Sedimentology of a saline playa in the northern Great Plains, Canada

Ceylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present-day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite. Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate-clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na-rich solution to a mixed Mg-Na system. The most important post-depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.     

Detrital modes and provenance of the Stilo-Capo d'Orlando Formation (Miocene), southern Italy

The Stilo-Capo d'Orlando Formation (SCO Fm) is a Miocene clastic unit nonconformably covering the basement of the southern portion of the Calabria-Peloritani Arc (CPA), a small-scale terrane which was accreted onto the Apennines-Maghrebides orogenic system during Neogene time. The SCO Fm is composed of conglomerate, sandstone and mudrock deposited in a marine environment through the action of gravity currents. Detrital modes of sandstones and conglomerates of the SCO Fm indicate a local provenance. In fact, petrological parameters can be matched directly with nearby basement lithologies. Two petrofacies with distinctive parameters are present. A northern petrofacies is predominantly composed of plutonic and low-grade metamorphic detritus, and was derived from the Serre massif, which is mostly made of late Hercynian plutons intruded into metasedimentary rocks. A southern petrofacies is mostly composed of metamorphic detritus and was derived from the Aspromonte and the Peloritani Mountains, which are predominantly composed of middle-to high-grade metamorphic rocks. Petrological results place important constraints on the geodynamic evolution of the CPA. According to several authors, the arc is composed of two sectors (microterranes) with different geological histories and partly composed of contrasting rock types, whose timing of juxtaposition is debated (Early Cretaceous against middle-late Miocene; that is, much before or soon after deposition of the SCO Fin). The SCO Fm crops out only in the southern sector of the CPA and does not contain any detritus derived from the northern sector, in spite of a favourable sediment palaeodispersal system. This fact indicates that the amalgamation of the two sectors probably took place after deposition of the SCO Fm.     

Uranium oxidation and probable subaerial weathering of phosphatized limestone from the Pourtales Terrace*

Phosphatized limestones from the Pourtales Terrace, Straits of Florida, have undergone fresh-water diagenesis, as shown by several selective petrologic features such as dissolution of aragonitic skeletal material and preservation and recrystallization of Mg-calcite. This interpretation is supported by the fact that greater than 94% of the uranium in these samples is in the U(VI) oxidation state. On the deeper Pourtales Escarpment, phosphatized limestones show no alteration features indicative of fresh-water diagenesis. The uranium in these samples is a mixture of U(IV) and U(VI) with oxidation-state ratios similar to other sea-floor phosphorites. Our interpretation of these data is that uranium in the terrace samples was oxidized when that area was emergent. The karsted surface of the terrace, therefore, is a result of subaerial exposure of this portion of the Floridan platform. The escarpment samples were never above sea level, and so more closely retain their original uranium oxidation-state ratios.     

The relation between the crystallography of quartz, and upturned aeolian cleavage plates

The relation between the crystallography of artificially abraded quartz sand grains and aeolian surface textures has been studied using an X-ray precession camera and the scanning electron microscope (SEM). Crushed Brazilian quartz was sieved to between 250 and 350 μm, eroded in a paddle wheel device which simulated aeolian action at 8 m s^?1 for 3 h, and photographed with the SEM. A typical grain was selected and over 1100 photographs were combined to create a 3 × 3 m photomosaic; because of the large size, it was possible to observe and measure the angular and linear relations between the various features. After the c axis direction in the grain was determined by precession X-ray analysis, the most important aeolian features were related to the crystallography of quartz. The upturned plates or cleavage plates probably represent traces parallel to r(10ī1) and z(01ī1) cleavages in quartz. Blocky areas appear to be an expression of a cleavage parallel to m(10ī0). In addition, plate spacing on thirty-five experimental, modern, and ancient quartz sand grains is fairly constant. Assuming that abrasion in most sedimentary environments acts in the same manner with respect to quartz crystallography, much of the fine silt and clay in sediments and sedimentary rocks may be the result of cleavage following abrasion parallel to the r and z cleavage planes, while the less common blocky particles are probably the result of cleavage parallel to the m planes.     

Geochemistry and origin of a black mudstone in a volcaniclastic environment, Ordovician Lower Rhyolitic Tuff Formation, North Wales, UK

Black mudstones in marine volcaniclastic environments have been interpreted both as non-volcanic ‘background’sedimentation and as fine grained vitric dust from the waning stages of primary volcanism. Although difficult to distinguish by standard petrographic techniques, differentiation between the two is crucial when attempting to determine sedimentation rates or to infer periods of volcanic quiescence. In the Ordovician Lower Rhyolitic Tuff Formation of North Wales such a fine grained black unit at Cwm Idwal is geochemically similar to an underlying rhyolite ash flow tuff. Its chemical index of alteration (CIA) is identical to that of the tuff. These data suggest that the black mudstone unit is a vitric tuff related to the underlying ash flow tuff. Use of a CIA in addition to trace element geochemistry should, in most cases, serve to distinguish tuffs from silicified mudstones.     

The Rio Dell Formation: a Plio-Pleistocene basin slope deposit in Northern California

The Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope.     

Transformation of Archaean Lithospheric Mantle by Refertilization: Evidence from Exposed Peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneissesat several localities in the Western Gneiss Region (WGR) ofwestern Norway; garnet peridotites typically occur as discretezones within larger bodies of garnet-free, chromite-bearingdunite and are commonly closely associated with pyroxenitesand eclogites. The dunites of the large Almklovdalen peridotitebody have extremely depleted compositions (Mg-number 92–93·6);the garnet peridotites have lower Mg-number (90·6–91·7)and higher whole-rock Ca and Al contents. Post-depletion metasomatismof both rock types is indicated by variable enrichment in thelight rare earth elements, Th, Ba and Sr. The dunites can bemodelled as residues after very high degrees (>60%) of meltextraction at high pressure (5–7 GPa), inconsistent withthe preservation of lower degrees of melting in the garnet peridotites.The garnet peridotites are, therefore, interpreted as zonesof melt percolation, which resulted in refertilization of thedunites by a silicate melt rich in Fe, Ca, Al and Na, but notTi. Previous Re–Os dating gives Archaean model ages forthe dunites, but mixed Archaean and Proterozoic ages for thegarnet peridotites, suggesting that refertilization occurredin Proterozoic time. At least some Proterozoic lithosphere mayrepresent reworked and transformed Archaean lithospheric mantle. KEY WORDS: Archaean mantle; Proterozoic mantle; Western Gneiss Region, Norway; mantle metasomatism; garnet peridotite     

EDWARD BELLAMY AND THE WEATHER OF UTOPIA

ABSTRACT. Utopian thinkers have often assumed that radical geoengineering is necessary for the creation of a perfect world. This assumption necessarily puts them at odds with environmentalism, but the conflict is not inescapable. Human difficulties with the biophysical world can instead be interpreted as arising from the interaction of environment with society and thus as capable of being eradicated simply by reforming the latter. One notable early exponent of this kind of social constructionism was the American utopian novelist and publicist Edward Bellamy (1850–1898). His fictional and nonfictional writings analyzed the ways in which the troubles that Americans of his time had with weather and climate grew out of their ways of life and political‐economic institutions and would disappear if these were reformed. This line of thought allowed Bellamy to portray a utopia where human beings had ceased to suffer serious harm and inconvenience from the weather yet had not tampered with the atmospheric environment itself.