Proximal Slope Research Articles

Formation of glaciolacustrine Late Pleistocene end moraines in the Tasman Valley, New Zealand

Detailed examination of the Tekapo Formation in the Tasman Valley, New Zealand has identified 20 facies, and five facies associations. These associations are delta foresets and bottomsets, sediment density flows, ice-contact lake sediments with ice-rafted debris and resedimentation deposits, and outwash gravels. Interpretation of the sediment-landform associations informed by observations at modern glacier termini suggests that the Late Pleistocene Tekapo Formation moraines have been formed by downwasting of a more expanded Tasman Glacier. During the early stages of glacier retreat, ponds on the glacier surface develop into thermokarst lakes which enlarge and coalesce to form a large supraglacial lake. Continued downwasting causes the lake outlet river to entrench into the impounding latero-frontal ice-cored moraine, lowering the lake level. This exposes lake-bottom sediments and forms shorelines on the proximal slopes of the ice-cored moraine. As the ice-cored moraine melts, these lake sediments are deformed and deposited against the Mt. John moraine. The observations and interpretations reported here suggest the Late Pleistocene end moraine is a constructional feature not a structural (glaciotectonic) feature as suggested by previous studies.

Impacts from the 2004 Indian Ocean Tsunami: analysing the potential protecting role of environmental features

The tsunami that deeply impacted the North Indian Ocean shores on 26 December 2004, called for urgent rehabilitation of coastal infrastructures to restore the livelihood of local populations. A spatial and statistical analysis was performed to identify what geomorphological and biological configurations (mangroves forests, coral and other coastal vegetation) are susceptible to decrease or increase coastal vulnerability to tsunami. The results indicate that the width of flooded land strip was, in vast majority, influenced by the distance to fault lines as well as inclination and length of proximal slope. Areas covered by seagrass beds were less impacted, whereas areas behind coral reefs were more affected. The mangroves forests identified in the study were all located in sheltered areas, thus preventing to address the potential protecting role of mangroves forests.

Stratigraphic and geochemical framework of the Agouron drill cores, Transvaal Supergroup (Neoarchean-Paleoproterozoic, South Africa)

Two long drill cores through the Transvaal Supergroup (Neoarchean-Paleoproterozoic) record the vertical transition from a mixed siliciclastic-carbonate ramp (Schmidtsdrif Subgroup) through the Campbellrand-Malmani carbonate platform (Campbellrand Subgroup) to deeper-water banded iron formations (Asbestos Hills Subgroup). The cores were drilled in proximal (core GKF01) and distal (core GKP01) positions relative to the margin of the Campbellrand-Malmani carbonate platform. Correlation between cores is based on three types of marker horizons: sequence stratigraphic, volcanic/impact and lithologic markers. Several surfaces can be correlated to the platform succession. Gamma-ray data and bulk-rock XRF geochemistry of siliciclastic mudstones and tuff beds support the correlation. Following flooding of a shallow-water carbonate ramp (Boomplaas Formation of Schmidtsdrif Subgroup), deposition of slope carbonates with minor siliciclastic mudstones, chert and banded iron formation (BIF) occurred below storm wave base. Cycles of in situ microbialites and reworked slope carbonates are prominent in both cores. A final transgressive pulse near the top of the succession led to deposition of thick cherts and iron formations across the former platform, slope and basinal areas. Deposition on the slope was controlled by cementation potential in microbial facies and by sediment input, which in turn was linked to variations in platform margin geometry and changes in accommodation space. Results indicate preferred sediment accumulation in the proximal slope position, while chemical sediments were deposited preferentially beyond the reach of carbonate and siliciclastic detrital sediment. Similarly, sediment input and possibly early marine cementation influenced cycles of microbialites and reworked carbonates. Sequence stratigraphic analysis and siliciclastic mudstone geochemistry suggest a possible sea level fall and a change in provenance of the siliciclastic detritus at the transition from the Monteville Formation to the Nauga Formation (basal Campbellrand Subgroup). These changes were probably related to a major depositional hiatus on the platform.

Quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope

AbstractTwo‐dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows. The results suggest that in natural flows, turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser‐grained beds in the bottomsets of Gilbert‐type fan deltas. Experimental flows were generated on 0°, 3°, 6° and 9° slopes of equal length which terminated abruptly on a horizontal bed. Two‐component velocities were measured on the slope, at the slope break and downstream of the slope break. Flows were depositional and non‐uniform, visibly slowing and thickening with distance downstream. One‐dimensional continuous wavelet transforms of velocity time series were used to produce time‐period variance maps. Peaks in variance were tested against a background red‐noise spectrum at the 95% level; a significant period banding occurs in the cross‐wavelet transform at the slope break, attributed to increased formation of coherent flow structures (Kelvin–Helmholtz billows). Variance becomes distributed at progressively longer periods and the shape of the bed‐normal‐velocity spectral energy distribution changes with distance downstream. This is attributed to a shift towards larger turbulent structures caused by wake stretching. Mean velocity, Reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents. A turbulent kinetic energy transfer balance shows that flow non‐uniformity arises through the transfer of mean streamwise slowing to mean bed‐normal motion through the action of Reynolds normal stresses. Net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed‐normal motion is limited. At the slope break, an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free‐shear boundaries at the bottom and top of the flows, respectively. A rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope. The increased turbulent kinetic energy, limited bed‐normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period. Thus, mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break. The internal effects of flow non‐uniformity therefore can be separated from the external influence of the slope break.

Lipid biomarkers and carbon-isotopes of modern travertine deposits (Yellowstone National Park, USA): Implications for biogeochemical dynamics in hot-spring systems

Lipid biomarkers and 13C fractionation patterns were used to understand the dynamics of carbon cycling during microbial metabolisms in different environments of travertine precipitation (called facies) at Spring AT-1 on Angel Terrace in the Mammoth Hot Springs complex of Yellowstone National Park, USA. Microbial mats that encrust travertine deposits were collected for analyses of lipid biomarkers and carbon isotopes along the continuous drainage outflow system of Spring AT-1. The spring water exhibits a continuous temperature drop from 71°C in the vent at top to 24°C in the distal slope at bottom. Phospholipid fatty acids (PLFA) and glycolipid fatty acids (GLFA) exhibit distinctly different compositions in each of the facies, which are consistent with partitioning of the bacterial 16S rRNA gene sequences in the Spring AT-1 travertine facies (Fouke et al., 2003). The δ 13C composition of total biomass within the microbial mats decreases from −16.1‰ in the vent to −23.5‰ in the distal slope. However, lower values occur in the pond (−26.0‰) and the proximal slope (−28.0‰) between the vent and the distal slope. Isotopic compositions of PLFA and GLFA have variations similar to those of total biomass. The average δ 13C values of PLFA are −12.4 ± 5.2‰ (n = 10 individual fatty acids, same below) in the vent, −33.0 ± 3.1‰ (n = 11) in the pond, −33.7 ± 3.8‰ (n = 16) in the proximal slope, and −22.4 ± 3.4‰ (n = 10) in the distal slope; the average δ 13C values of GLFA are −19.6 ± 3.0‰ (n = 3) in the vent, −30.4 ± 4.7‰ (n = 8) in the pond, −36.9 ± 2.8‰ (n = 12) in the proximal slope, and −27.9 ± 3.1‰ (n = 13) in the distal slope. In particular, fatty acids in the vent are enriched in 13C relative to the total biomass, which is consistent with the notion that the biosynthetic pathways of the extant microbial community in the vent may be dominated by Aquificales using the reversed tricarboxylic acid cycle. Fractionations between fatty acids and total biomass in the pond, the proximal slope and the distal slope suggest the involvement of other biosynthetic pathways for CO 2 fixation by extant microbial populations. The results indicate that lipid biomarkers provide valuable information on the changing diversity and activity of microbial communities in different depositional environments. Carbon-isotope fractionations, on the other hand, can provide insight into the operating biosynthetic pathways associated with different organisms in the changing environment. This integrated approach may serve as a powerful tool for identifying functional metabolism within a community and identify shifts in microbial community structure in modern hot-spring systems.

