Clast Form Research Articles

The Rheic riddle: ocean closed, but no orogen

Abstract Although the Rheic Ocean was the widest to be closed during the assembly of Pangaea, its suture zone is dominated by a younger (Rhenohercynian) Wilson Cycle. The present paper briefly summarizes the Rhenohercynian and reviews the relicts of the Rheic. The absence of a Rheic orogen is mainly due to bipolar subduction of the Rheic Ocean, which created two magmatic arcs and largely destroyed the passive margin prisms, thus depriving the collision zone of building material. Also, displacement of Variscan zones in England, France and Germany along a curvature of the dextral Bristol/Bray/Bohemia shear zone has effected their anticlockwise rotation. Younger, ENE-trending reverse faults have truncated the NE-trending structural boundaries and excised pre-Devonian, Rheic-related rocks west of the River Rhine, while the northern Rheic Arc and Cambrian–Silurian deposits of the Rheic Ocean have been preserved further east. Opening of the Rhenohercynian successor ocean to the Rheic put a brake on plate convergence and hindered formation of a large collisional belt. Scarce relicts of the southern Rheic Arc and rocks with Silurian deformation exist in the (largely eroded) upper plate of the Rhenohercynian collisional belt, mostly in the form of clasts and detrital zircons in Upper Devonian/lower Carboniferous flysch and in tectonic mélange at the base of the allochthon.

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

Exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern Iceland provides diagnostic sedimentological signatures of: a) debris transport pathways through active temperate glacier snouts; and b) till production in subglacial traction zones dominated by deforming layers. Three till end members are recognised based on stratigraphic architecture: 1) thin and patchy tills over eroded bedrock; 2) single push moraines and complexes; and 3) overridden moraines or outwash fans. Typical till thicknesses are 0.10–1.40 m, with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub-marginal shearing, freeze-on, squeezing and bulldozing. Clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits. Clast macrofabric strengths are variable, rarely matching those of laboratory shearing experiments, except where obviously lodged clasts are abundant. They also consistently record former glacier flow directions. But localized variability is introduced by bedrock protuberances, cavity infill, clast interference and freshly imported plucked clasts. Within tills, macrofabrics strengthen from lower (B horizon) to upper (A horizon) tills but at the outer edges of sub-marginally thickening till wedges or push moraines, seasonally-driven cycles of squeezing/flowage, freeze-on/melt-out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures. This reflects two extremes of till emplacement including the more mobile, flowing and often liquefied matrixes in push/squeeze moraines and, in contrast, the lodgement, deformation and ploughing at the thin end of sub-marginal till wedges.

Sources of volcanic detritus in the basal Chinle Formation, southwestern Laurentia, and implications for the Early Mesozoic magmatic arc

The Upper Triassic Chinle Formation in southwestern Laurentia is the oldest distinctive record of Early Mesozoic Cordilleran arc magmatism, in the form of detrital zircons and volcanic clasts. Initial deposition of the basal Shinarump and Mesa Redondo members, herein collectively called the Shinarump conglomerate, began in Late Triassic time, yet the earliest known arc magmatism is older by as much as 40 m.y. Analysis of detrital zircons from eight sites in southeastern Nevada, southern Utah, and northeastern Arizona and volcanic-clast zircons from four of these sites provides a basis for understanding the evolution of the Early Mesozoic arc. Most Permian and Triassic detrital zircons from the Shinarump conglomerate have ages from ca. 260 to 220 Ma with rare grains as old as 280 Ma. These ages are compatible with derivation from sources in the magmatic arc to the west and southwest, including plutons of corresponding age in the Mojave Desert. Volcanic clasts are uniformly in the range 232–224 Ma; their age and zircon geochemistry argue against a source in currently exposed Mojave Desert Triassic plutons. As a further test, we compared Th/U ratios of clast and detrital zircons with those of possible sources to the west. Th/U values of many detrital grains support their derivation from Triassic Mojave Desert plutons. Some detrital grains and those from the clasts, however, have Th/U values that are uniformly higher than those in Permo-Triassic Mojave Desert plutons and therefore argue for a different, unexposed source. We propose that the early arc lay offshore of western Laurentia. Over time, plutons were emplaced across a range of continental crustal thicknesses that likely increased toward the east. At approximately 235–230 Ma, a land connection between the arc and retro-arc areas was established and fluvial sedimentation began. The observation that the youngest grain ages in our detrital samples are variable suggests that this land connection was tenuous for perhaps 10 m.y. until well into Chinle Formation sedimentation.

