Vertical Exposures Research Articles

Estimating per-act HIV transmission risk: a systematic review.

Effective HIV prevention programs rely on accurate estimates of the per-act risk of HIV acquisition from sexual and parenteral exposures. We updated the previous risk estimates of HIV acquisition from parenteral, vertical, and sexual exposures, and assessed the modifying effects of factors including condom use, male circumcision, and antiretroviral therapy. We conducted literature searches to identify new studies reporting data regarding per-act HIV transmission risk and modifying factors. Of the 7339 abstracts potentially related to per-act HIV transmission risk, three meta-analyses provided pooled per-act transmission risk probabilities and two studies provided data on modifying factors. Of the 8119 abstracts related to modifying factors, 15 relevant articles, including three meta-analyses, were included. We used fixed-effects inverse-variance models on the logarithmic scale to obtain updated estimates of certain transmission risks using data from primary studies, and employed Poisson regression to calculate relative risks with exact 95% confidence intervals for certain modifying factors. Risk of HIV transmission was greatest for blood transfusion, followed by vertical exposure, sexual exposures, and other parenteral exposures. Sexual exposure risks ranged from low for oral sex to 138 infections per 10,000 exposures for receptive anal intercourse. Estimated risks of HIV acquisition from sexual exposure were attenuated by 99.2% with the dual use of condoms and antiretroviral treatment of the HIV-infected partner. The risk of HIV acquisition varied widely, and the estimates for receptive anal intercourse increased compared with previous estimates. The risk associated with sexual intercourse was reduced most substantially by the combined use of condoms and antiretroviral treatment of HIV-infected partners.

Sequential fragmentation/transport theory, pyroclast size–density relationships, and the emplacement dynamics of pyroclastic density currents — A case study on the Mt. St. Helens (USA) 1980 eruption

Pyroclastic density currents (PDCs) are the most dangerous hazard associated with explosive volcanic eruptions. Despite recent advancements in the general understanding of PDC dynamics, limited direct observation and/or outcrop scarcity often hinder the interpretation of specific transport and depositional processes at many volcanoes. This study explores the potential of sequential fragmentation/transport theory (SFT; cf. Wohletz et al., 1989), a modeling method capable of predicting particle mass distributions based on the physical principles of fragmentation and transport, to retrieve the transport and depositional dynamics of well-characterized PDCs from the size and density distributions of individual components within the deposits. The extensive vertical and lateral exposures through the May 18th, 1980 PDC deposits at Mt. St. Helens (MSH) provide constraints on PDC regimes and flow boundary conditions at specific locations across the depositional area. Application to MSH deposits suggests that SFT parameter distributions can be effectively used to characterize flow boundary conditions and emplacement processes for a variety of PDC lithofacies and deposit locations. Results demonstrate that (1) the SFT approach reflects particle fragmentation and transport mechanisms regardless of variations in initial component distributions, consistent with results from previous studies; (2) SFT analysis reveals changes in particle characteristics that are not directly observable in grain size and fabric data; and (3) SFT parameters are more sensitive to regional transport conditions than local (outcrop-scale) depositional processes. The particle processing trends produced using SFT analysis are consistent with the degree of particle processing inferred from lithofacies architectures: for all lithofacies examined in this study, suspension sedimentation products exhibit much better processing than concentrated current deposits. Integrated field observations and SFT results provide evidence for increasing density segregation within the depositional region of the currents away from source, as well as for comparable density-segregation processes acting on lithic concentrations and pumice lenses within the current. These findings further define and reinforce the capability of SFT analysis to complement more conventional PDC study methods, significantly expanding the information gained regarding flow dynamics. Finally, this case study demonstrates that the SFT methodology has the potential to constrain regional flow conditions at volcanoes where outcrop exposures are limited.

