Seismites in continental sand sea deposits of the Late Cretaceous Caiuá Desert, Bauru Basin, Brazil

99 (2007) 51–64www.elsevier.com/locate/sedgeo

Sedimentary Geology 1

Seismites in continental sand sea deposits of the Late CretaceousCaiuá Desert, Bauru Basin, Brazil

Luiz Alberto Fernandes a,⁎, Alice Bonatto de Castro b, Giorgio Basilici c

a Universidade Federal do Paraná- UFPR, Setor de Ciências da Terra- Depto. de Geologia,Centro Politécnico – Jardim das Américas, Mail box 19 001, 81531-990 Curitiba Paraná, Brazil

b Petróleo Brasileiro S.A. — PETROBRAS, Brazilc Universidade Estadual de Campinas – Unicamp, Instituto de Geociências, Brazil

Abstract

Two large-scale sediment deformation structures, minor fold occurrences in cross-bedded sand dune deposits and complexconvolute folds, are observed in red sandstones, in a zone about 1.5 km long in floodway cuts at the Sérgio Motta/Porto Primaveradam, São Paulo state, Brazil. The most important structures are confined to planar zones, up to 10 m thick, in undeformed duneforeset strata were they can be traced laterally for about 50–60 m. The sandstones are part of the Rio Paraná Formation, CaiuáGroup, which accumulated in a great sand sea of about 100,000 km2. The Caiuá Desert developed during the Late Cretaceous inthe southern part of the Bauru Basin, an intracontinental subsiding area in the central-southern part of the South-AmericanPlatform. The basin was filled by a sandy sequence about 300 m thick. The sand sea deposits correspond to the Caiuá Group andcomprise: a) deposits of dry sand sheets (Santo Anastácio Formation), b) deposits of medium-sized dunes and humid interdunes ofthe sand sea peripheral zones (Goio Erê Formation), and c) deposits of large-sized complex aeolian dunes and draas, thatcorrespond to the central part of the inland sand sea (Rio Paraná Formation). The deformations in the sediments are attributed to theeffects of fluidization, liquefaction and shear stress, which are interpreted as being earthquake-induced structures, mainly because:(1) the deformed horizons are confined between undeformed cross-bedded strata, (2) the complex convolute folds sometimesinclude nappe-like structures that overlie foreset facies, (3) during the Bauru Basin infilling there was tectonic activity associatedwith alkaline volcanism on the borders of the basin and related silicification in the central-southern part. The main silicificationzones are aligned to regional lineaments that cross the area near the large-scale sedimentary deformation structures.© 2006 Elsevier B.V. All rights reserved.

Keywords: Seismites; Earthquake; Aeolian; Cretaceous; Caiuá Group; Brazil

1. Introduction

The term seismitewas proposed by Seilacher (1969) torefer to in situ shock deformation of sediments and

⁎ Corresponding author. Tel.: +55 41 33613235, +55 41 33613386;fax: +55 41 33613642.

E-mail addresses: [email protected],[email protected] (L.A. Fernandes), [email protected](A.B. de Castro), [email protected] (G. Basilici).

0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2005.12.030

excludes the products of seismic-induced phenomena.According to the author, vibratory ground motions lead toliquefaction and fluidization of unconsolidated sedimentsprior to or soon after burial, causing penecontempora-neous deformation. For example, tsunamite, seismically-induced slides and turbidites lose their signature of seismicor impact origin in the depositional phase (Seilacher,1991). Vittori et al. (1991) suggested that the mostcommon processes associated with seismic shocks are: 1)liquefaction; 2) compaction of unconsolidated granular

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Fig. 1. Lithostratigraphic map of the Bauru Basin eastern part and seismites occurrences (after Fernandes, 2004).

52 L.A. Fernandes et al. / Sedimentary Geology 199 (2007) 51–64

sediment; and 3) mass movements (rock fragmentaccumulations of avalanches at the foot of mountains,debris and mud flows, sub-aqueous slumps etc.).

Horowitz (1982) described exposures of deformedcross-beds in aeolian dunes in the Navajo Sandstone(Jurassic), Utah State, USA, which he interpreted as adirect result of paleoseismic activity. According to Sims(1975), the correlation of deformational structuresshould be based on factors such as: (1) proximity to aseismic zone; (2) presence of a potentially liquefiablesediments; (3) small-scale structures within deformedzones that suggest liquefaction; (4) structures restricted

to a single stratigraphic horizon; (5) zones of deforma-tional structures correlatable over large areas; and (6)absence of detectable influence by other deformationalmechanisms.

