Bioclastic Deposits Research Articles

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

AbstractPost‐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.

Timing of uplift of the Troodos Massif (Cyprus) constrained by sedimentary and magnetic polarity evidence

Abstract: Plio-Pleistocene sediments exposed around the ophiolitic Troodos Massif document spectacular uplift from below sea level to a maximum height of c . 2000 m. Sedimentation reflects a dominant control of focused tectonic uplift, modified by the effects of glacio-eustatic sea-level change and climatic change. Understanding the uplift and controls on deposition has been hindered by an inadequate age model. Here, we present a preliminary investigation of the polarity of remanent magnetizations recorded within the upper part of the Pliocene succession in the Pissouri and Mesaoria basins and from the Pleistocene marine terrace deposits that border the Troodos Massif. When integrated with available lithostatigraphic and biostratigraphic data, the main results are as follows. Focused uplift of the Troodos Massif began during the Late Pliocene, either between 2.14 and 1.95 Ma or immediately prior to 1.77 Ma. Shallow-marine, ophiolite-derived clastic and bioclastic sediments accumulated in both the Mesaoria and Pissouri basins, implying that the Troodos Massif was uplifted as a single tectonic entity. Non-marine, deltaic and fluvial facies prograded into both of the basins during the Pleistocene (1.77 Ma–recent). Marine terraces in SW and south Cyprus were cut and covered by littoral sediments from <0.78 Ma, suggesting that high rates of uplift of the Troodos Massif persisted into mid- and late Pleistocene time.

BENTHIC FORAMINIFERAL BIOFACIES IN TEMPERATE CARBONATE SEDIMENT IN THE WESTERN PONTINE ARCHIPELAGO (TYRRHENIAN SEA, ITALY)

Recent foraminiferal assemblages are identified in 70 grab samples from 20–380 m depth in the Pontine Archipelago (Tyrrhenian Sea, Italy). All broken and relict tests of benthic foraminifera and other bioclastic sediments were counted for each sample, in order to provide a more complete description of the benthic community. Hierarchical cluster analysis (HCA) was performed on the relative abundances (live + dead) of 53 taxa of benthic foraminifera that were common in the samples. Q-mode HCA recognizes three distinct foraminiferal biofacies, each associated with a particular bioclastic lithofacies: 1) the Asterigerinata-Lobatula-Rosalina biofacies occurs at 20–150 m and is dominated by benthic foraminifera, most notably epiphytic species such as Asterigerinata mamilla , Lobatula lobatula , and Rosalina bradyi ; other bioclasts include rhodoliths, bryozoans, and echinoid spines, which constitute the rhodalgal lithofacies; 2) the Cassidulina carinata biofacies at intermediate depths of 100–250 m includes benthic and planktonic foraminifera among branches of bryozoans that constitute the bryomol lithofacies; also present are high percentages of reworked glauconitic tests of shallow-water taxa (e.g., Elphidium , Lobatula , and Asterigerinata ), possibly related to Late Quaternary sea-level still-stands; 3) the Uvigerina-Bulimina biofacies at 200–380 m, that is characterized by Uvigerina mediterranea and Bulimina marginata ; in addition to benthic and planktonic foraminifera, it has a molechfor lithofacies of bivalves and gastropods.

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

AbstractThe Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross‐stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out‐of‐phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol‐type cool‐water carbonate factories. The cross‐strata sets represent small to medium‐sized (10 to 60 cm thick) two‐dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud‐bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest‐term, diurnal and semi‐diurnal tidal cycles. Thirdly, the record of diurnal and semi‐diurnal tidal cycles is included within the pattern of neap‐spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud‐free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub‐tidal environment of straits and embayments and the shortest‐term tidal cycles.

