Showing papers in "Sedimentology in 2010"

Dolomite formation within microbial mats in the coastal sabkha of Abu Dhabi (United Arab Emirates)

Abstract: Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral-template properties of exopolymeric substances play a crucial role.

The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data

Abstract: The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and at many sites only suspended load data are available. Often the bedload fraction is estimated with rule-of-thumb methods such as Maddock’s Table, which are inadequately field-tested. Here the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short- and long-term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year-to-year averages. Using published simultaneous short-term field measurements of bedload and suspended load transport rates, Maddock’s table is reviewed and updated. Long-term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel bed rivers, whereby in large catchments the bedload fraction is insignificant at ~1%. For sand bed rivers, the bedload fraction may be substantial (30-50%) even for large catchments. However, available data are scarce and of varying quality. Long-term partitioning varies widely among catchments and the available data are currently not sufficient to effectively discriminate control parameters.

Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites

Abstract: Modern Ca:Mg carbonate stromatolites form in association with the microbialmat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). Thestromatolites, although showing diversified fabrics characterized by thin orcrude lamination and/or thrombolitic clotting, exhibit a pervasive peloidalmicrofabric. The peloidal texture consists of dark, micritic aggregates of veryhigh-Mg calcite and/or Ca dolomite formed by an iso-oriented assemblage ofsub-micron trigonal polyhedrons and organic matter. Limpid acicular crystalsof aragonite arranged in spherulites surround these aggregates. Unlike thearagonite crystals, organic matter is present consistently in the dark, micriticcarbonate comprising the peloids. This organic matter is observed as sub-micron flat and filamentous mucus-like structures inside the interspaces of thehigh-Mg calcite and Ca dolomite crystals and is interpreted as the remains ofdegraded extracellular polymeric substances. Moreover, many fossilizedbacterial cells are associated strictly with both carbonate phases. These cellsconsist mainly of 0AE2to4lm in diameter, sub-spherical, rod-like andfilamentous forms, isolated or in colony-like clusters. The co-existence offossil extracellular polymeric substances and bacterial bodies, associated withthe polyhedrons of Ca:Mg carbonate, implies that the organic matter andmicrobial metabolism played a fundamental role in the precipitation of theminerals that form the peloids. By contrast, the lack of extracellular polymericsubstances in the aragonitic phase indicates an additional precipitationmechanism. The complex processes that induce mineral precipitation in themodern Lagoa Vermelha microbial mat appear to be recorded in the studiedlithified stromatolites. Sub-micron polyhedral crystal formation of high-Mgcalcite and/or Ca dolomite results from the coalescence of carbonatenanoglobules around degraded organic matter nuclei. Sub-micron polyhedralcrystals aggregate to form larger ovoidal crystals that constitute peloids.Subsequent precipitation of aragonitic spherulites around peloids occurs asmicro-environmental water conditions around the peloids change.Keywords Bacterial fossils, biomineralization, microbialite, nanoglobules,peloids, stromatolites.

Controls on the alluviation of oxbow lakes by bed-material load along the Sacramento River, California

Abstract: Differences in the nature and quantity of sediment filling oxbow lakes have significant implications for the evolution of meandering rivers and the development of floodplains, influencing rates of meander migration and the valley width over which migration takes place. In an effort to identify the controls on the alluviation of oxbow lakes by coarse bed material, this study examined the sedimentary records stored within oxbow lakes of the Sacramento River of California, USA, and found that the volume of gravel in storage correlated negatively with the diversion angle separating flow between the river channel and the entrance into each lake. A method was devised for estimating the original channel bathymetry of the studied lakes and for modelling the hydraulic and sediment-transport effects of the diversion angle within channels recently abandoned by meander cut-off. The diversion angle determines the width of a flow separation within the abandoned-channel entrance, reducing the discharge diverted from the river channel and thus limiting the ability of the abandoned channel to transport bed material. Aggradation rates are faster within entrances to abandoned channels with high diversion angles, resulting in the rapid isolation of lakes that store only a small volume of coarse-grained sediment. Aggradation rates are slower within channel entrances where diversion angles are low, resulting in the slow transitioning of such channels into oxbow lakes with a larger and more extensive accumulation of coarse-grained sediment. These findings compare well with observations in other natural settings and the mechanism which is described for the control of the diversion may explain why some oxbow lakes remain as open-water environments for centuries, whereas others are filled completely within decades of cut-off.

Aeolian dune interactions and dune‐field pattern formation: White Sands Dune Field, New Mexico

Abstract: Pattern formation is a fundamental aspect of self-organization in fields of bedforms. Time-series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off-centre collision; (vi) defect creation; and (vii) dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off-centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of dunes, crest length, defect density, crest spacing and dune height), dune migration rates, and the type and frequency of dune interactions within a 3500 m box transect from the upwind margin to the core of the dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo-lake topography. Pattern evolution is most strongly coupled to the pattern parameters of dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the dunes become further apart and have fewer defects.