Possible evidence of paleoseismic activity in Brazi-lian sedimentary basins includes: 1. Convolute lamina-tion in Cretaceous aeolian sandstones (Pflug, 1961;Coimbra et al., 1992); 2. Lower Cretaceous turbidite“megabeds” (Della Fávera, 1984); 3. Convolute lami-nation, dish structure, load structure, flame structure,pseudo-nodules, pillar, ball and pillow, slide and slump,overfold cross-bedding, and sand dikes, in Lower

53L.A. Fernandes et al. / Sedimentary Geology 199 (2007) 51–64

Cretaceous continental lacustrine deposits (Raja Gaba-glia, 1992); 4. Clastic dikes in Permian coastal shallowplatform deposits (Chamani et al., 1992; Riccominiet al., 1992); 5. Clastic dikes, “sills”, and boudinagebeds, in Mesoproterozoic rocks (Schöll and Walde,1980); 6. Horizons of deformational structures, dish-and-pillar-shaped, between thinly inter-bedded siltstone,with shale and very fine sandstone in Permian units(Fernandes and Coimbra, 1993). Suguio and Barcelos(1983) described boudinage beds in the Santo AnastácioRiver valley (São Paulo State), in the same region as thestructures here described.

In this paper we describe two large-scale structures,and a few minor ones, observed in exposures of aeoliandune deposits located at Sérgio Motta or PortoPrimavera hydroelectric construction site, in the RioParaná Valley, São Paulo State, southwest of Brazil.They occur in the deposits of the Late Cretaceous CaiuáDesert, mainly in the Rio Paraná Formation. In this area,the Rio Paraná Formation consists of very well sorted,fine to medium red sandstones, with large-scale cross-bedding. The large-scale convolute folds within thesestructures are oriented nearly parallel to the cross-beddips. The deformation of the aeolian strata is hereinterpreted as an earthquake-induced phenomena,suggesting that they should be interpreted as seismites.

Fig. 2. Stratigraphy of the Bauru Basin (af

2. Geological setting

2.1. The Bauru Basin and the Caiuá Desert

The Bauru Basin is a Coniacian–Maastrichtian (LateCretaceous) continental basin developed as a result ofsubsidence within the central-southern part of the South-American Platform (Fernandes, 2004), following thebreak up and separation of Gondwana. The depressionwas created in part as an isostatic response to theaccumulation of almost 2000 m of the Early Cretaceousbasaltic lavas of the Serra Geral Formation. The basinhas an area of about 370,000 km2 (Fig. 1; CPRM, 2004)and was filled with sandy strata that have a preservedmaximum thickness of almost 300 m. The basinsubstratum consists of volcanic rocks (mainly basalts)of the Serra Geral Formation, from which the sedimen-tary pile is separated by an erosive regional surface(non-conformity). Its upper erosive limit is demarcatedby the Sul-Americana (King, 1956) or Japi (Almeida,1964) Surface, and by its posterior dissection. Accord-ing to Fernandes and Coimbra (2000a), the siliciclasticsequence is composed of two partially contemporaneousgroups (Fig. 2). These are the Caiuá (Rio Paraná, GoioErê and Santo Anastácio Formations) and the BauruGroup (Uberaba, Vale do Rio do Peixe, Araçatuba, São

ter Fernandes and Coimbra, 2000a).


José do Rio Preto, Presidente Prudente, and MaríliaFormations, including the Taiúva Analcimites).

Within the southern part of the basin a sand sea (Fig. 3)of about 100,000 km2 was developed, under hot desertconditions. This was named the Caiuá Desert byFernandes and Coimbra (2000b). The desert sand stratainclude: a) deposits of dry sand sheets (Santo AnastácioFormation); b) deposits of medium size dunes and humidinterdunes in the sand sea peripheral zones (Goio ErêFormation); and c) deposits of complex large aeoliandunes and draas, corresponding to the central part of theinland sand sea (Rio Paraná Formation). These formationstogether form the Caiuá Group (Fig. 4).