PROVENANCE OF THE NEOPROTEROZOIC ROCKS OF THE GIFBERG GROUP (WESTERN SOUTH AFRICA)

New petrographic and lithogeochemical data are presented for siliciclastic units of the Gifberg Group (western South Africa), a succession rarely studied in the past. The main detrital source for the oldest succession of the Gifberg Group, the Karoetjes Kop Formation, has been identified as sourced by local metamorphic rocks with a felsic geochemical composition. Nearly unweathered detritus (Chemical index of alteration: 50 to 60; K/Cs >>5000) occurs in the metadiamictite of this formation and is interpreted as palaeovalley infill, deposited only a short distance from its sources. The provenance signature of the overlying Aties Formation indicates significant mafic and/or intermediate sources. Possible sources are rocks of the Bushmanland Group and an unknown unmetamorphosed mafic to intermediate rock succession, which might have been exposed during the deposition of the Aties Formation. The overlying Bloupoort Formation is characterised by a geochemically homogenous metadiamictite (Swartleikrans Bed) with only slightly weathered detritus (K/Cs >10000) and overlying mature silica-rich metarenites and metaconglomerates. It is proposed that the Gifberg basin formed as a small-scale rift basin and then widened through time with subsequent longer sedimentary transport of the detritus which have caused the higher compositional maturity. The predominance of detrital input from local sources in the Karoetjes Kop Formation hinders correlation based on its mineralogical and geochemical content alone. A glacial origin for the Swartleikrans Bed was proposed previously based on the occurrence of possible dropstones. Certain criteria for a glacial depositional environment as outlined by Arnaud and Etienne (2011) were met in our study and include poor sorting, rapid lateral facies changes, crude stratification, variable thicknesses, compositional immaturity, and a gradational contact plus a chaotic texture with large boulders. While these observations point to a ‘rainout’ deposit, there is still a lack of evidence for glacial transport on grain surfaces or in the form of faceted clasts and pebbles.

Simulating the formation of overdeepenings with coupled ice, water, and sediment models

Eyles identified Kvı́árjökull, an outlet of the icecap Öræfajökull, southeast Iceland, as an exemplar of the glaciated valley landsystem, emphasising rockfall debris supply, passive transport and reworking of a thick cover of supraglacial morainic till. In this paper, we take a fresh look at the sediments and landforms of Kvı́árjökull, focusing on clast form analysis to reconstruct primary transport pathways. New data demonstrate the importance of active transport of debris derived from the glacier bed. Substantial portions of the supraglacial debris cover can be traced back to englacial channel fill deposits or anomalously thick exposures of basal ice. These two types of sediments also make up much of Kvı́árjökull's large Neoglacial moraine rampart. We attribute the abundance of these non-rockfall sediments to the presence of a terminal overdeepening, and the switch from predominantly subglacial to predominantly englacial drainage that this induces. This hydrological switch enhances retention of debris within ice transport, preventing it from being washed away and making it available to feed ice-marginal moraine formation. The complexity of sediment transport processes observed at Kvı́árjökull and the contrast between it and alpine glaciers undermines any single concept of the glaciated valley landsystem; accordingly, we propose Kvı́árjökull as the type example of a distinct subclass, defined by its moderate relief, high-debris turnover, and drainage behaviour characteristic of an overdeepened basin.

A revised approach to discriminating sediment transport histories in glacigenic sediments in a temperate alpine environment: a case study from Fox Glacier, New Zealand

ABSTRACTClast shape measurements have developed into a standard method for reconstructing the transport histories of sediments in glacial environments. The majority of studies use the ‘RA‐C40’ covariance approach, with some researchers routinely including clasts of varying lithologies within their samples. The corollary is that variable lithological properties may control clast form and roundness, rather than debris‐transport mechanisms. Despite this, the role of lithology on clast shape in glacial environments has rarely been analysed. Furthermore, some studies have reported difficulties in using the RA‐C40 co‐variance plot in discriminating clasts that have undergone subglacial transport, and clasts that have been modified by fluvial activity. Results from a glacierized valley in a temperate alpine setting indicate that detailed analysis of clast shape where samples are of uniform lithology, although time consuming, is a useful tool in the investigation of deposits in glaciated environments. Copyright © 2012 John Wiley & Sons, Ltd.