Sediment dispersal and quantitative stratigraphic architecture across an ancient shelf

Abstract: Two large (200 to 300 km), near-continuous outcrop transects and extensive well-log data (ca 2800 wells) allow analysis of sedimentological characteristics and stratigraphic architecture across a large area (ca 60 000 km2) of the latest Santonian to middle Campanian shelf along the western margin of the Western Interior Seaway in eastern Utah and western Colorado, USA. Genetically linked depositional systems are mapped at high chronostratigraphic resolution (ca 0·1 to 0·5 Ma) within their sequence stratigraphic context. In the lower part of the studied interval, sediment was dispersed via wave-dominated deltaic systems with a ‘compound clinoform’ geomorphology in which an inner, wave-dominated shoreface clinoform was separated by a muddy subaqueous topset from an outer clinoform containing sand-poor, gravity-flow deposits. These strata are characterized by relatively steep, net-regressive shoreline trajectories (>0·1°) with concave-landward geometries, narrow nearshore belts of storm-reworked sandstones (2 to 22 km), wide offshore mudstone belts (>250 km) and relatively high sediment accumulation rates (ca 0·27 mm year−1). The middle and upper parts of the studied interval also contain wave-dominated shorefaces, but coeval offshore mudstones enclose abundant ‘isolated’ tide-influenced sandstones that were transported sub-parallel to the regional palaeoshoreline by basinal hydrodynamic (tidal?) circulation. These strata are characterized by relatively shallow, net-regressive shoreline trajectories ( 190 km) and relatively low sediment accumulation rates (ca ≤0·11 mm year−1). The change in shelfal sediment dispersal and stratigraphic architecture, from: (i) ‘compound clinoform’ deltas characterized by across-shelf sediment transport; to (ii) wave-dominated shorelines with ‘isolated’ tide-influenced sandbodies characterized by along-shelf sediment transport, is interpreted as reflecting increased interaction with the hydrodynamic regime in the seaway as successive shelfal depositional systems advanced out of a sheltered embayment (‘Utah Bight’). This advance was driven by a decreasing tectonic subsidence rate, which also suppressed autogenic controls on stratigraphic architecture.

Morphological patterns of Aptian Lithocodium–Bacinella geobodies: relation to environment and scale

Abstract: Lithocodium aggregatum and Bacinella irregularis are now extinct, shallow marine life forms of unknown taxonomic origin. Forming part of the tropical platform biota during much of the Mesozoic, these organisms experienced bloom periods and temporarily replaced rudist–coral assemblages during parts of the Early Aptian. Within the limitations of time resolution, this ‘out-of-balance’ facies is coeval with the Oceanic Anoxic Event 1a-related black shale deposition in oceanic basins but the triggering factors remain poorly understood. Here, a platform-wide comparison of Lithocodium–Bacinella geobodies and morphotypes from the Sultanate of Oman is presented and placed in its environmental, bathymetric and physiographic context. Lithocodium–Bacinella geobodies reach from kilometre-scale ‘superstructures’ to delicate centimetre-sized growth forms. Clearly, scale matters and care must be taken when drawing conclusions based on spatially limited observational data. Whilst the factors that cause Lithocodium–Bacinella expansion should probably be considered in a global context, regional to local factors affected growth patterns in a more predictable manner. Here, the unresolved taxonomic relationship remains the main obstacle in any attempt to unravel the response of Lithocodium–Bacinella to specific or interlinked environmental parameters as different organisms respond differently to changing environment. Acknowledging these limitations, the following tentative patterns are observed: (i) Lithocodium–Bacinella tolerated a wide range of hydrodynamic levels and responded to differences in energy level or physiographic settings (margin, intrashelf basin, inner platform) by obtaining characteristic growth forms. (ii) Lithocodium–Bacinella favoured low-sediment input but had the ability to react to higher sedimentation rates by enhanced upward growth; a feature perhaps pointing to a phototrophic metabolism. Circumstantial evidence for continuous growth within the upper-sediment column is debated. (iii) The availability of accommodation space had a direct influence on the maximum size of geobodies formed. (iv) Fluctuating nutrient levels and sea water alkalinity may have affected the growth potential of Lithocodium–Bacinella. Understanding the relationship between Lithocodium–Bacinella morphogenesis on a wide range of scales and local environmental parameters allows for better prediction of the spatial distribution of reservoir properties and also results in an improved interpretation of palaeoenvironments. This study might represent a useful first step in this direction.

