Pebble Fabric Research Articles

A review and synthesis of strain studies in the Egyptian Nubian Shield: materials, methods, problems and prospects

More than seventy strain studies in the Egyptian Nubian Shield (ENS) are reviewed in this contribution. For the Hammamat basin strain studies, Rf/ϕ and Fry methods have been applied to pebble sections, mudcracks and raindrop prints. Consideration of pebble fabrics in undeformed outcrops suggests that RXZ estimates of less than 1.7 based on pebble ellipse data are doubtful. Extracted pebbles are also popular strained objects for analysis. Average Wadell sphericity and percentage of blades assist recognition of pebble strains in the Kareim, Queih, Igla and Seleimat basins. Feiran gneissic complex strains, measured using boudins and folds are much higher (X/Z > 15) than the reported strains for the Hafafit gneissic complex (X/Z usually < 2.5) that used metamorphic grain shapes and centres. The low Hafafit strains may result from progressive resetting of grain shapes and relocation of grains by diffusional effects during HT metamorphism. The Barramiya-Mubarak thrust belt provides a good example of focused study on ENS shear zones strain, with variable correlations between Rf/ϕ and Fry results, but consistently low k values. South Eastern Desert (SED) shear zone prolate strain ellipsoids record the effects of multiple deformation. The extensional shear zone overlying the Meatiq complex shows extreme strain and strongly prolate strain ellipsoids. ENS vorticity studies, applying PAR and RGN methods, have yielded higher Wm (> 0.5) for transpressional shears (Barramiya-Mubarak belt and Atalla shear zone) than for transtensional shears (Wm < 0.5). Volume strains have rarely been considered in the ENS, though evidence exists for negative (due to pressure solution) and positive (due to microveining) volume change during deformation. The ENS strain database can be improved by better documentation of sampling methods; better knowledge of primary fabrics; and by testing strain results using a range of materials. AMS-strain studies may broaden the range of lithologies suitable for strain measurement.

Reappraisal of strain estimations and measurement methods in the Hammamat Group sediments: Comparison of primary and secondary grain fabrics with new data from Wadi Zeidun and Wadi Arak basins, CED, Egypt

In this study, pebble and sand grain shape and orientation data from undeformed and deformed conglomerates and sandstones in the Egyptian Central Eastern Desert (CED) Wadi Zeidun and Wadi Arak basins were collected, with the aim of providing more accurate tectonic strain estimations for deformed Hammamat sediments by comparing their primary and secondary sedimentary fabrics. Common assumptions about primary pebble fabrics and pebble deformation behaviour in previous strain studies of the region are shown to be invalid. This raises doubts about the accuracy of earlier reported strain estimates for the Hammamat that were based on such assumptions. For low strains in the Hammamat sediments, the best strain measurement method is normalized Fry (NFry) analysis of sand grain centre-to-centre distance data. An alternative is conventional Fry analysis of pebble ellipse centre-to-centre distance data, provided that large numbers of pebbles (∼300) are used, and the pebbles form closely packed aggregates with high degrees of anticlustering. This study also provides systematic guidelines for pebble axial length and orientation measurements in the field. The progressive changes in the mechanical behaviour of pebbles with increasing bulk strain are discussed.

Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA

Gentoso, M. J., Evenson, E. B., Kodama, K. P., Iverson, N. R., Alley, R. B., Berti, C. &amp; Kozlowski, A. 2012 (January): Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA. Boreas, Vol. 41, pp. 31–41. 10.1111/j.1502‐3885.2011.00221.x. ISSN 0300‐9483.Thick, relatively homogeneous basal tills exposed in the drumlins and flutes of the Weedsport drumlin and flute field in New York State exhibit anisotropy of magnetic susceptibility (AMS) and pebble fabrics that are consistently oriented parallel to the streamlined bedforms. The pebble fabrics and AMS fabrics are concordant. In this study, six drumlins and five flutes were sampled. Thermally induced, incremental reduction of isothermal remanent magnetization indicates that AMS is caused by primarily elongate maghaemite grains. The orientations of principal axes of maximum susceptibility (k1) are generally parallel to pebble long‐axis orientations, and tend to plunge mildly up‐glacier. Fabric directions are generally parallel to drumlin long‐axis orientations, but deviate by 12°–23° from flute directions. Fabrics of the flutes are stronger and more unidirectional than those of the drumlins. These results support the use of AMS as a fast and objective method for characterizing fabrics in tills, and suggest hypotheses about basal processes linked to glacially streamlined landforms.

