Modern sedimentation in the Lower Negro River,
Amazonas State, Brazil
Elena Franzinelli*, Hailton Igreja
Department of Geociences, University of Amazonas, Manaus, 69077-000 Brazil
Received 10 May 2000; received in revised form 26 June 2001; accepted 27 June 2001
Abstract
The Negro River, which flows through the north central Amazon Basin, is one of the largest tributaries of the Amazon. The
name ``Negro'' comes from the colour of its water, which reflects the large quantity of dissolved humic acids and iron oxides
that also gives the water its characteristic acid pH. The river is estimated to have the fifth largest water discharge in the world,
about 30,000 m3
/s. The Negro River is characterized by a high dissolved load but a low energy system. Neotectonics and water
quality are the principal factors that control the modern sedimentation in the Lower Negro River. The Lower Negro River is
controlled largely by a NW­SE tectonic lineament, that is a segment of a major tectonic transcurrent dextral megasystem of the
Amazon Basin. Neotectonism in this area is responsible for the depth of the river and for the occurrence of steep ``falésias''
(cliffs), along some parts of its borders. It also seems that neotectonics have influenced the origin of the Anavilhanas Islands,
which are a series of anastomosed, elongated silty clayey channel bars, with internal round or long narrow lakes. The ``igapó'',
which is the forested area flooded during part of the year, appears to have a neotectonic origin as well. Igapós are located on
intermediate blocks between the high blocks of the ``terra firme'' and the low blocks of the channel. The absence of clastic
sediments carried in suspension is related to the rare appearance of floodplains, which are limited to very thin layers of fine
sediments, located on the abrasion shelfs carved in clastic deposits of the Alter do Cha~o Formation. Sand bars occur in places
along the base of the cliffs and along the edges of the channel system. These sand bars are composed of quartz sand, derived
from the reworking of the sand of the Alter do Cha~o Formation. D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Lower Negro River; Amazon Basin; Neotectonism; Black water; Anavilhanas Archipelago
1. Introduction
The Negro River is one of the largest tributaries of
the Amazon and the largest on the north side. The size,
width and colour of its water caught the attention of the
explorers of the Amazon region (von Spix and von Mar-
tius, 1823­1831; Bates, 1982; von Humbold, 1829).
Sioli (1954) pioneered scientific studies regarding
the water quality, soil, vegetation and ecological prob-
lems of Amazonia with a special reference to the Negro
River region. However, modern data on the hydrology,
climatology, sedimentology and ecology of the Negro
River and its basin, are also included in papers that deal
with the whole of Amazonia. Sternberg (1950), observ-
ing the NW­SE rectilinear direction of the Negro and
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0169-555X(01)00178-7
*
Corresponding author.
E-mail addresses: elena@fua.br, elena@tecnonet.it
(E. Franzinelli), hligreja@argo.com.br (H. Igreja).
www.elsevier.com/locate/geomorph
Geomorphology 44 (2002) 259­271
other rivers to the east flowing in parallel directions,
first speculated on the effect of tectonics on the lower
course of the Negro River. Franzinelli (1994) empha-
sized the great differences between the Negro and the
Solimo~es especially regarding water quality, mineralo-
gical characteristics, rates of transported load and com-
position and texture of modern deposits.
Franzinelli et al. (1999) showed that the modern
neostructural directions that control the fluvial mega-
system of the Amazon Basin can be summarized as
follows: (1) Solimo~es River: N60W; (2) Negro River:
N45W; (3) Amazon River: N80E; (4) Madeira River:
N50E; (5) Taruma~ River: N10E. These directions are
the result of transcurrent faults affecting parts of the
South American Plate under tension caused by the col-
lision of the Nazca Plate to the west and the Caribbean
Platetothenorth,whichbeganintheTertiary(Miocene).
The central belt of the Amazon ecosystem has deve-
loped on the Amazonian Structural Province (Palaeo-
zoic sedimentary rocks) situated between the Branco
River and Tapajós River Structural Provinces, north
and south respectively, on Precambrian igneous and
metamorphic rocks) (de Almeida, 1977). These pro-
vinces have undergone cyclic tectonic reactivation.
The aim of this paper is to discuss the sedimentation
that currently occurs in the Negro River, and to dem-
onstrate how the modern deposition and water quality
in this river system are controlled by regional neotec-
tonism.
