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Bioturbation has long been considered an antagonist of microbialite development and preservation, because metazoan grazing and burrowing destroy benthic microbial communities. However, metazoan bioturbation, in conjunction with microbial accretion, may have had a significant role in the morphogenesis of some columnar microbialites, as suggested by the case study presented and by some Phanerozoic and Upper Proterozoic analogues discussed here. Late Miocene in age, the studied microbial biostrome developed in a western Mediterranean restricted shallow-water platform dominated by grainy sediments and with a notable influence of bioturbation. This study is focused on the complex accretionary history of the columnar microbialite biostrome and on its striking dark grey colour, which is attributed to Mn-oxyhydroxides precipitated during meteoric diagenesis linked to subaerial exposure. The characteristic columnar structure of the microbialite biostrome has features consistent with an accretionary origin of the columns, but also has features suggesting metazoan disruption. Therefore, a new morphogenetic model for columnar microbialites is presented, highlighting the concomitant roles of microbial accretion, bioturbation and grainy sediment infill of the intercolumn space. Whether this model is an exception or a rule, should be tested on other examples of Phanerozoic and Upper Proterozoic columnar microbialites. Nevertheless, this model is a step forward in understanding the complex microbe–metazoan interactions as constructive coexistence rather than just as destructive competition.

Speleothems, mostly composed of calcium carbonate, are widely present in modern karst-originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee-deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle-like pendants and ground-up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro-crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg-calcite). Meanwhile, the carbon-oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high-frequency sea-level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.

Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical as it represents the transition from shoreline to proximal domains of a hydrologically-closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with δ13C–δ18O chemostratigraphy from matrix carbonates across five closely-spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back-scatter electron microscopy were performed to discriminate primary (syn-)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark-blue luminescent sparry cements carrying negative δ13C and δ18O. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix δ13C and δ18O range from ca. -7.5 to +4.0‰ and show clear positive covariance (R = 0.97) whose nature differs from that of previous basin-oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values likely represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are capable to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation with those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis. This article is protected by copyright. All rights reserved.

Exposures of the Pleistocene Miami oolite in South Florida provide excellent examples of preserved primary sedimentary features and subsequent diagenetic changes of a ‘fossilized’ ooid sand body that has been subaerially exposed in a tropical climate since its deposition during the last interglacial highstand – Marine Isotope Stage 5e. Using a robust airborne light detection and ranging digital terrain model and select outcrops, a detailed analysis of the morphologies and dimensions of the different portions of the Miami oolite was conducted. The sand body, extending 95 km north to south and approximately 15 km wide, consists of shoals (or bars) separated by tidal channels and is partly bounded on the ocean-facing side by a prograding barrier bar that collectively cover an area of approximately 1000 sq. km. Results of the quantitative interrogation of the Miami oolite are compared with those of previously published work from modern sand bodies on Great Bahama Bank including the Exumas, Schooners Cays and Tongue of the Ocean. Digital terrain models developed for these modern sand bodies on the Great Bahama Bank and the ancient Miami oolite were used as the basis for the definition of shoals, shoal crests (peaks) and channels which were examined with respect to their size, shape and orientation. The Exumas sand body is the best morphometric match when analysing the bars and the channels, and the Joulter Cays area is an analogue for the formation of the barrier bar and for burrow reworking of cross-bedding within stabilized sand shoals. The Exumas is a particularly compelling analogue for the Miami oolite with respect to length and overall visual comparison between the morphologies of the sand bodies, shoal (or bar) shape, number of tidal channels, channel length and width, and high areas parallel to the strike trend of the sand body (islands in the Exumas and possible islands as part of the Miami oolite barrier bar). Despite ca 115 kyr of subaerial exposure and meteoric diagenesis including karst, the morphology of the Miami oolite is still relatively intact to the point where it can be easily analysed and meaningful comparisons to modern systems can be made. The Miami oolite serves as a key reference example for comparison to Holocene sand units in the Bahamas – it validates the concept of comparative sedimentology and in particular emphasizes how results from the modern can improve the interpretation of a fossilized example.

International audience; The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium-series dates obtained and subsequently analysed. Sea-level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age-depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 +/- 50yr bp; accretion rates average 565mkyr(-1). The barrier reef consists of >30m thick Holocene coralgal and microbial successions. Holocene barrier-reef growth began 10 030 +/- 50yr bp and accretion rates average 615mkyr(-1). The underlying Pleistocene reef formed 116 900 +/- 1100 yr bp, i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 005 to 014mkyr(-1). In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea-level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier-reef systems in Tahiti and Mayotte exhibit more differences than similarities.

Since 1964, the Iron Ore Company of Canada has deposited iron tailings resulting from mining operations into Wabush Lake, Western Labrador, Canada. This activity has resulted in a continuous hyperpycnal flow flowing over the lake floor. Bathymetric surveys were carried out between 1999 and 2008 as part of the overall environmental Iron Ore Company of Canada plans to maintain safe disposal strategies of tailings into the lake. Here, this dataset is used to study the evolution of different channels present in Wabush Lake and their link with knickpoints found at the head of most of these channels. The channels show notable evolution from one survey to the next. Some knickpoints and channels may be followed throughout more than one survey; the evolution of these knickpoints is the process responsible for the creation of erosional channels. The context in which they appear, as well as the evolution of some of the subaqueous channels is analysed. Cyclic steps and sediment waves are also found in the lake. These rhythmic bedforms, also created by the passage of turbidity currents, are analysed. Sediment waves found in Wabush Lake show values for both wavelength and wave height that are approximately one order of magnitude smaller than most of those found in the marine environment.

A method that integrates elliptic Fourier and principal component analysis is a new development in the analysis of the shapes of sand grains. However, conventional elliptic Fourier and principal component analysis based on the variance–covariance matrix of the elliptic Fourier results can determine only the form of sand grains, and fails to quantify fine-scale boundary smoothness of grains. In this study, sand grains from glacial, fluvial, foreshore and aeolian environments were analysed using both elliptic Fourier and principal component analysis and an extension of elliptic Fourier and principal component analysis based on the correlation matrix to extract information on grain form (macroscopic) and grain boundary smoothness (microscopic) separately. Conventional elliptic Fourier and principal component analysis based on the variance–covariance matrix produces macroscopic particle shape descriptors, such as the elongation index and bump indices. These indices indicate that sand grains exposed to subaqueous transportation (fluvial and foreshore) have forms that are more elongated than those exposed to subaerial transportation (aeolian dunes). However, elliptic Fourier and principal component analysis based on the correlation matrix is, in addition, able to extract microscopic particle features, which can be interpreted in terms of a boundary smoothness index. The boundary smoothness index indicates that the surfaces of glacial grains are the most rugged, whereas the surfaces of aeolian grains are the smoothest. On bivariate plots of the boundary smoothness and elongation indices, samples from fluvial, foreshore, aeolian and glacial environments cluster in discrete regions. In addition, the analysis reveals that glacial grains are exposed to different morphological maturation pathways than those from fluvial, foreshore and aeolian environments.

