Sand Laminae Research Articles

Cliff-foot sandy cones: A proxy to study the time frames, patterns and rates of sandstone caprock decay?

Constraining time frames, patterns and rates of sandstone caprock disintegration in tablelands under temperate climate remains challenging. This is due to the scarcity of material in the appropriate geomorphological context that could be dated. Sandy cones are widespread below the sandstone plateaus and may serve as a direct sedimentological evidence of caprock disintegration via subsurface erosion. However, they have not been utilised to their full potential. The paper presents the results of investigation of two sandy cones in the Stołowe Mountains tableland, SW Poland, and soil profiles developed within them. The study also included collecting material for charcoal 14C and single grain Optically Stimulated Luminescence (OSL) dating. Both soil profiles (Dystric Arenosols (Humic)) are dominated by sand fraction. The lens–like laminae of light-coloured sand and darker material enriched in organic matter result from changing sedimentation dynamics. Charcoals were found to be widespread and occur down to the bottom of the studied soil profiles. While they record some of the oldest fire events ever documented in the tableland (mostly from Preboreal and Boreal, in one case from the Younger Dryas), we show that they are inappropriate to determine the timing of sediment deposition because of long residence time in the fissure network. Contrarily, single grain OSL dating is far better suited to trace the sedimentological history of the cones that initiated 10.5 ± 1.5 ka ago. The overall sedimentation trends are similar, likely reflecting climate fluctuations throughout the Holocene and the impact of deforestation. Although sandy cones may serve as a universal geochronological proxy in sandstone tablelands under temperate climate, the patterns and rates at which they develop are site-specific and controlled primarily by local geomorphological and topographical circumstances, governing the efficiency of the sediment supply. This finding was supported by a novel approach to the analysis of FMM components.

The influence of sedimentary and diagenetic heterogeneity on the radionuclide diffusion in the sandy facies of the Opalinus Clay at the core scale

Molecular diffusion is an important transport mechanism for radionuclide migration in low-permeable argillaceous host rock such as Opalinus Clay (OPA). In this study, the influence of sedimentary and diagenetic heterogeneity on heterogeneous diffusion in sandy facies of OPA (SF-OPA) from lamina scale to drill core scale is investigated using an upscaling workflow to model diffusive transport from the pore scale to the core scale. Our numerical results based on the simplified structural model show fast diffusion fronts in clay laminae (7 mm displacement after 6 days of diffusion) and slow diffusion fronts in carbonate lenses and sand laminae (4 mm displacement after 6 days of diffusion), demonstrating the endmembers of heterogeneous diffusion patterns in SF-OPA. Moreover, our results show that the diffusion fronts begin to homogenize after 22 days of diffusion with the specific influence of carbonate lenses (here: length = 1 cm, thickness = 3 mm). This example illustrates how material heterogeneities affect heterogeneous diffusion on a small temporal and spatial scale. The sensitivity studies show that the diffusion length and homogenization time increase by up to 190% when the length and thickness of the carbonate lenses are doubled. Using four compositional endmembers, we show the generalized diffusion behavior to demonstrate the influence of thin laminae and thick layers as well as dispersed small and large diagenetic concretions on the homogeneity of diffusion. These results demonstrate that the geometry of sedimentary and diagenetic material and the subfacies composition are the controlling factors for quantifying diffusion length and homogenization time. This study provides quantitative constraints on the temporal and spatial evolution of heterogeneous diffusion at the core scale. This quantitatively improves the predictability of radionuclide migration in host rocks as a function of compositional and pore network-specific parameters.

Depositional setup and characteristics of the storm deposit by the 2007 Cyclone Sidr on Kuakata Coast, Bangladesh

This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135–277 m from the coast. Laboratory analyses of grain size, TOC, TN, δ13C, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1–17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, δ13C values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.