The Rio Paraná Formation is composed of fine tomedium-grained quartz sandstones. It is characterized bymedium to large-scale tabular cross-bedding. The GoioErê Formation consists of fine to medium-grained quartzsandstones, with medium to small-scale trough cross-bedding, with medium to low dips. It contains numerouscalcite-cemented horizons, which may be associatedwith centimetric calcite nodules, here interpreted ascalcretes. The Santo Anastácio Formation is composedpredominantly of fine to very fine-grained quartzsandstones with medium to small-scale cross-bedding,and secondarily metre-scale silty massive tabular strata,

Fig. 3. The main regional tectonic structures a

occasional and poorly defined stratification, with low dipplane-parallel or cross-bedding stratification.

Regional paleocurrent analyses of cross-beddingregional data indicate a remarkable stability of SWaeolian transport direction (Fernandes and Coimbra,1999; Fig. 5), that is consistent with paleowinddirections observed in dune structures associated withthe seismites described bellow.

Only icnofossil marks have been found within theRio Paraná Formation (Leonardi, 1989; Fernandes et al.,2003). They indicate that some humidity was present inorder to preserve these traces.

Regional study of thin-section fabrics indicate thatthe Coniacian–Maastrichtian sequence only reached theinitial stages of mesodiagenesis (Fernandes et al., 1994).The main diagenetic transformations included mechan-ical compaction; incipient chemical compaction andpressure solution of unstable mineral grains, claymineral authigenesis, and poikilotopic carbonate ce-mentation of pedogenic origin (calcrete).

2.2. The Rio Paraná Formation

In the area with large deformation structures (Fig. 6)the sandstones of the Rio Paraná Formation overlie Serra

nd cretaceous alkaline rock occurrences.

Fig. 4. Caiuá Group type-sections: A) Rio Paraná Formation, located in top floodway of the Eng. Sérgio Motta Hydroelectric plant (also known asPorto Primavera; see “1” on Fig. 6), Pontal do Paranapanema region (São Paulo State); B) Goio Erê Formation, BR 272 Road, km 58.7; (Paraná State;see “12” on Fig. 6); C) Santo Anastácio Formation (after Suguio and Barcelos, 1983), SP 158 Road, 2 km from the Santo Anastácio River, towardMarabá Paulista town (São Paulo State; see “8” on Fig. 6).


Geral Formation basalts, at altitudes between 225 and230 m. However, mainly due to the effects of reactivationof the Ponta Grossa Arch (Ferreira et al., 1981), a regionaltectonic high that coincides with the south-southwestBauru Basin limit, the top of the basalt substratum canreach 525 m, on the southwest basin border.

In the Porto Primavera floodway area, the Rio ParanáFormation is 45 m thick. In the central part of the RioParaná Formation is almost 300 m thick, based on drill-core (277 m; Bigarella and Mazuchowski, 1985).Continuous exposures of silicified sandstones are rareat the surface, except in the Três Irmãos and Diabo Hills,which are about 200 m high.

The Rio Paraná Formation consists of a fine tomedium-grained very-well-sorted quartz sandstones.These are reddish or purple brown in color (red beds,Fig. 7), characteristically with medium to large-scaletabular cross-bedding (Fig. 8). They include anassociation of two main lithofacies (Fig. 9): 1) Df(dune and draas foreset deposits), that is predominant,and 2) Di (interdune deposits). The best exposures aresituated along the left bank of the Paraná River,between Porto São José and Porto Rico towns, along

near Porto Camargo town (Paraná State), and in the cutsof the Porto Primavera hydroelectric plant floodway(Fig. 8).

The stratification and laminae reflect deposition bygrain fall processes. Lenticular beds related to grain floware presented locally mainly in the basal parts of duneforeset. In some places centimetric-thick intervals withlandslide deformational features or collapse breccias arefound between undeformed strata (Fig. 10). Curled mudflakes are rare, probably due to the scarcity of water andmud in the inner desert environment. Adhesion ripplesare present on some low-dip laminated facies.

In the basal part of the Rio Paraná Formation,immediately above the contact with the Serra GeralFormation basalts, there occurs a bed of pseudo-breccia0.25 to 1.3 m thick, supported by muddy sand matrix.Centimetre-scale angular fragments of basalt and/orsmectite and calcium carbonate nodules form the brecciaframework. Massive and poorly-sorted muddy sand-stone frequently overlies the pseudo-breccia bed. Boththe breccia and the muddy sandstone are commonlycemented by calcium carbonate. These basal beds areinterpreted as paleosol horizons developed by weak

Fig. 5. Paleocurrents in the eastern Bauru Basin. Analyses of cross-stratification data indicate a consistent transport direction toward the SW.


chemical weathering in a semi-arid environment, asmainly indicated by the textural immaturity, claymineral type and presence of calcrete.

