Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.     

Neogene palaeoceanographic changes recorded in a carbonate contourite drift (Santaren Channel,Bahamas)

The Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats.     

Progradational Holocene carbonate tidal flats of Crooked Island,south‐east Bahamas: An alternative to the humid channelled belt model

The stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits. Representing accumulation at or near sea‐level, these deposits have provided insights into past palaeoenvironments, sea‐level and climate change. To expand understanding of carbonate peritidal systems, this study describes the geomorphology, sedimentology and stratigraphy of the tidal flats on the Crooked‐Acklins Platform, south‐east Bahamas. The Crooked Island tidal flats extend continuously for ca 18 km on the platformward flank of Crooked Island, reaching up to 2 km across. Tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics: (i) supratidal (continuous supratidal crust and pavement); (ii) upper intertidal, with the mangrove Avicennia germinans and the cyanobacteria Scytonema; (iii) lower intertidal (with the mangrove Rhizophora mangal) and (iv) non‐vegetated, heavily burrowed subtidal (submarine). These zones have gradational boundaries but follow shore‐parallel belts. Coring reveals that the thickness of this mud‐dominated sediment package generally is <2 m, with depth to Pleistocene bedrock gradually shallowing landward. The facies succession under much of the tidal flat includes a basal compacted, organic‐rich skeletal‐lithoclast lag above the bedrock contact (suggesting initial flooding). This unit grades upward into rhizoturbated skeletal sandy mud (subtidal) overlain by coarsening‐upward peloid‐foraminifera‐gastropod muddy sand (reflecting shallowing to intertidal elevations). Cores from landward positions include stacked thin indurated layers with autoclastic breccia, root tubules and fenestrae (interpreted as supratidal conditions). Collectively, the data reveal an offlapping pattern on this prograding low‐energy shoreline, and these Holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats. Nonetheless, their vertical facies succession is akin to that present beneath channelled belt examples, suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology, lateral facies changes and heterogeneity in stratigraphic analogues.     

Geomorphology of carbonate platform‐marginal uppermost slopes: Insights from a Holocene analogue,Little Bahama Bank,Bahamas

As the product of a variety of sediment sources and sedimentation (and re‐sedimentation) and erosion processes, the geomorphology and sedimentology of carbonate slopes are highly variable. The purpose of this study is to describe sub‐bottom profiles and side‐scan sonar, multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present‐day platform‐marginal, uppermost slope environments (< 240 m water depth) on the north, open‐ocean facing flank of Little Bahama Bank, Bahamas. Although at time scales of greater than 100 ka this margin is progradational, the data illustrate a complex juxtaposition of erosional and depositional processes and features. Erosion is evidenced by two prominent escarpments (70 m and 120 m) that expose eroded, bedded rocky outcrops. These escarpments are interpreted to represent relict features, related to past sea‐level positions, although they still may be shedding debris. Aside from erosional remnants, sedimentation and active transport is indicated by several features, including active bedforms (especially above the 70 m escarpment, but ripples occur to depths of ca 200 m), several mass transport complexes that overlie and cover the lower escarpment, gravity flow deposits and rare slump features. Similarly, a thick (up to 20 m) onlapping sediment wedge, interpreted to be Holocene in age, suggests lateral accretion of the slope by more than 75 m in this period. Data illustrate that this open‐ocean margin is distinct from windward margins in the Bahamas, which typically include near‐vertical walls of erosion or bypass, flanked downdip by rubble and talus, and leeward margins, which have onlapping muddy wedges, but that lack marked terraces or escarpments. Collectively, the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in Holocene uppermost slope systems, concepts potentially applicable to ancient analogues.     

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.     