Latitudinally different responses of Tethyan shoal‐water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a)

Abstract: Ocean-wide anoxic events represent intensively investigated anomalies in the global carbon cycle. Most previous research has focussed on hemipelagic and pelagic settings and on the relationship between black-shale deposition and carbon-isotope excursions. The study of ocean-wide anoxic events and coeval shallow-water settings is now increasingly seen as an interesting complementary approach, but one that is not without problems. Whereas platform drowning characterizes the Early Aptian of the northern Tethyan margin, Lithocodium-Bacinella-rich facies and ongoing shoal-water sedimentation at the southern Tethyan margin (Oman) bears important information on potential causes of carbon-cycle perturbations. The present paper seeks to test the supra-regional relevance of the Oman data by investigating coeval central Tethyan limestones. Three Lower Aptian shoal-water sections in Istria (Croatia), deposited on the isolated Adriatic Carbonate Platform, are investigated applying chemostratigraphy (carbon and strontium) and detailed sedimentological analysis. The focus is on peritidal to lagoonal facies characterized by mass occurrences of Lithocodium-Bacinella, an enigmatic microencruster community. Lithocodium-Bacinella facies occurs predominantly in layers ranging from one to several centimetres in stratigraphic thickness, with several layers merging to metre-thick packages. Growth fabrics within the layers include oncoidal morphotypes, lumps, interconnected patches and columns, layers and rare nodular to massive bindstone facies. These growth patterns show a remarkable regional extent and consistency over study sites distributed several kilometres apart. This widespread distribution suggests that specific Lithocodium-Bacinella morphotypes might serve as regional stratigraphic markers. The high-resolution carbon-isotope chemostratigraphy presented here is based on pristine rudist shells and matrix micrite samples and calibrated against strontium-isotope data obtained from screened rudist low-Mg calcite. The chemostratigraphic data are consistent with existing biostratigraphic data and place the studied strata at the onset of Early Aptian oceanic anoxic event 1a. Moreover, results indicate the near-coeval nature of Lithocodium-Bacinella bloom facies in Istria and Oman. The outcomes of this study point to latitudinally different responses of Tethyan shoal-water carbonate systems (platform drowning versus Lithocodium-Bacinella blooms) to the ocean-wide anoxic event 1a.

Architecture and sedimentology of the Kerinitis Gilbert‐type fan delta, Corinth Rift, Greece

Abstract: The Kerinitis Delta in the Corinth Rift, Greece, is a footwall derived, coarse-grained, Gilbert-type fan delta deposited in the hangingwall of a linked normal fault system. This giant Gilbert-type delta (radius 3-8 km, thickness > 600 m) was supplied by an antecedent river and built into a brackish to marine basin. Although as yet poorly dated, correlation with neighbouring deltas suggests that the Kerinitis Delta was deposited during a period of 500 to 800 ka in the Early to early Middle Pleistocene. Facies characterizing a range of depositional processes are assigned to four facies associations (topset, foreset, bottomset and prodelta). The dominantly fluvial topset facies association has locally developed shallow marine (limestone) and fluvial-shoreface sub-associations. This delta represents a subsidence-dominated system in which high fault displacement overwhelmed base-level falls (creation of accommodation predominantly > 0). Stratal geometries and facies stacking patterns were used to identify 11 key stratal surfaces separating 11 stratal units. Each key stratal surface records a landward shift in the topset breakpoint path, indicating a rapid increase in accommodation/sediment supply. Each stratal unit records a gradual decrease in accommodation/sediment supply during deposition. The cyclic stratal units and key stratal surfaces are interpreted as recording eustatic falls and rises, respectively. A 30 m thick package of foresets below the main delta records the nucleation of a small Proto-delta probably on an early relay ramp. Based on changes in stratal unit geometries, the main delta is divided into three packages, interpreted as recording the initiation, growth and death of the controlling fault system. The Lower delta comprises stacked, relatively thin, progradational stratal units recording low displacement on the young fault system (relay ramp). The Middle delta comprises vertically stacked stratal units each recording initial aggradation–progradation followed by progradation; their aggradational component increases up through the Middle delta, which records the main phase of increasing rate of fault displacement. The Upper delta records pure progradation, recording abrupt cessation of movement on the fault. A major erosion surface incising basinward 120 m through the Lower and Middle delta records an exceptional submarine erosion process (canyon or delta collapse).

Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin

Abstract: The relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the Lower Cretaceous Missisauga Formation was investigated using a combination of electron microprobe, bulk geochemistry and X-ray diffraction data. The succession studied comprises several stacked parasequences. The delta progradational facies association includes: (i) fluvial or distributary channel sandstones (some with tidal influence); (ii) thick-bedded delta-front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms; and (iii) more distal muddier delta-front and prodeltaic facies. The transgressive facies association includes lag conglomerate, siderite-cemented muddy sandstone and mudstone, and bioclastic sandy limestone. Chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles. Good quality chlorite rims are well correlated with Ti in bulk geochemistry. Ti is a proxy for Fe availability, principally from the breakdown of abundant detrital ilmenite (FeTiO3). Under conditions of sea floor diagenesis, the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate-depletion level and of the overlying Eh-controlled diagenetic zones, resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones. This early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite. Thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation. This study thus shows that the preservation of porosity by chlorite rims is a two-stage process. Rapidly deposited delta-front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims, but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis.

Deciphering the depositional environment of the laminated Crato fossil beds (Early Cretaceous, Araripe Basin, North-eastern Brazil)

Abstract: The laminated limestones of the Early Cretaceous Crato Formation of the Araripe Basin (North-eastern Brazil) are world-famous for their exceptionally well-preserved and taxonomically diverse fossil fauna and flora. Whereas the fossil biota has received considerable attention, only a few studies have focused on the sedimentary characteristics and palaeoenvironmental conditions which prevailed during formation of the Crato Fossil Lagerstatte. The Nova Olinda Member represents the lowermost and thickest unit (up to 10 m) of the Crato Formation and is characterized by a pronounced rhythmically bedded, pale to dark lamination. To obtain information on palaeoenvironmental conditions, sample slabs derived from three local stratigraphic sections within the Araripe Basin were studied using high-resolution multiproxy techniques including detailed logging, petrography, μ-XRF scanning and stable isotope geochemistry. Integration of lithological and petrographic evidence indicates that the bulk of the Nova Olinda limestone formed via authigenic precipitation of calcite from within the upper water column, most probably induced and/or mediated by phytoplankton and picoplankton activity. A significant contribution from a benthonic, carbonate-secreting microbial mat community is not supported by these results. Deposition took place under anoxic and, at least during certain episodes, hypersaline bottom water conditions, as evidenced by the virtually undisturbed lamination pattern, the absence of a benthonic fauna and by the occurrence of halite pseudomorphs. Input of allochthonous, catchment-derived siliciclastics to the basin during times of laminite formation was strongly reduced. The δ18O values of authigenic carbonate precipitates (between −7·1 and −5·1‰) point to a 18O-poor meteoric water source and support a continental freshwater setting for the Nova Olinda Member. The δ13C values, which are comparatively rich in 13C (between −0·1 and +1·9‰), are interpreted to reflect reduced throughflow of water in a restricted basin, promoting equilibration with atmospheric CO2, probably in concert with stagnant conditions and low input of soil-derived carbon. Integration of lithological and isotopic evidence indicates a shift from closed to semi-closed conditions towards a more open lake system during the onset of laminite deposition in the Crato Formation.

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

Abstract: The links between large-scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Rio Parana, Argentina, time series of three-dimensional velocity, and at-a-point suspended sediment concentration and particle-size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee-side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune-related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.

Recognition and significance of sharp-based mouth-bar deposits in the Eocene Green River Formation, Uinta Basin, Utah

Abstract: Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth-bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive-transgressive sequence. Individual sandstone bodies are texturally sharp-based towards mudstone substratum. In proximal parts, the mouth-bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth-bar clinothems. The clinothems pass gradually into greenish-grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex-upward sandstone bars. The mouth-bar deposits are connected to terminal distributary channel deposits. Together, these mouth-bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth-bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.

Grain-size sorting within river bars in relation to downstream fining along a wandering channel

Abstract: The textural variability of river bed gravels at bar scales is poorly understood, as are the relations between variability at this scale and at reach and river scales. Surface and subsurface grain-size distributions were therefore examined at reach, bar and bedform scales along lower Fraser River, British Columbia, Canada. Grain-size variations within compound bars are conditioned by longitudinal position, elevation and morphological setting. Surface and subsurface sediments tend to decrease in median size from bar head to bar tail by 33% and 17%, respectively. Sediment size is constrained at some upper limit that is inversely related to bar surface elevation and which is consistent with competence considerations. The surface sediments on unit bars are finer and better sorted than the bed materials in bar-top channels and along the main bar edges. Secondary unit bars tend to have a lower sand content than other features, a consequence of sediment resorting. Individual unit bars and gravel sheets exhibit streamwise grain-size fining and lee-side sand deposition. Over time, significant amounts of cut and fill do not ipso facto cause changes in surface grain sizes; yet, sediment characteristics can change without any significant morphological adjustment taking place. At the reach scale there is a clear downstream fining trend, but local variability is consistently high due to within-bar variations. The surface median grain-size range on individual bars is, on average, 25% of that along the entire 50 km reach but is 68% on one bar. While the overall fining trend yields a downstream change in surface median size of 0·76 mm km−1, the average value for ‘head-to-tail’ size reduction on individual bars is 6·3 mm km−1, an order of magnitude difference that highlights the effectiveness of bar-scale sorting processes in gravel-bed rivers. Possibilities for modelling bar-scale variability and the interaction of the different controls that are identified are discussed.