Formation, infill, and dissection of a latest-Pleistocene landslide-dammed reservoir (Betic Cordillera, Southern Spain): Upstream and downstream geomorphological and sedimentological evidence

A slope collapse in a narrow, mountainous valley led to the formation of a latest-Pleistocene (14,500 cal. BP) lake dammed behind this mass-movement deposit in the Betic Cordillera (SE Spain). The lithostructural conditions of the carbonate bedrock, a warm and more humid period at the end of the last glaciation (Bølling interstadial), and its location very close to a seismic mountain front provide the ideal context for the destabilization and failure of bedrock and the subsequent formation of this ephemeral mountain basin. It was filled in 260 years. Estimated specific sediment yield (SSY) is at least two orders lower than SSY nowadays in SE Spain. The calculated value of 81 t/km 2/y is similar to calculated values from artificial reservoirs in more humid and temperate drainage areas, such as in Central Europe. Geomorphological and sedimentological features of the siltation of the landslide-dammed reservoir have been preserved close to the Betic Cordillera mountain front. The reservoir fill consists of over 30 m of lacustrine and alluvial deposits. The lake created upstream of the landslide dam was silted by mainly terrigenous (Gilbert-type delta gravels and turbiditic sands) and mixed terrigenous–bioclastic sedimentation (horizontally laminated carbonate mudstones). After the lake was silted, a meandering river caused linear erosion over the natural dam and deeply dissected the lacustrine deposits above the river’s pre-landslide profile. This process has allowed the study of the well-preserved internal structure of the natural reservoir and identification of the stages in the construction and degradation of a semi-closed basin. The stages involved in the evolution of the landslide-dammed reservoir may provide insights into studying the development of other closed or semi-closed basins such as mountain basins or artificial dams.

A Record of Holocene Benthic Foraminifera in Vibrocore SG#279, Spencer Gulf, South Australia

Core SG#279 recovered 1.1 m of Holocene bioclastic sediment from northern Spencer Gulf, South Australia. Benthic foraminifera preserved in these sediments provide proxies for the postglacial marine transgression. Species that are characteristic of tidal dominated, coastal seagrass meadows, Cribrobulimina mixta + Nubecularia lucifuga + Peneroplis planatus + Discorbis dimidiatus decrease in numbers upcore confirming their potential to signify a rising sea. Massilina milletti, which favours the deeper waters of northern Spencer Gulf, increases in abundance upcore, thus also signifying the transgression. Quinqueloculina lamarckiana, which positively correlates with increasing water depth in northeastern Gulf St Vincent, similarly provides evidence of the transgression. Maximum water depth is indicated for the core sediments at 40 cm. Data based on the distributions and relative numbers of Elphidium crispum and Elphidium macelliforme were not in accord with those of the other proxy species. It is concluded that the Elphidium ratio is unreliable when applied to sediments deposited at depths of <15 m. Above 40 cm in the core, the distribution of M. milletti provides evidence of marine regression that has been attributed to hydroisostatic uplift of northern Spencer Gulf. The foraminifera provide no credible evidence of any late Holocene sea level rise.

Mangroves as an agent of rapid coastal change in a tidal-dominated environment, Gulf St Vincent, South Australia: implications for coastal management