Punctuated environmental change along the Holocene (<6 ka) bank margin of south-west Exuma Sound, Bahamas

Characteristics of environmental change along the Holocene (<6 ka BP) bank margin of south‐west Exuma Sound, Bahamas, are defined by comparing mineral, isotope and lithologic stratigraphic patterns in deep‐water (1081 m) slope strata in ODP Hole 631A (Exuma Sound) with proximal (≈6 km) shoal‐water and insular geology of the Lee Stocking Island (LSI) region. After platform submergence, a bank‐top transition from non‐skeletal‐ to skeletal‐dominated deposition occurred in the LSI region by ≈3·8 ka, and is recorded in the adjacent slope sediments by a contemporaneous change in the abundance of transported allochems. The transition at Site 631 is punctuated by an anomalous shift in foraminiferal δ¹³C (–1‰) and δ¹⁸O (–3‰), which may represent a period of climate change that established a brackish coastal zone at least 6 km wide, coincident (≈3·8–4 ka) with bank‐top erosion and pedogenesis. Non‐skeletal allochems reappeared in Hole 631A sediments as the dominant transported allochem type by ≈2·4 ka, then again over the last several hundred years, and are contemporaneous with periods of increased platform‐margin circulation in the LSI region. Such rejuvenation of platform circulation may have resulted from rapid relative rises in sea level, superimposed on the net gradual post‐glacial rise. At Site 631, a prominent (25%) decline in aragonite abundance occurred during the period from 2·4 ka to the last few hundred years, and coincided with an increase in magnesian calcite and skeletal debris, and a positive shift in foraminiferal δ¹⁸O values. These patterns lend support to the hypothesis that, during this period, the LSI margin was barred by skeletal shoals, allowing bank‐top trapping of aragonite and hypersaline water, with increased coastal salinity (resulting from reflux of hypersaline waters through the shoals) and/or cooled coastal waters (caused by reduced exchange of warmed bank‐derived water relative to wind‐generated upwelling). Along the Exuma slope, a diagenetic overprint may further enhance the recent decline in aragonite accumulation. Integration of Holocene deep‐water slope and bank‐top stratigraphies along the bank margin of south‐west Exuma Sound reveals a pattern of punctuated environmental change superimposed on longer term development of interglacial conditions.     

Silicification of sulphate evaporites and their carbonate replacements in Eocene marine sediments, Tunisia: two diagenetic trends

Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ¹³C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ¹⁸O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ¹⁸O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ¹⁸O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ¹⁸O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ²⁹Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.     

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 10⁵ m³ of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s^?1. At least 300–400 m³ of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.     

Geomorphology of a modern carbonate slope system and associated sedimentary processes: Example of the giant Great Abaco Canyon,Bahamas

The large acoustic data set acquired during the Carambar cruises is composed of high resolution bathymetry, backscatter data and very‐high resolution seismic lines which allow for an overview of the morphology and sediment transfer processes from the shallow upper slope to the abyssal plain of a modern carbonate system: the north‐eastern slope of the Little Bahama Bank. Surficial distribution of the acoustic facies and echofacies reflects a wide variety of sedimentary processes along and across the slope. The western sector of the Little Bahama Bank is dominated by depositional processes whereas its eastern sector, which is incised in the lower slope by giant canyons, is affected by erosion and bypass processes. Datasets suggest that currents play an important role both in along‐slope sedimentary processes and in the abyssal plain. The Antilles Current appears to affect a large part of the middle and lower slopes. The absence of sizeable present‐day channel/levée complexes or lobes at the mouth of the canyon – revealed by the bathymetric map – indicates that the southward flowing Deep Western Boundary Current influences modern abyssal sediment deposition. Based on depositional processes and indicators of canyon maturity observed in facies distribution, the current study proposes that differential subsidence affects the eastern versus western part of the bank. The morphology of the Great Abaco Canyon and Little Abaco Canyon, which extend parallel to the platform, and the Little Bahama Bank slope appears to be related to the Great Abaco Fracture Zone.     

Environmental controls on the Late Holocene carbonate sedimentation of a karstic lake in the Middle‐Atlas Mountains (Lake Afourgagh,Morocco)

