CANNIBALIZATION OF A CONTINENTAL MARGIN BY REGIONAL SCALE
MASS WASTING: AN EXAMPLE FROM THE CENTRAL TYRRHENIAN SEA
F. L. CHIOCCI, E. MARTORELLI, A. BOSMAN
Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, P.le Aldo
Moro 5, 00185, Roma, ITALY
Abstract
A morpho-stratigraphic study carried out on the Western Pontine Archipelago
continental slope (Eastern Tyrrhenian Margin) revealed a suite of instability/erosional
features producing the complete cannibalization of a whole span of continental slope,
from the shelf break down to the abyssal plain (located at 3600 m w.d.); only 2% of the
whole area (2.000 sq. km) is not affected by these processes. One of the main
controlling factors seems to be the slope gradient that produce different instability
features, ranging from gravity-driven failure (simple and complex slides) to gravityflow (debris and grain flows).
Keywords: Submarine instability, slide, gravity flow, continental slope, Tyrrhenian Sea
1. Introduction
Instability phenomena can be extensive processes active on continental margin,
principally on continental slope (Weaver et al., 2000) and on volcanic island flanks
(Masson et al., 2002), producing features extremely different in size and typology (i.e.
slide, slump, debris avalanche, debris flow and turbidity current; Mulder and Alexander,
2001; Mulder and Cochonat, 1996). The Western Pontine Island continental slope
(Central Tyrrhenian Sea) represents a very special area where a suite of
instability/erosive features, different in terms of size and processes, affect the whole
seafloor producing the cannibalization of almost all the continental margin. Thus the
continental slope, from the shelf break down to the abyssal plain (for a total of about
2.000 sq. km), was first investigated by a long range side scan sonar mounted on TOBI
(Towed Ocean Bottom Instrument, for details see Murton et al., 1992) and a high
resolution seismic reflection system, during T.I.VOL.I cruise on board of R/V Urania,
1998 (Chiocci et al., 1998; Fig. 1). In a second cruise, carried out in 2001, very high
resolution side scan sonar data on shallower areas (up to 1.000 m), as well as grab and
core samples on selected features, were collected. Low-resolution multibeam data
(courtesy of M. Marani) were also used to support long-range side scan sonar data.
2. Study area
2.1 GEOLOGY
Pontine continental slope is located in the central part of the Eastern Tyrrhenian Margin,
mainly made up of Plio-Quaternary terrigenous units (Marani et al., 1986). The slope
connects the continental shelf, surrounding the Western Pontine Archipelago, to the
Vavilov Abyssal Plain (up to 3.600 m w.d.). The Western Pontine Archipelago is a Plio409
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Quaternary volcanic apparatus made up of rhyolitic and trachytic rocks (De Rita et al.,
1986). Vavilov basin is the northern apex of the Tyrrhenian back-arc basin, of UpperTortonian age, characterized by thin oceanic lithosphere (Doglioni, 1991; Kastens et al.,
1988; Sartori, 1986). The opening of the basin caused a strong distensive tectonic
activity that affected the continental margin since lower Pliocene up to the Quaternary,
creating a very steep morphology of the continental slope (Zitellini et al., 1984).
Figure 1. Study area and location of images shown in the figures. Lines indicate the route of TOBI.
2.2 PHYSIOGRAPHY
Pontine continental slope is a NW-SE structurally-controlled regional feature. It is
narrow (up to 20 km) and steep (5°-10°, locally up to 30°) so that it represents the
steepest continental slope of the Eastern Tyrrhenian continental margin. The slope
extents from the shelf break, located at 150 m w.d., down to the abyssal plain, located at
3.550 m w.d. It is characterized by a complex morphology due to erosive/instability
features. The main ones are: 1) an erosional shelf break characterized by a complex
morphology and deep incisions; 2) four along-slope ridges (NE-SW) 15-40 km long; 3)
three NE-SW U-shaped valleys up to 6 km wide, extending from the shelf break to the
abyssal plain (20-40 km). Within the main valleys, two abrupt change s in slope were
found: the first at 2.500/2.850 m w.d. from 6°-10° to 3°-4°, the second at 3.100 m w.d.
from 3°-4° to 0.5-1°. These differences in slope gradient are mirrored by a quite
distinctive change of instability features.
