Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia

Spencer Gulf is a large (ca 22 000 km²), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m^?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m^?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m^?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec^?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide.     

Warm‐temperate,marine, carbonate sedimentation in an Early Miocene,tide‐influenced,incised valley; Provence,south‐east France

The Saumane‐Venasque compound palaeovalley succession accumulated in a strongly tide‐influenced embayment or estuary. Warm‐temperate normal marine to brackish conditions led to deposition of extensive cross‐bedded biofragmental calcarenites. Echinoids, bryozoans, coralline algae, barnacles and benthic foraminifera were produced in seagrass meadows, on rocky substrates colonized by macroalgae and within subaqueous dune fields. There are two sequences, S1 and S2, the first of which contains three high‐frequency sequences (S1a, S1b and S1c). Sequence 1 is largely confined to the palaeovalley with its upper part covering interfluves. Each of these has a similar upward succession of deposits that includes: (i) a basal erosional surface that is bored and glauconitized; (ii) a discontinuous lagoonal lime mudstone or wackestone; (iii) a thin conglomerate generated by tidal ravinement; (iv) a transgressive systems tract series of cross‐bedded calcarenites; (v) a maximum flooding interval of argillaceous, muddy quartzose, open‐marine limestones; and (vi) a thin highstand systems tract of fine‐grained calcarenite. Tidal currents during stages S1a, S1b and S1c were accentuated by the constricted valley topography, whereas basin‐scale factors enhanced tidal currents during the deposition of S2. The upper part of the succession in all but S1c has been removed by later erosion. There is an overall upward temporal change with quartz, barnacles, encrusting corallines and epifaunal echinoids decreasing but bryozoans, articulated corallines and infaunal echinoids increasing. This trend is interpreted to be the result of changing oceanographic conditions as the valley was filled, bathymetric relief was reduced, rocky substrates were replaced as carbonate factories by seagrass meadows and subaqueous dunes, and the setting became progressively less confined and more open marine. These limestones are characteristic of a suite of similar cool‐water calcareous sand bodies in environments with little siliciclastic or fresh water input during times of high‐amplitude sea‐level change wherein complex inboard antecedent topography was flooded by a rising ocean.     

Pliocene sedimentation in a shallow, cool-water, estuarine gulf, Murray Basin, South Australia

The Pliocene Norwest Bend Formation is a well‐preserved succession of terrestrial and shallow‐marine deposits in the Murray Basin, South Australia. Sediments in this unit consist of two discrete terrigenous clastic‐rich, decametre‐scale sequences, or informal members, which record episodes of marine incursion during the Early and Late Pliocene respectively. The base of each sequence is a transgressive lag and/or strandline deposit that is transitional upwards into a highstand, subtidal, terrigenous clastic and cool‐water carbonate sediment accumulation. The top of each sequence is incised by fluvial channels that are filled by river deposits which formed as relative sea‐level fell and terrestrial environments prograded basinward. Sedimentological data suggest that gross stratigraphic architecture was primarily determined by glacioeustasy. Differences in sedimentary style between these two sequences, however, reflect a major climatic change that took place in southern Australia during the mid‐Pliocene. The lower quartzose sand member is formed of siliciclastic sediment derived from prolonged, deep, subaerial weathering and contains a bivalve‐dominated, cool‐temperate, open‐marine mollusc assemblage. These sediments accumulated under an equitable, relatively warm, humid climate. The Murray Basin during this time, because of high fluvial discharge, was a salt‐wedge estuary with typical estuarine circulation. In contrast, the upper, oyster‐rich member is typified by large monospecific oyster buildups that grew in restricted coastal environments. Strandline deposits contain a warm‐temperate skeletal assemblage. Contemporaneous aeolian sediments accumulated under warm, semi‐arid climatic conditions. Well‐developed ferricrete, silcrete and calcrete horizons reflect cyclic conditions of rainwater infiltration and evaporation in the seasonally dry climate that typifies southern Australia today. Highly seasonal rainfall produced an estuary that fluctuated annually from being well to partially mixed. These Pliocene sediments support the notion that mollusc‐rich facies are the signature of cool‐water carbonate accumulations in inboard neritic environments. Unlike bryozoans that dominate the outer parts of Cenozoic cool‐water carbonate shelves, molluscs evolved to exploit an array of coastal ecosystems with wide salinity variations and variable sedimentation rates.     

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.     

