Evidence for the Uplift of the Volcanic Plateau

Pesquisas em Geociências ISSN 1518-2398

, 40 (3): 189-208, set./dez. 2013 Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brasil E-ISSN 1807-9806 Três Forquilhas Valley in Southern Brazil - evidence for the uplift of the volcanic plateau

Paul Edwin POTTER¹, Roberto VERDUM², Michael HOLZ3, Débora Pinto MARTINS4 & Nelson LISBOA5

1.Geology Department, University of Cincinnati. 2600 Clifton Ave. 45221, Cincinnati, Ohio, USA. E-mail: [email protected].

000, Porto Alegre, RS, Brasil. E-mail: [email protected]. 2. Departamento de Geografia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500, CEP 91.540- 020, Salvador, BA, Brasil. E-mail: . 3. Instituto de Geociências, Universidade Federal de Bahia. Rua Barão de Jeremoabo, s/n, Campus Universitário de Ondina, CEP 40.170- 000. Porto Alegre, RS, Brasil. E-mail:[email protected] [email protected]. 4. Departamento de Geografia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500, CEP 91.540- 000, Porto Alegre, RS, Brasil. E-mail: [email protected]. 5. Departamento de Geodésia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500, CEP 91.540-

www.pesquisasemgeociencias.ufrgs.br) Recebido em 07/2011. Aceito para publicação em 07/2013. Versão online publicada em 04/12/2013 (

Abstract - Areia on the seaward edge of the volcanic plateau of northeastern Rio Grande do Sul. We studied this area to ex- plain why theIn Brazilescarpment drainage of the patterns plateau and is closestslopes wereto the studied sea here in and the toValley establish of Três the Forquilhas erosional historynear Terra of this de

small part of the escarpment. Here two small rivers, Três Forquilhas and Maquiné, follow the Torres Syncline to the Atlantic Ocean. These rivers probably started to significantly erode the syncline and the volcanic plateau watershedsstarting in the much Middle as short to Late bursts Miocene of intense in response heavy rainfallto far field “pulse” Andean local tectonics erosion ofand the epirogenic escarpment uplift every across few most of South America. Subsequent erosion probably occurred in pulses rather than at an uniform rate in both

years today. Study of the offshore sequence stratigraphy of the Tertiary fill of the adjacent Pelotas Basin helped us establish this local erosional history (seven unconformity bound sequences, thickness variations between these unconformities, and large scale slumping). Study of an onshore subsurface cross section also identified totwo our paleovalleys understanding in Mesozoic of drainage rocks evolution beneath and the thepresent origin valleys of the of escarpment Três Forquilhas of the and volcanic Maquiné plateau. Rivers. There Keywordsis a strong structural control on drainage at all scales. Consideration of slope process contributed significantly- ments, drainage system. : Três Forquilhas Valley, volcanic plateau, Paraná Basin, Pelotas Basin, Torres Syncline, mass move Resumo -

localidade Valede Terra Três de Forquilhas Areia, entre noo mar Sul e do a borda Brasil nordeste - evidência do planalto do soerguimento vulcânico, no do Rio planalto Grande dovulcânico. Sul. Esta Os padrões de drenagem e a encosta foram estudados no vale do rio Três Forquilhas que é situado próximo da-

área foi estudada para explicar por que a escarpa do planalto vulcânico é mais próxima ao mar nesta sua por- ção nordeste e estabelecer a história erosional desta porção da escarpa. Aqui dois rios relativamente curtos, os rios Três Forquilhas e Maquiné, seguem a Sinclinal de Torres em direção ao Atlântico. Esses rios, provavelmen te, iniciaram a erodir significativamente a sinclinal e a borda do planalto a partir do Mioceno Médio a Superior,- em resposta à tectônica Andina de soerguimento tectônico de colisão ao longo da maior parte da América do Sul. A erosão subsequente, provavelmente, ocorreu em pulsos e não em uma fase uniforme em ambos os divi sores de águas, muito como as chuvas torrenciais atualmente, que a cada dois ou três anos “impulsionaram” a erosão local da escarpa. O estudo da sequência estratigráfica no Terciário quando da sedimentação da Bacia- de Pelotas adjacente à área de estudo, nos ajudou a estabelecer esta história erosiva local (sete discordâncias entre as sucessões de depósitos, variações de espessura entre estas discordâncias e a grande escala de es pessuras). O estudo de uma seção transversal do continente em direção ao litoral identificou dois paleovales em rochas do Mesozoico abaixo do nível de base dos atuais rios Três Forquilhas e Maquiné. Há um controle daestrutural escarpa forte do planalto da drenagem vulcânico. em todas as escalas. Essas considerações sobre os processos que se desenvolvem Palavras-chavesobre a vertente contribuem, significativamente, para a compreensão sobre a evolução da drenagem e a origem- res, movimentos de massa, sistema de drenagem. : Vale Três Forquilhas, planalto vulcânico, Bacia do Paraná, Bacia de Pelotas, Sinclinal de Tor

189 Potter et al.

1 Introduction et al., Long coastal escarpments on passive continen- Ouro, Maquiné and Arroio Teixeira quadrangles plus- - two 1:100,000 geologic quadrangles (Horn Filho phic features comparable in importance to continent- 1984a and b) someet al .,special 1998), kinds onshore of spatial core drillingimages, anden -spanning,tal margins Alpine-type are recognized mountains. as major Such global escarpments geomor offshorevironmental seismic and sectionsengineering plus reports insights (Har from Engenharia, the Neoge- ne1990; uplift Figueró history of South America. are well developed and best known mostly in southern Africa (Ollier & Marker, 1985; Partridge & Maud, 2000), western peninsular India (Widdowson, 1997), and- eastern Australia (Bishop, 1988; Ollier & Pain, 1994).- Southwestern Brazil, also a passive margin, has compa thatrable includes coastal escarpmentsthe Serra do rangingMar and from the seawardRio de Janei bor- ro to near Porto Alegre, a distance of over 1.500 km, of the drainage of this plateau is away from the coast der of the volcanic plateau of southeastern Brazil. Most aand long flows used a informalmuch longer name distance for the tolava-sandstone Buenos Aires pla via- the many tributaries of the Paraná River. “Planalto” is teau of the Paraná Basin (Ab’Saber, 1969; IBGE, 1990,- 1993; Ross, 1990). Other local names used include the: “Planalto We Meridional”, studied only “Planalto a very Basáltico” small portion and “Planal of this escarpmentto do Paraná”. where it comes closest to the South Atlan- tic Ocean in the Torres Syncline of northeastern Rio