3. Instability on higher gradient areas (typically upper continental slope and ridge
flanks)
These areas (with slope gradient of 6°-10°, up to 30°) are characterized by huge
instability/erosive phenomena affecting almost all the seafloor (so that only some tens
of sq. km of the seafloor are covered by hemipelagic/low backscatter sediments). In the
upper continental slope, erosion causes the shelf break to retreat from some hundred
meters to one km. A complex pattern of instability/erosive features, characterized by
different size and typology, were recognized: linear scar-channel systems, dendritic
gullies networks, canyons, simple and complex slides.
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3.1 LINEAR SCAR-CHANNEL SYSTEMS
These features are characterized by a failure scar and a related channel which develops
at its toe (Fig. 2). The channels are usually filled by coarse sediment, often arranged in a
linear pattern. Sometimes, at channel head, several coalescing scars were observed.
Linear channels represent the most common features affecting the seafloor, in areas with
average slope angle of 10° (up to 22° at channel head), with incisions of some tens of
meters deep. Their length ranges from 2 to 8 km, their width from 200 to 800 m.
Figure 2. Upper continental slope sonar image. Linear scar-channel systems (§ 3.1), complex slides
(§ 3.4) and grain flows (§ 4.1) are indicated.
Headscarps have a similar width of some to several hundred meters (sometimes more
than 1 km); their height range from some to several tens of meters.
The scar-channel systems seem to be produced by successive failure events, causing the
retrogression of the scar and its upward migration along the maximum slope gradient. In
this sense they can be assimilated to the successive overlapped scars of Mulder and
Cochonat (1996).
The scar-channel systems develop along the maximum slope gradient and converge
downslope into the main valleys of the continental slope. There the coalescence of the
debris carried by the multiple distinct channels produces the huge mass flow deposits
described hereafter (see § 4.1).
3.2 DENDRITIC GULLIES NETWORKS
These features are characterized by a complex network of rather linear gullies
- sometimes filled by coarse debris -, separated by small ridges, arranged in a dendritic
pattern (Fig. 3); sometimes at gullies head a scar was found, usually not as well defined
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Figure 3. Dendritic gullies networks (§ 3.2)
on upper continental slope.
Figure 4. Sonar and seismic image of a canyon
(§ 3.3) whose head produces a retreat of the shelfbreak of several hundreds meters.
as it is in the linear scar-channel systems. Gullies length varies from some hundred
meters to some chilometers.
They carve the seafloor for 10-75 m in the steepest areas (10°-30°). Although not
common in the study area (i.e. they cover only 70 km2 of the slope), the dendritic pattern
produces diffuse erosion of the seafloor, especially in areas located near the shelf break
that appears extensively retreating. Gullies distribution seems to be strongly controlled
by slope angle because the networks were found only in the steepest areas and within
the same area become more widespread and branched as the slope angle increases. This
fact is well evident in the lower continental slope; there the dendritic networks were
found in areas close to the continental slope-abyssal plain boundary where the gradient
is higher (up to 28°).
3.3 CANYONS
Two canyons were recognized in the eastern part of the area (offshore Ponza Island),
between 150 and 2.300 m w.d., developing linearly on smooth-low backscatter seafloor,
dipping 4-10°. They are deep erosional valley - as deep as 100-250 m respect to the
surrounding seafloor -, 70-350 m wide and up to 20 km long. The canyon head is made
up of a number of smaller tributaries that affect the shelf break, producing its extensive
retreat (Fig. 4).
