Coarse Debris Research Articles

Temperate Glacimarine Varves: An Example from Disenchantment Bay, Southern Alaska

ABSTRACT The sediment record of Disenchantment Bay, Southern Alaska, a large marine calving embayment, contains distinctive annual deposits. Each year a glacimarine couplet forms with a clast-rich stratified to massive diamicton deposited in winter by intense iceberg rafting and a summer, meltwater deposit of thinly laminated mud and turbidite sand beds. Although iceberg rafting occurs throughout the year, coarse debris is deposited in high concentrations and forms diamicton only during the winter because of minimal fine sediment from meltwater discharges and a longer residence time of icebergs in the Bay due to winter fjord circulation and meteorological factors. When meltwater discharge commences in summer, laminated mud with dropstones is deposited. Spring and fall conditions are recorded a transitional phases between winter and summer by clast-poor diamictons. Average sediment accumulation rates calculated from varve thicknesses range from 48 cm/yr, 3.4 km from Hubbard Glacier at the head of the Bay, to 14 cm/yr, 15 km away. Sediment accumulation rates estimated from 210Pb dating of core sediments are in the same range. Deposits from the 1986 Russell Fiord outburst flood are identified from anomalously high 210Pb activities in sediments indicating an alternative sediment source to Disenchantment Bay. Glacimarine varves are a dating tool for the barren proximal sediments commonly deposited near calving glacier termini. They yield paleoclimatic information because they are a product of both glacial ice in the sea and laminated sediments deposited from significant melt-water discharge found in a temperate to subpolar climate.

Formation of miniature sorted patterns by shallow ground freezing: A field experiment

At an experimental site near sea level in Scotland, a miniature sorted net with cell diameters of 56–172 mm and stony borders 4–9 cm deep developed in frost-susceptible regolith over two winters. Pattern formation was evident after only 8 freeze-thaw cycles, during which the minimum surface temperature was −5.3°C and freezing extended to a maximum depth of 16–17 cm. The primary process of pattern development was repeated differential growth of near-surface needle ice, causing upcoming of a cover of coarse debris. During subsequent thaw, bending and melting of ice needles resulted in net migration of upheaved clasts towards dome margins, causing progressive concentration of clasts in stony borders surrounding finer cells. This differentiation of near-surface soil texture appears self-sustaining, promoting preferential growth of ice lenses and thus preferential heave in fine cells. The experiment suggests that miniature frost-sorted patterns are not necessarily diagnostic of present or former periglacial environments, but may develop on unvegetated soil of suitable composition in a wide range of mid- or even low-latitude environments where shallow ground freezing is accompanied by the growth of needle ice. © 1996 John Wiley & Sons, Ltd.

Factors controlling dry sliding wear behaviour of a leaded tin bronze

The sliding wear behaviour of a leaded tin bearing bronze was investigated over a range of applied pressures and sliding speeds with respect to the influence of microconstituents such as lead on the wear response. Significantly high wear rates were found at the minimum sliding speed due to extensive microcracking. This was evinced by the formation of coarse debris and considerable subsurface/wear surface cracking. The (micro) cracking tendency of the alloy prohibited the occurrence of subsurface deformation. The absence of a lead film was primarily due to the lead particles being engulfed in the coarse debris. Higher sliding speeds led to increased frictional heating making the alloy matrix viscoplastic. This in turn greatly suppressed the tendency of the alloy to exhibit microcracking, thereby facilitating interaction between the materials of the mating surfaces through wear induced plastic deformation. As a result, a stable transfer layer formed on the specimen surface. Interestingly, the formation of a...

