Geological Survey of Denmark and Greenland Bulletin 17, 2009, 81-84 The evolution of the landscape of north-east Brazil in relation to the burial and exhumation history of both onshore and offshore areas is the focus of a research project carried out for StatoilHydro do Brasil and Petrobras from 2007 to 2009 by the Geological Survey of Denmark and Greenland in collab- oration with Geotrack International. In hydrocarbon explo- ration it is important to understand the regional tectonic framework and thus also to consider the volumes of rocks that may have been present and then removed during the geological past. For example, the timing of hydrocarbon gen- eration and changes in migration routes can be assessed when the timing and magnitude of uplift and erosion is known. Studies in West Greenland have demonstrated the usefulness of large-scale, low-relief, high-level landscapes as markers of uplift events, and in particular the strength of combining the denudation history from landscape analysis with the cooling history from apatite fission-track analysis (AFTA) data and the stratigraphic record (Bonow et al. 2006, 2007; Japsen et al. 2006, 2009). In the study area, there are two plateaux with elevations up to c. 1300 m above sea level (a.s.l.). The plateaux are cur- rently being dissected by deeply incised fluvial valleys, and escarpments separate the two plateaux. The lowlands cut across Early Cretaceous rift systems along the Atlantic margin, including the intracontinental Recôncavo–Tucano–Jatobá (RTJ) Rift and also the Camamu Basin, of which the western margin is exposed onshore (Fig. 1). The RTJ Rift is a mature hydrocar- bon province, whereas the deep-water parts of the Camamu Basin are the target of frontier exploration (e.g. Magnavita et al. 1994; Davison 1999; Cobbold et al. 2008). The post-rift sequence in the RTJ Rift and the inshore Camamu Basin is thin or absent. However, it has been estimated that up to 2000 m of sedimentary cover once was present, but has now been removed (Magnavita et al. 1994). The Atlantic margin of Brazil is characterised by elevated plateaux cut by deeply incised valleys, but this landscape has a pattern common with many other passive continental mar- gins with elevations from 1000 to 2000 m a.s.l. or more around the world, for example in Norway, East and West Greenland and south-east Australia. Mesozoic–Cenozoic rift systems parallel to the coast are generally present offshore 81 Post-rift landscape development of north-east Brazil Johan M. Bonow, Peter Japsen, Paul F. Green, Peter R. Cobbold, Augusto J. Pedreira, Ragnhild Lilletveit and Dario Chiossi © GEUS, 2009. Geological Survey of Denmark and Greenland Bulletin 17, 81–84. Available at: www.geus.dk/publications/bull ~100 km m a.s.l. Fig. 2A Fig. 2B Fig. 3 Brazil 225 km 42°W 10°S 12°S 14°S 40°W RTJ 38°W Neogene deposits Cenozoic laterites Barreiras Fm (Neogene) Sabiá Fm (Lower Miocene) Mesozoic deposits Palaeozoic deposits Precambrian rocks A BBB Chapada Diamantina PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP lllllllllllllllllllllllllllllllllllllllllllllllllllllllll aaaaaaaaaaaaaaaaaaaaaaaaa nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa llllllllllllllllllll ttttttttttttttttttttt oooooooooooooooooooooo dddddddddddddddddddddd eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss tttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttttt aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa P l a n a l t o d e C o n q u i s t a Camamu Basin 2000 1800 1600 1400 1200 1000 800 600 400 200 0 10°S 12°S 14°S 16°S 42°W 40°W 38°W Fig. 1. A: Geological map of the study area (based on CPRM 2001, 2003). Precambrian basement is covered by younger sedimentary sequences, which are important age constraints for the different peneplains. RTJ, Recôncavo – Tucano–Jatobá Basin. The arrow points at the small Sabiá Formation outrcrop. B: Topography of the study area. Two topographical features dominate the landscape: a lower surface, which is a plain mainly at 200–500 m a.s.l. (greenish and light yellow) and the higher surface which is a plain mainly at 900–1200 m a.s.l. (orange and reddish). Pronounced escarpments separate the two features. Elevation data source: Jarvis et al. (2008). ROSA_2008:ROSA-2008 01/07/09 15:49 Side 81 with a transition from continental to oceanic crust farther offshore. Several aspects related to elevated, passive continen- tal margins are controversial: the origin of the plateaux, the timing of their uplift to their present elevation and their rela- tion to the adjacent rift systems (Japsen et al. 2009). For example, Gallagher et al . (1994) found that the almost 3 km high mountain chain Serra do Mar, near Rio de Janeiro, is the remnant of a rift shoulder from the Early Cretaceous break- up in the South Atlantic, whereas Cobbold et al. (2001) argued that these mountains were formed during Neogene block-fault tilting. King (1967) mapped stepped surfaces (i.e. erosion sur- faces at distinct levels in the landscape) through the