Pliocene-Quaternary Fault Control of Sedimentation and Coastal Plain Morphology in NE Brazil

Journal of South American Earth Sciences 14 :2001) 61±75 www.elsevier.nl/locate/jsames Pliocene-Quaternary fault control of sedimentation and coastal plain morphology in NE Brazil

Francisco H.R. Bezerraa,*, Venerando E. Amaroa, Claudio Vita-Finzib, Allaoua Saadic

aDepartamento de Geologia, CCET, Universidad Federal do Rio Grande do Norte, Campus UniversitaÂrio, Natal RN, 59072-970, Brazil bDepartment of Geological Sciences, University College, Gower Street, London WC1E 6BT, UK cDepartamento de Geogra®a, Instituto de GeocieÃncias, Universidad Federal de Minas Gerais, Av. AntoÃnio Carlos 6627, Belo Horizonte MG, 31270-901, Brazil Accepted 18 January 2001

Abstract Although the last major tectonic event in the Brazilian passive margin was the South America±Africa breakup during the Mesozoic, there is pervasive evidence in northeastern Brazil for pronounced faulting since the late Tertiary. The faulting was partitioned between strike-slip and normal-slip and it reactivated Precambrian shear zones as well as generating new structures. A 040±0608 -trending fault set and a 300±3208 -trending set have strongly in¯uenced both the deposition of alluvial and aeolian sediments and coastal evolution. Vertical throws have attained 260 m since the Pliocene, and topographic breaks resulting from cumulative late Tertiary to Quaternary slip have attained 30±40 m. Fault control of sediment deposition and coastal morphology may have affected the entire South American passive margin. q 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Cenozoic; Sedimentation; Coastal plain morphology; Fault control; NE Brazil

1. Introduction region and in shaping the low-elevation coastal plain is also unclear. We addressed the question by analyzing the In eastern South America and West Africa the last major littoral zone between Natal and JoaÄo Pessoa :Fig. 1) in an tectonic event took place during plate separation in the attempt to trace fault evolution since the Pliocene in this part Mesozoic. Several studies :e.g. Popoff, 1988; Chang et al., of the South American passive margin. The evidence comes 1992; Matos, 1992) have shown that the Gondwana breakup from remote sensing, borehole logs, and ®eld mapping of reworked continental shear zones under a transtensional geological and geomorphological features. regime, leading to the formation of passive margin basins in both Africa and South America. Since then, both conti- nents have enjoyed relative tectonic quiescence. Neverthe- 2. Tectonic setting and lithostratigraphic units less, recent studies in South America that have focused on ®ssion track data :e.g. Harman et al., 1998), intraplate The coastal plain is composed of Precambrian crystalline volcanism :e.g.Almeida et al., 1988), the uplift of coastal basement overlain by Cretaceous basins and a Cenozoic deposits :Martin et al., 1986; Bezerra et al., 1998), paleo- sedimentary cover :Fig. 1). The basement consists of drainage patterns :Potter, 1997), and the instrumental seis- Archean and Proterozoic fold belts composed mainly of mological record :AssumpcËaÄo, 1992; Ferreira et al., 1998) volcanic-sedimentary terrains and granite plutons deformed have shown that Precambrian shear zones and major faults by one or more orogenic cycles dating from 2.3±2.15 Ga, were reactivated and new faults generated long after the 1.90±1.95 Ga, and 650±550 Ma :Jardim-de-SaÂ, 1994). The breakup. Even so, it is not clear whether northeastern Brazil basement orogenies indicate strain partitioning between :Fig. 1), the last part of the South American plate to be domains of folding and strike-slip or oblique-slip mylonitic separated from the African plate :FrancËolin and Szatmari, belts. The last of these cycles :Brasiliano/Panafrican 1987; Szatmari et al., 1987), underwent a clearly de®ned, orogeny) strongly deformed and overprinted older struc- widespread tectonic event in the Cenozoic. The role of faults tures throughout the area :Jardim-de-SaÂ, 1994; Amaro, in the deposition of Cenozoic sedimentary units in the 1998). The shear zones are generally marked by strong, pervasive foliation; present sigmoidal shapes :Fig. 1), and * Corresponding author. Tel.: 155-84-215-3831; fax: 155-84-215-3806. their deep roots are indicated by strong gravity anomalies E-mail address: [email protected] :F.H.R. Bezerra). :Jardim-de-SaÂ, 1994; Amaro, 1998).

Fig. 1. Tectonic framework of northeastern Brazil :modi®ed from Schobbenhaus et al., 1984; Matos, 1992) showing the location of study area. Insert: the South American continent.

