A large-scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn-rift or early post-rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation. The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low-stand of sea-level. They were subsequently filled by a fining-upwards suite of sediments which reflects subsequent relative rise of sea-level and carbonate platform development. Ultimately the canyon complex was buried by north-westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle. It is considered possible that the series of isolated outcrops of canyon cross-sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge-like Patsy Springs Canyon lies in more proximal regions of the basin-slope, whereas 40 km to the north-east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel-fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg. The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel-fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.     

Stable isotope study of carbonate sediments from the Coorong Area, South Australia

ABSTRACT
The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange.
The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ¹³C = -1 to -2% 0 ; δ¹⁸O=+3 to +5%₀) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ¹³C=+3 to +4%₀; δ¹⁸O=+5 to + 6%₀) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO² probably by the dolomitization of aragonite.     

The Zircon-Bearing Chromitites of the Phlogopite Peridotite of Finero (Ivrea Zone, Southern Alps): Evidence and Geochronology of a Metasomatized Mantle Slab

The phlogopite peridotite unit of the Finero Complex is a restiticharzburgite that records two metasomatic events. The first eventis related to the intrusion of basaltic magma, which reactedwith the pyroxene of the host harzburgite to produce chromititepods with dunite haloes. It also produced secondary clinopyroxeneand amphibole in the harzburgite and enriched harzburgite inNa and the light rare earth elements. The second metasomaticevent is related to the later intrusion of clinopyroxeniticdykes. During this event, water-rich vapour penetrated the harzburgitealong fractures and reacted with it to form phlogopite, thusenriching the rock in K. Chromitites host zircons that yieldan age for the first metasomatic event of 207·9 + 1·7/-1·3Ma, during which time extensional tectonics prevailed in theSouthern Alps. KEY WORDS: metasomatism; chromitite; zircon; geochronology; Finero     

Spherular modern dolomite from the Coorong area, South Australia

Scanning electron micrographs show that the youngest and apparently least altered of the Coorong dolomite is in the form of spherular bodies about 0.2–1.0 μm in size which themselves are composed of spherules about 100 nm in diameter. Older and more lithified sediments show sharply defined dolomite crystals suggesting an origin as primary dolomite spherules followed by aggregation and diagenetic alteration to well crystallized dolomites.     

OBER DIE PALAEOZOISCHE FLORA DER PROVINZ SUIYUAN~1

Die palaeozoischen Pflanzen der Provinz Suiyuan sind bisher sehr wenig bekannt. Die in der vorliegenden Arbeit beschriebenen Pflanzen stammen aus dem sogenannten "Tachingshan Range" namentlich aus den Localitaten Tungshengmao und Kumenyintien. Die Fundorte liegen also etwa an der Grenze     

Metamorphic Petrology of Xenoliths from Kenya and Northern Tanzania and Implications for Geotherms and Lithospheric Structures

Pyroxenitic and peridotitic xenoliths from the Quaternary volcanicfield of Marsabit (northern Kenya) bear strong evidence of decompressionand cooling. Pyroxenites are mostly garnet (grt) websteritesand grt clinopyroxcnites with some olivine (ol) and amphibole(amph). Grt is mostly rimmed by kelyphitic reaction zones butotherwise appears to have been in stable association with thepyroxenes. Along contacts between grt and rare ol, medium-grainedsymplectites consisting of orthopyroxene (opx), clinopyroxene(cpx), and spinel (spl) occur. Garnets do show significant compositionalvariations from core to rim. Primary pyroxenes are strained,have exsolution lamellae, and are chemically zoned. Integratedcore compositions of pyroxenes and grt compositions yield temperaturesof 1065–950 C and pressures of 28–23 kb (stage1). Pyroxene rims in contact with grt or kelyphite show Ca concentrationssimilar to, but Al concentrations higher than pyroxene rimsremote from garnet. Grt-opx contacts yield pressures of 11.5–9.0kb, and temperatures of 860–770C are obtained from pyroxenerims (stage 2). Peridotites from Marsabit show various stages of transformationfrom the garnet peridotite to the spinel peridotite stabilityfield. On the basis of differences in textures and mineral compositionsthey can be grouped into four types. Type I has a granular textureand contains fine-grained opx-cpx-spl symplectites frequentlysurrounding kelyphite which, in turn, may enclose relict grt.Rare matrix spl has higher Cr/(Cr + Al) ratios (0.25–0.32)than symplectitic spl (0.09). As in grt pyroxenites, matrixpyroxenes are strained, show exsolution lamellae, and have rimcompositions which are dependent on their positions relativeto former garnet. Integrated core compositions of matrix pyroxenessuggest former equilibration temperatures between 1050 and 880Cand pressures between 25 and 19 kb (opx—grt barometryusing composition of relict grt; stage 1). Pyroxene rims yieldsignificantly lower temperatures of 920–785 C (stage2). These P—T estimates and the occurrence of one compositexenolith consisting of type I peridotite and grt pyroxenitepoint to a common P—Tevolution of both grt pyroxenitesand type I peridotites. Granular type II peridotites are characterizedby medium-grained clusters of opx + cpx + spl amph and containmatrix spl, too. Pyroxenes are never strained and are free ofexsolution lamellae. All minerals are homogeneous and thereare no compositional differences between pyroxenes and spinelsof the matrix and those of the spl—opx—cpx clusters.Cr/(Cr+Al) ratios of spl are between 0–07 and 0.11. Two-pyroxenetemperatures are relatively uniform (970–925 C at anassumed pressure of 12 kb; stage 2). Type III peridotites arecoarse-grained granular spl peridotites without any indicationof the former presence of grt. Cr/(Cr + Al) ratios of spl aresimilar to those of peridotite type II. Pyroxenes show minorchemical zoning with Ca increasing in opx but decreasing incpx from core to rim indicating temperatures of 960–900C for pyroxene cores and of up to 1000C     

Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM

Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May–10 June 2006) at the site Porte au Sahara (30.237°N, 5.607°W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.     

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.