Selective sediment transport during Hurricane Sandy on Fire Island (New York, USA): Inferences from heavy-mineral assemblages

ABSTRACT In October 2012, Hurricane Sandy caused severe erosion on beaches and dunes of Fire Island (New York, USA). Major shoreline changes occurred with erosional dominance in the upper shoreline and aggradation in the lowermost section of the beach due to the deposition of eroded upper-beach and dune sediment. Sand laminae with a high concentration of heavy minerals (“black sand laminae”) were observed in three excavated trenches located on a washover terrace and fan on the east side of Fire Island. The mineral composition of these laminae reveals the presence of high quantities of magnetite, ilmenite (as opaque minerals), and garnet (as the main translucent mineral). These heavy-mineral-enriched laminae were formed as waves eroded and transported sand from the primary dune and smaller relict dunes under specific hydrodynamic conditions that promoted grain sorting according to differences in size and specific gravity. Based on the concentrations of certain heavy minerals, the threshold for primarily density-driven sorting probably lies between the specific gravity of less dense opaque mineral (ilmenite, 4.7) and almandine (4.3), the most common transparent heavy mineral. The number of laminae and concentrations of heavy minerals vary between trenches on the overwash terrace and appear to be controlled by their distance from sediment sources. The trench with the greatest number of laminae and higher heavy-mineral concentrations is located farthest from the main dune but is just 10 m inland of a relict dune that acts as the primary source of sediment of the washover deposit in this trench. A conceptual model for deposition of heavy-mineral layers is presented based on geomorphological and sedimentological evidence allowing the definition of a density threshold.

Emplacement and dewatering of the world's largest exposed sand injectite complex

Sandstone injectites form by up or down‐section flow of a mobilized sand slurry through fractures in overlying rock. They act as reservoirs and high‐permeability conduits through lower permeability rock in hydrocarbon systems. The Yellow Bank Creek Complex, Santa Cruz County, California is the largest known exposure of a sandstone injectite in the world. The complex contains granular textures that record processes of sand slurry flow, multiple pore fluids, and dewatering after emplacement. The injection was initially mobilized from a source containing both water and hydrocarbons. The water‐sand slurry reached emplacement depth first, due to lower fluid viscosity. As the sand slurry emplaced, the transition from slurry flow to pore water percolation occurred. This transition resulted in preferred flow channels ∼6 mm wide in which sand grains were weakly aligned (laminae). The hydrocarbon‐sand slurry intruded the dewatering sands and locally deformed the laminae. Compaction of the injectite deposit and pore fluid escape caused spaced compaction bands and dewatering pipes which created convolutions of the laminae. The hydrocarbon‐rich sand slurry is preserved today as dolomite‐cemented sand with oil inclusions. The laminae in this injectite are easily detected due to preferential iron oxide‐cementation of the well‐aligned sand laminae, and lack of cement in the alternating laminae. Subtle textures like these may develop during sand flow and be present but difficult to detect in other settings. They may explain permeability anisotropy in other sand deposits.

Evaluation of Thin Bed Using Resistivity Borehole and NMR Imaging Techniques

In the study of sedimentary units to evaluate thin beds, there lie a lot of uncertainties, primarily, sand, shale or their combination in terms of their petrophysical properties and lateral extant. Good thin beds are capable of yielding a reasonable amount of hydrocarbon if discovered and evaluated. Regrettably, most thin beds are usually bypassed if direct log interpretation techniques are applied in their evaluation due to their low vertical resolutions and ability to only characterize an average of multiple beds which often result to a significant under estimation of oil and gas in the sedimentary unit. To better characterize thin beds and maximize reservoir potential, the resistivity borehole imaging and NMR techniques were adopted in this research study. In addition, the Thomas Stieber equations using laminated sand-shale sequence were employed in calculating parameters such as sand porosity, hydrocarbon saturation and shale volume. Results show that at depths of about 4350m and 4500m (Fig. 2), thin beds were identified, whereas, the conventional logs identified the points as spikes of sand. The NMR imaging technique further confirms the presence of hydrocarbon in the thin beds. It is therefore suggested that the resistivity borehole imaging and NMR techniques should be employed in characterizing thin beds since their use in this study yielded results that are reliable and dependable. Keywords: Thin bed, Borehole image, Sand laminae, Conventional logs, lateral resistivity, Vertical resistivity.

MORPHOLOGIC VARIATIONS OF THE TRACE FOSSIL RUTICHNUS IN CM-THICK TURBIDITES FROM THE VERGHERETO FORMATION (NORTHERN APENNINES, ITALY)

The analysis of 56 specimens of the branched, walled post-depositional trace fossil Rutichnus , sampled from 31 thin-bedded, overbank turbidites from the Verghereto Formation, Romagna Apennines, indicates high morphologic variability of this ichnotaxon in hypichnial preservation. The variability concerns the arrangement of outer and inner walls, number and shape of annulations, distribution of pustules, the (false) branching arrangement and the general external shape. Highest variability has been observed mainly in the Rutichnus rutis ichnospecies, clearly less in the R. irregularis ichnospecies that is rare. The Dino-Lite microscope camera analysis of oriented thin sections indicates that endichnial preservation exhibits many structures due to burrowing in soft substrate, destroying laminae of sand. Consequently Rutichnus probably was produced by a worm or arthropod, inducing strong deformation in turbidite sand during the feeding activity on phytodetritus and organic matter transported by the turbidity flow.