The historical and instrumental seismic record indicates Several basins that formed during the South Atlantic an important level of seismicity in the region when opening overlie the Precambrian basement. To the north compared with other areas in Brazil :Fig. 2). It occurs within of the study area, the Potiguar basin, whose age ranges the ®rst 1±12 km of the upper crust :Ferreira et al., 1998) from Early to Late Cretaceous, lies along the east-trending and has been known since 1808, when an earthquake of coast. Its structural framework comprises half-grabens sepa- estimated magnitude mb 4:0 and intensity MM . VI rated by intrabasinal highs related to the Early Cretaceous occurred near AcËu :Ferreira and AssumpcËaÄo, 1983). Many rifting episode of the Brazilian marginal basins :Araripe and earthquake swarms, lasting for at least six months, have FeijoÂ, 1994) :Fig. 1). The stratigraphic column of the Poti- been described :e.g. Ferreira et al., 1998). The most impor- guar basin consists of continental rift sediments and marine tant of them was the JoaÄoCaÃmara earthquake swarm from post-rift sediments. Deposition was controlled by NE-trend- 1986 to 1989, when nearly 4,000 building were damaged ing structures during the rift phase in the mid-and Late and more than 40,000 events were recorded. Two main Cretaceous :Matos, 1992). To the south of the study area, events of 5.0 mb and 5.1 mb and their aftershocks were the Pernambuco-ParaõÂba basin is divided into several faulted concentrated along the 30 km long, 0408 -trending Samam- blocks :Rand and Manso, 1990). It occupies a narrow zone baia fault :Takeya et al., 1989), but no surface offset has along the coastal plain, which crops out to the south of JoaÄo been detected so far. Pessoa and near Recife :Fig. 1); it also occurs offshore :Fig. 1). F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 63

Fig. 2. Historic and instrumental seismicity and secondary ground failure :liquefaction and landslide) in northeastern Brazil :modi®ed from Ferreira, 1983; Ferreira et al., 1998; Bezerra, 1998).

Stratigraphic studies based mainly on outcrop data indicate of debate. Relative dating by palaeomagnetism and micro- that it includes a rift clastic sequence capped by marine post- pollen indicates that it dates from Miocene to Pliocene rift sediments :FeijoÂ,1994). times. A peat layer found in the Barreiras Formation at Eocene±Oligocene magmatic activity is represented by Natal, described by Salim et al. :1975), yielded angiosperm the Macau volcanism :Fig. 1) of 44.6 ^ 6.6±29.0 ^ 0.9 Ma pollen of Zonocostites ramonae. Although this species :Mizusaki, 1989) and consists of ma®c volcanic rocks such ranges form the Eocene to the Holocene, it is abundant in as basalt, basanite, and ankaratrite that occur as plugs, dikes late Miocene sedimentary rocks. More recently, a palyno- and necks :Sial, 1976; Almeida et al., 1988). The Macau logical study of the upper Barreiras Formation in the Poti- Formation is also encountered in boreholes and is indicated guar basin yielded Retisteplanocolpites gracilis, indicative by geophysical studies :e.g. Salim et al., 1975). of a Pliocene age :Lima et al., 1990). In Bahia state, The late Tertiary sedimentary rocks of northeastern 1000 km to the south of the study area, a Pliocene age Brazil formed late in the history of Atlantic opening was proposed for the base of the Barreiras Formation by and are represented mostly by a sedimentary sequence Suguio et al. :1986) on the basis of palaeomagnetic dating. called the Barreiras Formation, which crops out along The dates they obtained range from 4.5 to 5.0 Ma :early to more than 4,000 km of the Brazilian littoral zone from middle Pliocene) near the base of the deposit to 3.0±3.4 Ma the Amazon delta to the Rio de Janeiro coast :Mabesoone :late Pliocene) near the top. For the purpose of the present et al., 1972). study, the age of the top of the local Barreiras Formation is The age of the Barreiras Formation has long been a source therefore taken to be Pliocene. 64 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75

Fig. 3. Geological map of the coastal plain and adjacent crystalline basement between Natal and Canguaretama, showing the location of geological cross- sections. The city of Natal is omitted for clarity.

