Tafonomia de biválvios em calcários oolíticos da formação Teresina (Bacia do Paraná, Permiano Médio, Prudentópolis, PR)

Herein, it is presented the first detailed taphonomic study on bivalve mollusk shells preserved in the oolitic limestones of the Teresina Formation (probably Kungurian-Roadian, Lower-Middle Permian) in the eastern margin of the Parana basin. The selected beds are located in two quarries (informally named PRU 1 and PRU 2) in Prudentopolis municipality (Center-South Parana State), and positioned approximately in the middle of the formation and probably in the Pinzonella illusa Zone. The PRU 1 limestone ([approximately]30 cm thick), which is partially silicified and intercalated with predominantly pelitic rocks, is classified as a bivalve oolitic grainstone. The basal contact is erosive and the top shows symmetrical ripple marks, which are draped by shale with mud cracks. There are two fining-upwards successions characterized by dense to dispersed packing of the shells, which are usually disarticulated, randomly oriented (many nested/stacked) and mixed with some Formapelitic intraclasts. Microhummocky cross-stratification occurs a little below the top of the bed. The PRU2 bed is classified as ooidbivalve rudstone[approximately] (~5 cm thick), where all shells are disarticulated and fragmented, showing dense packing. The bivalves probably inhabited a muddy substrate and were mixed (as parautochtonous and allochthonous bioclasts) with ooids during high-energy storm events, including posterior shell displacement as a result of bioturbation. Thus, the calcareous beds represent amalgamated proximal tempestites with a complex taphonomic history, strong temporal/spatial mixing of bioclasts and limited paleoecological resolution. They are a typical example of shell beds generated in a huge epeiric sea, which was not necessarily connected to the ocean and where very low depositional-slope gradient, very slow subsidence and minimum sediment accommodation space caused frequent sediment reworking by storm related processes.

Tafonomia das rochas carbonáticas conchíferas das formações Teresina e Rio do Rasto (Permiano, Bacia do Paraná)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Estudo petrográfico e geoquímico dos ironstones da base da Formação Pimenteiras, Borda Oeste da Bacia do Parnaíba, Tocantins

Ironstones, que ocorrem na base da Formação Pimenteiras (Devoniano), na borda noroeste da Bacia do Parnaíba, foram investigados ao longo dos perfis Xambioá-Vanderlândia e Colinas do Tocantins-Couto Magalhães. Esses ironstones formam camadas de espessura decimétrica, descontínuas e intercaladas em arenitos e siltitos, que jazem sobre rochas do cinturão Araguaia. Além da textura oolítica, os ironstones de Xambioá-Vanderlândia diferem dos de Colinas do Tocantins-Couto Magalhães por conterem menores quantidades de material terrígeno, notadamente quartzo, e maiores proporções de oxi-hidróxidos de Fe. São ainda mais enriquecidos em V, Sr, Zr e Σ_ETR e mais empobrecidos em Al₂O>sub>3 e Rb. Diferem também no padrão de distribuição dos ETR normalizados ao North American Shale Composite (NASC), especialmente com relação aos valores de (ETRI)_N, os quais, mais altos nos ironstones oolíticos e mais baixos nos não oolíticos, geram curvas convexas e côncavas, respectivamente. No campo, não foram estabelecidas as relações espaciais entre as duas variedades de ironstones, porém sugere-se que elas representem diferentes fácies da mesma formação ferrífera. Possivelmente, a deposição da fácies não oolítica ocorreu mais afastadamente da borda continental, em ambiente de águas mais profundas e calmas, onde foram descarregadas maiores quantidades de sedimentos detríticos; a deposição da fácies oolítica transcorreu em águas mais rasas e agitadas, com menor suprimento de material terrígeno. O transporte do Fe poderia ter resultado, em grande parte, da erosão fluvial de áreas continentais marcadas por ambientes redutores, o que teria favorecido a solubilidade daquele metal na forma de complexos orgânicos.