Glacial-Marine Sedimentation, Canadian Polar Margin, North of Axel Heiberg Island

Sediment cores, taken at depths of 140 to 300 m across the northwestern shelf of Axel Heiberg Island (82° N), record the deposition of sediments under perennial sea ice. Five sedimentary fades are recognized: (A) soft pebbly-sandy-mud with dropstone structures; (B) bioturbated silty muds; (C) wispy-laminated silty clay/clay; (D) laminated sands/silts and mud; (E) firm pebbly-sandy-mud with chaotic pebble fabrics. Other sediments include terrestrial bedrock of Paleogene Eureka Sound Group, and a younger Tertiary deposit, possibly the Beaufort Formation. Ages range from 1530 ± 60 BP (Fades A) to 9950 ± 80 BP (Fades D). Sedimentation rates vary as follows: - 0.8 cm ka-1, Fades B; 4 cm ka"\ Fades A; 90 cm ka-1, Fades C; 134 cm ka~', Fades D. The sedimentation history, as interpreted from the sedimentology, palynology and foraminiferal results, suggests intervals of more continuous ice cover, with a reduced influx of coarse ice-rafted detritus, alternating with more open water conditions, and high sediment input from meltwater and/or floating icebergs. Only marine sediments overlie Neogene bedrock in the cores. The absence of diamictons at the core sites suggests that grounded ice perhaps never occupied this part of the Axel Heiberg Island shelf. The interpreted history of sedimentation generally corresponds to the land-based record from Ellesmere Island, but differs significantly from marine-based studies in more southern latitudes.

Fabric, paleomagnetism, and interpretation of pre-illinoian diamictons and paleosols on Cloudy Ridge and Milk River Ridge, Alberta and Montana

Pebble fabrics and sedimentological properties indicate that pre-Wisconsinan diamictons (Kennedy Drift) on Cloudy Ridge (Alberta) and Milk River Ridge (Montana) are of glacial rather than colluvial origin. S1 and S3 eigenvalues of the upper units on the two ridges are typical of undeformed lodgement till whereas those of the lower unit on Milk River Ridge are typical of glacigenic sediment flow. Other properties, including compact matrices, striations on stones, mean pebble dip angles, and Schmidt equal-area stereonet patterns, suggest each unit is lodgement or basal till. Pedogenic features indicate weathering zones capping the tills are paleosols. Degree of rubification, clay, iron and aluminum buildup, and clay mineral alteration resembles those of very strongly developed soils formed in warmer and moister environments. The argument that “soil-like features” of the Cloudy Ridge unit resulted from post-burial diagenesis is disproven because nearly identical paleosols occur at the surface on Milk River Ridge and other interfluves to the south. Each unit examined has normal polarity. Based on comparisons with similar till/paleosol sequences exposed in Kennedy Drift sections on nearby interfluves, the Cloudy Ridge till and the upper till on Milk River Ridge were probably deposited during the early to middle Bruhnes Normal Chron (780 ka to present) whereas the lower till on Milk River Ridge is of earlier Brunhes age or dates back to the Olduvai (1.98 to 1.76 Ma) Normal Subchron or the Gauss Normal Chron (3.6 to 2.6 Ma).

Fabric and origin of multiple diamictons within the pre-Illinoian Kennedy Drift east of Waterton-Glacier International Peace Park, Alberta, Canada, and Montana, USA

Pebble fabrics and sedimentological properties indicate a glacigenic origin for multiple, superposed diamictons within the (pre-Illinoian) Kennedy Drift east of Waterton-Glacier International Peace Park, Alberta, Canada, and Montana, USA. Determining the origin of these diamictons is particularly important in light of their antiquity. Eight diamictons on Mokowan Butte, Saint Mary Ridge, and Two Medicine Ridge have reversed magnetic polarity and are correlated with the Matuyama Chron (2.6–0.78 Ma). On Mokowan Butte, two diamictons with normal polarity underlying two reversed diamictons are correlated with the Gauss Chron (3.6–2.6 Ma) or possibly the Reunion or Olduvai events (2.23–2.20 and 1.93–1.76 Ma, respectively). Fabrics were taken from 17 units within the Kennedy Drift on five interfluves. On the basis of comparison of properties of these diamictons with those of modern glacial deposits, eight, including the uppermost one or two units on each interfluve, are probably undeformed lodgment tills. These are typically massive, matrix-supported bouldery sandy loam with strong unimodal fabric orientations, relatively high eigenvalue (S 1 /S 3 ) ratios, and striations on resistant clasts. Some include boulder pavements, bullet-shaped boulders, and weak fissility. Eight other units have weaker fabrics typical of either deformed lodgment tills or glacigenic sediment flows. Because these also include bullet-shaped boulders, stone pavements, and striated clasts, most are probably deformed lodgment tills. A colluvial or mudflow origin for the material is ruled out on the basis of the lateral and vertical extent of the diamictons (at least 300 m wide and 5–10 m thick), wide range of clast lithologies and sizes, and distance from the mountain front (2–10 km), as well as pebble fabrics and sedimentological properties.