Quaternary sedimentary rocks overlie the Barreiras 362 of 06 August 1994) which were normalized for haze Formation and are composed of alluvial and aeolian removal according to the method of Chavez :1988). The sediments and beachrock. The deposits range in age digital images were processed on ER-Mapper 5.5 run on a from Pleistocene to Holocene :e.g.Mabesoone and unix workstation. Digital manipulation included principal Campos-e-Silva, 1972). Silva :1991) obtained an age of component 1 :PCl), using a high-pass ®lter for edge 30,190 ^ 370 yr for post-Barreiras Formation deposits enhancement after Schowengerdt :1983). Equalizing trans- t200 km northwest of the study area, but this must be forms were also applied to the above image in order to considered a minimum value. The alluvial deposits increase contrast between stratigraphic units. Further infor- include sediments deposited in ¯uvial channels, deltas, mation was extracted from the topographic maps of and estuaries :Fig. 3). They are usually oxidized owing SUDENE :scale 1:100,000) and aerial photos :1979, scale to dry conditions during deposition, and therefore 1:70,000). contain little or no organic matter outside the deltas and Borehole logs were supplied by the Water Resources estuaries. Department of the State of Rio Grande do Norte and the state Water and Sewage Company :CAERN); reinter- preted geoelectric soundings were derived from Queiroz 3. Materials and methods et al. :1985) and the Instituto de Pesquisas TecnoloÂgicas of State of SaÄo Paulo :IPT, 1982). The available bore- The images used in the current study were X-band holes and geoelectrical soundings refer mainly to the airborne SAR and digital SPOT images :path/row 731± area from Natal to Canguaretama :Fig. 3) and were F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 65

Fig. 4. Detail of the sedimentary cover showing stereonets :equal-area lower hemisphere); S1, ®rst striae generation; S2, second striae generation; s, striae; f, pole to fault. the letter of each stereonet is plotted on the top right-hand side of each net. The city of Natal is omitted for clarity.

used to map subsurface faults. Most of the data are 4. General features of the Cenozoic faults concentrated in the Cenozoic units and the top of the Cretaceous stratigraphic units, but a few bear on the crys- Two sets of shear zones occur in the area. The NE-trend- talline basement. ing set and the NW-trending set display right-lateral and 66 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75

Fig. 5. Cross-sections of the area from Natal to Canguaretama :see Fig. 3 for location). Geoelectrical data of cross-section A±A0 from Queiroz et al. :1985); cross-section H±H0 from IPT :1982); borehole logs fom CAERN and CDM-RN. left-lateral slip :Fig. 3) related to tectonic extensional and In the interior of the continent, part of this brittle reacti- transtractional deformation, respectively :Amaro, 1998). vation may predate the Cenozoic sedimentary cover, as this Mylonitic foliation dips are mainly subvertical and myloni- is con®ned to patches bounded by remobilized shear zones. tic zones display evidence of brittle reactivation provided by Along the coastal plain, however, pronounced faulting post- fault breccia, cataclasite, and pseudotachylite veins, and dates or is coeval with Cenozoic deposition. Three main they form the basement framework where Cenozoic reacti- fault sets can be recognized: a 040±0608 :NE) trending vation has taken place. set; a 300±3208 :NW) trending set; and a 350±0108 :N) F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 67 again in the Pleistocene±Holocene :Mabesoone and Lobo, 1980). Syndepositional faulting is indicated in Barreiras cliffs near BaõÂa Formosa, where layers of faulted sandstone and conglomerate underlie and overlie undeformed layers. Liquefaction features such as sand dikes and pillar structures, which point to coseismic faulting, are also associated with the faults in places :Figs. 6 and 15). In addition, repeated changes in sediment package thickness across fault zones suggest synsedimentary faulting :see Fig. 5).