Controlled moraine development and debris transport pathways in polythermal plateau icefields: examples from Tungnafellsjökull, Iceland

AbstractA modern landsystems analogue for plateau icefield glaciation is presented, based upon Tungnafellsjökull in the arid, cold interior of Iceland. Landform mapping and intensive clast form analysis on three separate forelands occupied during the Little Ice Age reveal variability in the morphology of moraines and the degree of clast modification, which is interpreted as a function of the complex interplay between: (a) the efficacy of glacial erosion; (b) passive versus active debris transfer mechanisms; and (c) the volume of debris provision for ice‐marginal freeze‐on mechanisms. A steep outlet bounded by rockwalls produced a landform assemblage and debris transport signature typical of valley glaciers, whereas two unconfined outlets produced a landform‐sediment imprint typical of polythermal conditions. This comprises subglacial bedforms, relating to the initiation of warm‐based ice by strain heating below icefalls at the plateau edge, terminating at arcs of ice‐cored (controlled) moraine representative of frozen snout conditions. Patterns in clast form data on the unrestricted forelands reflect the characteristics of glacier dynamics and thermal regimes at the margin of a plateau icefield, where bedrock steps of sufficient size to initiate strain heating and bedrock abrasion and plucking are located only a short distance from the marginal freeze‐on conditions of a snout creating debris‐rich folia and controlled moraine. Additional englacial debris may well have been carried to the glacier margin through the development of ogives and englacial thrusting at the base of the icefall, thereby explaining the occurrence of striae on clasts that have otherwise been little modified by glacial abrasion. Copyright © 2010 John Wiley & Sons, Ltd.

Automated Sizing of Coarse-Grained Sediments: Image-Processing Procedures

This is the first in a pair of papers in which we present image-processing-based procedures for the measurement of fluvial gravels. The spatial and temporal resolution of surface grain-size characterization is constrained by the time-consuming and costly nature of traditional measurement techniques. Several groups have developed image-processing-based procedures, but none have demonstrated the transferability of these techniques between sites with different lithological, clast form and textural characteristics. Here we focus on image-processing procedures for identifying and measuring image objects (i.e. grains); the second paper examines the application of such procedures to the measurement of fluvially deposited gravels. Four image-segmentation procedures are developed, each having several internal parameters, giving a total of 416 permutations. These are executed on 39 images from three field sites at which the clasts have contrasting physical properties. The performance of each procedure is evaluated against a sample of manually digitized grains in the same images, by comparing three derived statistics. The results demonstrate that it is relatively straightforward to develop procedures that satisfactorily identify objects in any single image or a set of images with similar sedimentary characteristics. However, the optimal procedure is that which gives consistently good results across sites with dissimilar sedimentary characteristics. We show that neighborhood-based operations are the most powerful, and a morphological bottom-hat transform with a double threshold is optimal. It is demonstrated that its performance approaches that of the procedures giving the best results for individual sites. Overall, it out-performs previously published, or improvements to previously published, methods.

Sediments and landforms at Kvı́árjökull, southeast Iceland: a reappraisal of the glaciated valley landsystem

Abstract Eyles identified Kviarjokull, an outlet of the icecap Oraefajokull, southeast Iceland, as an exemplar of the glaciated valley landsystem, emphasising rockfall debris supply, passive transport and reworking of a thick cover of supraglacial morainic till. In this paper, we take a fresh look at the sediments and landforms of Kviarjokull, focusing on clast form analysis to reconstruct primary transport pathways. New data demonstrate the importance of active transport of debris derived from the glacier bed. Substantial portions of the supraglacial debris cover can be traced back to englacial channel fill deposits or anomalously thick exposures of basal ice. These two types of sediments also make up much of Kviarjokull's large Neoglacial moraine rampart. We attribute the abundance of these non-rockfall sediments to the presence of a terminal overdeepening, and the switch from predominantly subglacial to predominantly englacial drainage that this induces. This hydrological switch enhances retention of debris within ice transport, preventing it from being washed away and making it available to feed ice-marginal moraine formation. The complexity of sediment transport processes observed at Kviarjokull and the contrast between it and alpine glaciers undermines any single concept of the glaciated valley landsystem; accordingly, we propose Kviarjokull as the type example of a distinct subclass, defined by its moderate relief, high-debris turnover, and drainage behaviour characteristic of an overdeepened basin.

Clast size characteristics of gravel within rivers of the Waimea Plains, Nelson

Gravel within rivers of the Waimea Plains near Nelson, New Zealand, was derived through the Wairoa Gorge from Paleozoic‐Mesozoic basement rocks to the southeast, and to a lesser extent from Pleistocene gravels to the west. A downstream reduction of mean clast size and an increase in sorting occurs in three rivers of the Waimea Plains. Clast form is dominated by bladed clasts, with a slight tendency with transport distance to more elongate forms. Clast sizes are apparently larger in the modern rivers than in fluvial gravels of the Last Glacial period. The smaller clast size in the Last Glacial deposits may indicate more efficient clast‐size reduction in glacial as opposed to modern‐day rivers.

A small glacial cirque basin on Exmoor, Somerset

Geomorphological and sedimentological investigations at the Punchbowl on Exmoor, north Somerset, have demonstrated evidence of cirque basin glaciation, some 30 km south of the accepted southern limit of Anglian ice cover in this area. The Punchbowl is a steep-sided, north-facing basin that has a subdued arcuate ridge at its mouth. Analysis of macrofabrics, clast form and roundness and particle size on sediments forming the ridge indicate it to consist of glacial diamict deposited as a terminal moraine. Further evidence supporting the presence of a former cirque basin glacier at this site is derived from basin morphology and a striated sandstone boulder. Palaeoglacier reconstruction gives a surface area of 0.38 km 2 , a maximum thickness of 46.5 m and a maximum basal shear stress value of 51 kPa. The ELA of the former glacier has been calculated at 334 m OD. There is no evidence for multiple glaciations at this site, and, in the absence of dating controls on the moraine, the glacier is provisionally assumed to have formed during severe climatic conditions associated with the southerly advance of ice sheets during the Anglian cold stage or during the Dimlington Stadial. Development of a glacier at this location appears to have been a rare event during the Quaternary, and may have been facilitated by accumulation of windblown snow from the adjacent plateau.

Reconstructing the transport history of glacigenic sediments: a new approach based on the co-variance of clast form indices

Two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts: the C40 index (the percentage of clasts with ca ratios ⩽ 0.4) and the RA index (the percentage of very angular and angular clasts in a sample). Analysis of the between- and within-sample co-variance of these indices provides a powerful tool for discriminating actively- and passively-transported clasts in glacigenic deposits. Use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at Storbreen, Jotunheimen. These fall between those of actively transported clasts in basal till (low C40 and RA values) and those of unmodified frost-weathered clasts (high C40 and RA values). Both indices decline down-moraine. Within-sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts. These analyses also indicate that actively transported clasts form the dominant component of the moraines, particularly near the former glacier terminus. Further applications of the approach developed here are suggested.

Form and size characteristics of clasts on stone-banked solifluction lobes, Okstindan, North Norway

Subtle yet systematic differences in clast form and size in riser and tread zones on two stone-banked solifluction lobes in Okstindan, North Norway are observed. Clasts located within risers arc larger and tend to be ‘blockier’ (more equidimensional) than those found upon tread surfaces. Such large clasts may act as ‘braking blocks’; retard soil movement in the frontal zone and hence initiate the lobate form.

Variability in Neoglacial Moraine Morphology and Composition, Storbreen, Jotunheimen, Norway: Within-Moraine Patterns and their Implications

The nature and origin of the outermost 'Little Ice Age' moraine in front of a NE-flowing glacier (Storbreen, Jotunheimen) are investigated by analysing the variability along the length of the moraine of a range of characteristics (crossprofile form, surface composition, clast form and quartz sand grain characteristics). Concentrated application of a range of techniques in relation to a single moraine has enabled inferences to be made about glacial transport paths of debris in the moraine and processes of moraine-building. Moraine form is consistent within lateral and latero-terminal sections. Origin is attributed to a combination of some pushing of valley-side debris and dumping of glacially-transported material, in which the ratio of modified subglacial to unmodified supraglacial/englacial debris increases downglacier. A larger southern than northern side of the outer moraine is attributed to a difference in the quantity of debris being supplied from headwall areas, which are steep and heavily-eroded on the southern side but low-angled and vegetated on the northern side.

Debris transport by Loch Lomond Readvance glaciers in Northern Scotland: Basin form and the within‐valley asymmetry of lateral moraines

AbstractMoraines in six cirques in Northern Scotland are shown to be asymmetrically developed, being larger below north‐ or east‐facing valley sides. Moraine asymmetry is strongly correlated with the distribution of free faces in the valleys, a relationship that is interpreted as the result of variation in slope retreat rates. Analysis of clast form and roundness demonstrates that following initial release from bedrock, debris entered both passive high‐level glacial transport and tractive transport at the ice‐bed interface. The importance of glacier velocities and medial moraines on debris flux is discussed. Consideration is also given to the implications of process asymmetry to long term landform development.

Near-infrared spectroscopy of probable impact melt from three large lunar highland craters

The composition of small areas within three large lunar highland craters are compared using near-infrared, Earth-based telescopic data. Spectra from many areas in the walls and central peaks of the craters indicate the presence of crystalline components which include olivine, two types of pyroxene, and sometimes Fe-bearing feldspar. Spectra from other regions suspected of being impact melt deposits, located on the floors and walls of the three craters, have remarkably similar, yet anomalous features. These features are interpreted to indicate the presence of pyroxene, Fe-bearing glass, and Fe-bearing feldspar. Pyroxene and feldspar are believed to occur primarily in the form of lithic clasts and rapidly recrystallized impact melt. The Fe-bearing glass is interpreted as impact melt glass.

