Injection clastic dykes in the Lower Oxford Clay (Jurassic) of central England: relationship to compaction and concretion formation

An injection dyke of fine-grained sandstone derived from the Kellaways Sand Formation intrudes overlying organic-rich shales and shell beds of the Lower Oxford Clay. The dyke shows cross-cutting relationships with early carbonate concretions, and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales. Internally the dyke displays flow-like features, and the walls show lobate flow structures. Clasts of uncompacted Lower Oxford Clay and fragments of pyrite-rich concretions occur within the sandstone intrusions. The sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions. This cementation took place prior to final compaction of the Oxford Clay. The dyke has a sub-parallel relationship to the nearby Tinwell-Marholm fault suggesting that the dyke may be related to local tectonic events during the Middle Jurassic.     

The Medusa effect: instantaneous fossilization

Rapid fossilization of fishes and other animals in the Lower Cretaceous of the Chapada do Araripe, north-east Brazil, has preserved the most delicate structures known in the fossil record. Gills, muscles, stomachs and even eggs with yolks have been found. Interestingly, the material is not particularly rare, and specimens can be purchased from your local rock shop.     

Palaeo-yardangs: wind-scoured desert landforms at the Permo-Triassic unconformity

In Canyonlands National Park, south-east Utah, at least 29 partly exhumed, aligned sandstone ridges trending generally N20°W occur at the upper unconformable surface of the Lower Permian (Leonardian) White Rim Sandstone. The ridges are at least 1·5 km long, 250 m wide and have up to 14 m of vertical relief (mean of 9 m). A thin lag of coarse sandstone that contains wind-ripple laminae and granule ripples directly overlies the ridges. Angular blocks of sandstone within the lag and sand-filled fissures immediately below the lag, within the ridges, attest to early cementation of the ridge-forming material. SE-dipping aeolian cross-strata within the White Rim Sandstone and within the lag closely parallel the ridge trend. The ridges are interpreted as wind-sculpted desert landforms (yardangs) that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian.     

Modeling synthetic aperture radar (SAR) scattering from a seasonally varying snow-covered sea ice volume at 5.3 and 9.25 GHz

A series of sensitivity analyses using dielectric, mixture and microwave scattering models is presented. Data from the Seasonal Sea Ice Monitoring and Modeling Site (SIMMS) in 1990 and 1991 are used to initialize the models. The objective of the research is to investigate the role of various geophysical and electrical properties in specifying the total relative scattering cross section (ρ') of snow covered first-year sea ice during the spring period.
The seasonal transition period from the Winter SAR scattering season to Early Melt was shown to signal a transition in dielectric properties which caused the snow volume to become a factor in the microwave scattering process. The effect of the thermal insulation of a snow cover on sea ice was shown to be significant for both ε' and ε'. Higher atmospheric temperatures caused proportionally greater changes in the dielectric properties of the sea ice at the base of the snow cover. Model ρ⁰ was computed for a range of sensor, sensor-earth geometry, and geophysical properties. In the Winter season the surface roughness terms (o_hand L) were shown to have a significant impact on ρ⁰ when the ice surface was the primary scattering mechanism. Once the snow cover began to warm and water was available in a liquid phase, the ice surface became masked because of the decrease in microwave penetration depths. During this period the water volume variable dominated ρ⁰, both from its impact on ρ_v⁰, and due to its control over the dielectric mismatch created at the air/snow interface.     

Microbiological effects on slope stability: an experimental analysis

A natural, pure quartz sand has been seeded with the bacterium Pseudomonas atlantica and the fungus Penicillium chrysogenum, and angles of avalanche and repose have been measured under water using a laboratory clinometer. The lowest angles of avalanche occur in freshly packed clean sediment (control), with the seeded sediments having higher values. Among the latter, the lowest angles of repose occur in the bacterial seeded sediments, and the highest in the fungal seeded sediments. The largest differences between the angle of avalanche and angle of repose occur in the bacterial seeded and media control sediments. The smallest differences occur in the fungal seeded sediment. In most cases the second angle of avalanche is lower than the first angle of avalanche, whilst the second angle of repose is higher than the first angle of repose. The bacteria bind particles together with their extracellular polymeric material, while the fungus binds particles by holding them together with a network of hyphal filaments. In the bacterial seeded sediment growth is uniform over the sediment surface. In the fungal seeded sediment growth occurs as discrete colonies separated by bare sediment, and the fungal hyphae penetrate the sediment to a significant depth. On avalanching, the fungal colonies move down the slope with the hyphal filaments trailing behind them in the sediment. Overall, both the bacterium and the fungus increase slope stability. However, the fungal colonies maintain slope stability after avalanching more effectively than does the uniform bacterial growth. The results are discussed in relation to the wide range of biological effects that stabilize flat sediments and to laboratory and field studies on the stability of sediment slopes.     