The northeastern shore of Gulf St Vincent, South Australia is a mesotidal, low-wave-energy coastline, characterised by tidal-dominated bioclastic carbonate sedimentation. Distinctive assemblages of subtidal, intertidal and supratidal marine plants and invertebrate animals have resulted in well defined, adjacent bio- and litho-facies that lie parallel to each other across broad areas of coastal sedimentation. At Port Gawler, the intertidal zone is bounded by a landward high-tide beach and a subtidal Posidonia australis seagrass meadow seaward. Thirty years ago, within the immediate study area, this intertidal zone comprised an inner sandflat and an outer muddy seagrass meadow dominated by Zostera muelleri, the two areas separated by a shallow tidal distributary. Avicennia marina is a species of mangrove that has colonised much of the protected, tidal-dominated coasts of South Australia. Propagules of this tree readily take root in intertidal mud, with a natural ecological succession of Z. muelleri followed by A. marina. Over the time of observation, juvenile mangroves have progressively colonised a narrow zone along the landward side of the muddy seagrass meadow; they appear unable to colonise a sandy substrate. Juveniles advanced rapidly southward from mature woodland in the north towards equally mature woodland in the south. From the initial tap root, juvenile plants send out radial roots from which project, snorkel-like, an array of vertical pneumatophores. Redistributed algal, seagrass and other organic flotsam collect around these vertically protruding pneumatophores, promoting accumulation of organic sediment. Within the newly forming mangrove woodland, there has been pronounced sediment aggradation of >0.5 m, greatly altering the dynamics of tidal flow. Tidal flow is now largely constrained to a channel between the mature southern woodland and that formed most recently. Higher velocity flows through this channel have exhumed populations of infaunal bivalves, and floodtide and ebbtide deltas of coarse bioclastic sediments are a consequence. The aggraded sediment of the new woodland shelters the area landward. Consequently, more muddy sediments are now covering what was formerly bare sandflat, thus providing new areas for mangrove colonisation. Migration of mangroves towards the high-tide beach is inevitable and will be limited only by their need for regular flushing of accumulated salt from their roots by seawater at high tide. Thus, A. marina drives rapid geomorphic change in the tidal-dominated coasts of the South Australian gulfs. In similar coastal areas of southeast Asia, where intertidal muddy sediments predominate, this species appears to be an ideal candidate for reforestation projects. Juvenile plants should grow successfully and rapidly, so preparing coastal environments for the establishment of more ecologically diverse mangrove communities.

Early Ordovician reef construction in Anhui Province, South China: A geobiological transition from microbial- to metazoan-dominant reefs

The Lower Ordovician records a transition from microbial- to metazoan-dominant reefs immediately prior to the Great Ordovician Biodiversification Event. The Hunghuayuan Formation (late Tremadocian-early Floian) of the Jianxin section in Anhui Province, South China, includes well-preserved reefs, which consist of a combination of microbial boundstones and microbial-lithistid sponge-receptaculitid boundstones. The microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of Girvanella-rich layers and thick micrite layers in which the filamentous calcimicrobe Girvanella and bioclasts occur sporadically. Microbial biofilms, including the filamentous microbes, may have trapped and bound lime mud and/or precipitated micrite during their growth and metabolic activities. These activities contemporaneously contributed to substrate stabilisation, encrustation, and the construction of microbialite frameworks (stromatolites). In contrast, the microbial-lithistid sponge-receptaculitid boundstones are characterised by more micritic constituents than skeletal reef-building constituents. Thin and/or domal crusts produced by peloid and dense micrite, together with various proportions of Girvanella, occur upon bioclastic sediments and reef-building skeletal organisms. These micritic crusts with Girvanella therefore played roles as binders and stabilisers, and as consolidators by encrusting reef-building skeletal organisms. In the boundstones, it is remarkable that the degradation of lithistid sponges, due to microbial decay, produced micrite in the sponge bodies. As a result, lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors. These Lower Ordovician reefs, in which microbialites (e.g., stromatolites) dominate but coexist with metazoans (e.g., sponges), were widespread at the time. They provide excellent examples for understanding reefal palaeoecology during the transition from microbe-dominant reefs to reefs enriched in metazoans.

Filament lumachelle on top of Middle Jurassic oolite limestones: event deposits marking the drowning of a Tethysian carbonate platform (Subbetic, southern Spain)

