Juliana Okubo - Academia.edu (original) (raw)

Papers by Juliana Okubo

Albian carbonates rocks of Quissama Formation from the southern of Campos Basin (RJ) and the Oute... more Albian carbonates rocks of Quissama Formation from the southern of Campos Basin (RJ) and the Outeiro Formation are described in detail, with emphasis on petrology, petrography, cycles, paleoenvironments and diagenesis. Six lithofacies were recognized in the Quissama Formation, mainly grainstones and packstones (rare wackestones) with carbonate grains composed of ooids, oncoids, peloids and some bioclasts (mollusks, echinoids, benthonic microfossils). This study confirms previous interpretations that the Quissama Formation was deposited in a shallow carbonate ramp with a set of banks oriented parallel to the coast composed of carbonate grains. The other facies (mainly packstones) represent the flanks of banks and interbank regions, deposited under lower energy conditions. Rudaceousoncoids were most commonly formed in the protected areas at the western flank of the bank, where microbial communities found the most favorable life conditions. There is evidence of reworking of grains by s...

Durante o final do Periodo Ediacarano, sao identificados pelo menos tres importantes eventos glac... more Durante o final do Periodo Ediacarano, sao identificados pelo menos tres importantes eventos glaciais de expressao global, denominados de Sturtiano, Marinoano e Gaskiers, que sao sucedidos por uma incomum sedimentacao carbonatica de ampla distribuicao. Nas ultimas decadas, a Formacao Sete Lagoas (Grupo Bambui) tem sido associada a glaciacao Sturtiana ou Marinoana atraves de estudos sedimentologicos, geocronologicos e isotopicos realizados principalmente na porcao sul da Bacia do Sao Francisco. A ocorrencia de fosseis do genero Cloudina sp. tem trazido a discussao da idade ediacarana terminal para a Formacao Sete Lagoas. Este trabalho visa compreender a evolucao sedimentar da Formacao Sete Lagoas, a partir da analise de facies e estratigrafica, ciclicidade e geoquimica de isotopos estaveis, na porcao norte de Minas Gerais, localidade carente de estudos com esta abordagem. Devido a essa falta de dados, a plena correlacao da Formacao Sete Lagoas com outras unidades coevas e tambem com ...

In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), ... more In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows strik...

This work aims to identify and photograph grains that compose important Cretaceous carbonate unit... more This work aims to identify and photograph grains that compose important Cretaceous carbonate units of the Potiguar Basin, represented by the Ponta do Mel and Jandaira formations (Albian-Campanian). Petrographic investigation of thin sections was essential. The samples studied come from wells and surface samples from the collection UNESPetro – UNESP, Rio Claro. In the Ponta do Mel Formation, the grains consist of ooids, oncoids, peloids and bioclasts. Regarding to the identified bioclasts, the solenoporacean red algae, mollusks (bivalves and gastropods), echinoids, foraminifera, ostracods and worms were the dominant elements. In the Jandaira Formation, the grains are composed by ooids, peloids and bioclasts, which are represented by green algae, mollusks (bivalves and gastropods), benthic foraminifera miliolids, worms, echinoderms and ostracods. The grains found in the Ponta do Mel Formation are somewhat similar to those found in the Jandaira Formation, with the exception of calcareo...

Scientific Reports, 2021

We combine U–Pb in-situ carbonate dating, elemental and isotope constraints to calibrate the syne... more We combine U–Pb in-situ carbonate dating, elemental and isotope constraints to calibrate the synergy of integrated mountain-basin evolution in western Gondwana. We show that deposition of the Bambuí Group coincides with closure of the Goiás-Pharusian (630–600 Ma) and Adamastor (585–530 Ma) oceans. Metazoans thrived for a brief moment of balanced redox and nutrient conditions. This was followed, however, by closure of the Clymene ocean (540–500 Ma), eventually landlocking the basin. This hindered seawater renewal and led to uncontrolled nutrient input, shallowing of the redoxcline and anoxic incursions, fueling positive productivity feedbacks and preventing the development of typical Ediacaran–Cambrian ecosystems. Thus, mountains provide the conditions, such as oxygen and nutrients, but may also preclude life development if basins become too restricted, characterizing a Goldilocks or optimal level effect. During the late Neoproterozoic-Cambrian fan-like transition from Rodinia to Gon...

$Research paper thumbnail of Mechanical stratigraphy and structural control of oil accumulations in fractured carbonates of the Irati Formation, Paraná Basin, Brazil$