Batholith tectonics: Formation and deformation of ghost stratigraphy during assembly of the mid-crustal Andalshatten batholith, central Norway

The Andalshatten batholith (322 km 2 , >700 km 3 ) is a predominantly granodioritic high-K, calc-alkaline igneous body that was assembled in the mid-crust across four lithologically distinct nappes within the Helgeland Nappe Complex, central Norway. Extensive vertical and horizontal exposures of metamorphic screens and xenoliths within the batholith provide an unparalleled view of the nature of magma emplacement, host rock displacement, and batholith assembly, i.e., batholith tectonics. The mapped intrusion consists of at least five distinct lithologic phases, including schlieren-banded to gneissic granodiorite (11% of batholith area), coarse-crystalline to K-feldspar megacrystic granodiorite (69%), amphibole-bearing diorite (11%), tonalite (2%), and minor leucogranite. Contacts between phases are both sharp and/or gradational and are interpreted to reflect comagmatic behavior over the duration of crystallization of the phases separated by a given contact. New chemical abrasion–thermal ionization mass spectrometry 206 Pb/ 238 U zircon weighted mean ages of 442.67 ± 0.14 Ma and 441.53 ± 0.40 Ma for 2 samples of the voluminous megacrystic granodiorite from disparate localities indicate distinct periods of zircon crystallization separated by ∼1 Ma; titanite ages for these samples are 441.30 ± 0.21 Ma and 436.10 ± 2.80 Ma, respectively. No observable contacts were identified between these two lithologically similar localities. Of the mapped intrusion area, ∼8% (>24 km 2 ) comprises screens (kilometer scale) and xenoliths (subkilometer scale) of metamorphic rocks that reflect the skeletal framework of the host rock nappes into which the granodioritic magmas intruded. This ghost stratigraphy maintains broad continuity with host rock lithology and structural trends. The largest screens show no evidence of internal, emplacement-related ductile deformation, but appear to be rigidly rotated into subparallelism with the western host rock contact, presumably during subsequent magma injection into the batholith. In contrast, xenoliths underwent rotation, translation, and internal deformation in the magma. The scale dependence of synmagmatic deformation of screens and xenoliths is likely the result of smaller blocks becoming thermally equilibrated with the surrounding magma and thus deforming by ductile mechanisms in a magma with increasing yield strength due to crystallization. We interpret the Andalshatten batholith to have been assembled by at least five spatially distinct, elongate batches of magma over a minimum duration 600 ka to 1.7 Ma, including significant recharge events involving dioritic magmas. Local space for batholith assembly was accommodated by brittle and ductile deformation, including viscous flow of host rocks in a dynamothermal contact aureole. Viscous flow was facilitated by reactivation of existing structures (e.g., tightening of interlimb fold angles), recrystallization, and penetrative foliation development, resulting in near-field lateral and downward-directed displacement of host rocks along the western margin during batholith expansion and growth. Emplacement of dioritic magmas added heat and mass to the growing reservoir, enabling significant magnitudes of internal, hypersolidus flow, magmatic foliation development, mechanical mingling, and screen deformation. These observations and data sets are consistent with the hypothesis of multiple recharge events in a magma chamber that was partially molten over reasonably large spatial scales, thereby allowing screens and xenoliths to be incorporated and displaced and/or deformed.

Automatic Grain Sizing of vertical exposures of gravelly deposits

The Grain Size Distribution (GSD) is a key indicator in stratigraphy and morphology; it is a basic tool used by a variety of disciplines: geology, geomorphology, archeology, ecology and engineering. Automatic Grain Sizing (AGS) has been developed to estimate the GSD of surfaces in a rapid and effortless manner. Yet the traditional bulk method, time consuming and laborious, is still in use for the determination of the GSD of coarse-grained sedimentary deposits and of the near-subsurface of landforms, principally river beds. The AGS technique is examined here in its application to the stratigraphic record and to the subsurface. Images processed by AGS and manual samples of coarse-grained deposits in section were compared for evaluation of accuracy and determination of the extent of validation of the AGS technique.The AGS often achieves results very similar to field counting, especially when the largest grains are accounted for in the size of the sampling area and when photographs are taken afar without flash. Best results are provided when two images of the same sediment are taken at close range and also from afar to combine their GSDs, hence overcoming errors generated by resolution, sampling area and grain size. The computed errors are larger than those in prior studies yet remain reasonably small, even though results are not bias corrected, axes exposed in an outcrop very likely differ from those exposed on the surface, and sediment properties and lighting conditions vary. Although results indicate a 25% burial effect, the performance of the AGS for sections is high.Unlike prior application of AGS for riverbed surface openwork gravels, conglomerate sections and the near-subsurface include a matrix, resulting in a somewhat larger mean irreducible error. The technique may be utilized in studies of conglomerate texture, allowing the evaluation of entire GSDs under difficult sampling conditions. An example use of AGS to subsurface GSD evaluation is provided, among others enabling an easy and fast method to calculate extent of armoring.

