Maastrichtian facies succession and sea-level history of the Hossein-Abad,Neyriz area,Zagros Basin

The Maastrichtian shallow-water carbonate platform (Tarbur Formation) is described from outcrop in southwest Iran. It is characterised by eight microfacies types, which are dominated by larger foraminifera, rudist debris and dasycladacean algae. They are grouped into four distinct depositional settings: tidal flat, lagoon, barrier and open marine. The depositional settings include stromatolitic boundstone of tidal flat, peloidal dasycladacean miliolids wackestone and peloid bioclastic imperforate foraminifera wackestone of restricted lagoon, Omphalocyclus miliolids bioclast packstone–grainstone and miliolids intraclast bioclast packstone–grainstone of open lagoon, rudist bioclast grainstone of inner-platform shoals and rudist bioclast floatstone–rudstone and bioclastic wackestone of open-marine environments.

The facies and faunal characters are typical of a ramp-like open shelf. The lack of reef-constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. On the basis of facies analysis, three depositional sequences (third order) are defined.     

Relationships between foraminiferal assemblages and depositional sequences in Jahrum Formation,Ardal area (Zagros Basin,SW Iran)

The Jahrum Formation was deposited in the foreland basin in southwest Iran (Zagros Basin). The Zagros mountain belt of Iran, a part of the Alpine–Himalayan system, extends from the NW Iranian border through to SW Iran, up to the strait of Hormuz. The various facies of the Jahrum Formation were deposited in four main genetically related depositional environments, including: tidal flat, lagoon, shoal and open marine. These are represented by 14 microfacies. The Jahrum Formation represents sedimentation on a carbonate ramp. Tidal flat facies are represented by fenestral fabric, stromatolitic boundstone and thin-bedded planes. Carbonate deposition in a shallow marine lagoon was characterised by wacke–packstone, dominated by various taxa of imperforate foraminifer. The shoals are made up of medium- to coarse-grained skeletal and peloidal grainstone. This facies was deposited predominantly in an active high energy wave and current regime, and grades basinward into middle ramps facies are represented by wackestones–packstones with a diverse assemblage of echinoderm and large benthic foraminifers with perforate wall. Outer ramp facies consist of alternating marl and limestones rich in pelagic foraminifera. There is no evidence for resedimentation processes in this facies belt. The sequence stratigraphy study has led to recognition of three third-order depositional sequences.     

Offshore sedimentary facies of a modern carbonate ramp, Kuwait, northwestern Arabian-Persian Gulf

The Kuwait example studied here may serve as a model for ancient carbonate ramp systems just as the classical—but markedly different—southern Arabian-Persian Gulf ramp of the Trucial Coast (United Arab Emirates). Five sedimentary facies may be distinguished on the modern southern Kuwait carbonate ramp based on quantitative sedimentological, mineralogical, and geochemical analyses of 130 surface sediment samples and by using multivariate statistics. These facies include (1) inner ramp ooid-skeletal grainstone with common aggregate grains, peloids, and molluscs, (2) limited occurrences of nearshore quartz-ooid sand, (3) mid ramp mollusk packstone to grainstone, (4) outer ramp mollusk-marl wackestone with abundant siliciclastic fines, and (5) coralgal grainstone that is found on small nearshore patch reefs and outer ramp pinnacle and platform reefs. In addition to facies (1), an aggregate grain packstone to grainstone sub-facies is mapped out where abundances of this grain type exceed 20%. Ooid-skeletal grainstone, mollusk packstone to grainstone, and coralgal grainstone are predominantly aragonitic with 5–10% insoluble residue on average. Mollusk-marl wackestone has 55% insoluble residue on average with aragonite and low-magnesium calcite predominating in the carbonate fraction. Dolomite in this facies is interpreted to be of eolian origin derived from the upwind deserts of Syria and Iraq. Facies distribution is correlated with water depth, and hence controlled by depositional energy, primarily wavebase. This correlation is seen in the results of statistical analyses and in the fact that facies boundaries are more or less parallel to depth contours. Ooid-skeletal grainstones are found in depths from 0 to <10 m. The boundary between the mollusk packstone to grainstone and the mollusk-marl wackestone, which also marks the transition from grain-supported to mud-supported textures, is situated between 15–20 m depth and is much sharper than the boundary between the ooid-skeletal and the mollusk packstone to grainstone facies. Carbonate-dominated facies may also be distinguished geochemically as indicated by significantly different carbon and oxygen isotope compositions. The latter should be kept in mind when using bulk isotope values for chemostratigraphy or for paleo-environmental reconstructions in fossil carbonate ramps and platforms.An erratum to this article can be found at .     