Lake levels and sedimentary environments during deposition of the Trego Hot Springs and Wono tephras in the Lake Lahontan basin, Nevada, USA

The Wono and Trego Hot Springs (THS) tephras are widespread in the Lahontan basin and have been identified in a variety of sedimentary environments at different elevations. Davis (1983) reported lake level to be at about 1256 m when the THS tephra was deposited, an interpretation questioned by Benson et al. (1997) who interpreted lake level to be ≤ 1177 m at that time. This is a significant difference in lake size with important implications for interpreting the climate that prevailed at that time. Based on new interpretations of depositional settings of the THS bed at multiple sites, the larger lake size is correct. Additional sites containing the Wono tephra indicate that it was deposited when lake level was at about 1217 m in the western subbasins and at about 1205 m in the Carson Sink. Sedimentary features associated with progressively deeper paleowater depths follow a predictable pattern that is modulated by proximity to sediment sources and local slope. Fine to coarse sands with wave-formed features are commonly associated with relatively shallow water. Silty clay or clay dominates in paleowater depths > 25 m, with thin laminae of sand and ostracods at sites located adjacent to or downslope from steep mountain fronts.

Formation of mud ridge and runnels in the intertidal zone of the Severn Estuary, UK

Flow-parallel linear bedforms, consisting of ridges and runnels have been reported widely in soft and consolidated cohesive sediments within intertidal and subtidal coastal waters. Less frequent are reports of these systems incised into soft bedrock. Despite their common occurrence there are few data concerning the hydrodynamic controls on the initiation and maintenance of ridge–runnel systems. Rather, several hypotheses have been advanced to account for them. In tidal environments a theory based upon the evolution of ‘streamwise vorticity’ transverse to the main flow direction is favoured, and has been supported recently by detailed hydrodynamic data obtained from above a ridge–runnel system on an intertidal zone of the Severn Estuary, UK. In the present paper, a detailed description of the sedimentology of the same ridge–runnel system is provided and interpreted within the framework of the ‘streamwise vorticity’ hypothesis. This objective is accomplished by measurements of the morphology and sampling the bulk characteristics of the sedimentary ridges and runnels but also with reference to ridges and runnels incised into soft bedrock beneath the sediment body. Grain-size composition, sediment compaction, radionuclide distributions and elemental distributions and X-radiography are used to describe the sediment structures within ridges and their inferred histories of development. These descriptions are interpreted with respect to the known hydrodynamic climate and the deposition and erosion behaviour of the sediments in laboratory experiments using an annular flume. Runnels contain liquid mud at low tide but contain very little fine gravel and shell debris. In contrast, fine suspended sediment is present at high concentrations in the flow. Thus fluid evorsion, aided by a dense suspension of fine sediment that impacts the mud bed, must be the primary tool to initiate and maintain the runnels, rather than scour by a coarse bedload. The ridges are composed primarily of fine silt or sand laminae some of which are approximately horizontal but most of which are slightly domed upwards such that they drape the ridge topography. This relationship between lamination and form demonstrates that the ridges are distinct accretion units and not residual masses of un-eroded planar-laminated mud incised by runnels. Laminae thicken, or thin and pinch-out, or are truncated, laterally at the sides of the runnels. Truncation occurs probably by damped spanwise turbulence structures which were recorded at this location. A notable characteristic is that the mud in the upper portion of the ridges is of low density and low compaction whilst the sediment in the lower part is more dense and well-compacted; often the two units are distinct, being separated by major reactivation surfaces; the latter indicative of occasional stripping of the surface sediments from above a residual ‘core’ at the base. The presence of coarse-grained laminae and lenses and minor reactivation surfaces throughout the sediment body indicates that high energy conditions, possibly other than the Neap–Spring cycle, are also important in developing the ridges. In this respect, consideration of the erosion potential of deposited sediment, using field and flume data, shows that the relatively low density, low-compaction mud constituting the crests of the ridges is not readily eroded by the present-day Neap–Spring tidal flow regime. Further, for the same flow regime, when the shear stresses on the bed fall below critical for suspension, potential deposition rates are very low, which supports the interpretation of the coarse-grained layers and reactivation surfaces as indicators of the significant role played by additional high-stress events, such as storm surges, in ridge-erosion and rebuilding.