3. TheRio ParanáFormation large-scale deformationstructures

3.1. The Porto Primavera sections

Large cuts in aeolian red sandstones were openedduring the building of the Eng. Sérgio Motta hydro-electric plant, also known as Porto Primavera, on the leftbank of the Paraná River. The area is situated on the SãoPaulo and Mato Grosso do Sul states boundary. Thewalls of the floodway are about 1.5 km in length, and areup to about 30 m high. Unfortunately, most of these cutswere covered by concrete or are under the hydroelectricplant.

Two remarkable zones exposing large-scale defor-mational features in cross-bedded sandstone dunedeposits of the Rio Paraná Formation were found onthe walls of the cuts (Fernandes and Coimbra, 2000a). Inthe floodway area the sandstones are up to 45 m thick,and are composed of fine to medium-grained very-well-sorted quartz sandstone, with medium to large-scaletabular cross-bedding (Fig. 8).

Section A is exposed along the left wall of the flood-way (Fig. 11). The exposure is about 13 m high, situatedbetween the 23 and 28 building marks, at an altitude of250 m. The deformation zone is restricted to a planarelement dipping to SW, parallel to the main beddingplanes that bound cross-bedding sets. It is up to 10 mthick and is exposed laterally for about 60 m. Relativelycomplex recumbent, convolute folds and pillar-likefeatures characterize it. The upper boundary of thedeformed zone is a planar surface parallel to the foresetlaminae. The lower part of the deformed zone shows agradual decrease in deformation towards the base, androughly parallels the main bedding surface which mayhave acted as a slide surface. A horizontal surfacetruncates all sections as result of a deflationary episodethat followed deformation and removed part of the upperbeds. The deformation complexity and stretching featuresincrease upslope towards the NE (proximal) suggestingthat the movement was toward SW.

Section B is situated between the 43 and 47 buildingmarks, in the right wall of the floodway, also at an altitudeof 250 m (Fig. 12). The deformation zone has a lenticularshape and cuts the cross-bedding sets. It is up to 10 mthick and can be traced laterally for about 50 m. It isconfined between undeformed foreset units, and is char-acterized by very complex recumbent folds, and drag fold.

Fig. 6. Location map of the Bauru Basin seismite occurrences and main regional tectonic structures.


As in the section A, the deformation complexityincreases to NE. It was interpreted as the proximal partof the movement, which probably has been toward SW.Convolute and drag folds climb foreset stratification.The lower limit of the deformed zone is abrupt and mostlikely actuated as a thrust ramp. The drag-fold foresetsare situated directly above the slide surface.

3.2. Other seismites occurrences

Other minor deformed zones are confined betweenundeformed cross-bedding sets in Rio Paraná Formationsandstones in the region (Fig. 6). Typically they show

open to closed parallel folds of metre-scale as seen inexposures situated in road cuts near Umuarama town(Figs. 6 and 13), in the river banks southeast of Iguaraçutown, and the left bank cuts of the Paraná River. Locally,axial planes parallel to fractures were observed,suggesting that they are post-sedimentary disturbances,not deformations attributed to landslides on duneforesets.

A small clastic dike, about 70 cm long and 5 cmwide, was observed in the SP 563 and SP 294 road cuts,in the Santo Anastácio Formation (Fig. 6). In the sameunit, Suguio and Barcelos (1983) described a boudinagezone restricted between undeformed sandstone beds in

Fig. 7. Rio Paraná Formation lithofacies: a) well-sorted fine sandstone from the middle part of foreset facies (Df), showing typical large-scale cross-bedding, and b) similar laminated sandstone, from the basal part of a large-scale foreset. Drilling core is from holes near the Porto Primaverahydroelectric plant building.


the Santo Anastácio River valley, which they suggestedwas the result of seismic-induced processes.