Lower Jurassic Bahamian-type facies in the Choč Nappe (Tatra Mts,West Carpathians,Poland) influenced by paleocirculation in the Western Tethys

The Lower Jurassic (upper Sinemurian) of the Hronicum domain (Tatra Mts., Western Carpathians, Poland) represents typical tropical shallow-water carbonates of the Bahamian-type. Eight microfacies recognized include oolitic-peloidal grainstone/packstone, peloidal-bioclastic grainstone, peloidal-lithoclastic-bioclastic-cortoidal grainstone/packstone, peloidal-bioclastic packstone/grainstone, peloidal-bioclastic wackestone, spiculitic wackestone, recrystallized peloidal-oolitic grainstone and subordinate dolosparites. The studied sediments were deposited on a shallow-water carbonate platform characterized by normal salinity, in high-energy oolite shoals, bars, back-margin, protected shallow lagoon and subordinately on restricted tidal flat. Some of them contain the microcoprolite Parafavreina, green alga Palaeodasycladus cf. mediterraneous (Pia) and Cayeuxia, typical of the Early Jurassic carbonate platforms of the Western Tethys. The spiculite wackestone from the upper part of the studied succession was deposited in a transitional to deeper-water setting. The studied upper Sinemurian carbonates of the Hronicum domain reveal microfacies similar to the other Bahamian-type platform carbonates of the Mediterranean region. Thereby, they record the northern range of the Lower Jurassic tropical shallow-water carbonates in the western part of the Tethys, albeit the thickness of the Bahamian-type carbonate successions generally decrease in a northerly direction. The sedimentation of the Bahamian-type deposits in the Hronicum domain, located during the Early Jurassic at about 28°N, besides other specific factors (i.e., light, salinity, and nutrients) was strongly controlled by the paleocirculation of warm ocean currents in the Western Tethys.     

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian,NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.     

Controls on diagenesis and dolomitization of peritidal facies,Early Cretaceous Lower Edwards Group,central Texas,USA

The Early Cretaceous Fort Terrett Formation of Mason County, central Texas, is a succession of subtidal to peritidal mud-dominated facies with minor intervals of bioclastic packstone–grainstone, rudist floatstone, and interbedded chert nodules. The strata conformably overlie the Hensel Formation, which was deposited unconformably on Precambrian basement. The Hensel Formation also contains a significant percentage of dolomite, precipitated within a fine-grained clayey matrix. The Hensel and Fort Terrett Formations were deposited during a transgressive episode, which provided the conditions for the extensive shallow-water Comanche carbonate platform. Siliciclastic and carbonate sediments were deposited along the coastal margin in subtidal, intertidal to supratidal areas. Previous dolomitization models have suggested that high permeability layers are required for dolomitizing brines to flow through a carbonate succession. Although, interparticle porosity in muddy tidal-flat successions can be significant, it has a limited flow capacity. However, interconnected fenestral porosity can allow sufficient fluid flow to move dolomitizing fluids more efficiently through the succession. Thus, it is hypothesized that interconnected fenestral porosity could have had a significant impact on permeability within this muddy succession and provided the pathways and conduits for Mg-rich brines. Four types of dolomite are recognized in the Fort Terrett succession. Three of these dolomite types formed largely by replacement and they occur throughout the succession. Features such as crystal size, crystal face geometry and zonation reflect the progressive development and recrystallization of the dolomite types. Only type 4 dolomite formed as a cement in void spaces during a late diagenetic stage. The direction of the dolomitizing fluid movement is difficult to determine, but it was likely downward in this case, controlled by a density-head driving-mechanism generated by dense hypersaline fluids from an evaporating lagoon.     

