Caracterização estrutural do Lineamento de Piúma e sua influência na porção norte da Bacia de Campos

The detailed study of the Brazilian continental margin basins became possible with the advancement of geophysical tools, with emphasis on seismic reflection. Characterizing the structural and stratigraphic elements of Brazil’s marginal basins they realized that there was a relationship with structures present on the adjacent continental basement. So many works began to be made to understand this relationship and know the major factors that influenced the evolution of the continental margin. The study area of this work includes the northern portion of the Campos Basin and the continental outcropping adjacent areas, which corresponds to the northern of Rio de Janeiro state and the southern of Espirito Santo state. This area stands an important structural feature of NW-SE direction with a projection to the Campos Basin called Lineamento de Piúma. The outcropping basement rocks belongs to the Ribeira Belt which was bonded to other mobile bands forming the continent Western Gondwana during the Brasiliano Cycle, which later fragmented giving rise to the Atlantic Ocean. The opening of the ocean results on the formation of the marginal basins of Brazil. These basins have continental, transitional and marine facies. On Campos Basin the continental phase resulted on the formation of horsts and grabens bounded by synthetic and antithetic faults. Continuing rifting formed the saline lakes that deposited siliciclastic and carbonate sediments. The transitional phase resulted on thick packages consist of evaporites (halite and anhydrite) that was deposited in lagoon environment, tectonically quiet arid and semi-arid. The marine phase it deposited siliciclastic and carbonate in the Campos Basin resulting in shales, marls, limestones, ritmito, turbidites, sandstones and others. The objective of this study is to investigate the possible continuation... (Complete abstract click electronic access below)

Microbialitos da formação Codó (Aptiano Superior) às margens do rio Tocantins em São Miguel do Tocantins (TO)

The southwestern region of the São Luís-Grajaú Basin has a rare outcrop of the Codó Formation (upper Aptian) with seven outstanding microbialite bioherms along the left margin of the Tocantins river, near Imperatriz (MA). Resting on sandstones of the Grajaú Formation, the Codó Formation presents: 1) a 20 cm thick basal calcilutite with gypsite pseudomorphs and some fossil tree stems; 2) metric dark shales with carbonate nodules and thin intercalated carbonate layers, enclosing some microbial laminites; 3) a 2 cm thick upper breccia composed of microbialite fragments and other carbonate clasts, with halite hoppers on the top; 4) the carbonate bioherms, which partially overlie the extensive shales and interrupt them laterally, as well as the breccia. The bioherms in the northern part of the outcrop are thicker (<2 m) and have interbedded dark shales, whereas the southern are thinner and continuous in the vertical direction. In general, they are composed of irregular gently to strongly wavy microbial laminites, sometimes with pseudocolumnar to conical lamination. All microbialites with highest synoptic relief (<20 cm) look like columnar stromatolites on weathered lateral expositions. In plan view, the horizontal sections of these microbialites are circular to slightly elliptic, sometimes forming very small channels (N60W) filled with fine breccia. The highest bed of the northern bioherm has mixed microbial laminites and columnar stromatolites, where intercolumnar spaces were filled with microbialite clasts, fish bones, plant fragments and very small probable crustacean coprolites. Several fractures and deformation in this upper bed indicate an initial brecciation process probably caused by subaerial exposure. In microscopic scale, the lamination is smooth, diffuse, defined by subtle granulation differences of very fine granular calcite crystals within micrite, but oxide levels, dissolution surfaces or thin precipitated calcite veneers...

Paleozoic-aged brine and authigenic helium preserved in an Ordovician shale aquiclude

Consideration of the geosphere for isolation of nuclear waste has generated substantial interest in the origin, age, and movement of fl uids and gases in low-permeability rock formations. Here, we present profi les of isotopes, solutes, and helium in porewaters recovered from 860 m of Cambrian to Devonian strata on the eastern fl ank of the Michigan Basin. Of particular interest is a 240-m-thick, halite-mineralized, Ordovician shale and carbonate aquiclude, which hosts Br–-enriched, post-dolomitic brine (5.8 molal Cl) originating as evaporated Silurian seawater. Authigenic helium that has been accumulating in the aquiclude for more than 260 m.y. is found to be isolated from underlying allochthonous, 3He-enriched helium that originated from the rifted base of the Michigan Basin and the Canadian Shield. The Paleozoic age and immobility of the pore fl uids in this Ordovician aquiclude considerably strengthen the safety case for deep geological repositories, but also provide new insights into the origin of deep crustal brines and opportunities for research on other components of a preserved Paleozoic porewater system.