5. The Graben-Horst structure of the coastal plain

Along the coastal plain, several faults form the boundary between the graben and horst that are responsible for the coastal structural framework. The Canguaretama graben is bounded by two 0608-trending faults which display strike- slip and normal movement. Strike-slip movement is suggested by the linear geometry of faults, and normal movement is indicated by the high concentration of striae at a high pitch to the fault plane :Fig. 4, stereonets f, k, l, m, n, and o) and by cross-section A±A0 :Fig. 5) :see also Bezerra and Vita-Finzi, 2000). Transtensional structures related to the normal and strike-slip component of movement are common :Figs. 6±8). The vertical offsets of the two faults that bound the Canguaretama valley range from t60 m in the southern part of the valley to t15 m in the Fig. 6. Faulted rocks indicating strike-slip and normal faulting: :a) plan northern part :Fig. 5). view of right-lateral strike-slip fault in the Barreiras Formation showing C Â Â :main shear plane), R :Riedel) and T :tension) fractures :outcrop located in The Jacu river and Guaraõra lagoon form the Guaraõra the northern border of the Goianinha valley t7 km northeast of Goianinha); valley, which is bounded by two 0508 -trending faults :b) cross-section of normal fault in the Quaternary alluvial sediment :road :Figs. 3 and 4). These faults show a vertical offset of more cut 1 km south of Goianinha). than 40 m in the Barreiras Formation :cross section B±B0 in Fig. 5). There is a scattered pattern of mesoscopic faults trending set :Figs. 3 and 4). The NE- and the NW-trending owing to the occurrence of NE-, NW-, and N-trending faults are dominant. The amount of strike-slip offset was faults, but there is a clear dominance of the normal compo- dif®cult to assess owing to the lack of markers and the nent in all three sets :Fig. 4, stereonets i and h; see also Fig. ¯at-lying nature of the sedimentary layers. Vertical 5). As in the Canguaretama graben, faults in the GuaraõÂra offsets were easy to estimate in cross sections. NE- and graben are the result of right-lateral and normal late Tertiary NW-trending faults display systematic cross-cutting to Quaternary reactivation of NE-trending Precambrain relationships, suggesting they are contemporaneous and shear zones which occur underneath these valleys and have acted as conjugate faults. Fault planes are relatively crop out to the west in the crystalline basement :Fig. 3). smooth polished surfaces but sometimes also irregular or The TrairõÂ valley occupies a NW-trending fault-bounded listric. They often display dips of more than 808.The graben limited by two 3458 -striking normal faults. The faults provide the main boundary between the Pliocene faults are parallel to NW-trending Precambrian shear Barreiras Formation and Quaternary alluvial sediments zones to the west :Fig. 3). The Barreiras Formation has a and are buried in places by Quaternary aeolian sediments thickness of up to 80 m in the downfaulted block and 20± :Figs. 3±5). 30 m in the uplifted blocks, probably owing to synsedimen- The faulting processes that have affected Cenozoic strati- tary faulting and erosion; its base is offset by up to 40 m in graphic units occurred at very shallow depths, as indicated one of the faults :cross section C±C0 in Fig. 5). In the by fault zones marked by non-cohesive fault breccia and Bon®m lake area, both NE- and NW-trending sets of faults clay gouge as well as clear evidence of surface rupture, with a normal component were mapped. The latter set is probably coseismic, such as soil disruption. Most of the more pervasive :Fig. 4, stereonets e and f). A similar pattern mesoscopic faults in outcrops of the Barreiras Formation occurs around NõÂsia Floresta lake, where there is a relatively cut across layers previously affected by the products of high concentration of NW-trending faults mainly with 3208 weathering that took place mainly during the Pliocene and strike :Fig. 4, stereonet g). 68 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75

Fig. 7. Negative ¯ower structure indicating transtensional slip affecting the Barreiras Formation. Sea cliff 1 km north of BaõÂa Formosa.

A NW-trending graben is found to the north of TrairõÂ.Itis uplifted blocks, including fault scarps :Figs. 3 and 5). bounded by two 3208 -striking faults truncated, in places, by Another small downfaulted block, where the base of the NE-trending faults. The 3208 -striking faults display vertical Barreiras Formation is offset by 20 m, also occurs to the offsets that range from 30 m :cross section D±D0, Figs. 3 northeast of Parnamirim :Fig. 3, and cross sections E±E0 and 5) to 35 m :cross section F±F0 in Fig. 5). Quaternary to and F±F0 in Fig. 5). Holocene aeolian sediments cap part of the downfaulted and The JundiaõÂ half-graben extends for at least 20 km from the shoreline to MacaõÂba and is bounded by the 0608 -trending JundiaõÂ fault located on the right margin of the JundiaõÂ river :Fig. 3). This fault, which is truncated in places by N- trending faults, cuts across the Barreiras Formation, a lime- stone and a lower sandstone of Cretaceous age, and the crystalline basement. The main topographic expression of the JundiaõÂ fault is a topographic break about 40±60 m high, which is frequently covered by Quaternary alluvial sediments. As can be seen from cross sections G±G0 and H±H0 :Fig. 5), up to 350 m of sedimentary rocks cap the crystalline basement and ®ll the JundiaõÂ half-graben, showing sharp changes across the JundiaõÂ fault. The thickness of the Barreiras Formation changes from an average of 100 m in the uplifted block to more than 250 m in the downfaulted block :cross-section H±H0), and its base displays a vertical offset of about 250 m. A less abrupt Fig. 8. Pervasive conjugate joint system related to transtensional structures. change in thickness is seen upstream, where the Barreiras Sea cliff 1 km north of BaõÂa Formosa. formation is 75 m thick in the uplifted block and 80±85 m in F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 69 brecciation of sediment and soil from an early phase of ®lling. Normal reactivation is also indicated by R and T :tension) fractures affecting the sediment-®lled faults them- selves, the granite, and its weathered layers. On the other hand, the N-trending faults moved as right-lateral strike-slip structures only, as revealed by R fractures on fault zones, although a pull-apart effect allowed sediment and soil ®lling here, too. Away from the JundiaõÂ master fault, this pattern of dominant NE-trending faults changes completely in the Jundiai half-graben footwall, as indicated by a dominant normal N-trending set which shapes the present shoreline and affects the Barreiras Formation :Fig. 4, stereonet d).