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta-scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud-prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta-scale subaqueous clinoforms are common in recent tide-influenced, wave-influenced and current-influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient delta-scale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super-giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening-upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave-dominated or current-dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic ‘envelope’ of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform ‘envelope’ rolls over to define a topset–foreset–toeset geometry. All clinoforms are consistently oriented sub-parallel to the edge of the Horda Platform (N005–N030). In the eastern half of the field, individual foresets are relatively gently dipping (1° to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine-grained, hummocky cross-stratified sandstones. Towards the west, clinoforms gradually become steeper (5° to 14°) and bound thicker (15 to 60 m) clinothems that comprise medium-grained, cross-bedded sandstones. Topsets are consistently well-developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near-linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north-east and sculpted by the action of currents sub-parallel to the clinoform strike.

This study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre-scale) and amalgamated mounds composed primarily of hydromagnesite [Mg(CO)(OH)·4HO], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca-Mg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO·5HO] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0·4 mm yr. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.

Lower and middle Eocene ironstone sequences of the Naqb and Qazzun formations from the north-east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea-level changes. These sequences represent the only Palaeogene economic ooidal ironstone record of the Southern Tethys. These ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies. The distribution of low-energy ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow-mottled mud-ironstone and laterally equivalent tidal flat, stromatolitic ironstones. The agitated water ironstone facies consist of shallow subtidal–intertidal nummulitic–ooidal–oncoidal and back-barrier storm-generated fossiliferous ironstones. The formation of these marginal marine sequences was associated with major marine transgressive–regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea-level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate-rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions.

Facies architecture and platform evolution of an early Frasnian reef complex in the northern Canning Basin of north-western Australia were strongly controlled by syn-depositional faulting during a phase of basin extension. The margin-attached Hull platform developed on a fault block of Precambrian basement with accommodation largely generated by movement along the Mount Elma Fault Zone. Recognition of major subaerial exposure and flooding surfaces in the Hull platform (from outcrop and drillcore) has enabled comparison of facies associations within a temporal framework and led to identification of three stages of platform evolution. Stage 1 records initial ramp development on the hangingwall dip slope with predominantly deep subtidal conditions that prevented any cyclic facies arrangements. This stage is characterised by basal siliciclastic deposits and a major deepening-upward facies pattern that is capped by a sequence boundary towards the footwall (north-west) and a major flooding surface towards the hangingwall. Stage 2 reflects the bulk of platform aggradation, significant platform growth towards the hangingwall and the development of reef margins and cyclic facies arrangements. Thickening of this stage towards the hangingwall indicates that accommodation was generated by rotation of the fault block and overlying platform. Stage 3 records a major flooding and backstep of the platform margin. The Hull platform illustrates important elements of margin-attached carbonate platforms in a half-graben setting, including: (i) prominent, but limited, coarse siliciclastic input that does not have a major detrimental effect on carbonate production near the rift margin in arid to semi-arid settings; (ii) wedge-shaped accommodation created by syn-depositional rotation of fault blocks and tilting of the hangingwall dip slope, resulting in shallow-water facies and subaerial exposure up-dip of the rotational axis and deeper water facies down-dip; and (iii) evolution of a ramp to rimmed shelf, coincident with a sequence boundary–flooding surface, that is accelerated by tilting of the hangingwall dip slope during fault-block rotation.

This study, based in the Haushi-Huqf area of central east Oman, aims to characterize the controls on facies distribution and geometries of some of the best preserved examples of Lower Cretaceous tidal flat facies within the Tethyan epeiric platform. Field, petrographic and geochemical data were acquired from the Barremian–Aptian Jurf and Qishn formations that crop out in a 500 × 1000 m2 butte, thus allowing for pseudo three-dimensional quantitative data acquisition of the dimensions and spatial distributions of discontinuity surfaces and sedimentary bodies. The interpretation presented here suggests that the main processes impacting sedimentation in the Lower Cretaceous peritidal environment of the Haushi-Huqf were transport and erosion processes related to storm waves and currents. The vertical evolution of the carbonate system is organized into six types of metre-scale depositional sequences, from subtidal dominated sequences to supratidal-capped sequences, which are bounded by regional discontinuity surfaces. At subaerial exposure and submarine erosion surfaces associated with a base level shift, sedimentary horizons along the entire depositional profile are cut by scours possibly created by storm events. Chemostratigraphy allows correlation between the Haushi-Huqf and the age-equivalent sections logged in the interior of the platform in Oman. The correlation suggests that the change from subtidal to intertidal depositional sequences during the late highstand is coeval with the development of rudist dominated shoals on the shelf. This study is the first to discuss the controls on Lower Cretaceous peritidal carbonate cyclicity of the Arabian epeiric platform. The results presented here also offer a unique quantitative dataset of the distribution and dimensions of peritidal carbonate shoals and storm scours in a regional sequence stratigraphic context.

RICHARD H. T. CALLOW*, MARTIN D. BRASIER , and DUNCAN MCILROY*Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen,AB24 3UE, UK (E-mail: r.callow@abdn.ac.uk) Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive,St John’s, NL A1B 3X5, CanadaINTRODUCTIONThe late Ediacaran Period is a critical interval inthe history of life on Earth and its poorly under-stood fossils have potential to shape our under-standing of early biosphere evolution. Critical tothis process is an accurate assessment of theenvironments in which these organisms flour-ished, prior to the radiation of animal life duringthe Cambrian explosion. Few successions can bemore historically important for this than theEdiacara Member of the Rawnsley Quarzite inthe Flinders Ranges, South Australia (Glaessner,1984). The depositional settings for these rockshave hitherto been established, and widelyaccepted, as being below fair-weather wave base,delta front, prodelta, shoreface to intertidal(Gehling, 2000). The new interpretation advancedby Retallack (2012), is thought-provoking andincludes a diverse dataset, which is used to arguefor deposition of the Ediacara Member within ashallow marine, coastal, and subaerial suite ofenvironments. This discussion examines a num-berofthe conclusions insupportofthatclaim,andconsiders that the established marine depositionalsetting of the Ediacara Member remains valid.‘DEEP MARINE’ CONTEXT OF THEEDIACARA MEMBERThe title of the paper implies that Gehling (2000)interpreted the Ediacara Member to be a ‘deepmarine’ unit (see also Mount, 1989). The term‘deep marine’ is rarely used in a strict sense bygeologists. It has been used variously to refereither to environments situated beyond the shelfbreak (e.g. Pickering et al., 1989) or to environ-ments affected by processes that are typical ofslope and basin floor environments (i.e. sedimentgravity flows). The depositional environmentsrecognized by Gehling (2000) include sandstonedeposition below fair-weather wave base inincised valley fills (palaeovalleys), delta front topro-delta palaeoenvironments, delta-top sandsheets, braid-deltas and intertidal flats. It is clearfrom this list of palaeoenvironmental interpreta-tions that Gehling (2000) did not consider theEdiacara Member as ‘deep marine’. For example,intertidal environments were identified from theinference of ancient microbial mats and sandgypsum crystals (Gehling, 2000; Gehling & Dro-ser, 2009). Most importantly, the fossiliferousbeds were interpreted as tempestites that formedin delta-front palaeoenvironments between stormand fair-weather wave base (Gehling, 2000).Sediment gravity flows including, but not lim-ited to, turbidites were identified as componentsof the succession by Gehling (2000), as parts ofFacies Associations C, D, and E. However, it wasstated explicitly that the massive channelizedsands (0AE5 to 8 m thick beds) were not depositedin a basinal setting, but rather in sub-wave basesubmarine channels. Similarly, the ‘micro-turbi-dites’ (Facies Association E) were interpreted aspart of a distal pro-delta environment. Suchshelfal prodelta turbidites are common in depo-sitional systems with high sediment supply andhigh rates of accommodation generation (e.g.McIlroy et al., 2005). We concur with Gehling(2000) that the thick-bedded, massive, and dewa-tered channelized sandstones are not readilycompatible with fluvial facies models and aremore consistent with deposition from submarineflows (e.g. Kneller & Branney, 1995). The pres-ence of hummocky cross-stratified beds (Gehling,2000, fig. 10d) has been confirmed by manyauthors. Such bedforms were not observed bySedimentology (2013) 60, 624–627