Assessing Forest Soil Disturbance through Biogenic Gas Fluxes

AbstractSoil damage from wet‐weather timber harvests may persist after some physical and chemical properties have been restored through tillage and fertilization. This study's objective was to determine if the gaseous products of aerobic and anaerobic soil biological activity could reveal harvest‐damage effects, even after applying costly mitigation treatments. Fluxes of CO2, CH4, and N2O across the soil surface were measured in Ultisols of a Coastal Plain pine flat damaged during timber harvest, then mitigated by bedding and fertilization (100 kg ha‐1 N, P, and K). Gas fluxes were measured with large static chambers (0.5 by 1.0 m sampling area) to compensate for high microsite variability and the presence of coarse debris on the forest floor. Carbon dioxide evolution was a robust and consistent indicator of residual damage, declining an average of 34% in planting beds on damaged vs. undamaged soils. For example, in a late summer reading, efflux of CO2‐C from beds installed over former skid trails was 143 mg m‐2 h‐1 vs. 258 mg m‐2 h‐1 from undamaged beds and 231 mg m‐2 h‐1 from undisturbed forest floor. Methane and N2O fluxes were ephemeral and, thus, generally unreliable as indicators of harvest damage — though bedding produced scattered high peaks in both. Carbon dioxide was also the only gas flux that responded significantly to fertilization, with an average 26% increase up to 4 mo after fertilization. These results suggest that suppression of gross soil biological activity by harvest damage was not restored by intensive mitigation in the next rotation's establishment phase.

Preliminary Investigation of Geochemical Process Responses to Potential Environmental Change in Kärkevagge, Northern Scandinavia

Geochemical responses to potential global environmental change predicted by Global Circulation Models (GCM's) for high altitude/latitude environments are being assessed for Kärkevagge (literally translated as “boulder valley”), Swedish Lappland on the basis of a series of static and dynamic field and laboratory studies. Static studies provide baseline information on the geochemical process system, essential for understanding process responses to environmental change. Preliminary studies involve the determination of the nature and origin of lime crusts, rock varnish, and weathering rinds occurring extensively on coarse debris in Kärkevagge.

In-situ strength tests of coarse, cohesionless debris on scree slopes

Debris strength is a key factor in the initiation of debris flows. Therefore debris strength must be measured to assess the initiation conditions of debris flows. It is difficult to test the strength of coarse debris in the laboratory, as large quantities of material are needed to eliminate single-particle effects. Therefore in-situ strength tests have been conducted on scree slopes in the southern French Alps to measure the strength of dry, coarse, matrixless debris. The test method consisted of bringing a debris mass into movement parallel to the slope surface on slopes at or near the critical slope angle. As dry, coarse, matrixless debris is essentially cohesionless, its strength can be characterized by its internal friction angle. Mean kinetic internal friction angles vary from 360° to 387° for five debris types with mean stone sizes ranging from 33–50 mm. Stone size sorting is the most important cause of variations in kinetic internal friction angle. Stone shape also influences the kinetic internal friction angle, but it is less important. Stone size, stone shape sorting and rock type have no influence. However, rock type may indirectly influence kinetic internal friction angle through stone size sorting and stone shape.

The healing of disturbed hillslopes by gully gravure

Results of accelerated erosion on certain constructed surfaces in southeastern Arizona appear similar to those described by Bryan (1940) as gully gravure. Twenty cross-section excavations in eight rills incised into silt-rich lacustrine and fluvial deposits reveal partial filling of the rills by debris derived from overlying fluvial sand, gravel, and cobbles. Interstices of the coarse material gradually fill with fine-grained erosion products, decreasing permeability of the fill and deflecting subsequent runoff to the margins of the fill. Rills and rill fillings thus increase in width with time, and complete veneering of the surface by coarse debris ultimately may occur. Through incision, filling, lateral planation, and armoring, channels of the dissected surface heal and the new hillslope approaches an equilibrium condition. Natural hillslopes in the area with similar geologic conditions have inclinations of 16°-22°, have generally unbroken veneers of coarse debris, and appear subject to the same erosional processes identified at constructed hillslopes. Field measurements and the revised universal soil loss equation (RUSLE) were used to estimate soil loss on natural and constructed hillslopes. Results suggest that erosion may be almost three orders of magnitude greater on steep, non-veneered, nonvegetated constructed surfaces than on natural hillslopes. Attributes of stable natural hillslopes provide criteria for the design of stable constructed hillslopes.