Diagenesis of a light, tight-oil chert reservoir at the Ediacaran/Cambrian boundary, Sultanate of Oman

The Al Shomou Silicilyte Member (Athel Formation) in the South Oman Salt Basin shares many of the characteristics of a light, tight-oil (LTO) reservoir: it is a prolifi c source rock mature for light oil, it produces light oil from a very tight matrix and reservoir, and hydraulic fracking technology is required to produce the oil. What is intriguing about the Al Shomou Silicilyte, and different from other LTO reservoirs, is its position related to the Precambrian/Cambrian Boundary (PCB) and the fact that it is a ‘laminated chert‘ rather than a shale. In an integrated diagenetic study we applied microstructural analyses (SEM, BSE) combined with state-of-the-art stable isotope and trace element analysis of the silicilyte matrix and fractures. Fluid inclusion microthermometry was applied to record the salinity and minimum trapping temperatures. The microstructural investigations reveal a fi ne lamination of the silicilyte matrix with a mean lamina thickness of ca. 20 μm consisting of predominantly organic matter-rich and fi nely crystalline quartz-rich layers, respectively. Authigenic, micron-sized idiomorphic quartz crystals are the main matrix components of the silicilyte. Other diagenetic phases are pyrite, apatite, dolomite, magnesite and barite cements. Porosity values based on neutron density logs and core plug data indicate porosity in the silicilyte ranges from less than 2% to almost to 40%. The majority of the pore space in the silicilyte is related to (primary) inter-crystalline pores, with locally important oversized secondary pores. Pore casts of the silica matrix show that pores are extremely irregular in three dimensions, and are generally interconnected by a complex web or meshwork of fi ne elongate pore throats. Mercury injection capillary data are in line with the microstructural observations suggesting two populations of pore throats, with an effective average modal diameter of 0.4 μm. The acquired geochemical data support the interpretation that the primary source of the silica is the ambient seawater rather than hydrothermal or biogenic. A maximum temperature of ca. 45°C for the formation of microcrystalline quartz in the silicilyte is good evidence that the lithifi cation and crystallization of quartz occurred in the fi rst 5 Ma after deposition. Several phases of brittle fracturing and mineralization occurred in response to salt tectonics during burial. The sequences of fracture-fi lling mineral phases (dolomite - layered chalcedony – quartz – apatite - magnesite I+II - barite – halite) indicates a complex fl uid evolution after silicilyte lithifi cation. Primary, all-liquid fl uid inclusions in the fracturefi lling quartz are good evidence of growth beginning at low temperatures, i.e. ≤ 50ºC. Continuous precipitation during increasing temperature and burial is documented by primary two-phase fl uid inclusions in quartz cements that show brines at 50°C and fi rst hydrocarbons at ca. 70°C. The absolute timing of each mineral phase can be constrained based on U-Pb geochronometry, and basin modelling. Secondary fl uid inclusions in quartz, magnesite and barite indicate reactivation of the fracture system after peak burial temperature during the major cooling event, i.e. uplift, between 450 and 310 Ma. A number of fi rst-order trends in porosity and reservoir-quality distribution are observed which are strongly related to the diagenetic and fl uid history of the reservoir: the early in-situ generation of hydrocarbons and overpressure development arrests diagenesis and preserves matrix porosity. Chemical compaction by pressure dissolution in the fl ank areas could be a valid hypothesis to explain the porosity variations in the silicilitye slabs resulting in lower porosity and poorer connectivity on the fl anks of the reservoir. Most of the hydrocarbon storage and production comes from intervals characterized by Amthor et al. 114488 preserved micropores, not hydrocarbon storage in a fracture system. The absence of oil expulsion results in present-day high oil saturations. The main diagenetic modifi cations of the silicilyte occurred and were completed relatively early in its history, i.e. before 300 Ma. An instrumental factor for preserving matrix porosity is the diffi culty for a given slab to evacuate all the fl uids (water and hydrocarbons), or in other words, the very good sealing capacity of the salt embedding the slab.

Micro-facies and µXRF element scanning data of Dead Sea sediment cores DSEn and 5017-1

Laminated lake sediments from the Dead Sea basin provide high-resolution records of climatic variability in the eastern Mediterranean region, which is especially sensitive to changing climatic conditions. In this study, we aim on detailed reconstruction of climatic fluctuations and related changes in the frequency of flood and dust deposition events at ca. 3300 and especially at 2800 cal. yr BP from high-resolution sediment records of the Dead Sea basin. A ca. 4-m-thick, mostly varved sediment section from the western margin of the Dead Sea (DSEn - Ein Gedi profile) was analysed and correlated to the new International Continental Scientific Drilling Program (ICDP) Dead Sea Deep Drilling Project core 5017-1 from the deep basin. To detect even single event layers, we applied a multi-proxy approach of high-resolution microscopic thin section analyses, micro-X-ray fluorescence (µ-XRF) element scanning and magnetic susceptibility measurements, supported by grain size data and palynological analyses. Based on radiocarbon and varve dating, two pronounced dry periods were detected at ~3500-3300 and ~3000-2400 cal. yr BP which are differently expressed in the sediment records. In the shallow-water core (DSEn), the older dry period is characterised by a thick sand deposit, whereas the sedimentological change at 2800 cal. yr BP is less pronounced and characterised mainly by an enhanced frequency of coarse detrital layers interpreted as erosion events. In the 5017-1 deep-basin core, both dry periods are depicted by halite deposits. The onset of the younger dry period coincides with the Homeric Grand Solar Minimum at ca. 2800 cal. yr BP. Our results suggest that during this period, the Dead Sea region experienced an overall dry climate, superimposed by an increased occurrence of flash floods caused by a change in synoptic weather patterns.

(Table 1) Mineralogy at DSDP Leg 79 Holes

Four sites in the region of the Mazagan Plateau off northwest Africa were drilled during Leg 79 of the Deep Sea Drilling Project. Bulk mineralogy and clay mineralogy were analyzed from the Cenozoic sediments recovered from the four sites.

Cloride and bromide concentrations and Br-/Cl- molar ratios in pore fluids at ODP Site 134-833

Other than halite diagenesis and organic matter degradation, Cl- and Br- are considered to be conservative in marine pore fluids. Consequently, Br-/Cl- ratios should remain constant during most diagenetic reactions. Nonetheless, Br-/Cl- molar ratios decrease to 0.00127 (~18% less than seawater value) in pore fluids from Site 833 in the Aoba Basin of the New Hebrides convergent margin despite the lack of halite diagenesis and little organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. These hydration reactions remove sufficient water to increase the concentrations of most solutes including Cl- and Br-. The resulting concentration gradients drive diffusion, but calculations indicate that diffusion does not decrease the Br-/Cl- ratio. Some Cl- may be leached from the ash, but insufficient amounts are available to cause the observed decrease in Br-/Cl- ratio. The limited source of Cl- suggests that proportionately more Br- than Cl- is lost from the fluids to the diagenetic solids. Similar nonconservative behavior of Cl- and Br- may occur during fluid circulation at ridge crests and flanks, thereby influencing the halide distribution in the crust.

Major and trace elements and minerals at DSDP Hole 79-544A

Fifty m of basement rocks underlying 185 m of Neogene and Mesozoic sediments were drilled seaward of the Mazagan Slope about 100 km west of Casablanca during Leg 79. These rocks are metagranites with mylonitic textures consisting dominantly of quartz, plagioclase, and potassium feldspar. Chemically, they are strongly peraluminous. This along with the absence of hornblende suggest that these rocks are similar to the S-type granites. Petrographic and chemical data suggest the possible existence of a former weathering surface on top of the Mazagan metagranite.

Sedimentological and geochemical characteristics of upper Cretaceous and lower Tertiary sediments of ODP Hole 108-661A

Based on sedimentological and geochemical data, the Upper Cretaceous and Tertiary sequence at Ocean Drilling Program Site 661 was subdivided into four intervals: Interval I (Campanian age) is characterized by sediments deposited below the calcite compensation depth (CCD) inside a high-productivity area and well-oxygenated bottom waters, indicated by the absence of carbonate, the major occurrence of zeolites and opal-CT, and intense bioturbation. Very fine-grained siliciclastic sediments and the lack of any erosional features suggest a low-energy environment. The terrigenous fraction was probably supplied by winds from the nontropical areas in South Africa. Interval II (Maestrichtian age) is characterized by high-amplitude variations in the carbonate content indicative of a deposition above the CCD, superimposed by (climate-controlled) short-term fluctuations of the CCD. The absence of both zeolites and opal-CT imply a position of Site 661 outside high-productivity areas. The first occurrence of higher amounts of kaolinite (especially during the middle Maestrichtian) suggests the onset of a terrigenous sediment supply from tropical areas. Interval III (between uppermost Cretaceous to early Tertiary) is characterized by the absence of carbonate and zeolites, interpreted as deposition below the CCD and outside an oceanic high-productivity belt. The kaolinite-over-illite dominance suggests a terrigenous sediment supply from tropical areas. Interval IV (between early Tertiary and Miocene age) is characterized by the occurrence of black manganeserich layers, major nodules/pebbles, and erosional surfaces, indicating phases of extremely reduced sediment accumulation and bottom-current activities. In the lower part of this interval (?Eocene age), higher amounts of zeolites occur, which suggest a higher oceanic productivity caused by equatorial upwelling. The source area of the terrigenous sediment fraction at Site 661 was the tropical region of northwest Africa, as suggested by the kaolinite-over-illite dominance.