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool-rim dams, is sub-fossil (relict) and undergoing erosion, but calcite-encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8*3 to 8*9), Na-HCO3 waters containing little Ca ( 80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg-silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO2 degassing, which led to high-disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non-deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca2+ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty-three facies are defined and grouped into four categories (wave-influenced, river-influenced, tide-influenced and basinal). Wave-dominated deltaic deposits occur mostly in the Tacata Field. The delta plain was characterized by tide-influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm-wave reworking and suspended sediment fallout. Delta-front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid-generated ichnogenus Scolicia. River-dominated deltaic deposits are present in the Santa Barbara, Mulata, Carito and El Furrial Fields. Low-sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher-sinuosity rivers. The subaqueous delta plain includes distributary channels and tide-influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary-channel and distributary mouth-bar deposits, as well as by delta-front and prodelta deposits showing evidence of sediment gravity-flow and fluid-mud emplacement. Delta-front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river-dominated deltas, tide-influenced delta plains, wave-dominated deltas and wave-dominated strandplain–offshore complexes.

The Permian records one of the most extreme climate transitions of the Phanerozoic, yet the climatic and environmental dynamics of this interval and their effects on neritic carbonate systems remain poorly constrained. New stratigraphic and carbon isotope records from uppermost Pennsylvanian to Lower Permian strata of the Orogrande Basin in south-central New Mexico, USA, provide fresh insights into the response of palaeotropical carbonate systems to the growth and demise of ice sheets on Gondwana. The earliest Permian acme of glaciation is recorded in the Orogrande Basin as a series of subaerial exposure surfaces marked by highly variable δ13C values that tend toward values as low as −6‰. These stratigraphic and isotopic patterns are interpreted to reflect the retreat of the sea from the basin during glacial expansion. Younger (Lower Sakmarian to Kungurian) strata, deposited during the long decline of the Late Paleozoic Ice Age, are characterized overall by higher and more uniform δ13C values (+2 to +5‰) and show no evidence of long-term subaerial exposure. However, these strata record two short-lived excursions toward more negative δ13C values. These excursions occur through intervals that record shifts toward shallower and more restricted conditions in the Orogrande Basin. The timing of these shifts corresponds to two periods of glacial expansion across eastern Australia during Late Sakmarian to Early Artinskian and Kungurian time. These relationships suggest that pulses of glacial expansion in eastern Gondwana were of sufficient magnitude to have caused relative sea-level fall, and possibly environmental change, in the Orogrande Basin. The results from this study suggest that marine chemistry, depositional environments and sea-level in the Orogrande Basin were profoundly influenced by epochs of Gondwanan glaciation. These inferences help improve the understanding of how palaeotropical carbonate systems responded to the effects of cryospheric changes during the acme and waning stages of the Late Palaeozoic Ice Age.

Subaerial unconformities are used widely for palaeoenvironmental and palaeogeographic reconstructions, sequence stratigraphy and petroleum reservoir assessments. Recognition and interpretation of these unconformities, particularly those with associated palaeosols, may be problematic in Lower and Middle Palaeozoic carbonate successions because of the collective effect of limited land plant development, superficial similarities between some pedogenic and marine features, and overprinting by later diagenesis. The isolated Judy Creek reef complex in the Lower Frasnian Swan Hills Formation in west-central Alberta, Canada, contains two subaerial unconformities, R0.5 and R4, which formed as a consequence of relative sea-level falls of at least regional scale. Deposits beneath these unconformities have distinctive palaeosol and palaeokarst features. The lower unconformity, R0.5, occurs at the top of a progradational reefal phase of stromatoporoid rudstones–floatstones and peloidal packstones–grainstones and has been recognized in at least one other isolated Swan Hills reef complex (Snipe Lake). Palaeosol–palaeokarst profiles beneath this unconformity extend as deep as ca 2 m below the unconformity. These profiles are characterized by the presence of small rhizoliths, laminar calcretes, ferroan dolomite glaebules, desiccation cracks, breccias, green shale and solution vugs. The upper unconformity, R4, occurs at the top of a backstepping phase of reef growth and has been correlated widely between isolated reefs and carbonate banks on both the western and eastern shelves of the Central Alberta Basin. Palaeosol–palaeokarst profiles, extending as deep as ca 9·5 m beneath the R4 unconformity, are distinguished by abundant, sub-horizontal desiccation cracks filled with green shale, occurring in peloidal wackestones–packstones. Comparison of the R0.5 and R4 profiles indicates that the major intrinsic controls on the development and modification of the profiles are parent-material lithology, particularly the prior degree of induration and particle size; the low topographic relief at the top of the reef interior; and limited vegetation of the exposed reef top due to unfavourable growth conditions and geographic isolation. In addition to climate, the major extrinsic controls are the extent of relative sea-level fall, estimated to be 2·5 to 3 m and 13 to 14 m associated with the R0.5 and R4 unconformities, respectively, and the degree of shoreface erosion during the ensuing marine transgression, estimated to be up to 3 m. This study highlights the complex interplay of mainly physical and chemical processes influencing the formation of subaerial unconformities in carbonate environments during the Devonian, before major evolutionary innovations among vascular land plants led to more intense pedogenesis.

An inclined borehole through the margin of Moruroa Atoll, French Polynesia, in which the ages of units are constrained by radiometric dates, provides a unique opportunity to examine the relationship between sea-level change and diagenetic history. The oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence, culminating in the deposition of the Holocene sequence. Successive intervals of emergence provided the driving force for diagenetic change, promoting the through-flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale. These locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures. In contrast to some sequences elsewhere, the upper part of the Moruroa Atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present. Three distinct diagenetic signatures can be recognized as follows: (i) fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity, thus preventing the deposition of later cements and conferring relative stability; (ii) vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth, attesting to the relative duration of exposure and the conditions of ground water flow; and (iii) deep in the core, sequences reflect lowstand deposits that suffered sub-sea diagenesis, generating pores in which multi-layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes. All cements appear to be of narrowly constrained ages. These results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea-level stands and erosion surfaces. It seems that, in the Moruroa core, some cycles of sea-level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea-level stands.

Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward-directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward-oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.

Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow-water carbonate-platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer-platform shoal-barrier complex, integrated facies analysis and digital field technologies have been applied to a high-relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross-bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone-rudstone with tepees that together formed a 350 to 420 m wide shoal-barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low-energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base-level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low-energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief-building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated-grain rudstone bar facies. Lateral to the bar facies, pisoidal-grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea-level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.

International audience; The study of eight stratigraphic sections at the margin of the semi-enclosed Zsambek Basin (Hungary) allows the sedimentary anatomy of oolitic-bioclastic systems in the Sarmatian of the Central Paratethys to be reconstructed. The mollusc, foraminiferal and ostracod associations indicate that the carbonate systems are Latest Badenian to Late Sarmatian in age. The Lower-Upper Sarmatian deposits are organized in superimposed subaqueous dunes prograding towards the basin on a low-angle ramp. During the Late Sarmatian, the ramp underwent subaerial erosion linked to a moderate relative fall in sea-level. Lagoonal deposits were later formed and microbial-nubeculariid-bryozoan-serpulid buildups were emplaced. The 'abnormal' marine conditions of the Sarmatian, conducive to the development of a poorly diversified flora and fauna and dominant non-skeletal grains, are linked to fluctuating salinities, mesotrophic to eutrophic conditions and perhaps high alkalinity.

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.

Jeju Island is a Quaternary shield volcano built upon the Yellow Sea continental shelf off the Korean Peninsula. Decades of borehole drilling reveals that the shield-forming lavas of the island are underlain by extensive hydrovolcanic deposits (the Seoguipo Formation), which are about 100 m thick and show diverse depositional features. This study provides criteria for distinguishing between hydrovolcanic deposits formed by primary (pyroclastic) and secondary (resedimentation) processes in subaerial and submarine settings based on the observations of several selected cores from the formation. Five facies associations are identified, including: (i) primary hydrovolcanic deposits formed by pyroclastic surges and co-surge fallouts in tuff rings (facies association PHTR); (ii) primary hydrovolcanic deposits formed by Surtseyan fallout and related pyroclastic transport processes in tuff cones (facies association PHTC); (iii) secondary hydrovolcanic deposits formed by debris flows, hyperconcentrated flood flows, sheet floods and rill flows in subaerial settings (facies association RHAE); (iv) secondary hydrovolcanic deposits formed in submarine settings under the influence of waves, tides and occasional mass flows (facies association RHMAR); and (v) non-volcaniclastic and fine-grained deposits formed in nearshore to offshore settings (facies association NVMAR). The primary hydrovolcanic facies associations (PHTR and PHTC) are distinguished from one another on the basis of distinct lithofacies characteristics and vertical sequence profiles. These facies differ from the secondary hydrovolcanic and non-volcaniclastic facies associations (RHAE, RHMAR and NVMAR) because of their distinctive sedimentary structures, textures and compositions. The depositional processes and settings of some massive and crudely stratified volcaniclastic deposits, which occur in many facies associations, could not be discriminated unambiguously even with microscopic observations. Nevertheless, these facies associations could generally be distinguished because they occur typically in packets or sequences, several metres to tens of metres thick and bounded by distinct stratigraphic discontinuities, and comprise generally distinct sets of lithofacies. The overall characteristics of the Seoguipo Formation suggest that it is composed of numerous superposed phreatomagmatic volcanoes intercalated with marine or non-marine, volcaniclastic or non-volcaniclastic deposits. Widespread and continual hydrovolcanic activity, together with volcaniclastic sedimentation, is inferred to have persisted for more than a million years in Jeju Island under the influence of fluctuating Quaternary sea-levels, before effusion of the shield-forming lavas. Extensive distribution of hydrovolcanic deposits in the subsurface of Jeju Island highlights that there can be significant differences in the eruption style, growth history and internal structure between shelfal shield volcanoes and oceanic island volcanoes.

This paper reports a series of 700 porosity–permeability analyses and supporting petrographic and sedimentologic descriptions from Early to Late Miocene carbonate strata cored on the Marion Plateau, offshore from north-eastern Australia, during Ocean Drilling Program (ODP) Leg 194. The samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform-top facies, but also include 79 plugs from deeper-water slope to hemipelagic drift facies. Outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones, the large degree of short-range heterogeneity typical of these strata, and the better porosity–permeability correlation of dolostones than limestones. The platforms have experienced widely varying calcite cementation, dolomitization and dissolution but show little clear evidence of meteoric diagenesis, suggesting that subaerial exposure may have played little role in porosity–permeability evolution. Permeability-for-given-porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud-rich samples. Dolomitization tends to reduce the variation of permeability-for-given-porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system. As a result, there are no differences in permeability–porosity trends for different dolostone textures, whether dominated by intercrystalline, vuggy, or preserved intergranular pore types. Two platform-top sites separated by only 5 km display a major lateral variation in dolostone porosity–permeability characteristics within the youngest dolostone units. This difference is interpreted as reflecting a relatively ‘windward’ (current-facing) setting of the site with the overall higher permeability-for-given-porosity (Site 1199) that led to less muddy depositional facies, greater cementation, and lesser grain dissolution. Pore-geometry parameters measured by petrographic image analysis confirm that the ‘windward’ dolostones have pores that are both larger and less intricate than dolostones comprising the more current-protected location.

Stromatolite morphology is usually controlled by a number of biological and environmental factors. In the south-eastern part of the Korean Peninsula, three stromatolite units of the Cretaceous Sinyangdong Formation have been studied with regard to the effect of diagenesis on their morphology. Here, it is proposed that subaerial exposure and meteoric diagenesis are the most significant factors in shaping the stromatolites of the Cretaceous Sinyangdong Formation. Most previous palaeontological and sedimentological studies on stromatolites have concentrated on the environmental and biogenic controls on stromatolite morphology. These include extrinsic factors such as sedimentation rates or current velocities. The main controlling factor on the morphology of the stromatolites in the Sinyangdong Formation is apparently transient subaerial exposure and related meteoric diagenesis. Textural examination of stromatolite samples from three stratigraphic horizons shows a characteristic repetitive pattern of cycles with gradual transition from fibrous calcite to micrite layers, reflecting changes in the hydrological cycle of the lake in which the stromatolites grew. Stromatolite growth was terminated by corroded surfaces indicative of subaerial exposure related to a fall in lake level. The growth pattern of the stromatolites was mostly determined by the morphology of the corroded substrates during subaerial exposure. Furthermore, the internal stromatolite structure was strongly modified by the process of leaching. As a result, growth forms mostly changed from stratiform to columnar mesostructure. This study strongly implies that diagenesis could make a very significant influence on the morphogenesis of lacustrine stromatolites in the geological past.