1 and 2). We chose this area for two reasons: there is someGrande rare do Sulsubsurface and adjacent data Santahere both Catarina on and states offshore (Figs. and here the escarpment is closest to the sea, it is only wide stripe of coastal lagoons, swamps, well develo- pedseparated beaches from and the some South low Atlanticdunes. The Ocean escarpment by a 13 kmri- ses abruptly 800 m above this coastal landscape. Here two small, parallel rivers Três Forquilhas and Maquiné (Fig. 3) flow directly into the coastal 2lagoons, and that both of Ma in- GrossaFigure 1.Arch, The Sul-Rio-Grandense State of Rio Grande Shield) do Sul and on studythe passive area. margin of spectacular, short valleys,2. Both where watersheds the watershed are almost of allTrês in South America. Regional setting, major structural features (Ponta Forquilhas River covers about 380 km thequiné volcanic River 422 plateau km most of which drains inland and aslopes. small Includedarea of eastward in the study drainage are adjacentof the headwaters portions of northward and inland drainage behind the rim of the escarpmentMampituba River,is into which the Tainhas flows into River, Santa a tributary Catarina. to The the -

Antas-Taquari River which drains some 340 km sou -thwest why is into the the escarpment Jacuí River so and close the to Guaíba the sea Lagoon here, nearhow Porto Alegre. This setting suggests three questions when were they so spectacularly were the twin valleys of Três Forquilhas and Maquiné- latedRivers to localized, the time andof uplift of the volcanic plateau. Be- plateau from the shores of Quadros Lagoon. causeentrenched? both valleys The last appear of these to be questions identical istwins, directly we refo- Figure 2. Looking northeast along the dissected front of the volcanic regionalcused on geology,the valley used of Três the 1:50,000Forquilhas. topographic maps To answer these questions we turned first to of the Tainhas, Aratinga, Três Cachoeiras, Barra do190 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

al 2 - guay,., 2000; and MilaniUruguay & Zalán,and everywhere 2000). These form flows a distinctive cover an area of about 1,100,000 km in Brazil, Argentina, Para-

escarpment some(Fig. 1). of which,Locally, in in the the upper Torres part Syncline of the thesec- re appear to be between 15 to 20 m such flows in the lower half of the escarpment their composition is do- minantlytion, can bethat traced of basalt, as far but as higher10 to 15 in kmthe or section more. rhyoli In the-

et al - nite iset dominant al., 2000). and These there compositional are even some variations, glass-rich alongflows (Belliene ., 1986; CPRM & FEPAM, 1998; Mantova-

with variations in texture and fracture density, marke dly affect the weathering profile of the escarpment and highthe surface as 30 to processes 40 m or more,acting and on it. have The steep upper intervening acid flows slopes,form prominent, whereas the near lower vertical slopes cliffs developed (Fig. 6), on some basalt, as

- have fewer cliffs, gentler slopes and thicker colluvium.- geConsequently, from easily theweathered overall topographicbasalt to more profile resistant of the acid es volcanic.carpment steepens upward as its volcanic rocks chan The study area has three prominent fracture sets, the strongest generally trends N 60°E and is approxi-

Figure 3. Watersheds of Três Forquilhas and Maquiné rivers. 2 Regional setting fracturemately at systems right angles trend to N the 10 °trend-20E andof Maquiné N 40°-50 and° Três- Forquilhaset al., rivers1998). andThe N Josafaz 60°E fracture Stream system (Fig. 7). strongly Other W (Fi 2.1 Geology alsogueró may be related to the origin of the abrupt termi- influences drainage systems at virtually all scales and

nation Unconsolidated of the escarpment Quaternary above the sands, coastal silts plain and muds(Figs. the PontaThe Torres Grossa Syncline Arch to the(Fig. northeast 4 – A, B, C) and lies the about Sul-Rio half- 2 and 7). -Grandenseway between Shield two majorto the regionalsouthwest. structural Both directly features, in- base of the escarpment and the South Atlantic Ocean andform consist a narrow of a13 large km wide,Holocene coastal coastal plain sandy between barrier the the escarpment and thus affect its retreat. To the nor- theastfluence the the Ponta morphology Grossa Archof the carries valley andthe escarpmentthe location toof - redbehind terrace which deposits are several and remnantslarge shallow of coastal lakes, swamps,barriers - a few small isolated dune fields and someet low al., scatte1984a leover to the1600 southwest m above of sea the level Torres and Syncline, Proterozoic, the Sul-Rio mostly- and b). Except for a few dunes and low terraces, most -Grandensecrystalline rocks Shield extend is the inland dominant to about structural 350 km, feature whi ofat thisthe basecoastal of theplain escarpment is only a few (Horn meters Filho above sea level, and also causes the escarpment to extend inland some although the city of Terra de Areia lies on a low terrace some 10 to 15 m above sea level. The general geology of the volcanic plateau and its escarpment become gra- dually370 km. lower. As traced Above southwestward the Precambrian from basement Santa Catarina, in the sedimentsthe unconsolidated are about coastal 1000 mdeposits of Triassic is summarized and Permian by Villwock & Tomazelli (1995). Below these coastal plain Torres Syncline is a thin section of Upper Paleozoic and Mesozoic rocks, mostly sandstones, of which only the andsediments Triassic above sediment Precambrian preserved basement beneath (Aboarrage the Torres & Jurassic-Cretaceous Botucatu Sandstone, an eolianite SynclineLopes, 1998), shows fig. that 4A. a Thislow thickerhas been section in existence of Permian as a outcrops(Almeida, of 1952; the Botucatu Scherer, 2000),are near has the a fewcity outcropsof Terra dein topographic low since at least since the Triassic. Areiaand near and the none syncline. are higher In Três than Forquilhas 60 m above Valley sea all level. the The Pelotas Basin, a marginal Atlantic Basin et al -

Overlying the Botucatu are about 700 to 900 m of lava- Grande(Dias do., Sul1994; State Fontana, into Santa 1990, Catarina 1996; Cainelli State almost& Mo flows of the Jurassic-Cretaceouset al Serra Geral Formationet hriak, 1998), lies offshore and extends the length of Rio (Fig. 5), the widespread flood volcanics of the intra cra 191 tonic Paraná Basin (Bellieni ., 1986; Mantovani to Florianópolis. The Pelotas Basin is known from 17 Potter et al. wells and from extensive seismic study. It contains ce range from about 880 m in the southwest to more a Cretaceous section and thick Cenozoic section. We seathan and 1020 weathering m in the crusts.northeast Only (Fig. remnants 8). These of the surfaces plate- forfound understanding the stratigraphic nearby sequence Tertiary and erosional geologic history are defined by elevation, dissection, distance from the onshore.of the Tertiary fill of the Pelotas Basin to be essential In broad overview, the relief of Rio Grande do areau remain minor remnantsin most of of the a surfacewatershed between of Três 200 Forquilhas and 300 - m,so thatprobably most theof it Velhas is in slopes surface. (Fig. The 9). Sul In Americana addition, there sur- − Gon- face has been considered to range from Cretaceous to dwana,Sul State Post and mostGondwana, of southeastern Sul Americana Brazil hasand been Velhas as Pliocene in age and the Velhas to be Plio-Pleistocene in −signed to four major erosion surfaces or cycles et al. topographic contrast between the plateau and direct study following area the King volcanic (1953, plateau 1956), as Ab’Saber a Sul Americana (1969) sur and- Atlanticage (Brown, drainage 1971). to theEverywhere southeast in is the clear study and areagreat the as face.Justus In our study (1986). area We elevations tentatively of the recognize plateau insurfa the-

are contrasts in soils and surface processes (Table 1).

