Megaslides in the Foz do Amazonas Basin,
Brazilian Equatorial Margin
C.G. Silva, E. Araújo, A.T. Reis, R. Perovano, C. Gorini, B.C. Vendeville,
and N. Albuquerque
Abstract Recent analysis of multi-channel seismic data provides new evidence that
mass-transport deposits have been recurrent elements in the Foz do Amazonas Basin
during the Middle-Miocene to Recent. In regions located to the NW and SE of the
Amazon Deep-sea Fan, mass movement processes remobilized thick siliciclastic
series (up to 1,000 m) as huge megaslide deposits over areas up to 90,000 km2.
The Pará-Maranhão Megaslide in the SE shows a displaced block (>104 km2) in
association with large mass transport deposits covering an area of more than 105 km2.
These deposits are distally bounded by thrust faults, which propagate upwards
eventually offsetting the sea floor. In addition, NW of the deep-sea fan, the Amapá
Megaslide Complex presents a series of recurrent megaslides in the stratigraphic
succession, bounded by listric normal faults and tear zones on the upper slope.
Associated remobilized deposits extend from more than 300 km downslope, partially
involving the upper channel-levee units of the Amazon Deep-sea Fan.
Keywords Submarine mass-movements • megaslides • Amazon Deep-sea Fan • Foz
do Amazonas basin
C.G. Silva (), E. Araújo, R. Perovano, and N. Albuquerque
Universidade Federal Fluminense, Departamento de Geologia, Av. Gen. Milton Tavares de Souza,
s.n., Niterói, RJ, CEP: 24210–346, Brazil
e-mail: cleverson@igeo.uff.br
A.T. Reis
Universidade do Estado do Rio de Janeiro, Faculdade de Oceanografia, Rua São Francisco Xavier,
524, 4° Andar, bl E, Rio de Janeiro/RJ. CEP: 20.550–013, Brazil
C. Gorini
Université Pierre & Marie Curie, Paris VI,.Laboratoire de Tectonique et Modélisation des Bassins
Sédimentaires, UMR 7072, Place Jussieu, Case 117 tour 46–00, 75252 Paris, Cedex 05, France
B.C. Vendeville
Université de Lille1, 4 UMR 8157 Géosystèmes, Bat. SN5, USTL, 59655, Villeneuve d’Ascq
Cedex – France
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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Introduction
The Foz do Amazonas Basin on the Brazilian Equatorial Margin (Fig. 1) has been
influenced by immense sedimentary fluxes coming from the Amazon River, resulting in the seaward progradation of the margin and the deposition of the Amazon
fan. The submarine fan is a >10 km thick turbiditic complex that started to develop
during the Upper Miocene (Silva et al. 1999). Such a sedimentary context resulted
in a rather smooth central basin, while to the NW and SE of the fan the margin is
narrower and exhibits steep slope gradients.
Submarine mass movements and associated deposits are important elements in
the Foz do Amazonas Basin. In the Amazon fan, exceptionally high sedimentation
rates can induce submarine failures and sediment fluxes that contribute to the sedimentary evolution and architecture of the deep-sea fan (Damuth and Embley 1981;
Fig. 1 Location map. Regional bathymetry (100 m and every 1,000 m) and seismic dataset used
Megaslides in the Foz do Amazonas Basin
583
Damuth et al. 1988; Piper et al. 1997; Maslin and Mikkelsen 1997; Pirmez and
Imran 2003; Maslin et al. 2005).
The scientific literature, on the other hand, does not provide examples of studies
dealing with the sedimentary series along the steep NW and SE slope segments of
the Foz do Amazonas Basin margin. We used regional seismic lines to investigate
features of gravitational collapse occurring along such steep margins, focusing on
the recognition and description of extensive megaslides.
1.1
Database and Methods
The 2D multi-channel seismic data (Fig. 1) include approximately 15,000 km of
seismic-reflection profiles. The Brazilian Navy and the Brazilian Petroleum
Exploration Company (Petrobras) collected part of the data. This data set penetrates to
a maximum of 13 s (TWT) and shows 5 to 10 m of vertical resolution. Geophysical survey
companies operating in Brazil (FUGRO and GAIA) provided additional industrial
seismic data of up to 9 s of signal penetration and the same vertical resolution. For
the seismic interpretation we used SMT (8.0) Kingdom Suite®.
Regional bathymetric data are from ETOPO1 (Smith and Sandwell 1997) and
higher-resolution bathymetric data on the Upper to Middle Amazon fan were compiled
by the Brazilian Navy from different sources, including Petrobras, the Geophysical
Data System – GEODAS (www.ngdc.noaa.gov/mgg/geodas) and from the General
Bathymetric Charts of the Oceans – GEBCO (www.gebco.net).