The ability of ice‐flow indicators to record complex, historic deglaciation events, Burroughs Glacier, Alaska

Known changes in ice‐flow direction during a 100‐year interval have been used to evaluate how well ice‐flow indicators record complex deglaciation events. At Burroughs Glacier, nunataks emerging from a thinning Neoglacial ice mass and differential ice‐surface lowering caused by calving ice margins have produced major changes in ice‐flow direction sincc 1892. Cross‐cutting striae with angles of divergence of up to 105′ reflect the past range of flow directions in the area. Striae from the oldest flow events are deepest, and striae from some late‐stage flow events are missing. This may be caused by overprinting during late‐stage reversals in the direction of ice movement. The orientation of flutes and surficial bullet boulders reflects the final ice‐flow direction, but boulder orientations are less clustered than flute orientations. Surficial till pebble fabrics are weakly to moderately developed, but till fabrics vary with depth and record ice‐flow direction changes with time.

Basal till fabric and deposition at Burroughs Glacier, Glacier Bay, Alaska

Research Article| December 01, 1994 Basal till fabric and deposition at Burroughs Glacier, Glacier Bay, Alaska NELSON R. HAM; NELSON R. HAM 1Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar DAVID M. MICKELSON DAVID M. MICKELSON 1Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1994) 106 (12): 1552–1559. https://doi.org/10.1130/0016-7606(1994)106<1552:BTFADA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation NELSON R. HAM, DAVID M. MICKELSON; Basal till fabric and deposition at Burroughs Glacier, Glacier Bay, Alaska. GSA Bulletin 1994;; 106 (12): 1552–1559. doi: https://doi.org/10.1130/0016-7606(1994)106<1552:BTFADA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Basal till and debris-rich ice in the basal transport zone of Burroughs Glacier, southeast Alaska, were studied to develop a model for till deposition and to acquire new pebble fabric measurements from till at a modern glacier. Studies of till deposition and fabric measurements from modern glacial deposits are important for studying the genesis of Pleistocene and pre-Pleistocene glacial sediment.Basal till at Burroughs Glacier is deposited at the ice margin by the melting of basal debris-rich ice that stagnated progressively, from the bottom upward, beneath moving ice during deglaciation. The debris-rich ice is typically ∼1-3 m thick and has very high sediment content ranging from 60% to 70% by volume. The long axes of pebbles in the debris-rich ice, and in other parts of the basal transport zone of the glacier, are typically well oriented parallel to ice flow. The average S1 eigenvalue for nine fabrics measured in the basal transport zone is 0.8092 ± 0.067σ. The basal till is massive, matrix-supported, silty-sandy diamicton. Striated, lodged boulders, and crag and tails are common on the till surface. S1 eigenvalues calculated from 27 pebble fabrics measured in the till range from 0.4930 (no fabric) to 0.8261; the average value is 0.6757 ± 0.092σ. The range of values is nearly identical to that reported for fabrics from "undeformed lodgment till" by Dowdeswell and Sharp (1986).Although the final process of till deposition at Burroughs Glacier is passive melt out from stagnant, debris-rich ice, the nature and pebble fabric of the till are inherited from active ice processes, specifically the progressive stagnation of debris-rich ice beneath active ice. Evidence of differential movement (shear) within the debris-rich ice during deposition is reflected in a change in fabric azimuth with depth in the till, beveled bullet boulders, and shear surfaces. These features are typically thought to be characteristic of lodgment till or brittle deformation of subglacial sediment. The melt-out process is passive because of the very high debris content of the debris-rich ice. If the same sequence of till deposition occurred in older glacial deposits and the debris content of the basal ice was high, the final process of melt-out would not be reflected in properties of the sediment. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fabric and Origin of Diamictons in End Moraines, Animas River Valley, Colorado, U.S.A.