Figure 4. Cross sections based on Aborroage & Lopes (1998, fig. 96): A) Structural cross section and topography of Torres Synclinal seen from the coast; B) stratigraphic cross section using the middle of the Irati Formation as a level line; C) plateau surface (gray) with line of cross sections A and B. Note the three paleochannels below the location of Três Forquilhas River.

192 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

Figure 6. Massive thick flows in upper part of Serra Geral volcanics (A) and vertical cliffs (B).

Figure 5. Generalized stratigraphic column of Serra Ge ral Formation near Terra de Areia, Rio Grande do Sul.

Figure 7. Fractures and geomorphology in the study area. 193 Potter et al.

Figure 8. Generalized elevations of the volcanic plateau surface rise to the northeast toward the Ponta Grossa Arch.

Figure 9. Plateau surface and its dissection: A) typical topography of the volcanic plateau east of São Francisco de Paula a few kilometers- rectlynorthwest to the of Atlantic its scarp; in B)front wide of valleythe scarp. of Antas-Taquari River draining the back slope of the plateau about 18 km from its scarp (note open, shallow, swampy valley and contrast with narrow valleys flowing directly to the Atlantic Ocean); C) and D) valleys of streams flowing di

194 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

Table 1. Geomorphic summary (adapted from HAR Engenharia, 1990: tables 1.1 to 1.3, and Uberti, 1980). Morphology Soils Drainage Processes Broad, low relief, Thin to locally P gently inclined Minor mass wasting L surface between 880 strongly acid humic low gradients, 0.012 A little erosion and brownmoderately Cambisols, thick with Open networks with N wide to shallowly downon interfluves cutting near and – 1020 m with open, A entrenched valleys to 0.050 m/km, flow escarpment, where L and low slopes there is probably low base saturation; Rivers420 km to to Patos Atlantic Lagoon T commonly less than much internal arenon very plastic plastic. A horizons, Soils onlyvia Taquari 2 to 3 m and above Jacuí O 5° to 10°. Some small drainage in fracture typicallybut AB and between B horizons 2.5 sea level. irregular hills of systems. rhyodacite and 4.0 m thick. Almost total Thin soils between Mostly down cutting T dissection of plateau cliffs above 400 to 600 in upper tributary R surface by steep- valleys, but some Ê lateral planation in S commonlym, but thicker thin, soils slightly lower half of principal sided (20 to 30°) acidon colluvium incept sols below; and of steep gradient 85°)tributary on upper valleys; slopes, reddish Brunosols with Dense networks O butcliffs colluvium, common (60°2 to 8to high base saturation. S volcanicsvalley; episodic high on RF streams, 0.18 to 0.27 L slopesrock failures cause indebris acid Q below 300 to 500 m/km, flow to Atlantic O U m on thick gentle, dominates almost The A horizons are controlonly 30 atto all50 scales.km P side valley scars, and I concave upward andplastic, are butnon B plastic. horizons Soil distant; strong fracture E seemflows, to create the principal long L typicallycontain angular 3.0 to 4.5 blocks m process of slope H produce stepped retreat? A reliefslopes. on Volcanic all slopes. flows almost everywhere. S thick. Rocky substrates Well developed reddish Brunosols are V Mostly straight to half of valley largely moderatelyTwo major types: acid to B A freefloodplain of terraces in lower neutral with high base terraces suggests A L except at mouth, but saturation where as valleyLack of not widespread closely S L wellweakly developed, meandering low some debris fans and Entisols, developed on I E energyexcept justdelta above in Itapeva abandoned channels unconsolidated sand sea level changes? N Y Lagoon. in upper reaches. are acid to neutral with linked to Pleistocene low base saturation.

2.2 Climate and Vegetation Successive days of high rainfall are more dangerous - range from 91 to 206 mm for the years 1975 to 2010. high rainfall, the soil is already largely or totally satu- have Thetemperatures valley of Três betweenForquilhas 25 to River 30°C hasand a winterssubtro and damaging, however, because after the first day of betweenpical climate 5 and with 15°C. marked Average seasons; rainfall summers in the typicallyvalley is Departamentorated. For example, Nacional in 2010, de Águas324 mm e Energiain 11 days Elétrica caused - - widespread slope failures and debris flows (data from- 1800 mm (one of the wettest regions in Rio Grande do Sul). Year to year variation is great, however; for exam DNAEE and Companhia de Pesquisa de Recursos Mine mm,ple, betweenalmost three 1975 times and as2010 much. the Intense driest yearrainfalls at Terra have rais - CPRM, 1975-2010). alsode Areia been had recorded 970 mm and, of becauserainfall and of the the great wettest local 2768 re- altitudeThree and types climate of vegetationand are the are Campos recognized de Cima in theda lief, they play an important role in local geomorphic valley of Três Forquilhas. These vary with landscape, processes. It is intense single and successive day rain- - ticSerra is the (the most open widespread. grasslands ofOriginally the volcanic these plateau) two forests and the Araucária and Atlantic Forests of which the Atlan- fall; however, that saturates slopes to failure. In Três Forquilhas Valley, single day rainfall maxima (Table 2)195 covered all the valley floor and all but the steepest slo Potter et al. - almost all of the valley bottom and most of the lower pes and cliffs of Três Forquilhas Valley. Today, however Araucária Forests have dense understory of vegeta - tion that reduce runoff; these two forest types also and criptionsslopes have of beenvegetation cleared. and See its IBGE distribution (1993, p. 123-129),in coastal forestequally has have been dense cut, interpenetratinghowever, this root networks system rots of roots and, southernDomingos Brasil. (1997) and Verdum (2009) for detailed des afterthat bind a few and years, stabilize soil and soil colluvium and colluvium. are much Where less this re- Key for geomorphology is that Atlântica and sistant to creep and failure.