Whole Body Vibration Exposure and Seat Effective Amplitude Transmissibility of Air Suspension Seat in Different Bus Designs

A number of studies have shown that whole body vibration (WBV) exposures contribute to low back pain in vehicle operators. Bus design may be an important factor in determining the WBV exposures that a driver receives. The purpose of this study was to determine whether differences exist in WBV exposures among three buses commonly used in long urban commuter routes: a high-floor coach bus, a low-floor coach bus, and a low-floor articulating bus. Each bus had the same new air-suspension installed and was driven over a standardized test route which included four road types: a smooth freeway, a rough freeway, a city street segment, and a road segment containing several speed humps. WBV exposures were evaluated per ISO 2631-1 action limits for acceptable WBV exposure levels. In this study, there were statistically significant differences among buses in WBV exposures. The high-floor coach bus had the highest fore-aft (x-axis) exposures, the low-floor articulating bus had the highest lateral (y-axis) exposures and the low-floor coach bus had the highest vertical (z-axis) exposures. With respect to ISO action limits, the z-axis WBV exposures did not exceed the 8-hour action limit (0.5 m/s2). The study also found that the air suspension seat did not perform well in the coach buses. The air suspension seat transmitted 92% of the floor measured vibration to the seat of the operator on the high-floor coach bus, 88% on the low-floor bus, and 76% on the low-floor articulating bus. Due to the low vibration attenuation performance of the air suspension seat, an evaluation of the different types of seats and seat suspensions may be merited in future research.

Effects of 50-Hz magnetic field exposure on superoxide radical anion formation and HSP70 induction in human K562 cells

Epidemiological studies suggest a correlation between exposure to low-level extremely low-frequency (ELF) magnetic fields (MF) and certain cancers and neurodegenerative diseases. Experimental studies have not provided any mechanism for such effects, although at flux density levels significantly higher than the ones encountered in epidemiological studies, radical homoeostasis and levels of stress response proteins can be affected. Here, we report on the influence of MF exposure (50-Hz sine wave; 1 h; 0.025-0.10 mT; vertical or horizontal MF exposure direction) on different cellular parameters (proliferation, cell cycle distribution, superoxide radical anion, and HSP70 protein levels) in the human leukaemia cell line K562. The positive control heat treatment (42°C, 1 h) did not affect either cell proliferation or superoxide radical anion production but caused accumulation of cells in the G2 phase and increased the stress protein HSP70. MF exposure (0.10 mT, 1 h) did not affect either cell cycle kinetics or proliferation. Both vertical and horizontal MF exposures for 1 h caused significantly and transiently increased HSP70 levels (>twofold), at several flux densities, compared to sham controls and also compared to heat treatment. This exposure also increased (30-40%) the levels of the superoxide radical anion, comparable to the positive control PMA. Addition of free radical scavengers (melatonin or 1,10-phenantroline) inhibited the MF-induced increase in HSP70. In conclusion, an early response to ELF MF in K562 cells seems to be an increased amount of oxygen radicals, leading to HSP70 induction. Furthermore, the results suggest that there is a flux density threshold where 50-Hz MF exerts its effects on K562 cells, at or below 0.025 mT, and also that it is the MF, and not the induced electric field, which is the active parameter.