Arenigian (Early Ordovician) sea-level history and the response of conodont communities, western Newfoundland

Four cluster analyses were performed, which recognized 17 conodont communities in the Arenigian (Lower Ordovician) of western Newfoundland. The analyses include 69 598 identifiable conodont specimens recovered from 153 conodont-bearing samples from four stratigraphical sections representing the environmental settings of the platform, upper proximal slope, lower proximal slope, and distal slope. The distribution of conodont communities along the platform to slope environmental gradient shows that sea-level changes simultaneously affected the development and replacement of the conodont communities in the different facies. The pattern of change in conodont communities allows an interpretation of sea-level change that is correlated precisely into the detailed graptolite biozonation. A gradual transgression lasted most of Tetragraptus approximatus Zone time, which was followed by a brief regression; a transgression–regression cycle occurred in the T. akzharensis Zone time; a major transgression caused a highstand during the entire Pendeograptus fruticosus Zone time, which was followed by a major regression in the early Didymograptus bifidus Zone time; Isograptus victoriae lunatus Zone time included repetitive oscillations of sea level; a severe regression in the earliest I. i. victoriae Zone time was represented by the St. George unconformity on the platform and the Bed 12 megaconglomerate on the slope, reaching the lowest sea level during the I. i. maximus Zone time. The Arenigian sea-level curve developed by this study only partly agrees with that from the Baltic region and central Australia based on trilobite communities.

Glaciolacustrine kame terraces as an indicator of conditions of deglaciation in Lithuania during the Last Glaciation

Over the recent years, while executing the geological mapping of the Lithuanian territory at a scale of 1:50 000, attention have been given to kame terraces adjoining the recessional marginal ridges left by glacier lobes of the Last (Upper Nemunas, Late Weichselian) Glaciation. For a long time, only the kame terraces formed between dead ice blocks and the internal (with respect to the glacier lobe), i.e. proximal slopes of the marginal morainic ridges have been known. During the process of mapping, the kame terraces have been subdivided into the glaciofluvial (sand, gravel) and glaciolacustrine (clay, silt) according to their lithological composition. The glaciolacustrine kame terraces adjoining the distal slopes of the recessional marginal morainic ridges of the Middle Lithuanian, North Lithuanian and Pajūrio phases of the Last Glaciation have been found and examined. The origin of these terraces could be explained only by the hypothesis that the accumulation of the above-mentioned marginal ridges and kame terraces took place between the margin of active ice lobes and the blocks of dead ice. Supposing that when the glacier of the Last Glaciation was melting, it was the arial and not the frontal deglaciation that was definitely dominating. The cold periods of glacier activation (so-called stadials and phasials) were changed with warmer periods. However, the latter periods were too cold and too short that the territory could be deglaciated completely, that is why they cannot be interpreted as interstadials or interphasials according to the climatostratigraphic criterion.

Late Cambrian and Early Ordovician conodont communities from platform and slope facies, western Newfoundland: a statistical approach

Abstract Twelve conodont communities and assemblages in the Cambrian-Ordovician interval of western Newfoundland are recognized by four cluster analyses, which include 18 468 identifiable conodont specimens recovered from 230 conodont-bearing samples from four stratigraphic sections. Each section represents a different facies: platform, upper proximal slope, lower proximal slope, and distal slope. The 12 conodont communities and assemblages were differentiated into pelagic and nektobenthic communities as well as both the assemblages with a mixture of pelagic and nektobenthic communities and transported assemblage. The distribution of conodont communities along the platform-to-slope environmental gradient shows some gradational relationships. Relative sea-level curves derived in part from the distribution pattern of conodont communities reveal a major sea-level drop in the latest Cordylodus proavus Zone time, and two major sea-level rises in the early Iapetognathus fluctivagus Zone time and early Cordylodus angulatus Zone time, respectively.