Pleistocene aeolianites at Cape Spencer,South Australia; record of a vanished inner neritic cool‐water carbonate factory

Aeolianites are integral components of many modern and ancient carbonate depositional systems. Southern Australia contains some of the most impressive and extensive late Cenozoic aeolianites in the modern world. Pleistocene aeolianites on Yorke Peninsula are sculpted into imposing seacliffs up to 60 m high and comprise two distinct imposing complexes of the Late Pleistocene Bridgewater Formation. The lower aeolianite complex, which forms the bulk of the cliffs, is a series of stacked palaeodunes and intervening palaeosols. The diagenetic low Mg‐calcite sediment particles are mostly bivalves, echinoids, bryozoans and small benthic foraminifera. This association is similar to sediments forming offshore today on the adjacent shelf in a warm‐temperate ocean. By contrast, the upper aeolianite complex is a series of mineralogically metastable biofragmental carbonates in a succession of stacked lenticular palaeodunes with impressive interbedded calcretes and palaeosols. Bivalves, geniculate coralline algae and benthic foraminifera, together with sparse peloids and ooids, dominate sediment grains. Fragments of large benthic foraminifera including Marginopora vertebralis, a photosymbiont‐bearing protist, are particularly conspicuous. Palaeocean temperatures are interpreted as having been sub‐tropical, somewhat warmer than offshore carbonate factories in the region today. The older aeolianite complex is tentatively correlated with Marine Isotope Stage 11, whereas the upper complex is equivalent to Marine Isotope Stage 5e. Marine Isotope Stage 5e deposits exposed elsewhere in southern Australia (Glanville Formation) are distinctive with a subtropical biota, including Marginopora vertebralis. Thus, in this example, palaeodune sediment faithfully records the nature of the adjacent inner neritic carbonate factory. By inference, aeolianites are potential repositories of information about the nature of long‐vanished marine systems that have been removed due to erosion, tectonic obliteration or are inaccessible in the subsurface. Such information includes not only the nature of marine environments themselves but also palaeoceanography.     

Downslope‐migrating sandwaves and platform‐margin clinoforms in a current‐dominated,distally steepened temperate‐carbonate ramp (Guadix Basin,Southern Spain)

During the Late Tortonian, platform‐margin‐prograding clinoforms developed at the south‐western margin of the Guadix Basin. Large‐scale wedge‐shaped deposits here comprise 26 rhythms of mixed carbonate–siliciclastic bedset packages and marl beds. These sediments were deposited on a shallow‐water, temperate‐carbonate distally steepened ramp. A downslope‐migrating sandwave field developed in this ramp, with sandwaves moving progressively down the ramp to the ramp‐slope, where they destabilized, folded and occasionally collapsed. Downslope sandwave migration was induced by currents flowing basinwards. During the Late Tortonian, the Guadix Basin was open north to the Atlantic Ocean via the Dehesas de Guadix Strait and connected east to the Mediterranean Sea through the Almanzora Corridor. According to the proposed current circulation model for the Guadix Basin for this time, surface marine currents from the Atlantic entered the basin from the northern seaway. These currents moved counter‐clockwise and shifted the sediment on the ramp, forming sandwaves that migrated downslope. The development of platform‐margin prograding clinoforms by the basinward sediment‐transport mechanisms inferred here is known relatively poorly in the ancient sedimentary record. Moreover, these wedge‐shaped geometries are similar to those found in some shelves in the Western Mediterranean Sea and could represent an outcrop analogue to (sub)‐recent, platform‐margin clinoforms revealed by high‐resolution seismic studies.     

Cenozoic temperate and sub‐tropical carbonate sedimentation on an oceanic volcano – Chatham Islands,New Zealand