The geological effects of hurricanes on coral reefs and the interpretation of storm deposits

Hurricanes occur in belts 7° to 25° north and south of the equator. Reefs growing in these belts suffer periodic damage from hurricane-generated waves and storm surge. Corals down to 20m depth may be broken and removed, branching colonies being much more susceptible to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to link with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and grooves, on the intertidal reef flat as storm ridges of shingle or boulders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-reef lagoons, as drapes of carbonate sand and mud in deep off-reef locations in the fore-reef and lagoonal areas. In addition to the coarse debris deposited, other features may aid the recognition of former hurricane events, including the assemblage of reef biota, its species composition and the structure of the skeletons; graded internal sediments in framework cavities; characteristic sequences of encrusting organisms; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000–4000 y BP in certain parts of the world most likely relate to a slight fall in relative sea level rather than an increase in storminess during that period. A higher frequency of storms need not result in more reef flat storm deposits. The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previous major storm influences the amount of coral debris created; the length of time after the hurricane, and before a subsequent storm influences the degree of stabilization of reef-top storm deposits and hence their chances of preservation.

Sorted stripes in a snow-accumulation area on Niwot Ridge, Colorado Front Range. Altitude 3490 m. Fines move more rapidly than coarse debris on the 12–13° slope. (Photo by James B. Benedict, 17 August 1991.)

Sorted stripes in a snow-accumulation area on Niwot Ridge, Colorado Front Range. Altitude 3490 m. Fines move more rapidly than coarse debris on the 12–13° slope. (Photo by James B. Benedict, 17 August 1991.)

Antarctic sector of the Pacific

DSDP Leg 35 drilling results and seismic reflection profiles from the Bellingshausen Basin allow definition of the sedimentary processes which have shaped much of the deepwater West Antarctic continental margin.The continental rise exhibits three distinct types of acoustic signature characteristic of specific sedimentary processes. The upper rise has a smooth to irregular seafloor above highly reflective sediments with poor acoustic lamination or no coherent reflectors; the sediment probably has been emplaced chaotically by slumping from the adjacent continental slope and has been modified by local slumping and turbidity currents. The central continental rise exhibits moderately reflective but finely laminated sediments containing large sediment waves, smaller-scale migrating sediment dunes and leveed channels. The primary constructional processes are deposition from channel-bank overflow of turbidity currents and from the nepheloid layer maintained by the westward-flowing Antarctic Rise Countercurrent. The lower rise has very reflective sediments with moderate to strong acoustic lamination which represent proximal deposition from turbidity currents and local reworking by bottom currents.Development of the continental rise has caused seaward migration of these acoustic provinces with time. In addition, a marked change in depositional style on the central continental rise probably during Late Oligocene or Early Miocene time may have been caused by the formation of the Antarctic Rise Countercurrent at the time when the Drake Passage was deeply breached to the flow of bottom water. Correlation of specific reflecting horizons with events of Antarctic glaciation generally is not possible, although one widespread and highly reflective horizon may represent coarse debris deposited during a glacial maximum in the latest Miocene or Early Pliocene.The Bellingshausen Abyssal Plain contains highly reflective sediments which generally have well-defined, closely spaced and horizontal acoustic laminae; reflector definition is best in distal parts of the plain. These sediments represent proximal to distal deposition of turbidites, the most seaward of which are ponded into the Eltanin and adjacent fracture zones. Frequently discontinuous reflectors within the abyssal plain may indicate sediment reworking by flow at the base of the Antarctic Circumpolar Current.

