Showing papers in "Sedimentology in 2001"

The physical character of subaqueous sedimentary density flows and their deposits

Abstract: The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

Hyperpycnal plume formation from riverine outflows with small sediment concentrations

Abstract: A series of laboratory experiments has been conducted in order to elucidate the sediment-induced mixing processes accompanying riverine outflows; specifically, the discharge of a warm, fresh, particle-laden fluid over a relatively dense, cool brine. In a parameter regime analogous to recently acquired field measurements, hypopycnal (surface) plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm, fresh, sediment-laden plume and particle settling within it. Convection was robust in the presence or absence of intense turbulence, at sediment concentrations as low as 1 kg m−3, and took the form of millimetre-scale, sediment-laden fingers descending from the base of the surface plume. A consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal (bottom-riding) flow. The experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient. The resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond.

Microbial signatures in peritidal siliciclastic sediments: a catalogue

Abstract: A catalogue of microbial structural signatures is presented, based upon the coupling of fundamental biogeochemical–microbial processes and local morphogenetic determinants. It summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones (temperate humid: Mellum Island, southern North Sea; subtropical arid: coast of southern Tunisia). Textural geometries reveal a high structural diversity, but their determinants are primarily based upon six major parameters: (1) intrinsic biofactors: structural diversification of sedimentary microbial films and mats inherent in the organisms, i.e. their construction morphology, growth, taxis and behaviour, and local abundance of specific morphotypes. Most prominent are the ensheathed filamentous cyanobacteria Microcoleus chthonoplastes and Lyngbya aestuarii, and the sheathless filamentous cyanobacterium Oscillatoria limosa. (2) Biological response to physical disturbances: sediment supply, erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats. (3) Trapping/binding effects: physicobiological processes give rise to grain orientations and wavy to lenticular lamina, lamina-specific grain arrangements and ‘sucrose’ calcium carbonate accumulations. (4) Secondary physical deformation of biogenic build-ups: mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures. (5) Post-burial processes: textural fabrics that evolve from mechanical effects of gas formation from decaying mats, and features related to the formation of authigenic minerals (calcium carbonates, calcium sulphates, pyrite). (6) Bioturbation and grazing: post-depositional structures, such as Skolithos-type dwellings, traces of burrowing insects, gastropod grazing traces and faecal pellets. In synopsis, the catalogue firstly comprises a sound set of ubiquitous signatures. This uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings. The catalogue secondly documents signatures that are extremely sensitive to tidal position, hydrodynamic regime and overall climatic conditions. These kinds of signature indicate narrow facies zones, which often coincide with the activity or dominance zones of certain organisms. An overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings.

Microbial-silica interactions in Icelandic hot spring sinter: possible analogues for some Precambrian siliceous stromatolites

Abstract: Silicified deposits, such as sinters, occur in several modern geothermal environments, but the mechanisms of silicification (and crucially the role of microorganisms in their construction) are still largely unresolved. Detailed examination of siliceous sinter, in particular sections of microstromatolites growing at the Krisuvik hot spring, Iceland, reveals that biomineralization contributes a major component to the overall structure, with approximately half the sinter thickness attributed to silicified microorganisms. Almost all microorganisms observed under the scanning electron microscope (SEM) are mineralized, with epicellular silica ranging in thickness from < 5 lm coatings on individual cells, to regions where entire colonies are cemented together in an amorphous silica matrix tens of micrometres thick. Within the overall profile, there appears to be two very distinct types of laminae that alternate repeatedly throughout the microstromatolite: ‘microbial’ layers are predominantly consisting of filamentous, intact, vertically aligned, biomineralized cyanobacteria, identified as Calothrix and Fischerella sp.; and weakly laminated silica layers which appear to be devoid of any microbial component. The microbial layers commonly have a sharply defined base, overlying the weakly laminated silica, and a gradational upper surface merging into the weakly laminated silica. These cyclic laminations are probably explained by variations in microbial activity. Active growth during spring/ summer allows the microorganisms to keep pace with silicification, with the cell surfaces facilitating silicification, while during their natural slow growth phase in the dark autumn/winter months silicification exceeds the bacteria’s ability to compensate (i.e. grow upwards). At this stage, the microbial colony is probably not essential to microstromatolite formation, with silicification presumably occurring abiogenically. When conditions once again become favourable for growth, recolonization of the solid silica surface by free-living bacteria occurs: cell motility is not responsible for the laminations. We have also observed that microbial populations within the microstromatolite, some several mm in depth, appear viable, i.e. they still have their pigmentation, the trichomes are not collapsed, cell walls are unbroken, cytoplasm is still present and they proved culturable. This suggests that the bulk of silicification occurred rapidly, probably while the cells were still alive. Surprisingly, however, measurements of light transmittance through sections of the microstromatolite revealed that photosynthetically active light (PAL) only transmitted through the uppermost 2 mm. Therefore the ‘deeper’ microbial