4. Discussion

4.1. Genetic mechanisms

Liquefaction is a process that causes in situ disruptionin the system ofmutual support between grains, usually asa consequence of either seismic shock or the build-up byothermeans of high pore-fluid pressures, with a result thatshear strength is lowered or reduced to zero. Liquefactionusually occurs in sediments that are superficial, cohe-sionless, fine-grained, and loosely packed (Allen, 1984,1986) and has been considered as induced by seismicactivity (Maltman, 1994; Collinson, 1994). If this

liquefaction is vigorous enough the moving fluid maycarry and support particles within it, a process known asfluidization. Many authors consider that in order togenerate structures similar to those described above, thesand must be unconsolidated and water-saturated duringdeformation. However, there is no clear consensus on themechanisms involved. Fluidization, as a result of sedi-ment-dewatering is, necessarily, short-lived and usuallyassociated with seismic activity (Collinson, 1994).

Horowitz (1982) described similar structures fromthe Jurassic Aztec and Navajo Sandstones in the USA,and divided the deformed zone into three segments: (1) ahead portion marked by collapse features and smallantithetic faults; (2) a middle portion marked by a thrustramp or a large recumbent fold; and (3) a toe portionmarked by a planar shear zone with small recumbent

Fig. 8. Exposure of the Rio Paraná Formation in the righthand wall of the floodway of the Porto Primavera Hydroeletric plant. Large-scale planarcross-stratification commonly has tangential contacts at the set base. Crescent dune foresets facies deposits (Df). Note the minor horizontal beds ofmassive interdune deposits (Di). The outcrop is parallel to the dominant paleowind SW direction.


folds, drag folds, and possibly small horizontal shearfaults, that pass laterally into undeformed portions.Allen and Banks (1972) suggested that recumbent-folded and deformed cross-bedding in sandy sedimentshas probably been produced when a sand dune wassuddenly liquefied during an earthquake. Horowitz(1982) suggested that the deformation might be causedby earthquake-induced liquefaction and collapse of thedune, in areas that are most susceptible to liquefaction(situated below the water table, in the interdunal zone,where overburden stress is least), with compensatorylateral squeezing of liquefied sand in response to theunequal surface load, induced by dune topography.

The stratigraphically restricted occurrence of structuressuch as convolute bedding, sandstone dykes, and dish andpillar structures, in combination with the large areal extentof the deformed layers are considered by many authors(e.g. Seilacher, 1969; Sims, 1975; Bhattacharya andBandyopadhyay, 1998) as unmistakable evidences of aseismic origin. Some of the largest examples of convolutebedding occur in sandy deposits of aeolian dunes thatwere liquefied beneath the water table following burial;and those examples may involve sedimentary units up toseveral tens of meters thick (Collinson, 1994), like thoseobserved in the Rio Paraná Formation.

4.2. The Bauru Basin seismites

The large-scale deformation structures at PortoPrimavera correspond to the middle and the distal

portions of the Horowitz (1982) model. According tothis model the liquefied lenses straddle two cross-bedsets but were restricted in the upper set which was moreresistant to liquefaction. During the initial collapse,shearing due to lateral compression of sand occurs in thelower part of the liquefied lens, partially destroying theoriginal lamination. Continued subsidence of thecollapsed block forced out liquefied sand and water,which were squeezed over unliquefied segments alongthe base of the upper cross-bed set, forming a nappe-likestructure. The upper part of the disturbed sand lens waslater removed during a deflationary period. Theoccurrence of liquefaction and concomitant fluidizationbelow a deflationary zone and the absence of deformedhorizons above and underneath those structuresstrengthens the hypothesis of a seismic origin for theconvolutions (Cojan and Thiry, 1992).

The other minor deformation structures found in theregion were probably caused by the same geneticprocesses in the same depositional setting. It is proposedthat all of these structures were generated by earth-quake-induced liquefaction and collapse of foresetelements within aeolian dune deposits. The deformedzones correspond to thrust ramp and large recumbentfolds, which can pass laterally into a planar shear zonewith small recumbent and drag folds.

It is known that aeolian sand tends to be very wellsorted, and because of it, the deposits have a high initialporosity. According to Collinson (1994), these proper-ties make them susceptible to loss of strength when

Fig. 10. Intraformational breccia of small-scale deformation structurescreated by collapse on foreset dune facies. Rio Paraná Formation.

Fig. 9. Detail of main lithofacies in the Rio Paraná Formation aeoliansandstones: large-scale planar cross-bedding of crescent dune foresetsfacies deposits (Df), and minor horizontal beds of bad structured tomassive interdune deposits (Di).


waterlogged, so that liquefaction may be triggered by ashift in the water table, by seismic shock or by rapidsediment loading.