Contexto deposicional da sequência Balbuena III (Maastrichtiano/ Daniano) da formação Yacoraite na sub-bacia metán-alemania, na região de Salta, Argentina

Pós-graduação em Geologia Regional - IGCE

Análise estratigráfica em depósitos lacustres maastrichtianos da Formação Yacoraite (Bacia de Salta - Argentina): definição e rastreabilidade de sequências de alta resolução

Pós-graduação em Geologia Regional - IGCE

Petrologia das fraturas associadas a feições típicas de dissolução hipogênica na Formação Salitre, Bacia de Irecê, Bahia, Brasil

The recognition of karst reservoirs in carbonate rocks has become increasingly common. However, most karst features are small to be recognized in seismic sections or larger than expected to be investigated with borehole data. One way forward has been the study of analogue outcrops and caves. The present study investigates lithofacies and karst processes, which lead to the generation of the largest system of caves in South America. The study area is located in the Neoproterozoic Una Group in central-eastern Brazil. This province comprises several systems of carbonate caves (Karmann and Sanchéz, 1979), which include the Toca da Boa Vista and Barriguda caves, considered the largest caves in South America (Auler and Smart, 2003). These caves were formed mainly in dolomites of the Salitre Formation, which was deposited in a shallow marine environment in an epicontinental sea (Medeiros and Pereira, 1994). The Salitre Formation in the cave area comprises laminated mud/wakestones, intraclastic grainstones, oncolitic grainstones, oolitic grainstones, microbial laminites, colunar stromatolites, trombolites and fine siliciclastic rocks (marls, shales, and siltites). A thin layer and chert nodules also occur at the top of the carbonate unit. Phosphate deposits are also found. Our preliminary data indicate that folds and associated joints control the main karstification event at the end of the Brasiliano orogeny (740-540 Ma). We recognized five lithofacies in the cave system: (1) Bottom layers of grainstone with cross bedding comprise the main unit affected by speleogenesis, (2) thin grainstone layers with thin siltite layers, (3) microbial laminites layers, (4) layers of columnar stromatolites, and a (5) top layer of siltite. Levels (1) to (3) are affected by intense fracturing, whereas levels (4) and (5) seal the caves and have little fracturing. Chert, calcite and gipsite veins cut across the carbonate units and play a major role in diagenesis. Our preliminary study indicate that hypogenic spelogenesis is the main process of karst development and contributed significantly to the generation of secondary porosity and permeability in the carbonate units.

Revisiting the Late Jurassic-Early Cretaceous of the NW South Iberian Basin: new ages and sedimentary environments

The study of the Upper Jurassic-Lower Cretaceous deposits (Higueruelas, Villar del Arzobispo and Aldea de Cortés Formations) of the South Iberian Basin (NW Valencia, Spain) reveals new stratigraphic and sedimentological data, which have significant implications on the stratigraphic framework, depositional environments and age of these units. The Higueruelas Fm was deposited in a mid-inner carbonate platform where oncolitic bars migrated by the action of storms and where oncoid production progressively decreased towards the uppermost part of the unit. The overlying Villar del Arzobispo Fm has been traditionally interpreted as an inner platform-lagoon evolving into a tidal-flat. Here it is interpreted as an inner-carbonate platform affected by storms, where oolitic shoals protected a lagoon, which had siliciclastic inputs from the continent. The Aldea de Cortés Fm has been previously interpreted as a lagoon surrounded by tidal-flats and fluvial-deltaic plains. Here it is reinterpreted as a coastal wetland where siliciclastic muddy deposits interacted with shallow fresh to marine water bodies, aeolian dunes and continental siliciclastic inputs. The contact between the Higueruelas and Villar del Arzobispo Fms, classically defined as gradual, is also interpreted here as rapid. More importantly, the contact between the Villar del Arzobispo and Aldea de Cortés Fms, previously considered as unconformable, is here interpreted as gradual. The presence of Alveosepta in the Villar del Arzobispo Fm suggests that at least part of this unit is Kimmeridgian, unlike the previously assigned Late Tithonian-Middle Berriasian age. Consequently, the underlying Higueruelas Fm, previously considered Tithonian, should not be younger than Kimmeridgian. Accordingly, sedimentation of the Aldea de Cortés Fm, previously considered Valangian-Hauterivian, probably started during the Tithonian and it may be considered part of the regressive trend of the Late Jurassic-Early Cretaceous cycle. This is consistent with the dinosaur faunas, typically Jurassic, described in the Villar del Arzobispo and Aldea de Cortés Fms.