Late Quaternary sediment, sediment mass flow processes and slope stability on the Scotian Slope, Canada

The study area, just to the west of the Verrill Canyon on the Scotian Slope, eastern Canada, exhibits both large and small scale sediment mass movement features. Study of high resolution seismic reflection and sidescan sonar data shows that a large portion (approximately 70%) of the near surface sediment (<20 m) in the area has undergone erosion, rotational slumping and internal deformation. Remoulded sediment observed in physical properties profiles of piston cores and sediment deformation structures are further evidence of slumping. Small scale mass flow events are recorded by abundant turbidites and debris flow deposits noted in piston cores. Sediment physical properties are highly dependent on sediment type (lithofacies). Frequent facies changes, both temporally and spatially, make correlation between cores difficult. Although the small scale mass movement events correlate with glacial recession on the continental shelf and lower relative sea levels, the triggering mechanisms for the large scale events are less obvious. Slope stability analyses indicate that, at present, the seabed is stable. The most plausible explanation for large scale slope failures in this region are ground accelerations related to earthquake shock. Our analyses demonstrate that it is unlikely that large magnitude, distant earthquakes, such as those previously proposed in the Laurentian Slope Seismic Zone (LSP) model, could initiate failure of sediment in the study region. Our data support the interpretation that more frequent, lower magnitude earthquakes, closer to the study region, as previously proposed in the Eastern Slope Experimental Source Zone (ESX) model, are the likely causes of large scale slope failures. Furthermore, excess pore pressures resulting from shallow gas and/or high sedimentation rates during deglaciation contribute to slope failure.     

Synsedimentary dolospar cementation: a possible Devonian example in the Camsell Formation, Northwest Territories, Canada1

A unique association of beds containing fenestral pore-filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the Lower Devonian Camsell Formation. Many of these sandstones consist almost entirely of dolospar sand. Sedimentological, petrographic, cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during Camsell deposition from fenestral dolostone beds that directly underlie these sandstones. Erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization, or dolomite cementation of the pore-filling carbonate cements within fenestrae of the fenestral fabrics, occurred before erosion and deposition of dolospar sand. The fine details of the cathodoluminescent zonations suggest that the pore-filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures. This unique occurrence may document synsedimentary Devonian cementation by medium to coarsely crystalline dolomite.     

Facies-controlled shrinkage-crack assemblages in Middle Proterozoic mudstones from Montana, USA

Shrinkage-crack morphotypes in the Libby Formation (upper Belt Supergroup) are confined to distinct environmental facies. The lower facies is characterized by flat rip-up clasts, stromatolites, oolites, small-scale symmetrical ripples, and fenestral fabric. These rocks were deposited above fair-weather wave base on a periodically exposed mudflat. Shrinkage cracks in this facies are predominantly branching, incompletely connected features in plan view, except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare, long spindle-shaped cracks on bed tops of dark grey argillite. The upper facies was deposited below fair-weather wave base and contains mainly unconnected, short spindle-shaped shrinkage cracks, and rare slightly branching cracks. Restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks. The cracks in the upper facies were strongly influenced by sediment loading, and may have formed by compaction-induced expulsion of water from pore space, resulting in synaeresis cracks. In the underlying shallower facies, polygonal cracks formed by desiccation. Elsewhere in this facies, incomplete, partially connected cracks and long spindle-shaped cracks on the same bedding plane are interpreted as having formed by desiccation. Shrinkage cracks are an under-used source of environmental information, but confusion as to their origin sometimes restricts their potential. More intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology. Crack cross-sections, which are often more commonly exposed than bedding plane cracks, may provide critical additional information on crack genesis. Better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in Precambrian and lacustrine settings.     

Petrology of Ultramafic Xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo_{84–88 6} and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO₂ Dunitecontains Fo_{83 5–88?5} olivine, magnesiochromite with l?5–6?9wt.% TiO₂ (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo_{85 9} olivine and magnesiochromitewith 1?9–4?7 wt.% TiO₂ poikilitically enclosed in chrome-richdiopside (Wo_{45 4}En_{48 0}Fs_6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo_{89 5–926} olivine, enstatite (Wo_{2 0–2?7}En_{90?0–88 7}Fe_8?0–8?6),Cr-rich endiopside (Wo_{43 4–44 5}En_{52 0–50 0}Fs_{4 6–45}), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle.     

The Atomic Ratios of Natural Ferruginous Biotites with reference to 'The Stability Relations of the Ferruginous Biotite, Annite'

The atomic ratios given by Eugster and Wones (1962) for analysesof high FeO-low MgO micas cited by Foster (1960) are in error.The recalculated atomic ratios herein presented show that onlythree of the thirteen have low (OH+F), and only one has characteristicsthat may possibly be due to loss of water during determination.None of the analyses exhibit characteristics suggestive of oxidationof iron and loss of H, as in progression towards oxyannite. The average recalculated octahedral occupancy is 2.69, not 2.91as in the previous calculation. The differences between atomic ratios calculated on the basisot determined H₂O⁺ and F and those calculated on the basis ofthe theoretical O₁₀ (OH)₂ content are not great enough to changethe essential compositional character of these micas as interpretedby Foster (1960). However, they do affect significantly thecalculated octahedral occupancy.