2. Regional setting
The drainage area of the Negro River is about
600,000 km2
, which covers parts of Colombia, Ven-
ezuela and Brazil. The Negro River joins the Solimo~es
close to the city of Manaus, forming the Amazon (Fig.
1). Based on its average discharge, the Negro ranks as
the fifth largest river in the world (Meade et al., 1991).
Its sources are located in the llanos of Colombia where
it is called the Guainia. It receives the designation
``Negro'' after its confluence with the Brazo Cassi-
quiare, in Venezuela. The Cassiquiare connects the up-
per course of the Negro River with the Orinoco Basin.
It marks the boundary between Colombia and Vene-
zuela until it enters Brazil at about 1000 km from its
confluence with the Solimo~es River. The Upper Negro
River drainage basin is on the sedimentary rocks of the
Colombian Llanos, a flat savanna region bounded to
the west by the Andes Mountains. Most of the upper
course of this river, however, is in the Guyana Shield
region, a low lying granitic and granulitic Precambrian
craton, covered by rainforest (Fig. 1). The region con-
sists of a wide pediplain, between 80 and 160 m above
sea level. On this plain there are numerous inselbergs
whose tops occur at two levels, about 460 and 700 m.
The Negro River and its tributaries are well entrenched
into the truncated surface of the Guyana Shield. The
majority of rivers and tributaries follow fractures or
faults formed by regional tectonism. Most of the tribu-
taries have rapids or falls, which are less than 8-m high.
The middle and lower courses of the Negro River
developed in the deposits of the Amazon sedimentary
basin. This region consists of wide plateaus, upper
Pleistocene in age (Projeto Radambrasil, 1978), that
are composed of interfluvial tabular areas alternating
with hills carved in the clastic deposits of the Amazon
Basin. To the north of this area, a narrow strip of white
sandstone and yellow siltstone of the Silurian Trombe-
tas Formation. (Caputo et al., 1972) occurs in contact
with the granulitic rocks of the Guyana Shield. To the
south Cretaceous deposits of the Alter do Cha~o For-
mation and Tertiary deposits of the Solimo~es Forma-
tion outcrop. The Cretaceous continental deposits (red
clayey sandstones, siltstones and claystones of the
Alter do Cha~o Formation with some silicified beds in
the upper portion) cover the older sedimentary rocks of
the basin. The Cenozoic fine gray sandstones and clay-
stones of the Solimo~es Formation cover the Alter do
Cha~o Formation in the western part of the Amazon
Basin.
The climate of this region is tropical humid. Rainfall
in the Negro River basin varies from 3500 mm/year in
the upper basin to 2137 mm/year in the lower basin
(Mota, 1996). The average temperature of the region
varies from 24 to 32 °C. The vegetation of the region is
predominantly tropical forest.
According to the geology of the area, the course of
the Negro River can be divided into three parts:
(1) The Upper Negro River, from its sources to the
locality of Santa Isabel, where the river leaves the crys-
talline rocks of the Guyana Shield and flows across the
sedimentary deposits of the Amazon Basin. In this
reach the channel is straight with slow-flowing sections
that alternate with rapids and waterfalls. The maximum
depth is about 12 m in the dry season.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271260
(2) The Middle Negro River flows on the Tertiary
sedimentary deposits, in a general NW­SE direction.
This reach has a very large channel, with numerous
bars, especially near its southern end at the confluence
with the Branco River, where the crystalline rocks of
the basement outcrop in a small area and force the river
Fig. 1. A--The Amazon Basin and Negro River. B--Geological map of the Lower Negro River Region.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271 261
into a narrow channel. In the reach upstream from the
Branco River, where it reaches a width of 20 km and
more, and a maximum depth of 18 m in the dry season,
it resembles an active sedimentation basin, with the
crystalline rocks at the mouth of the Branco River
forming a barrier causing an upstream accumulation
of sediments (Franzinelli and Latrubesse, 1998).
(3) The lower course of the Negro River (Fig. 1B),
between 62°W and its confluence with the Solimo~es
(60°W), displays tectonic control. According to Fran-
zinelli and Igreja (1990), this region is a structural
lineament trending NW­SE, confining the river. The
lineament belongs to a larger transcurrent system of
geological features (faults, folds and rhombochasms)
that occur in the whole Amazon Basin (Fig. 2). In this
section the river is up to 20-km wide and it is bordered
by discontinuous cliffs (falésia), formed by the Alter
do Cha~o deposits.
