An Abandoned Estuary Within Marajo´ Island

Estuaries and Coasts
Vol. 30, No. 5, p. 813–826
October 2007
An Abandoned Estuary Within Marajó Island: Implications for
Late Quaternary Paleogeography of Northern Brazil
DILCE F. ROSSETTI1,*, MÁRCIO M. VALERIANO1, and MARCELO THALES2
1
2
Instituto Espacial de Pesquisas Espaciais-INPE, Centro de Observação da Terra, Divisão de
Sensoriamento Remoto-DSR, Rua dos Astronautas 1758, Jardim da Granja-CP 515, São José dos
Campos- CEP 12245-970 São Paulo Brazil
Museu Paraense Emı́lio Goeldi, Unidade de Sensoriamento Remoto-UAS. Av. Perimetral, 1901,
Terra Firme 376- CP 399, 66040-170 Belém, Pará, Brazil
ABSTRACT:
Several previous studies have recognized paleochannels as the most typical geomorphologic features
imprinted in the modern landscape of Marajó Island, northern Brazil. A characterization of these paleochannels by detailed
mapping has not yet been accomplished, despite their great potential for helping reconstructing the succession of events
responsible for the origin and evolution of the drainage system at the mouth of the Amazon River during the Quaternary. The
present study aims to provide a detailed mapping and characterization of these paleochannels using remote sensing (i.e.,
SRTM, Landsat 5-TM, and Landsat 7-ETM+), integrated with sedimentological and radiocarbon data obtained from outcrops
and cores. The mapped paleochannels allowed for the first time correlation of the channel-like morphologies with finingupward sedimentary successions, typical of channel deposits. The study focuses on the Lake Arari area located in the
northeast of Marajó Island, where a complex fluvial network dominated by a funnel-shaped structure attributed to a Late
Pleistocene paleoestuarine system was recognized. The inner estuary was connected to three fluvial channels that ran from
the east-southeast throughout an area currently occupied by Marajó Bay, implying that this bay did not exist when the
estuarine system was active. Separation of the eastern side of Marajó Island due to tectonic causes related to capture of
Tocantins River by northeastward-orientated faults would have interrupted the fluvial inflow, ultimately causing the
abandonment of the estuarine system. Today, the central part of the funnel-shaped structure related to the Late Pleistocene
paleoestuary is occupied by Lake Arari, where sediments started to accumulate no earlier than the mid-Holocene. It is
proposed that this lake might have developed from the Late Pleistocene estuary as the fluvial inflow was cut off during the
detachment of Marajó Island. Marajó Island serves as an example of a very dynamic site of changing landscapes throughout
the Quaternary as a result of tectonic activity.
An adequate characterization of the Marajó
Island paleochannels can be approached through
a combination of remote sensing and sedimentological information. In general, radar data have
been preferentially used for mapping of geological
and geomorphological features in Amazonian areas,
because the acquisition of Landsat images with
a good resolution has been a major problem due to
the frequent and abundant clouds and dense
vegetation. In contrast with most other Amazonian
areas, where dense forests dominate, northeastern
Marajó Island consists of open vegetation, mostly
grasslands, cerrados, and savanna woodlands. This
physiography makes the application of Landsat data
particularly suitable for morphological studies.
This paper provides a geomorphological characterization of the Lake Arari area on northeastern
Marajó Island (Fig. 1) based on remote sensing
interpretation (mostly Thematic Mapping (TM)
and Enhanced Thematic Mapping (ETM+) Landsat
imagery, and whenever possible, Shuttle Radar
Topographic Data (SRTM), combined with sedimentology and radiocarbon dating. The aim is to
use an integrated approach to assess the modern
Introduction
The northern Brazilian region near the mouth of
the Amazon River (Fig. 1) is dominated by a complex of islands, Marajó Island being the largest one.
This island, bounded by the lower Amazon River to
the west, the Tocantins River to the east, the Pará
River to the south, and the Atlantic Ocean to the
north, displays a landscape dominated by a network
of paleochannels that provide important information about tectonic mechanisms of Quaternary
sedimentation in northern Brazil, despite being
located on a tectonically passive margin.
The record of paleochannels on Marajó Island
is not new (e.g., RADAM 1974; Bemerguy 1981;
Porsani 1981; Vital 1988), but their mapping and
characterization remain to be presented. This is
important for reconstructing the paleogeographic
evolution of the lower Amazonas drainage system
during the Quaternary.
* Corresponding author; tele: 012/394-56451; fax: 012/39456488; e-mail: [email protected]
ß 2007 Estuarine Research Federation
813
814
D. F. Rossetti et al.
Fig. 1. A–B) Location map of the study area in the northeast of Marajó Island, northern Brazil. C) Chart summarizing the stratigraphy
of the study area, with the nomenclature applied to the surface and subsurface Cenozoic units.
geomorphological configuration of this area and
determine the succession of events responsible for
its paleogeographic evolution during the Late
Quaternary. A particular topic of importance is
evidence of tectonics on a passive margin.