Sedimentation on Rasdhoo and Ari Atolls, Maldives, Indian Ocean

A first systematic study of composition, texture, and distribution of modern sediments in two Maldivian atolls reveals the predominance of skeletal carbonates. Fragments of corals, calcareous algae, mollusks, benthic foraminifera, and echinoderms are identified in the grain-size fraction >125 μm. Non-skeletal grains such as cemented fecal pellets and aggregate grains only occur in small percentages. Fragments of skeletal grains, aragonite needles, and nanograins (<1 μm) are found in the grain-size fraction <125 μm. Needles and nanograins are interpreted to be largely of skeletal origin. Five sedimentary facies are distinguished (1–5), for which the Dunham-classification is applied. Fore reef, reef, back reef, as well as lagoonal patch reef and faro areas in both atolls are characterized by the occurrence of coral grainstones (1), which also contain fragments of red coralline algae, the codiacean alga Halimeda, and mollusks. On reef islands, coral-rich sediment is cemented to form intertidal beachrock and supratidal cayrock. Skeletal grains in atoll-interior lagoons are mainly mollusks and foraminifera. The lagoon of Rasdhoo Atoll is covered in the west by mudstones (2), in the center by mollusk packstones (3) and mollusk wackestones (4), and by hard bottoms with corals in the east adjacent to channels through the atoll reef margin. The interior lagoon of Ari Atoll contains mollusk wackestones (4) in the center and mollusk-foraminifer packstones (5). Marginal lagoon areas are characterized by hard bottoms with corals. Facies distribution appears to be an expression of depositional energy, which decreases from the atoll margin towards the center in Ari Atoll, and towards the west in Rasdhoo Atoll. Predominant sediment mineralogies include aragonite and high-magnesium calcite. Mean aragonite content decreases from 90% in coral grainstone to 70–80% in mollusk packstone, mollusk wackestone, and mudstone, and to 50% in mollusk-foraminifer packstone. Stable isotopes of oxygen and carbon in bulk samples range from −3 to −1.5 (δ¹⁸O) and from +0.4 to +3.2 (δ¹³C). It is not possible to delineate facies based on O- and C-isotopes.     

Pleistocene facies of Belize barrier and atoll reefs

The knowledge of Pleistocene reef facies of Belize, Central America, is largely limited to outcrops in the northernmost part of the country. Otherwise, Pleistocene limestone, which forms the basement of the modern barrier and atoll reefs, occurs in the subsurface and is to a major extent unstudied. Based on the study of 40 m of core from 25 rotary core holes collected on central and southern Belize barrier and on atoll reefs, five Pleistocene reef facies are distinguished in the present study. They include (1) Acropora palmata grainstone, (2) Acropora cervicornis grainstone, (3) biogenic grainstone, (4) mollusk packstone, and (5) mollusk-foram wackestone. Facies 1 and 3 occur on marginal reefs, facies 2 is found on marginal and lagoonal reefs, and facies 4 and 5 mark lagoon shoals and lagoons, respectively. Most of the facies have equivalents in the Pleistocene of the wider Caribbean and also in the modern of the study area. Diagenetic features include dissolution, caliche formation, laminated blocky low-magnesium-calcite and dogtooth spars. Age data from Pleistocene corals obtained during earlier studies are discussed, and indicate deposition during marine isotope stage 5, between 140–80 ka bp.     

Paleocene and Lower Eocene shallow-water limestones of Tibet: Microfacies analysis and correlation of the eastern Neo-Tethyan Ocean

Microfacial investigations of the Lower Paleogene sediments were based on four sections of the passive Indian (Ladakh, Tingri County, Gamba County and Yadong County) and one of the active Asian continental margin (Zhongba County). Eleven microfacies from the Tethyan Himalaya (prefixed with P for passive continental margin) and four from the Xigaze forearc basin (prefixed with A for active continental margin) were observed. The distribution of fossil assemblages in the environment ranges from the tidal flat and restricted lagoonal part of the inner carbonate ramp to the outer carbonate ramp: (P1) green algae pack-/grainstone with small miliolids, (P2) bioclast grainstone, (P3) Rotaliidae packstone, (P4) Miscellaneidae-Rotaliidae-Nummulitidae pack-/grainstone, (P5) laminated and bioturbated mud- and grainstone, (P6) Alveolina wacke-/packstone with Soritidae, (P7) Nummulites-Alveolina-Orbitolites pack-/floatstone, (P8) Discocyclinidae-Nummulitidae pack-/floatstone, (P9) Rhodolith wacke-/packstone, (P10) mudstone with anhydrite nodules, (P11) planktonic foraminiferal wackestone, (A1) molluskan float-/rudstone, (A2) Nummulitidae wacke-/packstone, (A3) rhodolith wacke-/packstone, (A4) Discocyclinidae-Nummulitidae float-/rudstone. The correlation of our observations provides a detailed overview of the paleoenvironmental development and the sedimentary history of the eastern Neo-Tethyan Ocean, showing a deepening trend in two stages from Lower Paleocene to Lower Eocene.     