The turbidite fill of the Annot sub-basin (SE France): a sequence-stratigraphy approach

Abstract New sedimentological and biostratigraphical data on the Annot sub-basin (southern Alps, France) provide a good overview of a fore-deep basin infill with well-known Annot turbiditic complex. This basin shows a total thickness of more than 1100 m, where the Grès d’Annot exceeds 500m with well-longitudinal and transversal outcrops. Our study is to define a chronostratigraphical framework, which allows comparisons of local signatures of sea-level fluctuations with eustatic curve, and also identification of tectonic effects. The Annot succession could be interpreted as a second-order cycle. We identify a deepening and transgressive Priabonian interval, represented by the transition from platform of the Calcaires Nummulitiques to hemipelagic marls of the Marnes Bleues where tectonic subsidence rate outpaces third-order eustatic fluctuations and sedimentary influx. Priabonian to early Rupelian Annot turbidites progressively appear with a thickening- and coarsening-upward stacking pattern of four different sand-rich turbiditic systems with lobes, megaslide, channelized lobes and erosional/depositional channel. Each prograding system is separated by an erosive surface corresponding to a third sequence boundary. A possible sequence stratigraphy interpretation may be modelled as a forced regression during foreland basin closure, where the upper turbiditic systems correspond to lowstand prograding complexes from basin floor fan to proximal slope fan.

The Beyşehir–Hoyran–Hadim Nappes: genesis and emplacement of Mesozoic marginal and oceanic units of the northern Neotethys in southern Turkey

The Beyşehir–Hoyran–Hadim Nappes crop out over 700 km from NW to SE. Above a regionally autochthonous Tauride carbonate platform the Beyşehir–Hoyran Nappes begin with a thrust sheet ( c . 400 m) of mainly redeposited carbonates, quartzose sandstones and mudstones of Mid–Late Triassic age, interpreted as a proximal slope–base-of-slope succession. Above is a thrust sheet ( c . 1 km) of Middle–Upper Triassic intermediate–acidic extrusive rocks, volcaniclastic rocks and minor pelagic carbonates, interpreted as a continental rift. Thin (<100 m) Upper Triassic–Upper Cretaceous pelagic carbonate and radiolarian chert lie depositionally above. The uppermost thrust sheet comprises broken formation and mélange, including Jurassic shallow-water carbonate, radiolarian chert and Upper Cretaceous pelagic limestone. Zones of tectonic–sedimentary mélange separate higher units. The Beyşehir–Hoyran Nappes document Triassic rifting and Jurassic–Cretaceous passive margin subsidence bordering the Northern Neotethys. A harzburgitic ophiolite probably formed above a north-dipping subduction zone within the ocean basin. The ophiolite was emplaced southwards onto the northern margin of the Tauride platform in latest Cretaceous time. The nappe pile and underlying platform (Hadim Nappe) were thrust further south in Late Eocene time. Assuming in-sequence thrusting, the Beyşehir–Hoyran Nappes restore to a location north of a Neotethyan spreading axis. More probably, they originated near the south margin of the Northern Neotethys, but reached their position by out-of-sequence thrusting. Formation within a southerly strand of the Northern Neotethys (Inner Tauride ocean) is more probable than within the main Northern Neotethys further north.

Experimental quasi-steady density currents

Hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths. Their deposits are probably very common in the rock record, although they are not frequently recorded in the literature. These currents may last longer and be steadier than surge-like turbidity flows and consequently their deposits are likely to be significantly different. In this paper simple laboratory experiments are used to investigate how flows and deposits may vary. The thickness of the experimental flows near the flow front varies with distance from the source depending on slope, effluent discharge, grain size and sorting. Runout distance varies with the proportion of fine-grained sediment rather than with mean grain size. Flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting. The variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area (approximating to bed thickness) is inversely dependent on slope angle. Variation in initial suspended particle concentration results in deposits of similar shape but different volume. Although the total mass of the deposit depends on the discharge the maximum mass per unit area (approximating to maximum bed thickness) does not. The distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges, perhaps relating to more efficient turbulent suspension at higher Re (more complete energy cascade), in addition to longer horizontal component of settling trajectory at higher velocity. The amplitude of mass variation depends on the grain size. As with laboratory surges [Gladstone et al., Sedimentology (1998) 833–844], the finer the sediment the more efficient is the sediment transport.