Four diamicton facies occur in end moraines located in the Animas River valley of the San Juan Mountains, southwestern Colorado. Pebble fabric, grain size, sedimentary structures, and stratigraphic relationships with associated fluvial sediment are used to interpret diamicton origin. (1) A massive, matrix-dominated diamicton with abundant silt is ubiquitous in the Animas City II and III end moraines. This facies has a weak fabric (Si eigenvalues of 0.4 to 0.7), and is interbedded with sandy and pebbly fluvial sediment. This facies is interpreted to be sediment-gravity-flow deposit. (2) A matrix-dominated diamicton with color bands and strong fabric (Si = 0.7 to 0.9) occurs at one site in the Animas City III end moraine, and is interpreted to be meltout till. (3) A clast-dominated diamicton with weak fabric (Si = 0.4 to 0.6) and interbedded silty to pebbly fluvial sediment occurs in places in the Animas City II and III moraines and is interpreted to be either a sediment-gravity-flow deposit or fluvial sediment. (4) A compact, matrix-dominated diamicton with weak fabric (Si = 0.5 to 0.7) occurs in the Baker's Bridge moraine. Bedrock striations at the base of the diamicton, lodgedboulder striations within the diamicton, and a prominent pebble-fabric mode reveal a strong N 30? W trend. This sediment is interpreted to be lodgement till.

Multiple Wisconsinan Glacigenic Sequences at Wedron, Illinois

Lithofacies and their relationships, sedimentological and glaciotectonic structures, pebble fabrics, and laboratory data were used to delineate seven facies associations in a classic upper Wisconsinan glacigenic succession near Wedron, Illinois. Each association is interpreted to represent a different depositional environment. Proglacial facies include massive and laminated silt (lacustrine), bedded sand (fluvial), and massive silt (eolian). Subglacial facies consist of homogeneous diamicton (lodgement till) and variable diamicton and sorted sediment deformed by folding and shearing (deformed till). An ice-marginal facies of heterogenous diamicton and sorted sediment (supraglacial redeposited sediment) and a paraglacial facies of laminated silt (lacustrine) also are present. Stratigraphic relationships among the facies allow the delineation of three glacigenic sequences from which three glacial events are inferred. Each sequence consists of proglacial fluvial and/or lacustrine sediment, subglacial till and fluvial deposits, and supraglacial and ice-marginal sediment-flow, fluvial, and lacustrine deposits. The complete nature of these glacigenic sequences suggests that the area was located primarily in the depositional zone of the Laurentide Ice Sheet during the late Wisconsinan. Erosional contacts and localized sorted sediment between three lithologically distinct till facies in the lowest sequence are interpreted to be subglacial in origin; the three facies were deposited during the same event, rather than separate events as previously inferred. Spatial and temporal variability among the subglacial processes of erosion, transportation, deformation, and lodgement is evident within that sequence.

Magnetic and pebble fabrics and origin of the Sunnybrook Till, Scarborough, Ontario, Canada

The early Wisconsinan Sunnybrook Till is a mud-rich, pebble-poor, dark gray to gray–brown massive diamicton that is exposed in the Scarborough Bluffs, Ontario. It is interpreted here to be till.Previous separate, magnetic, and pebble fabric studies made on the till suggest a westward ice movement ranging from west of south to northwest. In this study, two dominant directions are found, northwest by west and northeast by north. Of these two directions, the northwest by west direction is the most common and this combined with the direction of shear in the underlying sediments and lithology of the till suggests that the glacier moved in a westerly direction.However, the fabrics are not typical of a lodgement till and to some extent resemble fabrics found in subaquatic debris-flow deposits. It is suggested that the glacier that deposited the Sunnybrook Till was buoyed up by high pore water pressure in the accumulating sediment at the sole of the glacier. Under these conditions, the till either flowed away from the zone of release at the base of the glacier and (or) was being lightly smeared by a partially grounded glacier.

Characterization of pebble fabrics in modern terrestrial glacigenic sediments

ABSTRACT Pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers, where sedimentary processes can be observed or inferred with relatively little ambiguity. Over 100 samples from contemporary environments illustrate fabrics characterizing melt‐out till, deformed and undeformed lodgement till, sediment flow deposits and ice slope colluvium. Lodgement till fabric variability is related to the two‐layer structure of these sediments; a structureless, friable upper layer with low shear strength and high consolidation coefficient, overlying a very compact material of horizontal platy structure. Fabric strength (assessed by eigenvalue analysis) is weaker and pebble dip is more dispersed in the upper structureless horizon. Stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon. Alternatively, upper horizon fabrics may be characteristic of all recently‐deposited lodgement tills, with stronger fabrics developing at depth by dewatering and consolidation. There is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt‐out tills, through undeformed and deformed lodgement tills, to sediment flow deposits and ice slope colluvium. There is, however, considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills. Fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some Quaternary glacial sediments. Relatively strong fabrics characteristic of melt‐out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments.