GEOLOGICAL FRAMEWORK
The northeast of Marajó Island belongs, in great
part, to the Pará Platform (Fig. 1), which corresponds to a large area of basement rocks that
remained tectonically stable relative to adjacent
sedimentary basins. This platform displays numerous small, but deep, troughs that are up to 3,500 m
deep. An example is the case of the Lake Arari area,
which is located along a large, north-south elongated depression connected to the Mexiana Basin to
the north (Azevedo 1991). This basin is part of the
Marajó Graben System formed by crustal stretching,
related to the opening of the South Atlantic Ocean,
which occurred mostly during the Cretaceous
(Szatmari et al. 1987).
The sedimentary fill of the Mexiana Basin, still
poorly known, is mostly based on subsurface data
(Galvão 1991; Villegas 1994). It includes sandy
deposits of the Breves-Jacarezinho Formations (Aptian-Cenomanian) and silty mudstones of the Anajás
Formation (Early Cretaceous; Fig. 1). These units,
attributed to depositional environments ranging
from fluvial to shallow marine (Villegas 1994), are
overlain by sandstones, mudstones, and conglomerates of the Limoeiro Formation (Late Cretaceous),
which are also related to fluvial and shallow marine
settings (Villegas 1994). Tertiary deposits overlying
the Cretaceous succession consist of mixed siliciclastic-carbonates of the Marajó Formation (Paleocene-Eocene), as well as the Pará Group (Miocene
to Holocene), altogether included in transitional to
shallow marine depositional settings. The latter
encompasses sandstones of the Tucunaré Formation and mudstones of the Pirarucu Formation.
These deposits are equivalent in age to the Barreiras
Formation and the post-Barreiras sediments (Rossetti et al. 1989, 1990; Rossetti 2000, 2001a).
The Lake Arari area is dominated by Quaternary
deposits related to the latest phase of deposition of
the Tucunaré-Pirarucu succession (cf., Vital 1988).
There are only a few studies available from this
region. These include hydrodynamic reconstruc-
Quaternary Paleogeography of Northern Brazil
tions of the lake system (Vital 1988), as well as
geophysical studies of sandy deposits related to
paleochannels (Bemerguy 1981; Porsani 1981).
Materials and Methods
Lake Arari is a north-south elongated feature
forming a 1,900 km2 basin surrounded by lowlands
only 4–6 m high. The water volume in the lake has
been estimated at 600 3 106 m3, but the lake is
shallow, being only 7 m deep during wet seasons.
During dry seasons, there is a reduction in water
volume of up to 60%, and the lake desiccates during
prolonged dry seasons. To the south, it forms the
headwaters for the Arari River, which runs to the
southeast draining into Marajó Bay.
Lake Arari is located in a region of tropical
climate characterized by a mean annual temperature of 28uC and precipitation of 2,500 to
3,000 mm yr21, 90% of which is concentrated
between January and July. In the Koppen (1931)
classification (Instituto Nacional de Pesquisas Espaciais [INPE]; Global Land Cover Facility [GLCF]),
the southern part of the island is described as Af
(forest tropical climate) and the remaining areas
are classified as Amw’ (relatively drier than Af, with
dry winters).
The morphological characterization presented in
this paper is based on the analysis of Landsat 5-TM
(Ref. 224-060 and 225-061, INPE) and Landsat 7ETM+ (Ref. 223-060 and 223-061, GLCF) images,
collected in August 2001.
SRTM-90 m were also used to help with the
interpretations. These are of poor quality, though,
due to the very low topography of the study area,
compatible with data noise ranges. Geostatistical
refinements (Valeriano et al. 2006) only slightly
improved relief characterization at detailed scales.
Given these adverse conditions, the study area had
to be visualized using customized shade schemes
and palettes to efficiently highlight the morphologic features. Color schemes were rearranged to
present strong hue transitions near terrain unit
boundaries, often requiring adjustments from one
location to another. Image interpretation of elevation data was made possible by the use of the
software Global Mapper (Global Mapper Software
LLC, Greenwood Village, Colorado).
The sedimentological data were based on descriptions of a few outcrops along ephemeral river banks,
and on information obtained from cores collected
from nine shallow drills using a Robotic Key System
(RKS) percussion drilling system, model COBRA
mk1 (COBRA Directional Drilling Ltd., Darlington,
U.K.). This system allowed sampling of cores with
5 cm in diam. up to a depth of 18 m. The
lithological and structural characteristics of the
sedimentary facies, as well as their vertical distribu-
815
tion, were photographed and recorded in lithostratigraphical profiles.
The chronology of events in the study area was
based on radiocarbon analysis undertaken at the
Beta Analytic Radiocarbon Dating Laboratory, Florida (U.S.A.). Nineteen samples of peat, wood, and
organic sediments were dated by scintillation
spectrometer, with 13 being dated by accelerator
mass spectrometer (AMS). The samples were pretreated with acid to remove carbonates and weaken
organic bonds, washed with alkali to remove
secondary organic acids, and combined with acid
again to provide more accurate dating. Conventional 14C ages were calibrated to calendar years using
the Pretoria Calibration Procedure program, based
on tree ring data as calibration curves (Talma and
Vogel 1993).
MORPHOLOGIC CHARACTERIZATION
Existing topographic maps available for the study
area (1:1,000,000 or 1:250,000; contour spacings of
100 and 50 m, respectively) display no contour
lines, since heights vary under these levels. SRTM
data show heights averaging 4 to 8 m. Occasional
elevations of 30 m reflect altitude enhancement due
to vegetation-relief interactions; in fact, actual
terrain heights hardly reach 10 m in this region.