3.4. SIMPLE AND COMPLEX SLIDES
They are instability features developing in areas with slope angle less then 10° (typically
4°-8°), where scars are largely coalescent, channels or gullies are absent and small
quantity of failure debris may be present (Fig. 4). Only in few cases single scars were
observed.
The height of coalescing headscarps ranges from 20 to 30 m and is typically lower than
that of the linear scar-channel systems. They are small in dimension (their width is
about 100-300 m) and they affect the seafloor for few sq. chilometers.
According to the Mulder and Cochonat (1996) classification, isolated scars can be
related to simple slides, whereas coalescent scars can be related to complex slides.
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The absence of a channel at the scar toe could suggest either that these slides produce a
small volume of debris or that the failure debris do not move downslope for a long
distance.
4. Instability on lower gradient areas (typically lower continental slope and valleys
floor)
In the lower part of the continental slope (with slope gradient less than 3°-4°) instability
phenomena dominate the seafloor as well; however they are characterized more by
transport and sedimentation processes than by instability/erosive features. Mass flow
deposits are the main features recognized, they are located inside the wide valleys
carving the continental slope. Two types of mass flow were recognized, differing both
in side scan sonar and seismic facies: 1) very high backscatter-seismic soundless mass
flow interpreted as non-cohesive density flow deposits (grain flow deposits); 2) medium
backscatter-seismic transparent mass flow deposits interpreted as debris flow deposits.
4.1 VERY HIGH BACKSCATTER-SEISMIC SOUNDLESS MASS FLOWS DEPOSITS (NONCOHESIVE DENSITY FLOWS -GRAIN FLOWS DEPOSITS-)
These mass flows represent the most impressive features of the Pontine continental
slope developing for 15-20 km, from about 1.000 to 3.100-3.500 m w.d., reaching a
maximum width of 6 km (covering a total surface of about 160 sq. km). They are
actually present also in the higher gradient area on the upper slope (as coarse debris
infilling the linear scar-channel systems). However they are more developed in the area
below 2.500/2.850 m w.d., where the mean slope angle is 3°-4°, and they cover the
valley seafloor almost completely (Fig. 5). In several areas well-defined backscatter
lineation, parallel to the flow direction, were recognized, suggesting the presence of
distinct debris streams and patches within the main mass flow. Some large blocks (up to
200 m wide and up to 50 m high) were found, at 3.000-3.100 m w.d. where a sharp
change in slope angle is located (from 3-4° to about 1°); from the available data it is not
possible to define their origin (i.e. bedrock outcrops or rafted blocks). According to side
scan sonar (very high backscatter) and seismic (soundless) facies, a coarse texture (sand
to gravel) of the mass flow material can be hypothesized. This hypothesis is supported
by the presence of bedforms, normal respect to the flow, which have a wavelength of 50
m and an extension of 100-500 m. Moreover a core recovered at 2.950 m w.d. comprise
heterometric, sub-rounded volcanoclastic gravel (up to 10 cm in grainsize) and sand,
below 30 cm of mud. Excluding the upper 30 cm of mud, related to present-day
hemipelagic sedimentation, all these characters suggest non-cohesive density flow
(grain flow).
4.2 MEDIUM BACKSCATTER-SEISMIC TRANSPARENT MASS FLOW DEPOSITS (DEBRIS FLOW
DEPOSITS)
These deposits are less common than grain flows along the continental slope. However,
in a low gradient area (0.5-2°) of the central valley, a rather large flow was recognized
(Fig. 5).
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Figure 5. Lower continental slope where the grain flow (§ 4.1) and debris flow (§ 4.2) merge.
Pressure ridges and rafted blocks are present within the debris flow.
It starts at about 2.800 m w.d. on a ridge flank characterized by several coalescing scars
and extents more than 25 km downslope reaching the abyssal plain. The mass flow
deposits range in width from 2 to 6 km and covers an area of about 65 sq. km.