Table 2. Extreme Rainfall (mm) at Terra de Areia 1975 to 2010 (Data from DNAEE and CPRM, 1975-2010). Single Days Sequential Days

94 - 24/02/75 75 - 12/05/76 162 - 12, 13 & 14/12/75 84 - 21/07/80 159 - 11, 12 & 13/05/76 140 - 03/12/80 156 - 12 & 13/06/83 125 - 28/06/82 172 - 19 & 20/06/84 136 - 13/06/83 192 - 16, 17 & 18/01/87 140 - 09/08/85 198 - 13 & 14/05/94 150 - 21/02/93 130 - 05 & 06/11/99 100 - 14/05/94 124 - 29 & 30/11/02 120 - 05/11/99 108 - 22, 23 & 24/03/05 116 - 04/10/00 180 - 18, 19 & 20/11/06 103 - 29/11/02 165 - 01 to 04/03/07 103 - 04/05/04 231 - 16 to 21/03/07 108 - 20/11/06 202 - 02, 03, & 04/01/09 117 - 04/03/07 324 - 14 to 24/02/10 142 - 22/04/08 122 - 03/01/09 3 Results et al., 1999). of the Uruguay Another and example Paraná ofRivers, drainage both away of which from take the 3.1 Drainage pattern and profiles escarpmentthe “long way” rim to isthe the Atlantic northward (Araújo drainage down its - There is a close relation between drainage pat- ver. Here drainage pattern is mostly open and dendritic tern and structure in the Torres Syncline that is appa- backslope into the headwaters of the Antas-Taquari Ri - of the volcanic plateau) and gradients are low. These shallow(instead valleys of dense, on steepthe plateau and subparallel are separated on theby a front few dipsrent upat allto 15°scales along (Figs. some 4 and of the 7). straight Both the reaches Três Forqui of the- low, irregular scattered hills of acid volcanics. Although lhas and Maquiné Rivers flow south observation shows outside of the main volcanic plateau and now draining Basin. Thus it seems that many, perhaps all, of the lar- se small streams as reported elsewhere in the Paraná nearby developed along important fractures in under- directly to the Atlantic, Josafaz Stream flows northwest- ger tributaries in the watershed of Três Forquilhas and 13 km following a fracture before abruptly turning down cutting and headward stream erosion into the eastward to the Atlantic (Fig. 8). We interpret this nor plateaulying densely surface. fractured Notable bedrock; is that the this eastern greatly tributaries facilitated itsthwest capture trending by the segment Mampituba of Josafaz River inStream the present to be a cycle relic of an earlier tributary to the Antas-Taquari River before - onof Trêssteeper Forquilhas gradients River of the are eastern longer limb than of its the western syncli- of erosion. The main trunk of the Três Forquilhas River ne?ones. Overall, Could thethis fracture-lineament be a consequence ofsystem flowing in thedowndip study whereand its there principal are two tributaries short tributaries appear to of lack an earlier any signifi cycle area is directly responsible for its rectangular-dendri- cant knickpoints except at the lip of the plateau surface,

of erosion (Fig. 10). tic to directional-trellised drainage pattern (Howard,- In sum, the watersheds of Três Forquilhas and- 1967). Also of importance is that west of the watershed- tingMaquiné to the Rivers nearby are South limited Atlantic by the Ocean, Torres whereas Syncline many and of Maquiné River, drainage parallels the coast and flo flow sub parallel to its axis following fractures or faul smallerws into scale,Patos thisLagoon, mirrors a distance closely ofthe about drainage 160 patternkm, ra spaced, south-southwest trending fracture system. Be- ther than flowing directly into the Atlantic. On a much of their larger tributaries are localized by the densely 196 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013 cause the lithologies are similar in the two watersheds, - der stream, where it erodes all unconsolidated mate- - typically funneled into the steep channel of a first or brium.so too are their stream densities. Lack of knickpoints et al in longitudinal profiles points to a landscape in equili ofrial surface down processes to bedrock on to the form lower a distinctive, slopes and long how scar the (Fig. 11). See Figueró . (1998) for a good summary- truction. In addition, an alternating wet and dry climate indifferent the Quaternary volcanic rockhas beentypes inferred weather from and bothaffect valley cons-

− see the summary -side deposits and their topographic profile in nearby- Santa Catarina and Paraná States by Clapperton (1993, p. 221-228) and the original re Valleysearch haveby Bigarella a history & - Mousinhohave a stratigraphy (1966). Thus - that we deser fully- recognize that the slope deposits of Três Forquilhas understanding slope processes in the valley is the de- ves consideration in future studies. Fundamental to to transmit water and their susceptibility to chemical gree of fracturing of its underlying flows, their capacity three principal tributaries. Note low remnant gradients in several Figure 10. Longitudinal profiles of Três Forquilhas River and its − weathering (Chart 1). There is still another key factor headwaters upstream from knick points at edge of volcanic plateau. whether the flows locally dip into the valley or away 3.2 Slopes from it (Fig. 12). Whether or not a flow brings− wet water collu (is- viuman aquifer) weighs to more a slope and is also important, has lower because effective wet stresses slopes - are much more likely to fail than dry ones processesThe study that ofact slopes on the in escarpmentthe watershed of ofthe Três plateau For (Hunt, 2005, p. 721-722). andquilhas how Valley it retreats. provided These many processes insights of tomass the wasting surface

Virgili,include 1998,creep, Ch. rotational 15). The and different translational lithologies slides, of rock the falls and debris flows (Hunt, 2005, table 1-9; Filho & - Volcanic Plateau (basalts and rhyolites) control the weathering processes and, consequently, the topogra phic profile. Very broadly, the valleys− an of riversupper Trêspart consistingForquilhas ofand near Maquiné vertical and cliffs, their some larger as tributarieshigh as 30 allto 40share m separated a common by cross steep valley slopes profile above a lower, almost - convex upward, smoother profile that largely lacks cli moreffs. These basic two below), parts but reflect also different not only surface a compositional processes change in the flow composition (more acid above and the escarpment form prominent, long landslide scars as well. In addition rock falls from acid cliffs high on upper steep slopes has mostly shallow translational slideswith debriswhereas flows low on at the their escarpment base. Thin rotational colluvium slides on occurs on all the slopes, but is most important low on in thicker colluvium prevail. Creep, on the other hand, Where deforested, these lower slopes have many small shallowthe escarpment rotational where slumps colluvium as well is theas some thickest deep (Fig. ones 5). prominent, long landslide scars that extend from the acidin their volcanics thicker high colluvium. on the Inescarpment addition, there to the are bottom a few of large tributaries such as Pinto Stream, where they south of Aratinga. terminate in debris cones some of which displace the Figure 11. Debris scar and valley bottom deposit in Pinto Stream principal stream of the valley. The sudden release of debris from high above is

197 Potter et al.

Figure 12. Gentle dips of fractured lava flows can bring water to a hillside, when the dip is toward the valley. et al., 1990).