3. Neotectonism
A neotectonic model for this region has been pro-
posed based on remote sensing and verification of neo-
tectonic features (Fig. 2) in the field (Franzinelli and
Igreja, 1990).
The Lower Negro River region is governed by two
distinct neostructural domains that control its physio-
graphy: (1) normal fault domain and (2) dextral fault
domain. The normal domain, located on the left side of
the river, maintains and directs the flow of the major
tributaries (NE­SW) from the east towards the west,
and the smaller NW­SE oriented tributaries, which
flow southwest as well as towards the southeast. The
dextral fault domain, consisting of dextral transcurrent
faults N45W and secondary faults arranged en echelon
in the direction N70E, promotes the fluvial flow pre-
dominantly from west to east.
These two principal domains, which have formed a
half graben, have been altered by the younger third set
of faults in a general west­east orientation, resulting in
a transtensional basin (rhombochasm). This third set is
difficult to identify on radar and satellite images in the
north part of the area, but is fairly easy to observe in the
field. It is essentially a set of dextral transcurrent faults
and governs the southern part of the region, where it
restricts the channels of both the Negro and Solimo~es
rivers. Very wide reaches of the river, which are the re-
sult of the third set of faults, together with the first and
second sets forming a NE­SW distension and a NW­
SE oblique transtension confining the Negro valley,
influence the inflow/evaporation balance of the river.
The formation of these wide lake-like features involved
the inclination and rototranslation of blocks, which
tended to form wide depressions along the main faults
that are concentrated along the riverbed. These dextral
transtensional processes resulted in negative flower
structures (a flower structure is similar in form to a
flower with various-sized overlapping blocks (petals)
radiating outward and upward from a central fault
(stem)), from meters to kilometers in size, the petals
of which form many collapsed structures, which are
prominent at the entrance of the Solimo~es River Basin
(Igreja, 1999).
The tectonosedimentary evolution of the Lower
Negro River with an auto-cycling tendency is similar
to the processes of politaxitic and oligotaxitic alterna-
tion observed in some Cenozoic basins around the
world (Fischer and Arthur, 1977; Miall, 1990) that
have an accelerated subsidence and an alternation of a
population explosion and high biodiversity during the
rainy season, and decimation and low biodiversity dur-
ing the dry season.
4. Hydrology
The name ``Negro'' comes from the colour of its
water, caused by the large quantity of dissolved humic
acids and iron oxides, which also give the water its
characteristic acidic pH. According to Bringel et al.
(1999), the pH value of the Negro River varies from 4.2
for the water of the central channel to 5.8 for the water
close to the confluence with the Branco River. The hu-
mic acids come from organic material, such as roots,
leaves and wood that decay slowly in the water. The
headwaters of the numerous small tributaries generally
drain wide, shallow swamps situated on the acid rocks
of the Guyana Shield or on sandy sheets derived from
the weathering of these rocks. According to Sioli
(1991), the origin of black water is largely related to
the podzolic soil of the headwater area. In this type of
soil, the organic material rapidly undergoes decompo-
sition to humus, which is easily dissolved by the perco-
lating rain water and transported to the water table,
finally emerging on the surface as small streams.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271262
The mean annual discharge of the Negro is about
30,000 m3
/s (Meade et al., 1991). The maximum
annual fluctuation in river level measured at the port
of Manaus is about 14 m (Trisciuzzi, 1979). The height
of the water during the maximum annual flood reaches
about 28 m above sea level at the port of Manaus. Ac-
Fig. 2. Neotectonic model of Lower Negro River, principal structural lineaments: A= High Block, B = Low Block; Strike-slip movements;
555 Significant reverse component; ??? Significant normal component (from Franzinelli and Igreja, 1990). This region is a smalll sub-
system of the transcurrent Amazon mega-system.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271 263
cording to Sternberg (1998), the maximum fluctuation
of this century occurred in 1953, when the level of the
water reached 29.50 m above sea level. During the
rainy season the high water level facilitates the devel-
opment of a high amount of biodiversity. During the
dry season the low water level results in a decrease of
biodiversity due to hydrogeochemical factors.
Sediment load transported by the Negro River is
mostly dissolved load. According to Martinelli et al.