Bioclastic accumulations often occur on top of Tethysian carbonate platforms and crinoids are a common constituent of these bioclastic deposits on Lower and Middle Liassic carbonate platforms. In contrast, the relevant literature contains few examples in which the main constituent of the lumachels is thin-shelled bivalves (filaments). This paper presents a study of a filament lumachelle cropping out on top of a Middle Jurassic carbonate platform. The carbonate platform is represented by the Jabalcuz Formation, found in one of the northernmost Subbetic units (south of Jaen city). The lumachelle marks the demise of the carbonate platform and has special features that characterize the drowning phase. This process has been related with the syn-rift extensional tectonics associated to the opening of the Tethys westwards. Stratigraphically, the lumachelle occurs on top of shallow-water oolite limestones (Middle Jurassic) and is overlain by radiolarian-rich pelagic and resedimented deposits (Latest Callovian–Oxfordian). It occurs as a body (about 1.5 km wide and up to 8 m thick) made up entirely of densely packed thin bivalve shells. A remarkable feature of the bivalve shell beds is stromatolite-like crumpled lamination at the outcrop. The observations made at the outcrop scale, by microscope under transmitted light, and by cathodoluminiscence favor a diagenetic origin for this striking structure. Other hypotheses, such as its possible relation with seismicity, cannot be confirmed. The filaments would have filled one of the former basins that originated in relation with syn-rift fault-block tectonics leading to the demise of the carbonate platform. Tectonics was one of the main factors setting in motion a carbonate productivity crisis and the inhibition of a diverse benthic community. Once production failed in the carbonate factory, storms and probably hurricanes as well, swept shell deposits from the shallowest areas of the shallow-water carbonate platform and accumulated them in a coevally formed small half-graben basin. At least three main depositional stages can be differentiated in the fill of this half-graben basin, which was a sediment trap for the accumulation and preservation of the tiny bivalve shells against ebb surges. The massive accumulation of valves, the shortage of micrite around the filaments, outcrop morphology and facies relationships, along with regional geology, are among the arguments supporting this interpretation.

Very high‐resolution seismo‐acoustic imaging of seagrass meadows (Mediterranean Sea): Implications for carbon sink estimates

Posidonia oceanica is a widespread coastal Mediterranean seagrass which accumulates in its subsurface large quantities of organic material derived from its roots, rhizomes and leaf sheaths embedded in sandy sediments. These organic deposits may be up to several meters thick as they accumulate over thousands of years forming the matte, whose high content in organic carbon plays a major role in the global ocean carbon cycle. In this study, very high‐resolution seismo‐acoustic methods were applied to image the subsurface features of a P. oceanica seagrass meadow at Portlligat (Cadaqués, Girona, Spain), in the NW‐Mediterranean Sea. Our findings yield fresh insights into the settling of the P. oceanica meadow in the study area, and define with unprecedented detail the potential volume occupied by the matte. A strong reflector, located from 4.3 to 11.7 m depth, was recognized in several seismo‐acoustic profiles as the substratum on which P. oceanica first settled in the study area. A 3D bathymetric model of this substratum allowed us to reconstruct the Portlligat palaeo‐environment prior to the settling of P. oceanica, which corresponded to a shallow coastal setting protected from the open sea. A core drilled in the meadow at Portlligat revealed the presence of a 6 m thick dense matte composed of medium to coarse sandy sediments mixed with plant debris and bioclasts. Radiocarbon datings revealed a constant accretion rate of the matte of about 1.1 m/kyr. Gravelly bioclastic deposits observed at the base of the core correspond to the base of the matte and gave a date of 5616 ± 46 Cal yr BP. For the first time, very high‐resolution marine geophysical techniques allowed us to accurately define the volume occupied by P. oceanicamatte, which in the study area reaches up to almost 220,000 ± 17,400 m3. This result is an important step forward in our efforts to estimate the size of the carbon sink represented by P. oceanica meadows along the Mediterranean coasts significantly contributing to the biosphere carbon cycle.

Le Devonien Inferieur des Ouled Abbou (Maroc). Organisation sédimentaire, modalités diagénétiques et impact sur la porosité

La region de Ouled Abbou se caracteriza por la intercalacion de niveles carbonatados en pelitas silurico-devonicas. Esta serie esta estructurada en una secuencia de tres niveles de depositos: SD1, SD2, SD3, que son respectivamente de edad siluro-lochkoviense, praguiense y emsiense, desarrollados en una plataforma mixta de typo rampa monoclinal. La hidrodinamica, en relacion con las variaciones del nivel del mar, controlo la formacion y distribucion de las facies carbonatadas (depositos bioclasticos, de pendiente, etc.). El analisis de los niveles carbonatados ha permitido establecer la secuencia de desarrollo de las tranformaciones diageneticas. Estas son esencialmente el resultado de multiples fases de cementacion y recristalizacion, que afectan principalmente a las facies granulares. La dolomitizacion temprana aparece a techo de las secuencias SD2 y SD3, produciendo una roca cristalina (dolomicroesparita), seguido de un reemplazamiento-recristalizacion del lodo carbonatado. Estas transformaciones son mas tarde afectadas por compactacion, fracturacion tectonica y dolomitizacion tardia asociada. El resultado final es una litificacion practicamente completa, de ahi su bajo potencial como reservorio.