Transverse architecture of lahar terraces, inferred from radargrams: preliminary results from Semeru Volcano, Indonesia

AbstractSemeru Volcano is the highest mountain of Java (Indonesia), and a vulcanian explosion occurs every 15 minutes on average, since 1967. Thus a constantly renewed stock of material and the heavy monsoon rainfall [3700 mm yr−1 at 1500 m above sea level (a.s.l.)] provide a perfect setting for the study of lahars and their deposits. Hence, we examined the architecture of lahars' terraces 9·5 km from the summit in the Curah Lengkong Valley. We first used ground penetrating radar (GPR) over vertical exposures of the lahars cut‐bank terraces. This allowed us to better understand transversal radargrams across terraces, which are not visually accessible in the field.Preliminary results from a single radargram are very instructive, since (1) they prove that the lateral architecture does not correspond to that observed from banks only; (2) we could observe the presence of lenses and stratigraphic discontinuities; (3) the setting of the various units can also help reconstruct deposition processes and the chronology of different units. In order to finalize these preliminary results, we however need to perform multiple GPR radargrams and provide a complete set of results. Copyright © 2010 John Wiley & Sons, Ltd.

Scatter radiation from chest radiographs: is there a risk to infants in a typical NICU?

To evaluate the dose of scatter radiation to infants in a NICU in order to determine the minimal safe distance between isolettes. Dose secondary to scattered radiation from an acrylic phantom exposed to vertical and horizontal beam exposures at 56 kVp was measured at 93 cm and 125 cm from the center of the phantom. This corresponds to 2 and 3 ft between standard isolettes, respectively. For horizontal exposures, the dosimeter was placed directly behind a CR plate and scatter dose at 90-degrees and 135-degrees from the incident beam was also measured. Exposures were obtained at 160 mAs and the results were extrapolated to correspond to 2.5 mAs. Four measurements were taken at each point and averaged. At 125 cm and 93 cm there was minimal scatter compared to daily natural background radiation dose (8.493 microGy). Greatest scatter dose obtained from a horizontal beam exposure at 135 degrees from the incident beam was still far below background radiation. Scatter radiation dose from a single exposure as well as cumulative scatter dose from numerous exposures is significantly below natural background radiation. Infants in neighboring isolettes are not at added risk from radiation scatter as long as the isolettes are separated by at least 2 ft.

Mapping subsurface karst features with GPR: results and limitations

Ground penetrating radar (GPR) has been applied, with relative success, to locate paleo-collapses and cavities and to detect and characterise karst. One of its main advantages is that, while the penetration depth is limited to several tens of meters or even just several meters, the obtained resolution can be in the scale of centimeters. In this paper, we illustrate the applicability of GPR prospecting to the study of alluvial karst and the structures associated with subsidence areas. GPR radargrams obtained with two central frequency antennas (50 and 100 MHz) are balanced against direct observation of geological features of collapse structures in vertical exposures of gravel quarries. GPR-surveys offer the possibility of obtaining nearly continuous vertical cross-sections of the subsoil, and integration of data within a 3D frame. However, the study of the internal structure of the subsoil by means of the GPR-profiles has been usually neglected. In this work, we show that some hints about the evolution of individual dolines can be established from the study of the geometry of the sedimentary filling by means of GPR. The obtained results indicate that GPR allows to characterise the structures associated with karst features and can therefore be useful evaluating hazard susceptibility in doline fields, because: (1) when no surface evidences exists, it permits the detection of karst hazards in the subsoil, and (2) when surficial evidences of karst activity are present, it permits the characterisation of processes associated with subsidence.