Intra-parasequence architecture of an interpreted asymmetrical wave-dominated delta

Abstract: Although modern wave-dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface-shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave-dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high-resolution stratigraphic architecture and interpreted palaeo-geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave-dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south-trending, wave-dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave-dominated shoreface-shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial-dominated delta-front deposits, which have previously been assigned to a separate ‘stranded lowstand parasequence’ formed by a significant, allogenic change in relative sea-level. High-resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea-level. Interpreted normal regressive shoreline trajectories within the wave-dominated shoreface-shelf deposits are marked by aggradational stacking of bedsets bounded by non-depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave-base scour. Fluvial-dominated delta-front deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave-dominated shoreface-shelf and fluvial-dominated delta-front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave-dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.

Stratigraphy and depositional setting of slurry and contained (reflected) beds in the Marnoso‐arenacea Formation (Langhian‐Serravallian) Northern Apennines, Italy

Abstract: This work presents the stratigraphy and facies analysis of an interval of about 2500 m in the Langhian and Serravallian stratigraphic succession of the foredeep turbidites of the Marnoso-arenacea Formation. A high-resolution stratigraphic analysis was performed by measuring seven stratigraphic logs between the Sillaro and Marecchia lines (60 km apart) for a total thickness of about 6700 m. The data suggest that the stratigraphy and depositional setting of the studied interval was influenced by syndepositional structural deformations. The studied stratigraphic succession has been subdivided into five informal stratigraphic units on the basis of how structurally controlled topographic highs and depocentres, a consequence of thrust propagation, change over time. These physiographic changes of the foredeep basin have also been reconstructed through the progressive appearance and disappearance of thrust-related mass-transport complexes and of five bed types interpreted as being related to structurally controlled basin morphology. Apart from Bouma-like Type-4 beds, Type-1 tripartite beds, characterized by an internal slurry unit, tend to increase especially in structurally controlled stratigraphic units where intrabasinal topographic highs and depocentres with slope changes favour both mud erosion and decelerations. Type-2 beds, with an internal slump-type chaotic unit, characterize the basal boundary of structurally controlled stratigraphic units and are interpreted as indicating tectonic uplift. Type-3 beds are contained-reflected beds that indicate different degrees of basin confinement, while Type-5 are thin and fine-grained beds deposited by dilute reflected turbulent flows able to rise up the topographic highs. The vertical and lateral distribution of these beds has been used to understand the synsedimentary structural control of the studied stratigraphic succession, represented in the Marnoso-arenacea Formation by subtle topographic highs and depocentres created by thrust-propagation folds and emplacements of large mass-transport complexes.

Diagenetic formation of gypsum and dolomite in a cold-water coral mound in the Porcupine Seabight, off Ireland

Abstract: Authigenic gypsum was found in a gravity core, retrieved from the top of Mound Perseverance, a giant cold-water coral mound in the Porcupine Basin, off Ireland. The occurrence of gypsum in such an environment is intriguing, because gypsum, a classic evaporitic mineral, is undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore-water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21·6% and 28·6% of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment-water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate-rich sediments. These observations demonstrate that fluid flow, steered by environmental factors, has an important effect on the diagenesis of coral mounds.

Middle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco

Abstract: The response of shallow-water sequences to oceanic anoxic event 2 and mid-Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep-water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ13Corg for mid-Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ13C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea-level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow-water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid-Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north-east trade winds in tropical areas.

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures

Abstract: Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea-level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite-rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre-scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite-filled fenestrae; (ii) dolomicrite with bacterium-like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa-like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm-like remains occur in voids between the filaments. The dolomite consistently shows negative d 13 C values (down to )11AE3&) and very positive d 18 O (mean value 7AE9&) that suggest formation as primary precipitate with a substantial contribution of organic CO2. Very negative d 13 C values (down to )31AE6&) of early diagenetic calcite associated with the dolomite suggest contribution of CO2 originating by anaerobic methane oxidation. The shale-normalized rare earth element patterns of Sutera stromatolites show features similar to those in present-day microbial mats with enrichment in light rare earth elements, and M-type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation.