6. Fault control of alluvial and coastal morphology

The widespread faulting events that have taken place in the region since the Pliocene are responsible for the current con®guration of the coastal plain. The alternation of graben and horst produces plateaux as high as 200 m composed of Barreiras material. The Barreiras Formation has been dissected in the uplifted blocks and capped by alluvial terraces, sand dunes, or both along the downfaulted blocks, as already indicated by Lima et al. :1990). The Barreiras Formation forms cliffs up to 15 m high which rise abruptly from the foreshore zone where the horst meets the ocean :Fig. 9). One of the best examples of these plateaux is depicted in cross sections A±A0,B±B0 :Fig. 5) and in 3D view :Fig. 10), where the central blocks, which range in altitude between 2 m and 30 m asl, were downfaulted Fig. 9. Barreiras Formation cliff in :a) the horst north of the GuaraõÂra along NE-striking faults, whereas the upper blocks :foot- graben, and in :b) the horst south of the Canguaretama graben. wall) have altitudes of up to 70 m asl. Furthermore, another visible effect of the graben-horst structure is the retreat of the downfaulted block :cross section G±G0, Fig. 5), which shoreline along downfaulted blocks :Bezerra et al., 1999) can indicate either preferential uplift and erosion upstream :Fig. 3). or seaward thickness variations in the Barreiras Formation. Scarps are also associated with the uplifted and down- The JundiaõÂ fault is exposed as sediment-®lled fractures at faulted blocks. Several of the faults display evidence of least 30 m deep in several granite quarries at the boundary surface rupture such as normal fault scarps marked by between the Precambrian granite and the Cenozoic sedi- steep slopes and abrupt topographic breaks and, in places, mentary cover south of MacaõÂba. Cross-cutting relationships by triangular facets. Most of them are degraded and capped in sediment-®lled faults point to multiple faulting and ®lling by debris slopes, vegetation, and soil, indicating that they processes in the Holocene. The slickensides of the NE- have not been active recently despite their late Tertiary to trending faults contain as many as two different sets of striae Quaternary age. Free faces are relatively uncommon, and Ð namely striae at a low pitch to the fault plane :S1 in the shape of fault scarps as well as fault plane dips depicted stereonets a and b in Fig. 4) and striae at high pitch to the in cross section give a rough representation of the general fault plane :S2 in stereonet a in Fig. 4), implying strike-slip topography. The best examples of such topographic breaks and normal movement, respectively. To judge from the are seen in the Canguaretama and GuaraõÂra valleys, where Riedel :R) fractures synthetic to the main fault plane, as faults cut across the ¯at-lying beds of the Barreiras forma- well as the geometry of the releasing and restraining tion plateau :Fig. 10). bends along the fault zones, the NE-trending sediment-®lled The drainage pattern and alluvial morphology are faults ®rst moved by right-lateral strike-slip. These fault also strongly controlled by faults. The relatively dissected planes probably represent a pull-apart geometry which and rough surface of the crystalline basement displays a opened and allowed the ®rst episode of sediment and soil high-density, dendritic stream pattern, which in places is ®lling. Normal reactivation followed the ®rst movement, as de¯ected along Precambrian shear zones. On the indicated by striae S2 overprinted on striae S1, as well as sedimentary terrain, which is characterized by a smooth 70 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75

Fig. 10. Block diagram showing the topography of the GuaraõÂra and Canguaretama graben, including plateaux :horsts) and faulted-controlled valleys :graben). The area displayed is 30 km wide.

Fig. 11. Block diagram showing the topography of the Bom®m lake area. Arrows indicate main faulting directions. The area displayed is 24 km wide. F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 71

Fig. 12. SPOT principal component :PC1) image of the Bon®m lake area showing the strong structural control of lake shape and orientation by NE- and NW- trending faults. Arrows indicate the main directions of faulting. The area displayed is 48 km wide. surface, NE- and NW-trending faults form a rectangular northern uplifted block display 060- and 3208 -oriented drainage pattern associated with right-angle shaped lagoons margins, especially Bon®m lake :Figs. 3, 11 and 12), also and lakes. The margins of the NõÂsia Floresta lake, for exam- indicative of tectonic control of the morphology. In addi- ple, the northern part of the GuaraõÂra lagoon, as well as the tion, the cross-sectional geometry and surface characteris- river channels in the downfaulted block of several grabens, tics of alluvial sediments to some extent indicate that are oriented parallel to the faults and suggest active tectonic sedimentation in the JundiaõÂ half-graben is controlled by a control :Figs. 11 and 12). A further cluster of lakes in the master fault :Fig. 13): while they occupy a zone less than

Fig. 13. Block diagram showing the topography of the JundiaõÂ graben area :JR, JundiaõÂ river). The topographic break that marks the JundiaõÂ fault is indicated by arrows. The area displayed is 24 km wide. 72 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75

Fig. 14. SPOT principal component :PC1) image of the JundiaõÂ fault, which runs across the city of Natal. Large arrows indicate fault trace and small arrows indicate fault scarp. The area displayed is 16 km wide.