Reconstruction of the AD 1600 Huaynaputina eruption based on the correlation of geologic evidence with early Spanish chronicles

The largest historical eruption (VEI 6) in the Andes began on February 19 and continued until March 6 or 15, AD 1600 at Huaynaputina, a dacitic stratovolcano located on a high volcanic plateau in south Peru. Tephra falls, pyroclastic flows and surges disrupted life in an area of ∼4900 km2 around the volcano, and ash-fall was reported 200–500 km away in south Peru, west Bolivia, and north Chile. The aftermath of the large-scale eruption was severe and protracted for the people and colonial economy of south Peru. By linking up the series of events inferred from Spanish chronicles with the lithofacies and composition of the tephra (bulk volume 11.4–12.1 km3, dense rock equivalent (DRE) 4.6–4.95 km3), we distinguish five eruptive phases. (1) During the plinian phase, a sustained plinian column 27–35 km high on February 19–20 delivered a dacitic pumice fall of ∼3.1 km3 DRE volume. The plinian pumice formed a widespread lobe of ∼95 000 km2 within the 1-cm isopach; strong winds carried fine ash >500 km to the west, and west-northwest into the Pacific Ocean. The computed volumetric eruption rate was in the range of 5.4–6.6×104 to 1×105 m3/s and the mass eruption rate 1.3–1.6×108 kg/s. The onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre-AD 1600 amphitheater. The plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock, >1600 m below the vent area. (2) During the second phase, a dwindling column sent ash-falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes. (3) During the third ignimbrite-forming phase with interspersed hydromagmatic events, pyroclastic flows 1.5–2 km3 in volume were channeled into the Rı́o Tambo canyon and tributaries. The flows with interbedded base-surge deposits in proximal tributaries probably produced vigorous columns over high, rugged relief around the Huaynaputina plateau. Winds winnowing the columns dispersed a widespread co-ignimbrite ash, probably mixed with co-plinian ash, over an area of ∼265 000 km2. (4) During the fourth phase, an unusual crystal ash-fall was deposited when the residual magma with a crystal content as high as 80% was tapped near the end of the eruption. (5) During the fifth phase, ash-flows produced surge deposits and lag-fall breccias near vent, small-volume ash-flow deposits in proximal catchments, and a thin ash-fall layer in medial to distal areas. The proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent. The ignimbritic and hydromagmatic phases greatly modified the ≤400-m-diameter plinian vent. Tapping of the crystal-rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes. Geochemistry and mineralogy of the plinian and post-plinian units point to an unusual zoned magma sequence. The ignimbrite-forming phase tapped a magma batch richer in silica than the less differentiated plinian magma. The crystal-rich magma of unit 4 was fed by ‘crystal mush’ in a layered magma reservoir and (or) from two magma reservoirs at distinct depths. The geochemical and mineralogical trend throughout the eruption, and preliminary measurements of geobarometers suggest a complex model linking a shallow (6–7 km) magma reservoir to a deeper (∼15 km) reservoir. The total DRE volume (4.6–4.95 km3) of erupted tephra did not lead to caldera collapse. Ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock. This suggests the onset of a funnel-type or piecemeal collapse.

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100–150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the −120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ 18O=+1‰) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution accompanied by calcite cementation. The sediments examined were deposited well above the aragonite compensation depth, so seawater entering the sediment is saturated with respect to aragonite. The under-saturation needed to drive diagenesis is likely the result of bacterial oxidation of organic matter using sulfate. (4) Microspar forms in these sediments as a cement based on petrographic and SEM examination of partly to completely altered samples. This contradicts the common assumption that microspar forms by aggrading neomorphism of micrite. (5) Strontium content of sediments altered in marine pore fluids can show an extreme range of values, formerly thought to indicate different environments. The opportunity to finally examine the diagenesis of aragonite-rich sediments buried in seawater challenges current diagenetic paradigms and emphasizes the importance of integrated studies.