Depositional systems, composition and geochemistry of Triassic rifted‐continental margin redbeds of the Internal Rif Chain, Morocco

Abstract: The Middle to Upper Triassic redbeds at the base of the Ghomaride and Internal ‘Dorsale Calcaire’ Nappes in the Rifian sector of the Maghrebian Chain have been studied for their sedimentological, petrographic, mineralogical and chemical features. Redbeds lie unconformably on a Variscan low-grade metamorphic basement in a 300 m thick, upward fining and thinning megasequence. Successions are composed of predominantly fluvial red sandstones, with many intercalations of quartzose conglomerates in the lower part that pass upwards into fine-grained micaceous siltstones and massive mudstones, with some carbonate and evaporite beds. This suite of sediments suggests that palaeoenvironments evolved from mostly arenaceous alluvial systems (Middle Triassic) to muddy flood and coastal plain deposits. The successions are characterized by local carbonate and evaporite episodes in the Late Triassic. The growth of carbonate platforms is related to the increasing subsidence (Norian-Rhaetian) during the break-up of Pangea and the earliest stages of the Western Tethys opening. Carbonate platforms became widespread in the Sinemurian. Sandstones are quartzose to quartzolithic in composition, testifying a recycled orogenic provenance from low-grade Palaeozoic metasedimentary rocks. Palaeoweathering indices (Chemical Index of Alteration, Chemical Index of Weathering and Plagioclase Index of Alteration) suggest both a K-enrichment during the burial history and a source area that experienced intense weathering and recycling processes. These processes were favoured by seasonal climatic alternations, characterized by hot, episodically humid conditions with a prolonged dry season. These climatic alternations produced illitization of silicate minerals, iron oxidation and quartz-rich red sediments in alluvial systems. The estimated burial temperature for the continental redbeds is in the range of 100 to 160 °C with lithostatic/tectonic loading of ca 4 to 6 km. These redbeds can be considered as regional petrofacies that mark the onset of the continental rift valley stage in the Western Pangea (Middle Triassic) before the opening of the western part of Tethys in the Middle Jurassic. The studied redbeds and the coeval redbeds of many Alpine successions (Betic, Tellian and Apenninic orogens) show a quite similar history; they identify a Mesomediterranean continental block originating from the break-up of Pangea, which then played an important role in the post-Triassic evolution of the Western Mediterranean region.

Fluvial and submarine morphodynamics of laminar and near-laminar flows: a synthesis

Abstract: The interaction of flow with an erodible bed in alluvial rivers and deep-sea channels gives rise to a wide range of self-formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple-row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near-laminar flow analogues of morphologies observed in the field. Laminar-flow analogues of turbulent-flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent-flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.

Controls on platform‐scale patterns of surface sediments, shallow Holocene platforms, Bahamas

Abstract: Isolated carbonate platforms occur throughout geological history, and commonly exhibit considerable spatial variability. To evaluate the controls on the nature of sediment accumulation across the expansive, shallow platform tops, this study systematically compares and contrasts patterns in surface sediments from several shallow (<10 m) Holocene Bahamian examples. Remote-sensing data, field observations, petrographic characterization and quantitative grain-size analyses reveal the spatial patterns of sediment accumulation on Crooked–Acklins Platform and the Berry Islands Bank. Integration of these data with synoptic observations of waves, tides and currents, along with regional geochemical data, provides a means to explore the factors that influence platform-scale sedimentary patterns. These data illustrate that the platform interiors of both Crooked–Acklins Platform and Berry Islands Bank are blanketed with medium to coarse sand size sediment. Peloids are most common in the interior of Crooked–Acklins Platform, whereas the Berry Islands Bank includes more abundant composite grains. In both areas, very little mud is present, with surface sediments averaging <2% mud. Comparison of these results with published data from Little Bahama Bank, Great Bahama Bank and Caicos Platform suggest that, contrary to previous interpretations, the presence of open margins and/or brisk winds are not necessary for the occurrence of a platform top with little mud. Although the muddy sediment fraction of the interior can be suspended by elevated wave energy, wind-generated current speeds in protected platform interiors are relatively low. Instead, in parts of the platform interiors, transport and winnowing of fines is enhanced greatly by tidal currents, which carry suspended sediments off the shallow platforms, even if shielded by islands. Beyond physical influences, however, regional geochemical compilations suggest that the Bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors. This exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids, marine cements and hardened peloids across vast expanses of the platform interiors. Such fundamental controls on Holocene platform-scale sediment dynamics are likely to have influenced carbonate systems through the geological record.

Bedload transport and bed resistance associated with density and turbidity currents

Abstract: Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream-migrating antidunes and downstream-migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open-channel flows and, in fact, is fitted well by an existing relation determined for open-channel flows. In the case of dunes and downstream-migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand-bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream-migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open-channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field-scale sand-bed rivers at bankfull flow and field-scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well.