Chart 1. Geomorphic and engineering characteristics of lava flows (adapted from Figueró Composition Rhyolites and rhyodacites

– Weather notably less than basalt and form prominent cliffs high on the escarpment or low, Basalt – irregular hills on the volcanic plateau; thin to moderately thick soils and colluvium. Relatively few cliffs and thick colluvium and soils low on escarpment. Structure and Texture

Little fractured Hard, dense and fresh rocks Much fractured – Weather slowly into large blocks; likely to form a ledge or cliff and be an aquiclude. transmit much water. – Mechanically unstable, closely packed blocks, which weather rapidly; may form steep talus and commonly

Amygdaloidal beds – Commonly fairly thin, but likely to be aquicludes unless much fractured. Volcanic breccias – Tend to be aquifers, especially where fractured. Glasses – Weather readily to clay and form reentrants; generate much colluvium Laminated rocks – Weather chemically and physically more rapidly than massive acid or basic volcanic rocks Paleosoils and altered rocks – Act as aquicludes and readily form colluvium were exposed. Sandstones are thin and uncommon, but are good aquifers

the extensive vertical fracture system of the lavas of the nor tooWhere impermeable), aquifer beds but dip where into they a slope, dip toward they may the slope,drain waterthey add from water it (if to colluvium it − not only is neitherduring rains, too thick but waterSerra Geralwith aFormation high hydraulic and thehead great from local a wide relief area of andthe throughout the year − so that wet colluvium and soil valley, a fractured flow dipping into a slope can collect are never too far from failure. Conversely, where water is diverted from a slope, colluvium and soil can adsorb thus willTopographic bring an enhancedcross sections flow rate provided to its outcrop insight andfor quickly destabilize it. C). Eight cross sections were made in the watershed greatermuch more its transmission water from a of rain water and and thus its be internal less likely solu to- a better understanding of slope processes (Fig. 13A- fail. In either case, the more fractured the aquifer, the were constructed from enlargements of the 1:50,000 all of which contribute to additional instability. Greater topographicof Três Forquilhas maps. River Spacing and threeof control outside points of it. variesThese tion and weathering (yielding more expandable clays), - - caldensity fractures of fractures that dip also toward produces a valley smaller reduce blocks its stabili that- with slope ranging from 0.1 to 0.95 km apart for much tyweaken more athan face. those Additionally, that dip intonear thevertical side ofto thesub valley.verti of the plateau to as close as 0.05 km for some of the ste endedepest slopes at the dependingplateau surface on topography or close to (Fig.it. All 13B). sections For Três Forquilhas Valley, these sections always began and In Três Forquilhas Valley, the worst case for failure is a well fractured flow dipping into the valley. Because of were made along interfluves. 198 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

Figure 13. Topographic cross sections: A) index map to cross sections; B) Três Forquilhas valley and similarity to the model of parallel retreat (Davis, 1908, apud King & Schumm, 1980) and; C) possible Velhas surface just to the east of the watershed. Sections 1, 1A and 2 are at right angles to small, - vations decrease from over 800 m to almost sea level. stream valleys not yet consumed by the new encroa- pment slopes uniformly seaward and in only 4 km ele- first order tributaries that drain to the Atlantic, relic- - hillsProfile provide 8 has detaileda similar, insight but more to the irregular, above topographicabrupt des ching cycle of erosion. Profile 2 illustrates rapid entren cent to sea level. Two detailed profiles of isolated, small chment just below the escarpment only 4.6 km downs tream. Here valley sides slope at about 30°. Profiles° to 3 cross sections (Fig. 14). - and 4 are 9 and 14 km downstream both with distinctly We also made three cross sections, profiles 9, 10, wider valleys. Along profile 4, slopes range from 5 distinctand 11, alongshoulder, the easta surface side of between the escarpment 200 and just 300 out m, 45°. Profile 5 has an even wider valley with a convex possiblyside of the the Três Velhas Forquilhas surface, watershed, along the eastThese side show of the a upwards surface while Profile 6.35 km downstream, lacks remnants of the volcanic plateau surface, shows a thebroad front bench of the at about escarpment 450 m andinto hasQuadros a wide and flood Itapeva plain escarpment as it trends north of Três Cachoeiras. This of 3 to 4 km. Sections 7 and 8 (Fig. 13C) extend down thedistinct broad surface, lowland 2 todrained 4 km bywide Mampituba and over River.10 km long, - parallels the east side of the escarpment and overlooks Lagoons. Here, in all of southern Brazil, the volcanic plateau is closest to the Atlantic. In profile 7 the escar199 Potter et al.

Hermann et al et al - in Santa Catarina (Bigarella & Becker, 1975, p. 200-206; Primeira Hora,., 1993;2001, Pellerinp. 14). Intense., 1997), rainfall and triggers recen tly in Rio Grande do Sul (Correio do Povo, 2000, p. 17;- cause it adds weight to the slope or face and at the same failure (Fig. 15) on both colluvial slopes and cliffs, be in near surface deposits, so that a few hours of intensi- vetime rain reduces will create effective ideal stress conditions (Hunt, for1984, closely p. 721-723) spaced - vium and soil from a slope of the escarpment and re- landslides and debris flows. These locally strip collu − the deposits of large isolated of boulders seen on the deposit it on valley floors as thick debris flow deposits FigureThree 14. Two principal detailed profiles observations of small basaltic result hills. from the se- stillfloodplains wet from of earlierthe larger rains. tributaries See Pellerin represent et al old splay deposits. This is most likely to happen when slopes are upper resistant acid volcanics were consumed by ero- . (1997) for rial cross sections of Três Forquilhas Valley. As the sion, topographic relief within the valley was reduced, such an event in nearby Santa Catarina, Jones (1973) in athe hurricane coastal mountains in the Appalachian of the city Mountains of Rio de ofJaneiro, Virginia. and - Williams (1973) for erosion from extreme rainfall from slopes became lower, and floodplains widened. In Três Forquilhas Valley the rate of valley widening downs tream, calculated from the junction of Carvalho and- Pinto Streams, is about one km per 7 km downstream seemswhereas to inincrease the Maquiné exponentially valley this downstream. rate is about one ki lometer per 10 km downstream. In both valleys width of slope development in a landscape underlain by al- ternatingDavis`s resistant 1908 modeland less (see resistant King & beds Schumm, seems 1980)to ap- ply well to the escarpment of the plateau and suggests

- lythat and the generate resistant more acid flowscolluvium. retreat Additional mostly by colluviumrock falls, whereas the lower basaltic flows weather more easi and blocks are added from above by debris flows and creep so that most massive flows on lower slopes are- siblyburied non by parallelit. This retreatprocess for invokes the more parallel easily retreat weathe for- the upper, resistant, acid cliff-forming flows and pos where a stream impinges against a valley wall, or whe- red basaltic flows of the lower slopes. Exceptions occur re an acid flow is exceptionally closely fractured and thus never forms a cliff. We fully recognize, of course, Figure 15. Debris scars and debris flows in nearby Santa Catarina that more superficialet al mapping on scales of 1:10,000 to State after intense rainfall (A). Abandoned channel and flash flood deposition in Pinto Stream, Três Forquilhas Basin (B and C). to1:2000, fully understand such as that in made detail by all Uberti the processes (1981) for acting soils onor 3.3 Age and origin of Três Forquilhas Valley: implications theby Figueróslopes of the .escarpment, (1998) for construction,their relative importance,are needed for the origin of the escarpment and the scales on which they operate. Thus the above - at departure for more detailed future studies. See King discussion of slope evolution should be taken as a point Four lines of evidence provide insights to the drai et al nage history of Três Forquilhas and Maquiné Valleys:- for(1967, an introductionp. 158-169), toSelby the (1985,vast literature p. 233-238 of the and larger 576- the far field effects of the uplift history of the Andes 586), Bloom (2004, p. 197-204), and Nott . (1996) theMountains, broad regional the subsidence geology historyof the Ponta (seismic Grossa stratigra Arch When slope retreat on the escarpment occurs see- andphy evidence)the Rio Grande of the do bordering Sul Shield, Pelotas and an Basin onshore (Fig. sub16),- question of back wearing and how slopes retreat. - surface cross section. tion of how it occurs. We suggest that the slope retreat ofms the much plateau clearer is largely to us, however, episodic thanand mostlythe process occurs ques during intense rainfall such as has been well documented