Microfacies of a Permian calcisponge reef in Lichuan,western Hubei,South China

The Lichuan Jiantianba reef is located at the platform margin between the carbonate platform and the marine trough in western Hubei, China. The water depth of this area became shallow in the late Permian Changhsingian Age, and a huge aggradation-progradation platform marginal reef developed. Based on precise field measurements and microscopic observation, this paper describes the petrological characteristics and biological assemblages of the reef in detail and distinguishes 10 microfacies: small echinoderm wackestone, sponge floatstone, bound sponge bioliestone, bound sponge framestone, large echinoderm wackestone, red algal limestone, bioclastic grainstone, dasycladales wackestone, shelly wackestone, and microbialites. Sponge floatstone and bound sponge bioliestone are defined as toppled sponge limestone. Comparisons of the petrological characteristics and biotic association of toppled sponge limestone, bound sponge framestone and bioclastic wackestone and grainstone revealed that the toppled sponge limestone and the bound sponge framestone are similar in sponge content in terms of the types and contents of reef-dwellers, except that the sponge content is slightly lower, and the preservation state is mainly toppled for the former and upright or inclined for the latter. The toppled sponge limestone is dominated by tabular calcite, and the bound sponge framestone is dominated by fibrous calcite. The bioclastic wackestone and grainstone do not contain reef-building sponge organisms, and the bioclast content is very high and often dominated by a certain class, such as echinoderms, foraminifers, green algae or shells. The toppled sponge limestone below the framework, which was classified as fore-reef breccia or bioherm bafflestone-bindstone in previous studies, should be defined as reef-core sponge limestone deposited in situ that experienced serious post-karstification. The vertical evolution of the sedimentary facies of the reef is analyzed based on the microfacies and sedimentary environment. The toppled sponge limestone and the bound sponge framestone should be classified as reef core, which is the only subfacies of the reef facies. The underlying small echinoderm wackestone should be classified as the shelf facies, whereas the overlying bioclastic wackestone and grainstone should be classified as the open platform facies. These classifications represent a modification of the sedimentary facies subdivision of the Jiantianba reef in Lichuan, Hubei Province, South China, and provide a new reference model for the subdivision of the Permian calcisponge reefs on platform margin.     

Sedimentary facies and paleoenvironmental interpretation of the Oligocene larger-benthic-foraminifera-dominated Qom Formation in the northeastern margin of the Tethyan Seaway

The well-exposed outcrops of the Bujan, northern Abadeh, and Varkan stratigraphic sections of the Qom Formation in the Iranian part of the “northeastern margin” of the Tethyan Seaway were characterized by abundant biogenic components dominated by foraminifers, coralline red algae, and corals. The Qom Formation is Rupelian–Chattian in age in the study areas. Based on the field investigations, depositional textures, and dominant biogenic components, fifteen (carbonate and terrigenous) facies were identified. These facies can be grouped into four depositional environments: open marine, open lagoon, restricted lagoon, and continental braided streams. The marine facies were deposited on a ramp-type platform. The euphotic inner ramp was characterized mainly by imperforate foraminifera, with co-occurrence of some perforate taxa. These facies passed basinward into a mesophotic (middle) ramp with Neorotalia packstone (F5), coral, coralline algae, perforate foraminiferal packstone (F4), and coral patch reefs (F7). The deeper, oligophotic ramp facies were marly packstones with planktonic and hyaline benthic foraminifera, including large lepidocyclinids and nummulitids. The abundance of perforate foraminifera and the absence of facies indicating restricted lagoonal or intertidal settings suggest that the Varkan section was deposited mainly in open marine settings with normal salinity. The prevalence of larger benthic foraminiferal and red algal assemblages, together with the coral facies, indicates that carbonate production took place in tropical–subtropical waters.     