elevated terrains along many passive continental margins, e.g. in east- ern Australia, southern Africa and north-east Brazil. King used remnants of sedimentary rocks to constrain the ages of these surfaces and concluded that the main surfaces were formed during the Cenozoic, and consequently that the mar- gins had been uplifted in that same time interval. Further - more, King found that the highest peaks in the interior of the continents represented remnants of a pre-break-up topogra- phy. Subsequent geomorphological research into the devel- opment of the passive margins of southern Africa and eastern Australia has, however, regarded the elevated terrains along these passive margins as mainly reflecting preserved rift- shoulders (e.g. Ollier 1985), a model that is commonly used as input to thermochronological studies (e.g. Gallagher et al. 1994). The burial and exhumation history of the study area is currently investigated by combining the cooling history from apatite fission-track analysis from both outcrop and borehole samples with the denudation history from landform analysis and the stratigraphic record. Based on field work carried out during four weeks in July and August 2007, we here report results focused on the rift systems near the Atlantic margin and on the interior highlands in north-east Brazil. Large-scale landforms A geomorphological analysis based on the method described by Bonow et al. (2006) has led to the identification of two major erosion surfaces (peneplains) of low, relative relief and of regional extent within the study area: a lower surface extending from near-coast areas and far into the interior (up to 500 m a.s.l., greenish colours in Fig. 1B) and a higher sur- face that includes the plateaux (‘planaltos’) of Chapada Dia - mantina (c. 1200 m a.s.l., reddish colours in Fig. 1B) and Planalto de Conquista (Fig. 1B; c. 900 m a.s.l.). Both surfaces cut across rocks of different ages and resistances and must therefore be erosional features (Fig. 2). These observations show that the surfaces were originally formed by denudation to a near-horizontal plain because even slightly tilted surfaces will be incised by fluvial valleys and the relief rejuvenated, until a new and younger peneplain is formed. The location of these surfaces near the present coast indicates that sea level was the most likely baselevel to which the surfaces graded. After uplift of what is now the higher surface, the lower sur- face was developed by incision along the main rivers in the area (Figs 1, 3, 4). The higher surface can be correlated from Chapada Dia - mantina to Planalto de Conquista at a slightly lower elevation (Fig. 2). It is characterised by an undulating plain with shal- low and wide valleys, and it is preserved on high ground with resistant rocks. The higher surface must have developed across a larger area than that presently preserved, as the lower surface has developed at the expense of it. Escarpments often 82 W W E E ? ? 0 1 2 0 1 2 Precambrian basement Jurassic–Cretaceous sediments Maximum topography in corridor Minimum topography in corridor Higher surface Lower surface A B 40°W 39°W 38°W 42°W 41°W 40°W 39°W km a .s .l. km a .s .l. Fig. 2. Two profiles to illustrate the surface mapping. The dotted lines show the interpreted peneplains. The surfaces cut across both the basement, consisting of rocks that have different resistance to erosion, as well as the sedimentary sequence of the RTJ Basin. This shows that the peneplains are erosional features. See Fig. 1B for profile location and corridor width. ROSA_2008:ROSA-2008 01/07/09 15:49 Side 82 separate the lower surface from the higher surface (Figs 3, 4). In detail, these escarpments usually coincide with bedrock boundaries, thus reflecting bedrock resistance. The valley patterns and the incision of rivers in the Recôncavo and Tucano basins also show that the lower surface is rapidly being dissected by incising rivers, due to a change in baselevel after formation of the lower surface. Geological constraints The lower surface cuts across post-rift strata within the rift and Precambrian basement outside the rift (Figs 1, 2). The formation of the surface thus post-dates the Aptian Marizal Formation (e.g. Magnavita et al. 1994). The age of the sur- face may, however, be further constrained by an outlier of the early Miocene, marine Sabiá Formation within the Recôn - cavo Basin (Fig. 1A) where this sedimentary unit has been found in deep trenches (Viana et al. 1971). This outlier testi- fies to a marine transgression that occurred before the forma- tion of the lower surface because the outlier is truncated by that surface, and thus the lower surface is younger than early Miocene. The areas where the higher surface is defined are characterised by laterites (deep weathering profiles) of Cenozoic age (Fig. 1; CPRM 2001, 2003). Both the higher surface and the laterites are currently being destroyed by erosion along the escarpments that outline the plateaux. Consequently, the laterites must have formed