Bedforms and associated sedimentary structures formed under supercritical water flows over aggrading sand beds

Abstract: Bedforms and associated sedimentary structures, formed under supercritical water flow over an aggrading sand bed, were studied in a laboratory flume. Although the geometry and hydraulic characteristics of these bedforms (antidunes, chutes-and-pools) are well known, their internal structures are not. The objectives of the study were to: (1) describe the three-dimensional geometry of the sedimentary structures and examine their mode of origin; (2) develop a relationship between the geometries of the sedimentary structures and the formative bedforms and; (3) identify criteria that distinguish these sedimentary structures from similar types, such as hummocky and swaley cross-strata. Sedimentary structures associated with antidunes are primarily lenticular laminasets with concave-upward erosional bases (troughs) in which laminae generally dip upstream or fill the troughs symmetrically. These laminasets are associated with growth and upstream migration of water-surface waves and antidunes, and with surface-wave breaking and filling of antidune troughs respectively. In addition, sets of downstream-dipping laminae are produced by rapid migration of asymmetrical bedwaves immediately after wave breaking. Rare convex-upward laminae define the shape of antidunes that developed under stationary water-surface waves. The laminasets and internal laminae extend across the width of the flume, but vary in thickness and inclination, indicating that the antidunes have some degree of three dimensionality. The length and maximum thickness of the lenticular laminasets are approximately half of the length and height of formative antidunes, providing a potentially useful tool for palaeohydraulic reconstructions. The sets of downstream-dipping laminae formed under antidunes are distinctive and do not occur in hummocky and swaley cross-strata. Sedimentary structures associated with chutes-and-pools are sets of upstream-dipping laminae and structureless sand.

Anatomy, geometry and sequence stratigraphy of basin floor to slope turbidite systems, Tanqua Karoo, South Africa

Abstract: The Tanqua area of the Karoo basin, South Africa, contains five Permian deep-water turbidite fan systems, almost completely exposed over some 640 km2. Reconstruction of the basin-fill and fan distributions indicates a progradational trend in the 450 m+ thick succession, from distal basin floor (fan 1) through basin-floor subenvironments (fans 2, 3 and 4) to a slope setting (fan 5). Fans are up to 65 m thick with gradational to sharp bases and tops. Facies associations include basin plain claystone and distal turbidite siltstone/claystone and a range of fine-grained sandstone associations, including low- and high-density turbidite current deposits and proportionally minor debris/slurry flows. Architectural elements include sheets of amalgamated and layered styles and channels of five types. Each fan is interpreted as a low-frequency lowstand systems tract with the shaly interfan intervals representing transgressive and highstand systems tracts. All fans show complex internal facies distributions but exhibit a high-frequency internal stratigraphy based on fan-wide zones of relative sediment starvation. These zones are interpreted as transgressive and highstand systems tracts of higher order sequences. Sandy packages between these fine-grained intervals are interpreted as high-frequency lowstand systems tracts and exhibit dominantly progradational stacking patterns, resulting in subtle downdip clinoform geometries. Bases of fans and intrafan packages are interpreted as low- and high-frequency sequence boundaries respectively. Facies juxtapositions across these sequence boundaries are variable and may be gradational, sharp or erosive. In all cases, criteria for a basinward shift of facies are met, but there is no standard ‘motif’ for sequence boundaries in this system. High-frequency sequences represent the dominant mechanism of active fan growth in the Tanqua deep-water system.

The hydrodynamic and sedimentary setting of nearshore coral reefs, central Great Barrier Reef shelf, Australia: Paluma Shoals, a case study

Abstract: The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the highly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Shoals, a shore-attached ‘turbid-zone’ coral reef, and at Phillips Reef, a fringing reef located 20 km offshore, have been studied to document the mechanisms controlling turbidity. At each reef, waves, currents and near-bed turbidity were measured for a period of ≈1 month. Bed sediments were sampled at 135 sites. On the inner shelf, muddy sands are widespread, with admixed terrigenous and carbonate gravel components close to the reefs and islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of inner-shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fascicularis, up to >50% coral cover) and is broadly similar to that at Phillips Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef flats consist of mixed terrigenous and calcareous gravels and sands, with intermixed silts and clays, whereas the reef slope is dominated by gravelly quartz sands. The main turbidity-generating process is wave-driven resuspension, and turbidity ranges up to 175 nephelometric turbidity units (NTU). In contrast, at Phillips Reef, turbidity is 40 NTU probably occurs for a total of >40 days each year, and relatively little time is spent at intermediate turbidities (15–50 NTU). The extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotrophy) in response to different levels of turbidity. Sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 000 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of <3 BTF. As elsewhere along the nearshore zone of the central GBR, dry-season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments. The location of modern nearshore coral reefs is controlled by the presence of suitable substrates, which in Halifax Bay are Pleistocene and early Holocene coarse-grained (and relatively stable) alluvial deposits.