1 km wide on the right margin of the JundiaõÂ river, they are after which boulder falls were reported in local and national at least 5 km wide on the left margin where the presence of newspapers; and :c) the Senador Pompeu earthquake of 23 palaeomeander loops :Fig. 14) may re¯ect tilting and lateral February 1968 :MMI VII and mb 4:6; during which migration of the river channels towards the JundiaõÂ fault. blocks of crystalline rocks were dislodged from steep slopes in the epicentral area :Ferreira, 1983) :Fig. 2). Although no landslides have been identi®ed in the local 7. Secondary ground failure geological record so far, there is widespread evidence for earthquake-induced liquefaction both in the Barreiras There is evidence for secondary ground failure from both Formation and in Quaternary alluvial deposits. The earth- the historical and the geological record of northeastern quake-induced features include water-escape structures Brazil. A few valuable reports of earthquake-induced lique- such as pillars, dikes, and pockets. Liquefaction pillars faction and landslides have been recognized in the catalogue :Lowe and LoPiccolo, 1974) are the most common type. of historical seismicity by Ferreira :1983). Liquefaction They are generally marked by elongated vertical columns occurred during the Araticum earthquake swarm in March of pebbles which die out upwards or are slightly inclined and April 1969, which caused soil collapse and disrupted towards the horizontal near the top. One example occurs at soil that silted up minor streams :Fereira, 1983). Landslides the contact between the Barreiras Formation and overlying were observed during at least three events: :a) the Araticum alluvial terraces and resulted from liquefaction of both units earthquake; :b) the Pereiro earthquake of 2 January 1968, :Fig. 15). Similar structures were described by Fonseca :1996) about 200 km to the west of the study area but in the same stratigraphic and tectonic setting. Some of these features, such as ¯ame structures, load casts associated with passively deformed beds, and gravity sliding, may be interpreted as sedimentary features in the area; however, coseismic surface faulting is the most probable cause for water-escape features. The vast majority of cases occur in gravelly sediments underlain and overlain by undeformed beds. The collective occurrence of liquefaction structures in a variety of lithological, sedimentological, and topographic conditions strongly suggests a paleoseismic origin. In addition, structures Fig. 15. Liquefaction features of the study area: syndepositional faults such as sand dikes have been interpreted elsewhere as affecting the Barreiras Formation sandstone. Note that liquefaction pillars unequivocal proof of seismically induced features occur along small faults. Cliff about 1 km north of BaõÂa Formosa. :Obermeier 1996), and their spatial and stratigraphic F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 73 association with other soft-sediment structures indicates The tectonic evolution of the area since the Holocene can that they were generated by earthquakes. now be described with more precision, thanks to the radio- carbon ages obtained by Bezerra :1998), and Bezerra et al. :1998). Although there is clear continuity with earlier 8. Discussion events, the record is detailed enough to reveal the interplay between sea-level change, coastal sedimentation, and fault- Several studies have already revealed Late Cretaceous ing. The in¯uence of sea-level changes, as we have seen, to early Tertiary tectonism in the area associated with some can be divided into two major phases. kind of tectonic uplift and faulting.Cremonini :1995), for From 10,000 yr BP sea-level rose, passed its current posi- example, described two major tectonic events that affected tion about 7,000 yr BP, and reached a 2 m highstand at 5,000± the Potiguar basin and were caused by heat from the ocea- 5,500 yr BP. The second phase is characterized by a steady fall nic plate during the Late Cretaceous :Mesocampanian) :Bezerra et al., 1998). During the regressive phase, signi®cant after the formation of the Potiguar rift. The ®rst and stron- coastal sedimentation and faulting took place. Strong winds ger event was responsible for regional erosion during the removed sediments from the exposed shelf, redepositing a Mesocampanian as well as extensive uplift and reverse large quantity of sand onshore; Holocene surface fault reacti- faulting. The second event, during the Tertiary, resulted vation of several NE-and NW-trending faults, some of them in E±W folding. probably associated with liquefaction features, trapped aeolian In the Potiguar basin, Oliveira et al. :1993) described the sediments in downfaulted blocks. NW-trending Afonso Bezerra fault, which affected the The pervasive faulting has