1.2
Geological Setting
Open marine conditions in the Foz do Amazonas Basin started at the end of the
Albian, with the dominance of siliciclastic sedimentation during the Upper
Cretaceous (Brandão and Feijó 1994; Figueiredo et al. 2007). A carbonate platform
installed during Upper Paleocene times prevailed during the Paleogene and part of
the Neogene. An increased rate of the Andean uplift since the Upper Miocene
(10.7 Ma – Tortonian) was responsible for increased siliciclastic input to the
Amazon River that finally led to the formation of the Amazon fan (Hoorn et al. 1995;
Figueiredo et al. 2007).
The Amazon fan occupies an area of approximately 160,000 km2, from the continental shelf break down to 4,800 m water depth (Damuth et al. 1988). With a total
of 10 km of sedimentary thickness deposited in 10 Ma, the average sedimentation
rate is 1 mm/year (Cobbold et al. 2004). However, sedimentation rates of 0.05 to
0.1 mm/year during interglacial periods and 50 mm/year during glacial periods have
been reported (Mikkelsen et al. 1997; Piper et al. 1997).
The sedimentary sequence of the Amazon fan glides along multiple detachment
surfaces in a linked extensional-and-compressional system (Da Silva 2008). The
extensional domain is located on the outer continental shelf and upper slope (up to
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500 m deep), consisting of predominantly seaward-dipping normal listric faults.
Distally, folds and thrusts running along the upper submarine fan (up to 2,000 m)
characterize the compressional domain. Thrust structures can reach the sea floor,
forming scarps as high as 500 m (Araújo 2008).
Previous research works on mass transport deposits in the Foz do Amazonas Basin
have been focused on the Amazon fan (Damuth and Embley 1981; Piper et al. 1997).
Recurrent mass-wasting events and mass-movement deposits are interbeded with
channel-levee deposits (Damuth et al. 1983, 1988; Maslin and Mikkelsen 1997; Flood
and Piper 1997; Piper et al. 1997; Pirmez and Imran 2003; Maslin et al. 2005). Each
deposit can result in remobilized sediment masses (slumps, slides and debris flows)
as large as 15,000 km2 and reaching thicknesses of up to 200 m. Most of the surficial
and subsurface mass transport deposits are recent, dated as either 41–45 or 35–37 ka,
and formed during Upper Quaternary low sea-level stands (Piper et al. 1997; Maslin
and Mikkelsen 1997; Maslin et al. 2005). Some even younger slides (11 to 14 ka)
formed during the last sea-level rise (Maslin and Mikkelsen 1997; Maslin et al.
2005). Piper et al. (1997) and Maslin and Mikkelsen (1997) consider that during low
seal-level stands, gas hydrates destabilization, higher sedimentation rates and erosion
of canyon heads are the principal causes of mass movement initiation. During relative
sea-level rise and high stands the rise of the hydrostatic pressure and the migration
of depocenters would generate instabilities. Piper et al. (1997) cited the gravity
tectonics observed in the Amazon fan as another possible cause; however, the authors
did not explore this hypothesis.
2
Results
The Amazon fan sedimentation greatly affects the morphology of the Brazilian
Equatorial Margin. The steep gradients (3° to 5.6°) observed on the continental
slope to the NW and SE of the fan are totally subdued by the thick sedimentary
sequences responsible for the fan construction (average gradients on the upper
deep-sea fan are in the order of 0.8°). In the steep continental slope to the NW and
SE of the Amazon fan, mass transport mobilized thick siliciclastic series (up to
1,000 m, considering a layer sound velocity of 1,800 m/s for the upper 2 s of the
seismic sections) as megaslides deposits (Fig. 2) over areas up to 90,000 km2 that
are for the first time mapped in this work. These deposits are considerably thicker
than the mass transport deposits mapped on the Amazon fan (Damuth and Embley
1981; Piper et al. 1997; Araújo 2008).
2.1
The Pará-Maranhão Megaslide
The shelf break to the SE of the deep-sea fan occurs at 100 m water depth and the
upper slope gradients are in the order of 3° to 3.5°. On the upper slope, an abrupt
erosional scarp (maximum vertical relief of 1,000 m), continuous along 180 km, is the
headwall scarp of megaslide deposits, which extend downslope over approximately
Megaslides in the Foz do Amazonas Basin
585
Fig. 2 Mass transport deposits and megaslides in the Foz do Amazonas Basin. Numbers indicate
locations of Figs. 3–7
600 km. The megaslide has a maximum width of 200 km, covers an estimated
area of 90,000 km2 comprising about 60,000 km3 of remobilized sediments. This
megaslide was named Pará-Maranhão Megalisde since a great part of it extends to
the neighbor Pará-Maranhão basin (Figs. 1 and 2).