Brazilian Journal of Geology, 2020

Precambrian Research, 2020

Abstract Most Precambrian sediment-hosted barites have been interpreted to be diagenetic, hydroth... more Abstract Most Precambrian sediment-hosted barites have been interpreted to be diagenetic, hydrothermal exhalation, or methane-seepage in origin. Seafloor barite precipitates and void-filling barite cements in basal Ediacaran cap dolostones have been interpreted as sedimentary and early diagenetic in origin, and they have been used to infer ocean geochemistry in the aftermath of the terminal Cryogenian (or Marinoan) Snowball Earth glaciation. Barite crystals precipitated syndepositionally from seawater or authigenically from marine porewaters can potentially offer insights into Neoproterozoic ocean geochemistry. In this study, we analyzed void-filling barite cements from the basal Ediacaran Sete Lagoas cap dolostone to determine whether the barite records seawater geochemical signatures and whether it experienced post-depositional hydrothermal alteration. Sete Lagoas barite occurs as veins and major void-filling cement in multiple horizons, sometimes interbedded with carbonate fans. Barite crystals, commonly forming rosettes, grew upwards and downwards as radiating bladed crystals, isolated crystals in the matrix, and inclusions within carbonate fan crystals. Sulfur isotope compositions of coexisting barite and carbonate-associated sulfate (CAS) show similar values, suggesting a similar sulfate source, potentially seawater sulfate from a shallow marine environment. Assuming that CAS was sourced from seawater sulfate, we infer that the Sete Lagoas barite was precipitated from marine porewaters with seawater as a source of the sulfate. Carbonaceous material (CM) and fluid inclusions in barite crystals share a similar thermal history with a peak temperature in the range of 206–257 °C. CM was not detected in the host rock, possibly due to pervasive dolomitization, thermodegradation of CM or the fine-grained nature of the host rock. Estimated temperatures and salinity based on fluid inclusions are both higher than seawater, suggesting that hydrothermal activity, perhaps those related to Mississippi-Valley type mineralization in the Sao Francisco Craton, may have influenced the Sete Lagoas barite. Thus, although the Sete Lagoas barite may have originally precipitated from seawater or marine porewaters in communication with seawater, it may have been subsequently influenced by hydrothermal activity and therefore a more thorough diagenetic assessment must be done before the Sete Lagoas barites can be used to infer seawater geochemistry and environment in the aftermath of Marinoan Snowball Earth.

Journal of South American Earth Sciences, 2019

Abstract The Sete Lagoas Formation (Ediacaran), located in the central part of the Sao Francisco ... more Abstract The Sete Lagoas Formation (Ediacaran), located in the central part of the Sao Francisco Craton (Brazil), consists of limestones and dolostones deposited in very shallow waters in the inner part of the carbonate platform. Four breccia types occur throughout the stratigraphic succession: evaporitic breccia with tepees, flat-pebble breccia, hydrothermal breccia and brecciated stromatolites. Here we combine a detailed sedimentological and stratigraphic analyses of the flat-pebble breccia in order to determine its origin and the processes and environmental conditions that originated these brecciated facies. The studied interval consists of a 20 m thick succession of tabular beds composed of flat-pebble breccia interbedded with laminated microbialites. In these breccia beds, the clasts are usually platy or oblate with angular edges and are mainly disposed horizontally within the sedimentary bed, suggesting that they were not transported or reworked. The presence of microbialite clasts with sharp edges and vertices in the Sete Lagoas flat-pebble breccia suggests that the lithification process started very early in diagenesis and, even the sediments exposed at the bottom were, at least, partially lithified. Some breccia levels show bidirectional imbrication and clast size analyses reveal a NE-SW long-axis clast orientation whereas square clasts tend to fill the space among oriented clasts. Breccia clasts are vertically oriented and show deformation features increasing upwards, typically of deforming beds formed by ascendant expulsion of liquefied sediment. Disrupted layers or presenting folds and synsedimentary faults commonly occur confined between undeformed beds. Other evidences of liquefaction and soft-sediment deformation are the injection structures, as flame and load cast-like features, in the base of the brecciated beds. These structures commonly penetrate the upper bed and disrupt the sedimentary layer immediately above promoting local brecciation. These features are found both in modern and ancient deposits of seismic influence, which suggests a similar origin for the Sete Lagoas flat-pebble breccia. Thus, the processes that led to the formation of the studied flat-pebble breccia are interpreted as seismically triggered, since: a) the breccia beds are laterally continuous and extend for several kilometers; b) the breccia beds are restricted to a 20 m thick stratigraphic interval; c) the clasts of the breccia are the same lithology of non-deformed beds below and above the interval of breccia; d) the interbedding of breccia beds and laminated microbialite beds is recurrent; e) the breccia beds are subhorizontal and present irregular upper and lower contacts; f) the presence of liquefaction structures and dyke injection. Thus, this seismic-triggered breccia deposits represent the product of the synsedimentary tectonism occurred within the Sao Francisco Craton during the terminal Ediacaran and correspond to a very well-defined local stratigraphic marker in the Bambui Basin. The seismic activities could be related to the NW regional faults in the regional Paleoproterozoic basement of the study area, which were reactivated during the deposition of the Sete Lagoas Formation in the Ediacaran Period.