PLIOCENE ALLUVIAL TO MARINE DEPOSITS OF THE VAL D’ORCIA BASIN (NORTHERN APENNINES, ITALY): SEQUENCE STRATIGRAPHY AND BASIN ANALYSIS

Since the Middle-Late Miocene, the Apennine chain was characterised by the development of several basins. These basins have been alternatively explained as extensional post-orogenic basins or syn-orogenic compressional basins. In this framework, the Pliocene depositional history of the Val d'Orcia Basin (Southern Tuscany) is described, based on a sequence stratigraphic approach. Three depositional sequences (Vd3a-c) have been recognized. The Vd3a sequence records marine ingression and subsequent relative sea-level highstand in a narrow W-E trending depression. Subsequent relative sea-level fluctuations led to a progressive basin widening and deposition of Vd3b and Vd3c sequences. Basin scale sedimentation was mainly controlled by relative sea-level changes, although accumulation of bioclastic deposits of the Vd3c sequence could have been influenced by climatic factors. Based on its wide bowl-shaped geometry, the Pliocene Val d'Orcia Basin has been interpreted as an inherited depression. This depression is thought to be originated by erosion at the Late Miocene-Early Pliocene transition.

Stratigraphy and sedimentology of Muschelkalk carbonates of the Southern Iberian Continental Palaeomargin (Siles and Cehegín Formations, Southern Spain)

The Triassic sediments of the External Zones of the Betic Cordillera were deposited on the Southern Iberian Continental Palaeomargin. Two coeval Ladinian formations, namely the Siles Formation and the Cehegin Formation, are described to illustrate the facies and lithostratigraphic variability in the Muschelkalk carbonates. There has been some dispute over the number of carbonate units present in the Siles Formation. Our studies assign a tectonic origin to these recurrent carbonate units. Both formations comprise only one carbonate unit, which is correlated to the Upper Muschelkalk of the Catalan and Germanic basins and some Iberian Range sections. To characterize the sedimentological features of these formations, 14 facies were defined. The most widespread sediment was originally lime mud, although bioclastic deposits are also common. In the facies succession, a main transgressive-regressive sequence could be identified. According to the facies model proposed here, a muddy coastal and shallow-water platform prograded over mid ramp deposits. There is no evidence for a seawards reefal or oolitic-bioclastic sandy barrier. The most significant feature of this sedimentary interpretation is that these carbonate facies show clear characteristics of an epicontinental platform.

Paleoenvironmental implications of the guano phosphatic cementation on Dongdao Island in the South China Sea

The phosphatic cement in the bioclastic sediment sequence on the northeastern shore of Dongdao Island in the South China Sea was studied and its paleoenvironmental implications discussed. Petrological characteristics and major, trace, REE element data unequivocally supported the notion that phosphatization was closely associated with avian guano decomposition and leaching, whereas carbon and oxygen isotope results further revealed that meteoric water were involved in these processes. AMS 14C dates on the brown phosphate cements indicate that they were formed around 5700, 5000–5100 and 2900 yr BP, respectively. The multi-episodes of phosphatization very likely correspond to intermittent seabird occupation on this island — possibly reflecting Holocene sea-level oscillation and/or long term climate changes in the South China Sea that have controlled seabird habitat. The phosphate cementation, which occurs widely in tropical islands, may be another useful monitor for sea-level and/or paleoclimate changes.