200 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

South America in the Middle and Upper Miocene o - val with extensive granitic intrusions) between 11to 6 The uplift of the Andes Mountains in the Middle Andes in westernet al Argentina between 37 to 39 S (coe and Late Miocene included development of high plate- - aus, new and reactivated thrust belts, igneous activity Ma (Melnick ., 2006, p. 91); the etincrease al at 7.5 Ma - inof headwardthe central erosion Andes fromacross the the Pacific Antiplano in northern Plateau co of ble 2, Chart 2). Starting at its southern end, examples of astal Chile (Mortimer, 1973; Kober ., 2006); uplift and major faulting plus new intermontane basins (Ta et al et al Bolivia was mostly between 10 and 7 Ma in the Upper these events include the thrusted and uplifted Neuquén south-centralMiocene (Ghosh Peru, the., 2006; last two Garzione at about 13., 2008,and 6 Fig.Ma 4); threeet periods al of folding occurred in the Miocene in - nuad(Tosdal et al., 1995).., 1984, Fig. 10); and uplift occurred in the Middle Miocene in the Merida Andes to Colombia (Par

the transitionFar field consequences between the eastMiddle of the and Andes Late includeMioce- the developmentet ofal the present Amazonas River near Orinoco from the Caribbean to the Atlantic in the late ne (Figueiredo ., 2009); the course change of the- thdrawal of the Miocene seaway in Argentina and Para- Middle Miocene (Diazet al de., 1999) Gomero, as well 1994), as the and abandon the wi-

guay (Sprechmann ment of several Andean embayments (Marengo, 2000). etNearby, al the thickest, 1200-to-3800 m fill of the Chaco changedbasin in Bolivia from fossiliferous was deposited gray-green in the late to Miocene non fossilife (Uba- rous., reddish2006, Fig. brown 4) and near in thethe Llanos Middle-Upper Basin mudstones Miocene et al., 1995, p. 1435). Along much on

boundary (Cooper andthe Brazilianbelow the coast, widespread quartz-rich Tortonian reddish unconformity brown gravels at - the Barreiras Formation - were deposited both above epirogenic coastal uplift far from the Andes. Such re- - gional10.5 Ma evidence (Arai, 2006), points thus to probable demonstrating early Middlewidespread Mio- thern Pelotas Basin offshore of the states of Rio Grande do Sul and Figure 16. Location map of seismic lines in the central and nor - southsouthern side Santa and the Catarina, large Rio southernmost Grande Cone Brazil. 9 Line Note 3. the important cene deeping of Três Forquilhas and Maquiné Valleys Rio Grande High with the Quintão Escarpment (Lines 1 and 2) on its with likely accelerated deepening in the Upper Mioce Chart 2. Relevant South American geomorphic and paleogeographic events.ne. See Potter & Sztamari (2009) for other evidence of

Chamberland, 2006). • Principal uplift of the Andes in Middle and Upper Miocene (Tosdal et al., 1984; Silva et al., 1999; Poage &

• Altiplano of Bolivia uplifted between 10.3 to 6.7 Ma as shown by carbon and oxygen isotopes of caliches in Late MudstonesMiocene (Ghosh of Llanos et al., Basin 2006; of Garzione Columbia et change al., 2008). from marine, greenish grey to non marine, brownish red at about • Mérida Andes rise in Middle Miocene (Parnaud et al., 1995). • 10 Ma (Cooper et al., 1995, fig. 4 and p. 1445). • Molassic Chaco basin of Bolivia principally filled in Late Miocene. • Amazonas River System takes present form in Late Miocene (Figueiredo et al., 2009) and the Orinoco River System in Middle Miocene (Di Croce, 1995; Diaz de Gomero, 1996, figs. 6 and 9) • Entrerriense-Paranense seaway of parts of Paraná-Paraguay river basins abandoned in Late Miocene (Marengo, 2000; Sprechmann et al., 2001) • Present canyons of Atacama Desert started in Late Miocene (Mortimer, 1973; Kober et al., 2006) • Far field fracture pattern of Miocene age inferred across much of southern South America (Costa & Hasui, 1997) • Barreiras-type gravels of Brazil deposited as earlier Tertiary regolith is stripped from interior highlands in Middle and Late Miocene (Costa et al., 1993; Arai, 2006).

201 Potter et al. tectonic activity in South America and globally in the Middle and Upper Miocene. - dentiality17 and 18). restrictions. These lines were supplied by ANP and are Evidence from offshore seismic stratigraphy simply identified as line 01, 02 and line 03 due to confi - - ces. First, This allimplies three seismicaccelerated lines erosion show that onshore sequences and mic lines in the Pelotas Basin off the coast of Rio Gran- more4F and accommodation 4G are thicker than in the the basin earlier offshore. Tertiary Secondly, sequen Sequence stratigraphy study of the three long seiset - al et al aled that in the northern part of the basin, close to the recordde do Sul of (Fontana,the erosional 1990, history 1996, ofp. coastal113-183; Rio Dariva Grande seismic study of seismic lines 01 and 02 (Fig. 18) reve ., 2008; Holz ., 2008) provides a time equivalent - Our analysis of the offshore is based on a recently con- Florianópolis High (which delimits the Pelotas Basin- do Sul and southern Santa Catarina (Figs. 16 and 17).- from the adjacent Santos Basin) there is a sedimenta sedimentaryry buildup forming feature ais huge located plateau-like seawards structurein front of histhe Basincluded to research update its project, stratigraphy sponsored and byto reevaluate Brazilian Peits torically known as the Rio Grande High (Fig. 17). This- troleum Agency (ANP), was carriedet outal., 2008).in the PelotasEarlier - study area. Here sequences 4A to G show a very inte possible petroleum systems (Holz et al. ofresting the two pattern nearby regarding valleys depositionduring Early and to erosion Middle quiteMio- published studies include the pioneer work by Fonta cenepossibly time, linked to the onshore uplift and entrenchment na (1990, 1996, p. 113-183) followed by Dariva - - mic(2008). stratigraphic Four second-order analysis integrated depositional to well sequences log analy (1- mentary nature of the Rio Grande High, with deposi- sisto 4) permitted form the theinfill uppermost of the Pelotas Tertiary Basin depositional (Table 3). Seis se- Seismic line 01 (Fig. 18) clearly reveals the sedi labeled 4A to 4G. This shows that the sedimentation tional sequences 4A to 4G forming a thick sedimentary duringquence theto be drift divided period into of seven the basin third recordedorder sequences, at least pile suggesting intense sediment influx at that part of- i.e., modationthe basin. Thespace), thickness showing of thateach this depositional structural sequence features - wasis greater controlling at their the flanks sedimentation than over the in high that (less area accom since seven major base level falls and unconformities; i.e - sequence boundaries. The age of each depositional se- cequence is also implies the age a of fall the of unconformity base level and at itsthus base. enhanced Each of the LateWithin Cretaceous; this scenario, ., the a bulk particular of sediments feature were seems de thesepotential unconformities for erosion onshore. represents The episodes age of each of base sequen level posited at the flanks of that structure. -