Despite the overall very smooth topography,
northeastern Marajó Island displays impressive
morphological features that provide important
information for deciphering its mode of formation.
The morphological features of Lake Arari area are,
in general, characterized by an abundance of
channellized features that are particularly well
preserved (Fig. 2). The channels are recognized
on Landsat images by virtue of their intricate
meandering drainage network. A digital elevation
model based on SRTM data reveals elevations that
are slightly higher on the paleochannels relatively to
surrounding areas. Fieldwork showed that this
gradient is, in great part, exaggerated due to
vegetation, because radar C band, used for the
acquisition of SRTM-90 m, refers to wavelengths
that are unable to penetrate the canopy (Le Toan et
al. 1992). The fieldwork also revealed that the
paleochannels show a very smooth, convex relief
that can reach up to 2–3 m high over paleochannel
areas. These slightly higher lands were favorable for
the development of arboreal vegetation, as they can
escape from frequent floods that inundate the
entire area during wet seasons.
A particular feature of interest is Lake Arari,
which appears as the central part of a larger
channelled morphology (Fig. 3). The lake dominates the modern landscape, occurring near the
locality of Santa Cruz do Arari (Fig. 1), where it is
part of the most important drainage basin of the
816
D. F. Rossetti et al.
Fig. 2. Characterization of the paleochannels in the study area based on Landsat 5(R), 4(G), and 3(B) composition (A) and SRTM (B)
data. C) Drawing over the data presented in A and B, highlighting the elongated, meandering morphologies attributed to paleochannels.
D) Profiles transversal to a paleochannel morphology characterized by a SRTM relief gradient of up to 20 m relative to surrounding areas,
resulting from a combination of smooth topography and vegetation. E) A detail of one vegetated, sandy paleochannel showing the
substratum that stands up to 3 m higher than muddy surrounding areas.
island. The lake developed within a north-south
elongated depression averaging 4 km wide and
25 km long, which falls within a much larger,
funnel-shaped structure that reaches as much as
twice the lake length (Fig. 3). The southern portion
of the funnel coincides with the main, deeper lake
body. At the southern edge, the funnel-shaped
structure bends to the east-northeast, and becomes
progressively narrower as it grades into a complex of
meandering paleochannels (Fig. 4). Three main
branches of this paleochannel (see 1 to 3 in Fig. 3)
occur in an area presenting several small lakes (i.e.,
, 5 km in length). The segment located to the
north (i.e., segment 1 in Fig. 3) is the main
continuation of the funnel-shaped structure, forming an elongated, northeastward-orientated tail that
becomes highly sinuous in its southeastern end,
where closed loops are present. From this point, the
segment turns to the east-west direction, and
disappears abruptly to the west of a northward
affluent of the Paracauari River, around 6 km from
Marajó Bay. Along its course, the width of this main
trunk is variable, ranging from more than 4.5 km to
the west to 2–3 km on average to the east, with
intermediate widths as narrow as 1.2 km. Rather
than representing a single paleochannel, this part of
the funnel displays several superimposed paleochannel scars that are as narrow as 0.5 km.
The intermediate segment (i.e., segment 2 in
Fig. 3) of the funnel consists of a single, mostly
northeast-southwest orientated, meandering paleochannel, the eastern edge of which is projected into
the modern Paracauari River. A second paleochannel superimposed on this segment runs to the west-
Quaternary Paleogeography of Northern Brazil
817
Fig. 4. A–C) Detail of the southernmost portion of the funnelshaped structure interpreted as scar of a middle estuarine
depositional setting as seen in the SRTM (A) and Landsat (B)
5(R), 4(G), and 3(B) composition, as well as the corresponding
drawing (C). See Fig. 3 for location.
Fig. 3. Landsat 5(R), 4(G), and 3(B) composition from the
northeast of Marajó Island, indicating the morphological features
discussed in the text. The inside rectangles indicate Figs. 4 and 5.
TSM-3, TSM-5, and TSM-7 locate the lithostratigraphic profiles
shown in Fig. 6. Arrows in the northeast of Lake Arari indicate
places where the paleochannels are sharply interrupted.
northwest, joining with the meandering loops of the
main trunk.
The southernmost segment of the funnel-shaped
structure (i.e., segment 3 in Fig. 3) is composed of
two paleochannels: a larger one averaging 0.6–
0.8 km wide to the west, and a narrower one
averaging 0.3–0.4 km wide to the east. These
paleochannels, which mostly display a main southeast-northwest orientation, join together 4.5 km
from the main funnel segment described above.
As Lake Arari becomes shallower northward, the
opposite side of the funnel-shaped structure enlarges substantially, varying from 10 to 40 km wide
in its middle and outer reaches, respectively.
Paleochannels are scarce in this area and, when
present, are not clearly traceable. Exceptions are
those mapped further north, which might form
paths for modern rivers. This is well illustrated by
the Cururu River, which is a near east-west
orientated ephemeral river connected to a large,
elongated feature that truncates the funnel shaped
structure described herein (Fig. 3). This superimposed feature also displays a funnel shape that
opens up to the east-northeast, revealing a trace of
a pre-existing drainage with channels much larger
than the corresponding active channels, suggesting
that it might represent remnants of a paleodrainage
system.
