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Evidence for the Uplift of the Volcanic Plateau

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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 Atlantic dunes. TheOcean escarpment by a 13 km ri- 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).
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