Evidence of Tsunami Sedimentation on the Southeastern Coast of Australia

In coastal regions, the highest magnitude storms cannot always be invoked to account for large-scale, anomalous sediment features. Any coastline in the Pacific Ocean region can be affected by tsunamis, including Australia, which historically lacks evidence of such events. Geologically, tsunamis along the New South Wales coast have deposited a suite of Holocene features that consist of anomalous boulder masses, either chaotically tossed onto rock platforms and backshores or jammed into crevices; highly bimodal mixtures of sand and boulders; and dump deposits consisting of well-sorted coarse debris. In addition, many coastal aboriginal middens were distributed by such events. Within estuaries, tsunamis have left a record of stranded run-up ridges that have been interpreted mistakenly as cheniers. Dating of such deposits indicates that several events have affected this coastline since 3000 BP. In contrast to storm waves, tsunamis can leave a depositional imprint of their passage characterized by chaotic sort...

Neoproterozoic glaciation in the Broken Hill area, New South Wales, Australia

The Neoproterozoic sedimentary rocks on the eastern margin of the Adelaide Geosyncline include two major glacigenic successions. The lower of these, the Yancowinna Sub-Group (as much as 5,000 m thick), can be divided into four stratigraphic units deposited in a northwest-trending graben. Dramatic thickness variations are attributed to contemporaneous normal fault movements. The thin basal granite-boulder diamictite/conglomerate of Unit 1 is considered to represent ice-proximal deposition in a temperate glacial setting. Unit 2 is a heterogeneous mudstone-dominated succession formed from fine material provided by meltwater streams during glacial recession when the basin was starved of coarse debris. Coarser grained facies were deposited by sediment gravity-flow mechanisms. The thick diamictites of Unit 3 are interpreted as rain-out deposits formed during a second glacial advance. Unit 4 records the terminal glacial recession. Paleo-current measurements suggest that the sediment source was the Curnamona Cratonic Complex to the northwest. These rocks are the most easterly preserved Neoproterozoic glacigenic deposits of the Adelaide Geosyncline and are therefore relevant to comparisons with regions such as western North America, which were possibly adjacent, prior to the breakup of a Neoproterozoic supercontinent.

Ichnology of a glacially-influenced continental shelf and slope; the Late Cenozoic Gulf of Alaska (Yakataga Formation)

The study of trace fossils in glaciomarine strata is important because of the low preservation potential of fauna but glaciomarine settings have received little attention from ichnologists. Widespread biological activity is recorded by ichnofossil assemblages in glaciomarine strata of the Late Cenozoic (6 Ma to present) Yakataga Formation, Alaska. Glacial sedimentation began in the Gulf of Alaska in the Miocene along a deep water active margin basin dominated by turbidity currents and debris flows. Later strata record progradation of the continental slope by mass flow processes and the establishment of a broad shelf influenced by repeated glaciation, ice-rafting of coarse debris, changes in water depth and salinity, storms, earthquakes, and high sedimentation rates. Yakataga Formation strata show overall, a limited ichnological diversity and low degree of bioturbation in relation to comparable non-glacial settings although certain beds are highly bioturbated. Twenty-four ichnotaxa are identified and grouped into nine assemblages characteristic of different depositional settings and substrate types. Ichnogenera and body fossil evidence shows that polychaetes, arthropods, molluscs and echinoderms were the dominant infaunal benthic organisms with deposit feeding being the predominant feeding strategy; poorly consolidated muddy sediments excluded suspension feeders and limited the development of the Skolithos ichnofacies typical of non-glacial shelves characterized by clean, shifting sand substrates. Evidence of rapid sedimentation is given by escape structures (fugichnia), the presence of Diplocraterion, and by the mass mortality of bivalves. Firm-ground burrows, typical of the Glossifungites ichnofacies, are associated with erosional surfaces, boulder pavements and coquinas. Diminutive feeding traces may reflect salinities lowered by glacial meltwater inputs. The downslope transport of food by turbidity currents and mass flows may explain the development of a diverse Cruziana ichnofacies in water depths where in non-glacial settings the Nereites ichnofacies is found. An ichnological summary is presented for the Late Cenozoic Gulf of Alaska continental margin; similar ecological conditions obtain in the modern setting and provide valuable supportive data. Ichnofacies identified from the Yakataga Formation are probably widespread in Late Paleozoic and Late Cenozoic glacially-influenced marine strata.