The final phase of dead‐ice moraine development: processes and sediment architecture, Kötlujökull, Iceland

Abstract: Consecutive phases of de-icing of ice-cored moraines and the formation of dead-ice moraine were monitored over a 4-year period at the terminus of the Kotlujokull glacier, Iceland. Particularly, the transition from partially ice-cored moraine with isolated dead-ice blocks to the ice-free landscape receives attention in this paper in order to link the final melting processes to the architecture of the sedimentary end product. In the current humid sub-polar climate of south Iceland de-icing of partially ice-cored moraines results chiefly from melting along the bottom surface of ice-cores with an annual average rate of 25 cm. The final de-icing is associated with an interrelated group of re-sedimentation processes and surface features. Series of sinkholes evolve at the toe of dead-ice blocks, which initiate retrogressive rotational sliding or backslumping of the ice-cored slopes and the formation of distinct edges and fractures in the adjacent basins. Although backslumping is the dominant process in this phase of re-sedimentation, structures resulting from this process are rarely recognized in the ice-free landscape. As ice-cores gradually diminish the effect of the latest re-sedimentation events will overprint or destroy most existing sedimentary characteristics. Thus, in the ice-free landscape, structures mainly related to the formation of sinkholes and fractures remain imprinted on the sediment succession. Generally, no inversion of the topography occurs during the final phase of de-icing. The overall topography recognized in the late phase of the fully ice-cored terrain is merely lowered and the amplitude of the relief reduced as de-icing progresses. The sediment architecture of the ice- free landscape is characterized by heterogeneous and often slumped diamict sediments with variable thickness and lateral distribution; clast orientation is related to the direction of slopes, and boulders are found in isolated groups or in linear arrangements.

Depositional systems and sequence stratigraphy of coarse‐grained deltas in a shallow‐marine, strike‐slip setting: the Bohemian Cretaceous Basin, Czech Republic

Abstract: Deposits of coarse-grained, Gilbert-type deltas showing varying degrees of reworking of foresets by basinal currents were identified in Middle Turonian to Early Coniacian sandstones of the Bohemian Cretaceous Basin. The progradation of the deltaic packages, earlier interpreted as large-scale subaqueous dunes, shelf ridges or subaqueous fault-scarp ‘accumulation terraces’, was controlled by high- and low-frequency, relative sea-level changes in a relatively slowly subsiding, intracontinental strike-slip basin. End-member types of the Bohemian Cretaceous coarse-grained deltas are deep-water deltas, characterized by thick (50–80 m) foreset packages with steep (10–30°) foresets, and shallow-water deltas, which deposited thin (<15 m) packages with foresets typically between 4° and 10°. The differences in thickness and foreset slope angle were controlled predominantly by the accommodation available during progradation. The depositional regime of the deltas was governed by (i) the fluvial input of abundant sand bedload, with a minor proportion of gravel; (ii) gravity flows, most probably caused by liquefaction of the upper part of the unstable foreset slope; and (iii) migration of sandy bedforms on the foreset slopes. The bedform migration was driven by unidirectional currents of possible tidal origin. Individual foreset packages represent systems tracts, or parts of systems tracts, of depositional sequences. A variety of stacking patterns of high-frequency sequences exists in the basin, caused by low-frequency relative sea-level changes as well as by local changes in sediment input. Because of generally low subsidence rates, fluvial or beach topset strata were not preserved in the cases studied. The absence of preserved fluvial facies, which has been one of the main arguments against the fluvio-deltaic origin of the sandstone bodies, is explained by erosion of the topsets during transgression and their reworking into coarse-grained lags of regional extent covering ravinement surfaces.

The emission and vertical flux of particulate matter <10 mum from a disturbed clay-crusted surface

Abstract: Arid and semi-arid environments are important sources for the atmospheric loading of PM10 (particulate matter <10 lm), although the emission of this material is often limited by surface crusts. This study investigates the emission and vertical flux of PM10 from a clay-crusted playa, with and without saltating grains to abrade the surface. Using a portable field wind tunnel, it was found that, despite disturbance to the surface, the emission of PM10 decays rapidly without abrasion. Only in the presence of saltating grains was PM10 continuously liberated from the surface, such that the emission rate (the total amount of PM10 emitted from the surface expressed as a horizontal flux) varied linearly with the saltation transport rate. Although the emission of PM10 was found to depend on saltation abrasion, past studies have tended to focus on the relationship between the vertical flux of PM10 (the amount of PM10 being transported vertically through the boundary layer) and the shear velocity. In this study, the vertical flux of PM10 was found to vary with the shear velocity to the power of 2AE14. Although the vertical PM10 flux is a proportion of the emission rate (the horizontal flux), no statistically significant relationship was observed between the emission rate and the shear velocity. The disparity of these results is explained by the lack of a consistent relationship between the shear velocity and the saltation transport rate in this supply-limited environment. This suggests that the observed relationship between the vertical PM10 flux and the shear velocity is a spurious correlation, resulting from the use of shear velocity to calculate the vertical dust flux. It is thus concluded that shear velocity is not an appropriate variable for emission modelling in supply-limited environments and that improvements in dust emission modelling will only be realized if the abrasion process is the focus of a concerted research effort.

Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?

Abstract: Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. ‘Meniscus-type cements’ (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure.

Sedimentology, stratigraphy and landscape evolution of a Holocene coastal dune system, Lodbjerg, NW Jutland, Denmark

Abstract: The stratigraphy and landscape evolution of the Lodbjerg coastal dune system record the interplay of environmental and cultural changes since the Late Neolithic. The modern dunefield forms part of a 40 km long belt of dunes and aeolian sand-plains that stretches along the west coast of Thy, NW Jutland. The dunefield, which is now stabilized, forms the upper part of a 15–30 m thick aeolian succession. The aeolian deposits drape a glacial landscape or Middle Holocene lake sediments. The aeolian deposits were studied in coastal cliff exposures and their large-scale stratigraphy was examined by ground-penetrating radar mapping. The contact between the aeolian and underlying sediments is a well-developed peaty palaeosol, the top of which yields dates between 2300 BC and 600 BC. Four main aeolian units are distinguished, but there is some lateral stratigraphic variation in relation to underlying topography. The three lower aeolian units are separated by peaty palaeosols and primarily developed as 1–4 m thick sand-plain deposits; these are interpreted as trailing edge deposits of parabolic dunes that moved inland episodically. Local occurrence of large-scale cross-stratification may record the head section of a migrating parabolic dune. The upper unit is dominated by large-scale cross-stratification of various types and records cliff-top dune deposition. The nature of the aeolian succession indicates that the aeolian landscape was characterized by alternating phases of activity and stabilization. Most sand transported inland was apparently preserved. Combined evidence from luminescence dating of aeolian sand and radiocarbon dating of palaeosols indicates that phases of aeolian sand movement were initiated at about 2200 BC, 700 BC and AD 1100. Episodes of inland sand movement were apparently initiated during marked climate shifts towards cooler, wetter and more stormy conditions; these episodes are thought to record increased coastal erosion and strong-wind reworking of beach and foredune sediments. The intensity, duration and areal importance of these sand-drift events increased with time, probably reflecting the increasing anthropogenic pressure on the landscape. The formation of the cliff-top dunes after AD 1800 records the modern retreat of the coastal cliffs.

Sharp-based, tide-dominated deltas of the Sego Sandstone, Book Cliffs, Utah, USA

Abstract: The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east-central Utah contains three 10- to 20-m thick layers of tide-deposited sandstone arranged in a forward- and then backward-stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave-influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide-influenced river deltas during a time of fluctuating relative sea-level. Shale-dominated offshore marine deposits gradually shoal and become more sandstone-rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea-level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp-based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic-sandstone are exposed, a sharp-based upward-coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide-eroded estuaries, formed during relative sea-level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward-stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward-stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Abstract: Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an »460-m-thick platform succession. The fine crystalline dolomite has d 18 O values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and d 13 C values together with 87 Sr/ 86 Sr ratios that overlap with sea-water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution-collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72‐199&). The medium crystalline dolomite has some d 18 O and d 13 C values and 87 Sr/ 86 Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of d 18 O, d 13 Ca nd 87 Sr/ 86 Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (»500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Abstract: Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian‐ Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole-rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque‐Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole-rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well-preserved belemnite shells (non-luminescent to slightly luminescent) typically show stable isotope values of +4AE3& to ‐0AE7& d 13 C, +0AE7& to ‐3AE2& d 18 O, and trace element contents of 950 l gg ‐1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low-Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non-luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their d 13 C and d 18 O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or highfrequency secular variations in sea-water chemistry superimposed on the longterm isotopic trend. A pronounced positive carbon-isotope excursion (up to +4AE3&) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian d 13 C maximum reported in other European and Tethyan regions.

The formation of large mudwaves by turbidity currents on the levees of the Toyama deep‐sea channel, Japan Sea