Biostratigraphy and paleoecology of the Oligo-Miocene succession in Fars and Khuzestan areas (Zagros Basin,SW Iran)

Paleontological and biostratigraphical studies on carbonate platform succession from southwest Iran documented a great diversity of shallow-water benthic foraminifera during the Oligocene–Miocene. Larger foraminifera are the main means for the stratigraphic zonation of carbonate sediments. The distributions of larger benthic foraminifera in two outcrop sections (Abolhayat and Lali) in the Zagros Basin, Iran, are used to determine the age of the Asmari Formation. Four assemblage zones have been recognized by distribution of the larger benthic foraminifera in the study areas. Assemblage 3 (Aquitanian age) and 4 (Burdigalian age) have not been recognized in the Abolhayat section (Fars area), due to sea-level fall. The end Chattian sea-level fall restricted marine deposition in the Abolhayat section and Asmari Formation replaced laterally by the Gachsaran Formation. This suggests that the Miocene part of the formation as recognized in the Lali section (Khuzestan area) of the Zagros foreland basin is not present in the Abolhayat outcrop. The distribution of the Oligocene larger benthic foraminifera indicates that shallow marine carbonate sediments of the Asmari Formation at the study areas have been deposited in the photic zone of tropical to subtropical oceans. Based on analysis of larger benthic foraminiferal assemblages and microfacies features, three major depositional environments are identified. These include inner shelf, middle shelf and outer shelf. The inner shelf facies is characterized by wackestone–packstone, dominated by various taxa of imperforate foraminifera. The middle shelf is represented by packstone–grainstone to floatstone with a diverse assemblage of larger foraminifera with perforate wall. Basinwards is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Planktonic foraminifera wackestone is the dominant facies in the outer shelf.     

Microfacies and sequence stratigraphy of the Amapá Formation, Late Paleocene to Early Eocene, Foz do Amazonas Basin, Brazil

On the basis of thin-section studies of cuttings and a core from two wells in the Amapá Formation of the Foz do Amazonas Basin, five main microfacies have been recognized within three stratigraphic sequences deposited during the Late Paleocene to Early Eocene. The facies are: 1) Ranikothalia grainstone to packstone facies; 2) ooidal grainstone to packstone facies; 3) larger foraminiferal and red algal grainstone to packstone facies; 4) Amphistegina and Helicostegina packstone facies; and 5) green algal and small benthic foraminiferal grainstone to packstone facies, divisible locally into a green algal and the miliolid foraminiferal subfacies and a green algal and small rotaliine foraminiferal subfacies. The lowermost sequence (S1) was deposited in the Late Paleocene–Early Eocene (biozone LF1, equivalent to P3–P6?) and includes rudaceous grainstones and packstones with large specimens of Ranikothalia bermudezi representative of the mid- and inner ramp. The intermediate and uppermost sequences (S2 and S3) display well-developed lowstand deposits formed at the end of the Late Paleocene (upper biozone LF1) and beginning of the Early Eocene (biozone LF2) on the inner ramp (larger foraminiferal and red algal grainstone to packstone facies), in lagoons (green algal and small benthic foraminiferal facies) and as shoals (ooidal facies) or banks (Amphistegina and Helicostegina facies). Depth and oceanic influence were the main controls on the distribution of these microfacies. Stratal stacking patterns evident within these sequences may well have been related to sea level changes postulated for the Late Paleocene and Early Eocene. During this time, the Amapá Formation was dominated by cyclic sedimentation on a gently sloping ramp. Environmental and ecological stress brought about by sea level change at the end of the biozone LF1 led to the extinction of the larger foraminifera (Ranikothalia bermudezi).     

Early Triassic peritidal carbonate sedimentation on a Panthalassan seamount: the Jesmond succession,Cache Creek Terrane,British Columbia,Canada

The Jesmond succession of the Cache Creek Terrane in southern British Columbia records late Early Triassic peritidal carbonate sedimentation on a mudflat of a buildup resting upon a Panthalassan seamount. Conodont and foraminiferal biostratigraphy dates the succession as the uppermost Smithian to mid-Spathian. The study section (ca. 91 m thick) is dominated by fine-grained carbonates and organized into at least 12 shallowing-upwards cycles, each consisting of shallow subtidal facies and overlying intertidal facies. The former includes peloidal and skeletal limestones, flat-pebble conglomerates, stromatolitic bindstones, and oolitic grainstone, whereas the latter consists mainly of dolomicrite. The scarcity of skeletal debris, prevalence of microbialite, and intermittent intercalation of flat-pebble conglomerate facies imply environmentally harsh conditions in the mudflat. The study section also records a rapid sea-level fall near the Smithian-Spathian boundary followed by a gradual sea-level rise in the early to mid-Spathian.     