Downslope from the headwall scarp, slided blocks extend over 12,500 km2, having
a thickness around 1,000 m. These blocks are faulted (normal listric faults) and rotated
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(Fig. 3). The downslope limit of the displaced blocks is located at around 2,100 m
deep, where there is another prominent scarp, with 700 m of vertical relief and extending
laterally along 260 km (Fig. 2). Further downslope the allochtonous mass has a
chaotic-to-transparent seismic facies, with a few preserved blocks showing parallel
seismic reflectors. An upper unit (150 to 200 m) of parallel discontinuous reflectors
still preserves the original layers; however, irregularities of the seafloor suggest a continuous activity of the mass transport processes and associated deformation (Fig. 3).
Both the headwall scarp and the displaced blocks are the source area for considerably more deformed mass transport deposits occurring downslope. These deposits
are internally chaotic, suggesting debris flow, but locally present large displaced
blocks that still maintain their internal layering (Fig. 3). These deposits extend to
water depths of 3,800 m. The megaslide is laterally and frontally confined, favoring
the development of a series of reverse faults rooted on detachment levels that
progressively become shallower, thus decreasing the thickness of the mobilized
deposits from 1,000 m to about 300 m basinward. Eventually they reach the seafloor
forming pressure ridges (up to 50 m of relief) (Fig. 4).
Fig. 3 Headwall scarp, displaced and rotated blocks, debris flow deposits and preserved block (bl)
over detachment surface (dotted line) in the upslope portion of the Pará-Maranhão Megaslide
Fig. 4 Pará-Maranhão megaslide lateral confinement with reverse faults and pressure ridge
Megaslides in the Foz do Amazonas Basin
2.2
587
The Amapá Megaslide Complex
To the NW of the Amazon fan, the shelf-break is located under 200 m water
deep, and the upper continental slope shows the highest local gradients in the
Foz do Amazonas Basin (circa 5.5°). A continuous erosive scarp, as high as
1,800 m, extends laterally along the upper continental slope for at least 120 km
(Fig. 2). Beyond 2,000 m water depths, the seafloor is again smooth, with low
gradients around 0.93°, in response to the deposition in the distal portions of
the Amazon fan.
The megaslide deposits mapped in this region correspond to a complex of several
mass transport deposits encompassing the upper sedimentary section between
1,800 to 2,700 m thick. These successive deposits present a total estimated area of
about 80,000 km2, from the base of the slope (2,600 m of water depth) to a depth of
about 4,000 m below sea level. Each individual deposit can vary in thickness from
approximately 300 to 700 m, and present variable extensions (Fig. 5). This
megaslide was named Amapá Megaslide Complex, after the adjacent coastal Amapá
State in northern Brazil (Figs 1 and 2).
This megaslide complex starts on a headwall scarp located on the upper continental slope, where a large displaced block moved along a basal sliding surface
(Fig. 5). This basal megaslide represents the first gravity collapse event in the area
and is separated from the upper megaslide deposit by a thick in situ non-deformed
unit (900 m), with layered, continuous reflectors. Above this non-deformed unit, at
least five discrete megaslide deposits were recognized, each one departing from a
removal scar and presenting sharp internal transitions from layered reflectors to
chaotic or transparent seismic facies (Fig. 5). Further downslope the detachment
Fig. 5 Headwall scarp and upslope deposits of Amapá Megaslide Complex (bl = preserved
blocks). Detachment surfaces (small dashed lines) and megaslide deposits (darker gray)
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level shifts upward, decreasing the thicknesses of the mass transport deposits until
they completely disappear in continuity with non-deformed layered reflectors.
These distal domains of megaslides are still marked by prevalent, although
discontinuous, layered reflectors deformed by thrusts related to frontal and
lateral shortening.
3
Discussion
In contrast to mass movement deposits recognized on Amazon fan area (Damuth
and Embley 1981), the Pará-Maranhão and Amapá megaslides complexes are
located along portions of steep continental slopes of the Foz do Amazonas basin.