Abstract: The development of mudwaves on the levees of the modern Toyama deep-sea channel has been studied using gravity core samples combined with 3·5-kHz echosounder data and airgun seismic reflection profiles. The mudwaves have developed on the overbank flanks of a clockwise bend of the channel in the Yamato Basin, Japan Sea, and the mudwave field covers an area of 4000 km2. Mudwave lengths range from 0·2 to 3·6 km and heights vary from 2 to 44 m, and the pattern of mudwave aggradation indicates an upslope migration direction. Sediment cores show that the mudwaves consist of an alternation of fine-grained turbidites and hemipelagites whereas contourites are absent. Core samples demonstrate that the sedimentation rate ranged from 10 to 14 cm ka−1 on the lee sides to 17–40 cm ka−1 on the stoss sides. A layer-by-layer correlation of the deposits across the mudwaves shows that the individual turbidite beds are up to 20 times thicker on the stoss side than on the lee side, whereas hemipelagite thicknesses are uniform. This differential accretion of turbidites is thought to have resulted in the pattern of upcurrent climbing mudwave crests, which supports the notion that the mudwaves have been formed by spillover turbidity currents. The mudwaves are interpreted to have been instigated by pre-existing large sand dunes that are up to 30 m thick and were created by high-velocity (10°ms−1), thick (c. 500 m) turbidity currents spilling over the channel banks at the time of the maximum uplift of the Northern Japan Alps during the latest Pliocene to Early Pleistocene. Draping of the dunes by the subsequent, lower-velocity (10−1ms−1), mud-laden turbidity currents is thought to have resulted in the formation of the accretionary mudwaves and the pattern of upflow climbing. The dune stoss slopes are argued to have acted as obstacles to the flow, causing localized loss of flow strength and leading to differential draping by the muddy turbidites, with greater accretion occurring on the stoss side than on the lee slope. The two overbank flanks of the clockwise channel bend show some interesting differences in mudwave development. The mudwaves have a mean height of 9·8 m on the outer-bank levee and 6·2 m on the inner bank. The turbidites accreted on the stoss sides of the mudwaves are 4–6 times thicker on the outer-bank levee than their counterparts on the inner-bank levee. These differences are attributed to the greater flow volume (thickness) and sediment flux of the outer-bank spillover flow due to the more intense stripping of the turbidity currents at the outer bank of the channel bend. Differential development of mudwave fields may therefore be a useful indicator in the reconstruction of deep-sea channels and their flow hydraulics.

Organic matter, putative nannobacteria and the formation of ooids and hardgrounds

Abstract: Modern ooids from Joulters Cay, when baked at 500 °C, turn various shades of black depending upon the organic content. Mucus-rich laminae occur at quasi-regular intervals of a few micrometres within the cortex. When mucus is still present, it turns black; when it is absent, there is a gap. The cortex consists of two types of aragonite: (1) ‘batons’ of circular cross-section capped by a single 0·1-μm (100-nm) ball, which can be interpreted as a single nannobacteria cell that precipitated the baton; (2) elongate crystals made of multiple rows of minute balls of about 0·03 μm (30 nm), which may or may not have been small organisms in the size range of viruses. There are also some crystals that show no evidence of organic precipitation. Hardground cementation begins with the formation of a terminal mucus-rich ring on the ooid that bakes black and is crowded with 0·1-μm (100-nm) balls. Some ooids are then joined by a meniscus also made of mucus with aragonite crystals. The final, most abundant hardground cement forms a fur of inorganic aragonite crystals often shaped like plywood sheets, although some ‘organic’, elongate crystals composed of ≈0·03 μm (≈30 nm) balls are also found in the later cement. For a century, ooids have been known to be closely associated with organic matter; this paper goes further and proposes that the bulk of the ooid may be precipitated by nannobacterial processes. Hardground formation, in the beginning, may also be a microbiological process, but most cementation is accomplished inorganically.

Variations in shoreface progradation and ravinement along the Texas coast, Gulf of Mexico

Abstract: Shoreface architecture, evolution (mid-Holocene to present) and depths of transgressive ravinement were examined from Sabine Pass, at the Texas–Louisiana border, to South Padre Island, near the Texas–Mexico border, using 30 shoreface transects. Shoreface transects extend out to 16-m water depth, each created from an echo-sounding profile and, on average, seven sediment cores. The shoreface is composed of three broad sedimentological facies: the upper shoreface, composed almost entirely of sand; the proximal lower shoreface, composed of sand with thickly to medium-bedded (50–10 cm) mud; and the distal lower shoreface, composed dominantly of mud with medium- to thinly bedded (20–3 cm) sand. Shoreface architecture and evolution is extremely variable along the Texas coast. Shoreface gradients increase from 2·25 m km–1 in east Texas to 3·50 m km–1 in south Texas. Shoreface sands coarsen towards south Texas. East and south Texas shoreface deposits are thin and retrograding whereas central Texas shoreface deposits are thicker and prograding. Central Texas is characterized by stacked shoreface successions, whereas in east Texas, lower shoreface sands are preserved only in offshore banks. Preservation of shoreface deposits is low in south Texas. Although eustatic fluctuations and accommodation space have a strong impact on overall mid-Holocene to present shoreface evolution and preservation potential, along-strike variations in sediment supply and wave energy are the main factors controlling shoreface architecture. The transgressive ravinement surface varies from –6 to –15 m along the Texas coast.