The Chatham Islands, at the eastern end of the Chatham Rise in the South‐west Pacific, are the emergent part of a Late Cretaceous to Cenozoic stratovolcano complex that is variably covered with limestones and fossiliferous tuffs. Most of these deposits accumulated in relatively shallow, high‐energy, tide‐influenced palaeoenvironments with deposition punctuated by periods of deeper‐water pelagic accumulation. Carbonate components in these neritic deposits are biogenic and dominated by molluscs and bryozoans – a heterozoan assemblage. The widespread Middle to Late Eocene Matanginui Limestone contains local photozoan elements such as large benthonic foraminifera (especially Asterocyclina) and calcareous green algae, reflecting the general Palaeogene sub‐tropical oceanographic setting. More localized Late Eocene to Oligocene deposits (Te One Limestone) as well as Pliocene carbonates (Onoua Limestone) are, however, wholly heterozoan and confirm a generally cooler‐water oceanographic setting, similar to today. Early sea floor diagenesis is interpreted to have removed most aragonite components (infaunal bivalves and epifaunal gastropods). Lack of aragonite resulted in the absence of intergranular calcite cementation during subaerial exposure, such that most carbonates are friable or unlithified. Cementation is, however, present at nodular hardground–firmground caps to metre‐scale cycles. Such cements are microcrystalline or micrometre‐thick isopachous circumgranular rinds with insufficient definitive attributes to pinpoint their environment of formation. The overall palaeoenvironment of deposition is interpreted as mesotrophic, resulting in part from upwelling about the Chatham volcanic massif and in part from nutrient element delivery from the adjacent volcanic terrane and coeval volcanism. Biotic diversity in tuffs is two to three times that in limestones, supporting the notion of especially high nutrient availability during periods of volcanism. These mid‐latitude deposits are strikingly different from their low‐latitude, tropical, photozoan counterparts in the volcanic island–coral reef ecosystem. Ground water seepage and fluvial runoff attenuate coral growth and promote microbial carbonate precipitation in these warm‐water settings. In contrast, nutrients from the same sources feed the system in the Chatham Islands cool‐water setting, promoting active heterozoan carbonate sedimentation.     

An epeiric ramp: low-energy, cool-water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia

The Murray Supergroup records temperate‐water carbonate deposition within a shallow, mesotrophic, Oligo‐Miocene inland sea protected from high‐energy waves and swells of the open ocean by a granitic archipelago at its southern margin. Rocks are very well preserved and exposed in nearly continuous outcrop along the River Murray in South Australia. Most facies are rich in carbonate silt, contain a background assemblage of gastropods (especially turritellids) and infaunal bivalves, and are packaged on a decimetre‐scale defined by firmground and hardground omission surfaces. Bioturbation is pervasive and overprinted, resulting in rare preservation of physical sedimentary structures. Facies are grouped into four associations (large foraminiferan–bryozoan, echinoid–bryozoan, mollusc and clay facies) interpreted to represent shallow‐water (<50 m) deposition under progressively higher trophic resource levels (from low mesotrophy to eutrophy), and restricted marine conditions from relatively offshore to nearshore regions. A large‐scale shift from high‐ to low‐mesotrophic conditions within lower Miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow, land‐locked sea. Overall, low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone, a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity. Inshore, heterozoan facies became progressively muddier and restricted towards the shoreline. In contrast, periods of wet climate led to rising trophic resource levels, resulting in a shallowing of the euphotic zone, a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids. Rates of carbonate production and accumulation were relatively low. The Murray Basin is best described as an epeiric ramp. Wide facies belts developed in a shallow sea on a low‐angled slope reaching many hundreds of kilometres in length. Grainy shoal and back‐barrier facies were absent. Internally generated waves impinged the sea floor in offshore regions and, because of friction along a wide and shallow sea floor, created a low‐energy expanse of waters across the proximal ramp. Storms were the dominating depositional process capable of disrupting the entire sea floor.     

Quaternary aeolianites in south‐east Australia – a conceptual linkage between marine source and terrestrial deposition

Calcareous aeolianites are an integral part of many carbonate platforms and ramps. Such limestones are particularly common in heterozoan, Late Cenozoic carbonate systems, and it has been postulated that they could contain a particularly sensitive record of their offshore source. This hypothesis is tested herein by documenting and interpreting part of the most extensive and temporally longest such system in the modern world. The deposits are a combination of extraclasts and biofragments. Extraclasts are detrital quartz, relict allochems, older Pleistocene particles and Oligocene–Miocene limestone clasts. Biofragments are penecontemporaneous coralline algae, echinoderms, small benthic foraminifera, molluscs and bryozoans. The aeolianites differ in composition from distant, open shelf sediments because they contain more mollusc fragments and many fewer bryozoans. This difference is interpreted to be due to (i) most sediment was derived from near‐shore seagrass meadows and macroalgal reefs; (ii) all sediments were modified by hydrodynamics in near‐shore and beach environments; and (iii) fragments of infaunal, beach‐dwelling bivalves were added to the sediment at the strandline. Extraclasts should be expected in older Pleistocene and Cenozoic heterozoan deposits, because the limestones are poorly lithified, largely due to the lack of meteoric cementation, and so easily eroded. Thus, cool‐water aeolianites ought to contain more extraclasts than their warm‐water, tropical cousins. Seagrasses in temperate environments are more productive than in the tropics and thus potentially might contribute many more particles to the beach and dunes than do tropical systems. Although particle breakage in the surf zone cannot be proven, herein the abundance of whole benthic foraminifera and delicate bryozoans implies that suspension and flotsam shoreward transport was an essential process. The similarity of Pleistocene aeolianites over such a long time period herein suggests that the combination of postulated sedimentological, biogenic and hydrodynamic processes could be universally important.     

