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JOURNAL OF QUATERNARY SCIENCE (2007) 22(9999) 1–15Copyright � 2007 John Wiley & Sons, Ltd.Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.113262
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FSQuaternary tectonics in a passive margin:Marajo Island, northern BrazilDILCE F. ROSSETTI,1* ANA M. GOES,2 MARCIO M. VALERIANO1 and MARIA CAROLINA C. MIRANDA21 Instituto Nacional de Pesquisas Espaciais-INPE, Sao Jose dos Campos, SP, Brazil2 Universidade de Sao Paulo-USP, Instituto de Geociencias – Programa de geologia Sedimentar e Ambiental Rua do Lago, SaoPaulo, SP, Brazil
Dilce F. Rossetti, Ana M. Goes, Marcio M. Valeriano and Maria Carolina C. Miranda. 2007. Quaternary tectonics in a passive margin: Marajo Island,northern Brazil. J. Quaternary Sci., Vol. 22 pp. xxx–xxx. ISSN 0267-8179.
Received 27 December 2006; Accepted 14 March 2007
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PROABSTRACT: Marajo Island is located in a passive continental margin that evolved from riftingassociated with the opening of the Equatorial South Atlantic Ocean in the Late Jurassic/EarlyCretaceous period. This study, based on remote sensing integrated with sedimentology, as well assubsurface and seismographic data available from the literature, allows discussion of the significanceof tectonics during the Quaternary history of marginal basins. Results show that eastern Marajo Islandcontains channels with evidence of tectonic control. Mapping of straight channels defined four maingroups of lineaments (i.e. NNE–SSW, NE–SW, NW–SE and E–W) that parallel main normal and
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CTEDstrike-slip fault zones recorded for the Amazon region. Additionally, sedimentological studies of lateQuaternary and Holocene deposits indicate numerous ductile and brittle structures within strati-graphic horizons bounded by undeformed strata, related to seismogenic deformation during or shortlyafter sediment deposition. This conclusion is consistent with subsurface Bouguer mapping suggestiveof eastern Marajo Island being still part of the Marajo graben system, where important faultreactivation is recorded up to the Quaternary. Together with the recognition of several phases offault reactivation, these data suggest that faults developed in association with rift basins might remainactive in passive margins, imposing important control on development of depositional systems.Copyright # 2007 John Wiley & Sons, Ltd.95
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EKEYWORDS: tectonics; Quaternary; Marajo Island; sedimentary structure; soft sediment deformation; spatial analysis.97
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Introduction R101
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UNCOThe origin of the Brazilian coast relates to the South Atlanticopening initiated in the Late Jurassic to Early Cretaceous (e.g.Szatmari et al., 1987; Zanotto and Szatmari, 1987). After themain Aptian to Albian rifting (Chang et al., 1990; Aranha et al.,1990), this region became established as a passive margin(Azevedo, 2001Q1). When compared to the eastern andsouthern Brazilian coast, the northern Equatorial Brazilianmargin seems to have had a more complex tectonic evolution,with strike-slip deformation promoting development of pull-apart basins (Szatmari et al., 1987; Aranha et al., 1990;Azevedo, 2001). This is the case for the Marajo graben systemand adjacent areas located at the mouth of the Amazon River,where an increasing number of studies have supported Tertiarytectonic reactivation (e.g. Igreja, 1992; Villegas, 1994; Costa115
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* Correspondence to: D. F. Rossetti, Instituto Nacional de Pesquisas Espaciais-INPE, Centro de Observacao da Terra, Divisao de Sensoriamento Remoto (OBT/DSR), Rua dos Astronautas 1758, Jardim da Granja CP 515, Sao Jose dos CamposCEP 12245-970, Brazil.E-mail: [email protected]
and Hasui, 1997; Goes and Rossetti, 2001; Costa et al., 1993,1995, 2002; Bemerguy et al., 2002). Several publicationssuggest that this area might have remained tectonically activeeven in the Holocene as a result of reactivation of old faultzones (Costa et al., 1996, 1997, 2001; Bezerra, 2003; Silva,2005). This interpretation has been suggested based on the factthat many Amazonian river systems are oriented according tomain tectonic structures of this region (Projeto Radam, 1974;Bemerguy, 1981, 1997; Bemerguy et al., 2002), as revealed byrivers that follow E–W to ENE–WSW and NE–SW dextraltranscurrent fault zones, NE–SW and NNE–SSW reverse faults,as well as NW–SE and NE–SW normal faults (Costa and Hasui,1997).
Taking the foregoing studies into account, it is intriguing tosuggest that tectonic stress produced by reactivation of basinfaults during the Quaternary might be more common in passivemargins, as in the northern Equatorial Brazil, than initiallythought. Given the significance of the study, a more detailedinvestigation is required in order to fully demonstrate thatthe tectonic reactivation of Cretaceous and Tertiary faultswas important in defining the modern geomorphology inthis region.
This study focusing on eastern Marajo Island (Fig. 1) aimsto integrate geomorphological and sedimentological data
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Figure 1 Location map of the study area with a generalized geological map showing the Quaternary deposits of eastern Marajo Island. Box shows thestudy area
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UNCORRECobtained from remote sensing and cores, respectively, in orderto demonstrate the importance of tectonic reactivation ondevelopment of both the modern landscape and the deposi-tional systems during the latest Quaternary. When combinedwith subsurface information compiled from the literature, thistype of work reveals that the tectonic setting of this area mightbe more complex than suggested. Eastern Marajo Island islocated in the Para Platform, which is considered a tectonicallystable area of shallow basement bounded by major basinboundary faults (Azevedo, 1991). However, the present authorhas raised the possibility that this platform, still poorly studiedin all aspects, might contain sedimentary basins that have notyet been described.
In this study we provide a detailed mapping of geomorpho-logical lineaments in eastern Marajo Island, attempting todemonstrate that they might reflect tectonic structures derivedfrom latest Quaternary fault reactivation. Likewise, analysis ofsedimentary features that could record influence of seismicactivity during or shortly after sediment deposition has not yetbeen provided for this area. Sedimentary features have longbeen used as evidence to interpret the effects of seismic activityin many depositional settings throughout the world (e.g.Seilacher, 1969; Sims, 1975; Ben-Menahen, 1976; Johnson,1977; Doe and Dott, 1980; Visher and Cunningham, 1981;Allen, 1982, 1986; Hempton and Dewey, 1983; Mohindra andBagati, 1996; Obermeier, 1996; Plint, 1985; Brodzikowskiet al., 1987; Nocita, 1988; Ringrose, 1989; Karling and Abella,1992; Owen, 1996; Blanc et al., 1998). Therefore, recognitionof seismically induced soft sediment deformation structures in
Copyright � 2007 John Wiley & Sons, Ltd.
latest Quaternary deposits of the study area might help inevaluating the importance of tectonic reactivation in passivemargins. This study also contributes to discussion of therelationship between neotectonic events and the origin andevolution of the largest fluvial island in the world, whichdetermined the modern configuration of the lowest Amazon.
Regional geology
The Para Platform, located in the northeast of the State of Para innorthern Brazil, corresponds to a large area of crystalline andPalaeozoic sedimentary basement that remained generallytectonically stable relative to adjacent Cretaceous andCenozoic sedimentary basins. It is bounded by the Vigia–Castanhal trough to the east, Mexiana Sub-basin to the north,Limoeiro Sub-basin to the southwest, and Cameta Sub-basin tothe southeast (Fig. 2(A)). The three latter basins are part of theMarajo graben system, while the Vigia–Castanhal troughrepresents a northward extension of the Cameta Sub-basin.
The Marajo graben system covers an area of 1.5� 106 km2,consisting of a NW–SE and then a NE–SW oriented rift in thenorthward direction. This structure is defined by NW–SE andNE–SW normal faults reactivated from Precambrian basement,but E–W to ENE–WSW and NE–SW strike-slip faults have alsobeen recorded (Azevedo, 1991; Villegas, 1994; Costa andHasui, 1997; Figs 2(A) and (B)). The rift experienced two
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Figure 2 Structural framework map (A) and seismic regional cross-sections of the Marajo graben system (B). (B modified from Azevedo, 1991)
QUATERNARY TECTONICS IN A PASSIVE MARGIN 3
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extensional phases. The first one is related to the opening of theEquatorial South Atlantic Ocean in the Late Jurassic and EarlyCretaceous (Szatmari et al., 1987; Azevedo, 1991), but asecond and more important extensional episode took place inthe Aptian–Albian transition. In addition, seismic data from allsub-basins of the Marajo graben system show indications offault reactivation along strike-slip zones throughout theCenozoic (Villegas, 1994).
The sedimentary fill of the Marajo rift basins (Fig. 3), which ismostly based on subsurface data (Galvao, 1991; Villegas,1994), is represented by Cretaceous to Quaternary fluvial tonearshore deposits (Villegas, 1994). Sandstone of the Breves/Jacarezinho Formations (Aptian–Cenomanian) and silty mud-stones of the Anajas Formation (Cenomanian) are overlain bysandstones, mudstones and conglomerates of the Upper
UNCOR
Figure 3 Simplified stratigraphic section of the Marajo graben system
Copyright � 2007 John Wiley & Sons, Ltd.
ED PCretaceous to Palaeocene Limoeiro Formation. Other Tertiarydeposits consist of mixed siliciclastic–carbonates of the MarajoFormation (Palaeocene–Pliocene), as well as the Para Group(Quaternary). The latter includes sandstones and mudstones ofTucunare and Pirarucu formations, respectively. On thesurface, correlatable deposits are the Pirabas and Barreirasformations and the post-Barreiras sediments (e.g., Rossetti et al.,1989, 1990; Rossetti, 2001).
Study area and methods
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Most of eastern Marajo Island is characterized by lowlands thataverage only 4–6 m in elevation (Fig. 4(A)). This region has atropical climate with a mean annual temperature of 288C andprecipitation of 2500–3000 mm yr�1, 90% of which isconcentrated between January and July. During this rainyseason, accessibility is possible only by boat, but in the dryseason the area can be easily explored by vehicles on numerousunpaved roads. The area is mainly open vegetation of cerrados,grasslands and savanna woodlands, which are mixed withnarrow, elongated belts of dense ombrophyla forests.