High resolution seismics depict a seismically transparent body having a rather constant
thickness of 6-7 m. According to these measures the mass flow deposits have a volume
of about 0.5 km3.
Within the mass flow deposits, features interpreted as rafted blocks and pressure ridges
were observed. Rafted blocks have individual widths of 50-100 m and usually are
aligned in the flow direction, covering areas up to 5 km long and 250-600 m wide.
Pressure ridges are characterized by lengths ranging from 200 to 450 m and width
ranging from 30 to 100 m and are aligned perpendicular to the flow.
With regard to the transparent seismic facies, flow dimension (a rather tabular, thin
body with wide areal distribution) and occurrence of rafted blocks and pressure ridges,
the deposits have been interpreted as debris flow deposits.
In comparison with other submarine debris flows deposits (i.e. the Saharan and Canary
debris flow; Gee et al., 1999; Masson et al., 1998 and the Norwegian debris flow;
Vorren et al., 1998), the Pontine debris flow represents a small event (volume of about
0.5 km3), however, other deeper bodies, not resolved by seismics because of the low
penetration of the seismic signal, could be present.
4.3 SHALLOW TRANSLATIONAL SLIDES
On restricted areas of the seafloor, for a total of 6-8 sq. km, characterized by low
gradient slope (typically 1.5-3°), large (0.5-2 sq km) sub-elliptical areas were found.
They are characterised by a well-defined failure headscarp, a flat surface lying 20-30 m
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lower than the surrounding seafloor and the presence of several blocks/ridges near the
scar toe (maximum distance from the scar 900 m). The blocks/ridges are 20-600 m long
and 20-180 m wide, with the longer side always parallel to the scar (Fig. 6).
Figure 6. Sonar and seismic image of a shallow translational slide (§3.3). Note how the detached
block shows a stratified seismic facies
A direct relationship between blocks size and dimension of the whole feature has been
found , as block sizes increase as the area increases. One of the larger blocks, which has
relief of 10-15 m respect to the seafloor, shows a stratified seismic facies similar to the
facies of the seafloor outside the depression (Fig. 6). The described features are
interpreted as being produced by shallow translational slides occurring over a basal
detachment surface (weak layer) and may be assimilated to the shallow slab slides of
Prior (1984). In fact the surface within the depression is always very flat with the same
dip of the surrounding seafloor and blocks shape very often mirrors the shape of the
headscarp, suggesting a detachment and rather short translation. These slides could be
considered as complex if one image a retrogressive process producing successive block
detachment.
5. Conclusion
In the Western Pontine continental margin instability and erosive processes affect
almost all (98%) of the continental slope seafloor causing the slope retreat. They
produce a complex pattern of features characterized by an extremely variable extension
(ranging from less than one sq. km to several tens of sq. km) and typology (from
processes driven only by gravity –simple and complex slides- to processes driven by
gravity and fluid motion -debris and grain flow-).
Temporal and spatial relationships between different events are poorly understood.
However, a relationship between the distribution of the different mass movements and
the slope gradient has been found. In fact, steepest areas (mean value 6°-10°, max. value
30°), such as the upper continental slope and the flanks of the main along-slope ridges,
are characterized by pervasive sliding and punctual mass transport processes developed
mainly by linear channels, gullies and canyons. On the contrary, more gently sloping
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areas (3°-4°), i.e. the lower continental slope from 2.500/2850 m to 3.100 m w.d, are
characterized by amalgamation of the debris transfer features (mainly linear channels)
and deposition of such debris, producing very wide grain flows which cover an area of
about 150 sq. km.
In areas with even more gentle slope gradient lower then 3° (typically 0.5-2°), usually
located down to 2.800-3.100 m w.d., the mass movements develop mainly as debris
flow deposits (covering an area of about 65 sq. km) even though other small scale
feature such as shallow translational slides were observed.
