2. Physiographic setting
2.1. Seabed topography and
geological setting
The continental rise in the south-western Brazil Basin
confined between the Rio Grande Rise in the south, the Vitoria-Trindade
seamounts in the north and the escarpment of the São Paulo Plateau in the west
is characterized by a rather smooth surface gently sloping eastward. In
addition to the W-E extended Vitoria-Trindade chain there are two single conically
shaped seamounts: Almirante Saldanha and São Tomé (Fig. 1a, b). The São Tomé
seamount (STS) has dimensions of about 37 km x 42 km and a relative height of
2800 m (with a minimal water depth of 1350 m), while the height of the
Almirante Saldanha seamount exceeds 3600 m and its dimensions are about 47 x 58
km.
The large São Paulo Plateau is located on the continental
slope at a depth range of 2000-3200 m. Its eastern boundary is marked by the
200 m high escarpment showing an N-S trend. The plateau is affected by numerous
halokinetic structures related to the Aptian salt deposits. The active diapiric
structures control courses of manifold channels forming a complex network on
the plateau (Viana et al., 2003). A part of these channels crosses the escarpment
and extends to the rise where further converges and forms several major channels
(Mello, 1988; Castro, 1992; Miller et al., 1996; Alves, 1999). The most
prominent one in the region is the Columbia (Trindade) channel with a depth of
200-400 m and width up to 20 km. It runs in the WNW-ESE direction at a depth
range of 4200-5000 m (Faugeres et al., 2002; Lima et al., 2009). The Rio de
Janeiro channel has much smaller dimensions. It is oriented in the W-E
direction and related to the Rio de Janeiro fracture zone (Bassetto et al.,
2000, Gonthier et al., 2003). Their largest tributaries are the Maçae channel
running along the foot of the Almirante Saldanha seamount and the Carioca
channel located southward (Fig. 1a).
This study embraces the area confined between the
Almirante Saldanha and São Tomé seamounts, the Vitoria-Trindade seamounts and
the Maçae channel (Fig. 1b).
2.2. Regional Oceanography
Bottom circulation in the deep-water part of the
south-western Brazil basin is generally controlled by the Antarctic Bottom
Water (AABW). The term AABW in this paper comprises the Weddell Sea Deep Water
and the lower part of Circumpolar Water described by Reid et al. (1977).
AABW propagates from the Argentine basin to the Brazil
Basin mainly through the Vema channel and the Hunter channel located eastward
of the Rio Grande Rise. Northward flowing bottom currents of Antarctic water
are deflected eastward turning around the Vitoria-Trindade chain and passing through
the deep passages between Dogaressa bank, Columbia seamount and the Trindade Island
(Fig. 1a). Modern AABW current velocities don’t exceed 10 cm/s in the open
basin, while in channels and transform fault valleys they can reach 20 cm/s
(Tarakanov, Morozov, 2015).
AABW is overlaid by highly saline, oxygen-rich and
nutrient poor North Atlantic Deep Water (NADW) flowing southward at a velocity
of less than 5 cm/s (Reid, 1989, 1996). A boundary between AABW and NADW in the
southern Brazil basin corresponds to a depth of about 4000 m and well
correlates with a neutral density isosurface of γn = 28.16 g/cm3
(Morozov et al., 2010). Проверить глубину границы по нейтральной плотности
2.3. The sedimentary system
According to seismic data the thickness of sediments deposited
on the continental rise in the south-western Brazil basin since the late
Cretaceous is 1–1.8 s TWT (two way time). The upper Pliocene-Quaternary seismic
unit has a thickness of less than 0.1 s TWT (Barker et al., 1983; Viana et al.,
2003, Lima et al., 2009). The Quaternary section recovered at the DSDP site 515
lying north of the northern outlet of the Vema channel is about 40 m thick (Barker
et al., 1983).