Palaeogeography, Palaeoclimatology, Palaeoecology, 2019

Abstract Significant Ediacaran paleoenvironmental changes recorded in the mixed carbonate-silicic... more Abstract Significant Ediacaran paleoenvironmental changes recorded in the mixed carbonate-siliciclastic Bambui Basin, east-central Brazil, outcrop in one of the best-preserved sites, the Januaria paleo-high. From a detailed stratigraphic and chemostratigraphic approach (carbon and oxygen isotopes, total organic carbon, total sulfur and selected elements abundances), we identified transgressive-regressive intervals and depositional settings within the Bambui Group stratigraphy and developed a new model for the origin of the extremely positive Middle Bambui Excursion (MIBE) present in the middle section of the basin. A post-Marinoan cap carbonate interval is recorded in the first ~10 m, preserving aragonite pseudomorph fans, barite-rich layers and negative δ13C anomaly. A hiatus separates the cap carbonate from the late Ediacaran succession that makes up the remaining Bambui Group. The younger intervals preserve both siliciclastic (middle Serra de Santa Helena Formation) and carbonate (middle/upper Sete Lagoas and Lagoa do Jacare formations) shallow-water depositional settings, requiring tectonic influence or climatic changes in source areas. The MIBE yields δ13C values as high as +14‰ and extends for about 350 m, from the upper Sete Lagoas Formation to the lower Serra da Saudade Formation. We suggest a model of a restricted basin setting that favored local carbon isotopic signals in the δ13C record of sedimentary carbonates through preferential weathering of ancient carbonate platforms on the continent and higher burial rate of authigenic carbonate. This scenario caused the global δ13C budget to be affected by an offset of, at least, +4‰ in sedimentary carbonates deposited on the Sao Francisco craton and along its margin during the terminal Ediacaran. Many previous studies suggested a restricted nature for the middle Bambui Basin, which probably resulted from the central position of the Sao Francisco paleocontinent within the mosaic of collisional blocks during the SW Gondwana amalgamation.

Precambrian Research, 2018

Carbonates in the Sete Lagoas Formation (São Francisco craton, Brazil) preserve a record of chemi... more Carbonates in the Sete Lagoas Formation (São Francisco craton, Brazil) preserve a record of chemical, biological, and oceanographic changes that occurred during the Ediacaran Period. The base of this formation constitutes a post-glacial cap carbonate, which contains seafloor precipitates (carbonate and barite crystal fans) as well as various authigenic and diagenetic minerals (apatite, pyrite, and barite). Here, we present petrographic and geochemical data on this unit, and discuss the significance of its mineral association for marine environments following the Marinoan ('Snowball Earth') glaciation. For the first time, we report well-developed apatitic cements in a Neoproterozoic cap carbonate. Isopachous and intergranular void-filling cements encrust and surround seafloor-precipitated fan crystals that precipitated as aragonite. We propose a model for the origin of this mineral association, which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions. According to this model, the boundary between oxic and anoxic conditions was located at or just below the sediment-water interface. Burial of iron (oxyhydr)oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction. Iron reduction released Fe 2+ , which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor. Concurrently, 'iron-pumping' shuttled phosphate from the water column to the sediment, and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways, created conditions conducive to phosphate mineralization. This model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation, in addition to high alkalinity, and that Fe 2+ serves as this inhibitor. Overall, our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the Ediacaran, illuminates the paleoenvironments of cap carbonates with seafloor precipitates, and contributes to understanding of phosphogenesis following low latitude glaciations.

Geological Society of America Abstracts with Programs, 2018

Brazilian Journal of Geology, 2015

Carbonate rocks from the Macaé Group (Albian) represent an example of carbonate sedimentation rel... more Carbonate rocks from the Macaé Group (Albian) represent an example of carbonate sedimentation related to the drift phase in Campos Basin. This study presents depositional features, integrating them with diagenetic and stratigraphic aspects of the Macaé Group carbonates including the upper part of the Quissamã Formation and the lower part of the Outeiro Formation. Macroscopic analyses in cores and microscopic ones in thin sections allowed the recognition of eleven sedimentary facies - nine of them corresponding to the Quissamã Formation and two of them representing the Outeiro Formation. These facies were grouped into five facies associations. Oolitic grainstones and oncolitic grainstones are interpreted to be deposited in shallow depth probably in shoals above the fair weather wave base. The interbanks between shoals were formed in less agitated waters and characterized by deposition of peloidal bioclastic packstones and wackestones representative of sedimentation in calm waters. Bi...

Journal of the Geological Society, 2022

Since the report of tubular structures interpreted as Cloudina from the Neoproterozoic Sete Lagoa... more Since the report of tubular structures interpreted as Cloudina from the Neoproterozoic Sete Lagoas Formation (SLF) in the Bambuí basin of eastern Brazil, this stratigraphic unit has become a focus of numerous geochemical, paleomagnetic, geochronological and sequence stratigraphic studies. Geochemical data from the SLF have been used to infer paleoenvironmental conditions in the Bambuí basin and the 13C enrichment reported in this and overlying units. To further address depositional environments of the SLF, we present high-resolution δ13C and δ34S data from the SLF in the Januária and Sete Lagoas regions. We report the presence of superheavy pyrite with δ34S value greater than carbonate associated sulfate (CAS) and observe a decrease in CAS abundance coupled with a δ34S change from ca. +20 to near +50‰. These data are interpreted as evidence for distillation of sulfate through CAS and pyrite removal from the epeiric Bambuí basin. These changes are qualitatively consistent with quanti...