(1988), the Negro River supplies an annual total load of
0.057 Â 108
t. The annual total load at Obidos for the
Amazon River was estimated to be 2.46 Â 108
t. Ac-
cording to Dunne et al. (1998), 7% of the mean annual
load is made up of sand, and 93% of silt and clay. The
Negro River carries very little sedimentary load to the
Amazon.
5. Geomorphology
Cross sections from different sites along the Negro
River have been plotted in Fig. 3. The sections il-
lustrate the control of bedrock and tectonics on chan-
nelmorphology.Suchfeaturesarefloodplain,sandbars,
Anavilhanas Islands, and igapó.
5.1. Floodplains
Floodplains are very poorly developed along the
banks of the Negro River because (1) the river is con-
fined in a channel (Fig. 4) formed by faulting (Fran-
zinelli and Igreja, 1990), and (2) the river transports
little suspended sediment load. Floodplains are devel-
oped above abrasion terraces that are the result of
erosional activities on the red claystone or silicified
sandstone of the Alter do Cha~o Formation. Thin fine
sandy beds cover these terraces that are completely
flooded during the rainy season (Fig. 5). The small areas
of floodplain are generally limited by cliffs (Fig. 6)
that can reach 20 m in height and are the edge of the
``terra firme'' or high terrains that are never inundated
during floods. The cliffs were originated by faults that
emphasize the confinement of the river. These small
areas of floodplain are more common on the south side
of the Negro due to the regional tilting of the neo-
tectonic blocks. One of these blocks contains the Ariau
River (Figs. 4 and 7), a tributary on the south side
whose upper drainage area is in the floodplain of the
Solimo~es River. The direction of flow of this tributary
depends on the timing of the Solimo~es or Negro River
floods. For example, when the Negro River is in flood
stage and its water level is higher than the Solimo~es,
the direction of flow is from the Negro to the Solimo~es.
When the Solimo~es is in flood stage, the flow is
reversed. When the Ariau drains into the Negro River
the sediments that the river transports are deposited at
its mouth, forming an elongate delta (Fig. 7). X-ray
analysis of the delta sediments (Franzinelli et al., 1996)
confirmed their origin from the Solimo~es floodplain.
5.2. Sand bars
Modern sand bars result from the deposition of
quartz sand along the banks of the river. These are la-
teral bars; other types of bars are very rare. Sand bars
are more prominent on the south side of the river due to
the dominant winds that blow from the northeast ge-
nerating waves on the river, which promote accumu-
lation of the sand along the edges of the tectonic blocks.
These sand bars are typically situated at the foot of the
cliffs (falésias) (Fig. 7) or are isolated along the shores
where the cliffs have been completely destroyed by
erosion forming the natural levees of the river. A long,
flat sand bar occurs on the right bank downstream from
the confluence with the Branco River. The mean grain
size of its white sand is 0.23 mm, the sand is very well
sorted, and the quartz grains show good roundness.
This bar is situated in an area of Paleozoic sandy rocks
and reflects the similar textural characteristics of the
source rocks. The best developed sand bars, also called
``beaches'', are located at Ponta Negra where these are
about 2-km long, situated on the north side of the river,
15 km upstream from downtown Manaus and at Praia
Grande, where the bars are about 12-km long, 200-m
wide and 16-m high, on the south side, 70 km upstream
from Manaus (Figs. 4 and 7). Grain size, sorting and
roundness analysis indicate that these bars were
derived from local reworking of sand beds of the Alter
do Cha~o Formation (Brito et al., 1994a,b; Brito, 1996).
From a mineralogical point of view, these sands are
very pure quartz sand (Potter and Franzinelli, 1985).
5.3. Anavilhanas Islands
The archipelago of the Anavilhanas Islands (Figs. 4,
7 and 8) is a prominent feature of the Lower Negro
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271264
River region. It is a complex of numerous silty clayey
channel bars each with a core or upstream head that is
generally occupied by a lake, and a very long and nar-
row downstream tail. The Anavilhana Island, for
example, is 40-km long and 130-m wide, probably
demarcating a fault line of the same dimensions. In the
dry season it is possible to see upstream the lateral
banks, which are very steep. Downstream the banks of
the islands can be steep, but generally they terminate
gradually. The vegetation zonation is marked by a
decreasing height and a transition from igapó (periodi-
cally inundated forest) to mangrove type vegetation, at
the extreme end of the islands. The internal lakes in the
cores of the islands have less steep edges than the island
banks. A section across the tail of the islands displays a
sinusoidal topography with crests and lows of the is-
lands parallel to the flow. These islands are completely
flooded during the rainy season. The islands are built
with yellow-gray clay and silt, and very rarely contain
finely laminated, reddish mottled fine sand. The lith-
ology of these islands shows that their origin is not
compatible with the actual dynamic conditions of the
river, which in its present state hardly transports any
material in suspension. Three elements were essential
Fig. 3. Schematic cross-sections in the Lower Negro River Region. Data from Projeto Radambrasil (1978) and from measurements during
fieldwork. Location of the cross sections I, II, and III shown in Fig. 4. K-Cretaceous, Transtensional Fault (Dextral).