Dinosaur tracks and possible lungfish aestivation burrows in a shallow coastal lake; lowermost Cretaceous, Bornholm, Denmark

The Mesozoic succession of the island of Bornholm in the Baltic Sea has hitherto yielded only very few remains after vertebrate life. We describe a new find of a well-exposed vertebrate trample ground and large, densely spaced, vertical to inclined burrows from shallow lake deposits belonging to the Berriasian Skyttegård Member of the Rabekke Formation. The succession is terrestrial, but a coastal setting is indicated by the great abundance of pyrite concretions and framboidal pyrite. Organic geochemical and palynological data show that the lake was surrounded by a low-diversity gymnospermous vegetation. The trampling ground is exposed in a vertical section at the boundary between light grey, structureless or trough cross-bedded fluvial sand and dark brown, organic-rich lacustrine mud with densely spaced vertical rootlets, wood fragments and large pyrite concretions. The tracks are bowl-shaped, have an irregular flat or slightly conical base, are on average 22 cm deep and on average 43 cm wide in the upper part and 23 cm in the lower part, but some reach widths up to about 1 m and have a highly irregular shape. The underlying sand in some cases shows a faint stratification that is bent downwards adjacent to the mud-filled tracks, which have near-vertical and commonly overhanging margins. The morphology of the bowl-shaped depressions suggests that they were made by dinosaurs, most likely sauropods. They were clearly emplaced after deposition of the dark brown mud. Long, vertical to inclined, commonly slightly curved, J-shaped or spiraling cylindrical burrows with a diameter averaging 4.4 cm and lengths up to 45 cm occur densely spaced in the mud. They are unbranched, have no mud or lag linings. The top of the burrows is indistinct or not preserved, whereas the base is rounded, concave-up, commonly with a slightly enlarged chamber. The length is difficult to estimate precisely because of the missing top and because the burrows cross-cut and go in and out of the plane of exposure. The burrows have a passive infill of dark mud and, at some levels, of irregularly alternating laminae of white sand and mud. The sand may be of aeolian origin or may represent distal crevasse splays or washovers from adjacent rivers or the nearby sea. Skeletal remains have not been found in or associated with the burrows, which are similar to burrows referred to lungfish or crayfish, and they are with hesitation interpreted as representing lungfish aestivation burrows based on the morphological characteristics. This is only the second report of dinosaur tracks in Denmark and the first in-situ find where environmental conditions can be interpreted with some confidence. Possible lungfish burrows are here reported for the first time from the Danish Mesozoic. The overall rarity of vertebrate remains in the Mesozoic succession of Bornholm may reflect that the area was a horst island or submerged shoal during much of the Jurassic and Cretaceous and may not always have had direct land connection with surrounding land areas of the Baltic Shield or mainland Europe.

Elucidating aeolian dune history from lacustrine sand records in the Lake Michigan Coastal Zone, USA

Aeolian sand in two small lakes within a Lake Michigan coastal-dune complex southwest of Holland, Michigan, provides a more detailed higher-resolution history of dune activity, during the later half of the Holocene, than do combined palaeosol and OSL ages from the dunes themselves. The sand signal from four cores within these lakes consists of visible sand laminae and invisible sand peaks within mud and sapropel units. Early in the history of dune growth the sand may have come from backdunes immediately adjacent to the lakes, but in the upper portions of the core aeolian sand is interpreted as having been transported from 30—40 m high parabolic dunes ~200 m from the lakes. All but one episode of dune mobility suggested by radiocarbon-dated dune palaeosols or OSL-dated dune sand are represented by a peak in the radiocarbon-dated lacustrine sand signal. However, sand peaks occur in the lacustrine record that do not correspond to palaeosol or OSL ages, suggesting that the sand record from small lakes gives a more complete record of aeolian activity. Heterogeneity in sand deposition related to bathymetry suggests that the study of multiple cores from the same lake will yield the best results . No strong correlation between rising lake levels on the Lake Michigan relative lake level curve and aeolian activity was found, although such a correlation has been reported elsewhere.