Carbonate sediments in a cool‐water macroalgal environment,Kaikoura, New Zealand

Brown and red, and to a lesser extent green, macroalgae are a hallmark of intertidal rocky coasts and adjacent shallow marine environments swept by stormy seas in middle and high latitudes. Such environments produce carbonate sediment but the sediment factory is neither well‐documented nor well‐understood. This study documents the general marine biology and sedimentology of rocky coastal substrates around Kaikoura Peninsula, a setting that typifies many similar cold‐temperate environments with turbid waters and somewhat elevated trophic resources along the eastern coast of South Island, New Zealand. The macroalgal community extends down to 20 m and generally comprises a phaeophyte canopy beneath which is a prolific rhodophyte community and numerous sessile calcareous invertebrates on rocky substrates. The modern biota is strongly depth zoned and controlled by bottom morphology, variable light penetration, hydrodynamic energy and substrate. Most calcareous organisms live on the lithic substrates beneath macroalgae or on algal holdfasts with only a few growing on macroalgal fronds. A live biota of coralline red algae [geniculate, encrusting and nodular (rhodoliths)], bryozoans, barnacles and molluscs (gastropods and epifaunal bivalves), together with spirorbid and serpulid worms, small benthonic foraminifera and echinoids produce sediments that are mixed with terrigenous clastic particles in this overall siliciclastic depositional system. The resultant sediments within macroalgal rocky substrates at Kaikoura contain bioclasts typified by molluscs, corallines and rhodoliths, barnacles and other calcareous invertebrates. In the geological record, however, the occurrence of macroalgal produced sediments is restricted to unconformity‐related early transgressive systems tract stratigraphic intervals and temporally constrained to a Cenozoic age owing to the timing of the evolution of large brown macroalgae.     

Spit-platform temperate carbonates: the origin of landward-downlapping beds along a basin margin (Lower Pliocene, Carboneras Basin, SE Spain)

ABSTRACT Lower Pliocene temperate carbonates exhibit landward‐downlapping beds at the southern margin of the Carboneras Basin in south‐eastern Spain. This rarely documented stratal geometry resulted from the accumulation of bedded bioclastic carbonate sand and gravel by longshore currents along a spit platform located a few hundred metres from the palaeoshoreline. The top of the spit platform was covered by shoals that extended over a gently dipping ramp inclined to the north. On the landward slope of the spit, sediments washed over from the shoal area were deposited in parallel‐laminated beds with a southward dip of 8–11°. These beds aggraded and retrograded after an increase in accommodation space, probably related to an Early Pliocene eustatic sea‐level rise. As a result, the beds downlap onto the underlying unconformity surface in a shoreward direction. Eventually, the depression between the shoreline and the spit platform was filled, and a gentle ramp became established. These Pliocene exposures in the Carboneras Basin and a similar Upper Miocene example in southern Spain suggest that landward‐downlapping stratal geometries can be expected in nearshore temperate carbonates along basin margins, and demonstrate a similarity in sedimentary dynamics to siliciclastic sands and gravels.     

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.     

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.     