The regime of oceanic sedimentation in this part of
the western South Atlantic has taken place since Aptian/Albian (113 Ma, Chang
et al., 1992). The Late Cretaceous-Eocene sedimentation was probably dominated
by hemipelagic settling and turbidity currents. The opening of the Drake and Tasman
oceanic seaways in the Southern Hemisphere in Eocene-Oligocene caused dramatic
changes in ocean circulation, and since that time deposition was controlled by
interplay between geostrophic contour currents (along slope processes), gravity
flows and mass-wasting processes from continental slope and seamounts
(down-slope gravity driven processes). Global climate changes and tectonic
modifications allowed a dramatic increase of the NADW and AABW production
resulting in episodes of a drastic erosion and formation of several major
regional unconformities. They represent time lines embracing the period from the
Eocene-Oligocene to the late Pliocene (Gamboa et al., 1983; Faugeres et al.,
2002; Viana et al., 2003; Lima et al., 2009). Lateral sedimentation processes are
responsible for formation of several depositional system: São Tomé turbidite
system at the foot of the São Paulo Plateau escarpment (Viana et al., 2003;
Gonthier et al., 2003), Columbia channel mixed contourite-turbidite system (…),
São Tomé seamount mixed system (Borisov et al., 2013), Vema contourite fan (Mézerais
et al., 1993; Massé et al., 1994) and Ioffe contourite drift (Ivanova et al.,
2016). Most of the channels in the area have been transporting sands during the
Late Quaternary and remain active till now (Machado et al., 1998; Viana, 1998).
3. Materials and methods
This study is based on seismic, lithological and
micropaleontological data acquired during cruises 33, 35, 37 and 43 of the RV
Akademik Ioffe (2010-2013) as well as on publicly available bathymetric data.
3.1. Bathymetry
The physiographic map of the Brazil oceanic margin
(scale 1:1883927) is used as a geomorphological background for the study area.
This map is prepared by the Navy Center of Hydrography of the Brazil
Directorate of Hydrography and Navigation and compiled of the LEPLAC data
(Brazilian Continental Shelf Survey Plan), nautical charts, ETOPO global relief
model and General Bathymetric Chart of the Oceans – GEBCO (
https://www1.mar.mil.br/dhn/?q=node/202).
3.2 Seismic profiling
High-resolution seismic lines running in the SW-NE
direction eastward and westward of the STS were collected by means of the SES 2000 deep (Innomar Technologie GmbH,
Germany) parametric sub-bottom echo-sounder (central frequency 4 kHz). Several
short intersecting profiles were collected at the foot of the northwestern
slope of the São Tomé seamount. The acoustic penetration into the sediments
reached 20-30 m with a vertical resolution up to 15 cm. Processing of the SES data was carried
out with the Interactive Sediment Layer Editor (ISE) software. A sound velocity
of 1500 m/s was assumed both for the water column and sediments.
The profiles were subjected to the seismic facies
analysis aimed to determine the geometry of the studied depositional features
and its internal acoustic structure. Various types of seismic facies have been
distinguished and interpreted according to Damuth (1975) and Damuth, Hayes
(1977). Since the acoustic images of the same deposits recorded using the SES 2000 deep profiler and echo-sounders
of 60-70’ss differ due to their technical characteristics, the classification
of seismic facies given by Damuth, Hayes (1977) has been adapted for seismic
facies distinguished in SES profiles (Fig. 2). This adaptation is based on
correlation between SES seismic data collected during cruises 32, 33, 35, 37,
40 of the RV Akademik Ioffe (2010-2013) in
the Western Atlantic (Levchenko, Murdmaa, 2012, 2013; Levchenko, 2014;
Levchenko et al., 2014) and map of seismic facies distribution at the east
Brazilian oceanic margin published by Damuth, Hayes (1977). This map has been improved according to
collected seismic data.
3.3 Lithology
Four gravity cores, with recoveries from 1.94 to 3 m,
were retrieved to the northwest and southeast of the STS at water depths from 3950
to 4108 m (Fig. 1a, b).