A 45° dual dipole decomposition scheme to improve image fidelity

Dipole exposure techniques are currently being explored as alternatives to the low k1 problem that hinders the manufacture of sub 100 nm logic technology. Recently, there has been a great deal of interest in using a combination of vertical and horizontal dipole exposures capable of producing manufacturing-robust image results for Manhattan features. In this technique, a design is split into two complementary masks that have enhanced contrast at a given dipole imaging condition. The dipole is a strong case of off-axis illumination, and significant resolution enhancement can be achieved, but only for one pattern direction. Complementary dipole exposures are required when dense pitches in horizontal and vertical direction are present. Dipole decomposition approaches that separate vertical and horizontal features in a layout are based on the fact that horizontal features form an improved aerial image with a vertical dipole illumination, and vertical features show the same aerial image improvement with a horizontal dipole illumination. This technique, though conceptually simple, requires a computer algorithm to decompose a design layout into two patterns consisting of features oriented mainly in the horizontal and vertical directions. Some layout features, landing pads or angled lines for example, cannot easily be assigned to a particular mask based on this simple orientation logic. In this work a new approach has been presented in which 45° decomposition of the pattern is used to improve image contrast, pattern fidelity and focus behavior. The results will be analyzed in terms of contrast, pattern fidelity and focus dependence in order to determine the feasibility of printing manhattan and angled features using a dual dipole approach for sub 100 nm processes.

Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars

On the summit of Kilauea volcano, sulfur dioxide, which is continuously emitted from Halemaumau crater and rapidly sequestered into sulfuric-acid–rich aerosol entrained in the prevailing trade winds, is subsequently precipitated as acid fog immediately downwind from Kilauea caldera in the Kau Desert. The characteristic pH of surface tephra deposits is <4.0 in Sand Wash, a region of nearly continuous, acidic aerosol fallout immediately southwest of the caldera. Vertical exposures of unconsolidated tephras of the Keanakakoi Ash found within fissures and small, dry gullies are coated with thin rock coatings of amorphous silica and jarosite. These rock coatings are formed via an evaporative mechanism whereby acidic pore fluids, circulating in the upper few meters within the highly porous tephra, are wicked toward the walls of the gullies. Geochemical modeling of the rock coating formation process implies that the sulfate formation via evaporation occurs subsequent to minimal interaction of acidic pore fluids with the basaltic tephra. This also suggests that the cycle from acid-fog fallout to precipitation of the siliceous-sulfate rock coatings must occur quite rapidly. Acid-fog deposition of sulfate and silica at Kilauea may provide one mechanism for the origin of jarosite-bearing outcrops on Mars.

Thresholds for 60 Hz magnetic field stimulation of peripheral nerves in human subjects

The goal of the research reported here is to narrow the range of uncertainty about peripheral nerve stimulation (PNS) thresholds associated with whole body magnetic field exposures at 50/60 Hz. This involved combining PNS thresholds measured in human subjects exposed to pulsed magnetic gradient fields with calculations of electric fields induced in detailed anatomical models of the body by that same exposure system. PNS thresholds at power frequencies (50/60 Hz) can be predicted from these data due to the wide range of pulse durations (70 mus to 1 ms), the length of the pulse trains (several tens of ms), and the exposure of a large part of the body to the magnetic field. These data together with the calculations of the rheobase electric field exceeded in 1% (E(1%)) of two anatomical body models, lead to a median PNS detection threshold of 47.9 +/- 4.4 mT for a uniform 60 Hz magnetic field exposure coronal to the body. The threshold for the most sensitive 1% of the population is about 27.8 mT. These values are lower than PNS thresholds produced by magnetic fields with sagittal and vertical orientations or nonuniform exposures.

Effiziente Erfassung der Kronendichte mit Hilfe fotografischer Senkrechtaufnahmen | Efficient data assessment of canopy density with the help of vertical photographic exposures

From the point of view of silvicultural and nature protection the canopies of forests represent an important characteristic value. However, conventional forestry methods do not allow for a differentiated ecological assessment of the changed situation of radiation following light tending interventions. The method presented here enables the collection of exact and reproducible values on the actual density of the canopy from the ground, which calls for no specialist demands on workers or equipment. The paper describes the technology of vertical exposures for both analogue and digital photographic technology. In addition, we present a specially developed programme,which allows the automatic evaluation of the digital canopy exposures. We give two examples for areas of application of the method (finely structured map of trees along a water course and documentation of forestry intervention for the resumption of traditional middle forest management). The presented methodology enables an objective assessment of measures to manage the conditions of light in forests (e.g., special forest reserves), which is significantly superior to a visual assessment with regard to accuracy and reproducibility.