Sedimentology of the NW margin of the Arabian plate and the SW–NE trending Nahr El-Kabir half-graben in northern Syria during the latest Cretaceous and Cenozoic

The Late Cretaceous (Maastrichtian) to Recent sedimentary record of NW Syria documents the palaeoenvironments and regional tectonic setting during a time when the Neotethys ocean to the north was closing, culminating in regional collision by mid-Miocene time. This detailed sedimentological study builds on a biostratigraphical framework established by Russian workers. During the mid-Maastrichtian the Baer-Bassit ophiolite and related melange were emplaced southwards from Neotethys onto the northern margin of the Arabian (African) plate, resulting in flexurally controlled deepening to the south. After short-lived emergence and erosion of emplaced units a renewed marine transgression ushered in shelf-depth, open-marine carbonate deposition during the Palaeocene–early Eocene. This deposition was characterized by generally westward-channelised mass flow from shallow-marine carbonate banks to the east, probably influenced by storm conditions. Flexurally controlled uplift and erosion took place during late Eocene–Oligocene, related to final closure of Neotethys. Shelf-depth carbonate deposition resumed during the Aquitanian (early Miocene), influenced by tectonic subsidence. During the Burdigalian, a SW–NE trending, narrow (up to 15 km wide), linear (∼ 60 km long) basin developed in the vicinity of the present-day Nahr-El Kabir river valley. This basin is attributed to extension or transtension along the Nahr El-Kabir Fault. This important lineament runs onshore from the southern limit of the Cyprus arc (Tartous Ridge), through NW Syria. It is then offset by the Dead Sea Transform in the Ghab Valley, before merging with the South Anatolian Transform Fault in Turkey further northeast. The basin in NW Syria developed into the asymmetrical Nahr El-Kabir half-graben during the mid-Miocene with uplift of the northern margin and rapid fault-controlled subsidence of the basin axis. Large volumes of terrigenous clastic and shallow-marine bioclastic material were redeposited from the flanks into the axis of the basin, mainly as gravity flows. Shelf-depth carbonate deposition characterized both basin flanks, especially the southern margin. Regression culminated in gypsum and marl precipitation in tectonically active local depocentres during the Messinian salinity crisis. Pliocene re-flooding established argillaceous shelf deposition within and around the pre-existing basin, coupled with local alkaline basalt volcanism and faulting in the southern, coastal area. Regional regression gave rise to prominently channelised bioclastic sedimentation by the late Pliocene. The Quaternary was marked by regional uplift and terracing, influenced by glacio-eustatic sea level change, climatic change and further movement along the Nahr El-Kabir Fault. The dominant control on basin development was thus local tectonics related to movements along the El-Kabir lineament and its interaction with regional-scale tectonic events, while eustasy played a subsidiary, modifying role.

Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples

Abstract One unresolved conceptual problem in some Palaeozoic sedimentary strata is the boundary between the concepts of ‘shell concentration’ and ‘reef’. In fact, numerous bioclastic strata are transitional coquina-reef deposits, because either distinct frame-building skeletons are not commonly preserved in growth position, or skeletal remains are episodically encrusted by ‘stabilizer’ (reef-like) organisms, such as calcareous and problematic algae, encrusting microbes, bryozoans, foraminifers and sponges. The term ‘parabiostrome’, coined by Kershaw, can be used to describe some stratiform bioclastic deposits formed through the growth and destruction, by fair-weather wave and storm wave action, of meadows and carpets bearing frame-building (archaeocyaths, bryozoans, corals, stromatoporoids, etc.) and/or epibenthic, non-frame-building (e.g. pelmatozoan echinoderms, spiculate sponges and many brachiopods) organisms. This paper documents six Palaeozoic examples of stabilized coquinas leading to (pseudo)reef frameworks. Some of them formed by storm processes (generating reef soles, aborted reefs or being part of mounds) on ramps and shelves and were consolidated by either encrusting organisms or early diagenesic processes, whereas others, bioclastic-dominated shoals in barrier shelves, were episodically stabilized by encrusting organisms, indicating distinct episodes in which shoals ceased their lateral migration.