On the inner side of the main funnel-shaped
structure, there is an elongated belt formed by a set
of nearly north-south orientated, parallel features
(Figs. 3 and 4) that reach up to 10 km in width and
35 km in length. These become progressively
curved to the south, conforming to the margin of
the funnel-shaped structure.
On both sides of the funnel-shaped structure,
a network of meandering to anastomosing paleochannels is present (Fig. 5). These vary from ,
0.4 km wide to the east of the lake and become
wider to the west, where paleochannels averaging
1 km wide are stacked upon each other and form
a belt up to 3 km wide. Most of these paleochannels
display an overall northeast-southeast orientation
on both sides of the lake, suggesting that they were
either contemporaneous or parts of a same drain-
818
D. F. Rossetti et al.
Fig. 5. A–B) Detail of the area to the east of Lake Arari,
illustrating the mostly northeastward-orientated paleochannel
network complex that predates the development of the funnelshaped structure as seen in Landsat 5(R), 4(G), and 3(B)
composition (A) data, as well as the corresponding drawing (B).
See Fig. 3 for location.
age basin. Except for a few narrower paleochannels,
in general the paleochannels do not continue on
the belt with north-south orientated parallel features that occur in the eastern side of Lake Arari.
They also disappear ca. 30 to 50 km from the
modern coastline in the southeastern edge of
Marajó Island.
SEDIMENTOLOGY
A combination of direct and indirect data reveals
that the features related to paleochannels on
Marajó Island contain an abundance of sandy
sediments. Indirect evidence was already obtained
by previous workers (e.g., Bemerguy 1981; Porsani
1981), who prospected many of these structures in
the study area, aiming to analyze their potential as
freshwater reservoirs. These investigations led to the
conclusion that the paleochannels, as well as the
area corresponding to the belt with a set of northsouth orientated features that parallel Lake Arari,
display geophysical properties that are typical of
fine-grain to medium-grain sands with a freshwater
content. This composition differs from surrounding
terrains, which are dominated by mixtures of muds
and sands with brackish to saline waters.
Sedimentological studies undertaken during the
present work confirm that the features related to
paleochannels on the basis of remote sensing
interpretation are consistently sandier than the
surrounding deposits. However, these channels are
not entirely composed of sand, as lenticular heterolithic bedded deposits are also present. The sands
exposed at the surface are dominantly fine-grained
to very fine-grained and well sorted and create
a sharp contact with the surrounding muds.
In order to better characterize the paleochannel
deposits, information from a few available exposures along river banks was combined with data
derived from 10 drills. The latter resulted in
continuous cores (Fig. 6). Based on the sedimentary
facies, paleochannel deposits typically encompass
fining upward successions that are, at least, up to
18 m thick (corresponding to the maximum depth
sampled), with continuous sand beds up to 7 m
thick. A sharp discontinuous surface was recorded
in several cores at depths varying from 8 to 13 m,
under which the sediments become muddier. At
one site located . 1 km to the west of Lake Arari, at
the locality of Santa Cruz do Arari, continuous cores
were obtained up to a depth of 118 m. These cores
record three complete fining-upward successions,
each reaching up to 40 m in thickness, with the
basal sandy parts being as thick as 25 m in the
youngest succession. The lowest fining upward
succession is basally bounded by an unconformity
that overlies a siliciclastic unit with variegated colors
ranging from red to yellow and purple. This unit is
typical of the Barreiras Formation (early to middle
Miocene), which crops out on the eastern side of
Marajó Island.
The lower sandy parts of the fining-upward
successions consist of gray-colored, micaceous,
fine-grain to medium-grain or medium-grain to
coarse-grain sands ranging from poorly to well
sorted. The sands are either massive or well
stratified, the latter encompassing cross lamination
or cross stratification of medium size (Fig. 7).
Foresets and set boundaries are marked by an
abundance of highly fragmented plant remains (i.e.,
coffee-grounds) that are usually associated with
Quaternary Paleogeography of Northern Brazil
819
Fig. 6. Lithostratigraphic profiles representative of the paleochannel successions in the study area (see Fig. 3 for location). Note the
upward gradation from sands to muds, characterizing fining upward successions formed by decreasing flow energy, typical of channel
fill deposits.
820
D. F. Rossetti et al.
Fig. 7. Sedimentary features of the paleochannel deposits, illustrating: A) cross laminated sand. B) Detail of cross-laminated sand with
set boundaries marked by accumulation of plant debris (arrows). C) Sand with interbedded mud drapes (arrows). Note that spacing
between drapes increases upward. D) Sand with two pairs of double mud drapes (arrows), attributed to flood-ebb tidal fluctuations. E)
Heterolithic bedded deposits from the upper part of the paleochannel succession. F) Sand with convolute bedding.
mud drapes. In some cores, organic rich, moderately to well sorted sands are interbedded with dark
gray muds that form beds up to 2 cm thick and
regularly distributed in spaces varying progressively
from 5 to 20 cm. Double mud drapes are locally
present. Where muds are more frequent, flaser
heterolithic deposits are interbedded with the
sands, which locally contain dispersed ferruginous
concretions 2–4 cm in diam.