controlled sedimentation and Cretaceous JandaõÂra Formation, and they concluded that geomorphological features across the region. The Barreiras early Tertiary movement occurred along the fault because Formation usually occurs in troughs controlled by NE- and of N±S subhorizontal compression. They suggested that the NW-trending faults. This pattern has been already observed tectonism was related either to Campanian erosion or to the in the Cenozoic record of southeastern Brazil, where sedi- magmatism that generated the Macau Formation during the ments occur usually in small graben :e.g. Riccomini et al., Eocene±Oligocene. Oliveira et al. :1996) have since 1989; Saadi, 1990,1993; Saadi and Pedrosa-Soares, 1990). described two tectonic events that affected the JandaõÂra Sediment deposition in fault-controlled troughs in north- Formation immediately northwest of the study area, the eastern Brazil is also consistent with post-breakup denuda- ®rst one of ESE±WNW extension and the second a phase tion rates for the Brazilian passive margin in the SaÄo of NNW±SSE compression. Francisco Craton :t1,000 km south of the study area), Evidence of late to post-Cretaceous deformation leading which amounted to between 2 and 7 km of denudation to uplift and reverse faulting has also been observed :Harman et al., 1998). offshore. Gomes :1997) found evidence in seismic surveys Both the NE- and the NW-trending faults are compatible for inversion tectonics on the east coast of Brazil which with ongoing E±W compression :AssumpcËaÄo, 1992; Lima affected the oceanic crust and overlying Cretaceous to et al., 1997; Ferreira et al., 1998). But, as some NW-trending post-Cretaceous sediments but not pre-Miocene age volca- faults depart by more than 458 from the optimum direction nic rocks :12.3 my) in oceanic islands at low latitudes. of fault reactivation :Bezerra, 1998), their reactivation may Despite all this evidence of Late Cretaceous to early- have been favored by high-¯uid pressure and weak fault middle Tertiary deformation, there is no consensus on material :Bezerra and Vita-Finzi, 2000). how many tectonic events took place and what caused them. But it was only after the Pliocene that widespread faulting shows association with sediment deposition and 9. Conclusions coastal plain morphology and can thus be traced more easily. Several tectonic pulses favored the development The background to Pliocene-Quaternary faulting in north- of strike-slip and normal faulting, but no subsequent eastern Brazil can be summed up as follows. Northeastern great kinematic or geometric change in the fault pattern Brazil is located on the Brazilian passive margin within the has been observed. The continuity is consistent with the E± intraplate part of South America. The region displays a W-oriented maximum compression and N±S-oriented Precambrian crystalline basement deformed mainly by shear extension :AssumpcËaÄo, 1992; Lima et al., 1997; Ferreira zones, overlain by Cretaceous sedimentary basins which et al., 1998) that are implicit in the shape and direction of formed during the South America-Africa breakup and by movement of the South American plate since the Pliocene. Cenozoic stratigraphic units. Faulting started in the Pliocene, The difference in altitude between the base of the when, to judge from the geometry and orientation of the South Barreiras Formation in several plateaux inland :up to American plate, the current stress ®eld was established. Since 650 m) and in the study area :from 50 m asl to then, the region has been under E±W-oriented compression t250 m bsl) may also indicate further faulting in the and N±S-oriented extension. crystalline basement and thus displacement of crustal There are three main sets of faults in the region, trending blocks. NE, NW, and N. The maximum horizontal compression 74 F.H.R. Bezerra et al. / Journal of South American Earth Sciences 14 :2001) 61±75 deduced from focal mechanisms is oblique to the NE- and Amaro, V.E., 1998. AnaÂlise Conjunta de Dados GeoloÂgicos, GeofõÂsicos e the NW-trending faults in a compressive direction, which de Sensoriamento Remoto do Setor Extremo Nordeste da ProvIÂncia Â favors right-lateral and left-lateral strike-slip, respectively. Borborema, Nordeste do Brasil, com Enfase nas Zonas de Cisalhamento DuÂcteis NeoproterozoÂicas. Ph.D. Thesis, Universidade de SaÄo Paulo, This obliquity in most cases is about 308 for the majority of SaÄo Paulo, Brazil. the fault planes. Araripe, P.T., FeijoÂ, F.J., 1994. Bacia Potiguar. Boletim de GeocieÃncias da Although strike-slip offsets could not be quanti®ed, verti- Petrobras 8, 127±141. cal offsets indicate signi®cant movement along major faults AssumpcËaÄo, M., 1992. The regional intraplate stress ®eld in South America. since the Pliocene. The base of the Barreiras Formation was Journal of