A eustatically driven calciturbidite sequence from the Dinantian II of the Eastern Rheinisches Schiefergebirge

Summary The Gladenbach Formation is an approximately 30 m thick, well-segregated calciturbidite sequence, restricted to the H?rre belt of the eastern Rheinisches Schiefergebirge. It is middle Tournaisian in age (lowerPericyclus Stage, lower cd II of the German Culm zonation) and is an equivalent of the Liegende Alaunschiefer. The sequence is composed predominantly of minor turbiditic fining-upward cycles. Cycles start with massive calciturbidite beds. They are composed of fine-grained intraclastic-bioclastic grainstone/packstone, more or less ooid-bearing in the top of the formation, and/or radiolarian-rich packstone. Cycles continue with platy, dense limestones consisting of radiolarian-rich wackestone/packstone and microlithoclastic-microbioclastic wackestone/packstone. Different types of shales finish the fining-upward development. Minor cycles can be grouped into several 4th order cycles, composing a single 3rd order cycle. Towards the top, abundance of resedimented platform components, like ooids, calcareous smaller foraminifers, echinoderms, brachiopods, bryozoans and critical conodont genera, increases. Simultaneously, the thickness of the minor cycles decreases. This indicates a transgressive phase, characterized by increasing over-production of carbonate on platform realms and a correlated increase in the frequency of resedimentation events in the basin. The transgression corresponds to the well-documented global eustatic transgression of the Lowercrenulata andisosticha-uppercrenulata Zone of the conodont chronology. Thus, the Gladenbach Formation is interpreted as a transgressive systems tract/highstand systems tract. The Liegende Alaunschiefer is the time-equivalent, starved basin facies. Predominating hemipelagic calciturbidites of the lower Gladenbach Formation derive from the deeper shelf slope or from an intrabasinal swell, which might constitute a flexural bulge in front of the shelf slope. Turbidite sediments from the upper part of the formation derive from shelf-edge sands and the upper shelf slope. The source might be related to the ancient Devonian reef complex of Langenaubach-Breitscheid in the southwest.     

Carbonate Mineralogy Along a Biogeochemical Gradient in Recent Lacustrine Sediments of Gallocanta Lake (Spain)

Three sedimentary subenvironments, palustrine (GP), marginal lacustrine (GML) and central lacustrine (GCL), were compared regarding water chemistry and microbial activity in order to explain the differences in the carbonate mineralogical composition of the upper sediment layer in Gallocanta Lake, a shallow hypersaline environment in Northeastern Spain. Horizontal heterogeneity was considerable, salinity ranged from 5 to 116 (‰) for the GP and GCL subenvironments respectively. Sulfate, Mg ^{2 +} , and Ca ^{2 +} concentrations covaried among them and with salinity. The relative abundance of Mg-bearing carbonates, including high-Mg calcite, dolomite and hydrated Ca-magnesite, increased with the salinity. They were absent from the GP subenvironment, where only calcite precipitates, and maximum abundances were found in the GCL subenvironment (61%), where salinity, sulfate, and Mg ²⁺ concentrations were highest. Every subenvironment presented specific microecological characteristics. The microbial community of the GCL subenvironment lacked of oxygenic photosynthesis, while the microbial communities of GML and GP subenvironments were photosynthetically active. Vertical profiles of sulfide and pH at the water-sediment interface revealed clear differences between the GCL and GML subenvironments as well. Sulfide was detected below the oxic layer in the GCL subenvironment and increased with depth, but it was undetected in the GML subenvironment. The precipitation of Mg-bearing carbonates with different Mg:Ca proportions occurs at different stage along a biogeochemical gradient, where increasing salinity and sulfate content favour the anaerobic oxidation of organic carbon by dissimilatory sulfate reduction.     