Liquefaction may also force the sand to intrude into acohesive host, forming sand dykes. These features arefound in a wide range of sedimentary environments, butthey are more common in marine environments (Jollyand Lonergan, 2002). A sand dyke was observed in thesand sheet deposits of the Santo Anastácio Formation(see 8 in Fig. 6).

The large-scale sedimentary deformation structuresdescribed here occur in ancient cross-bedded sand dunesdeposited in the central part of the Caiuá Desert, underarid to semi-arid conditions. The sand deposits aremainly characterized by dune foresets facies, with minorinter-bedded horizontal tabular beds that representhumid interdune deposits (Rio Paraná Formation), andmassive beds with nodular calcretes (Goio Erê Forma-tion). Sedimentary records of water action are not verycommon. However rare evidence includes minor mud

cracks and adhesion ripples horizons. Fernandes et al.(2003) described the tracks of Theropod dinosaurs andsmall mammals. The dinosaur tracks are additionalevidence of life and water availability in the desertregion, necessary for the maintenance of a relativelyshallow water table that is required in the Horowitz(1982) and Collinson (1994) models.

The distribution of larger-scale seismites appears tobe directly related to the local tectonic setting. Theprincipal sites are along a distinct lineation whichlocally controls the course of the Paraná River. Crosssections perpendicular to the Paraná River (Fig. 14) areapproximately perpendicular to NE lineaments like theRio Paraná Alignment, which is part of the Loanda–Presidente Epitácio Fault Zone (Marques et al., 1993),that apparently controls long segments of the Rio ParanáRiver.

There the basalt substratum is intensely fractured andfilled by palygorskite-rich material. This rock with ananastomosed framework of veins was informally named“low density basalt” by the companies involved in theconstruction of the hydroelectric plant (e.g. Marqueset al., 1987). These “basalts” occur in pockets restrictedto fractured zones above the contact with the Rio Paranásandstones, very near to the occurrences of seismites.The rock consists of 70–95% smectite and palygorskite.Hydrothermal activity is indicated by the presence ofcarbonate–fluorapatite cement in sandstones in twosubsurface core samples of the basal Rio ParanáFormation, in the Sérgio Motta plant region. Thefracturings and infillings may correspond to hydrother-mal activity related to tectonic events, probablypenecontemporaneous with the final infilling of theBauru Basin. Another possibility is that the palygorskiteis related to weathering of fractured bedrock by alkaline

Fig. 11. Porto Primavera section A, showing complex recumbent to convolute large folds and like pillar and flame features (as can be observed in thephoto c area). Deformation zone is within a planar confined element dipping parallel to the main bedding planes bounding cross-bedding, located inthe left wall of floodway, between the 23 and 28 building marks, at 250 m altitude. The deformation complexity and stretching features increase to NE(proximal) suggesting that the movement was toward SW.


to hyperalkaline ground water. In either case there is aclear argument for fracture control of the lineation.

Many occurrences of silicified sandstones have beenfound in the Caiuá and Bauru Groups (Fernandes et al.,

Fig. 12. Porto Primavera section B. Complex recumbent and drag folds insituated between 43 and 47 building marks, in the right wall of the floodway,NE, indicating that this was probably the proximal part of the structure. Convoof the deformed zone is abrupt and probably actuated as a thrust ramp.

1993, 2002), within the northwest part of the ParanáState and the extreme west of São Paulo State. Thesilicification occurs as pore linings, sometimes partiallyor totally filling the intergranular space. It appears to be

lenticular zone that cuts across the cross-bedding sets. The section isat an altitude of 250 m. The complexity of deformation increases to thelute and drag folds climb the foreset stratification. The lower boundary

Fig. 14. Cross-section perpendicular to the Paraná River at the Sérgio Motta Hand contact with underlying basalt. Note that there is about 12 m of verticMaranesi et al., 1983).