Magnetic and Pebble Fabrics in Subaquatic Debris-Flow Deposits

Magnetic and pebble-fabric analyses have been made on three subaquatic debris-flow deposits. The first is from the Late Pleistocene glaciogenic deposits exposed in the shore cliffs on the north side of Lake Erie. The second is from the Late Pleistocene Champlain Sea deposits near Ottawa, Canada, and the third is from the Pliocene Pico Formation, California. In pebble-rich debris-flow deposits with a sandy-mud matrix, both the long axes of prolate-shaped pebbles and magnetite particles are aligned parallel to each other and, in four out of five cases, parallel to the dip of the paleoslope. In pebble-poor debris-flow deposits with a mudmatrix, the long axes of prolate-shaped pebbles and magnetite particles may be parallel or normal to each other and parallel, normal, or at some other angle to the dip of the paleoslope. The results suggest that the two most important factors involved in the alignment of elongate particles are collision effects and the position in which a gyrating particle has the lowest angu...

Pebble fabric in an ice-rafted diamicton : Domack, E.W. and D.E. Lawson, 1985. J. Geol., 93(5):577– 591. Dept.of Geol., Hamilton College, Clinton, NY 13323, USA

Pebble fabric in an ice-rafted diamicton : Domack, E.W. and D.E. Lawson, 1985. J. Geol., 93(5):577– 591. Dept.of Geol., Hamilton College, Clinton, NY 13323, USA

Pebble Fabric in an Ice-Rafted Diamicton

Pebble fabric studies on ice-rafted diamictons have been limited to general observations, with authors noting preferences toward vertical, random, or horizontal orientations. To clarify such observations, pebble fabric data were collected from a fossiliferous diamicton of late Pleistocene age located on Whidbey Island, Washington. The ice-rafted origin of this unit is supported by several independent characteristics including in situ macrofauna and microfauna, conformity with subaqueous lithofacies containing dropstones, lower bulk densities and higher void ratios than associated tills, soft sediment deformation structures suggestive of iceberg dumping, textural gradations, and facies relationships. Analysis using the eigenvalue method indicates that ice-rafted fabrics are nearly random with little consistency of vector orientations between sites and without any relationship to the probable direction of glacial flow. The weak fabric is mainly the product of settling through the water column and impact with, or penetration of, the bed. Samples that possess a weak preferred long axis orientation with a low angle of dip, including those from laminated muds, can best be explained by the intermittent effects of bottom currents, a resistant substrate at the time of deposition and post-depositional flowage. Comparisons of pebble fabrics from basal tills, recent sediment flow deposits, and basal, debris-laden ice of an active glacier demonstrate that the ice-rafted fabrics are distinct from those of basal ice and till but are quite similar to those of sediment flow diamictons. Ice-rafted diamictons appear, however, to contain a greater number of elongate stones with long axis plunge angles exceeding 45° than other glacigenic diamictons.

Magnetic and pebble fabrics of glaciomarine diamictons in the Champlain Sea, Ontario, Canada

Glaciomarine diamicton(ite)s that contain articulated unbroken shell remains are usually assumed to have been deposited by the settling out of suspended mud with additions of coarse clasts derived from the melting of debris-laden floating ice. However, little is known about the magnetic and pebble fabrics of these composite sediments, and the purpose of this study is to describe and interpret the magnetic and pebble fabrics of fossiliferous glaciomarine diamictons found at three locations in the Champlain Sea, Ontario.At two of the three sites examined it is found that the long axes of the prolate and blade-shaped pebbles do not show well defined alignments, but they are not randomly distributed in either the horizontal or vertical planes. This observation and an analysis of the alignment of the magnetic particles in the matrix of the diamictons show that the diamictons have undergone syn- or post-depositional movement and fall into the category of glaciogenic subaquatic debris flows. At the third site the diamictons are stratified, and the magnetic fabric and field evidence show that post-depositional movement was caused by loading of sand layers into underlying muds.The results suggest that magnetic fabrics can be useful in interpreting pebble fabrics and that the dual approach may help to differentiate various types of glaciomarine diamicton(ite)s.