Sedimentary history of Upper Silurian biostromes of Podolia (Ukraine) based on stromatoporoid morphometry

The sedimentary history of stromatoporoid biostromal accumulations reflecting various depositional conditions (autoparabiostromes and parabiostromes) is studied in two isochronous, Late Silurian carbonate sections of the Malynivtsy Formation from Podolia (western Ukraine, Kam'janec' Podil'skyj area). This study focuses on morphometrical analysis of massive stromatoporoids. Various stromatoporoid attributes, such as growth form, volume, surface character etc., are interpreted in terms of growth environments. Attributes of redeposited specimens are also analysed in terms of their susceptibility to exhumation and redeposition, and new criteria are presented in this matter. The exposed facies succession, which can be subdivided into three units: an oncolitic–fenestral complex and the stromatoporoid–coral complexes that underlie and cover it, represents the belt of shoals located at a considerable distance from shore, and its transition to a narrow zone of back-shoal tidal flats. The facies patterns proved to be strongly obscured by an intensive process of onshore redeposition of material during high energy episodes. These events caused exhumation and landward transport of stromatoporoids inhabiting soft-sediment bottoms of outer shelf areas, which were afterwards accumulated in parabiostromes in calm waters on lee side of a zone of shoals. The main process governing the distribution of redeposited stromatoporoids is fractional (weight) segregation. The high energetic events had less effect on stromatoporoid–coral autoparabiostromes that formed the zone of shoals, which were inhabited by stromatoporoids better adapted to permanent wave action, but nonetheless, they caused their partial reworking and depletion from those forms that did not resist redeposition, on one hand, and supplementation by specimens derived from offshore areas, on the other.     

Lowermost Triassic (Griesbachian) microbial bindstone-cementstone facies, southwest Japan

Summary On the basis of the lithostratigraphy and microscopic characters, the paper describes the facies interpretation of the upper Upper Permian (Changhsingian) and Lower Triassic (Griesbachian to Spathian) carbonates of southwest Japan, with a focus upon the lowermost Triassic (Griesbachian) microbial bindstone-cementstone. We emphasize the significant sediment-binding and stabilizing agencies of microbes chiefly of cyanobacteria along with the syndepositional cementation for the carbonate deposition on a Panthalassan buildup in a period of the Scythian reef gap. Cyanobacteria flourished as postmass extinction disaster forms in the beginning of the Triassic. The Griesbachian microbial bindstone-cementstone we describe comprises the oldest known Triassic microbial facies. Examined were the Changhsingian Mitai Formation and the Triassic Kamura Formation (Griesbachian to Norian) in southwest Japan. These units consist entirely of carbonates and are reconstructed as relict of a shallowmarine buildup upon a seamount in the Panthalassa. The Changhsingian Mitai carbonates (ca. 35 m thick) consist mainly of grainstone and packstone with a small amount of lime-mudstone. The topmost part is intensely dolomitized. The carbonate succession is characterized by an upward-decrease in number and taxonomic diversity of shallow-marine skeletal debris and an increase up-section in an amount of peloidal particles. The lower Mitai rocks are interpreted to have accumulated as skeletal sand in an oxygenated subtidal environment and the upper Mitai carbonates are considered to have been formed in a quiet intertidal environment where peloidal particles predominantly accumulated. The facies interpretation suggests the late Changhsingian regression, which led to an increase of an inhospitable condition for shallow-marine benthic communities and to an intensive dolomitization. The Kamura Formation (ca. 38 m thick) disconformably rests upon the Mitai Formation with a drastic lithologic change. The Lower Triassic rocks we focused reach 15.5 m thick and comprise the Griesbachian and Dienerian to Spathian sections. The lower part (ca. 5.5 m) of the Griesbachian section consists of dark gray carbonaceous limestone composed of thinly layered triplets of a gastropod-bearing peloidal grainstone layer, a spar-cemented frame of clotted peloids, and a thin-laminated and occasionally stromatolitic cover of cryptomicrobial micrite in ascending order. The upper two members of a triplet often form a bindstone-cementstone layer characterized by a low-relief domed structure, or a broad hump. The upper part (ca. 2 m thick) of the Griesbachian section is composed of oncolitic limestone that contains laminae packed with gastropods. The Dienerian to Spathian section (ca. 8 m thick) consists of coquinites comprising an explosive flourish and accumulation of pectinacean bivalves. We interpret the Griesbachian rocks to have accumulated in a stagnant, ecologically rigorous tidal flat, where microbes, of possible cyanobacteria, flourished. The flourish of gastropods reflects an intermittent inundation by spring tide into the peritidal environment. The deposition of gastropods was followed by a dominant cyanobacterial activity that formed a microbial bindstone-cementstone layer along with the syndepositional cementation in an intertidal zone. The cyanobacterial activity contributed to the formation of gently undulated, sediment-binding and stabilizing mats. The oncolitic limestone in the upper part of the Griesbachian section also suggests the cyanobacterial, or algal activity. The Griesbachian microbial-controlled sedimentation was followed by the mass accumulation of bivalves that most possibly reflects a rapid transgression in Dienerian time. All the results permit us to conclude that possible cyanobacteria were the significant rock-forming organisms as post-mass extinction disaster forms on a panthalassan buildup in the beginning of the Scythian reef gap. The Griesbachian carbonates here described are similar in having the important microbial control on the sedimentation to the Lower Triassic stromatolitic and thrombolitic carbonates previously known in the Tethyan platform.     