The evaporitic Hessian Zechstein Basin is a sub-basin of the Southern Zechstein Basin, situated at its southern margin. Twelve facies groups were identified in the Zechstein Limestone and Lower Werra Anhydrite in order to better understand the sequence-stratigraphic evolution of this sub-basin, which contains economically important potassium salts. Four different paleogeographic depositional areas were recognized based on the regional distribution of facies. Siliciclastic-carbonate, carbonate, carbonate-evaporite and evaporite shallowing-upward successions are developed. These allow the establishment of parasequences and sequences, as well as correlation throughout the Hessian Basin and into the Southern Zechstein Basin. Two depositional sequences are distinguished, Zechstein sequence 1 and Zechstein sequence 2. The former comprises the succession from the Variscan basement up to the lowermost part of the Werra Anhydrite, including the Kupferschiefer as part of the transgressive systems tract. The highstand systems tract is defined by the Zechstein Limestone, in which two parasequences are developed. In large parts of the Hessian Basin, Zechstein sequence 1 is capped by a karstic, subaerial exposure surface, interpreted as recording a type-1 sequence boundary that formed during a distinct brine level fall. Low-lying central areas (Central Hessian Sub-basin, Werra Sub-basin), however, were not exposed and show a correlative conformity. Topography was minimal at the end of sequence 1. Widely developed perilittoral, sabkha and salina shallowing-upward successions indicate a renewed rise of brine level (interpreted as a transgressive systems tract), because of inflow of preconcentrated brines from the Southern Zechstein Basin to the north. This marks the initiation of Zechstein sequence 2, which comprises most of the Lower Werra Anhydrite. In the Central Hessian Sub-basin, situated proximal to the brine inflow and on the ridges within the Hessian Basin, physico-chemical conditions were well suited for sulphate precipitation to form a thick cyclic succession. It consists of four parasequences that completely filled the increased accommodation space. In contrast, only minor sulphate accumulation occurred in the Werra Sub-basin, situated further southwards and distal to the inflow. As a result of substantially different sulphate precipitation rates during increased accommodation, water depth in the region became more variable. The Werra Sub-basin, characterized by very low sedimentation rates, became increasingly deeper through time, trapping dense halite brines and precipitating rock salt deposits (Werra Halite). This ‘self-organization’ model for an evaporitic basin, in which depositional relief evolves with sedimentation and relief is filled by evaporite thereafter, contradicts earlier interpretations, that call upon the existence of a tectonic depression in the Werra area, which controlled sedimentation from the beginning of the Zechstein.

The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open-channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density-driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near-bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open

A new genetic facies model for deep-water clastic evaporites is presented, based on work carried out on the Messinian Gessoso-solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow-water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity-driven deposits ranging from debrisflow to low-density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep-water primary deposit, is here reinterpreted as the fine-grained product of high to low-density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow-water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi-closed thrust-top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep-water resedimented deposits rather than shallow-water to supratidal primary evaporites indicative of a complete basin desiccation.

This study utilized three-dimensional exposures to evaluate how sea-level position and palaeotopography control the facies and geometries of heterozoan carbonates. Heterozoan carbonates were deposited on top of a Neogene volcanic substrate characterized by palaeotopographic highs, palaeovalleys, and straits that were formed by subaerial erosion, possibly original volcanic topography, and faults prior to carbonate deposition. The depositional sequence that is the focus of this study (DS1B) consists of 7–10 fining upward cycles that developed in response to relative sea-level fluctuations. A complete cycle has a basal erosion surface overlain by deposits of debrisflows and high-density turbidity currents, which formed during relative sea-level fall. Overlying tractive deposits most likely formed during the lowest relative position of sea level. Overlying these are debrites grading upward to high-density turbidites and low-density turbidites that formed during relative sea-level rise. The tops of the cycles consist of hemipelagic deposits that formed during the highest relative position of sea level. The cycles fine upward because upslope carbonate production decreased as relative sea level rose due to less surface area available for shallow-water carbonate production and partial drowning of substrates. The cycles are dominated by two end-member types of facies associations and stratal geometries that formed in response to fluctuating sea-level position over variable substrate palaeotopography. One end-member is termed ‘flank flow cycle’ because this type of cycle indicates dominant sediment transport down the flanks of palaeovalleys. Those cycles drape the substrate, have more debrites, high-density turbidites and erosion on palaeovalley flanks, and in general, the lithofacies fine down the palaeovalley flanks into the palaeovalley axes. The second end-member is termed ‘axial flow cycle’ because it indicates a dominance of sediment transport down the axes of palaeovalleys. Those cycles are characterized by debrites and high-density turbidites in palaeovalley axes, and lap out of strata against the flanks of palaeovalleys. Where and when an axial flow cycle or flank flow cycle developed appears to be related to the intersection of sea level with areas of gentle or steep substrate slopes, during an overall relative rise in sea level. Results from this study provide a model for similar systems that must combine carbonate principles for sediment production, palaeotopographic controls, and physical principles of sediment remobilization into deep water.

Hiati of various duration in carbonates are commonly expressed as discontinuity surfaces. The understanding of processes that form and affect these surfaces leads to an improved sequence-stratigraphic interpretation, a reliable outcrop correlation, and better models for reservoir compartmentalization. Various intraformational discontinuities were analysed and interpreted in a well-exposed study window, 2·5 km in lateral length and 60 m in height comprising the Barremian-Aptian Qishn Formation (Haushi-Huqf area, central Oman). This study focuses on the lateral extent and morphology of the surfaces, the petrography of the underlying rocks, and the facies changes and geochemical trends across these discontinuities. Furthermore, the lateral variability of discontinuity surfaces was documented. Three genetic types of discontinuities are differentiated: (i) erosion surfaces; (ii) omission surfaces (hard- and firmgrounds); and (iii) composite surfaces with evidence for both subaerial exposure and submarine boring. Field observations, combined with petrographic and geochemical data, suggest that 17 surfaces are laterally extensive for at least 20 km and record relative sea-level fluctuations of regional scale. In contrast, a large number of laterally limited surfaces (

Silurian pinnacle reefs in the subsurface of the south-western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef-capping facies in the upper part of the Guelph Formation and the overlying A-1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non-isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south-western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.