The effects of source rocks and chemical weathering on the petrogenesis of siliciclastic sand from the Neto River (Calabria, Italy): implications for provenance studies

Abstract: Plutonic and gneissic rocks of the Sila Massif in the uppermost portion of the Neto drainage basin (Calabria, Southern Italy) weather and erode under a humid Mediterranean climate. During the development of weathering profiles, a combination of chemical weathering and granular disintegration processes occurred. Chemical weathering involves a loss of both plagioclase (mainly during grus generation) and K-feldspar (mainly during soil formation). This loss is attributed to transformation of plagioclase to clay minerals and to leaching and dissolution of K-feldspar. Sand composition is quartzofeldspathic and nearly homogeneous along the main channel of the Neto River, even where the river cuts across a blanket of sedimentary cover. Thus, fluvial transport does not alter sand composition within the Neto drainage basin. Petrographic indices are effective in (1) discriminating between contributions from similar (granite and gneiss) source rocks (Qm/F); (2) relating the provenance of plutoniclastic and gneissiclastic sand found in the headwaters to grus horizons (Qm/F; Q/Rg); and (3) distinguishing between upstream first-cycle and downstream multicycle sand (Q/Rg). This last distinction is further emphasized by considering both aphanitic and phaneritic varieties of rock fragments (RgRmRs diagram). Chemical weathering is the main sand producer within the regolithic environment in northern Calabria. In addition, rapid erosion resulting from steep slopes removes weathered products, and rapid and short transport leads to minimal sediment maturation. In general, the F/Q index is climate and relief dependent; thus, it should be used in conjunction with palaeoclimatic and palaeophysiographic evidence for provenance interpretations of ancient quartzofeldspathic sandstones.

Long-distance correlation between tectonic-controlled, isolated carbonate platforms by cyclostratigraphy and sequence stratigraphy in the Devonian of South China

Abstract: During the early Middle Devonian in South China, an extensive carbonate platform was broken up through extension to create a complex pattern of platforms, and interplatform basins. In Givetian and Frasnian carbonate successions, five depositional facies, including peritidal, restricted shallow subtidal, semi-restricted subtidal, intermediate subtidal and deep subtidal facies, and 18 lithofacies units are recognized from measured sections on three isolated platforms. These deposits are arranged into metre-scale, upwardshallowing peritidal and subtidal cycles. Nine third-order sequences are identified from changes in cycle stacking patterns, vertical facies changes and the stratigraphic distribution of subaerial exposure indicators. These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker-than-average subtidal cycles, and regressive packages by thinner-than-average peritidal cycles. Sequence boundaries are transitional zones composed of stacked, high-frequency, thinner-than-average cycles with upward-increasing intensity of subaerial exposure, rather than individual, laterally traceable surfaces. These sequences can be further grouped into catch-up and keep-up sequence sets from the longterm (second-order) changes in accommodation and vertical facies changes. Catch-up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies, and even deep subtidal facies locally within some individual sequences, recording long-term accommodation gain. Keep-up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences, recording long-term accommodation loss. Correlation of long-term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins. This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins. In the early Frasnian, extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.

Fault‐controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west‐central Alberta, Canada

Abstract: The partly dolomitized Swan Hills Formation (Middle-Upper Devonian) in the Simonette oil field of west-central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post-date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (Th=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (Tpeak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon-bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more 87Sr-rich fluid. Fabric-destructive and fabric-preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume-like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on 87Sr/86Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and 206Pb/204Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.

The Mackenzie Delta: sedimentary processes and facies of a high‐latitude, fine‐grained delta

Abstract: The Mackenzie Delta is a large fine-grained delta deposited in a cold arctic setting. The delta has been constructed upon a flooding surface developed on a previous shelf-phase delta. There are three principal depositional zones: the subaerial delta plain, the distributary channel mouth region and the subaqeous delta. The subaerial delta plain is characterized by an anastomosing system of high-sinuosity channels and extensive thermokarst lake development. This region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels, ice-jamming and flooding in the early spring and declining river stage during the summer and autumn. Deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable. In the channel mouth environment, deposition is dominated by landward accretion and aggradation of mouth bars during river- and storm surge-induced flood events. The subaqeous delta is characterized by a shallow water platform and a gentle offshore slope. Sediment bypassing of the shallow-water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine-grained sediments. Facies of the shallow platform include silty sand with climbing ripple lamination. Offshore facies are dominated by seaward-fining fine sand to silt tempestites. Sea-ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies. The low angle profile of the shallow-water platform is interpreted to be the combined response of a fine-grained delta to (1) storm sediment dispersal; (2) autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley; (3) limited water depth above the underlying flooding surface; and (4) efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge. Several indicators of the cold climate can be used as criteria for the interpretation of ancient successions, including thermokarst lake development, submarine channel scours, freeze–thaw deformation and ice-scour deformation structures. Permafrost inhibits compaction subsidence and, together with the shallow-water setting, also limits autocyclic lobe switching. The cold climate can thus influence stratal architecture by favouring the development of regional-scale clinoform sets rather than multiple, smaller scale lobes separated by autocyclic flooding surfaces.