Geophysical Research 97, 11,889±11,903. displaced by as much as 260 m in the JundiaõÂ half-graben Bezerra, F.H.R., 1998. Neotectonics in Northeastern Brazil. Ph.D. Thesis, University of London, London, UK. since the Pliocene. Bezerra, F.H.R., Vita-Finzi, C., 2000. How active is a passive margin? The geological record of northeastern Brazil is character- Paleoseismicity in northeastern Brazil. Geology 28, 591±594. ized by successive phases offault reactivation. The vast major- Bezerra, F.H.R., Lima-Filho, F.P., Amaral, R.F., Caldas, L.H.O., Costa- ity of late Tertiary to Quaternary faults represents reactivation Neto, L.X., 1998. Holocene coastal tectonics in NE Brazil. Coastal of preexisting zones of weakness in basement-controlled Tectonics, Stewartt, I.S., Vita-Finzi, C. :Eds.), Geological Society of London, Special Publication 146, 279±293. structures, which are in the range of optimum orientation for Bezerra, F.H.R., Diniz, R., Accioly, P.C.V., 1999. In¯ueÃncia de falhas reactivation, suggesting this should be an important condition cenozoÂicas na sedimentacËaÄo e morfologia da faixa sedimentar costeira for deformation of previous structures in the region. But do Rio Grande do Norte. VII Congresso de AssociacËaÄo Brasileira do important exceptions have been found that cut across existing QuaternaÂrio :Porto Seguro, Salvador BA, Brazil), Anais. structures, notably the JundiaõÂ fault. Chang, H.K., Kowsmann, R.O., Figueiredo, A.M.F., Bender, A.A., 1992. Geomorphological features of tectonic origin may be Tectonics and stratigraphy of the east Brazil rift system: an overview. Tectonophysics 213, 97±138. seen along the coastal plain. Valleys ®lled by alluvial and Chavez Jr., P.S., 1988. An improved dark object subtraction technique for aeolian sediments overlie the Barreiras Formation, and atmospheirc scattering correction of multispectral data. Remote dissection of the Barreiras plateaux has given rise to horsts. Sensing of Environment 24, 459±479. Extensive bodies of aeolian sediment that penetrate as far as Cremonini, O.A., 1995. A relativacËaÄo tectoÃnica da Bacia Potiguar no CretaÂ- 15 km inland are the combined product of downfaulting, ceo Superior. VSBG Simpo Âsio Nacional de Estudos TectoÃnicos :Gramado RS, Brazil), Anais, pp. 277±280. sea-level fall, and wind directions favorable to deposition FeijoÂ, F.J., 1994. Bacia de Pernambuco-ParaõÂba. Boletim de GeocieÃncias da along the graben. Petrobras 8, 143±147. The methods used during this study, including remote Ferreira, J.M., 1983. Sismicidade no Nodeste do Brasil. M.Sc. Thesis, sensing, borehole logging, and geophysical investigation, Universidade de SaÄo Paulo, Brazil. were useful for locating and determining the geometry of Ferreira, J.M., AssumpcËaÄo, M., 1983. Sismicidade no Nordeste do Brasil. late Tertiary to Quaternary faults. Although some seismologi- Revista Brasileira de Geo®sica 1, 67±88. Ferreira, J.M., Oliveira, R.T., Takeya, M.K., AsssumpcËaÄo, M., 1998. Super- cal studies indicate that the E±W coast in northeastern Brazil is position of local and regional stresses in northeast Brazil: evidence from more seismic than other coastal areas along the Brazilian focal mechanisms around the Potiguar marginal basin. Geophysical passive margin :AssumpcËaÄo, 1992; Ferreira et al., 1998), Journal International 134, 341±355. there is no reason to believe that the late Tertiary to Quaternary Fonseca, V.P., 1996. Estudos Morfo-TectoÃnicos na AÂ rea do Baixo Curso do evolution and pervasive faulting observed here is not repeated Rio AcËu :AcËu-Macau), Rio Grande do Norte. M.Sc. Thesis, Universi- dade Federal de Minas Gerais, Belo Horizonte, Brazil. in other coastal areas on the eastern part of the South American FrancËolin, J.B.L., Szatmari, P., 1987. Mecanismo de Rifteamento da PorcËaÄo plate. But new paleoseismological analyses are required to Oriental da Margem Norte Brasileira. Revista Brasileira de GeocieÃncias address this point. 17, 196±207. Gomes, P.O., 1997. O Projeto Leplac na margem continental nordeste: imagens sõÂsmicas de ¯exura litosfeÂrica junto a zonas de fratura. V Acknowledgements Congresso Brasileiro de GeofõÂsica, :SaÄo Paulo SP, Brazil), Anais 1, 18±21. Harman, R., Gallagher, K., Brown, R., Raza, A., Bizzi, L., 1998. Acceler- Field visits were supported by a CNPq-Brazilian project ated denudation and tectonic geomorphic reactivation of the cratons of :Vales TectoÃnicos do Rio Grande do Norte) coordinated by northeastern Brazil during the Late Creataceous. Journal of Geophysi- A. Saadi. C. Vita-Finzi thanks a British Council-CAPES cal Research 103, 27,091±27,105. link for support. F.H.R. Bezerra and V.E. Amaro thank IPT Ð Instituto de Pesquisas TecnoloÂgicas do Estado de SaÄo Paulo, 1982. CAPES-Brazil for their Ph.D. studentships. 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