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271 265
for their formation: (1) a sufficient amount of clay and
silt, (2) a very low energy environment, and (3) the li-
near accommodation space. According to Sioli (1991),
these very long bars originated by the deposition of the
fine sediments that the Branco River supplies to the
Negro River. Bringel et al. (1999) showed some signi-
ficant textural and mineralogical differences between
(1) the bottom sediments of several small lakes that re-
ceive water from the Branco River and (2) that of the
lakes along the Negro River far from the influence of
the Branco. For example, the composition of the bot-
tom sediments of the lakes under the influence of the
water of the Branco River is more sandy (53%) than of
those downstream in the Negro River. The Anavilhanas
Islands present the same clayey content as of all tribu-
taries and not only of the Branco River. These new data
indicate that the sediment that forms the islands did not
come from the Branco, but from the tributaries that
flow into the depression (rhombochasm).
5.4. Igapós
Igapós are another distinctive depositional feature
of the Negro River (Fig. 9). An igapó is a periodically
inundated forest, according to Prance (in Sioli, 1984),
inundated by regular annual flood cycles of black
water rivers. Igapó regions are believed to derive from
low dry land areas (``terra firme''), which developed
Fig. 4. Location map of the cross sections and of the principal modern sedimentary deposits (6) and neotectonic blocks.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271266
during the Pleistocene or even the Tertiary (Irion et al.
in Sioli, 1984). Igapó deposits normally consist of
clay, silt, rare fine sand, and a large quantity of
organic material derived from the decay of the veg-
etation that grows in these areas. Based on intensive
studies of igapó areas by remote sensing and verifi-
cation of geological and structural features in the
field, Igreja et al. (1998) concluded that the develop-
Fig. 5. Lower Negro River, south bank upstream from Praia Grande Beach bar. Sandy floodplain during flooding. Note the water mixed with
sand on the shore due to wave agitation. In the distance, the thin white line is a sandy lateral bar below the terrace of the Alter do Cha~o deposits.
Fig. 6. Lower Negro River, south bank. Sandy floodplain upstream from Praia Grande Beach bar, distal view. In the background is a cliff carved
in the Alter do Cha~o deposits.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271 267
ment of the igapós of the Negro River region was a
consequence of neotectonics. Specifically, igapós are
formed by the deposition of sediments and organic
material above low blocks, sometimes tilted, separated
by high blocks of the ``terra firme''. On these low
blocks the vegetation of the ``terra firme'' adapted itself
to the aquatic environment. The different elevations of
the tectonic blocks along the Lower Negro River
suggest different stages of neotectonic movement
and, in consequence, different stages of adaptative
Fig. 7. Anavilhanas Islands in the central part of the Lower Negro River. One can note the lakes, channels and anastomosing islands.
Downstream, where the islands form a long tail, the pattern turns deltaic. In the lower right-hand of the figure, on the right bank of the Negro
River, the elongate delta of the Ariau River is visible (j), and upstream, on the same bank, the half-moon shaped ``Praia Grande'' beach is
located (P). The tributaries of the right bank have wide mouths as they enter the Negro River, which is a consequence of the control by dextral
transcurrent faults. On the left bank, to the north, the Apuau River also indicates the influence of neotectonism in its channel, with a widening in
the orthogonal course. (Modified from Folha SA. 20-Z-B, Projeto Radambrasil, 1978).
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271268
evolution of the ``terra firme'' species composition into
igapó species.