At the top, the paleochannel successions grade
into intervals consisting of heterolithic bedded
deposits (Figs. 6 and 7) that display a progressive
decrease in sand lenses, as they vary upward
from flaser to lenticular, and then streak lamination. Thicker sand lenses might display current
cross-lamination. The uppermost 2–3 m of the
successions are dominated by massive pelite displaying reddish to yellowish mottling, where
roots are abundant and plant remains might be
present.
In some places, the paleochannel successions are
entirely represented by sands, though an overall
fining-upward tendency can still be recognized, as
indicated by the upward transition from mediumgrain to fine-grain sizes (e.g., TSM7 in Fig. 6).
Individual sand beds, which might reach up to
4.5 m thick, form a lower order of fining upward
cycles, defined by gradation of sands into heterolithic beds. The sands are usually massive or display
convolute bedding (Fig. 7), being locally highly
bioturbated. Ferruginous concretions and mud
clasts averaging 2–3 cm in diam. are dispersed in
these sands. Deposits surrounding the paleochannels consist of gray, plastic, massive muds with
abundant organic debris.
GEOCHRONOLOGY
14
Previous C dating has indicated an age of 35,000
C yr B.P. for subsurface samples obtained in areas
located to the east of Lake Arari (Tancredi et al.
1975). Other workers have suggested that Lake
Arari was established only around 6,000 yr. B.P.,
when the fluvial sand supply from mainland would
have been interrupted (Vital 1988).
During the present study, additional radiocarbon
dating was obtained for 19 samples derived from
14
Quaternary Paleogeography of Northern Brazil
821
TABLE 1. Radiocarbon ages obtained for the deposits of paleochannels in the Lake Arari area (the indicated depths are from the cores).
Profiles/Sample Number
Depth (m)
TR1/MR-153
M77/MR-125
M184/MR-291
M-90/MR-130
M-128/MR-151
M171/MR-166
M180/MR-241
TSM1/SM10
M172/MR-212
M172/MR-216
M181/MR-265
M172/MR-218
M177/MR-228
M180/MR-249
M182/MR-282
M184/MR-311
TSM1/SM1-42
TSM1/SM1-193
TSM1/SM1-214
0.7
0.8
1.3
2.4
2.6
4.7
7.7
7.8
8.8
9.8
10.3
10.6
12.0
12.5
12.5
17.8
50.6
76.0
117.0
14
Type of Material
Organic sediment
Peat
Peat
Wood
Wood
Wood
Organic sediment
Organic sediment
Peat
Peat
Plant remains
Organic sediment
Peat
Organic sediment
Peat
Organic sediment
Organic sediment
Organic sediment
Charred material
cores drilled in the Lake Arari area, which are
shown on Table 1. In general, the obtained ages
show very good consistency when all the sections are
stratigraphically correlated. Where more than one
dating was available for the same drill (i.e., cores
TSM1, M172, and M180, with 4, 3, and 2 datings,
respectively), the ages also decreased consistently
upward in the sections. The obtained data point
to a Holocene age for the uppermost deposits
that, in general, overlie the discontinuity surface
that occurs at 8 to 13 m. Two samples (MR-249 and
MR-311) collected from strata underlying this
surface displayed a late Pleistocene age, though
a sample collected from the upper portion of this
succession is as young as 9,770 (670) 14C yr B.P.
(sample MR-218). Interestingly, while these latest
Pleistocene-earliest Holocene deposits occur at
depths that, in general, vary from 11 to only
17.8 m, the 118 m-deep drill obtained nearest to
Lake Arari, at the locality of Santa Cruz do Arari,
records a similar time span (i.e., 7,900 (640) to
30,360 (6250) 14C yr B.P., see samples SM-10 and
SM1-193, respectively), but at depths that range
from 7.8 up to at least 76 m, where sandy channel
successions are up to 40 m thick. The lowermost
radiocarbon age obtained in this drill provided
an age . 40,200 14C yr B.P. (sample SM-214), but
this came from charred material deposited above
the unconformity at the top of the Barreiras
Formation.
Discussion
The results of this study provide a basis
reconstructing the Quaternary evolution of
Lake Arari area. The new data presented in
paper shed light on both the moment and
C yr B.P.
3,500 (640)
4,370 (650)
6,190 (660)
3,960 (640)
5,520 (660)
4,770 (640)
7,450 (640)
7,900 (640)
6,630 (670)
6,690 (660)
7,320 (640)
9,770 (670)
8,850 (6110)
42,580 (61430)
6,300 (680)
41,080 (6810)
30,560 (6330)
30,360 (6250)
. 40,200
for
the
this
the
Calendar Year B.P. (Type of Analysis)
3,870–3,670
5,050–4,840
7,250–6,900
4,520–4,290
6,410–6,200
5,600–5,460; 5,380–5,340
8,360–8,180
8,980–8,820; 8,800–8,600
7,610–7,420
7,940–7,680
8,190–8,020
11,250–11,100
10,220–9,550
(AMS)
(radiometric)
(radiometric)
(radiometric)
(AMS)
(AMS)
(AMS)
(AMS)
(radiometric)
(AMS)
(AMS)
(AMS)
(radiometric)
(AMS)
7,410–7,000 (radiometric)
(AMS)
(AMS)
(AMS)
(AMS)
possible mechanisms involved in the detachment of
Marajó Island from the mainland. This has implications for reconstructing the paleogeographic evolution of areas located at the mouth of the Amazon
River in northern Brazil.