Sedimentary furrows and organized flow structure: A study in Lake Superior

Investigations of the geology and physical limnology of a field of sedimentary furrows in Lake Superior have provided information on the characteristics of these bedforms and their relationship to the wind-driven currents and associated turbulence structures in the bottom boundary layer. The results generally support the existing conceptual model for furrow formation. Furrows in the study area (2 km2) at 100-m depth are spaced 20–100 m apart and exceed 800 m long, the troughs being ∼0.5 m deep and 3–5 m wide. Sediment cores collected by submersible are poorly sorted, cohesive, terrigenous silts with a mean grain size of 20 µm. Relative to sediments in the interfurrow area, those in the trough contain higher fractions of sand and coarse debris and accumulate at a rate 15% slower. Strong bottom currents (speed > 6 cm s−1) usually flow within 20° of the furrow direction and are most frequent between late October and early February, when wind-forced currents reach the bottom; annual peak speeds approach 30 cm The relationship of the current and temperature records to the wind field is consistent with predictions of coastal jet theory. Profiling current meters deployed on either side of a furrow showed near-bottom, cross-stream flow converging over the furrow with divergence higher in the boundary layer when the current speed exceeded 6 cm s−1. A preliminary numerical model exhibits a cross-stream flow pattern consistent with the profiler observations.

Tektite-bearing, deep-water clastic unit at the Cretaceous-Tertiary boundary in northeastern Mexico.

The hypothesis of Cretaceous-Tertiary (K-T) boundary impact on Yucatán, Mexico, predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves. An outcrop along the Arroyo el Mimbral in northeastern Mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic-marl sequence deposited at more than 400 m water depth. The marls were found to be unsuitable for determining magnetostratigraphy, but foraminiferal biostratigraphy places the clastic unit precisely at the K-T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact. The clastic unit comprises three main subunits. (1) The basal "spherule bed" contains glass in the form of tektites and microtektites, glass spherules replaced by chlorite-smectite and calcite, and quartz grains showing probable shock features. This bed is interpreted as a channelized deposit of proximal ejecta. (2) A set of lenticular, massive, graded "laminated beds" contains intraclasts and abundant plant debris, and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water. (3) At the top, several thin "ripple beds" composed of fine sand are separated by clay drapes; they are interpreted as deposits of oscillating currents, perhaps a seiche. An iridium anomaly (921 +/- 23 pg/g) is observed at the top of the ripple beds. Our observations at the Mimbral locality support the hypothesis of a K-T impact on nearby Yucatán.

Tectonic and Climate Changes Expressed as Sedimentary Cycles and Stratigraphic Sequences of the Paleogene Lake Uinta System, Central Rocky Mountains, Utah and Colorado

Lake Uinta strata record both long- and short-term changes in climate and tectonic regime. Late Paleocene to late Eocene deposits are characterized by evaporite units (including halite and bicarbonate salts) and organically derived carbonate with extreme positive {delta}{sup 13}C and slightly negative {delta}{sup 18}O values that serve as evidence the lake was the center of a closed hydrologic system. Large reconfigurations of the lake system were tectonically induced and gave rise to relatively thick, lithologically distinct stratigraphic sequences. Simultaneous climate changes initiated very rapid lake level expansions and contractions as well as shifts in lake-water alkalinity and salinity (or chemistry) which resulted in the development of small- to large-scale sedimentary and stratigraphic cycles. Maastrichtian to earliest Eocene phases formed in local depressions (piggy back basins) on the thrust sheets, and in the incipient Uinta basin. Lake system reached its greatest aerial and volumetric extent in the middle and late Eocene and was centered in the foreland formed in front of high-angle reverse faults that bounded the rising Laramide structural blocks. At this time chemical precipitates, including basin centered carbonate and evaporite facies, formed during episodes of tectonically induced subsidence at the center of the clastic sediment-starved basin. Some fault-bounded marginsmore » of the Uinta basin are marked by synorogenic coarse debris that extends from the mountain front to the clastic sediment-starved lake. Tectonically induced stratigraphic sequences of the Lake Uinta system express environments for several million years whereas climatic cycles reflect much shorter episodes and very rapidly changing conditions.« less

Internal-tide deposits in an Ordovician submarine channel: Previously unrecognized facies?