The morphological aspects of the study area were charac-terised based on the analysis of Landsat 5-TM (Refs 224-060and 225-061 from INPE, the Brazilian National Institute forSpace Research) and Landsat 7-ETM (Refs 223-060 and223-061, GLCF) images, collected in August 2001, as wellas topographic data acquired during the Shuttle RadarTopographic Mission (SRTM-90 m) distributed by the NationalAeronautics and Space Administration (NASA). The SRTM datawere processed using customised shading schemes and palettesto highlight topographic and morphological features. Theremote sensing analysis was particularly useful for mapping anetwork of palaeochannels that typify Marajo Island. Weinterpreted elevation data using the software Global Mapper(Global Mapper Software LLC, Olathe, KS, USA), and wecombined the remote sensing data with drainage maps
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Figure 4 (A) Digital topographic data from eastern Marajo Island acquired during the Shuttle Radar Topographic Mission (SRTM). Note the sharpboundary between two topographic regions: an eastern one of low topography, and a western one of higher topography. This topographic difference isartificial and is the result of the effect of vegetation (grassland to dense forest from east to west), combined with a smooth topographic gradient. (B)Modern drainage map (based on Amazonian Security Service, SIPAM/SIVAM). (C) Map of straight lineaments based on analysis of the modern drainagesystem
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available from the Amazonian Security Service (SIPAM/SIVAM).
Our sedimentological data are based on our descriptions ofthe few outcrops along ephemeral river banks and farm damscombined with data from continuous cores collected using apercussion drilling system. We used subsurface data from cores
Copyright � 2007 John Wiley & Sons, Ltd.
that are 4.5–6.0 cm in diameter and averaging 18 m deep, withone drill hole 120 m deep.
In the following sections, we present information derivedfrom drainage system analysis based on remote sensing anddrainage maps integrated with sedimentology, as well assubsurface and seismographic data available from the
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literature, in order to evaluate the significance of tectonicinfluence during the Quaternary evolution of the study area.
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Analysis of the drainage systemOFS
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The modern drainage system in eastern Marajo Island (Figs 4(B)and (C)) is superimposed on a palimpsest drainage system(Figs 5 and 6) consisting of a network of mostly anastomosing tomeandering palaeochannels that are, in general, larger than thepresent channels. For the purpose of describing the moderndrainage, the study area was subdivided into four sectors: I, II,III and IV (Fig. 4(B)). Sector I, corresponding to most of thecentral and western portions of the study area, is drained by theArari, Anajas, Mocoons, Atua, Cururu and Jenipacopu basins.These are characterized by low-density channels with manystraight segments that form a subdendritic pattern with trellisinfluence (cf. classification of Howard, 1967). A trellis pattern isparticularly developed in the Anajas drainage basin, whichshows the main stream rapidly changing course from asoutheast to a northwest direction, forming straight angles
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Figure 5 (A) Palimpsest drainage system in eastern Marajo Islandobtained from interpretation of remote sensing (Landsat and SRTM)data. (B) Map of straight lineaments based on analysis of palaeochan-nels
Figure 6 (A) Map of the straight lineaments in eastern Marajo Islandcombining information from modern and palimpsest channels. (B)Distribution of these lineaments in a rose diagram where four mainmodes are present: NNE–SSW, NE–SW, NW–SE and E–W
Q2
Copyright � 2007 John Wiley & Sons, Ltd.
before joining the southwestward-flowing MocoonsQ2 River.Another peculiar feature in this sector is the Cururu drainagebasin located to the north, which is formed by low-density,mostly westward drainage channels that rapidly change courseto southward to form, together with waters derived from theMocoons and Anajas rivers, a westward-flowing drainage basindisplaying higher-density channels organised into a subden-dritic pattern. Comparisons of sets of effluents from both sides ofthe main channels in this sector reveal dominance ofanomalous asymmetric distribution of straight and mostlyparallel channels.
An interesting characteristic of sector I is the abundance ofchannels associated with a well-preserved palimpsest drainagesystem. A main palaeochannel system occurs around the ArariLake area (Fig. 5(A)), where a series of meandering channels areconnected to a relatively straighter, but much wider channelsegment that is bent, forming a straight angle to the north toform a funnel-shaped, even wider channel (see black arrows inFig. 5(A)). Additionally, numerous other remains of palaeo-channels are mapped in this sector, some with patternssuggestive of water flowages contrary to the modern channels.An excellent example can be illustrated along the Cururu Riverarea, where there is an elongated funnel-shaped feature thatbecomes wider to the east (see open arrows in Fig. 5(A)),
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suggesting a water discharge in this direction, as opposed to themodern Cururu River, which is in continuity with this featurebut flows to the west.
Sector II corresponds to areas located in the southeast andnorth of the study area, which are drained by the lower courseof the Arari River, as well as the Marajo-Acu River, and theGanhoao and Tartaruga rivers, respectively (Fig. 4(B)).Although resembling the channels from sector II, this sectordisplays relatively higher-density channels. In addition, thedrainage here forms also a subdentritic pattern with trellisinfluence; likewise sector I, but a rectangular influence is alsopresent.
Sector III is characterised by a long belt in the extremenortheastern portion of the study area. It is drained by theParacauari River and an abundance of lakes and ponds thattypifies this marginal sector (Fig. 4(B)). This river, highlymeandering and up to 1 km wide, becomes suddenly straightand much narrower westward. Interesting is that the widersegment of the Paracauari River shows continuity with ameandering palaeochannel located to the south of the modernriver (Fig. 7(A)), and which is connected with the palimpsestdrainage of the Arari Lake area (Fig. 5(A)). In addition, severaldrainage basins displaying an annular pattern occur in thissector. Finally, sector IV characterizes the southern portion ofthe study area, being drained by several southward-flowingchannels represented by the Guajara, Piria and CaraticuGrande rivers. This sector displays a typical dendritic drainagepattern with high-density channels, but even in this sectorstraight channels are present.