Apart from the difference in texture (sonar backscatter and seismic facies) and in the
slope gradient of the area where they develop, the two types of mass flow recognized
are very different. The grain flow is fed by a very large number of point sources (i.e. the
scar-channel systems and the dendritic gullies networks), so that such a wide flow is
essentially the amalgamation of multiple linear debris streams; on the contrary the
debris flow originates from a single source area actually made up of coalescing scars,
and seems to be related to a single or few events.
6. References
Chiocci, F.L., Martorelli, E., Sposato, A., and T.I.VOL.I research group 1998. Prime immagini TOBI dei
fondali del Tirreno centro-meridionale. (settore orientale). Geologica Romana Vol. 34, 207-222.
De Rita, D., Funiciello, R., Pantosti, D., Salvini, F., Sposato, A. and Velonà, M., 1986. Geological and
structural characteristics of the Pontine Islands (Italy) and implications with the evolution of the
Tyrrhenian margin. Mem. Soc. Geol. It., 36: 55-65.
Doglioni C. 1991. A proposal for the Kinematic modelling of the Tyrrenyan-Apennines system. Terranova, 3:
423- 432.
Gee, M.J.R., Masson D.G., Watts, A.B., and Allen P.A., 1999. The Saharan debris flow: an insight into the
mechanics of long runout submarine debris flows. Sedimentology, 46: 317-355.
Kastens, K., Mascle, J., and Leg 107 Scientific Party, 1988. ODP Leg 107 in the Tyrrhenian Sea: insights into
passive margin and back-arc basin evolution. Geological Society of America Bulletin, 100: 11401156.
Marani, M., Taviani, M., Trincardi, F., Argani, A., Borsetti, A.M., and Zitellini, N., 1986. Pleistocene
progradation and postglacial events of the NE Tyrrhenian continental shelf between the Tiber River
delta and Capo Circeo. Mem. Soc. Geol. It., 36: 67-89.
Masson, D.G., Watts, A.B., Gee, M.J.R. Urgeles, R., Mitchell, N.C., Le Bass, T.P., Canals, M., 2002. Slope
failures on the western Canary Islands. Earth Science 57: 1-35.
Masson, D.G., Canals, M., Alonso, B., Urgeles, R., & Huhnerbach, V., 1998. The Canary Debris Flow:
source area morphology and failure mechanisms. Sedimentology 45: 411-432.
Mulder, T., and Alexander, J., 2001. The physical character of subaqueous sedimentary density flows and
their deposits. Sedimentology, 48: 269-299.
Mulder, T., and Cochonat, P., 1996. Classification of offshore mass movements. Journal of Sedimentary
Research, Vol. 66, N°1: 43-57.
Murton B.J., Rouse I.P., Millard N.W. and Flewellen C. 1992. Deep-Sea volcanoclastic sedimentary systems:
an example from La Fournaise volcano, Reunion Island, Indian Oceans. Sedimentology, 45: 293-330.
Prior D.B. 1984. Subaqueous landslides, State of the art paper. Fourth international Symposium on
Landslides, Univ. of Toronto, Canada, 179-196.
Sartori, R., 1986. Notes on geology of the acqustic basament in the tyrrhenian sea. Mem. Soc.Geol. It. 36: 99108.
Vorren, T.O., Laberg, J.S., Blaume, F., Dowdeswell, J.A., Kenyon, N.H., Mienert, J. Rumohr, J., and Werner,
F. 1998. The Norvegian-Greenland Sea Continental Margins: morphology and late Quaternary
sedimentary processes and environment. Quaternary Science Reviews, 17, 273-302.
Weaver, P.P.E., Wynn, R.B., Kenyon, N.H., Evans, J., 2000. Continental margin sedimentation, with special
reference to the north-east Atlantic margin. Sedimentology 47: 239-256.
Zitellini, L., Marani, M., Borsetti, A..M., 1984. Post-orogenic tectonic evolution of Palmarola and Ventotene
basins (Pontine archipelago). Mem. Soc. Geol. It. p. 27: 121-131.