The Lake Afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation. Afourgagh is a small, shallow permanent lake located in the Middle‐Atlas Mountains in Morocco in a karstic context. It is fed by ground waters that are relatively enriched in Mg resulting from the leaching of the Jurassic dolomitic bedrock of the catchment. This eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high Mg/Ca ratio. The maximum extension of the Holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi. Lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies: a littoral crust, palaeosols, palustrine silts and charophyte tufas, which reflect different environments from the shoreline toward the deeper water. In the more distal parts, the charophyte tufas display a well‐expressed lamination punctuated by the development of microstromatolites on algae thalli. The mineralogical composition of the carbonates is linked to the facies. While the charophyte tufas are characterized by a relatively high content in aragonite, in addition to low‐Mg calcite, the littoral crust is mainly composed of magnesite. This pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers. Calcium‐carbonate precipitation on algae thalli (both bioinduced and microbially mediated) progressively induces an increase in the Mg/Ca ratio of the lake water, while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline, producing magnesite during early diagenesis. This effect is characteristic of two episodes: part of the Roman Warm Period and the beginning of the Dark Age Cold Period. The carbonate mineralogy of the different depositional sequences at Afourgagh indicates lake‐level and water‐chemistry fluctuations under a climatic influence. Therefore, among other regional records, the Lake Afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes.     

Development of secondary porosity in the Fairholme carbonate complex (southwest Alberta, Canada)

Because of their economic importance as hydrocarbon reservoirs, the Upper Devonian dolomitized carbonate reefs in southwest Alberta have been the subject of several studies. Still, there is no consensus on the process of matrix dolomitization and furthermore, the process of vug development is not often addressed. The studied outcrops show features of an early diagenetic matrix-selective dolomitization by a Late Devonian seawater-derived fluid. Seepage reflux dolomitization combined with latent reflux is proposed, which best explains most chemical characteristics. The cements in the vugs are precipitated from warm saline, ⁸⁷Sr-enriched fluids and testify to thermogenic sulphate reduction based on the presence of sulphur, CO₂ and H₂S in inclusions, relatively high homogenization temperatures and depleted δ¹³C values, which sets constraints on the timing of vug formation. Secondary porosity may be created by the mixing of formation water with a tectonically and topographically driven fluid and by the dissolution of anhydrite nodules.     

Geochemical signal of seasonality in annually laminated organic- carbonate sediments in Shira lake(Khakasia)

正1 Introduction The most valuable for the task of climate reconstruction are the time series with an annual resolution,which allows to reveal natural periodicity and pass to the search for mechanisms of regional and global climatic changes.Bottom sediments of lakes are one of the best climate archives in addition to tree ring series,ice cores etc.     

Submarine lobes and feeder channels of redeposited, temperate carbonate and mixed siliciclastic-carbonate platform deposits (Vera Basin, Almería, southern Spain)

Temperate carbonates and mixed siliciclastics-carbonates of Upper Tortonian age were deposited on a narrow platform along the southeastern margin of the Sierra de los Filabres on the western side of the Vera Basin. The temperate carbonates were unlithified or were only weakly lithified on the seafloor and so were easily prone to synsedimentary removal. Part of the shelf sediments were eroded, reworked and redeposited in submarine lobes, up to 40 m thick and 1 km wide. The lobes consist of turbiditic carbonates (calcarenites and calcirudites) and mixed siliciclastics-carbonates, which contain up to 30% siliciclasts, derived from the Sierra de los Filabres to the northwest, and abundant bioclasts of coralline algae, bivalves and bryozoans. In the inner platform, the feeder channels of the lobes cross-cut beach and shoal deposits, and are filled by strings of debris flow conglomerates (up to 3 m thick and a few metres wide). These channels presumably developed as the continuation of river courses entering the sea. Further towards the outer platform, they pass into large channels (up to several hundred metres wide and 20 m deep) steeply cutting into the horizontally bedded strata of the platform. Significant quantities of platform sediment were removed by erosion during their excavation. Once abandoned, they were filled by new platform sediments. Further towards the basin, the channels associated with the lobes exhibit lateral accretion and internal cut-and-fill structures, and are intercalated between hemipelagic deposits. The channel-filling sediments are in this latter case coarse-grained carbonates and mixed siliciclastics-carbonates. Lobe development concentrated first at Cortijo Grande on the western side of the study area, and then to the east at Mojácar. This migration may relate to the uplift of the Sierra Cabrera, a major high occurring immediately to the south of the channel and lobe outcrops.     

Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene,Belgium)

The septarian carbonate concretions from the Boom Clay (Belgium) consist mainly of authigenic minerals such as micrite ( 70% bulk volume) and pyrite framboids ( 3%). These mineral phases occur between detrital grains and fossils. The septarian cracks are lined with calcite, which is sometimes covered with pyrite. The preservation of delicate sedimentological features in the concretion matrix (hardly compacted faecal pellets, burrows and uncrushed shells) points to an early origin of the concretions. Systematic geochemical variations from concretion centre to edge suggest that growth continued during shallow burial. The13C values (–17.5 to –20.5) of the concretionary carbonate show that bacterial sulphate-reduction processes were dominant. Sulphate-reduction-derived HCO3- was diluted by marine-related HCO3-, derived from dissolved bioclasts. A slight enrichment in 13C during growth is caused by the decreasing influence of sulphate reduction because of the progressive closure of the diagenetic system due to shallow-burial compaction. The 18O values (–0.5 to +1.0) of the concretionary carbonate point to a marine origin. The slightly 18O-depleted signature with respect to time-equivalent marine-derived carbonate relates to the incorporation of an 18O-depleted component, originating from sulphate and organic matter. The slight decrease in 18O during growth relates to an increasing influence of this component and to a decreasing influence of seawater-derived oxygen during early diagenesis.     

Microbial fossilization in carbonate sediments: a result of the bacterial surface involvement in dolomite precipitation

ABSTRACT Recent dolomitic sediment samples from Lagoa Vermelha, Brazil, were examined microscopically to study the process of bacterial fossilization in carbonate sediments. Bacteria‐like bodies were intimately associated with carbonate mineral surfaces, and coatings on the former demonstrate the calcification of single bacterial cells. The bacterial fossilization process in Lagoa Vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate‐reducing bacteria, which were isolated from the Lagoa Vermelha sediments. These cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process. In mixed culture experiments, bacterial colonies became calcified, whereas in pure culture experiments, single bacterial cells were associated with dolomite surfaces. Dolomite nucleated exclusively in bacterial colonies, intimately associated with extracellular organic matter and bacterial cells. Electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of Ca²⁺ and Mg²⁺ onto the cell surfaces, indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization. The affinity of the cells for Mg²⁺ was related to the capability of the strains to mediate dolomite formation. Combined with sulphate uptake, which dissociates the [MgSO₄]⁰ ion pair and increases the Mg²⁺ availability, the concentration of Mg²⁺ ions in the microenvironment around the cells, where the conditions are favourable for dolomite precipitation, may be the key to overcome the kinetic barrier to dolomite formation. These results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation, implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates.     

Sedimentary processes in a submarine canyon excavated into a temperate‐carbonate ramp (Granada Basin,southern Spain)

During the Late Tortonian, shallow‐water temperate carbonates were deposited in a small bay on a gentle ramp linked to a small island (Alhama de Granada area, Granada Basin, southern Spain). A submarine canyon (the ‘Alhama Submarine Canyon’) developed close to the shoreline, cross‐cutting the temperate‐carbonate ramp. The Alhama Submarine Canyon had an irregular profile and steep slopes (10° to 30°). It was excavated in two phases reflected by two major erosion surfaces, the lowermost of which was incised at least 50 m into the ramp. Wedge‐shaped and trough‐shaped, concave‐up beds of calcareous (terrigenous) deposits overlie these erosional surfaces and filled the canyon. A combination of processes connected to sea‐level changes is proposed to explain the evolution of the Alhama Submarine Canyon. During sea‐level fall, part of the carbonate ramp became exposed and a river valley was excavated. As sea‐level rose, river flows continued along the submerged, former river‐channel, eroding and deepening the valley and creating a submarine canyon. At this stage, only some of the transported conglomerates were deposited locally. As sea‐level continued to rise, the river mouth became detached from the canyon head; littoral sediments, transported by longshore and storm currents, were now captured inside the canyon, generating erosive flows that contributed to its excavation. Most of the canyon infilling took place later, during sea‐level highstand. Longshore‐transported well‐sorted calcarenites/fine‐grained calcirudites derived from longshore‐drift sandwaves poured into and fed the canyon from the south. Coarse‐grained, bioclastic calcirudites derived from a poorly sorted, bioclastic ‘factory facies’ cascaded into the canyon from the north during storms.     