Sedimentary mineralogy of argillaceous sediments at DSDP Legs 56-57 Holes

Mineral assemblages of DSDP Holes 436 and 438A and the upper section of Hole 439 (871.5-911.0 m sub-bottom) resemble each other and are composed of montmorillonite (probably a small portion of montmorillonite/illite mixed-layer clays), illite, chlorite, kaolinite, quartz, plagioclase, hornblende, calcite, dolomite, siderite, gypsum, pyrite, and halite. In the middle section of Hole 439 (933.5-1041.0 m), clinoptilolite is also found. In the lower section of Hole 439 (1077.5-1150.0 m), montmorillonite is not confirmed, and clinoptilolite and mixed-layer illite are found. These assemblages, which also contain detrital kaolinite, are generally found in sediments from brackish-water environments. At Site 439, more than 1000 meters of sediment might have been removed by erosion at the base.

Mineralogy and chemical composition of ODP Sites 129-800 and 129-801

Sites 800 and 801 in the Pigafetta Basin allow the sedimentary history over the oldest remaining Pacific oceanic crust to be established. Six major deposition stages and events are defined by the main lithologic units from both sites. Mineralogical and chemical investigations were run on a large set of samples from these units. The data enable the evolution of the sediments and their depositional environments to be characterized in relation to the paleolatitudinal motion of the sites. The upper part of the basaltic crust at Site 801 displays a complex hydrothermal and alteration evolution expressed particularly by an ochre siliceous deposit comparable to that found in the Cyprus ophiolite. The oldest sedimentary cover at Site 801 was formed during the Callovian-Bathonian (stage 1) with red basal siliceous and metalliferous sediments similar to those found in supraophiolite sequences, and formed near an active ridge axis in an open ocean. Biosiliceous sedimentation prevailed throughout the Oxfordian to Campanian, with rare incursions of calcareous input during the middle Cretaceous (stages 2, 4, and 5). The biosiliceous sedimentation was drastically interrupted during the Aptian-Albian by thick volcaniclastic turbidite deposits (stage 3). The volcanogenic phases are pervasively altered and the successive secondary mineral parageneses (with smectites, celadonite, clinoptilolite, phillipsite, analcime, calcite, and quartz) define a "mineral stratigraphy" within these deposits. From this mineral stratigraphy, a similar lithologic layer is defined at the top of the Site 800 turbidite unit and the bottom of the Site 801 turbidite unit. Then, the two sites appear to have been located at the same distal distance from a volcanic source (hotspot). They crossed this locality, at about 10°S, at different times (latest Aptian for Site 800, middle Albian for Site 801). The Cretaceous siliceous sedimentation stopped during the late Campanian and was followed by deposition of Cenozoic pelagic red clay (stage 6). This deep-sea facies, which formed below the carbonate compensation depth, contains variable zeolite authigenesis in relation to the age of deposition, and records the global middle Cenozoic hiatus events. At the surface, the red clay from this part of the Pacific shows a greater detrital component than its equivalents from the central Pacific deep basins.

Mineralogy and major element oxide composition of sediments from ODP Hole 172-1063D

Dansgaard-Oeschger (D-O) cycles in sediment at Site 1063 are characterized by distinct fluctuations in physical properties. Stadials are marked by low bulk density and interstadials by high bulk density. Compressional (P-)wave velocity is in phase with bulk density over some but not all depth intervals. Four of the D-O cycles straddling the oxygen isotope Stage 4/5 boundary have been studied in detail to understand the origin of the physical properties changes. Sediment on the Bermuda Rise is comprised of three main components: calcite, aluminosilicate minerals, and biogenic silica. Calcite concentrations vary from 1% to 43% of bulk sediment and are highest during interstadials. Aluminosilicate concentrations vary from 52% to 92% of bulk sediment and are highest during stadials. The major element ratios Al2O3/TiO2 and K2O/Al2O3 show increases across bulk density cycles, suggesting a change in the composition of aluminosilicates. This interpretation is supported by mineralogical analyses, which show a subtle change in clay composition. Biogenic silica concentrations vary from 0% to 23% of bulk sediment and are also highest during stadials. However, the abundance of silica varies significantly from one D-O cycle to another. Silt and fine sand abundance also increase during the first of the four stadials. This coarsening of sediment coincides with the increase in biogenic silica. The low grain density and high porosity associated with biogenic silica result in intervals of low bulk-sediment density. The abundance of biogenic silica closely matches P-wave velocity, suggesting that silica imparts a greater rigidity to the sediment.