Callow et al. (2012) conclude with two contradictory statements: (i) ‘‘It seems entirely probable, while as yet unproven, that soils or soil-like processes could be present in intertidal to supratidal portions of the Ediacara Member’’; and (ii) ‘‘On the basis of sedimentological observations, deposition of the fossil-bearing strata within fully marine, tempestite sandstones is considered to be the most parsimonious interpretation’’. My paper (Retallack, 2012a) presents evidence from a variety of geochemical, petrographic, granulometric and palaeopedological analyses, as well as sedimentological observations, that would appear to falsify their second statement, while supporting their first statement. ‘As yet unproven’ can be taken as an opinion of a working hypothesis, following Popper’s (1968) view that mathematics may offer formal proof, but science can only falsify. Thus, Callow et al. (2012) agree that there are non-marine as well as marine parts of the Ediacara Member, as also argued by Mawson & Segnit (1949), Goldring & Curnow (1967) and Jenkins et al. (1983), but not the entirely deep marine interpretation of Mount (1989), and Gehling (2000, and for Fedonkin et al., 2008). Callow et al. (2012) are mistaken in asserting that marine facies and hummocky cross-stratification were not mentioned or illustrated by Retallack (2012a). Hummocky cross-stratification illustrated by Gehling (2000) and heterolithic intertidal facies illustrated by Jenkins et al. (1983) were both mentioned and incorporated in an explicit reinterpretation of sequence stratigraphy of the Ediacara Member, that included horizons of marine influence (Retallack, 2012a, p. 1214, figs 4D and 6). In their original description of hummocky cross-stratification, Dott & Bourgeois (1982) used examples from deltaic sandstones of the Eocene Coaledo Formation of Oregon, familiar to this author from many years of student excursions. Subsequent discoveries of hummocky stratification have been in shoreface to shallow marine shelf palaeoenvironments, not the deep ocean (Higgs, 2011). Callow et al. (2012) also note: ‘‘Here it is concluded that analysis of demonstrably subaqueous, and most probably marine, microbial mat fabrics has led Retallack (2012a) towards mistaken conclusions about his subaerial palaeosols’’. This does not follow as a conclusion, because demonstration that mat fabrics of the Ediacara Member are subaqueous cannot be found in their preceding discussion. The South Australian Ediacaran mat fabrics are detailed by Retallack (2012a), as well as another recent publication (Retallack, 2012b), which develops general criteria for discrimination of microbial mats (marine-lacustrine) and microbial earths (non-marine subaerial). In summary, microbial earths have vertically oriented organisms intimately admixed with minerals of the soil, whereas microbial mats are laminated, and detachable from their mineral substrate as flakes, skeins and rollups. Microbial earths have irregular relief, healed desiccation cracks and pressure ridges even in clay-poor sandstones, whereas microbial mats have flexuous, striated domes and tufts. Microbial earths form deep soil profiles with downward variation in oxidation, clay abundance and replacive nodular subsurface horizons, whereas microbial mats form caps to unweathered, chemically reduced sedimentary layers. Microbial earths develop increasingly differentiated soil profiles through time, whereas microbial mats build upward in laminar to domed increments. On the basis of these criteria, the healed cracks and ridges characteristic of the microbially induced sedimentary structure designated ‘old elephant skin’ and its underlying stratal homogenization, nodularization and geochemical differentiation in the Ediacara Member are interpreted as subaerial microbial earths, and not subaqueous microbial mats (Retallack, 2012a). Sedimentology (2013) 60, 628–630 doi: 10.1111/j.1365-3091.2012.01362.x

Although shelf-edge deltas are well-imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand-prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf-edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand-prone, shelf-margin clinoforms. The shelf-edge deltas of the Battfjellet Formation on Litledalsfjellet and Hogsnyta produced a 3–5 · 15 km, shelf edge-attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth-bar deposits in its shelf-edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short-headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf-edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane-parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flatlaminated intervals. The examined shelf-edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity-flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward-directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive-to-transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf-margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re-established above the shelf edge.

The depositional organization and architecture of the middle–late Devonian Yangdi rimmed carbonate platform margin in the Guilin area of South China were related to oblique, extensional faulting in a strike-slip setting. The platform margin shows two main stages of construction in the late Givetian to Frasnian, with a bioconstructed margin evolving into a sand-shoal system. In the late Givetian, the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton). These grew upwards and produced an aggradational (locally slightly retrogradational) architecture with steep foreslope clinoforms. Three depositional sequences (S3–S5) are recognized in the upper Givetian strata, which are dominated by extensive microbialites. Metre-scale depositional cyclicity occurs in most facies associations, except in the platform-margin buildups and upper foreslope facies. In the latest Givetian (at the top of sequence S5), relative platform uplift (± subaerial exposure) and associated rapid basin subsidence (probably a block-tilting effect) caused large-scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups. Subsequently, sand shoals and banks composed of ooids and peloids and, a little later, stromatoporoid buildups on the palaeohighs, developed along the platform margin, from which abundant loose sediment was transported downslope to form gravity-flow deposits. Another strong tectonic episode caused further platform collapse in the early Frasnian (at the top of sequence S6), leading to large-scale breccia release and the death of the stromatoporoid buildups. Siliceous facies (banded cherts and siliceous shales) were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids. The platform-margin sand-shoal/bank system, possibly with gullies on the slope, persisted into the latest Frasnian until the restoration of microbial buildups. Four sequences (S6–S9), characterized by abundant sand-shoal deposits on the margin and gravity-flow and hemipelagic deposits on the slope, are distinguished in the Frasnian strata. Smaller-scale depositional cyclicity is evident in all facies associations across the platform–slope–basin transect. The distinctive depositional architecture and evolution of this Yangdi Platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy, environmental factors, oceanographic setting, biotic and sedimentary fabrics.

A high-energy Aptian-Albian platform margin in northern Oman fronted onto an open oceanic basin, making the area a valuable analogue for coeval guyot margins. Most similar aged carbonate margins described in the literature faced either intracratonic or minor oceanic basins. The studied margin is characterized by a stabilized outer rim, which, although it did not rise discernibly above the adjacent lagoonal deposits, flanked a steep upper slope (32-40°) basinwards with a relief of at least 30 m. Two main facies provided the rigidity of the outer margin: Lithocodium boundstones that constituted up to 50% of the rock volume; and marine fibrous cements that occluded up to 35% of primary pore space. In contrast, coral-rudist patches and other shelly sessile benthos were distributed irregularly, and the rudist bioherms of the outer margin were often disrupted, with shells being transported and redeposited. The inner margin is characterized by wedge-shaped storm layers that radiate from the platform top lagoonwards, where they interdigitate with carbonate sands and small rudist bioherms. Polygenetic discontinuity surfaces that bear evidence of both marine hardground and subaerial exposure stages are prominent features of the margin. Throughout the latest Aptian to Middle Albian, the platform succession recorded some 30 relative sea-level falls, of which seven reached amplitudes of many tens of metres. These seven high-amplitude falls in sea level are recorded across the entire south-eastern portion of the Arabian craton and are probably of eustatic origin.

A salt crust from an ephemeral saline lake in northern Namibia includes a basal thenardite layer (Na2SO4), followed by intervals consisting of burkeite (Na6(CO3)(SO4)2) with a palisade-type fabric that are separated by layers of fine-grained burkeite. All palisade burkeite formed as coatings along the sides of the neighbouring intervals and around thin lenses of fine-grained burkeite. During the development of these coatings, the original constituents of the intervals that they now occupy were dissolved. Textural features indicate that the dissolved deposits were composed at least in part of halite crystals. The study of this crust illustrates the complexity of evaporite sedimentation in ephemeral saline lakes where more than one dominant mineral is formed. In the studied basin, selective dissolution of the most soluble salts occurred during flooding stages, partly affecting subsurface occurrences that consisted of continuous beds. The final product is a nearly monomineralic deposit, derived from a layered non-monomineralic formation. The deposit developed to a large extent by subsurface crystal growth in a subaqueous setting, rather than by mineral formation along the brine–sediment interface or under subaerial conditions.