Owing to the abundance of straight channels and palaeo-channels in the above-described sectors, particularly in sector I,we proceeded with their detailed mapping in order to evaluatepossible lineaments that could follow regional tectonic trends,as many straight channels develop following tectonic structures(Howard, 1967; Ouchi, 1985; Summerfield, 1993). Thismapping (Figs 4(C), 5(B) and 6) revealed straight lineamentsdisplaying a wide distribution of directions, but these areclearly concentrated in four distinctive modes: NNE–SSW,NE–SW, NW–SE and E–W (Fig. 6). Noteworthy is that theboundary of Marajo Island is defined by short straight segmentsdisplaying these orientations.
Some observations related to the mapped lineaments arenoteworthy: (1) NW–SE lineaments are, in general, disrupted byNE–SW and E–W lineaments, which are particularly illustratedin sector I respectively by several NE–SW deviations along thecourses of the Atua and Mocoons rivers (see Fig. 4(B)) and theE–W funnel-shaped palaeochannel to the east of the CururuRiver that truncates NW–SE lineaments north of the Arari Lake(Figs 5(A) and 7(B)). (2) NNW–SSE lineaments disrupt E–Wlineaments (Fig. 7(C)). (3) The crossing of NW–SE and NE–SWlineaments results in rectangular to rhombic blocks that aremore than 25 km long and as much as 15 km wide (Fig. 7(D)).(4) NNE–SSW lineaments are particularly well developedaround the Arari Lake area, where they parallel the length ofthis lake (Fig. 7(E)). (5). NE–SW and E–W palaeochannels aredisrupted by NW–SE palaeochannels to the east of the ArariLake (Fig. 7E), as well as to the west of the study area (Fig. 7F),where there is a set of straight, short, but aligned, NW–SElineaments that parallel the margin of the Limoeiro Sub-basin ofthe Marajo graben system. Interesting to notice also is that thisset of NW–SE lineaments separate Marajo Island into twophysiographic areas (Fig. 4(A)): the western region, dominatedby older (i.e. Plio-Pleistocene) deposits of the post-BarreirasFormation, where dense forest prevails, and the eastern regionwith pervasive Holocene deposits, where vegetation is amosaic of grasslands and narrow elongated belts of ombro-phylas. Slightly curved lineaments produce rounded, arcade
Copyright � 2007 John Wiley & Sons, Ltd.
features that are as much as 10 km in diameter and about 3 mhigher than surrounding terrains (Fig. 7(G)).
It is noteworthy that the shape of Marajo Island is defined bymany short straight segments that have orientations asdescribed above.
Sedimentology and age
ED PROOFS
The present sedimentological characterisation will focus on thedescription of soft sediment deformation structures that areabundant in the deposits of the study area. However, a briefoverview on facies and their depositional setting is presented,although this will be the subject of a separate publication.
The strata in the study area consist mostly of well-sorted,rounded to subrounded, fine- to medium-grained, massive orcross-stratified sands that are interbedded with heterolithicdeposits forming lenticular, wavy and flaser lamination, andmudstones (Fig. 8(A)). Reactivation surfaces and mud drapes,locally configuring alternating thicker and thinner sedimentbundles, are present in association with these deposits(Fig. 8(B)). The lithologies are often organized into sharp-based,erosive, fining upward successions that might be as thick as45 m, but with an average thickness of 10–20 m. Coarseningupward succession might be also present, but only locally.Plant remains and pits are common in these strata. The top ofthe sedimentary successions might be represented by massive,endured deposits displaying root marks, which are related topalaeosols. Facies interpretation indicates the prevalence offluvial channel, flood plain and lacustrine depositionalenvironments, possibly with local influence of tidal currents,as suggested by the thicker/thinner bundles marked byreactivation surfaces and/or mud drapes, as recorded in manyother tidal settings (e.g. Yang and Nio, 1985; Leckie and Singh,1991; Shanley et al., 1992).
Radiocarbon dating indicates that the fluvio-lacustrinedeposits of the study area formed during the latest Pleistoceneand Holocene, with the base of the deposits recordingconventional ages that are up to 40 200 14C yr BP (Tancrediet al., 1975, and our unpublished data). The thickest welldrilled in this area penetrated the entire Quaternary section,reaching Miocene basement at 120 m. Radiocarbon-datedsamples from this well yielded conventional ages of 30 360(�250) yr BP and >40 000 14C yr BP at depths of 76 and 119 m,respectively (Fig. 8(A)).
Description of soft sediment deformationstructures
The studied deposits contain highly deformed horizons that areinterbedded either with undeformed or only slightly deformedstrata (Fig. 8). The deformed beds, which are often in sharpcontact with beds that display some evidence of disturbance,can be several metres thick. Several types of soft sedimentdeformation are present in these deposits and include bothductile and brittle structures, which are commonly found in thesame stratigraphic horizon.