EVIDENCE FOR A LATE QUATERNARY PALEOESTUARY
The paleochannels are the most impressive
geomorphogical features in northeast Marajó Island. The typical meandering geometry of individual channels, as recognized in many previous works
and described in more detail herein, leaves no
doubt that they are paleodrainages, as they resemble channels from many modern drainage
basins. The elongated nature and the sharp contact
with the surrounding muddy deposits further
demonstrate that these features correspond to
confined depositional settings.
The funnel-shaped structure developed in continuity with Lake Arari suggests the existence of
a previous depositional system (see proposed
evolution of the depositional system in Fig. 8).
Previous work has suggested that Lake Arari is
a modern expression of an ancient, much larger
lake setting (Porsani 1981; Vital 1988). The data
presented herein suggest a paleoestuarine system,
rather than a large paleolake, in the area now
occupied by Lake Arari. This is based on the fact
that the modern lake is installed on a much larger
structure characterized by a funnel-shaped morphology that ends to the north into a paleoshoreline
(see hatched line with indication of the paleoshoreline in Fig. 3), and to the south, and then to the
east, into a meandering channel system, a characteristic exclusive to estuarine settings (e.g., Dalrymple
et al. 1990, 1992).
822
D. F. Rossetti et al.
Fig. 8. Hypothesized reconstruction illustrating the proposed
succession of depositional events in the northeast of Marajó
Island from the Late Pleistocene to the Holocene. A) Development of a main northward-orientated, meandering to
anastomosing paleodrainage system. During this phase, Marajó
Island was still connected to the mainland, and the Guamá River
ran in an uncertain course to the northwest. The depicted
paleocoastline, located near 30 to 50 km from the modern
coastline, is based on the sharp interruption of the paleochannels,
but an alternative interpretation is that a subsequent transgression would have masked any evidence of the paleochannels
near the coastline due to either erosion or sediment accumulation. B) Development of an estuarine system in the Lake Arari
area, fed by fluvial channels flowing from an overall eastward
direction, a situation that implies in the linkage of Marajó Island
with the continent. It is very likely that the main channel was
connected to the Guamá River. A northward tectonic diversion of
this river would have formed almost straight angles near the
locality of Belém, a pattern reproduced in the Lake Arari area,
where the estuarine system was developed. C) Northeastward
tectonic faults would have led to development of the Tocantins
River and opening of Marajó Bay, an event that would have caused
the cut off of the fluvial inflow into the Lake Arari area, ultimately
leading to the abandonment of the estuarine system. Lake Arari
became established in a subsiding area, either as an immediate
reflection of estuary abandonment or as a consequence of
a following transgression. In the outer portion of the paleoestuarine system, located to the north, an east-northeast orientated
funnel-shaped drainage developed, which is now also in the
process of abandonment, where the ephemeral Cururu
River flows.
Although not exclusive, the sedimentologic characteristics described herein correspond to the ones
expected within estuary fills. Estuaries are confined
features presenting abundant channel scours and
fills having the mixed contribution of tidal currents
and fluvial inflows. The studied sedimentary successions are typical of channel deposits. In particular,
the upward gradation from sands to muds, forming
fining-upward cycles, records progressive flow decline, a characteristic of channel abandonment. The
great thickness of the fining-upward successions that
reach up to 40 m reveals the presence of channels
of considerable depths. Channel widths ranging
from 0.4 to 1 km are also consistent with large
channel systems. The abundance of poorly to
moderately sorted and medium-grain to coarsegrain sands rich in plant debris suggests a fluvial
influence. Occurrence of mud drapes between sets
and mantling foresets of cross strata and, in
particular, their local arrangement as a succession
of regularly-spaced mud couplets, resemble sedimentary structures attributed to ebb-flood tidal
fluctuations (e.g., Mowbray and Visser 1984; Yang
and Nio 1985; Kreisa and Moiola 1986; Leckie and
Singh 1991; Nio and Yang 1991). These characteristics are consistent with fluvial channels located
near the coastline and subject to the influence of
tidal processes. The well sorted nature of the sands,
suggestive of terminal drainages, supports this
interpretation.
The presence of thick channel fill deposits having
mixed tide and fluvial influence within a depositional system characterized by a large elongated funnel
shape with a wide opening in one edge (north),
which becomes progressively narrower in the other
edge as it grades into meandering channels, is
strongly suggestive of the presence of a paleoestuarine depositional setting in the Lake Arari area.
Based on the foregoing discussion, it can be
proposed that this lake is a relatively young
morphological feature on Marajó Island, which
was installed on a much larger ancient structure
related to an estuarine system.