In the middle Miocene Badenian gypsum basin of the Carpathian Foredeep, west Ukraine, three main zones of gypsum development occur in the peripheral parts of the basin. Zone I consists entirely of stromatolitic gypsum formed in a nearshore zone. Zone II is located more basinward and is characterized by stromatolitic gypsum in the lower part of the section, overlain by a sabre gypsum unit. Zone III occurs in still more basinward areas and is characterized by giant gypsum intergrowths (or secondary nodular gypsum pseudomorphs of these) in the lowermost part, overlain by stromatolitic gypsum, sabre gypsum and then by clastic gypsum units. Correlation between these facies and zones has been achieved using lithological marker beds and surfaces. Of particular importance for correlation is a characteristic marker bed (usually 20–40 cm thick) of cryptocrystalline massive gypsum occurring in zones II and III. The marker was not distinguished in zone I, possibly because this bed is older than the entire gypsum section of that zone. These new results strongly suggest that the deposition of giant gypsum intergrowth facies and stromatolitic gypsum facies was coeval. In some sections of zones I and II, limestone intercalations have been recorded within the upper part of the gypsum sections. Considerable scatter of the δ18O and δ13C values of these limestones indicates variable diagenetic overprints of marine carbonates, but a marine provenance of the limestones is confirmed by microfacies analysis. Some of the limestones are coeval with an intercalation of gypsarenitic, mostly laminated gypsum occurring in the sabre gypsum unit of zones II and III. Badenian gypsum formed in extremely shallow-water to subaerial environments on broad, very low relief areas of negligible brine depth, which could be affected by rapid transgressions. Stable isotope (δ34S, δ18O) studies of the gypsum demonstrate that the sulphate was of sea-water origin or was derived from dissolution of Miocene marine evaporites. Investigations of individual inclusions in the gypsum indicate decreased water salinity when compared with modern marine-derived, calcium sulphate-saturated water. Groundwater influences are indicated by high calcium sulphate contents of the brines in the evaporite basin. The chemical composition of Badenian waters was thus a mixture of relic sea water (depleted in NaCl), groundwater (enriched in calcium sulphate) and surface run-off.

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.

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.

The Maesan fan-delta-fed slope system in the Miocene Pohang Basin occurs between two Gilbert-type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, representing distinct depositional environments: alluvial fan (facies association I), steep-faced slope (facies association II), and basin plain (facies association III). Subaerial debris flows and dense, inertia-dominated currents were transformed into subaqueous sediment gravity flows in steep-faced slope environments. Further downslope, these flows were channelized and formed lobate conglomerate and sandstone bodies at the terminal edge of the channels (or chutes). Interchannel and interlobe areas were dominated by homogeneous mudstone and muddy sandstone, deposited by suspension settling of fine-grained materials. Part of the steep-faced slope deposits experienced large-scale slides and slumps. The chutes/channels, lobes and splays on the steep-faced slope of the Maesan system are similar to those in modern subaqueous coarse-grained fan-delta systems.

Summary The depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories: marginal (mixed shallow-subaqueous and subaerial), shallow and deep subaqueous. These three environmental groupings hold for both marine and nonmarine settings, although the details of continental evaporites may be far more complex than in most marine-fed water bodies. The primary evaporite morphologies from many continental (playa), hypersaline marine and marine-marginal depositional settings are reasonably well understood, because of the numerous detailed studies of recent, Holocene and Cenozoic deposits that serve as models for sedimentary interpretation. The sedimentological features that develop in deeper water settings are inferred, based on examination of unaltered Cenozoic deposits. Each environmental setting develops characteristic depositional features and patterns, and one facies grades into the next. Because there may be significant physiochemical changes in water composition during deposition as well as sudden change(s) in both water depth and basinal circulation, description and interpretation of evaporative rocks should not be based on mineralogy alone, but on the distinct sedimentary characteristics of each part of a deposit. In cases where sediments still reflect their primary mineralogy and morphology, most of the environments can now be recognized; however, geochemical studies are commonly required to determine the source(s) of the original water. The determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions, bromine content of chloride salts, and the stable isotopes of strontium, sulphur, carbon and oxygen. Only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled.

Marine siltstone successions, 1–20 m thick, form the upper part of sequences in the mid-Pleistocene Castlecliff section (≈ 0·98–0·35 Ma). The siltstones were deposited within a broad shelf embayment at and about glacioeustatic highstands and are interpreted as highstand systems tracts (HST). Shell-rich to relatively shell-poor contacts at the base of Castlecliff HST are interpreted as downlap surfaces, which mark the quenching of transgressive in situ biogenic accumulation (backlap shellbed). Nonetheless, the basal parts of Castlecliff HST successions are enriched in fossil content in the context of the highstand successions as a whole and represent downlap shellbeds. Castlecliff HST are truncated above by sequence-bounding ravinement surfaces, such that complete sandier-upward successions and subaerial exposure surfaces associated with sequence boundaries sensu stricto are never preserved. Modern highstand sedimentation in the Taranaki Bight offshore from Castlecliff is characterized by a mid-shelf mud depocentre and a coastal shore-connected sand prism, both of which are encroaching upon intervening shell-rich relict and palimpsest transgressive deposits. The mud depocentre is up to 9 m thick, and deposition is influenced by a gyre caused by bathymetric steering of storm-driven currents along the embayed coastline. Modern highstand deposition in the Taranaki Bight, in which the mud depocentre is in part detached from the contemporary shore-connected sand prism, is regarded as an analogue for the deposition of the preserved lower parts of Castlecliff HST. The inferred architecture of Castlecliff HST therefore need not refer to the shore-connected, progradational geometry predicted by traditional sequence models. The model proposed herein may have application to other shelf palaeo-embayments in which mid-shelf focusing of fine-grained sediment has resulted from coastal steering of currents.

The behaviour of subaerial particle-laden gravity currents (e.g. pyroclastic flows, lahars, debris flows, sediment-bearing floods and jokulhlaups) flowing into the sea has been simulated with analogue experiments. Flows of either saline solution, simple suspensions of silicon carbide (SiC) in water or complex suspensions of SiC and plastic particles in methanol were released down a slope into a tank of water. The excess momentum between subaerial and subaqueous flow is dissipated by a surface wave. At relatively low density contrasts between the tank water and the saline or simple suspensions, the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water. The cloud then collapses gravitationally to form an underwater gravity current, which progresses along the tank floor. At higher density contrasts, the subaerial flow develops directly into a subaqueous flow. The flow slows and thickens in response to the reduced density contrast, which is driving motion, and then continues in the typical gravity current manner. Complex suspensions become dense flows along the tank floor or buoyant flows along the water surface, if the mixtures are sufficiently denser or lighter than water respectively. Flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow. Material from the particulate-depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface, whereas material from the particulate-enriched lower sections forms a dense flow along the tank floor. Sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy, lifts off and becomes a secondary buoyant flow along the water surface. Jokulhlaups, lahars and debris flows are typically much denser than seawater and, thus, will usually form dense flows along the seabed. After sufficient sedimentation, the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface, leading to a decoupling of the fine and coarse particles. Flood waters with low particulate concentrations (