Lithofacies associations and stable isotopes of palustrine and calcrete carbonates: examples from an Indian Maastrichtian regolith

Abstract: Detailed information on semi-arid, palustrine carbonate–calcrete lithofacies associations in a sheetwash-dominated regolith setting is sparse. This is addressed by studying the Lower Limestone of the Lameta Beds, a well-exposed Maastrichtian regolith in central India. The general vertical lithofacies assemblage for this unit comprises: (a) basal calcareous siltstones and marls with charophytes, ostracods and gastropods; (b) buff micritic limestones associated in their upper parts with calcretized fissure-fill sandstones; (c) sheetwash as fissure-fill diamictites and thin pebbly sheets, locally developed over a few metres; and (d) sandy, nodular, brecciated and pisolitic calcretes at the top. The sequence is ‘regressive’, with upsection filling of topographic lows by increased sheetwash. Lateral lithofacies change is marked, but there are no permanent open-water lake deposits. In topographic lows close to the water table, marshy palustrine or groundwater calcretes formed, whereas on better drained highs, brecciation and calcretization occurred. Prolonged exposure is implied, suggesting that shrinkage was the main cause of brecciation. Evidence for rhizobrecciation and other biological calcrete fabrics is sparse, contrasting with the emphasis on root-related brecciation in many studies of palustrine lithofacies. Stable isotope (d18O and d13C) values are consistent with the palustrine limestones being fed from meteoric-derived groundwater with a strong input of soil-zone carbon. There is overlap of both d18O and d13C values from the various palustrine and calcrete fabrics co-occurring at outcrop. This suggests that, in groundwater-supported wetlands, conversion from palustrine carbonate to calcrete need not show isotopic expression, as the groundwater source and input of soil-zone carbon are essentially unchanged. Cretaceous–Tertiary d18O and d13C values from palustrine lithofacies and associated calcretes appear to be strongly influenced by the inherited values from lakes and wetlands. Hydrologically closed lakes and marine-influenced water bodies tend to result in low negative palustrine d18O and d13C values. During brecciation and calcretization, the degree of isotopic inheritance depends on whether or not alteration occurs in waters that are different from those of the original water body or wetland. Marked biological activity (e.g. rhizobrecciation or root mat development) during calcretization may lower d13C values where C3 plants are abundant but, in shrinkage-dominated systems, d13C values will be largely inherited from the palustrine limestones.

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Abstract: Magnesite forms a series of 1- to 15-m-thick beds within the »2AE 0G a (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680m-thick formation is composed of a stromatolite‐dolomite‐‘red bed’ sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by d 13 C values from +7AE1& to +11AE6& (V-PDB) and d 18 O ranging from 17AE4& to 26AE3& (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high d 13 C values ranging from +9AE0& to +11AE6& and d 18 O values of 20AE0‐25AE7&. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high d 13 C values reflect a combined contribution from both global and local carbon reservoirs. A 13 C-rich global carbon reservoir (d 13 C at around +5&) is related to the perturbation of the carbon cycle at 2AE0 Ga, whereas the local enhancement in 13 C (up to +12&) is associated with evaporative and restricted environments

Subaqueous mass flow origin for Lower Permian diamictites and associated facies of the Grant Group, Barbwire Terrace, Canning Basin, Western Australia

Abstract: The intracratonic Canning Basin is Western Australia’s largest sedimentary basin (>400 000 km2) and has experienced repeated episodes of Phanerozoic extension and subsidence, resulting in deposition of a number of first-order ‘megasequences’. A major phase of basin extension and sedimentation (Grant Group) occurred in the Late Carboniferous/Early Permian when Australia lay at high palaeolatitudes. Facies analysis of 5000 m of drill core from 25 continuously cored wells in Grant Group strata on the fault-bounded Barbwire Terrace in the northern Canning Basin identified three facies associations (FAs). These record the predominance of fault-generated, subaqueous mass flow and sediment reworking. The lowest association (FA I; up to 355 m thick) rests unconformably on tilted older strata and consists of coarse-grained, subaqueously deposited, sediment gravity flow facies. These include fault-generated breccias, massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization. FA I is overlain abruptly by relatively fine-grained deposits of FA II (up to 140 m thick), which consist of laminated to thin-bedded mudstone and sandstone turbidites, recording an abrupt increase in relative water depths. In turn, these facies coarsen upwards and are transitional into shallow-water, swaley cross-stratified and rippled sandstones of FA III (up to 125 m thick). The overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment (FA I), a subsequent phase of rapid subsidence, increasing water depths and ‘sediment underfilling’ (FA II) and, finally, a regressive phase of shoreface progradation. The occurrence of rare striated clasts in FA I suggests reworking of glacial sediment, but no direct glacial influence on sedimentation can be identified.

