Ernesto L.C. Lavina

Sequence stratigraphy of continental Triassic strata of Southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and palaeoclimate

Abstract The continental Triassic succession of southernmost Brazil comprises two second-order depositional sequences—the Sanga do Cabral (Early Triassic) and the Santa Maria (Middle to Late Triassic) supersequences. The first one includes ephemeral, low-sinuosity fluvial deposits developed on a low gradient plain. Based on fossil tetrapods, especially procolophonids, an Upper Induan age is estimated for this sequence. Facies association of the Santa Maria Supersequence indicates low-sinuosity fluvial rivers, deltas and lakes. This supersequence can be further subdivided into three third-order sequences (age provided by palaeovertebrate biostratigraphic data) as follows: Santa Maria 1 (Ladinian), Santa Maria 2 (Carnian to Early Norian) and Santa Maria 3 (probably Raethian or Early Jurassic) sequences. The Gondwanides paroxysms I and II in the Sierra de la Ventana–Cape Fold Belt are directly related to the development of both supersequences. The source area of the Sanga do Cabral Supersequence was located to the south. It consisted of an uplifted peripheral bulge situated landward of the retro-foreland system, from where older sedimentary rocks were eroded. The source area of the Santa Maria Supersequence was also positioned southwards and related to the uplifted Sul-Rio-Grandense and Uruguayan shields. The Santa Maria Supersequence stratigraphic architecture is comparable to the Triassic rift basins of Western Argentina. Diagenesis, facies and palaeontology of the studied succession suggest a dominantly semiarid climate during the Triassic.

Tectonic Control of the Triassic Santa Maria Supersequence of the Paraná Basin, Southernmost Brazil, and its Correlation to the Waterberg Basin, Namibia

Abstract The Middle and Late Triassic Santa Maria Basin, exposed in southernmost Brazil, and Waterberg Basin, in Namibia, are herein interpreted as part of en echelon small basins in southern West Gondwana. The main structures are the Waterberg-Omaruru Fault which can be connected to a NW-strike anastomosed fault zone in Brazil. Based on field structural and stratigraphic analysis two populations of NW-strike fractures, named A-type and B-type, are recognized. A-type fractures (Az = 280°-290°) occur in the Sanga do Cabral Supersequence, underlying units of Santa Maria Basin, as extension of the Waterberg-Omaruru Fault during the Early Triassic. B-type fractures (Az = 295°-345°) are observed in all studied stratigraphic units, from the Triassic Sanga do Cabral Supersequence to the Early Cretaceous Botucatu/Serra Geral formations. Based on the structural analysis we propose that NNE-SSW extension reactivated structures of the Damara Belt, Namibia, with a propagation towards Rio Grande do Sul State forming an anastomosing normal fault system and related-rift basin by Early-Middle Triassic time. The A-type fractures were preferentially active by this phase and the B-type ones are interpreted as secondary link segments within the anastomosing system. During the Gondwana break-up and South Atlantic opening (rift to proto-oceanic phase, Aptian) the B-type fractures were reactivated as normal faults by N50°E-S50°W extension.

Assinaturas icnológicas da sucessão sedimentar Rio Bonito no bloco central da jazida carbonífera de Iruí, Cachoeira do Sul (RS)

The subsurface deposits of the Rio Bonito sedimentary succession in the central block of the Irui coal mine (Cachoeira do Sul, Rio Grande do Sul State, S Brazil) is composed of sedimentary facies associations that represent deposition in dominantly marginal-marine and shallow-marine settings, the later in minor scale. Ichnofabrics of Chondrites , Chondrites-Helmintopsis-Planolites , Cylindrichnus-Thalassinoides , Helminthopsis , Macaronichnus , Ophiomorpha , Palaeophycus, Planolites , Thalassinoides , and Thalassinoides-Palaeophycus occur in the marginal marine settings , whereas the Thalassinoides-Palaeophycus-Helminthopsis ichnofabric is present in the shallow marine deposits. The marginal-marine ichnofabrics are characterized by the low bioturbation index, the reduced size of burrows, and the low ichnodiversity, whereas the marine ones show a moderate to high degree of bioturbation and low ichnodiversity. The ichnofabric pattern, its stratigraphic distribution, and its sedimentological relationships allow recognizing four ichnological signatures, three suggesting stress caused by changes in the salinity gradient, and one substrate-controlled. Ichnofabrics of Helmithopsis, Palapeophycus , and Planolites in lithofacies SiltArg suggest the dominance of oligohaline to freshwater conditions. Ichnofabrics of Thalassinoides-Palaeophycus and Cylindrichnus-Thalassinoides refl ect the dominance of mesohaline conditions, each one representing, respectively, more quiet and stable environments (impoverished Cruziana Ichnofacies suite), and moderate to high energy settings (mixed Skolithos-Cruziana Ichnofacies suite). The Thalassinoides-Palaeophycus-Helminthopsis ichnofabric represents an arquetypical Cruziana Ichnofacies suite, indicating shallow marine settings. The reduced size of the burrows, however, suggests dominance of polyhaline rather than stenohaline conditions and its local distribution allow infer deposition in embayments, which mimic the environmental conditions found in the lower shorefce / offshore transition zones. The Thalassinoides ichnofabric is substrate-controlled, representing a Glossifungites Ichnofacies suite. It demarks two autogenic stratigraphic surfaces, and one allogenic stratigraphic surface interpreted as a sequence boundary. The integrated ichnological and sedimentological analysis in high resolution scale, allowed refining the existent paleoenvironmental and stratigraphic interpretations of the Rio Bonito sedimentary succession in the study area. Key words: ichnology, sedimentology, ichnofabrics, ichnological signatures, coastal deposits.

Paleogeografia da borda leste da bacia do Paraná no tempo de formação das principais jazidas de carvão do permiano (formações Rio Bonito e Palermo)

Nos anos de 1983a 1986, a CPRM(SUREGs PAe Sp) desenvolveu umprojeto de integração de dados de sondagens e de afloramentos, ao longo de toda a borda leste da Bacia do Paraná, obtidos durante programas de pesquisa para carvão. Um dos principais objetivos consistiu na elaboração de umaseção paleoambiental, a partir de poços, nos estados do Rio Grandedo Sul, Santa catarina, Paranâ .e são Paulo. Dados relativos à parte sul desta seção paleoambiental foram apresentados emtrabalh:>s anteriores (WPESez aLU, 1986; IAVINA& WPES, 1987).