escarpmentto indicate times generated of increased by slides sediment or slumps influx. at the This sou fe- Basin.fall - implying, The indicated rejuvenation time frames of the of source table 3area are followedapproxi- thernature, marginhere named of the the Rio Quintão Grande Escarpment, High. Seismic is linea huge 02 mationsby increased based sediment mostly upon influx biostratigraphic to the adjacent data. Pelotas - Three of the Tertiary unconformities recorded in - crosses the Quintão Escarpment revealing two impor- point-of-view, the story of entrenchment of the onsho- tant stratigraphic relationships: first, depositional se rethe valleys basin areis punctuated widespread by and at leastpronounced. three episodes From this of quences 4A to C drape its relief decreasing in thicknes important uplifts and denudation of the source area onses thefrom steep its flank escarpment towards surface the capping at the structural southern high,mar- - and secondly, sequences 4D to 4G are partly deformed des led to the generation of three third order unconfor- mitiesinduce in base-level the Miocene changes; of the these Pelotas three Basin. erosional episo gin of the Rio Grande High. This deformation (collapse) - affects mainly sequences 4E and 4F, sequence 4D to a islesser a time extent, of base and level the basalfall generating part of sequence unconformities 4G. Hence, in The first of these important unconformities oc the stratigraphicalepoch of onset record) of each seems of these to besequences coincident (which with 24curs Ma), between as calibrated depositional by available sequences biostratigraphic 4D and 4E, data. and a very huge sediment input, which caused rapid sedi- has an Early Miocene age (Aquitanian, approximately - sedimentary pile, and thus the slides and slumps of the proximatelyThe second unconformity 15 Ma), and isthe between third relevant sequences unconfor 4E and- mentary up building and consequent instability of the F, with an age of Early Middle Miocene (Langhian, ap- med during the Middle to Late Miocene, Serravalian to Quintão Escarpment. Tortonianmity is between at approximately sequences 124F Ma.and Bueno4G which et al was for Seismic line 03, about 300 km to the southwest of p. 559), additionally noted, unconformities at 10 Ma the Quintão Escarpment, crosses a large deltaic feature . (2007, known as the Rio Grande Cone. This line shows both normal,increased synthetic sediment and influx antithetic during faults the deposition plus reverse of andthe and 5Other Ma, althoughevidence thefor threeperiods lines of increasedof figure 19sediment do not Mid- to Late Miocene sequences and a large system of confirm them. dé thrust faults, caused by collapse of the delta front; this influx is given by analysis of three seismic lines (Figs.202 collement affects mainly sequence 4F and the base Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013

des apparently due to overloading by rapid sediment In sum, we propose that the initiation of most of input. The dominance of shale in the basin in these se- of sequence 4G of latest Miocene age (Chart 3). - of the Serra Geral volcanics that form the escarpment. tothe the relief distant of the Andean valleys orogeny, of Três Forquilhasan orogeny and coincident Maqui quencesWe foundis consistent seven Tertiary with humid unconformity-bound climate weathering se- withné is amany far field other response events in throughout the Middle toSouth Late MioceneAmerica Chart 3). Two occurs when

quences in all three lines ( - Basin.(Chart In2). the Locally, Pelotas our Basin interpretation there are at is least fully three supported third- many Andean far field events across South America- and amplified by the seismic stratigraphy of the Pelotas were in progresset al - one at 15 Ma (Middle-Lower Mio- ddlecene boundaryboundary) at and 10 the Ma. other These at unconformities12 Ma (Middle Mioce result -order base level changes, which generated sequences ne); Bueno . (2007) reported one at the Upper-Mi 4E, 4F and 4G. The approximate ages of the implied- - unconformities are 24, 15, and 12 Ma (Chart 3). All of from relative sea level fall — implying rejuvenation of these three sequences were affected by slumps and sli the source area followed by increased flux to the Pelo et al., 1990).

Chart 3. Geomorphic and engineering characteristics of lava flows (adapted from Figueró Composition Rhyolites and rhyodacites

– Weather notably less than basalt and form prominent cliffs high on the Basalt – escarpment or low, irregular hills on the volcanic plateau; thin to moderately thick soils and colluvium. Structure and Texture Relatively few cliffs and thick colluvium and soils low on escarpment. Little fractured Hard, dense and fresh rocks. Much fractured – Weather slowly into large blocks; likely to form a ledge or cliff and be an aquicludes. talus and commonly transmit much water. – Mechanically unstable, closely packed blocks, which weather rapidly; may form steep

Amygdaloidal beds – Commonly fairly thin, but likely to be aquicludes unless much fractured. Volcanic breccias – Tend to be aquifers, especially where fractured. Glasses – Weather readily to clay and form reentrants; generate much colluvium. Laminated rocks – Weather chemically and physically more rapidly than massive acid or basic volcanic rocks. Paleosoils and altered rocks – Act as aquicludes and readily form colluvium were exposed. Sandstones are thin and uncommon, but are good aquifers.

Earlier evidence for paleorivers of all the Neogene units in all three seismic sections. Thistas Basin; large forrelative example, fall in Unit sea G level, (12 Maresulted <) is thein rapid thickest se- What can be inferred about the age of the oldest drainage in the study area? The uplift history of the - Ponta Grossa Arch and the Sul-Rio-Grandense Shield mentarydimentation pile up of buildingthe Pelotas (decrease Basin. inThis accommodation) caused slides and consequent instability (overpressure) in the sedi - - plus the cross section of figure 4 provide insights to this seand displaced slumps onunits the are Quintão capped Escarpment by latest Miocene and induced beds). interesting question. Remembering that the watershe Thuscollapse we ofsuggest the delta that front much of ofthe the Rio pre Grandes Cone (who theds of two Três structural Forquilhas highs and of Maquiné the Rio GrandeValleys occupyShield andthe Pontalowest Grossa structural Arch, point the age(the of Torres uplift ofSyncline) these two between regio- Middle Miocene or even possibly as lateent asrelief the ofUpper Três nal highs gives the earliest age of ancestral drainage. Forquilhas and Maquiné Valleyset al was initiated in the p. 91-93) found that initial uplift of the Ponta Grossa ThusMiocene three judging types of by offshore Bueno evidence. (2007, - unconformities, p. 558-559), ArchUsing occurred apatite fission at 110 tract Ma and dating its maximum (Vignol-Lelarge, rate was 1993, be- who also identified unconformities at 10 and 5 Ma. sometime in the late Cretaceous there must have been inenhanced the Middle thicknesses Miocene betweenwith probable these unconformitiesacceleration in atween river 90system to 110 occupying Ma (Turonian the present through structural Albian). low Thus, of and slumping - point to onshore rejuvenation starting narrow, but rapidly widening Atlantic Ocean. This im- late Miocene time (Fig. 18). Três Forquilhas and Maquiné Valleys that flowed into a