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uctile structures. This type of structure includes all fea-tures that have different degrees of crumpling or folding ofthe laminae; it is the most common soft sediment defor-mation structure in the study area. According tomorphology, styles of ductile structures include: convoluteJ. Quaternary Sci., Vol. 22(9999) 1–15 (2007)DOI: 10.1002/jqs
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Figure 7 (A) A wider segment of the Paracauari River in continuity with a meandering palaeochannel located to the south of the modern river. Notealso that the modern river changes its pattern from highly meandering to straighter westward. (B) Nearly E–W oriented lineaments (white traces)controlling the development of a funnel-shaped channel to the east of Cururu River. These lineaments sharply intercept NW–SE lineaments (arrows)located to the north of the Arari Lake area. (C) NW–SE, NE–SW and nearly E–W straight lineaments between the Paracauari and Camara rivers, easternside of Marajo Island. Note that NNE–SSW lineaments disrupt nearly E–W lineaments and that NW–SE lineaments are disrupted by NE–SW lineaments.(D) The intersection of NW–SE with NE–SW lineaments produces rectangular- to rhombic-shaped areas. Note the straight boundaries of these features,and the places where NE–SW lineaments have disrupted NW–SE lineaments (white arrows). (E) NNE–SSW lineaments defining the margin of ArariLake. Note also the several NE–SW straight segments of palaeochannels that are disrupted by NW–SE lineaments to the east of this lake. (F) NW–SElineaments (black lines) define two physiographic regions in Marajo Island, with the lowest SRTM elevations (mostly yellow to brown) being located tothe east. Note that these lineaments sharply disrupt (white arrows) many NE–SW oriented palaeochannels. (G) Arcade features defined by curvedlineaments are listric faults. The topographic profile A–A0 suggests a topographic high that separates the relatively flat rounded area from slightly loweradjacent terrain. (A–C, E and F: Landsat image, RGB 543); D and F: SRTM topographic data)
Copyright � 2007 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 22(9999) 1–15 (2007)DOI: 10.1002/jqs
QUATERNARY TECTONICS IN A PASSIVE MARGIN 7
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Figure 8 (A) Representative lithostratigraphic profile from the study area obtained from core description illustrating the sedimentary facies consistingof sands that grade upward into heterolithic bedded deposits and mudstones, configuring fining upward cycles that are up to 45 m thick. Note theseveral horizons with soft sediment deformation strata that are interbedded with non-deformed deposits. (B) A detail of alternating thicker and thinnersand beds defined by mud drapes (arrows), forming bundles probably resulting from ebb/flood tidal fluctuations
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fold, ball-and-pillow, ptygmatic fold, and oversteepenedundulation and lenses.
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upward morphologies (Figs 9(A) and (B)), similar to featuresdescribed in the literature (e.g. Visher and Cunningham,1981; Hempton and Dewey, 1983; Mills, 1983). The degreeof undulation varies upward within individual folds fromslightly undulating to strongly folded, and then again slightlyundulating to the top (Fig. 9(A)). The intermediate, highly
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Figure 9 Styles of ductile soft sediment deformation in the study area. (A, B) Convolute folds. Note the highly folded horizon sandwiched betweenslightly undulating strata. Note also several recumbent microfolds (arrows) formed in the anticline of the highly folded horizon. (The box in A locatespart B). (C, D) Deformed strata with a monoclinal fold. (E, F) Ball-and-pillow structures. Note the irregular shape of the larger pillow in figure F. (G)Ptygmatic folds (arrows). (H, I) Oversteepened folds (H) and lenses (I). Note the pronounced convex-up laminae in part H (arrows), and the steeplydipping sand lenses in part I (arrows). (In all figures, darker stippled areas indicate sandy deposits)
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UNfolded laminae have sharp, unconformable contacts and caninternally display a series of bed disruptions and recumbentmicrofolds (Fig. 9(B)). The relatively less deformed bedsabove and below the folded horizon form packages oflaminae displaying depositional angles that produce trunca-tions. We also included features resembling monoclinal folds(Figs 6(C) and (D)) in this category.
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all-and-pillow. This type of ductile structure is character-ised by sand bodies sunk down into underlying sedimentsthat are usually of muddy composition. The shape of thedownwarped deposits varies from pillow-like (Fig. 9(E)) toirregular-shaped masses of sediment (Fig. 9(F)) that are insharp contact with surrounding, often convolute-folded sedi-ments. Individual features can be up to 8 cm long at the corescale and are either detached from the upper strata, in which
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case they appear as floating in the underlying muddy depos-its, or are attached through a short and narrow branch.Sediments in these circular to semicircular features mightbe laminated, with individual laminae conformable to theexternal shape of the structure, highly distorted or evenentirely massive.