The coordinates, water depths, core lengths of each core site are provided in
Table 1. All the collected cores were subjected to a visual and smear-slide
description, as well as wet sieving through a 100 µm mesh for pilot
foraminiferal analysis onboard the vessel. After opening, the hue and chroma
attributes of sediment color were determined using the Munsell Geological
Rock-Color Chart (1995).
Onboard study of cores AI-2561, AI-2562, AI-3150 also included
measurements of sediment bulk density and water content by drying method at a
temperature of 60°C. A total of … samples taken at … cm intervals was analyzed.
The water content was also measured using the SVR 6M very high frequency microwave resonance device at … points along
the cores AI-2561 and AI-2562. For correlation between cores and seismic
profiles the obtained results were compared with values of actual amplitude of
acoustic signal recorded at coring sites.
Geochemical and grain-size analyses were carried out
in the Atlantic Branch of the P.P. Shirshov Institute of Oceanology RAS. Total
organic carbon (TOC) and calcium carbonate content (CaCO3) were
measured using the AN-7529 express
coulometric carbon analyzer (Gomel Plant of Measuring Equipment, Republic of Belarus) in 94
dried sediment samples taken from all the cores. Sampling intervals in
homogenous layers are 15-30 cm, while samples from sand/silt rich or laminated
layers were taken at intervals of 2-3 cm.
Grain-size distribution analyses were performed with
the SALD 2300 laser diffraction
particle size analyzer (Shimadzu, Japan) on 119 bulk sediment samples taken
from cores AI-2561, AI-2562, AI-3150. This type of analysis was carried out
every 10-30 cm in homogenous and bioturbated layers and every 1-2 cm in
suggested gravitite or contourite cycles. Core AI-2445 wasn’t subjected to the grain-size
analysis because it recovered almost the same section as core AI-3149. The grain-size distribution of the terrigenous
fraction was studied on samples from the same intervals after removal of CaCO3
by an HCl N1 solution. Samples for the analyses were dispersed by ultrasonation
in sodium tripolyphosphate solution.
3.4 Micropaleontology
(Елена Владимировна)
4. Results
4.1. Seismic profiling
The seismic facies analysis revealed three most
widespread types of seismic facies in the study area: IB, IIB and IIA. Type IB
facies characterized by continuous sharp parallel moderate-amplitude reflectors
persisting for tens of kilometers and an acoustic penetration of about 20–30 m
(Fig. 2) are revealed southward of the Columbia channel and at the foot of the
western slope of the São Tomé seamount. Seismic facies of type IIB generally
showing no sub-bottoms (with short intervals of single high-amplitude reflectors
occurring close to the bottom one) are characterized by the lowest acoustic
penetration of less than 5 m (Fig. 2). They are related to the Columbia
channel. Type IIA facies with an acoustic penetration of 10–15 m show intermittent
parallel reflectors with a moderate to high amplitude (Fig. 2). These seismic
facies are recorded in the area between the channel and Vitoria-Trindade
seamounts as well as at the base of the north-western slope of the São Tomé
seamount.
Large irregular overlapping and single hyperbolae with
widely varying vertex elevations above the sea floor correspond to type IIIA
facies that are related to the São Tomé seamount and Vitoria-Trindade chain. Type
IIIB facies characterized by regular single or slightly overlapping hyperbolae
with conformable reflectors (acoustic penetration of 20–25 m) are found in one
seismic profile north of the Maçae channel and in all three profiles – north of
the unnamed channel located further south. The height of the hyperbolae varies
from 5 to 15 m and wavelength is about 0.2–1 km. Seismic profiles recorded regular
overlapping hyperbolae with varying vertex elevations above the sea floor and
no sub-bottom reflectors (type IIIC) in the Maçae channel and its minor channel
deflected to the ESE. North and south of this minor channel as well in the area
of its conjunction with the Maçae channel the seismic lines crossed broad,
single, irregular hyperbolae with disconformable reflectors and acoustic
penetration of 10–20 m (type IIIF facies). The same facies were found northwest
of the São Tomé seamount. Describe Channels???
4.2 Lithology