Scanning electron microscopy of quartz grains in supraglacial debris, Adishy Glacier, Caucasus Mountains, USSR

The surface microtopography of quartz grains in glacial depostis has often been attributed to mechanical release from source rocks during weathering and prior to entrainment by glaciers. Fractures on quartz particles, including subparallel and conchoidal features, often attributed to cryostatic pressure in ice and stick-slip processes at the base of glaciers, have been considered, in part, to result from mechanical weathering of source rocks. To test this hypothesis we studied 15 samples of supraglacial debris from the Adishy Glacier in the central Caucasus Mountains, USSR. Clasts in transport on the surface of the glacier originate primarily from the cirque headwall by weathering and mass wasting processes; a minor amount is presumed to have been derived from aeolian influx following reworking of older sediments down valley. The results show unequivocally that mechanical weathering is only likely to produce grains with pronounced fracture faces that lack subparallel and conchoidal fracture features. No fresh glacial-crushing type features were observed on these samples. Grains that had been glacially crushed, weathered and retransported, presumably by aeolian processes, were observed in four samples of the data set.     

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.     

The sedimentology and alluvial architecture of the sandy braided South Saskatchewan River, Canada

Ground penetrating radar (GPR) surveys of unit and compound braid bars in the sandy South Saskatchewan River, Canada, are used to test the influential facies model for sandy braided alluvium presented by Cant & Walker (1978) . Four main radar facies are identified: (1) high‐angle (up to angle‐of‐repose) inclined reflections, interpreted as having formed at the margins of migrating bars; (2) discontinuous undular and/or trough‐shaped reflections, interpreted as cross‐strata associated with the migration of sinuous‐crested dunes; (3) low‐angle (< 6°) reflections, interpreted as formed by low‐amplitude dunes or unit bars as they migrate onto bar surfaces; and (4) reflections of variable dip bounded by a concave reflection, interpreted as being formed by the filling of channel scours, cross‐bar channels or depressions on the bar surface. The predominant vertical arrangement of facies is discontinuous trough‐shaped reflections at the channel base overlain by discontinuous undular reflections, overlain by low‐angle reflections that dominate the deposits near the bar surface. High‐angle inclined reflections are only found near the surface of unit bars, and are of relatively small‐scale (< 0·5 m), but can be found at a greater range of depths within compound bars. The GPR data show that a high spatial variability exists in the distribution of facies between different compound bars, with facies variability within a single bar being as pronounced as that between bars. Compound bars evolve as an amalgamation of unit bars and other compound bars, and comprise a facies distribution that is representative of the main bar types in the South Saskatchewan River. The GPR data are compared with the original model of Cant & Walker (1978) and reveal a much greater variability in the scale, proportion and distribution of facies than that presented by Cant & Walker (1978) . Most notably, high‐angle inclined strata are over‐represented in the model of Cant and Walker, with many bars being dominated by the deposits of low‐ and high‐amplitude dunes. It is suggested that further GPR studies from a range of braided river types are required to properly quantify the full range of deposits. Only by moving away from traditional, highly generalized facies models can a greater understanding of braided river deposits and their controls be established.