The main evolution of this estuary had already
taken place in the late Quaternary. This is proposed
based on the available radiocarbon dates, which
document sediments of late Pleistocene to Holocene age in Lake Arari area. Despite the young age
of most of the sediment fill, the data available are
not sufficient to determine the time when the
estuarine valley was established, as the lowermost
radiocarbon age obtained in the deepest part
provided an infinite age (i.e., . 40,200 14C yr
B.P.). On the other hand, sandy channel fill
successions up to 40 m thick suggest that at least
a major part of this estuarine paleovalley might have
been filled by high volumes of sands deposited in
large fluvial channels in relatively recent, i.e., late
Quaternary times. This would explain why this
feature is preserved so perfectly in the modern
landscape, despite the strong tropical weathering.
Quaternary Paleogeography of Northern Brazil
GENESIS OF THE PALEOESTUARY AND
PALEOGEOGRAPHIC IMPLICATIONS
Considering that a substantial fluvial sand supply
to the paleoestuary existed during the latest
Pleistocene-earliest Holocene, some questions arise
concerning the paleogeography of the study area.
This is because the three branches of paleochannels
that fed the estuary from northeast-southeast require a continental inflow in an area now occupied
by Marajó Bay (see Fig. 3). If interpretations are
correct, the Tocantins River could not have existed
by the time this estuary was active (Fig. 8), i.e., when
Marajó Island was still connected to the mainland.
Taking into account the infinite age of the sample
derived at the base of the studied succession as
representative of a maximum age for the valley fill
sediment, it could be claimed that the valley became
established most likely by fluvial incision associated
with the penultimate glacial episode. Considering
this hypothesis, the filling of the valley could have
occurred during the ensuing transgression linked to
the last interglacial (i.e., Eemian-Sangamonian). A
sea-level rise of 8 6 2 m above the present level has
been recorded along the eastern and southern
Brazilian coast in association with this transgressive
event (Bittencourt et al. 1979; Villwock et al. 1986;
Martin et al. 1996).
Despite the possible and apparently simpler
explanation provided above, a tectonic origin is
claimed as the most likely trigger mechanism for the
establishment of this paleovalley. This is because the
sediments that fill this depositional system are
younger than 30,360 (6250) 14C yr B.P. (disregarding the infinite age obtained at the base of the
sections, as it derives from charred material that
might have been reworked from underlying deposits). This does not coincide with the main last interglacial transgressive peak (i.e., around 126,000 years
ago). Depth gradients of almost 60 m among late
Pleistocene deposits within short distances is more
likely explained by fault activity. There are several
studies supporting Quaternary tectonics on eastern
Marajó Island, where the paleovalley is located
(Bemerguy 1981, 1997; Costa et al. 1993, 2002). A
recent publication (Rossetti et al. in press) has
shown that eastern Marajó Island contains four sets
of lineaments (i.e., NNW/N/NNE-SSE/S/SSW,
NW-SE, NE-SW and E-W/ENE-WSW/ESE-WNW)
that coincide with a tectonic pattern observed in
many other areas located at the mouth of the
Amazon (RADAM 1974; Bemerguy 1981; Costa and
Hasui 1997; Bemerguy et al. 2002). This work also
described thick intervals with penecontemporaneous soft sediment deformation structures, including both ductile (i.e., convolute fold, ball-andpillow, ptygmatic fold, oversteepened folds, and
823
lenses) and ruptile (i.e., fissure and fault with
vertical planes displaying ragged edges) structures
that occur within undisturbed strata, and are related
to a syn-sedimentary seismicity. A tectonic influence
in the development of Quaternary depositional
systems is consistent with the presence of a seismogenic zone near the mouth of Amazon River,
recorded by several earthquakes, including one of
magnitude up 4.8 in the northeastern side of
Marajó Island (Miotto 1993).
Based on the foregoing discussion, the origin of
the upper Quaternary estuarine paleovalley in the
Lake Arari area appears to be more likely related to
tectonics. It has been proposed that the lower
course of Tocantins River was captured by dextral
northeast-southwest strike-slip faults, reactivated
probably during the Pleistocene-Holocene (Costa
et al. 1993, 1996, 2002; Rossetti and Valeriano
2007). The interruption of fluvial sand supply into
the estuarine system, and its consequent sudden
abandonment, recorded by the sharp end of their
feeding channels in the easternmost side of Marajó
Island, might be a reflex of such an event.
Considering the above, it should be possible for
one to trace the continuity of the paleochannel
systems that fed the paleoestuary toward the
continent. The only suitable candidate for this
drainage is the Guamá River (Figs. 1 and 8). This
river is located further south relative to the main
branch of the paleoestuary, but if a dextral strikeslip motion is taken into account for the installation
of the Tocantins River, then we should expect
a corresponding continental drainage in the position occupied by the modern Guamá River. This
relation is based on the shape of this river, which
turns from east-west to north-south forming
a straight angle, a pattern also duplicated in the
paleoestuary of the Lake Arari area (see Fig. 1 for
a comparison). Such a deviation in the drainage
direction is typical of tectonically controlled channels; so it seems sound to presume that the
northward deviation of the Guamá River around
the Belém area and of the paleochannel characterized herein results from a correlatable tectonic
event. This hypothesis must be further investigated,
and a specific study is currently being carried out in
order to compare the heavy mineral assemblages
from sands of the Late Pleistocene paleochannel
deposits, as well as their clay mineral contents, with
those from the Guamá River in order to determine
sediment sources.