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tygmatic fold. This structure consists of narrow andelongated folds that sharply cross through the beds verticallyto subvertically (Fig. 9(G)). The ptygmas, which are as muchas 2 cm long and only a few millimetres thick, are usuallycomposed of very fine to silty sand. The host deposits aremostly heterolithic and display convolute folds.� O
versteepened folds and lenses. This category of soft sedi-ment deformation is distinguished based on deformed bedsdisplaying either unusual strong undulation (Fig. 9(G)) orJ. Quaternary Sci., Vol. 22(9999) 1–15 (2007)DOI: 10.1002/jqs
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lenses (Fig. 9(H)) that, instead of being parallel or subparallelto bedding planes, are deeply inclined and irregularly dis-tributed. In part, oversteepened folds resemble hummockycross-lamination, because they have laminae that are almosthorizontal or slightly inclined, and sharply mantled by a setof convex-up laminae (Fig. 9(G)). However, in this instance,the convexity is more pronounced than recorded for hum-mocky cross-lamination, rather recording a package of sedi-ment disturbed differentially from surrounding strata duringor shortly after deposition. In addition, the lithology isdominantly muddier than expected in hummocky cross-lamination, which consist mostly of sands. Similarly to thefolds, oversteepened lenses are present in heterolithic depos-
UNCORRECT
gure 10 Brittle deformation structures from the study area. (A, B) Planar faultults terminate upward with the overlying beds being only slightly deformed. Norming a series of normal microfaults. Observe also the reverse faults abovederlying deposits with ductile deformation forming ball-and-pillow structuresperimposed disrupted beds due to faulting. (H, I) Reverse fault (circle). (In a
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its, being characterised by silty to very fine sand lenses<1 cmin length that are unusually distorted and inclined, locally athigh angles (up to 458).
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rittle structures. This group of structures includes numer-ous microscale fractures and faults. Fissures are distributedthroughout the deposits, forming planes a few millimetres inlength that sharply cut the strata, usually at angles of >608,although low-angle fault planes are also present. Faultplanes are either straight (Figs 10(A) and (B)) or slightlycurved (Figs 10(C) and (D)), having offsets ranging betweena few millimetres to as much as 3 cm, and display inclination
ED PROOFS
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ith that extendsQ3 into a listric fault, forming a graben (circle). Thesete that the sand layer inside the graben is disrupted in several places,he coin. (C, D) Listric faults form a spoon-shaped graben that cuts(E) A long, ragged fault plane with small sharp peaks (arrows). (F, G)l figures, darker dotted areas indicate sandier deposits)
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angles similar to the fractures. Some straight fault planesbecome listric downward, forming spoon-shaped features(Figs 10(A) and (B)). Faults that are longer than a few tens ofcentimetres in length have extremely ragged planes withsmall sharp peaks (Fig. 10(E)). Faults occur either isolated oras a group of several faulted micro-blocks, with syntheticand antithetic faults together forming micrograbens(Figs 10(A) and (B)). These depressions are overlain by bedsthat are progressively less disrupted upward, ending inundeformed strata. Strata filling a graben may show bedsthat are disrupted by even smaller-scale faults. At somestratigraphic horizons, bed sets dip in different angles dueto faulting (Figs 10(F) and (G)). Both normal and reversefaults are present (Figs 10(H) and (I)).
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As opposed to the Marajo graben system, where several seismiclines are available, the tectonic framework of eastern MarajoIsland remains to be discussed. This area is located to the east ofthe Limoeiro Sub-basin, represented by a complex of deeptroughs in the central part of the Marajo graben system. Thesetroughs consist of semi-grabens formed by combination ofNW–SE and NE–SW oriented normal faults with NE–SW andENE–WSW strike-slip faults.
UNCORRECT
ure 11 Bouguer anomalies map of northern Brazilian margin, where thencides with negative anomalies below�40 mGal. Note also the presence oicated by the white arrow), northeastern Marajo Island, which continues
evedo, 1991)
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S
In contrast to the complex structural style that characteriseswestern Marajo Island, it has been suggested that its eastern sideis stable given its location in the eastern portion of the ParaPlatform (Fig. 2(A)). However, this platform appears to be notuniformly stable, as it might contain numerous small but deeptroughs that are as much as 3.5 km deep (Azevedo, 1991).Unfortunately, subsurface data from this area are not availablefor detailed studies, but analysis of Bouguer anomalies confirmsthe presence of a large north–south oriented area displayingnegative anomaly characterised by values between 0 and�40 mGal (Fig. 11). This anomaly, which coincides with theposition and the orientation of Arari Lake, dominates themodern physiography in the study area, extending northwardwhere it connects to the Mexiana Sub-basin. As shown inFig. 11, this north–south oriented negative anomaly does notrepresent an isolated subsurface feature but is part of a trend ofnegative anomalies that match well with the position of majorrift basins developed along the northern Brazilian margin.
ODiscussion
ROAll the above-described sectors of the study area have drainagebasins with channels and palaeochannels that lead to therecognition of several morphological lineaments suggestive ofdevelopment according to tectonic structures. These mostly
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study area is located. Note that the position of sedimentary rift basinsf an elongated anomaly with values of�40/0 mGal in the Arari Lake areanorthward where it joins with the Mexiana Sub-basin. (Modified from
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include rivers composed of rectilinear segments that areconnected forming straight angles creating the subdendriticpattern with trellis and rectangular influence, presence ofmeander channels that change suddenly into rectilinearchannels and sets of channels distributed asymmetrically whencomparing both sides of higher-order channels (Howard, 1967;Ouchi, 1985; Summerfield, 1993). Additionally, the four maindirection modes obtained from mapping of straight segments ofchannels are coincidental with regional tectonics. For instance,the NW–SE and NE–SW directions parallel main normal faultzones that define the Marajo rift system. In addition, the E–Wand NNE–SSW oriented lineaments parallel strike-slip faultzones recorded in northern Brazil. As previously mentionedhere, all these faults were reactivated through the Cenozoic,and even during the Holocene (Azevedo, 1991; Costa andHasui, 1997), having affected the Limoeiro Sub-basin in thewestern part of Marajo Island (Villegas, 1994).