Responses of coral reefs to increased amplitude of sea-level changes at the Mid-Pleistocene Climate Transition

We show responses of coral reefs to increased amplitude of sea-level changes at the Mid-Pleistocene Climate Transition (MPT) based on lithostratigraphic, sedimentologic and calcareous nannofossil biostratigraphic investigations on Pleistocene reef-complex deposits (Ryukyu Group) on the Motobu Peninsula, Okinawa-jima, Central Ryukyus. Our data show that reef growth started in earliest Quaternary time (1.45–1.65 Ma) and that extensive reef formation dates back to ∼ 0.8 Ma. The mode of Quaternary sedimentation changed at ∼ 0.8 Ma in the study area. Before this time, thick siliciclastics and mixed carbonate-siliciclastics accumulated, which were followed by the deposition of bioclastic sediments (detrital limestone). No indications have been found of episodic subaerial exposures in these deposits and no calcareous nannofossil biozones are lacking. Since the detrital limestone includes biogenic components characterizing fore-reef to shelf environments, the coastal areas of the northern Motobu Peninsula mostly lay in fore-reef to shelf environments for > 0.6 million years (between ∼ 0.8 Ma and 1.45–1.65 Ma), when the sediments had not been subaerially exposed due to sea-level changes characterized by relatively small amplitudes. Coral limestone that formed in the latest Early to Middle Pleistocene between 0.4 Ma and 0.8 Ma extends over the study area, ranging in elevation from 0 to 70 m. This coral limestone grades upward into fore-reef to shelf carbonates (rhodolith, Cycloclypeus-Operculina, and detrital limestones) which is in turn overlain by coral limestone. This succession, combined with configuration of the lithofacies and paleobathymetry inferred from lithology and biogenic components, implies that the reef-complex deposits formed responding to sea-level changes with amplitude of > 60 m. Consequently, we suggest that the change in the mode of sedimentation results from increased amplitude of sea-level fluctuations at ∼ 0.8 Ma. This timing corresponds roughly to the timing of the Mid-Pleistocene Climate Transition (MPT).

Foraminiferal record of the postglacial (Holocene) marine transgression and subsequent regression, northern Spencer Gulf, South Australia

Core SG120 recovered 3.65 m of Quaternary sediment from a northern, shallow-water environment of Spencer Gulf, a marine embayment into the southern continental margin of Australia. Previous investigations had revealed that the upper interval 0 – 148 cm is Holocene marine bioclastic sediment, and that the lower Late Pleistocene interval 250 – 365 cm, with its carbonate palaeosol, had a similar marine origin. However, the age and origin of the interval 148 – 250 cm remained subject to ambiguous interpretation. Re-examination of core SG120, employing detailed foraminiferal analysis, has revealed that this middle unit records the earliest sedimentation associated with the postglacial marine transgression into the northern gulf. These basal Holocene sediments, which incorporated broken, corroded and carbonate-encrusted tests from the underlying palaeosol, together with tests of more pristine appearance, were deposited in a shallow-water, seagrass sandflat environment similar to those in coastal settings of the modern gulf. The lithological change at 148 cm has therefore been reinterpreted as a facies change related to increasing water depth. Radiocarbon analyses of fossil molluscs support this interpretation and reveal that marine transgression, at the site of SG120, was initiated prior to 8600 y cal BP. Selected species of foraminifers (Nubecularia lucifuga, Massilina milletti, Peneroplis planatus, Discorbis dimidiatus, Elphidium crispum and E. macelliforme) together reveal a consistent record of the final stages of the transgression with maximum water depth indicated at a core depth of 90 cm. Subsequent regression, which has been attributed to the combined effects of hydroisostatic uplift and sediment aggradation, is equally recorded by the foraminiferal assemblages.