6. Sedimentation in the Lower Negro
The description of the neotectonic and geomor-
phological features and sedimentological and hydro-
logical aspects confirm that the Negro is an unusual
tropical river. The Negro can be divided into three sec-
tions (Fig. 1A). First, downstream of the confluence
with the Branco River, it runs straight in the NW­SE
direction in a narrow channel 5­8-km wide. Second,
when the river enters the Cretaceous deposits the width
of the channel first increases to 20 km for about 100 km
then it narrows downstream in the restraining zone
(Fig. 2). Neotectonics deeply influence this portion of
the river, forming a half-graben rift, located in the clas-
tic friable deposits of the Alter do Cha~o Formation. In
the wide section, the main channel bifurcates, the two
new channels flowing along the banks of the river, bet-
ween 18- and 22-m deep, surrounding the Anavilhanas
Islands. The major islands appear to follow faults in the
river bed along the principal zone of deformation. (Fig.
2). In these islands or channel bars, sedimentation oc-
curs by lateral accretion in linear deposits. It seems that
the deposition of the sediments starts upstream and
forms two bars, which act as levees and in some places
deflect the flow of water to form secondary channels.
Very fine material in suspension is deposited in the
middle in the internal lakes. Third, downstream, in the
direction towards the restraining zone the bars no
longer have any internal lakes. They taper towards
the end, giving to the set an aspect of deltaic config-
uration.
Presently, the provenance of the sediments of the
bars cannot be explained, nor the chronological date
of their formation. The Negro River does not have a
high sedimentary load due to the nature of weathering
of the rocks and the geomorphic aspects of the catch-
ment area (Stallard and Edmond, 1983; Stallard,
1985). The amount of sedimentary material that the
Branco River contributes to the Negro in the rainy sea-
son is not sufficient to explain the formation of the
Anavilhanas Islands.
Considering the enlargement of the river on entering
the friable deposits of the Alter do Cha~o Formation and
the confinement of the river, it seems reasonable to
suppose that these bars were formed by the reworking
of the previous sediments in the rift, along with the
material coming from the Branco River and other tri-
butaries. However, textural evidence indicates that the
Fig. 8. Anavilhanas Islands in the central part of the Lower Negro River. Note the linear arrangement of the bars and the long internal lakes. The
flow is right to left (photo by Antonio Iaccovazo).
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271 269
lateral bars in this reach were formed by local sand
deposited in an aquatic environment and the exposed
sand underwent aeolian transport (Brito et al., 1996). In
contrast, the igapós seem to have a tectonic origin, and
their sedimentological, depositional and vegetational
aspects are characteristic and different from those of the
floodplain. In spite of its large discharge, the Negro is a
low energy river. Erosion rarely happens. The occur-
rence of ``terras caídas'', bank failures due to the in-
teraction of the river water and groundwater, very
frequent in the Solimo~es (Sternberg, 1960), is uncom-
mon. Remarkably, in some periods of the year, gene-
rally at the beginning of the rainy season, due to the
nonsynchronous arival of high water (Meade et al.,
1991), the Solimo~es River water blocks the Negro
River exit, damming its water. The Negro is a very un-
usual river on many accounts.
7. Conclusion
Studies of the current sedimentation in the Lower
Negro River indicate that its hydrology, geomorphol-
ogy and evolution are strongly controlled by neotecto-
nic elements. Floodplains, sand bars, islands and igapó
forests may be considered as concurrent features inher-
ent in the geodynamics of a transtensional basin, which
isapartoftheAmazonianmegasystem.TheAmazonian
megasystem is in turn a part of a complex transcurrent
dextral fault system that covers all of the territory of
the Amazon Basin. This transcurrent dextral fault sys-
tem is a result of the collision of the northern part of
the South American Plate with the Nazca Plate to the
west and the Caribbean Plate to the north.
Depositional, morphostructural, and hydrochemical
processes make the Negro River unusual, with uncom-
mon characteristics associated with a confined tectonic
basin environment. Similar valleys, on a smaller scale,
may be predicted to occur in other areas with the same
neotectonics orientation within the Amazon ecosystem.
Acknowledgements
We thank the CNPq (Government National Coun-
cil of Brazil) for the financial support for the field trips
(Grant no. 520104/94-3NV), Dr. Avijit Gupta and Dr.
Andrew G. Warne for reviews leading to improvement
of this paper, and Daniel Ribeiro Fernandes for the
layout end reproduction of the figures.
Fig. 9. Cuieiras River, close to its confluence with the Negro River. The white trunks are those of igapó trees that have died due to the
subsidence of the low block.
E. Franzinelli, H. Igreja / Geomorphology 44 (2002) 259­271270
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