Sedimentology,palaeoenvironments and biostratigraphy of the Pliocene–Pleistocene carbonate platform of Grande‐Terre (Guadeloupe,Lesser Antilles forearc)

Pliocene and Pleistocene deposits from Grande‐Terre (Guadeloupe archipelago, French Lesser Antilles) provide a remarkable example of an isolated carbonate system built in an active margin setting, with sedimentation controlled by both rapid sea‐level changes and tectonic movements. Based on new field, sedimentological and palaeontological analyses, these deposits have been organized into four sedimentary sequences (S1 to S4) separated by three subaerial erosion surfaces (SB0, SB1 and SB2). Sequences S1 and S2 (‘Calcaires inférieurs à rhodolithes’) deposited during the Late Zanclean to Early Gelasian (planktonic foraminiferal Zones PL2 to PL5) in low subsidence conditions, on a distally steepened ramp dipping eastward. Red algal‐rich deposits, which dominate the western part of Grande‐Terre, change to planktonic foraminifer‐rich deposits eastward. Vertical movements of tens of metres were responsible for the formation of SB0 and SB1. Sequence S3 (‘Formation volcano‐sédimentaire’, ‘Calcaires supérieurs à rhodolithes’ and ‘Calcaires à Agaricia’) was deposited during the Late Piacenzian to Early Calabrian (Zones PL5 to PT1a) on a distally steepened, red algal‐dominated ramp that changes upward into a homoclinal, coral‐dominated ramp. Deposition of Sequence S3 occurred during a eustatic cycle in quiet tectonic conditions. Its uppermost boundary, the major erosion surface SB2, is related to the Cala1 eustatic sea‐level fall. Finally, Sequence S4 (‘Calcaires à Acropora’) probably formed during the Calabrian, developing as a coral‐dominated platform during a eustatic cycle in quiet tectonic conditions. The final emergence of the island could then have occurred in Late Calabrian times.     

Evolution of a carbonate delta generated by gateway‐funnelling of episodic currents

Cool‐water carbonate sedimentation has dominated Mediterranean shelves since the Early Pliocene. Skeletal sand and gravel herein consist of remains of heterozoan organisms, which are susceptible to reworking due to weak early cementation in non‐tropical waters. This study documents the Lower Pleistocene carbonate wedge of Favignana Island (Italy), which prograded from a 5   km wide passage between two palaeo‐islands into a perpendicular, 10 to 15   km wide strait between the palaeo‐islands at one side and Sicily at the other during the Emilian highstand (1·6   Ma to 1·1   Ma). The clinoformed carbonate wedge, which is 50   m thick and 6   km long, formed by east/south‐east progradation of a platform on the submarine sill by currents that were funnelled between the two palaeo‐islands. Platform‐slope clinoforms evolved from initial aggradation (thin and low‐angle) into a progradation phase (thick and high‐angle). Both clinoform types are characterized by a bimodal facies stacking pattern defined by sedimentary structures created by: (i) subaqueous dunes associated with dilute subcritical currents; and (ii) upper‐flow‐regime bedforms associated with sediment‐laden supercritical turbidity currents. Focusing of episodic currents on the platform by funnelling between the islands controlled the downstream formation of a sediment body, here named carbonate delta. The carbonate delta interfingers with subaqueous dune deposits formed in the perpendicular strait. This study uses a reconstruction of bedform dynamics to unravel the evolution of this gateway‐related carbonate accumulation.     

Stratal architecture and platform evolution of an early Frasnian syn‐tectonic carbonate platform,Canning Basin,Australia

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

Late Pleistocene and Holocene cool‐water carbonates of the Western Mediterranean Sea

Post‐glacial, neritic cool‐water carbonates of the Western Mediterranean Sea were examined by means of hydroacoustic data, sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea‐level rise. The analysed areas, located on the Alboran Ridge, in the Bay of Oran, and at the southern shelf of the island of Mallorca, are microtidal and bathed by oligotrophic to weakly mesotrophic waters. Seasonal water temperature varies between 13 °C and 27 °C. Echosounder profiles show that the Bay of Oran and the southern shelf of Mallorca are distally steepened ramps, while the Alboran Ridge forms a steep‐flanked rugged plateau around the Alboran Island. In the three areas, an up to 10 m thick post‐glacial sediment cover overlies an unconformity. In Oran and Mallorca, stacked lowstand wedges occur in water depths of 120 to 130 m. On the Alboran Ridge and in the Bay of Oran, highstand wedges occur at 35 to 40 m. Up to 5 m long cores of upper Pleistocene to Holocene successions were recovered in water depths between 40 and 81 m. Deposits contain more than 80% carbonate, with mixed carbonate‐volcaniclastics in the lower part of some cores in Alboran. The carbonates consist of up to 53% of aragonite and up to 83% of high magnesium calcite. Radiocarbon dating of bivalve shells, coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr bp . The carbonate factories in the three areas are dominated mostly by red algae, but some intervals in the cores are richer in bivalves. A facies rich in the gastropod Turritella, reflecting elevated surface productivity, is restricted to the Mallorca Shelf. Rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m; they form a 10 to 20 cm thick veneer overlying rhodolith‐poor bioclastic sediments which, nonetheless, contain abundant red algal debris. This rhodolith layer has been developing for the past 800 to 1000 years. Similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation. Sedimentary successions in the cores record the post‐glacial sea‐level rise and the degree of sediment exposure to bottom currents. Deepening‐upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth‐indicative red algae. There are only weak downcore variations of carbonate mineralogy, which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface. These new data support the approach of using the Recent facies distribution for interpretation of past cool‐water, low‐energy, microtidal carbonate depositional systems. Hydroacoustic data show that previous Pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea‐level lowstands and accumulated downslope as stacked lowstand wedges; this suggests that, under conditions of high‐amplitude sea‐level fluctuations, the stratigraphic record of similar cool‐water carbonates may be biased.     