Conodont biostratigraphy of Tournaisian shallow-water carbonates in central Guangxi,South China

The conodont faunas of Tournaisian shallow-water carbonates from central Guangxi are described mainly for biostratigraphic purposes. A complete series of samples was collected from the Long’an and Du’an formations in the Long’an section. These formations are characterized by lime-mudstone, skeletal and peloidal wackestone, packstone and grainstone with typical shallow-water biota. Overall, these samples produced 809 identifiable Pl elements, belonging to 50 species in 11 genera, of which one species and one subspecies are new. The fauna enables the establishment of seven biozones (in ascending order): Polygnathus spicatus, Siphonodella homosimplex, S. sinensis, S. dasaibaensis, Polygnathus communis carina Acme, Gnathodus cuneiformis, and P. communis porcatus zones. Based on these new collections from central Guangxi and on data from the literature, a conflated Tournaisian conodont zonation is proposed for shallow-water successions in South China. Most of the conodont zones correlate well with their counterparts recognized in Western Europe, which may be of greater significance in stratigraphic correlation than previously thought.     

Stratigraphic analysis of a fossil Neogoniolithon-capped patch reef and associated facies,San Salvador,Bahamas

An unusual Pleistocene patch reef is exposed in a coastal cliff at Grotto Beach, San Salvador, Bahamas. The reef is a coralline framestone constructed mainly by Porites astreoides together with a few large heads of Diploria strigosa and Montastrea annularis, and is capped by a dense thicket of Neogoniolithon strictum that is interpreted as marking the subtidal/intertidal boundary. The reef is flanked to the northeast by laminated to low-angle cross-laminated intraclastic grainstones and to the southwest by skeletal rudstone of reefal and interreefal derivation. Uranium-series dating of pure aragonite from a Diploria corallum yielded an age of 123 000±9000 years. Reef growth began on an erosional surface underlain by steeply crossbedded eolian grainstone. As the reef grew upward, it also grew laterally over adjacent penecontemporaneous subtidal sediments. The reef was eventually buried by 2.3 m of shallow subtidal and beach sediments that apparently prograded seaward during a highstand, or possibly while sea level was still rising. The shallow subtidal sediments are mainly peloidal, ooidal and skeletal grainstones that are pervasively bioturbated. The overlying beach facies comprises predominantly laminated, sparsely burrowed grainstone. The beach and shallow subtidal facies contain boulders of fine-grained laminated grainstone that are interpreted as storm-tossed blocks of beachrock. Living analogs of the Grotto Beach fossil reef lie off East Beach, San Salvador. Several of these have a flourishing cap of Neogoniolithon that extends above low-tide level and we believe that the Neogoniolithon cap of Grotto Beach reef did likewise. Wherever found in the stratigraphic record this facies should serve to identify the subtidal/intertidal boundary. The uppermost Pleistocene beach sediments associated with Grotto Beach fossil reef lie 5.8 m above present-day mean sea level, which ist strong evidence that this portion of San Salvador has undergone little subsidence since the Grotto Beach section was deposited.