at the end of the ero- sional process that shaped the higher surface. Based on the age of the laterites we can deduce that the higher surface formed during the Cenozoic. This time interval can be further narrowed if we take into account that the younger, lower surface was formed subse- quent to the deposition of the Sabiá Formation. The age of the higher surface may thus tentatively be estimated to be Palaeogene, which implies that the landscape at that time was a peneplain close to sea level. This suggestion is in agreement with observations from similar plateaux north and south of the study area where the plateau surfaces were exposed dur- ing the Palaeogene according to stratigraphic data (Sant’Anna et al. 1997; Morais Neto et al. in press) and geochronological constraints on deep weathering (Spier et al. 2006; Lima 2008). Alternatively, both the higher and the lower surface in the study area may have formed during the Neogene, which also agrees with the Cenozoic age of the laterites. Geo - morphological analysis alone cannot definitely conclude which alternative is correct, but we prefer the first alternative because it is consistent with independent constraints from outside the study area. 83 Fig. 3. Digital terrain model with a higher surface (HS) and a lower surface (LS) that are separated by escarpments in the Chapada Diamantina area. The higher surface forms a coherent plateau at 1200–1400 m a.s.l. (reddish) with only minor val- ley incisions. This plateau is presently being dis- sected by rivers along the escarpment, erod ing down to the lower surface, here at 500–400 m a.s.l. (greenish). Escarpments are also found above the higher surface, maybe representing steps towards older surfaces at higher elevations. The arrow indicates the location and direction of the photograph in Fig 4. Elevation data source: Jarvis et al. (2008). Fig. 4. The lower surface with the escarpment and the higher surface in the background. See Fig. 3 for location. 2000 1500 1000 500 m a.s.l. HS LS 42°W 41.5°W 41°W 14 °S 13 .5° S 13 °S ~25 km ROSA_2008:ROSA-2008 01/07/09 15:49 Side 83 Conclusions The landscape in the study area is dominated by two main peneplains, a higher surface and a lower surface that were both formed as low-relief erosion surfaces. The higher surface developed during the Cenozoic, probably during the Palaeogene as other similar plateaux in Brazil. The lower sur- face formed during the Neogene after the deposition of the Sabiá Formation and an uplift event that raised the higher surface to its present elevation around 1000 m a.s.l. The uplift resulted in rejuvenation of the relief and subse- quent formation of the lower surface. Progressive backward erosion along the main rivers has resulted in escarpments that separate the two surfaces. The escarpments are pronounced at geological boundaries with large differences of erosional resis- tance. Even the lower surface is presently under destruction due to minor subsequent uplift. In summary, we find that the passive margin topography in the study area was shaped many millions of years after the Early Cretaceous break-up of the South Atlantic. The conclusion that the landscape is mainly Cenozoic is thus in agreement with that of, e.g. King (1967). A better understanding of the timing of uplift events will be achieved from apatite fission-track data as well as the amount of exhumation involved in the formation of the ero- sion surfaces. Acknowledgements The study was funded by StatoilHydro do Brasil and Petrobras. References Bonow, J.M., Lidmar-Bergström, K. & Japsen, P. 2006: Palaeosurfaces in central West Greenland as reference for identification of tectonic movements and estimation of erosion. Global and Planetary Change 50, 161–183. Bonow, J.M., Japsen, P., Green, P.F., Wilson, R.W., Chalmers, J.A., Klint, K.E.S., van Gool, J.A.M., Lidmar-Bergström, K. & Pedersen, A.K. 2007: A multi-disciplinary study of Phanerozoic landscape development in West Greenland. Geological Survey of Denmark and Greenland Bulletin 13, 33–36. Cobbold, P.R., Meisling, K.E. & Mount, V.S. 2001: Reactivation of an obliquely rifted margin, Campos and Santos basins, southeastern Brazil. American Association of Petroleum Geologists Bulletin 85, 1925–1944. 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Viana, C.F., Gama Junior, E., Simões, I.A., Moura, J.A., Fonseca, J.R. & Alves, R.J. 1971: Revisão estratigráfica da bacia Recôncavo/Tucano. Boletim Técnico da Petrobras 14, 157–192. 84 Authors’ addresses J.M.B & P.J., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: jbon@geus.dk P.F.G., Geotrack International Pty Ltd, 37 Melville Road, Brunswick West 3055, Victoria, Australia. P.R.C., Géosciences-Rennes (UMR6118 du CNRS), Université de Rennes, 35042 Rennes Cedex, France. A.J.P., Geological Survey of Brazil (CPRM), Avenida Ulysses Guimaraes, 2862, 41213-000 Salvador, Brazil. R.L., StatoilHydro, Angola team, Stavanger, Norway. D.C., StatoilHydro do Brazil, Rio de Janeiro, Brazil. ROSA_2008:ROSA-2008 01/07/09 15:49 Side 84