Cryptic sequence boundaries in braided fluvial successions

Abstract: In braided fluvial deposits, consisting of monotonous successions of sandstone or conglomerate, it may be difficult to distinguish regionally significant bounding surfaces (sequence boundaries) from autogenic channel-scour surfaces. Major surfaces may be characterized by erosional relief and draped by lag deposits, but not all sequence boundaries show these characteristics. Other clues to the presence of a major surface are sharp changes in detrital composition, shifts in regional palaeocurrent trends and evidence of early diagenesis of the sandstones immediately below the sequence boundary. Examples of these attributes of cryptic sequence boundaries are illustrated from three Mesozoic units in the Colorado Plateau area of the United States. In the Chinle Formation (Triassic), near Moab, Utah, angular intraformational unconformities overlie sandstones showing evidence of early diagenesis. In the Castlegate Sandstone (Upper Cretaceous) of east-central Utah, a cryptic sequence boundary can be discriminated from other erosion surfaces by the evidence of detrital petrography and early diagenesis. Palaeocurrent data indicate changes in regional palaeoslope at two sequence boundaries within this unit. Evidence of early diagenesis is also present at a sequence boundary in the Kayenta Formation (Jurassic) of westernmost Colorado.

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda ‘white earth’ (Pliocene, central Spain)

Abstract: Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from −3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from −6·34‰ to −0·39‰ PDB (mean=−3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=−0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3−/CO32− groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.

High-resolution (2-7 kHz) acoustic and geometric characters of submarine creep deposits in the South Korea Plateau, East Sea

Abstract: A synthesis of high-resolution (Chirp, 2–7 kHz) seismic profiles in the South Korea Plateau reveals that large masses of wavy stratified sediment (≈60–90 m thick) cover broad, gently sloping (<0·5°) ridges in water depths of 1000–2000 m. The wavy stratified sediment (WSS) is characterized by wavy (0·2–5 km in wavelength and <15 m in relief), continuous reflective layers with a basal deformed zone that overlies undeformed, strong reflectors. The WSS exhibits systematic variation in wave dimensions and thickness of internal reflective layers with changes in slope gradient. The troughs of the waves are commonly associated with internal growth faults, and wave amplitude generally increases with subbottom depth. On steep slopes around the ridges, the WSS masses are bounded downslope by slide and slump deposits including slightly translated or rotated WSS blocks. The acoustic and geometric characters, and association with downslope slides and slumps on the steeper slopes, suggest that the WSS masses were most probably formed by slow creep movement before slope failure. In the absence of significant sediment input to the South Korea Plateau, the deep (1000–2000 m in water depth) mass movements were probably triggered by earthquakes that have occurred frequently in this region. Some slightly displaced, intact WSS blocks in the associated slides and slumps downslope reflect a progressive evolution from submarine creep into slide and slump.

Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non-marine deposition in saline lakes and saline pans

Abstract: The mid-Permian Nippewalla Group of Kansas consists of bedded evaporites, red-bed siliciclastics and grey siliciclastics deposited in a non-marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater-brackish perennial lake facies. Bedded anhydrites containing gypsum-crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red-bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red-bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red-bed sandstone, composed of well-sorted, well-rounded quartz grains cemented with halite, indicate aeolian and rare shallow-water deposition. Most deposition took place in halite-dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non-marine, acidic saline pan system in the mid-continent of North America. The problem of distinguishing between ancient marine and non-marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid-Permian palaeoclimate data in the form of short-term air temperature proxies within longer-term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid-Permian of western Pangaea.

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Abstract: Septarian concretions are abundant in many Phanerozoic marine and marginal- marine shales and mudstones. They range from a few centimetres to several metres in size and are spherical or ellipsoidal in morphology. In general, formation by localized calcite or siderite cementation in argillaceous sediments began under less than a few metres of burial. Septarian cracks vary widely in shape and configuration: included are networks of wide, vertically, radially and sometimes concentrically oriented, lenticular shrinkage cracks; and narrower, parallel-sided, straight to irregular tension cracks locally accompanied by brecciation, and plumose and en echelon sigmoidal cracks indicative of shear stresses. Crack types are intergradational; many concretions exhibit multiple cracking events. Enclosed macrofossils and isopachous fibrous calcite cement that lines earlier formed cracks are commonly broken and displaced. In some cases, cracks contain injected lime mud and silt. These features, taken together, testify that cracking involved a spectrum of responses in concretion interiors, from loss of shear strength, dewatering and shrinkage to brittle failure, demonstrating variations in, and contrasts between, the rheological properties of the matrix and enclosed objects. Localization to interiors and outward tapering of lenticular cracks make a case for fracture partitioning and indicate that interiors were softer than exteriors at the time of shrinkage. Parallel-sided cracks point to greater stiffness, and evolving crack shape in multiply cracked concretions shows that rigidity increased with time. Crack orientations indicate highly variable tensile and shear stress directions within individual concretions. Rupture, brecciation, displacement of fragments, loss of shear strength, liquefaction and injection of unconsolidated granular sediment suggest that deformation events were rapid, if not virtually instantaneous. Previous explanations for the internal cracking, such as gas generation, spontaneous chemical dehydration or localized overpressuring due to compaction, seem either untenable or fail to account for the spectrum of observed features. However, syndepositional earthquake-induced shaking of cementing bodies of varying rigidity at shallow burial depths seems to be a plausible source for the requisite short-lived, variable to anisotropic, high-stress regime inside the concretions. Septarian concretions may thus preserve a signature of basin seismicity as it relates to their cementation history.