Despite a low tidal range and relatively low wave conditions, the Mackenzie Delta is not prograding seaward but rather is undergoing transgressive shoreface erosion and drowning of distributary channel mouths. In the Olivier Islands region of the Mackenzie Delta the resultant morphology consists of a network of primary and secondary channels separated by vegetated islands. New ground is formed through channel infilling and landward-directed bar accretion. This sedimentation is characterized by seven sedimentary facies: (1) hard, cohesive silty clay at the base of primary channels which may be related to earlier, offshore deposition; (2) ripple laminated sand beds, believed to be channel-fill deposits; (3) ripple laminated sand and silt, interpreted as flood-stage subaqueous bar deposits; (4) ripple laminated or wavy bedded sand, silt and clay, representing the abandonment phase of channel-fill deposits and lateral subaqueous bar deposition from suspension settling; (5) a well sorted very fine sand bed, presumed to result from a single storm event; (6) parallel or wavy beds of rooted silt, sand and clay, interpreted as lower energy emergent bar deposits; and (7) parallel or wavy beds of rooted silt and clay, believed to represent present-day subaerial bar aggradation. The distribution of sedimentary facies can be interpreted in terms of the morphological evolution of the study area. Initial bar deposition of facies 3 and channel deposition of facies 2 was followed by lateral and upstream bar sedimentation of facies 3 and 4 which culminated with the deposition of the storm bed of facies 5. Facies 6 and 7 signify bar stabilization and abandonment. Patterned ground formed by thermal contraction and preserved in sediments as small, v-shaped sand wedges provides the most direct sedimentological indicator of the arctic climate. However, winter ice and permafrost also govern the stratigraphic development of interchannel and channel-mouth deposits. Ice cover confines flow at primary channel mouths, promoting the bypassing of sediments across the delta front during peak discharge in the spring. Permafrost minimizes consolidation subsidence and accommodation in the nearshore, further enhancing sediment bypass. Storms limit the seaward extent of bar development and promote a distinctive pattern of upstream and lateral island growth. The effects of these controls are reflected in the vertical distribution of facies in the Olivier Islands. The sedimentary succession differs markedly from that of a low-latitude delta.

Palaeokarst is an important feature of late Dinantian exposure surfaces. Soil-filled depressions are widely developed. These are comparable to modern day solution dolines and were probably interspersed by areas of relatively bare limestone pavement. The palaeokarst primarily exhibits a smooth, mamillated to potholed surface form, consistent with its formation beneath a soil cover. Areas between the depressions have been extensively stylolitised and would likely have originally been characterized by small-scale fretted and sculpted karren forms typical of subaerial karstification. Palaeokarst pits making up the depressions are thought to have been initiated through stem-flow drainage from trees. Rain water, intercepted by the crown of the tree, was concentrated at specific sites on the emergent surface and dissolution beneath the trunk produced cylindrical pits that propagated vertically downwards. Trees responsible for concentrating drainage may also have enhanced the acidic nature of the rain water through leaching of organic acids from foliar and woody tissues. Downward propagation of the pits was limited to the uppermost 1–2 m and enlargement primarily occurred through lateral amalgamation of adjacent pits. Once initiated, continued development of the depressions would have been self perpetuating; the preferential accumulation of volcanic ash and organic matter enhancing water retention and encouraging further vegetation growth. In contrast, intervening areas would have been characterized by slow vertical denudation only. Karstification likely took of the order of a few hundred years in the case of potholed palaeokarstic surfaces formed solely by stem-flow drainage, to a few tens of thousands of years where the palaeokarst is more mature.

The southern margin of the vast Yangtze platform in central Guizhou Province, China, retreated during the Anisian (early Middle Triassic) by shedding skeletal debris and boundstone blocks into the margin of the adjacent basin. Anisian platform deposits are shoaling-up cycles that commonly terminated in subaerial exposure. Platform-margin facies are obscured by massive dolomitization and mechanical erosion. Distal basin deposits are terrigenous mudstone and siltstone. At the basin margin a wedge of mixed carbonate and terrigenous rocks consists of (A) thin-bedded dolostone and limestone, (B) lime breccia with thin-bedded mudstone, and (C) lime mudstone with breccia. Blocks within the breccias indicate that the shelf margin contained extensive boundstone formed by ‘Tubiphytes,’encrusting organisms, and early marine cement. Interspersed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin. The profile of the shelf margin from detailed mapping indicates 1.7–2.7 km of platform-margin retreat during deposition of a basin-margin wedge 250 m thick. Intertonguing of various basin-margin facies reflects alternating minor episodes of advance and retreat of the margin. Near-parallelism of the tongues suggests low relief at the platform margin. An upward stratigraphic progression to more distal, carbonate-free, terrigenous basin facies indicates a cessation of carbonate production on the platform owing to emergence during early Anisian time. Retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick, the largest observed dimensions. A re-entrant in the margin 7 km wide and 10 km deep could also reflect collapse. Retreat occurred along 175 km of the platform margin. The lack of platform-margin facies along this front suggests 3–7 km of retreat and a total area of 875 km2. The Anisian platform-margin retreat in Guizhou is one of relatively few examples of platform-margin retreat in the geological record.

Aeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio-eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non-glacial. Numerous lenticular bodies of cross-stratified oolite lie near the contact between the lower and upper members of the mudstone-dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north-central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross-strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat-bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand-free deflation surface. When the Sundance Sea transgressed the dunes, a thin (

The clast fabrics of certain types of terrestrial slope deposits are reviewed and compared, including the deposits of rockfalls, solifluction, debris flows, dry grain flows, frost-coated clast flows and run-off. The analysis is based on modern deposits in active environments. The study shows that fabric characteristics allow discrimination between ‘collective’and individual movement of rock particles. The individual particle movement generally results in a random clast orientation, whereas the processes of ‘collective’movement typically create distinct preferred orientations. The highest fabric strengths together with low values of spherical variance are found in solifluction deposits. A survey of Pleistocene slope deposits indicates, however, that clast fabric has to be used with caution in the identification of past slope dynamics, because significant post-depositional changes may occur during ageing and burial of deposits. The diagnostic significance of fabric characteristics may also be low due to the overlap of the statistical values that typify different processes.

During the upper Pleistocene the Central Altiplano of Bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations. Development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands, discontinuously covering the 300-km-long and 100-km-wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of Uyuni. Distribution, size and shape of the bioherms are diverse due to various factors, e.g. the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth. On larger palaeoterraces, the build-ups coalesced to form platform-like carbonate accumulations. Although the morphologies closely resemble those induced by cyanobacteria, they were predominantly constructed by other plant communities, probably dominated by filamentous green algae. Cyanobacterial communities flourished in association with these plants, but they did not contribute significantly to the architecture of the bioherms; they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build-ups. A prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects. The build-ups located between 3660 and 3680 m elevation display up to three major parts: (1) a massive inner core formed during an early Minchin highstand, before 40 ka; (2) a large peripheral envelope deposited at about 40 ka (late Minchin) and (3) a thinner outermost crust formed during a late glacial event. Lake level dropped during interlacustrine stages, sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system, i.e. the present-day salar of Uyuni.

A superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea-level dynamics. In the first sea-level cycle, metre-scale reefs grew amongst mid-ramp calcarenites and outer-ramp shales during slow sea-level rise; a 25-m-thick oblate reef tract, separating mid-ramp and outer-ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea-level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea-level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high-standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea-level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst. These Proterozoic slope buildups are similar to Phanerozoic deep-water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well-developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand- and gravel-sized calcareous debris. Their responses to changes in sea-level were strikingly similar to those shown by their younger counterparts, and suggest that sequence-stratigraphic concepts derived from studies of Phanerozoic reefs can also be applied to the Proterozoic.