Wind deposition of mud aggregates and their role in development of lamellae in the Fair Oaks Dunes, Indiana

Abstract Three parabolic dunes from the Fair Oaks Dune field in northern Indiana were excavated, in order to study the properties and genesis of lamellae. Reddish lamellae with sharp upper boundaries and diffuse lower boundaries are intercalated with yellowish sand layers within the upper 3–5 m of each dune. The thicknesses of the lamellae decrease from > 2 cm in the east (Winamac dune) to + and Mg + led to breakdown of mud aggregates and clay dispersion from clay rich sand laminae through clay-poor sand laminae. The reddish color of lamellae results from oxidation of mafic grains. Sand-size mud aggregates were lighter than quartz or feldspar grains and were carried further downwind and, as a result, the clay in the dunes, and the thickness of the lamellae, increases to the east. At 5.6 m depth in the Winamac dune we found the straight depositional laminae transitioning into wavy lamellae. This is the most direct evidence of depositional lamellae (petrogenic) formation in the literature to date.

Catastrophic hurricane history revealed by organic geochemical proxies in coastal lake sediments: a case study of Lake Shelby, Alabama (USA)

We developed a new method for reconstructing millennia-long hurricane records from coastal environments that uses Organic Geochemical Proxies (OGPs) of organic carbon and nitrogen concentrations and their δ13C and δ15N compositions. The new method is independent of presence/absence of sand layers and improves significantly the severe-storm history resolution. The subject of this investigation is a 1.5 m long sediment core raised at 2.8 m water depth from the center of Lake Shelby, Alabama, a freshwater lake located approximately 250 m from the Gulf of Mexico, from which an overwash sand-layer based record was previously derived. The core contains two distinct sediment units; an upper 62 cm thick, fine-grained, organic-rich lacustrine sapropel (gyttja) that shows no visible structures except one sand lamina at 23.7 cm depth, and an underlying 90 cm thick, organic-poor lagoon/estuary clay unit. The sapropel unit was deposited over a 682 ± 30 cal year time interval (1320–2002 A.D.) with a mean sedimentation rate of 0.79 ± 0.04 mm/year. Lake Shelby’s water column exhibits two contrasting states based on water chemistry surveys (i) an “isolated”, stratified, mode under calm weather conditions with a relatively low trophic state, and (ii) a “flooded” mode occurring during storm surges when nutrient-rich seawater floods the lake. Statistically significant δ13C and δ15N positive excursions in organic matter, up to maximum values of −25 (‰ PDB) and 4 (‰ Air N2), respectively, are interpreted as geochemical responses to the marine intrusions that fertilize the lake, increase light availability, and cause eutrophication spikes. Detailed OGPs analyses crossing a sand layer that offers visual evidence of a catastrophic hurricane overwash event at 1717 A.D. exhibit large δ13C and δ15N positive shifts bounded by rapid returns to base values, thus confirming the validity of the hurricane identification by the OGPs model. Our data indicate that 11 catastrophic hurricanes hit the Alabama coast over the past 682 years with a rough recurrence interval of one in 62 years.

Statistical analysis of the temporal and spatial controls on tidal signal preservation in late-Holocene tidal rhythmites, Romney Marsh, Southeast England

A hierarchical series of tidal periodicities are preserved within laminated tidal flat deposits at the barrier/back-barrier interface of Romney Marsh and the Dungeness Foreland in southeast England. The sedimentary record of the tidal signature, extracted from variations in sand layer thickness, was found to be severely truncated with neap-spring periods typically represented by five or less sand layers and possibly only alternate neap-spring periods present. Despite the low number of sand layers deposited in these higher frequency tidal cycles, semi-annual periods are clearly preserved but tend to contain less than the expected 6 months of sedimentation. Annual accumulation rates of around 0.2–0.3 m/year are indicated. Local emplacement of storm beaches is considered to have created the protected conditions suitable for tidal rhythmite preservation. Ongoing foreland progradation and infilling eventually restricted tidal inundation to the point where distinct sand laminae were no longer deposited. Here, in contrast with other systems, accommodation space was not limiting and tidal flat elevation is reflected as a more subtle control on spatial changes in the resolution of the tidal signal.

