Submarine sedimentation on a developing Holocene fan delta

The submarine morphology, sediments, and three-dimensional geometry of a developing fan delta are described using data from acoustic surveys, bottom sampling, and observations from a manned submersible. The fan system is being built in a British Columbian fjord (water depth 410 m) supplied with coarse-grained sediments from a fjord-side river. Construction of the subaqueous fan began about 10–12,000 yr BP and is ongoing. The system is analogous to part of one fault-uplift sedimentation cycle in ancient fan deltas. Initially, when offshore relief was at a maximum, acoustically chaotic sediment wedges were emplaced over fjord-bottom glaciomarine deposits. Subsequent aggradation/progradation resulted in moderately dipping sequences interrupted by local chaotic units. The present fan surface (average slope 13°) is divided into six zones arranged concentrically from the fan apex, on the basis of form, sediment and process interpretations. Continued subaqueous fan growth results from settling of river-derived sediments from suspension and downslope sediment dispersal by episodic gravity flows, apparently fed by underflows from the river.     

A scanning electron microscope study of surface textures of quartz grains from glacial environments

Surface textures of quartz sand grains from several glacial environments at the Feegletscher, Switzerland examined by means of scanning electron microscopy are described. The difference between supraglacial and subglacial material is very slight. At the moment the interpretation of these textures must be done with care until the full statistical relation to environments can be ascertained. This does not necessarily invalidate earlier investigations although it does mean that glacial and extraglacial environments in the vicinity of glaciers are apparently not distinguishable. Examination of surface precipitation features suggests a sequence of events which can be used to help discriminate between different ages of deposits in an area. A similarity between some of the surface debris from moraine samples and those seen in loess and quickclay deposits is also suggested.     

Processes of late Quaternary turbidity current flow and deposition on the Var deep-sea fan, north-west Mediterranean Sea

High-resolution seismic boomer profiles, with a vertical resolution of less than 1 m, together with piston cores and previous side-scan sonar data, are used to describe late Quaternary sedimentation on the Var deep-sea fan. Chronological control is provided by foram biostratigraphy and radiocarbon dating in cores, and is extended over the fan by seismic correlation. Regional erosional events correspond to the oxygen isotopic stage 2 and 6 glacial maxima. Cores and seismic data define a widespread surface sand layer that is correlated with prodelta failure in 1979 and subsequent submarine cable breaks. Numerical modelling constrains the character of this 1979 turbidity current. It originated from a relatively small slide on the upper prodelta that put sufficient material in suspension to form an accelerating turbidity current which eroded sand from the Var Canyon. The turbidity current was only 30 m thick on the Upper Valley, but experienced significant flow expansion in the Middle Valley to thicknesses of more than 120 m, where it spilled over the eastern Var Sedimentary Ridge at a velocity of about 2·5 m s^?1. Other Holocene turbidity currents (with a recurrence interval of 1000 years) were somewhat muddier and thicker, but also deposited sand on the levees of the Middle Valley, and are inferred to have had a similar slide-related origin. Late Pleistocene turbidity currents deposited thick mud beds on the Var Sedimentary Ridge. The presence of sediment waves and the mean cross-flow slope inferred from levee asymmetry indicates that some of these flows were many hundreds of metres thick and flowed at velocities of about 0·35 m s^?1. This contrast with Holocene turbidites suggests that a slide origin is unlikely. Estimated times for deposition of thick mud beds on the levees are many days to weeks. The Late Pleistocene flows may therefore result from hyperpycnal flow of glacial outwash in the Var River. The variation in the Late Pleistocene to Holocene turbidite sedimentation is controlled more by variations in sediment supply than by sea-level change.     

Proximal to distal sedimentological variation in a linear turbidite trough: implications for the fan model

Determination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back-arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse-grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in T_b and T_c divisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end-member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical.     

Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica

Sandy lobe deposits on submarine fans are sensitive recorders of the types of sediment gravity flows supplied to a basin and are economically important as hydrocarbon reservoirs. This study investigates the causes of variability in 20 lobes in small late Pleistocene submarine fans off East Corsica. These lobes were imaged using ultra‐high resolution boomer seismic profiles (<1 m vertical resolution) and sediment type was ground truthed using piston cores published in previous studies. Repeated crossings of the same depositional bodies were used to measure spatial changes in their dimensions and architecture. Most lobes increase abruptly down‐slope to a peak thickness of 8 to 42 m, beyond which they show a progressive, typically more gradual, decrease in thickness until they thin to below seismic resolution or pass into draping facies of the basin plain. Lobe areas range from 3 to 70 km² and total lengths from 2 to 14 km, with the locus of maximum sediment accumulation from 3 to 28 km from the shelf‐break. Based on their location, dimensions, internal architecture and nature of the feeder channel, the lobes are divided into two end‐member types. The first are small depositional bodies located in proximal settings, clustered near the toe‐of‐slope and fed by slope gullies or erosive channels lacking or with poorly developed levées (referred to as ‘proximal isolated lobes’). The second are larger architecturally more complex depositional bodies deposited in more distal settings, outboard more stable and longer‐lived levéed fan valleys (referred to as ‘composite mid‐fan lobes’). Hybrid lobe types are also observed. At least three hierarchical levels of compensation stacking are recognized. Individual beds and bed‐sets stack to form lobe‐elements; lobe‐elements stack to form composite lobes; and composite lobes stack to form lobe complexes. Differences in the size, shape and architectural complexity of lobe deposits reflect several inter‐related factors including: (i) flow properties (volume, duration, grain‐size, concentration and velocity); (ii) the number and frequency of flows, and their degree of variation through time; (iii) gradient change and sea floor morphology at the mouth of the feeder conduit; (iv) lobe lifespan prior to avulsion or abandonment; and (v) feeder channel geometry and stability. In general, lobes outboard stable fan valleys that are connected to shelf‐incised canyons are wider, longer and thicker, accumulate in more basinal locations and are architecturally more complex.     