The sedimentation rates in these areas are considerably lower as compared to
the Amazon fan region. This may indicate that higher slope gradient is the main
controlling factor to initiate mega-events of mass wasting. These megaslides are
considerably larger (several 100’s km long and a few 100 km’s wide) and thicker
(up to 1,000 m) than the mass-transport deposits recognized by Damuth and
Embley (1981) and Piper et al. (1997) in the Amazon fan. Furthermore, the stratigraphic column in the Amapá and Pará-Maranhão Megaslides has recorded multiple
events that, unlike the Amazon fan, are not restricted to the Upper Quaternary
sedimentary section.
The Pará-Maranhão Megaslide shows 60,000 km3 of remobilized sediments,
located out of the sedimentary influence of the Amazon fan. It is comparable in
dimensions with other well-known megaslide such as the Storegga Slide, which has
25,000 to 35,000 km3 (Bryn et al. 2005). These scales apply to the young ParáMaranhão Megaslide, but other older megaslides occurred deeper in the sedimentary section. On the other hand, the Amapá Megaslide Complex differs markedly
from the Pará-Maranhão Megaslide, because the entire sedimentary section (1,800
to 2,700 m) was affected by a succession of megaslides. In addition the Amapá
Megaslide Complex was influenced by the Amazon fan sedimentation, as observed
by the presence of interlayered channel-levee systems, some of them partially
remobilized by the mass-transport deposits. The thinner and more frequent episodes of mass-transport deposits that occurred in the Amapá Megaslide Complex
could result from higher sedimentation rates from the Amazon fan system, leading
to increased frequency of slope instabilities.
The seismic horizons along the basal detachments of each individual allochtonous mass transport deposit mapped both in the Pará-Maranhão and Amapá
megaslides, present a characteristic negative polarity (Fig. 6), which may indicate
fluid overpressure conditions as recognized in the Amazon fan (Silva et al. 1999;
Cobbold et al. 2004; Da Silva 2008). As well as that, the regional detachment
surface that serves as the basal level of both megaslide complexes laterally correlates
with the regional upper detachment surface of gravity tectonics defined by Da Silva
(2008) and dated as 40 Ma in the Amazon fan (Fig. 7). This stratigraphic correlation suggests that different modes of gravitational collapse that occur in the
Megaslides in the Foz do Amazonas Basin
589
Foz do Amazonas Basin, either in the form of gravity tectonic deformation in the
Amazon fan or in the form of sediment layers disruption and mass transport in the
megaslides, may reflect the regional interaction of gravitational processes of different
scales and frequencies. The associated stratigraphic horizon can thus be interpreted
as an impermeable regional level at basin scale, enhancing fluid overpressure, and
thus potentially favoring gravitational instabilities and collapse at different domains
of the Foz do Amazonas Basin.
Fig. 6 Thrust sheets and reverse faults in the Amapá Megaslide Complex. Negative polarity
(white) on the detachment surfaces (dotted lines) may indicate fluid overpressure
Fig. 7 Gravitational collapse over upper detachment surface in the Amazon Deep-sea Fan (Da
Silva 2008). The base of Pará-Maranhão Megaslide and Amapá Megaslide Complex correlates
laterally with the upper (white dashed) detachment surface
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C.G. Silva et al.
Conclusions
This study reports, for the first time, the occurrence of megaslide complexes
capable of remobilizing huge allochtonous masses downslope the NW and SE
slope segments of the Foz do Amazonas Basin, such as the Pará-Maranhão
Megaslide and the Amapá Megaslide Complex. These megaslides have been recurrent events during the last 40 Ma, but were more frequent in the Amapá Megaslide
Complex, probably because of higher sedimentation rates under the influence of
the Amazon fan.
Both megaslides are located in areas of steep slopes (~3°–6°), show well-defined
headwall scarps, and rotated blocks sliding along seismically well defined surfaces.
Strongly deformed allochthonous masses present chaotic-to-transparent facies,
interbedded laterally and vertically with non-deformed units and blocks. Thrust
sheets and reverse faults, in the lateral and distal portions of the mass transport
deposits, indicate laterally- and frontally-confined sediment masses, eventually
developing pressure ridges that deform the seafloor.
Below both megaslides, the main base of slide correlates laterally to a regional
seismic horizon that corresponds to the upper detachment surface involved in the
Amazon fan gravity tectonics. This correlation suggests that the same surface acted
as an impermeable layer, favoring fluid overpressure and consequently the gravitational collapse.
Acknowledgements The authors greatly acknowledge financial support and scholarship from
CNPq/CTPETRO, CAPES, COFECUB and the Brazilian Petroleum Agency, ANP. We also thank
GAIA and FUGRO for the availability of additional seismic data and SMT Kingdom for the use
of educational licenses of the software Kingdom Suite. We acknowledge the reviewers Adolfo
Maestro and Lorena Moscardelli for their valuable comments.
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