Environmental interpretation and a sequence stratigraphic framework for the terminal Proterozoic Ediacara Member within the Rawnsley Quartzite, South Australia

Fossils of the Ediacara biota are confined to preservational windows in the Ediacara Member of the Rawnsley Quartzite (Pound Subgroup), from the Flinders Ranges, South Australia. The base of the Ediacara Member is a type 1 sequence boundary incised into the partly lithified Chace Quartzite Member of the Rawnsley Quartzite. The Ediacara Member and the upper half of the Rawnsley Quartzite comprise the Rawnsley depositional sequence, bounded above by the Early Cambrian Uratanna depositional sequence. The Rawnsley sequence developed over an erosional surface cut into the Chace Quartzite Member, with some 250 m of relief. Southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone. At lowstand, local channels were filled by massive sand. The highstand systems tract consists of several parasequence sets of laminated siltstone and channelized, amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well-bedded, clean sandstone. Impressions of soft-bodied Ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward-shoaling, delta-front environments between storm- and fairweather wavebase. The Ediacara Member is, in places, conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member. Distinctive microbial-mat-bound sand laminae, described as petee structures, are characteristic of the Rawnsley Quartzite above and below the Ediacara Member. During deposition of the Ediacara Member, colonization of the deeper substrates by microbial mat communities enabled preservation of the Ediacara biota. The erosional surface at the base of the Ediacara Member is one of at least four incision events recorded in the terminal Proterozoic and Early Cambrian succession of the Adelaide Geosyncline. The similarity of incision-filling lithofacies of the Rawnsley and Uratanna sequences has, in condensed sections, led to confusion between the depositional sequences. Characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the Proterozoic–Cambrian boundary.

Patterns and processes of sediment dispersal on the continental slope off Nice, SE France

The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice (SE France) have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material (gravel and cobble) directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles (suspension load) are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin.

Morphology and palaeoenvironmental interpretation of deformed soft-sediment clasts: examples from within Late Pleistocene glacial outwash, Tempo Valley, Northern Ireland

Abstract Glacial outwash, deposited during deglaciation of the late Devensian ice sheet, is present as a flat-topped valley fill in the Tempo Valley on the southern flanks of the Fintona Hills, Northern Ireland. Sedimentologically, the outwash comprises well-sorted and interbedded rippled to massive sands which record distal deposition within a proglacial water body. Beds of ripple-drift cross-laminated sands contain deformed (folded and contorted) soft-sediment clasts which are composed mainly of silt and clay. The soft-sediment clasts were deformed prior to final deposition because clast a–b planes lie conformable to sand laminae which are undeformed. Morphological characteristics of the soft-sediment clasts, and their facies context, provide evidence for transport mechanisms, depositional environment, and processes of clast deformation. The soft-sediment clasts were transported into a proglacial water body by unidirectional water currents (∼1.5–2.5 m s−1). Sediment transport processes include sediment bypassing within the water column, a low bedload component, and grain flow activity during waning flow stages. The overall morphology of soft-sediment clasts records between 1 and 3 distinct phases of hydroplastic deformation prior to emplacement. The deformation phases are recognised on the basis of morphologically `unrolling' the superimposed folds of the soft-sediment clasts. Deformation structures (i.e. fold style) and direction of the principal stress axis relative to clast axes suggest that clasts were reoriented with respect to water flow direction following each deformation phase. Processes of deformation include folding-over of the clast along its b axis into two or more components, crumpling and abrasion of the outer margins of the b plane, and squashing of the clast c axis (some of which may be post-depositional deformation). The presence of silt- and clay-rich soft-sediment clasts within the outwash succession suggests that they were ripped-up from shallow and irregular pools on the glacier forefield, into which fine sediments accumulated after flood or meltwater events, and transported distally into a proglacial water body. These inferences based on facies evidence and styles of hydroplastic deformation impact on reconstructions of local palaeogeography, and the wider interpretation of similar soft-sediment clasts in the geological record.

Acquisition of anhysteretic remanence and tensor subtraction from AMS isolates true palaeocurrent grain alignments

Abstract Anisotropy of magnetic susceptibility (AMS) is commonly used to detect subtle palaeocurrent directions. A Proterozoic subarkose shows extreme anomalous inverse AMS fabrics that cannot be attributed to the well-known effect of single-domain magnetite. This was confirmed by anisotropy of anhysteretic remanent magnetization (AARM). Further, by subtracting increasing proportions of the AARM tensor from the AMS tensor the paramagnetic/diamagnetic response of the silicate fabric was isolated. This reveals an inverse quartz fabric but analysis of the results shows that two grain alignments are present. A prominent inverse magnetic fabric is due to quartz long axes aligning with current flow revealed by the difference [AMS]-[AARM]. This is due to the avalanche flow of concentrated sand dispersions down the slip-face of cross-stratified units. Dispersive pressure caused by grain interactions and shear within the fluid produces a subhorizontal resultant force that aligns grains by frictional freezing during deposition. In some samples AARM isolates a secondary feeble alignment of magnetite nearly perpendicular to bedding. This is due to suspension rainout and sweeping by the reverse circulation separation eddy that supplies additional material to the top of the sand laminae. It particularly affects fine grains such as magnetite that infiltrate the framework’s pores to produce a secondary population of vertically oriented grains.

Facies and architecture of an estuarine tidal bar (the Trompeloup bar, Gironde Estuary, SW France)

This study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay-head delta of a wave- and tide-dominated estuary, the Gironde Estuary, in southwest France. Based on 51 cores (2 to 5 m long), this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers. The bar appeared in the study area around 1874. Since then, its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment. During the periods of active bar progradation, which commonly last between 10 and 20 years, the bar progrades up to 7 km seaward and forms a narrow (200–800 m wide) sand ridge up to 6 m thick. During abandonment phases which also last around 10–20 years, the bar is starved of sand and a mud layer (1 to 3 m thick) is deposited. The vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment. A typical vertical succession in the bar system is 8 m thick and comprises several superimposed, upward-coarsening units of cross-stratified sand, deposited during phases of seaward progradation, separated by a thick layer of thinner-bedded mud and sand laminae deposited during phases of bar abandonment. When a tidal bar builds up to the supratidal zone, marshes accumulate and the bar becomes a permanent island in the estuary. The large-scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals. The reservoir units are characterized by a sigmoidal shape. The muddy layer, which is deposited between the two sigmoidal units, is a thick, continuous potential internal seal. This complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay-head delta of the Gironde Estuary. It is thought to be representative of the architecture of tidal bars in the inner part of wave- and tide-dominated estuaries.

Induction Log Shoulder-Bed Corrections to Anisotropic Formations and the Effect of Shale Anisotropy in Thinly Laminated Sand/Shale Sequences

Summary We examine induction log responses to layered, dipping, and anisotropic formations analytically. The analytical model is especially helpful in understanding induction log responses to thinly laminated binary formations, such as sand/shale sequences that exhibit macro-scopically anisotropic resistivity. We discuss two applications of the analytical model. In one application, we examine special induction log shoulder-bed corrections for use when thin anisotropic beds are encountered. It is known that thinly laminated sand/shale sequences act as macroscopically anisotropic formations. Hydrocarbon-bearing formations also act as macroscopically anisotropic formations when they consist of alternating layers of different grainsize distributions. When such formations are thick, induction logs accurately read the macroscopic conductivity, from which the hydrocarbon saturation in the formations can be computed. When the laminated formations are not thick, proper shoulderbed corrections (or thinbed corrections) should be applied to obtain the true macroscopic formation conductivity and to estimate the hydrocarbon saturation more accurately. We use the analytical model to calculate the thinbed effect and to evaluate the shoulder-bed corrections. We show that the formation resistivity and, hence, the hydrocarbon saturation are greatly overestimated when the anisotropy effect is not accounted for and conventional shoulder-bed corrections are applied to the log responses from such laminated formations. In another application, we examine the effect of shale anisotropy in thinly laminated sand/shale sequences. How do induction logs respond to such laminated formations when shale laminae are anisotropic? How accurate are the estimates of the sand-laminae resistivity and of the hydrocarbon saturation in these sand laminae? To answer these questions, we used the analytical model to examine the effect of shale lamina anisotropy on induction log responses in thinly laminated formations. We learned that the macroscopic conductivity of a laminated formation is determined uniquely by the sandlaminae conductivity and the anisotropic shale conductivity, if the sand fraction is known. We also found that the sand resistivity estimate depends on the value of shale resistivity and the dip angle, if the shale is assumed to be isotropic and if the sand fraction is known. When the sand fraction is not known, the sand resistivity and the sand fraction can be estimated if the set of horizontal and vertical macroscopic resistivities is known. In this case, the sand resistivity tends to be overestimated if the shale anisotropy is not accounted for. On the other hand, the sand fraction estimate is generally insensitive to the shale anisotropy, except for low sand-laminae resistivity (Rsd/Rsh < 5).