A Middle Ordovician submarine-channel deposit in the southern Appalachians is capped by an interval of fine-grained, predominantly cross-laminated sandstones that are interpreted as internal-tide deposits. Two facies are recognized: (1) bidirectional cross-laminated, very fine grained sandstones, and (2) unidirectional cross-bedded and cross-laminated, medium- to fine grained sandstones. Facies 1 is dominated by bidirectional cross-laminations that dip both landward and seaward parallel to the paleochannel axis. This facies is related to up-channel and down-channel currents caused by internal tides. Facies 2 has both low-angle, tabular cross-beds and cross-laminations that dip landward. The formative up-channel currents are attributed to superimposition of internal waves on internal tides. The vertical transition from high-concentration gravity-flow deposits to intercalated low-concentration turbidites and internal-tide deposits is related to a rise in sea level that resulted in storage of coarse debris landward of the submarine channel.

Correlation between vegetation pattern and microtopography in periglacial areas of the Central Alps

The distributional patterns of plant species were analyzed within periglacial microforms belonging to the collective groups of patterned grounds and sorted stripes In the Ortles-Cevedale group (Lombardy, Italy). The edges of primitive polygons are practically free of vegetation, whilst the central areas are colonized by pioneer plants of detritic slopes. This is clearly correlated with a low degree of sediment sorting. The morphological evolution of the forms proceeds at a faster rate at the edge of polygons than in the central areas. Vegetation, on the other hand, colonizes the coarse debris at the polygons’ edges slowly but colonizes the fine material in the central areas actively. The centre of the most evolved polygons is covered by a carpet of Salix herbacea . Age determinations of Salix shoots are not useful for dating purposes, but provide Information about the local microclimate.

Clast size variations on talus: Some observations from northwest Ireland

AbstractClast size variations are reported for a relict talus slope in northwest Ireland. Alternating longitudinal zones of coarse debris below buttresses and less coarse debris below gullies give the talus pronounced lateral variations in clast size that are related to joint spacing variations in the talus source area. Longitudinal variations in clast size are also present. The talus accumulated predominantly by the process of rockfall. Other processes frequently associated with lateral size variations were not significantly effective during talus development. Talus sliding may have modified some patterns of rockfall size grading.

Sedimentation in an ice-contact subaqueous setting: The mid-Pleistocene ‘North Sea Drifts’ of Norfolk, U.K.

In East Anglia, the mid-Pleistocene ‘North Sea Drifts’ extend over some 1500 km 2 of the coastal zone of North Norfolk. They are the oldest Pleistocene glacial sequences exposed in Britain, deposited during the Anglian Glaciation (ca. 450 ka BP?). Laminated and stratified pebbly mud (diamict) facies of the North Sea Drifts are widely regarded as the product of subglacial deposition below a terrestrial ice margin. In this paper these facies are identified as the product of subaqueous deposition in front of the margin of an ice sheet terminating in a large water body. Laminated, stratified, and massive diamict facies were deposited by the ‘rain-out’ of fine-grained sediment from suspension and coarse debris from floating ice, combined with downslope sediment gravity flow. Intraformational folds are the product of episodic downslope slumping and identify a regional paleoslope to the east, away from a major ice marginal moraine ridge (The Cromer Ridge). Exposures at Cromer show bulldozed ice-contact subaqueous sediments and ice-thrust chalk rafts. A fall in water-level emplaced shelly shoreface sands and beach gravels which are loaded into the underlying diamicts as large pan-shaped ‘sag basins’ with diapiric structures at their margins. Comparable sedimentary sequences are exposed elsewhere around the southern North Sea Basin; a glaciomarine setting in response to glacio-isostatic depression around the Anglian ice sheet is suggested.