The action of wind and water in a mid‐Cretaceous subtropical erg‐margin system close to the Variscan Iberian Massif, Spain

Abstract: Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid-Cretaceous outer back-erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain-size distributions, mud playa, ephemeral floodplain, pebble-sand and cobble-sand bedload stream, pebble–cobble-sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub-environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind-driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back-erg depositional system comprises an outer back-erg close to the Variscan highlands, and an inner back-erg close to the central-erg area. The inner back-erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back-erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud-cracked surfaces, resulting in largely structureless sandstones with boulder-size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser-grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back-erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back-erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non-exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti-clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well-preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre-Cretaceous times.

Sea‐level reconstructions from the Peri‐Vocontian Zone (South‐east France) point to Valanginian glacio‐eustasy

Abstract: The Valanginian is a period of global environmental change as illustrated by sedimentary, palaeontological, geochemical and climatic perturbations. A production crisis in most of the carbonate platforms suggests important changes in palaeoenvironmental conditions. During the same time interval, a major positive excursion in δ13C, the Weissert Event, suggests perturbations of the carbon cycle from the latest Early Valanginian to the Early Hauterivian. In order to better understand the link between these changes, sea-level fluctuations have been reconstructed in detail from the Middle Berriasian to the earliest Hauterivian. Sections from the Peri-Vocontian Zone (South-east France) have been investigated because of the good quality of outcrops on the carbonate platforms, their margins and in the Vocontian Basin. Sections ranging from the most proximal zone (Swiss Jura) to the basin were interpreted in terms of sequence stratigraphy and cyclostratigraphy, and correlated at high resolutions. Using the identified small, medium and large-scale sequences as well as depositional geometries, sea-level fluctuations were reconstructed. Two main trends are evidenced during the studied interval: (i) the peak amplitude (magnitude) of the sea-level fluctuations increased gradually from the Middle Berriasian to the Early Valanginian, and reached a maximum (more than 50 m) from the middle Early Valanginian to the Valanginian/Hauterivian boundary; and (ii) sea-level variations were quite symmetrical during the Late Berriasian, slightly asymmetrical during the Early Valanginian and strongly asymmetrical (fast sea-level rise, slow fall) from the latest Early Valanginian to the earliest Hauterivian. Moreover, three orders of sea-level fluctuations were recognized in the sedimentary rocks of the Peri-Vocontian Zone. Platform-basin correlations and cyclostratigraphic interpretations of the basinal sections evidence an astronomical control on the sea-level variations, mainly by the two eccentricity cycles of 100 and 400 kyr. The increase in the amplitude of the sea-level fluctuations and their change from symmetrical to asymmetrical can be related to the onset of a major cooling event in the Early Valanginian. Fast transgressions followed by slower regressions would correspond to waxing and waning of high-latitudinal ice during most of the Valanginian, especially from the latest Early Valanginian to the latest Late Valanginian. Glacio-eustatic sea-level fluctuations in tune with the 100 and 400 kyr eccentricity cycles are in agreement with glaciations during the Valanginian.

Whiting events and the formation of aragonite in Mediterranean Karstic Marine Lakes: new evidence on its biologically induced inorganic origin

Abstract: This paper discusses the formation of whiting events, drifting milky clouds of water, and their role in the formation of recent aragonite sediments in a semi-enclosed, karstic, marine lake on the island of Mljet (Adriatic Sea). This study is based on detailed structural, morphological and sedimentological characterization of, and strontium distribution in, particles originating from suspended matter and sediments. The particles were examined by X-ray diffraction and electron diffraction analyses, scanning and transmission electron microscopy, as well as electron microprobe energy dispersive X-ray analyses and atomic absorption spectroscopy. Morphological features, granulometric characteristics, strontium enrichment and characteristic details of the aragonite structure in the needle-like particles were identical in both the suspended matter collected during whiting events and in the bottom sediments. The whiting events, which occasionally occurred in surface waters, were found to be sites of short-term active authigenic aragonite precipitation and the main source of fine-grained aragonite mud. This study exemplifies the role of biologically induced inorganic precipitation processes in the formation of recent aragonite mud in a restricted Mediterranean environment.

Late Ordovician climbing‐dune cross‐stratification: a signature of outburst floods in proglacial outwash environments?