Aggregate clast form characteristics of deposits near the margins of four glaciers in the Jotunheimen Massif, Norway

The form of a clast is defined by the relative dimensions of its three principal axes. Samples of clasts from deposits with different erosional ‘histories’ near the margins of four Jotunheimen glaciers revealed systematic differences in their aggregate form characteristics. Samples of frost-shattered detritus generally contain many ‘slabby’ clasts (low c:a axis ratios) but samples from subglacial deposits and talus do not. This distinction is relatable to the spacing of dilation joints in the parent rock. The potential for employing systematic differences in aggregate clast from characteristics in studies of glacial landforms and sediments is discussed.

The ellipsoidal form of clasts with practical applications to fabric and size analyses of fluvial gravels

The widespread assumption that most water-worn gravel clasts approximate ellipsoids is confirmed by a statistical analysis of available data. The analysis demonstrates a Gaussian distribution of V/Ve ratios, centred on unit ratio, where V is clast volume and Ve the volume of a symmetric ellipsoid with equivalent triaxial dimensions. For internally isotropic and unbroken clasts, ellipsoidal form evolves as the rounding due to abrasion reaches its final stages. There appears to be no other major control on the tendency towards ellipsoidal geometry. The ellipsoidal tendency assists the interpretation of fluvial gravel deposits, which depends greatly on accurate description of clast size and fabric.Firstly, it facilitates calculation of Ap, the plane area projected upstream by clasts, a key parameter in bed–flow interactions such as preferred fabric. Formulae are derived to calculate Ap for ellipsoidal clasts with any configuration relative to flow direction. Viewing fabric in terms of the Ap variable supports and explains earlier conclusions concerning the controls on variability of imbrication angle.Secondly, an investigation of the relative merits of six size measures as descriptors of areal trends and predictors of nominal diameter, dn, concludes that (abc)1/3(the formula for dn of an ellipsoid) is superior. Other measures, namely, a, b, c, (a + c)/2, and (a + b + c)/3, are all subject to error in proportion to the degree of shape variation. Also, since downstream fining is typically accompanied by a changing proportion of oblate, bladed, prolate, and equant forms, dn is subject to inconsistent levels of under- or overestimation. The commonly used b dimension is endorsed as an acceptable predictor of dn, but a severely overestimates dn and should be abandoned. Information on errors in size analysis is presented as nomograms in the form of contoured c/b versus b/a plots and as probability distributions based on the typical range of shape variation in fluvial gravel.

Channelized Submarine Carbonate-Debris Flow, Cretaceous, Mexico: ABSTRACT

Carbonate platforms, fringed by rudistid reefs, rose to more than 1,000 m above contemporaneous basinal carbonates in the middle Cretaceous of central Mexico. The platforms are surrounded by halos of debris forming wedges which extend as much as 5-15 km from the platform margin. An exception to the wedge morphology is a breccia-filled channel exposed in section perpendicular to the paleoslope, 1 km from the marginal escarpment of the El Doctor platform, Queretaro, Mexico. The main part of the channel deposit is 225 m wide and 16 m thick. Breccia clasts are 75% basinal components (pelagic limestone and chert) and 25% platform derived. The bedded clasts form flat plates up to several meters in maximum dimension. A few clasts are deformed, indicating that they were at most slightly lithified and presumably of low density; nevertheless, most are neatly stacked with little distortion. Unstable clast postures, with long axes approaching normal to bedding, and imbrication are common. Content of dolomitized muddy matrix is low; the deposit is grain supported. The morphology of the breccia deposit and truncation of pelagic limestone at the channel edge demonstrate erosional competence of the depositional mechanism. Locally in the channel base an incipient slump of slightly folded pelagic limestone and chert indicates slope instability and suggests a mechanism for incorporating basinal carbonates in the breccias. In the top few meters of the deposit, breccia grades upward into fine-grained, skeletal-fragment packstone, which extends several hundred meters beyond the channel limits as thin, graded bed. At least 3 episodes of channel fill are suggested by internal structure. This evidence places the following constraints on the depositional proces--high competence (large clasts, erosion), high viscosity (unstable clast postures), low turbulence (slight deformation and platy form of unconsolidated clasts), high clast concentration (grain support, clast interaction) and channelization (form of deposit, multiple episodes). A high velocity submarine debris flow is suggested. The nonchannelized part of the last depositional episode is a typical turbidite. End_of_Article - Last_Page 777------------