203 Potter et al.

LineFigure 3 have17. Offshore a Lower seismic Miocene lines: seismic note signature the greater showing thickness that of movement the third order was post sequences Lower Miocene.4G and 4F in all three lines, the mound structure and scarp of the Rio Grande High on Line 2, and the décollement of the Rio Grande Cone of Line 3. Faulted slices within the décollement of

Figure 18. Summary of drainage evolution in the study area. 204 Pesquisas em Geociências, 40 (3): 189-208, set./dez. 2013 plies that somewhere at sea there should be a delta, fan - delta or deep sea fan supplied by sediment from this cating even earlier drainage in this structural low. This - paleodrainageof the Jurassic-Cretaceous suggests that Serra there Geral may Formation,be hidden deltas indi or subsea fans offshore. early drainage system. From studies of vitrinite reflec c. The spatial coincidence of both present valleys possibletance along deposits the east might side be of considerable. the Paraná Basin, Zanotto and ancient buried valleys in the Torres Syncline de- (1993)The inferred stratigraphic as much and as 2,500topographic m of erosion cross -sections so such monstrates an exceptional long term, 200 Ma tectonic of the Torres Syncline permit us to identify even earlier paleodrainage near Terra de Areia. The stratigra- Drainage, at all scales in both watersheds, is stron- - glycontrol controlled of paleotopography by a fracture in pattern this part closely of coastal related Brazil. to that of the opening of the South Atlantic Ocean. phic cross sections of figure 4 show two erosional val In both valleys episodic erosion today and in the leys, one at the base of the eolian Botucatu Formation past powered headward erosion dominantly by paral- One(Jurassic-Cretaceous) is above the other and and the both other are at close the baseto the of axis the lavas of the Serra Geral Formation (Triassic-Jurassic).- cantly - both are directly below the present mouths of lel slopeToday, retreat most through headward creep, erosion rock fall,occurs slope in failures pulses of the Torres Syncline. In addition - and most signifi and debris flows. - Triassic time, the Torres Syncline was already a topo- lar processes operated on scales from 104 to 105 years the Três Forquilhas and Maquiné Rivers. Thus in late induring the past. heavy rainfall; it thus seems probable that simi - graphic low focusing drainage to the sea. Consequently, longwe infer ago thatas the the Late present Triassic valleys some of 200Três Ma Forquilhas ago. and gradedJudging to present by the sea absence level and of knickpoints seemingly noin longitudi record of Maquiné Rivers had their most distant beginnings as- nal profiles, both valleys have equilibrium landscapes and origins.In sum, Thewe proposebeginnings that of the much valleys of their of Três present For earlierAcknowledgments: rejuvenations. We appreciate the sustained support of landscapequilhas and came Maquiné into Riversexistence both in have response had similar to Andean ages - - pful commentaries of L.A. Bressani, C. Scherer, S. Dillenburg the Institute of Geociences for field transport and the hel rated uplift and erosion beginning in the Late Miocene. far-field tectonics in the Middle Miocene with accele - for(UFRGS) his earlier R. Lopes studies. and W. R. Wildner Dias introduced (CPRM-RS). us Weto thethank volca R.L.- ding fractures in a broad, gently, seaward-dipping syn- Fontana (PETROBRAS S.A.) for reading the manuscript and clineBoth towardrivers follow the Atlantic well defined Ocean. south The presence southeast of tren two broad paleovalleys in the subsurface - one as old as nic stratigraphy of the Serra Geral Formation. Michael Holz Triassic - implies that the Torres Syncline was a topo- acknowledges Agência Nacional do Petróleo (ANP-SDB) for supplying seismic lines, Conselho Nacional de Pesquisa e Desenvolvimento (CNPq) for a personal scholarship (Grant graphic low even in the middle of the Mesozoic. 3026666/04-4). Finally, we much appreciated the sustained 4 Conclusions support of the Instituto de Geociências, UFRGS. References - - Aboarrage, A.M. & Lopes, R.C. 1986. Projeto Borda Leste da teau Thein the initial region relief of ofthe the Torres present Syncline valleys was of Três uplifted For Bacia Paraná: Integração Geológica e Avaliação Econômi- quilhas and Maquiné was initiated as the volcanic pla ca - - ; Relatório Final. Comissão Pesquisa Recursos Mine in the Middle Miocene in response to far field Andean ment of the volcanic plateau as shown by unconformi- nasrais, paisagens São Paulo doe Porto Rio GrandeAlegre, v.do 1, Sul. 15p. Geomorfologia, 11: orogeny; this caused accelerated erosion of the escarp Ab’Saber, A.N. 1969. Participação das superfícies aplainadas - - ties, increased thicknesses and slumping in the nearby 1-17. Boletim Paulista de Geo- 10the and offshore 5 Ma. Miocene section. Judging by offshore seis Almeida,grafia, F.F.M.10: 3-32. 1952. Contribuição a geomorfologia da re mic unconformities,Inferences about later earlier rejuvenation drainage are followed based onat da12,- gião oriental de Santa Catarina. Geologia USP, Arai, M. 2006. A grande elevação eustática do Mioceno e sua cross section: Série Científica, influência na origem do Grupo Barreiras: ting by apatite fission track studies and a subsurface 6 (2): 1-6. Araújo, L.M., Franca, A.B., & Potter, P.E. 1999. Hydrogeology- a. Uplift of the Rio Grande de Sul Shield (74 Ma) Cretaceous left the Torres Syncline as a structural low of the Mercosul aquifer in the ParanaHydrogeology and Chaco-Parana Journal betweenand the Ponta two highs Grossa and Arch focused (90 todrainage 100 Ma) into in thea wide late- Basins, South America, and comparision with the Navajo ning South Atlantic Ocean. -Nuggett aquifer system USA. , 7: b. A subsurface cross section across the Torres 317-336. Bellieni,A. & Stolfa,G., Comin-Chioramonti, D. 1986. Petrogenetic L.S., Marques,aspects of L.S., acid Melfi, and A.J.,ba- Nardy, A.J.R., Papatrechas, C., Piccirillo, E.M., Roisenberg, Syncline shows two paleovalleys, one filled by the saltic lavas from the Paraná Plateau (Brasil): Geological, Triassic-Jurassic Botucatu Formation the other by lavas205 Potter et al.

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