Pleistocene coralline algal build-ups ( coralligène de plateau ) and associated bioclastic deposits in the sedimentary cover of Cutro marine terrace (Calabria, southern Italy)

Abstract Carbonate build-ups mainly constructed by encrusting coralline red algae are currently developing on Mediterranean soft bottoms, at depths ranging from 20 m to 160 m. They are usually referred to as ‘ coralligène de plateau ’. Few fossil examples of these bioconstructions have been described in the literature and their evolution in the context of a stratigraphic cycle has never been modelled in detail. Cutro marine terrace (Calabria, southern Italy) preserves mid-Pleistocene deposits assigned to Marine Isotope Stage (MIS) 7 or MIS 9. Extensive algal build-ups representing the deepest unit of the succession occur in the outer and central portion of the terrace, interpreted as the most distal setting from the palaeo-shoreline. Two studied sections, Vrica and Telegrafo, showed that the solid biogenic framework grew over a basal rhodolithic layer, which was stabilized by the binding activity and overgrowth of non-geniculate Corallinales (calcareous red algae). Therefore, these bioconstructions represent a rare fossil example of coralligène de plateau . At the Telegrafo section, Titanoderma pustulatum has been identified as the major rhodolith component. The build-ups are dominated by T. pustulatum , associated mainly with Mesophyllum spp. and locally with Lithophyllum stictaeforme . Rhodalgal bioclastic deposits are found in lateral contact with the build-ups. The two facies developed together under a hydrodynamic regime where phases of sedimentation from storm-driven currents alternated with phases of calm conditions. They were deposited during a single stratigraphic cycle beginning with the generation of a ravinement surface during the transgressive systems tract (TST) and ending with the burial of the coralligène by well-sorted shoreface bioclastic sands. Optimal and extensive growth of the coralligène took place during the highstand systems tract (HST).

Marked accumulation patterns characteristic of Lower Devonian stromatoporoid bindstone: Palaeoecological interactions between skeletal organisms and microbes

Stromatoporoid bindstones consisting of coarse alternations of both laminar stromatoporoids (“ Aculatostroma” and Syringodictyon) and microbialites (thrombolites and leiolites) are developed in a biohermal limestone of the Lower Devonian Garra Formation at the Catombal section, south of Wellington, New South Wales, Australia. The stromatoporoid “ Aculatostroma” typically shows a thinly laminar and ragged appearance, producing main frameworks having cryptic spaces within them, and Syringodictyon is also very thinly laminar. Thrombolites are composed of an association of peloids and Girvanella, whereas leiolites are made up of dense micrites with intensive microborings. Similar accumulation patterns are repeatedly shown at each observation scale: 1) the main framework-building “ Aculatostroma” and a combination of secondary framework ( Syringodictyon) and microbialites (leiolites and thrombolites) mesoscopically; 2) Syringodictyon and overlying leiolites and thrombolites at optical microscopic level; and 3) skeletons and intercalating dense micrite layers (leiolites) within Syringodictyon itself at SEM level. Microbialite accumulations on “ Aculatostroma” suggest disruptions of skeletal growth, corresponding to periods of harsh habitat environments, whereas the recurrent appearance of Syringodictyon over microbialites suggests somewhat ameliorated conditions. The resurgence of “ Aculatostroma” as a main frame-builder over a combination of secondary framework and microbialites suggests habitat recovery as well as the competitive superiority of this stromatoporoid. Successions such as “ Aculatostroma” – Syringodictyon – leiolites–thrombolites reflect differences in the tolerances of each responsible organism, and correspond to fluctuating environments. The growth history of stromatoporoid bindstones is as follows: 1) the laminar stromatoporoid “ Aculatostroma” first grew laterally, thereby stabilizing bioclastic sediments; 2) the stromatoporoid growths were partly or totally interrupted in relation to deteriorating habitat conditions, where micritization (leiolites with microborings), and encrustation by thrombolites occurred in succession. However, somewhat ameliorated conditions enabled Syringodictyon to temporarily dominate; 3) surviving parts of “ Aculatostroma” again spread vertically and laterally upon the recovery of optimum conditions and; 4) the ceilings of cryptic spaces produced within “ Aculatostroma” are affected by micritization (leiolites) and then encrustation of thrombolites. Repeated accumulations at various scales indicate antagonistic interrelationships within skeletal organisms and between skeletal organisms and microbes within their habitable ranges. These accumulation patterns thus provide invaluable clues for biotic interactions between skeletal frameworks and microbes, biological successions in accordance with microenvironmental changes, and resultant depositional sequences of bindstones.