Acoustic Properties of "Young" Carbonate Rocks, ODP Leg 166 and Boreholes Clino and Unda, Western Great Bahama Bank

Acoustic properties determined from logs and discrete measurements along a borehole transect across the slope of the western Great Bahama Bank margin (ODP Leg 166) show a distinct relationship with primary sediment composition and noncarbonate content. Dominant sediment types are alternating cemented and uncompacted calciturbidites with shallow-water derived grains, and poorly to uncemented and compacted periplatform ooze of globigerinid-dominated wackestone to packstone. Comparison of the velocity-porosity variation from logs and discrete measurements reveals that both tend to follow the trend of generally accepted velocity transforms. Primary control on the acoustic velocity is exerted by total porosity, whereas the remaining variation in velocity is explained by differential diagenesis. The less cemented to uncemented and compacted background sediments have velocities that, at a given porosity, fall below the time-average equation for calcite. The cemented turbidites, however, have velocities that are higher than those predicted by the time-average equation. Although it is unclear whether the original volume of aragonite or presence of organic matter controlled this difference, it is suggested that the acoustic properties reflect an input signal. The presence of noncarbonate material, some of which is clay, increases basinward and further reduces the acoustic velocities in the background sediment. In contrast, earlier work on the more proximal slope, and nearly pure carbonate, intervals in Clino and Unda cores documents diagenetic differentiation in compositionally similar sediment, resulting in alternating acoustically fast and slow layers. Marginal and nearly pure limestones have a more complex acoustic behavior and hence are difficult to predict. Though both velocity logs and discrete velocity measurements show comparable general trends, logging data show a considerably larger variation at a given porosity value (up to 3.5 km/s) than discrete data, which ranges between 1.0 and 2.5 km/s. To improve the quality of velocity logs it is suggested to groundtruth these data with other logging data, general velocity transforms, and, ideally, discrete measurements on core plugs.

Famennian mud-mounds in the proximal fore-reef slope, Canning Basin, Western Australia

Famennian (Late Devonian) carbonate buildups and, in particular, mud-mounds, are poorly known, in general, and few have been documented in detail. Relatively small Famennian mud-mounds occur in proximal fore-reef slope settings in the Canning Basin, Western Australia. The Famennian platform margin facies passes from typical shoaling carbonate facies in the back reef, through massive, calcimicrobial, cement-rich reef-margin facies, to relatively steeply dipping (20–30°), well-bedded fore-reef slope facies containing shelf-derived, winnowed grainy sediments and extremely coarse reef-block debris. Isolated or coalescing mounds occur in the proximal slope, immediately adjacent to and, in some cases, possibly grading into the margin facies. Mounds are elongate perpendicular to the margin and some had synoptic relief greater than 2 m. Mounds are lithologically variable and consist of varying proportions of micrite, multiple generations of marine cement, abundant Rothpletzella, Renalcis, poorly preserved sparry microbial crusts and sporadically distributed laminar stromatoporoids. Surrounding grainy slope facies abut and slope off of mound flanks. Mound facies are very similar to nearby reef-margin facies, with the exceptions that stromatoporoids have not been observed in margin facies and solenoporoid algae, which occur in the margin, have not been observed in the mounds. Stromatolites are conspicuously absent from both facies. Mound facies appear to be more closely related to Frasnian and Famennian calcimicrobe cement-dominated reef-margin facies than to Famennian deep-water stromatolite–sponge-mound facies, such as those that occur elsewhere in the Canning Basin. The observed Canning Famennian reef and mound frameworks were constructed by communities that appear to be very similar to earlier Frasnian communities, despite the Frasnian–Famennian extinction event, and provide good examples of microbial reef framework construction in a high energy setting.

Sedimentary successions in ice-marginal fans of the Late Saalian glaciation, southwestern Poland

Abstract The Late Saalian end moraines of southwestern Poland contain several features that may be attributed to the formation that is predominantly controlled by the deposition of sorted (waterlaid) sediments. These hills are essentially small alluvial fans with ice-contact facies. They have a semi-conical form, plano-convex geometry, their distal slopes are up to 5°, and proximal slopes are between 5° and 20°. The sedimentary sequences contain two units: the lower unit comprises coarse-grained material of gravel-bed sheetflow facies and gravel-bed channel facies with subordinate high-energy channel flow facies and boulders up to 1.8 m in diameter. The upper unit contains sand-bed sheetflow facies and sand-bed channel facies. The sediment bedding is usually conformable with and parallel to the inclined fan surface. These sequences represent typical proximal–distal fan facies associations, with a high-energy and pulsatory water discharge in the proximal zone. The fans, in spite of their formation in the proglacial zone, have geomorphological features and sedimentary sequences that are very similar to present-day fans, and are equivalent to sheetflow-dominated or ‘humid’ alluvial fans in non-glacial environments.

Minor end moraines of the Wisconsin Valley Lobe, north‐central Wisconsin, USA

Approximately 35 parallel, discontinuous glacial ridges occur in an area of about 100 km2 in north‐central Wisconsin. The ridges are located between about 6 and 15 km north (formerly up‐ice) of the maximum extent of the Wisconsin Valley Lobe of the Laurentide Ice Sheet. The ridges are between 1 and 4 m high, up to 1 km long, and spaced between 30 and 80 m apart. They are typically asymmetrical with a steep proximal (ice‐contact) slope and gentle distal slope. The ridges are composed primarily of subglacial till on their proximal sides and glacial debris‐flow sediment on the distal sides. In some ridges the till and debris‐flow sediment are underlain by sorted sediment that was deformed in the former direction of ice flow. We interpret the ridges to be recessional moraines that formed as the Wisconsin Valley Lobe wasted back from its maximum extent, with each ridge having formed by a sequence of (1) pushing of sorted ice‐marginal sediment, (2) partial overriding by the glacier and deposition of subglacial till on the proximal side of the ridge, and (3) deposition of debris‐flow sediment on the distal side of the ridge after the frozen till at the crest of the ridge melted. The moraines are similar to annual recessional moraines described at several modern glaciers, especially the northern margin of Myrdalsjokull, Iceland. Thus, we believe the ridges probably formed as a result of minor winter advances of the ice margin during deglaciation. Based on this assumption, we calculate the net rate of ice‐surface lowering of the Wisconsin Valley Lobe during the period when the moraines formed. Various estimates of ice‐surface slope and rates of ice‐margin retreat yield a wide range of values for ice‐surface lowering (1.7–14.5 m/yr). Given that ablation rates must exceed those of ice‐surface lowering, this range of values suggests relatively high summer temperatures along the margin of the Wisconsin Valley Lobe when it began retreating from its maximum extent. In addition, the formation of annual moraines indicates that the glacier toe was thin, the ice surface was clean, and the ice margin experienced relatively cold winters.

Morphology of riffle-pool sequences in the River Severn, England

Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero-crossing’, the ‘spectral analysis’ and the ‘control-point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ p) and channel width ( W), λ p = 3 W, applies to the River Severn. Reach-average riffle height ( H) is a constant proportion of bankfull depth ( h); typically H ≅ 0·16 h. Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed-level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L r) to water depth, i.e. L r = α2π h, wherein α ≅ 1 for steep near-equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent-flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self-similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.

Retrograde nailing of humeral shaft fractures: a biomechanical study of its effects on the strength of the distal humerus.

The purpose of this study was to evaluate the loss of strength in the distal humerus that resulted from the creation of two different entry portals used for retrograde humeral nailing. Nine pairs, treated as blocks of size two, of fresh frozen humeri from individuals free of musculoskeletal disease were randomly divided into three groups in a balanced incomplete block experimental design. The specimens were tested intact (control) or with an entry portal drilled in either the distal metaphyseal triangle or the proximal slope of the olecranon fossa. Two of these three conditions were applied to each pair of the bones. Therefore, three pairs of bones accommodated testing of all possible combinations of the two treatments. All specimens were tested in torsion at a rate of 30 degrees per second until failure or fracture. The creation of an entry portal reduced the ultimate torque to 63 percent of that of the intact specimens (p = 0.044) and the energy absorbed to failure to 27 percent of that of the intact specimens (p = 0.039). The metaphyseal entry portal reduced the torque to failure to 71 percent of that of the intact specimens (p = 0.143) and the energy absorbed to failure to 37 percent of that of the intact specimens (p = 0.073), The olecranon fossa entry portal reduced the torque to failure to 55 percent of that of the intact specimens (p = 0.035) and the energy absorbed to failure to 18 percent (p = 0.058) of that of the intact specimens. Surgeons should be aware of the loss of strength in the distal humerus after retrograde humeral nailing. This is especially important when prescribing postoperative mobilization in which the upper extremities will be used for weight-bearing in either transfers or ambulation.

Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand

Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.