A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault

We mapped and analyzed two vertical exposures—exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation—across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 ^(14)C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

Origin of post‐Minoan caves and volcaniclastic cave fill, Thera (Santorini), Greece

AbstractThe Aegean island of Thera (Santorini) was covered by tephra from its cataclysmic Late Bronze Age (ca. 3600 yr B.P.) eruption. Vertical exposures of the eruptive sequence show secondary, nonvolcanic, circular (in cross section) features composed of stratified sediment. Many are inaccessible from the floors of modern quarries and appear to be caves filled with younger sediment, but show no connection to the land surface. A filled cave was found in the wall of a modern gully outside the modern quarries, and a filled cave was found in a terrace scarp, well above the modern gully. Natural (and probably rapid) incision by gullies into the thick tephra deposit left many locations with lateral access to tephra. Inhabitants from post‐Minoan to recent times excavated tephra for materials and buildings, and caves were subsequently filled by sporadic (possibly seasonal) flood events that deposited sediment. These gullies may have provided access for modern tephra removal that isolated the filled caves high on the modern quarry walls. © 2003 Wiley Periodicals, Inc.

Données complémentaires sur les variations du niveau marin relatif, à l’holocène, à l’Anse de Bellechasse, sur la côte sud du moyen estuaire du Saint-Laurent

À l'embouchure du ruisseau de Bellechasse, trois petits glissements de terrain dans la berge escarpée de la terrasse de 10 m, survenus au printemps de 1998, ont permis de découvrir une couche organique (tourbe) et des troncs d'arbres, à la surface d'un dépôt argileux, marin (rythmites datées à environ 10 ka), recouvert de 2 à 3 m de sédiments fins intertidaux. L'âge de la tourbe et des troncs d'arbres semblable (6 à 7 ka) à celui des coupes de Montmagny et de Cap-Saint-Ignace permet d'affirmer que les événements mis en évidence dans ces deux localités (bas niveau suivi d'une transgression) ont une extension géographique plus grande, sur la rive sud de l'estuaire du Saint-Laurent, qu'on le croyait jadis.

Determination of Paleocurrent Direction from Oblique Sections of Trough Cross-Stratification--A Precise Approach

The study of paleoflow patterns is an essential part of basin analysis. Cross-stratification, both planar and trough, is the principal directional structure used for paleocurrent analysis. Trough cross-stratification is especially characteristic of fluvial deposits, where it is typically formed by the primary current and correlates strongly with the flow direction (Harms and Fahnestock 1965; Wermund 1965; Williams 1968; Barrett 1970; McGowen and Garner 1970; Dott 1973; Michelson and Dott 1973; High and Picard 1974). The only exception, reported by Meckel (1967), is an artifact of an erroneous sampling technique (High and Picard 1974). However, the paucity of three-dimensional exposures of trough cross-strata that are suitable for direct measurement of trough axis orientations quite often hinders the systematic collection of paleocurrent data. With a view to overcoming this problem, attempts have been made to estimate paleoflow directions from two-dimensional exposures of trough cross-strata (Slingerland and Williams 1979; DeCelles et al. 1983), because they are much more common. Slingerland and Williams (1979) proposed two specific ways of determining the orientations of trough axes from two-dimensional exposures. According to their first method, a single strike measured on a trough side can be corrected to give the trough axial azimuth, with reference to the plunge of the trough measured from a quasi-three-dimensional exposure (Slingerland and Williams 1979, p. 731). The plunge direction itself (after necessary tilt correction), however, indicates the flow direction, and if it can be measured beyond doubt no further measurement of any other part of the trough is required. This kind of exposure is also not qualitatively very different from a three-dimensional one. The second method proposed by Slingerland and Williams (1979) involves estimation of the trough axial azimuth by measuring, on perfectly two-dimensional vertical exposures, the face azimuths and two …