Quantifying sample bias in clast fabric measurements

Sampling elongate clasts that protrude from a planar face for clast macro‐fabric analysis introduces a bias into reconstructed fabrics because clasts aligned perpendicular to the sampling face are over‐represented relative to those aligned parallel to the face. This study develops a probability‐based mathematical analysis to quantify sample bias for a variety of clast shapes and population fabrics, including isotropic, clustered and girdled fabric styles. Bias is expressed in terms of sample eigenvalues and eigenvectors relative to those of the parent population. Results indicate that sampling always has the effect of artificially drawing fabrics towards perpendicularity to the sampling face relative to the populations from which they are drawn. This rotation generally has the effect of artificially strengthening population fabrics, by up to 30% in the case of very weakly clustered or girdled populations. However, fabric strengthening is by no means universal and sampling alters different parent populations in different, sometimes complex, ways: the analysis in this paper identifies situations where sampling can strengthen or weaken parent fabrics, where it can rotate parent fabrics (by up to 90° in the case of a very weak population fabric), and where it can even change the style of a population fabric. For example, near‐isotropic population fabrics can appear clustered; weakly clustered and weakly girdled population fabrics can appear isotropic; weakly clustered population fabrics can appear weakly girdled; and girdled population fabrics can appear clustered. Overall, this analysis indicates that weakly orientated population fabrics are most susceptible to bias. Thus, a weakly clustered population fabric aligned parallel to a sample face is particularly susceptible to major sample bias in both fabric strength (artificially overestimated) and orientation (artificially rotated perpendicular to the face). Bias can be reduced either by sampling all the clasts from a cube‐shaped block of sediment excavated from the face, or by sampling equal numbers of clasts from the three orthogonal faces of the same sedimentary unit.     

Viscous Energy Dissipation and Strain Partitioning in Partially Molten Rocks

We develop a steady-state fluid-mechanical analysis describingthe effect of strain partitioning on viscous energy dissipation.As observed in experimental studies of shear deformation ofpartially molten rocks, strain partitions when melt segregatesbecause viscosity is reduced in regions of elevated melt fraction.The equations derived here are based on parameters measuredin experiments, describing the evolution of melt distributionand rheological properties. We find that the dissipation dependsstrongly on the configuration of the melt-rich network of shearzones, including the average angle, volume fraction of meltand amplification of strain rate in the melt-rich bands. Minimain energy dissipation as a function of band angle develop, correspondingto configurations of melt networks that minimize the differencein mean stress between the band and the non-band regions. Wepropose that the organization of band networks occurs by theinterplay between strain localization and viscosity variationsassociated with melt segregation. The band networks maintaina steady-state angle during shear by continuously pumping meltthrough the network. The development of strain partitioningin melt-rich networks will modify the energetics of meltingand melt transport by efficiently extracting melt and reducingeffective viscosity. KEY WORDS: melt transport; rheology; self-organization; strain localization; strain partitioning     

Rinded iron‐oxide concretions: hallmarks of altered siderite masses of both early and late diagenetic origin

Iron‐bearing concretions are valuable records of oxidation states of subsurface waters, but the first concretions to form can be altered drastically during later diagenetic events. Distinctive concretions composed of heavy rinds of iron oxide that surround iron‐poor, mud‐rich cores are common along bases of fluvial cross‐bed sets of the Cretaceous Dakota Formation, Nebraska, USA. Concretion rinds thicken inward and cores contain 46 to 89% void space. Millimetre‐scale spherosiderites are abundant in palaeosols that developed in floodplain facies. Evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils, transported, abraded and deposited in river channels. Alteration of siderite and formation of rinds occurred much later, perhaps in the Quaternary when sandstone pore waters became oxic. Dakota concretions are analogous to ‘rattlestones’ in Pleistocene fluvial channels of The Netherlands, and their rinded structure is analogous to that of iron‐rich concretions in the aeolian Navajo Sandstone of Utah. In all three deposits, rinded concretions formed when pre‐existing, siderite‐cemented concretions were oxidized within a sand matrix. Unlike fluvial examples, siderite in the Navajo Sandstone was autochthonous and of late diagenetic origin, having precipitated from carbon dioxide and methane‐enriched waters moving through folded and jointed strata. Iron‐rich rinds formed in all these strata because concretion interiors remained anaerobic, even as oxygen accumulated in the pore waters of their surrounding, permeable matrix. Iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward‐thickening rind. Acid generated by the oxidation reaction drove siderite dissolution to completion, creating the iron‐poor core. Iron‐oxide rinds are indicators of the former presence of siderite, a mineral that forms only under reducing conditions, during either early or late diagenesis. Siderite is vulnerable to complete oxidation upon exposure, so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate.