basal high-amplitude reflectors occur above
the basal HST unit and under the reliefs. In cores this basal unit appears muddy and accompanied by low
values on magnetic susceptibility logs, suggesting a decrease of terrigenous component. The undeformed and
stratified unit below is muddy but has greater shear strength and reduced water content. Location in Fig.3.
Gas- charged sediment appears as a fuzzy area on the left, on all sections. The mfs dips
seaward at 0.2o and therefore its intersection with the horizontal plane appears
progressively displaced landward (moving upsection). The basal HST unit, above the
mfs, appears as a complex area affected by sinuous and elongated features that in about 2
m pass upsection into well-formed mud reliefs. These small-scale features are few tens
of m in width and as much as 250 m in a direction perpendicular to the regional contour
and subparallel to the long axis of the mud reliefs above them (Figs. 5 and 6).
Sediment type. Bottom sediment, mostly deposited from river plumes redirected by shelf
currents, is well layered and characterized by high water and clay content, low density
and low shear strength (Correggiari et al., 2001; etc.). The sediment cored through the
mud reliefs at several locations is very similar to the sediment in adjacent areas. Wholecore magnetic susceptibility logs have been correlated between acoustically transparent
reliefs and stratified units on their flanks, albeit minor variations exist in thickness of
selected sub-units (Fig. 3).
4. Discussion and Conclusion
The kind of mud reliefs described in the Adriatic shelf are not commonly observed on
continental margins for two possible reasons. First, the reliefs possibly develop as an
early deformation that is subsequently removed by increased mobilisation and
translation. Second, if occurring in deeper waters, these features would be difficult to
resolve, due to limited spatial resolution on conventional seismic surveys, and therefore
would appear as a set of overlapping hyperbolae on acoustically-transparent units.
Morphobathymetry of small-scale mud reliefs on the Adriatic shelf
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Figure 6. Z slices at 80-cm intervals from VHR 3-D seismic survey (Marsset et al., this volume). Upsection,
the apparent landward extent of the basal, deformed, HST unit increases because the mfs dips gently seaward.
The reliefs described from the Ortona and Vieste study areas share five similar
characteristics: 1) exclusive occurrence above the downlap surface at the base of the
HST (Fig. 5); 2) spatial distribution along shore-normal clusters (Figs. 2-4); 3) presence
of a restricted acoustically transparent area in their core (Figs. 5 and 6); 4) exclusive
muddy composition; 5) the possibility of correlating the stratigraphy from the relief
center to the adjacent regions (Fig. 3). Based on the above characters, these reliefs can
best be explained as fluid escape features related to one or repeated events of sediment
mobilization and fluid venting (Trincardi et al., 2000). Mud diapirs or mud volcanoes
(Milkov, 2000), would instead result in a more thorough remolding of the sediment
column and, perhaps, a more random spatial distribution. A possible field analogue of
the Adriatic reliefs is provided by the narrow pipes, up to 3-4 m high with sub-circular
diameters of 2-2.5 m, documented in Plio-Pleistocene clays of Rhodes (Hanken et al.,
1996). The preferential directions of the fluid escape features in the Adriatic reflect the
regional stress field in a fashion that may resemble the mechanism of formation of
polygonal faults (Cartwright and Dewhurst, 1998). In this view, the mfs acts as a
permeability barrier for fluids forming and/or circulating through the stratigraphic units
beneath. This mechanism of formation implies: 1) decay of organic matter and
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production of CO2 and CH4; 2) entrapment of a mixture of water and gas resulting in
density inversion near the base of the HST; 3) increase of internal pressure at the base of
the HST; 4) cyclic loading by earthquakes (and/or tsunamis or storm waves) was more
effective on previously-overpressured sediment; 5) definition of areas of preferential
fluid expulsion; 5) upsection flow of micro-bubbles along narrow conduits (pipes). No
shear extension is therefore implied, although shear planes can be observed, locally
(Correggiari et al., 2001). This mechanism explains why deformation appears
concentrated at the base of the reliefs. Fluid expulsion drives sediment contraction
thereby contributing to “re-consolidate” the sediment section and self-limiting
mobilization and preventing failure (Gardner et al., 1999; Correggiari et al., 2001).
The subtle but consistent differences in the structure of the mud relefs in the study areas
do not indicate contrasting mechanisms of formation but, rather, reflect different styles
of deposition following their formation. The uppermost HST unit records the interaction
between bottom currents and the micro-topography generated by the growth of the
reliefs (Marsset et al., this volume). This is more evident in the Ortona area where
asymmetric deposition on the flanks of the reliefs is recorded by thicker accumulations
on the NW side and thinner deposits or erosional moats on the SE side; in the Vieste
region, individual relief’s crests are progressively more elongated in a contour-parallel
direction, moving progressively eastwards (Fig. 3).
5. Acknowledgements
Data for this paper were collected within the EU COSTA project (Contract EVK3-CT-1999-00006) and within
the Italian GNDT project on geologic risks in offshore areas. We thank reviewers Dan Orange and Mike Field
for their inspiring comments and suggestions. This is IGM contribution n 1304.
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