A dissolution pipe palaeokarst of mid-Pleistocene age preserved in Miocene limestones near Staszów, Poland

Many hundreds of solution pipes are exposed in quarries near Staszów, SE Poland. The pipes are contained in late Badenian (middle Miocene) carbonates of the Chmielnik Formation and are filled with sediment derived from a weathered Sanian (Elsterian) till cover. The pipes average 1 m 3 in volume, though the largest is of the order of 15 m 3. The average depth is 1.9 m; the average diameter, 0.6 m. The host limestone is fractured by several sets of open master joints, but, in respect of the way inclined pipes cross the joints without deflection in vertical exposures, and the absence of x-axis elongation along master joints in plan, the evidence is unequivocal that the structural fabric of the host-rock had little, if any, influence in determining pipe locus and form. Neither is there any evidence that irregularities of the interface between host and cover determined the points of entry for the focused groundwater flows which created the individual pipes. Therefore, unless the entry points into the upper surface of the limestone host were determined by an irregular network of first-to-decay channels in a relict permafrost zone at the end of the Elsterian cold period, for which there is no independent evidence, the focused flows must relate to selectively permeable pathways through the till cover and/or to concentrations of acid radicals there. Small quantities of organic carbon in the pipe cortices hint at the former presence of an appreciable content of peat in the till cover at the time of piping; this could have been the source of the acidic groundwater flows. The Staszów piping represents a covered, intraformational palaeokarst system. It is postulated that it developed as a product of fast-acting deglaciation and, possibly also, of irregular permafrost decay below the Elsterian till, early in the Holsteinian warm period.

Sampling‐surface orientation and clast macrofabric in periglacial colluvium

AbstractTo isolate the influence of sampling‐surface orientation on the macrofabric of periglacial colluvial deposits, clast orientation measurements were obtained from seven paired horizontal and vertical exposures in turf‐banked solifluction lobes on Niwot Ridge, Colorado Front Range. Most samples form moderately strong, upslope‐plunging clusters aligned with the local slope orientation. Fabrics obtained from vertical faces were stronger than those from horizontal exposures in six of the pairs. Near‐horizontal sampling surfaces yield less biased results than vertical exposures, owing to operator perceptions, procedural difficulties, and the relative thinness of the layer affected by colluvial processes. Sampling procedures must be standardized before comparative studies of colluvium can yield reliable results. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Deep crustal reflectors under Reydarfjördur, eastern Iceland:crustal accretion above the Iceland mantle plume

Summary Deep seismic reflection profiling on the Faroe–Iceland Ridge Experiment (FIRE) imaged a sequence of dipping events in the upper to |-crust beneath the Tertiary lava pile in Reydarfjordur, eastern Iceland. These events dip towards the spreading centre at which the crust was formed, and continue down to ∼ 6 s two-way time, corresponding to depths down to 15–18 km. Similar crustal reflections have been observed previously within the crust in other areas of Iceland, although the reflective events located by previous surveys have not been interpreted rigorously. In this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava πle. The synthesized lava pile sections are based on field measurements made in the Reydarfjordur area: the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions, and some intercalated tuffaceous clastic horizons. We compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas. We conclude that the upper and mid-crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface, with the main reflections resulting from packets of flows of similar thicknesses. The reflectors that we have imaged beneath Iceland are similar to those defining the seaward-dipping reflectors on the volcanic continental margins, and we therefore suggest that the seaward-dipping reflections commonly result from packets of lavas with similar distributions of thicknesses. The occurrence of lavas down to 15–18 km beneath Reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust, although the volcanic processes forming the crust in Iceland generally lead to advection of extrusive material to mid-crustal levels. The production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of frozen lava flows to mid-crustal levels contributes to the maintenance of the subsolidus crust that is observed on Iceland, and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques.