SHEAR-STRENGTH SIGNATURES OF MASS MOVEMENTS,
CONTINENTAL SLOPE OF CAMPOS BASIN, BRAZIL
R.O. KOWSMANN, A.M. DA COSTA, C. S. AMARAL
Petrobras Research Centre (CENPES), Cidade Universitária, Ilha do Fundão, Rio de
Janeiro 21949-900, Brazil
Abstract
Downhole shear-strength profiles, obtained from cone-penetrometer and lab tests were
tied to sedimentary facies from adjacent continuous cores. The geotechnical response of
mass-transport deposits was investigated. In the Campos Basin, sediments have
evacuated from the upper continental slope and have accumulated as folded deposits on
the middle slope. Sediment removal is recognised by an abrupt step-like change in
shear-strength at the level of the unconformity. The folded deposits are characterised by
a belly-shaped increase in shear-strength coinciding with a zone of intense lamination
within the deposit, induced by internal shearing and fluid loss (strain hardening). In
contrast, highly disintegrated muddy debris-flow deposits are indistinguishable, in terms
of shear-strength, from normal hemipelagic slope sediments.
Keywords: mass movement, shear-strength, continental slope, Campos Basin
1. Introduction
The discovery and development of giant oil fields on the continental slope of the
Campos basin has led to extensive geohazard investigations preceding the installation of
production systems on the seafloor. During these investigations, various large-scale
slope instability features were observed on high-resolution seismic, sonar and swath
bathymetry records, which were targeted for geotechnical and geological
characterisation.
This paper focuses on the shear-strength signatures and the geological interpretation of
the most common types of mass-wasting deposits encountered.
2. Morphology of the continental slope
The shelf break in the Campos basin occurs in water depths of 180m, some 100 km
from the coastline. Beyond it, the 40km wide continental slope displays a concave
profile in the south and a convex profile in the north. The geotechnical data are located
in the latter (Fig.1). The underlying Miocene prograding wedge (Fig.2) shapes the
convex profile of the northern slope. Gradients in the narrow upper slope range from 35o. The wider middle slope, overlying the back of the Miocene clinoform has an
inclination of 2o. The lower slope, coincident with the face of the Miocene clinoform, is
8-10o (Fig 2).
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Figure 1. Bathymetry of the Campos Basin slope with location of boreholes and other figures (source: Marine
Geology Group, PETROBRAS). Boreholes GL-16 and GL-25 (not shown), are located 10 meters away from
GT-48, GT-211, respectively.
3. Mass-wasting on the continental slope
Mass-wasting on the northern Campos Basin slope is controlled by physiography and
seafloor gradient (Gorini et al., 1998; Kowsmann et al., 2002).
Stacked mounded to tabular deposits with internal hummocky reflections were
deposited on the gentle middle slope covering an area of 1600km2 south of São Tomé
Canyon (Castro et al., 1995). These deposits originate from failures on the steeper upper
slope. Sediment shedding and failure prevail along the steep, highly convex lower slope
(Kowsmann and Viana, 1992).
One of the most conspicuous surface features on the gentle middle slope is an arcuate
elongated scar, about 25m in relief that surrounds a 10 x 30km morphological
Shear-strength signatures of mass movements
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depression (Fig.3A). On the distal part of this depression sonar records image a
corrugated seabed indicative of compressional folds.
Echo-character study of the 3.5 kHz subbottom profiles (Kowsmann et al., 1996a) and
subsequent piston coring (Kowsmann et al., 1996b) confirm that the base of slope is
littered with muddy debris-flow deposits (Fig. 2, core 50). In sonar records these
deposits appear as tongues of high backscatter (Machado, 2001).
4. Data and Methods
Four cone-penetrometer boreholes (CPT) were collected by Fugro-McClelland, along a
475km grid of specially acquired high-resolution multi-channel seismic lines.
Each CPT stroke reached a maximum of 3 meters and on average every 3 strokes were
alternated with 60cm long thin-walled large diameter shelby samples. On board, these
samples were subject to lab tests including mini vane, torvane and triaxial UU tests.
Pore pressure generated during CPT penetration was measured by a filter sensor above
the cone tip and was used to correct the cone resistance. Further corrections were also
made to reference the data to the seafloor. The corrected CPT profiles, expressed both
as total and effective cone resistances, were converted to undrained shear strength (Su)
by using an empirical correlation with the shipboard triaxial UU tests.
Next to three of these geotechnical boreholes, continuously cored holes were drilled in
order to ensure the correlation of the geotechnical data with sedimentological and age
information. These cores were split lengthwise, photographed, described for color, grain
size, calcium carbonate content and sedimentary structures (Caddah et al., 1998;
Magalhães and Andrade, 1999).
Sediments were dated by Vicalvi (1997, 2001) using the planktonic foraminifer
zonation scheme of Ericson and Wollin (1968), calibrated to the oxygen isotopic stages
of Imbrie (1985).
5. Results
The geotechnical (GT and GS) and continuously cored (GL) boreholes, presented here
as examples, were drilled on several situations on the continental slope (Fig.1).
5.1 GS-28
GS-28 was drilled on the steeper upper slope from which sediments evacuated and
moved in a downslope direction. An abrupt step-like increase in shear strength is
observed at the subbottom depth of 57m (Fig.2). The projection of the shear-strength
profile below the step, towards the origin (0 Kpa) indicates that 90m of sediment
column were removed at one time, with the later accumulation of 57m at the site. Thus
the step represents an unconformity. In seismic records, this unconformity can be traced
downslope to the base of the mounded deposits with hummocky internal reflections and
most likely represents the glide plane for the displaced sediments.
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5.2 GT-32/GL-13
Boreholes GT-32/GL-13, downslope of GS-28, are located on the gentle middle slope,
where sediments accumulated as mounded to tabular mass-movement deposits with
hummocky seismic reflection patterns (Fig. 2).
Two of these stacked deposits were sampled in boreholes GL-13/GT-32 three
kilometers apart (Fig.2). The deposit nearest to the seafloor is 60m thick in GL-13.
Figure 2. Seismic section across the continental slope with boreholes GS-28, GT-32/GL-13. Vertical scale in
seconds. Location in Fig.1. MU- Late Miocene unconformity. PW- prograding wedge. Step in shear strength
profile (line- CPT, circles- triaxial UU) at GS-28 indicates decollement surface over which sediments moved
downslope by creep, accumulating as folded sediment mounds sampled in GL-13. The top section of GL-13 is
also illustrated in piston core 3, where folds (fd) and overlying debris-flow (df) were up-thrown by
compression. “Potbelly” of higher shear strength at zone of intense lamination in GT-32 is due to shearing and
fluid loss during creep. Sediment failure and shedding in steep (>10o) lower slope is documented by
unconformity in piston core 5. Debris-flows accumulate at the base of the slope (core 50). Numbers next to
GL-13 are oxygen isotopic stages. Based on Kowsmann and Viana (1992), Caddah et al. (1998).
Shear-strength signatures of mass movements
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The upper and lower parts of this deposit display inclined color bands and beds dipping
at various angles, with truncations and fold hinges. Fractures and small offset faults are
common in the lower part of the deposit, cross-cutting the inclined banding.
A 20m-thick zone of intense sub-horizontal lamination occurs at the centre of the folded
deposit. This lamination is produced by the textural segregation, due to shearing, of the
bioturbated sandy mud components. Structures such as discontinuous sand lenses
(boudins) interbedded with the mud and stretched Planolites burrows are typical
(Caddah et al., 1994). The zone of intense lamination in GL-13 correlates with a
“potbelly” of higher shear-strength in GT-32 (Fig.2).
According to Farrel and Eaton (1987) this highly sheared section within a slump deposit
forms when water migrates upward across the moving mass causing a differential
displacement between the more consolidated (dehydrated) base and its lubricated top.
The expulsion of fluids by deformation leads eventually to the “freezing” of the deposit.
Dating of the sediments revealed that the downslope mass-movement took place rather
slowly, by the process of creep instead of slumping. The initiation of the movement was
coincident with the significant drop in sealevel in isotopic Stage 4 and ended at a
relative rise in sealevel in isotopic Stage 3 (Vicalvi, 1997).
5.3 GT-48/GL16
Where sediments are regularly deposited and are subject to normal compaction the
downhole shear-strength profile should display a linear increase with depth, starting
with zero strength at the seafloor. This type of profile is observed in borehole GT-48,
where seismic reflections are concordant and sub-parallel to the seafloor (Fig.3B) and
lithology and biostratigraphy of GL-16 (not shown) indicate the continuity of deposition
and absence of disturbed sections.
5.4 GT-47 and GT211/GL-25
These boreholes were drilled into the disturbed sediment prism enclosed by the major
slope scar both in upslope (GT-47) and downslope (GT-211/GL-25) positions. They
contrast with the position of GT-48/GL16, which were drilled outside the major slope
scar (Fig.3A).
A high-resolution strike-oriented seismic line connecting GT-48 and GT-47 illustrates
their contrasting seismic facies, parallel in the former and chaotic in the latter (Fig.3B).
The shear strength profiles of GT-48 and GT-47 are identical from the seafloor to the
depth of 38m, where a sudden step-like increase occurs in GT-47 (Fig.3E). This step is
coincident with the base of the chaotic fill observed in the seismic record. The
reconstruction of the profile below the step suggests that a 60m thick section of
sediment was removed, the ensuing void being filled by 38 meters of chaotic sediment.
Thus, the 20m bathymetric escarpment represents only the visible part of a much more
profound scar on the slope.
The nature of the topmost 38 meters of sediment in GT-47 is uncertain, because no core
data exists in the area. While its seismic facies is chaotic, the shear-strength profile is
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identical to that of GT-48/GL-16 where mass-wasting deposits are absent. The chaotic
sediments seem to have originated from the collapse of the scar headwall but they are
geotechnically indistinguishable from normal slope sediment. The character of the3.5
kHz echogram at the site of GT-47, of seafloor tangent overlapping hyperbolae (Fig.
3C) is, according to Damuth (1980), indicative of debris-flow deposits.
GT-211is located inside the perimeter of the scar, further downslope from GT-47
(Fig.3A). It displays a similar step-like increase in shear strength at 17m; however this
step does not coincide with the actual top of the folded deposit sampled in nearby
borehole GL-25, which occurs at 7.5m (Fig.3F). A better correlation is observed at the
unconformable base of the folded deposit at 40m (u, Fig.3F), where another step in
shear strength is noted. As in GT-47, the topmost part of the mass-movement deposit
cannot be differentiated, in terms of shear-strength, from in situ slope sediments. In GL25 the mass wasting deposit consists of mud with faint inclined bedding that dips in
opposite directions forming fold hinges and is interpreted as a folded slump deposit.
This genesis is consistent with the corrugated seafloor morphology of compressional
folds observed in sonar and in 3.5 kHz echogram at the site (Fig. 3D). It is suggested
that within the perimeter of the scar, slope sediments moved downslope as a carpet slide
to the site of GT-211, leaving behind a steep scar on the upper slope that collapsed into
a rubble slide at the site of GT-47.
6. Conclusions
On the continental slope, the removal of sediment is easily recognised by an abrupt
downhole increase in shear-strength at the level of the unconformity. The magnitude of
the step is proportional to the magnitude of the section removed.
Folded deposits generated by slump or creep display higher shear-strengths. Where
these deposits are deformed into a zone of intense lamination due to shearing, a
potbelly-shaped increase in shear-strength develops through sediment de-watering and
strain hardening. The top of this zone does not coincide with the top of the folded
deposit. Matrix-rich muddy debris-flows cannot be distinguished, in terms of shearstrength, from normal slope sediments. However, muddy debris-flows bearing older,
more consolidated clasts do display higher shear-strengths with irregular, serrated
profiles.
7. Acknowledgements
The authors are indebted to the staff of Marine Geology Group at PETROBRAS for
sharing data and ideas. We thank reviewers Renato Cesar Salgado da Fonseca and
Dennis James Miller for their constructive comments.
Shear-strength signatures of mass movements
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Figure 3A. Swath bathymetry of upper slope scar with location of geotechnical boreholes. 3B. Seismic section
connecting GT-48 and GT-47. 3C- Echogram at site of GT-47 characteristic of debris-flow type chaotic fill.
3D. Echogram at site of GT-211 characteristic of folded sediments. Strong sub-bottom reflection (u) correlates
with unconformity at 40m in GL-25 (Fig. 3F). 3E. Shear-strength profiles of GT-48 and GT-47 (see coments
in text). 3F. Correlation of geotechnical borehole GT-211 core GL-25. Lithology from Magalhães and
Andrade (1999); hm- hemipelagic mud, m- mud; fd- folded deposit. Biozones Z (Holocene), Y (Glacial) and
X (Last Interglacial), from Vicalvi (2001).
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8. References
Caddah, L.F., Kowsmann, R. O., Viana, A. R. , 1994. Laminação secundária em sedimentos escorregados:
um exemplo do Pleistoceno da Bacia de Campos. Bol. Geociências da Petrobrás, .8, (2/4): 421-427.
Caddah, L.F., Kowsmann, R.O., Viana, A.R., 1998. Slope sedimentary facies associated with Pleistocene and
Holocene sea-level changes, Campos Basin, Southeast Brazilian margin. Sedimentary Geology 115:
159-174.
Castro, D.D., Rizzo, J.G., Heinerici, J., Caddah, L.F.G., 1995. Geometry of five submarine slump complexes,
Campos Basin, Brazil. 4o Congresso Internacional de Geofísica, Rio de Janeiro, Anais, p. 615-618.
Damuth, J.E., 1980. Use of high-frequency (3.5-12 kHz) echograms in the study of near-bottom sedimentation
processes in the deep-sea: a review. Marine Geology, 38: 51-75.
Ericson, C., Wollin, G., 1968. Pleistocene climates and chronology in deep-sea sediments. Science, 16 (3859):
1227-1234.
Farrel, S.G., Eaton, S., 1987. Slump strain in the Tertiary of Cyprus and the Spanish Pyrenees: definition of
paleoslopes and models of soft-sediment deformation. In: Jones, M.E., Preston, R.M.F. (Eds)
Deformation of Sediments and Sedimentary Rocks. Geological Society Special Publication, 29: 181196.
Gorini, M.A., Maldonado, P. R., Silva, C.G., Souza, E.A., Bastos, A.C., 1998. Evaluation of deep water
submarine hazards at Campos Basin, Brazil. OTC 86944, p.133-1141.
Imbrie, J., 1985. A theoretical framework for the Pleistocene ice ages. Geological Society of London Journal,
142: 417-432.
Kowsmann, R.O., Viana, A.R., 1992. Movimentos de massa provocados por cunhas progradantes de nível de
mar baixo: exemplo na Bacia de Campos. Bol. Geociências da Petrobrás, 6 (1/2): 99-102.
Kowsmann, R.O., Schreiner, S., Murakami, C.Y., Piauilino, P.O., Barrocas, S., Miller, D., Rizzo, J.G.,1996a.
Ecofácies de 3,5 kHz do talude da Bacia de Campos e do Plato de S. Paulo adjacente. 39o Congresso
Brasileiro de Geologia, Salvador, Anais v.3, p.463-465.
Kowsmann, R.O., Voelker, H.E., Magalhães, J.L.C., 1996b. Análise sedimentológica de testemunhos a pistão
do talude da Bacia de Campos e Plato de S. Paulo adjacente. Relatório técnico no 006/96,
Petrobras/E&P-BC/Gexp/Gelab, 46p.
Kowsmann, R.O., Machado L.C.R., Viana, A.R., Almeida Jr.,W., Vicalvi, M.A., 2002. Controls on masswasting in deep water of the Campos Basin. OTC 14030, 11p.
Machado, L.C., 2001. Sonar characterisation of modern subaqueous mass-movement deposits bordering the
Brazilian continental slope. 7th Intern. Cong. of the Brazil. Geophys. Soc., Salvador, Expanded
Abstract. SBGf.
Magalhães, J.L.C., Andrade, S.B., 1999. Análise sedimentológica dos furos geológicos GL-20, GL-22, GL-23,
GL-25, GL-27, GL-28, talude quaternário da Bacia de Campos e Platô de São Paulo adjacente. Macaé
E&P-BC/GEXP/GELAB Relatório técnico 004/99, 19p.
Vicalvi, M.A., 1997. Zoneamento bioestratigráfico e paleoclimático dos sedimentos do Quaternário superior
do talude da Bacia de Campos, R.J. Brasil. Boletim de Geociências da Petrobras, 11, (1/2) CD-ROM.
Vicalvi, M.A., 2001. Bioestratigrafia do furo geológico GL-25 (Talude de Barracuda). CT- BPA 079/01, 47p.