Storm- and Tide-Dominated Shorelines in Cretaceous Moosebar-Lower Gates Interval--Outcrop Equivalents of Deep Basin Gas Trap in Western Canada

The Moosebar and Gates Formations crop out in the deformed Foothills belt south of Fort St. John, British Columbia. They permit examination of the depositional environments of sandstones and conglomerates that are gas-bearing in the equivalent Wilrich-Falher interval of the Deep basin of Alberta and British Columbia. The Moosebar-Gates interval consists of several upward-coarsening and marine to nonmarine sequences. Moosebar bioturbated shales pass upward into turbidites and offshore storm deposits. Flow directions indicate a north-dipping paleoslope. A second coarsening-upward sequence at the top of the Moosebar and base of the Gates terminates in a lenticular fluvial conglomerate up to 30 m thick, which is overlain by coals, carbonaceous mudstones, and sandstones. Above the carbonaceous zone, a major lateral facies change is observed. Fluvial conglomerates in the south grade into beach conglomerates northward. The upper part of the beach conglomerate consists of exposed three-dimensional storm berms, at least 500 m long, up to 1.5 m high, with wavelengths of 12 to 24 m. The berms trend east-west. Seaward of the conglomerate beach are herringbone cross-bedded (tidal) sandstones with a marine trace fauna. Graded sharp-based conglomerates also are present, probably washed out to sea by storms. Farther north, at the same stratigraphic level, is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across. The tops of these bars are covered with storm-formed symmetric conglomeratic dunes. Details of the orientation and geometry of these conglomerate bodies, along with their internal structure and pebble fabrics, suggest that four major environments (fluvial, beach, offshore conglomerate bars, and offshore graded storm conglomerates) should be identifiable and predictable in the subsurface.

Distinguishing characteristics of Diamictons at the Margin of the Matanuska Glacier, Alaska

The origins of diamictons deposited at the Matanuska Glacier are identified in stratigraphic sequences mainly by the presence or absence of a pebble fabric, internal structure, and variation in gravel-size clast distribution. These properties correlate with major differences in depositional mechanisms and source material. Melt-out till mostly inherits fabric, internal structure, and grain-size distribution from its debris-laden basal ice source. Sediment flow deposits and ice-slope colluvium (deposited by ablational slope processes) have properties developed by resedimentation mechanisms. Melt-out till ranges from structureless to stratified with interspersed lenses and discontinuous laminae, and generally possesses a well-defined pebble fabric. Sediment flow deposits show various combinations of six sedimentologic units with distinct sedimentary features and clast dispersion. Pebble fabric is absent or poorly defined, depending upon the unit's origin. Ice-slope colluvium is usually featureless, except for randomly-dispersed laminated lenses and irregularly-shaped gravelly zones in an otherwise disarrayed assemblage of particles, and is without a pebble fabric.

Sandy gravel accumulation in a fluvial environment

AbstractParticle size, pebble shape, pebble fabric, discharge and flow velocity data are used to introduce a model of sandy gravel formation in Welsh gravel‐bed rivers. The development of contact‐imbrication of the typically very bladed and very platy cobbles and larger pebbles subsequently acts to significantly affect the depositional modes and patterns of small pebbles and sand particles. An important distinction is drawn between sand deposition, which can occur at or below the bed surface, and pebble and cobble deposition, which is merely a surface phenomenon.

Glacial and Glacial Marine Sediments of the Antarctic Continental Shelf

Until recently, contributions towards the understanding of glacial marine sedimentation have been limited. Sedimentologie studies of continental shelf deposits recently cored in the Weddell Sea, Ross Sea, and the George V region of Antarctica have enabled the distinction of three types of sediment. Type 1 sediments are massive, poorly sorted, and texturally and mineralogically homogeneous downcore; they are unfossiliferous or contain reworked fossils, lack a preferred pebble orientation, and are overcompacted. They are interpreted as being basal tills. Type 2 sediments are crudely stratified, contain a sorted mud fraction, are texturally and mineralogically heterogeneous downcore, contain distinctive microfossil assemblages, are typically normally compacted, and have a horizontal pebble fabric. Type 3 sediments are similar to Type 2 sediments except that they are depleted in silt and clay and contain a moderately sorted sand fraction. Both Type 2 and Type 3 sediments are deposited from floating ice and re...