Abstract: Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddy-related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune cross-stratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbing-dune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

Facies and stacking patterns of shelf‐deltas within the Palaeogene Battfjellet Formation, Nordenskiöld Land, Svalbard: implications for subsurface reservoir prediction

Abstract: Textural and mineralogical properties of the sediment, facies characteristics (both derived from well data), stacking patterns (derived from well and seismic data) and plan-view shapes (derived from seismic data) are all essential components when reconstructing reservoir geometries from subsurface data sets. Without the availability of all these sources of information, reservoir predictions can potentially deviate significantly from the true geometries. In particular, inferences about internal sandbody geometry from plan-view considerations (isopach maps and seismic attribute maps) may lead to erroneous conclusions without knowledge of facies and textural properties of the sediment. Analysis of well-exposed sedimentary systems has the potential to establish links between internal facies characteristics and large-scale geometry and to improve models coupling information at these variable scales. The Palaeogene Battfjellet Formation, Svalbard, has been studied with the aim to unravel internal characteristics and external form by investigating sediment properties, facies and stacking patterns. The formation shows a combination of texturally and mineralogically very immature sediments, a predominance of wave-generated or wave-induced sedimentary structures and a stacking pattern of highly variable numbers of parasequences at localities few kilometres apart that best can be explained in terms of small shifting deltaic lobes that produced a complex pattern of overlapping sandbodies. The strong evidence of wave action in the receiving basin could in itself indicate strike-extensive sandstone bodies; however, the complex sandbody arrangement and the immaturity of the sediments preclude such an interpretation. Traditional facies models coupling plan-view geometries with internal facies characteristics (such as the coupling of strike extensive barrier systems with wave-dominated sedimentary structures or the coupling of elongated fluvial-dominated deltas with offshore-directed current-generated structures) are much too simple and may lead to erroneous interpretations if the complexity revealed by all sources of data is not appreciated.

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

Abstract: The controlling parameters of early marine carbonate cementation in shoal water and hemipelagic to pelagic domains are well-studied. In contrast, the mechanisms driving the precipitation of early marine carbonate cements at deeper slope settings have received less attention, despite the fact that considerable volumes of early marine cement are present at recent and fossil carbonate slopes in water depths of several hundreds of metres. In order to better understand the controlling factors of pervasive early marine cementation at greater water depths, marine carbonate cements observed along time-parallel platform to basin transects of two intact Pennsylvanian carbonate slopes are compared with those present in the slope deposits of the Permian Capitan Reef and Neogene Mururoa Atoll. In all four settings, significant amounts of marine cements occlude primary pore spaces downslope into thermoclinal water depths, i.e. in a bathymetric range between some tens and several hundreds of metres. Radial, radiaxial and fascicular optic fibrous calcites, and radiaxial prismatic calcites are associated with re-deposited facies, boundstones and rudstones. Botryoidal (formerly) aragonitic precipitates are common in microbially induced limestones. From these case studies, it is tentatively concluded that sea water circulation in an extensive, near-sea floor pore system is a first-order control on carbonate ion supply and marine cementation. Coastal upwelling and internal or tidal currents are the most probable mechanisms driving pore water circulation at these depths. Carbonate cements precipitated under conditions of normal to elevated alkalinity, locally elevated nutrient levels and variable sea water temperatures. The implications of these findings and suggestions for future work are discussed.

The incised valley of Baffin Bay, Texas: a tale of two climates

Abstract: Baffin Bay, Texas is the flooded Last Glacial Maximum incised valley of the Los Olmos, San Fernando and Petronila Creeks along the north-western Gulf of Mexico. Cores up to 17 m in length and high-resolution seismic profiles were used to study the history of Baffin Bay over the last 10 kyr and to document the unusual depositional environments within the valley fill. The deposits of the Baffin Bay incised valley record two major and two minor events. Around 8·0 ka, the estuarine environments backstepped more than 15 km in response to an increase in the rate of sea-level rise. Around 5·5 ka, these estuarine environments changed from environments similar to other estuaries of the northern Gulf of Mexico to the unusual suite of environments found today. Another minor flooding event occurred around 4·8 ka in which several internal spits were flooded. Some time after 4·0 ka, the upper-bay mud-flats experienced a progradational event. Because of its semi-arid climate and isolation from the Gulf of Mexico, five depositional environments not found in the other incised-valley fills of the northern Gulf of Mexico are found today within Baffin Bay. These deposits include well-laminated carbonate and siliciclastic open-bay muds, ooid beaches, shelly internal spits and barrier islands, serpulid worm-tube reefs and prograding upper-bay mud-flats. Based on these unusual deposits, and other characteristics of Baffin Bay, five criteria are suggested to help identify incised valleys that filled in arid and semi-arid climates. These criteria include the presence of: (i) hypersaline-tolerant fauna; (ii) aeolian deposits; and (iii) carbonate and/or evaporite deposits; and the absence of: (iv) peat or other organic-rich deposits in the upper bay and bay-margin areas; and (v) well-developed fluvially dominated bayhead deltas.