Fig. 13. Metre-scale convolute folds confined between undeformedcross-bedding sets in sandstones of the Rio Paraná Formation. Stateroad cut near to Umuarama town, Paraná State.


restricted to points aligned along known regionaltectonic lineaments. Another interesting feature is thatsilicification processes appear to have developed in aframework without evidence of compaction, suggestingthat it occurred before significant burial of sediments.Geological distribution and petrographic analysis indi-cate a hydrothermal origin for the silicification. It islikely that silicification could have occurred during ahydrothermal event associated with Late Cretaceousalkaline magmatism. As the silicification affects almostall of the sedimentary strata within the Rio ParanáFormation, as observed in the Diabo and Três Irmãoshills, it probably was developed at the end ofsedimentation in the Bauru Basin. There is topographicevidence that the tectonism occurred during theCenozoic (Fernandes et al., 1993).

The known centers of alkaline Late Cretaceousevents are aligned and appear to be associated withregional highs along the basin margins. The neighboringeast marginal Santos Basin has Cenomanian to Maas-trichtian conglomerates and red sandstones facies withinthe Santos Formation (Pereira and Feijó, 1994) whichmay have developed in alluvial fans related to tectonicactivity, contemporaneous with the Bauru Basinsedimentation.

Studies of volcanic activity within adjacent marginalBrazilian basins have produced interesting results. Forexample, Santonian volcanic ash beds found in theCampos Basin are the same age as ash beds in the Santos

ydroelectric plant, showing the basal limit of the Rio Paraná Formational displacement of the basal contact across the fractured zone. (After


and Espírito Santo basins (Alves et al., 1994). It is likelythat they correspond to a period of intense uplift ofmarginal source areas, the highlands that separated thesecontemporaneous neighboring basins, recorded in theBauru Basin deposits by seismites.

The style of the deformation structures can be relatedto the intensity of earthquakes (Mohindra and Bagati,1996). Kuribayashi and Tatsuoka (1975 apud Jolly andLonergan, 2002), and Yould (1979 apud Allen, 1986)maintain that the formation of convolute structures byseismic activity can be related to events with amagnitudeof 5 to 6 or greater (Seed, 1968, in Raja Gabaglia, 1992),in close proximity to the active fault. Kuribayashi andTatsuoka, (1975 in Jolly and Lonergan, 2002) alsoconcluded that an earthquake of a magnitude less than 5may cause little or no liquefaction, whereas magnitude 6earthquakesmay cause liquefactionwithin a 4 km radius.We therefore conclude that the seismic events thatgenerated the deformation structures on the Bauru Basindescribed here reached at least magnitude 6.

5. Conclusions

It is known that most of the synsedimentary structuresdescribed in this paper may also develop in places withoutsignificant seismic activity. However, when structuressuch as sandstone dykes, convolute folds and dish-and-pillar structures, which record liquefaction and fluidiza-tion of unconsolidated sediments, are associated withlocal or regional faults and alignments, it is very probablethat they record the impact of a high-energy event such asan earthquake.

The large-scale deformed strata described here satisfythe criteria needed to link these deformational structureswith seismic events: (1) the deformed horizons arecontinuous and restricted between undeformed cross-bedded strata, (2) in the Porto Primavera section B theconvolute/recumbent folds are nappe-like structures thatoverlie foreset facies, and have an opposite polarity tothat expected from gravity-inducedmassmovements, (3)the deformational structures are similar and occur at thesame stratigraphic level, probably caused by the sameseismic shock, (4) they resemble the middle and distalparts of a seismically deformed zone as described byHorowitz (1982), and are attributed to processes ofearthquake-induced liquefaction of a sand lens situatedbeneath an interdunal area, (5) the large-scale sedimen-tary deformation structures straddle an important zone ofregional tectonic lineaments (Ponta Grossa Arch NWand Paraná River NE lineaments), and provide evidencethat this zone was active in the Late Cretaceous as well inthe Cenozoic Era.

Acknowledgements

We thank the researcher's team of the Instituto dePesquisas Tecnológicas do Estado de São Paulo (IPT)for the technical support, with special thanks to theGeologist Carlos Alberto de Oliveira. We thank theCompanhia Energética de São Paulo (CESP) for thelogistical support during the research, and also thank tothe Fundação de Amparo à Pesquisa do Estado de SãoPaulo (FAPESP) for the financial support. Finally, wealso wish to thank Dr. Darrel G. F. Long, of Departmentof Earth Sciences Laurentian University, Canada, for thecareful revision and critical observations, and to theeditors Françoise Bourrouilh-Le Jan, Christian Beck andDonn Gorsline.

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Documents

Seismites in continental sand sea deposits of the Late Cretaceous Caiuá Desert, Bauru Basin, Brazil