As the paleoestuary evolved, its middle segment
seems to have been constantly changing to a westernmost position, leaving behind the belt with the
set of parallel features disposed according to the
elongation of Lake Arari (Figs. 4 and 8). A previous
study had already interpreted these features as
824
D. F. Rossetti et al.
paleoshorelines formed due to the progressive
shallowing of Lake Arari through time (Porsani
1981). The belt occurs only on one side of the lake,
continuing beyond its area, conforming to the
shape of the paleoestuary (Fig. 4). This belt of
parallel bars occupies the inner side of the
paleoestuary, which is the most likely position if
the estuary migrated laterally to the west, reproducing the mechanism of point bar formation in
meandering channels, though on a larger scale.
As the Tocantins River became established, the
water supply at the estuary head was cut off (Fig. 8),
causing stagnation of the depositional setting, with
its consequent abandonment. Subsequently, the
water started to accumulate, promoting the development of Lake Arari. The time favorable for
lake formation needs to be determined, but the
increased volume of muddy deposits above the
discontinuity surface located at depths ranging from
8 to 13 m might record the onset of lower energy
sediment deposition under lacustrine condition.
Taking this into account, then Lake Arari would
have become established in the early Holocene, as
sediments overlying the discontinuity surface record
a maximum age of 7,450 (640) 14C yr B.P.
It is interesting to mention that, prior to the
development of the late Pleistocene estuary described herein, the study area was dominated by
mostly northeastward orientated fluvial channels
(Fig. 8). This is demonstrated by the network of
meandering to anastomosing paleochannels that
occur on both sides of the paleoestuary, which is
interrupted by this funnel-shaped structure. The
fact that these paleochannels disappear to the
northeast around 30–50 km from the modern
coastline suggests that either the paleocoast was
located much further inland than today or a further
transgression buried any evidence of these features
near the coastline. The superimposition of an eastnortheast orientated funnel shaped structure, representing the continuity of the Cururu River in an
area corresponding to the mouth of the Late
Pleistocene paleoestuarine system (Fig. 8), indicates
that northeastern Marajó Island still represents
a dynamic system.
Conclusions
Remote sensing and sedimentological data lead
to the conclusion that northeastern Marajó Island
has been a highly dynamic depositional area.
Changes in the physical setting were frequent, and
they appear to have been, at least in great part,
forced by tectonics. As tectonics took place, many
channels had their courses diverted, resulting in
a network of abandoned features that are nicely
preserved in the modern landscape. Many of these
paleochannels are sharply interrupted at the mar-
gins of the island, some of them even showing
continuity with the paleochannels mapped on the
mainland. The funnel-shaped morphology recorded
in this work is attributed to a large paleoestuary
developed in the northeastern portion of the island
mostly during the late Pleistocene to early Holocene. As the estuary became enlarged, it changed
rapidly from an east-west to a north-south orientation, with modern Lake Arari paralleling the latter
segment. This pattern is related to a tectonic
control, as these orientations parallel fault trends
mapped on Marajó Island. A tectonic influence on
the evolution of this paleoestuary is further indicated by the abundance of deformation structures
attributed to penecontemporaneous seismicity associated with the channel fill successions formed
within the estuarine system.
Some aspects of this research are of relevance to
paleogeographic reconstructions in northern Brazil,
as well as for a better understanding of the
establishment of estuarine valley systems. The
presence of a paleovalley fed by a fluvial sand
supply derived from east-southeast continental areas
implies a connection of Marajó Island with the
mainland up to the late Pleistocene-early Holocene,
when reactivations on northeast-southwest fault
systems would have given rise to Marajó Bay. This
instance serves to highlight the relevance of
tectonics in the preservation of sedimentary successions in very low-lying areas. As opposed to the
traditional view that an estuarine valley results from
fluvial incision followed by filling during a drop and
rise in sea level (e.g., Dalrymple et al. 1994),
respectively, more recent studies have highlighted
the importance of tectonics in the establishment of
such depositional systems (Ardies et al. 2002; Lukie
et al. 2002; Zaitlin et al. 2002; Carr et al. 2003). In
particular, it has been shown that the northern
Brazilian margin is characterized by successions of
tectonically-induced estuarine valley systems since
the Cretaceous (Rossetti 1998, 2000, 2001a,b;
Rossetti and Santos Jr. 2003). The evolution of late
Cretaceous and Tertiary depositional systems in this
region has suggested that changes in sea level alone
are probably not enough to produce a significant
sedimentary pile in low subsiding areas (Rossetti
2006). This proposition is now further supported by
the tectonically-influenced, late Pleistocene-early
Holocene estuarine valley mapped in the study area.
ACKNOWLEDGMENTS
This work was funded by Fundação de Amparo à Pesquisa do
Estado de São Paulo (Project # 004/15518-6), and had the logistic
contribution of the Goeldi Museum, and the Mayor of the town of
Santa Cruz do Arari. Two anonymous reviewers are thanked for
valuable help with the corrections and comments that substantially improved the text. The first two authors are scholarship
Quaternary Paleogeography of Northern Brazil
holders of Conselho Nacional de Desenvolvimento Cientı́fico e
Tecnológic.
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Revised, May 10, 2007
Accepted, July 26, 2007