Although seismic data from eastern Marajo Island are notavailable, the present study led to the suggestion that easternMarajo Island might also have been affected by Quaternarytectonics. The Bouguer map shows that, rather than beinglocated in tectonically stable areas of the Para Platform, thesubsurface of eastern Marajo Island is a deep sedimentary basinthat extends southward from the Mexiana rift basin, thusbelonging to the Marajo graben system. Taking this interpret-ation into account, it is expected that eastern Marajo Island hada tectonic evolution similar to other sub-basins in this riftsystem. As in those basins, it is expected that this area was alsoaffected by fault reactivation through the CenozoicQ4, asrecorded in several seismic lines obtained from the western sideof the island (Villegas, 1994).
In the absence of other data such as seismic lines that couldphysically attest to fault activity in eastern Marajo Island, welooked at the sedimentary record obtained from cores in orderto seek supporting evidence for Quaternary tectonism in thisarea. Our study shows that the strata in the study area containsedimentological features that indicate seismic activitiescontemporaneous with or shortly after deposition when thesediment was still unconsolidated or semi-consolidated.
The several types of deformation structures present in thestudy area are attributed to disturbances that occurred while thesediment was still in an unconsolidated to semi-consolidatedstate (e.g. Lowe, 1975; Blatt et al., 1980; Allen, 1982; Mills,1983; Owen, 1987). Convolute folds have been interpreted asextremely complex soft sediment deformation structures, mostfrequently produced from dewatering due to shear induced byloading or slumps (e.g. Lowe, 1975; Brenchley and Newall,1977; Visher and Cunningham, 1981; Mills, 1983; Scott andPrice, 1988; Nichols et al., 1994; Owen, 1996). Theball-and-pillow structures sustain reverse density loading inthe study area. These are extremely complex forms of loadstructures produced when a high-density layer overlies alower-density layer (Visher and Cunningham, 1981; Mills,1983). Such a condition was reached in the study area whenthicker sandy lenses were deposited onto water-saturatedmuds, causing gravitational collapses. The narrow branchattached to the ball-and-pillow structures attests to theirconnections to an overlying sandy layer. Similar ’bal-l-and-pillow’ structures have been extensively described inthe literature (e.g. Visher and Cunningham, 1981; Hemptonand Dewey, 1983; Mills, 1983; Allen, 1986; Scott and Price,1988; Moretti et al., 1995).
Recumbent folds and bed disruptions internal to convolutefolds similar to those recorded in this study have been observedin association with convolute folds elsewhere (e.g. Mohindraand Bagati, 1996) and reveal local horizontal stress. The degreeof stress might have varied through time while the sediment was
Copyright � 2007 John Wiley & Sons, Ltd.
ED PROOFS
being deposited, as suggested by strata that change upwardfrom slightly undulating to strongly folded, and then slightlyundulating. The truncating packages of folded laminae withvarying depositional angles in association with slightlyundulating strata are evidence of deformational stress takingplace contemporaneously with sediment deposition. Theoversteepened folds are also explained by a similar process,representing a variation of convolute fold where dipping stratabecame strongly undulating upward due to progressiveincreased stress. On the other hand, oversteepened lensesindicate areas with milder sediment deformation, when amuddy depositional setting was unstable, promoting a differentdegree of deformation in sand lenses formed by migration ofstarving ripples.
The abundance of faults and fractures attests to the resistanceof sediments to the applied force, at least to the degree ofcausing brittle behaviour. Experimental studies on shearunlithified deposits have resulted in comparable structures(Tchalenko, 1970). Faults and fractures in ductile deformedstrata attest to a genesis contemporaneous with sedimentdeposition. Such association of structures has been recorded inmany other soft sediment deformed deposits (e.g. Shiki andYamazaki, 1996; Grimm and Orange, 1997; Bhattacharya andBandyopadhyay, 1998; Stollhofen, 1998). Considering that thematerial deformed is the same, changes in stress rates, whichdirectly influence the value of the pore pressure within thesediment, a range of structural features (folds, faults, fractures)may be produced across the brittle/ductile spectrumQ5. Brittledeformation contemporaneous with sediment deposition isreinforced in the study area by the fact that the faults formgrabens filled by strata that are progressively less deformedupward. The presence of ragged fault planes with sharp peaksconfirms that the brittle deformation was caused by instan-taneous collapses (e.g. Demoulin, 1996). The occurrence ofboth normal and reverse microfaults in the study area recordslocal points of extension and compression taking place in asame stratigraphic horizon.
Ductile and brittle soft sediment deformation contempora-neous with sediment deposition might be due to differenttriggers, mostly including sudden sediment loading, gravity-induced mass movement, storm impact and seismic shocks. Inthis instance, the depositional settings represented by fluvialchannels, flood plains and lakes are not places that wouldnaturally favour the first three hypotheses. On the other hand,the fact that the soft sediment deformation structures areconfined to highly deformed strata that occur on specifichorizons interbedded with either undeformed or only slightlydeformed strata points to the influence of an event external todepositional environment. Therefore, a seismic event isproposed, which would have the power to produce instan-taneous sediment deformation affecting only specific beds withrecurrence through time between quiet moments, as recordedin many other areas in the world (Sims, 1975; Hempton andDewey, 1983; Anand and Jain, 1987; Scott and Price, 1988;Murakoshi and Masuda, 1991; Moretti et al., 1995; Mohindraand Bagati, 1996; Moretti and Tropeano, 1996Q6; Blanc et al.,1998).