Heterozoan carbonate deposition on a steep basement escarpment (Late Miocene,Almería,south‐east Spain)

Heterozoan temperate‐water carbonates mixed with varying amounts of terrigenous grains and muddy matrix (Azagador limestone) accumulated on and at the toe of an inherited escarpment during the late Tortonian–early Messinian (late Miocene) at the western margin of the Almería–Níjar Basin in south‐east Spain. The escarpment was the eastern end of an uplifting antiform created by compressive folding of Triassic rocks of the Betic basement. Channelized coralline‐algal/bryozoan rudstone to coarse‐grained packstone, together with matrix‐supported conglomerate, are the dominant lithofacies in the higher outcrops, comprising the deposits on the slope. These sediments mainly fill small canyon‐shaped, half‐graben depressions formed by normal faults active before, during and after carbonate sedimentation. Roughly bedded and roughly laminated coralline‐algal/bryozoan rudstone to coarse‐grained packstone are the main lithofacies forming an apron of four small (kilometre‐scale) lobes at the toe of the south‐eastern side of the escarpment (Almería area). Channelized and roughly bedded coralline‐algal/bryozoan rudstone to coarse‐grained packstone, conglomerates, packstone and sandy silt accumulated in a small channel‐lobe system at the toe of the north‐eastern side of the escarpment (Las Balsas area). Carbonate particles and terrigenous grains were sourced from shallow‐water settings and displaced downslope by sediment density flows that preferentially followed the canyon‐shaped depressions. Roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope, whereas the rest of the carbonate lithofacies were deposited by high‐density turbidite currents. The steep escarpment and related break‐in‐slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse‐grained, low‐transport efficiency skeletal‐dominated sediment in the apron lobes. Accelerated uplift of the basement caused a relative sea‐level fall resulting in the formation of outer‐ramp carbonates on the apron lobes, which were in turn overlain by lower Messinian coral reefs. The Almería example is the first known ‘base of slope’ apron within temperate‐water carbonate systems.     

Carbonate shedding and sedimentary cyclicities of a distally steepened carbonate ramp (Miocene, Great Bahama Bank)

Received: 10 May 1999 / Accepted: 16 October 1999     

Climatic and tectonic controls on carbonate deposition in syn‐rift siliciclastic fluvial systems: A case of microbialites and associated facies in the Late Jurassic

This work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins. Sedimentological and stable‐isotope data of microbialites and associated carbonate facies, along with regional geological information, are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings. The Vega Formation – a 150 m thick Lower Kimmeridgian siliciclastic fluvial sequence in Asturias Province (northern Spain) – constitutes a candidate for this approach. This unit includes varied facies (stromatolites; rudstones, packstones and wackestones containing oncoids, intraclasts, charophytes and shell bioclasts; marlstones and polygenic calcareous conglomerates) that formed in a low‐gradient fluvial–lacustrine system consisting of shallow, low‐sinuosity oncoid‐bearing channels and pools within marshy areas, with sporadic coarse alluvial deposition. The sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water‐discharge variations. The carbonate fluvial–lacustrine system developed near uplifted marine Jurassic rocks. The occurrence of the system was conditioned by normal faults (active during the deposition of the unit) that favoured: (i) springs of HCO₃–Ca‐rich water from a Rhaetian–Sinemurian carbonate rock aquifer; and (ii) carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system. The microbialite δ¹³C and δ¹⁸O values support deposition in a hydrologically open system, fed by ambient‐temperature meteoric water, with riparian vegetation. Three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities. The textural pattern of lamination parallels δ¹³C and δ¹⁸O changes, suggesting short‐term cycles of precipitation and temperature. A moderately to strongly contrasted seasonal and/or pluriannual precipitation regime is inferred from the cyclic δ¹³C pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids. Thus, the isotopic and sedimentological attributes of the carbonate deposits were linked to short‐term climate changes associated with semi‐arid conditions, consistent with the studied climatic zone.     

Marine carbonate systems in the Sarmatian (Middle Miocene) of the Central Paratethys: the Zsámbék Basin of Hungary

The study of eight stratigraphic sections at the margin of the semi-enclosed Zsámbék 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.