Top Of Section Research Articles

Authigenic carbonate sedimentation in Eravnoe group lakes (Western Transbaikalia): Response to Holocene climate change

Sediments of intracontinental closed lakes are unique natural archives that store climate history records. We study Late Holocene carbonate-bearing sediments of Lake Dolgoe, a small saline lake in the Eravnoe basin on the Vitim Plateau (western Transbaikalia). Sediment samples have been analyzed by several methods: X-ray powder diffraction, infrared spectroscopy, stable isotope (18O and 13C) determination, laser diffraction particle size analysis, XRF elemental analysis, etc. The mineral phases of bottom sediments include clastic and carbonate components and hydromagnesite at the section top; carbonates are authigenic varieties of the calcite-dolomite series. Modeling of complex XRD profiles reveal Mg-calcite with various Mg contents and excess-Ca dolomites, which are mixed-structure crystals compositionally close to stoichiometric dolomite. The crystal structure and relative percentages of carbonates from the dated sedimentary section controlled by climate and lake level changes allow reconstructing the Holocene history of Lake Dolgoe.

Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA

The McCoy Mountains Formation consists of Upper Jurassic to Upper Cretaceous silt- stone, sandstone, and conglomerate exposed in an east-west-trending belt in southwest- ern Arizona and southeastern California. At least three different tectonic settings have been proposed for McCoy deposition, and multiple tectonic settings are likely over the ~80 m.y. age range of deposition. U-Pb iso- topic analysis of 396 zircon sand grains from at or near the top of McCoy sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Moun- tains, all in western Arizona, identifi ed only Jurassic or older zircons. A basaltic lava fl ow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 ± 2.1 Ma. Geochemically similar lava fl ows and sills in the Granite Wash and southern Plomosa Mountains are inferred to be approximately the same age. We interpret these new analyses to indicate that Meso- zoic clastic strata in these areas are Upper Jurassic and are broadly correlative with the lowermost McCoy Mountains Formation in the Dome Rock, McCoy, and Palen Moun- tains farther west. Six samples of numerous Upper Jurassic basaltic sills and lava fl ows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains yielded initial e Nd values (at t = 150 Ma) of between +4 and +6. The geochemistry and geochronology of this igne- ous suite, and detrital-zircon geochronology of the sandstones, support the interpretation that the lower McCoy Mountains Forma- tion was deposited during rifting within the western extension of the Sabinas-Chihuahua- Bisbee rift belt. Abundant 190-240 Ma zir- con sand grains were derived from nearby, unidentifi ed Triassic magmatic-arc rocks in areas that were unaffected by younger Juras- sic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock Mountains (Arizona) yielded numer- ous 80-108 Ma zircon grains and almost no 190-240 Ma grains, revealing a major reor- ganization in sediment-dispersal pathways and/or modifi cation of source rocks that had occurred by ca. 80 Ma.

Geometria e cinemática da aba sul da estrutura divergente do Rio Paraíba do sul ao longo da Seção Areal-Três Rios, Rio de Janeiro

The Rio Paraiba do Sul structural divergence is a regional scale structure in which the mylonitic foliation delineate a fan geometry, those of the northern limb dipping to SE, and those of the southern limb dipping to NW. On the core of the structure occurs a large dextral ductile transpressional shear belt. The present work deals with the geometry and kinematics of structures in a section cross-cutting the southern limb of this structure. At the base of section, occurs a high-grade banded mylonite granitoids, with mylonitic foliation dipping moderately to NW and stretching lineation plunging to NNE. Northwards, the same geometry of ductile structures is seen in mylonitic S-type granitoids and paragneisses. In this region, however, the foliation steepens and the lineation increases in plunge with a slightly rotation towards the NE. Flattening strains leads to the formation of S and SL high-grade tectonites. At the base of section shear-sense indicators (S-C foliations, s -porphyroclasts and shear-bands) suggest a dextral component in horizontal planes and down-dip normal motion in a monoclinic shear-zone model. Towards the top of section, however, the asymmetry of the structures is not clear, but one oblique shearing with top-to-SW is also present. To the north the foliation have slow dips, and the stretching lineation plunges shallower to NE. In this domain the shearsense indicators suggest top-to-SW sense of shear parallel to the orogenic trend. Field relations suggests that ganitoids sheets were intruded lit-par-lit during this stage of the tectonic movements. In this domain also occur open to tight folds with axis sub-parallel to the stretching lineation in the same outcrop and axial surfaces with high dips preferentially to NW. This folding is interpreted as resulted from a shortening component at high angle to the regional shear-zones during transpressional strain, coeval with transpressive dextral shear-zones and thrust (ductile/ruptile) with top-to-SE sense of shear. The structural and field data suggests that the early movements were parallel and oblique to the belt in this region, and occurred near the peak temperatures of regional metamorphism accompanied by widespread crustal anatexis. This early stage of motion would be interpreted as horizontal and vertical extrusion of a partially molten crust during unconfined transpression.

Dinoflagellate cysts from the Rabot Member (Santa Marta Formation) of eastern James Ross Island

Palynomorph assemblages dominated by dinoflagellate cysts are described from seventeen samples from the Rabot Member of the Santa Marta Formation at Ekelöf Point, eastern James Ross Island, Antarctic Peninsula. Although the assemblages are of relatively low diversity, the dinoflagellate cyst taxa recorded indicate a mid to late Campanian (Late Cretaceous) age. Changes in species diversity, dominance and gonyaulacacean ratio suggest a gradually reducing distance from shore during deposition, with a return to more offshore conditions towards the section top. A new species of dinoflagellate cyst, Isabelidinium papillum, is described.

$$^{40}Ar/^{39}Ar$$Age Calibration of the Litho- and Paleomagnetic Stratigraphies of the Ngorora Formation, Kenya

Precise eruptive ages have been determined by the laser-fusion, single-crystal $^{40}Ar/^{39}Ar$method for juvenile volcanic feldspars from reworked and contaminated volcaniclastic rocks of the middle Miocene Ngorora Formation, Kenya Rift Valley. These ages range from 13.06 Ma at the base to 10.51 Ma toward the top of the type section near Kabarsero. Correlation of the local paleomagnetic stratigraphies with the geomagnetic reversal time scale yields magnetochronologic age estimates that are younger than the isotopic ages by an average of 0.18 Ma. Much of the discrepancy can be eliminated if an inferred change in sea-floor spreading rate occurred at 13 Ma or earlier, rather than at 10.42 Ma as previously suggested. Sedimentation rates at Kabarsero calculated from the $^{40}Ar/^{39}Ar$results decrease from initial values of ~25 cm/1000 yr to ~5 cm/1000 yr toward the top of the section. The initial rapid sedimentation rates characterize the first 0.1 to 0.3 m.y. following emplacement of the underlying, volu...

Contrasted mineralogy and textural relationships in the uppermost titaniferous magnetite layers of the Bushveld Complex in the Bierkraal area north of Rustenburg

Disseminated Fe-Ti oxides and ore-rich concentrations of these minerals in a 1,230-m vertical borehole section through the top of the upper zone of the Bushveld Complex are described. The succession consists of a layered sequence of highly fractionated olivine-rich rocks that range in composition from ferrogabbro to diorite. The Fe content of the olivine increases systematically from Fa 65 at the base to a maximum of Fa 94 at the top of the section. Fe-Ti oxides axe present throughout this interval and are divided into three distinct groups on the basis of their mineralogy and chemistry: (1) an apatite-poor, high Ti magnetite association, (2) an apatite-rich, high Ti magnetite association, and (3) an apatite-poor, low Ti magnetite association. The Ti magnetite in each of these associations exhibits a wide variety of ulvoespinel and/or ilmenite microintergrowths, the development of which can be interpreted in terms of the ambient f (sub O 2 ) at various stages of subsolidus cooling. Certain Ti magnetite crystals are characterized by the development of radiating, eutectoidlike intergrowths of magnetite and ilmenite or ulvoespinel that appear to represent duplex cell decomposition products of magnetite-ulvoespinel solutions.The apatite-poor, high Ti magnetite represents the dominant opaque oxide association and is present both as disseminations and distinct ore-rich layers, particularly toward the base of the study section. The greatest concentration of this association occurs at a depth of between 1,033 to 1,060 m below the top of the intrusion. The lowermost 14 m of this interval consist of a thick composite Ti magnetite layer that contains numerous silicate inclusions and partings. This is believed to represent the equivalent of the uppermost Ti magnetite layer (layer 21) in the eastern Bushveld Complex. Groups of minor Ti magnetite layers are also present at depths of 412 and 225 m below the top of the intrusion. The Ti content of the magnetite increases with increasing stratigraphic height from 55 mole percent ulvoespinel at the base to 65 mole percent near the top of the study section where it gives way to the Ti-poor association. The beginning of Ti-poor magnetite precipitation 225 m below the roof of the intrusion marks the onset of Ti depletion in the residual liquid. Ilmenite is virtually the only Fe-Ti oxide phase present in the uppermost 100 m of the complex. Both these associations formed in response to normal fractional crystallization processes, the formation of discrete ore-rich layers being triggered by episodic increases in f (sub O 2 ) . The apatite-rich, high Ti magnetite association is present both as disseminations and in ore-rich concentrations but is restricted to three relatively narrow horizons within the study section. These are developed at the following depths below the top of the intrusion: 233 to 283 m, 677 to 700 m, and 1,033 to 1,108 m. Discrete ore-rich concentrations consisting almost entirely of Ti magnetite + ilmenite ( approximately 70 vol %) and apatite ( approximately 30 vol %) are present in the lower 2 m of the middle horizon. The upper and lower horizons contain only disseminated opaque oxide-apatite assemblages. The formation of these assemblages is interpreted in terms of the separation of immiscible Fe-Ti-Mn-Ca-P-REE-enriched liquids from the late-stage dioritic residual melt.