Therefore, the deformed structures documented here aremost plausibly explained by seismic activity contemporaneouswith sediment deposition. Considering the overall flat topo-graphy of the study area, tectonics seems to have been crucialin producing space to accommodate new sediment. Thecomplex distribution of tectonic lineaments recognised fromthe analysis of modern and palaeodrainage systems indicatesrecurrence of seismic events through time.
Reconstructing the succession of tectonic events that affectedthe study area is problematic, particularly considering that
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some events might have taken place contemporaneously. Therelationships of the straight lineaments attributed to tectonicstructures that were mapped in the study area, as describedabove, might be used to make some suggestions.
In general, the study area was affected by a tectonic motionthat produced NW–SE structures that pre-dated the develop-ment of NE–SW and E–W structures. However, the presence ofNE–SW palaeochannels that are sharply disrupted by NW–SElineaments in the western part of the study area attests to therecurrence of these events through time. The tectonic motionthat gave rise to the NW–SE lineaments in this area seems alsoto have caused a slight subsidence of eastern Marajo Island, assuggested by the abundance of traces having this direction rightat the boundary between the two physiographic compartmentspresent in this island. The fact that the eastern compartment isdominated by Holocene sedimentation is taken as evidencethat this part of the island was slightly depressed relative to thewestern side, a process that culminated with the creation ofnew space to accommodate Holocene strata.
The combination of NE–SW and NW–SE lineaments createdmany small basins, as suggested by the rectangular andrhombic areas with straight boundaries oriented in thesedirections, which are still the locus of sediment accumulationduring floods. Slightly curved lineaments demarking roundedfeatures with SRTM topographies about 3 m higher than thebackground terrains are related to abandoned lake basins thatwere probably formed in association with listric faults.
The presence of E–W tectonic traces that intercept the CururuRiver to the north of Arari Lake suggests that this area wasaffected by a tectonic motion that produced E–W structures thatpost-date the development of NW–SE structures. The disruptionof E–W lineaments by NNE–SSW lineaments in the areabetween the Paracauari and Camara rivers indicates thatstructures displaying the latter orientation are possibly theyoungest in the study area.
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UNCORRThe integration of tectonic mapping and sedimentological datashows that northeastern Marajo Island records tectonic activitycontemporaneous with sediment deposition during the mid tolatest Quaternary.
Many previous publications have proposed that theQuaternary history of Marajo Island was strongly controlledby tectonics. This study, based on remote sensing (i.e. SRTMtopographic elevation models and Landsat images) interpret-ation combined with sedimentological studies, represented aneffort to provide data supporting that these tectonic events werecontemporaneous with sedimentation, having great control onthe evolution of the depositional systems. Tectonics wouldhave favoured the development of subsiding areas and createdsites to accommodate new sediment. As the study arearemained tectonically unstable during sedimentation, thickhorizons of deposits strongly marked by abundant soft sedimentdeformation structures were formed. A complex network oftectonic lineaments controlled and/or modified the course ofchannels in both the modern and palaeodrainage systems.
The tectono-sedimentary evolution took place in differentphases, as recorded by the four successive trends of lineamentsrecognised in the study area. Main tectonic episodes seem tohave been related to reactivations of NNE–SSW, NW–SE,NE–SW and E–W oriented fault normal and strike-slip zones,with NW–SE motion being responsible for the slight depressionof the eastern margin of Marajo Island, which caused a renewal
Copyright � 2007 John Wiley & Sons, Ltd.
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of sediment deposition during the Holocene. Interaction oflineaments having these orientations produced many subsidingareas that still act as depositional loci during seasonal floods.The complex interaction of tectonic lineaments recorded in thestudy area culminated in detachment of Marajo Island frommainland. Although dating of individual events is not available,radiocarbon ages reveal that this tectono-sedimentary historytook place mostly during the last 40 000 yr BP.
The proposed Quaternary tectonic influence in Marajo Islandis consistent with the seismographic record of northern Brazil,which reveals earthquakes as large as magnitude 4.8 innortheastern Marajo Island (Miotto, 1993). This seismic zone isnot isolated in Brazilian Amazonia, where seven others arepresent, and where earthquakes as large as magnitude 6.0 havebeen recorded (Miotto, 1993).
Therefore, the present work led to the conclusion that faultsdeveloped in association with rift basins might remain active inpassive margins, imposing important control on developmentof depositional systems. In the particular case of the EquatorialBrazilian margin, this process imposed a significant control ondevelopment of both modern and palimpsest drainage systems,which can be analysed as a tool for unravelling tectonicpatterns in sedimentary basins.
Acknowledgements This work was funded by FAPESP (project no.004/15518-6). The three first authors are scholarship holders of CNPq.The Mayor Assistant of Santa Cruz do Arari, Mr Leonardo, is acknowl-edged for providing logistic support during the fieldwork.
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Q5: Author: Sense? ‘changes in stress states directly influence the value of the pore pressure withinthe sediment, and a range of structural features (folds, faults, fractures) may be produced’?
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