Sea‐level rise,depth‐dependent carbonate sedimentation and the paradox of drowned platforms

A mathematical model of carbonate platform evolution is presented in which depth‐dependent carbonate growth rates determine platform‐top accumulation patterns in response to rising relative sea‐level. This model predicts that carbonate platform evolution is controlled primarily by the water depth and sediment accumulation rate conditions at the onset of relative sea‐level rise. The long‐standing ‘paradox of a drowned platform’ arose from the observation that maximum growth rate potentials of healthy platforms are faster than those of relative sea‐level rise. The model presented here demonstrates that a carbonate platform could be drowned during a constant relative sea‐level rise whose rate remains less than the maximum carbonate production potential. This scenario does not require environmental changes, such as increases in nutrient supply or siliciclastic sedimentation, to have taken place. A rate of relative sea‐level rise that is higher than the carbonate accumulation rate at the initial water depth is the only necessary condition to cause continuous negative feedbacks to the sediment accumulation rates. Under these conditions, the top of the carbonate platform gradually deepens until it is below the active photic zone and drowns despite the strong maximum growth potential of the carbonate production factory. This result effectively resolves the paradox of a drowned carbonate platform. Test modelling runs conducted with 2·5 m and 15 m initial sea water depths at bracketed rates of relative sea‐level rise have determined how fast the system catches up and maintains the ‘keep‐up’ phase. This is the measure of time necessary for the basin to respond fully to external forcing mechanisms. The duration of the ‘catch‐up’ phase of platform response (termed ‘carbonate response time’) scales with the initial sea water depth and the platform‐top aggradation rate. The catch‐up duration can be significantly elongated with an increase in the rate of relative sea‐level rise. The transition from the catch‐up to the keep‐up phases can also be delayed by a time interval associated with ecological re‐establishment after platform flooding. The carbonate model here employs a logistical equation to model the colonization of carbonate‐producing marine organisms and captures the initial time interval for full ecological re‐establishment. This mechanism prevents the full extent of carbonate production to be achieved at the incipient stage of relative sea‐level rise. The increase in delay time due to the carbonate response time and self‐organized processes associated with biological colonization increase the chances for platform drowning due to deepening of water depth (> ca 10 m). Furthermore this implies a greater likelihood for an autogenic origin for high‐frequency cyclic strata than has been estimated previously.     

Oxygenation of shallow marine environments and chemical sedimentation in Palaeoproterozoic peritidal settings: Frere Formation,Western Australia

The Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine‐grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea‐level fluctuations that produced five parasequences. Decametre‐scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite‐bearing, hummocky cross‐stratified sandstone that grades upward into interbedded hematite‐rich mudstone and trough cross‐stratified granular iron formation. Each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm‐generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind‐blown sediment input. Iron formation in the Frere Formation, in contrast to most other Palaeoproterozoic examples, was deposited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe‐rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine‐grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe‐rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline. Although essentially unmetamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean‐atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post‐depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.     

Palaeoenvironmental and diagenetic reconstruction of a closed‐lacustrine carbonate system – the challenging marginal setting of the Miocene Ries Crater Lake (Germany)

Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with δ ¹³C–δ ¹⁸O chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back‐scatter electron microscopy were performed to discriminate primary (syn‐)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative δ ¹³C and δ ¹⁸O. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix δ ¹³C and δ ¹⁸O range from ca −7·5 to +4·0‰ and show clear positive covariance (R  = 0·97) whose nature differs from that of previous basin‐oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis.