Primary Cavities Research Articles

Microbialite development patterns in the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310): Implications on reef framework architecture

The widespread occurrence of microbialites in the last deglacial reef frameworks (16–6KaBP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes.The present study is based on the sedimentological and chronological analysis (14C AMS dating) of drill cores obtained during the IODP Expedition #310 “Tahiti Sea Level” on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise.The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the “reefal microbialites” which developed a few hundred years after coralgal communities in shallow-water environments, whereas the “slope microbialites” grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities.The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (>50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~30%) and could not host a significant microbialite expansion.

Biological control of paleomagnetic remanence acquisition in carbonate framework rocks of the Tahiti coral reef

Integrated Ocean Drilling Program (IODP) Expedition #310 recovered 632 meters of Post Last-Glacial-Max (Post-LGM) and late Pleistocene coral reef framework rocks (37 holes at 22 sites) from the current fore-reef slopes surrounding the island of Tahiti. The cores were collected primarily to constrain the timing and amplitude of rising sea levels associated with the end of the last global glaciation (10–18,000 YBP), to identify the climatic and environmental changes during that period, and to evaluate the reef response to those combined changes. The cores, which were collected in three regions (Faaa, Tiarei, and Maraa), are remarkable in that more than 60% of the total core volume is associated with microbialites rather than macroscopic coral or coralline algae assemblages. We present here paleomagnetic data from 316 oriented paleomagnetic samples collected from the Post-LGM framework rocks of the Maraa region (12 holes at 6 sites). 96% of our samples were recovered from microbialites that have filled the primary cavities of the coral/coralgal framework. All samples appear to accurately record geomagnetic field variability at Tahiti (average inclination = −30.6°, alpha 95 ~ 2.9°, site axial dipole inclination = −32.6°) and yet the samples are composed of ~ 99% precipitated calcium carbonate. Our rock magnetic data indicate that the NRM resides almost entirely in detrital titanomagnetite grains derived from the Tahiti volcanic edifice. We present magnetic and microscopic evidence that the natural remanent magnetization in the carbonate rocks was locked in through mediation of organic biofilms (mostly bacterial) that held the grains in place until they were trapped by carbonate precipitation.

Comparative morphology of the axial complex and interdependence of internal organ systems in sea urchins (Echinodermata: Echinoidea)

BackgroundThe axial complex of echinoderms (Echinodermata) is composed of various primary and secondary body cavities that interact with each other. In sea urchins (Echinoidea), structural differences of the axial complex in "regular" and irregular species have been observed, but the reasons underlying these differences are not fully understood. In addition, a better knowledge of axial complex diversity could not only be useful for phylogenetic inferences, but improve also an understanding of the function of this enigmatic structure.ResultsWe therefore analyzed numerous species of almost all sea urchin orders by magnetic resonance imaging, dissection, histology, and transmission electron microscopy and compared the results with findings from published studies spanning almost two centuries. These combined analyses demonstrate that the axial complex is present in all sea urchin orders and has remained structurally conserved for a long time, at least in the "regular" species. Within the Irregularia, a considerable morphological variation of the axial complex can be observed with gradual changes in topography, size, and internal architecture. These modifications are related to the growing size of the gastric caecum as well as to the rearrangement of the morphology of the digestive tract as a whole.ConclusionThe structurally most divergent axial complex can be observed in the highly derived Atelostomata in which the reorganization of the digestive tract is most pronounced. Our findings demonstrate a structural interdependence of various internal organs, including digestive tract, mesenteries, and the axial complex.

Integration of core, logging and drilling data in modern reefal carbonates to improve core location and recovery estimates (IODP Expedition 310)

Palaeoclimate and sea-level studies commonly rely on interpretations based on depth. The location and quantity of core recovered during a drilling expedition has direct implications for subsequent applications of core measurements. A key objective of Integrated Ocean Drilling Program (IODP) Expedition 310 is to establish the course of postglacial sea-level rise for Tahiti in the South Pacific, for which assigning the correct depth to recovered core is fundamental. By convention, core is placed at the top of the core barrel run from which it was collected. Unless core recovery is 100%, this results in depth errors that are inversely proportional to the recovery. The difficulties of locating core are compounded for the Tahiti boreholes, where primary cavities in the carbonate reef successions are frequently present. Only through the integration of core with continuous geophysical measurements of the borehole can accurate depth positioning be achieved. High-resolution optical and acoustic images of the borehole wall allow effective visual correlation of the core and logging datasets. The final integrated depths form the underlying framework for all subsequent scientific analyses of recovered core employing evaluations that rely on depth. Furthermore, the integration process allows true core recoveries to be accurately estimated.

Concave Pit-Containing Scaffold Surfaces Improve Stem Cell-Derived Osteoblast Performance and Lead to Significant Bone Tissue Formation

BackgroundScaffold surface features are thought to be important regulators of stem cell performance and endurance in tissue engineering applications, but details about these fundamental aspects of stem cell biology remain largely unclear.Methodology and FindingsIn the present study, smooth clinical-grade lactide-coglyolic acid 85:15 (PLGA) scaffolds were carved as membranes and treated with NMP (N-metil-pyrrolidone) to create controlled subtractive pits or microcavities. Scanning electron and confocal microscopy revealed that the NMP-treated membranes contained: (i) large microcavities of 80–120 µm in diameter and 40–100 µm in depth, which we termed primary; and (ii) smaller microcavities of 10–20 µm in diameter and 3–10 µm in depth located within the primary cavities, which we termed secondary. We asked whether a microcavity-rich scaffold had distinct bone-forming capabilities compared to a smooth one. To do so, mesenchymal stem cells derived from human dental pulp were seeded onto the two types of scaffold and monitored over time for cytoarchitectural characteristics, differentiation status and production of important factors, including bone morphogenetic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF). We found that the microcavity-rich scaffold enhanced cell adhesion: the cells created intimate contact with secondary microcavities and were polarized. These cytological responses were not seen with the smooth-surface scaffold. Moreover, cells on the microcavity-rich scaffold released larger amounts of BMP-2 and VEGF into the culture medium and expressed higher alkaline phosphatase activity. When this type of scaffold was transplanted into rats, superior bone formation was elicited compared to cells seeded on the smooth scaffold.ConclusionIn conclusion, surface microcavities appear to support a more vigorous osteogenic response of stem cells and should be used in the design of therapeutic substrates to improve bone repair and bioengineering applications in the future.

Archaeocyathan buildups within an entirely siliciclastic succession: New discovery in the Toyonian Lalun Formation of northern Iran, the Proto-Paleotethys passive margin of northern Gondwana

Meter-scale buildups constructed exclusively by archaeocyaths have been recognized within the uppermost Lower Cambrian siliciclastic succession of Eastern Alborz in northern Iran. They are the only known Toyonian reefs in Iran and adjacent countries and occur in the lower part of the Shale unit of the Lalun Formation. The reefs are of limited lateral extent, reach a maximum thickness of 2.5 m and consist of several reef complexes containing cabbage- or sack-shaped buildups surrounded by well-bedded, colored shale. Each reef complex in the main reef zone consists of meter-scale individual and kalyptrate (compound) buildups that are overgrown by laminated stromatolite. The individual buildups are 12 to 75 cm thick and 5 to 50 cm in diameter, but the compound buildups are up to 2 m thick and their diameter ranges from 75 to 120 cm. In contrast to most Lower Cambrian reefs, these compound buildups demonstrate a complete ecological succession including pioneer and climax phases. Archaeocyaths in the buildups are solitary and colonial types showing great diversity of growth forms, with vase-, bowl- and cup-shaped and cylindrical forms present. The cups range from less than 1 mm to 1.5 cm in diameter and are up to 4 cm tall in the majority of the skeletons, but they may reach up to 4 cm in diameter in the massive colonial forms or in the branching forms of the upper outer part of the compound bioherms. The limited lateral extent of the reef horizons, and lateral facies changes in the colored shale toward various reefs supports deposition in drowned tidal channels in an estuarine depositional environment. A change of the individual bioherms to larger compound buildups suggests lateral depth variation of the tidal channels. Partial infilling of the primary inter-biohermal cavities by large reef clasts and cross-laminated siltstone to very fine sandstone suggests occasional disturbances by storms and periodic influx of coarse siliciclastics. In striking contrast to other Lower Cambrian reefs, the archaeocyathan buildups of the Alborz flourished in a completely siliciclastic setting and lack the skeletal calcimicrobes which dominated the Lower Cambrian reefs. Abrupt lateral and vertical facies changes to colored shale and fluvial red beds, the presence of infiltrated shale between the skeletons, and in central cavities and the intervallum of the archaeocyaths suggest highly turbulent and turbid incoming water with abnormally high concentrations of fine siliciclastic material during reef development. High concentrations of fine, suspended siliciclastics could well have prevented the light penetration which was necessary for calcimicrobial growth. In addition, the relatively small size of the archaeocyaths and absence of reef dwellers is very likely a consequence of the high terrigenous mud content and, perhaps, below-normal salinity of the ambient sea water. In the absence of calcimicrobes and suspension feeder metazoans in the dark and muddy water of the mid-estuarine setting, archaeocyathans became the only bioconstructors of the Iranian buildups. Abnormally high concentration of nutritious fine siliciclastics suggests that photosymbionts and oligotrophic conditions were not needed by archaeocyaths; particular hydrodynamic conditions along with high nutrient flux, rather than light, were essential for archaeocyathan communities.

Ironstone bodies of the Barberton greenstone belt, South Africa: Products of a Cenozoic hydrological system, not Archean hydrothermal vents!

Irregular bodies of goethite and hematite, termed ironstone pods , in the Barberton greenstone belt, South Africa, have been previously interpreted as the Earth9s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about Archean hydrothermal systems, ocean composition and temperature, and early life. This report summarizes geologic, sedimentological, and petrographic evidence from three widely separated areas showing that the ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems, probably during periods of higher rainfall in the Pleistocene. The deposits include a recently active spring terrace and outflow system containing iron oxide–encrusted microbial filaments, shallow subsurface groundwater areas showing ironstone deposited between chert blocks and replacing chert, degraded spring and outflow deposits, and goethite-cemented Pleistocene landslide breccias and regolith. The predominance of goethite, a thermally unstable iron oxide; widely developed slope-parallel stratification; abundant primary open cavities, some with vertical goethite dripstone; and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time. Veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict Archean vein and cavity-fill quartz after iron oxide replacement of surrounding chert. Iron was sourced by Archean sideritic sedimentary units that hold up high ridges on which the ironstone bodies occur. Siderite was dissolved by circulating groundwater, and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the Fe-rich water flowed onto the surface. These deposits represent a remarkable iron oxide–depositing Quaternary hydrologic system but provide no information about conditions or life on the early Earth.

Identification of large (2–10 km) rayed craters on Mars in THEMIS thermal infrared images: Implications for possible Martian meteorite source regions

Four definitive and three probable rayed craters have been identified on Mars using 100‐m resolution thermal infrared images obtained by the Mars Odyssey Thermal Emission Imaging System (THEMIS). These seven craters are similar to the previously discovered rayed crater Zunil and are best recognized by a distinct thermal contrast with respect to their surroundings. Martian rays, unlike their lunar counterparts, only exhibit minor contrasts in visible albedo. As a consequence, their presence on Mars most likely went unnoticed until substantial global coverage of THEMIS thermal infrared was achieved. Their presence has since been discerned in the coarser‐resolution Thermal Emission Spectrometer (TES) data set, which preceded THEMIS. Observations in visible images of the primary cavities, secondaries, and rays suggest that, like lunar ray counterparts, Martian rays are invariably young geomorphic features. Martian rays are typically greater than hundreds of kilometers in length and consist of numerous densely clustered secondary craters, and thereby are a physical manifestation of high‐velocity ejecta. Spallation accounts for a small fraction of the high‐velocity ejecta that experiences low‐shock compression due to interference from the rarefaction wave with the free surface. Spallation is currently the favored mechanism responsible for ejecting meteorites from Mars and is likely responsible for some of the ray‐forming secondaries. Additional observations and inferences based on Martian rayed craters are compared with current Martian meteorite delivery models and the Martian meteorites themselves. The correlations presented here suggest that Martian rayed craters are the most plausible candidate source craters for the Martian meteorites to date.

Stromatoporoid response to muddy substrates in Silurian limestones

Stromatoporoids grew on both soft and hard substrates, and recent work revealed primary cavities in Devonian reef systems. We present evidence that stromatoporoids also developed primary cavities in muddy sediments. Stromatoporoids initiated on two categories of material: (1) fine-grained sediment, usually covering large bioclasts; (2) uncovered bioclasts, forming a small area of clean substrate. In both, subsequent growth was previously usually interpreted to be across, and in direct contact with, adjacent fine-grained sediment, itself presumed to be unlithified in most cases. However, stromatoporoid basal surfaces show two types of morphology: (A) smooth, generally curved and flat, and (B) corrugated, with concentric growth rings as minor ridges projecting downwards. Both may be present in different stromatoporoid individuals of the same species. Smooth bases are interpreted in most cases to indicate growth on unlithified sediment, and are common in category 1 above. For corrugated bases, rings are interpreted to result from growth to form minor primary cavities by the stromatoporoid skeleton growing laterally a few mm above the substrate. The base then likely settled into the soft sediment as skeletal mass increased. Primary cavities are interpreted to be similar to those in muddy Devonian facies, but both differ from those in Devonian reefs (e.g. Canning Basin) where cavities may be tens of cm high, and were permanent. In the Silurian samples, a third morphology of concentric basal ring arrangement containing sediment wedges is interpreted as due to episodic sedimentation. Also, stromatoporoids grew on lithified and eroded intraclasts, indicating patchy early lithification of the sea floor in these open shelf settings. Evidence presented from the Silurian examples emphasises the dynamic response of stromatoporoids to their substrate, consistent with sediment-avoiding habits of modern calcified sponges.

Microbial primary dolomite from a Norian carbonate platform: northern Calabria, southern Italy

AbstractThe origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ18O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite.

When an epithelium ceases to exist – an ultrastructural study on the fate of the embryonic coelom in Epiperipatus biolleyi (Onychophora, Peripatidae)

AbstractIt is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda.

Palaeoecology of a post‐extinction reef: Famennian (Late Devonian) of the Canning Basin, north‐western Australia

Reefs were decimated by the Frasnian/Famennian (Late Devonian) mass extinction event (371 Ma), and are assumed to have survived only as depauperate calcimicrobial communities dominated by disaster taxa. Description of Famennian proximal reef‐slope communities within the Windjana Limestone, Canning Basin, north‐western Australia, shows that, notwithstanding the loss of large metazoans, novel ecologies were established in this setting by a rich biota of survivor and progenitor taxa. Diverse calcimicrobes together with algae, crinoids, bryozoans, brachiopods, and abundant sponges (stromatoporoids, inozoans, sphinctozoans, lithistids and hexactinellids) formed a reef framework of either elevated platy structures up to 4 m in diameter and 0.35 m thick, or mounds up to 15 m in diameter. This framework was dominated by a complex intergrowth of calcimicrobes, where Rothpletzella formed the primary framework, Ortonella, and Girvanella were secondary encrusters, and Shuguria spp. occupied small crypts 2–30 mm in diameter. Contiguous columnar stromatolites up to 50 mm in height and 1 m in width grew upwards from substrate sheltered beneath large sheltered primary cavities: based on minimum growth rates of 50–100 μm/year these are estimated to have been between 500 and 1000 years old. The elevated platy community is inferred to have grown in conditions of episodic siliciclastic sediment input; the reef mounds grew during either episodes, or in localised areas, of low sedimentation. At least 14 morphospecies of spicular sponges are now identified from the Windjana Limestone, where only two were previously documented. These fore‐slope reef communities exposed in Windjana Gorge flourished in high‐energy carbonate environments dominated by coated grain sediments, and where rapid, early lithification was pervasive.Such observations demonstrate that no protracted interval of time was necessarily required for post‐extinction ‘recovery’ in regions where some reef‐building taxa survived and suitable carbonate habitats persisted or returned. Moreover, they show that new ecologies, rather than remnants of the pre‐extinction community, could be established rapidly. The reef‐slope communities of the Windjana Limestone offer little evidence to support the ideas of resurgence or invasion of taxa from deeper waters after the Frasnian/Famennian extinction. Indeed, there is evidence to suggest that similar microbial‐sponge communities were already established in margin and reef slope communities in the latest Frasnian. As such, the most dramatic ecological changes caused by the extinction occurred in back‐reef communities.

Genesis of some Carboniferous dolomites in the south and east of Ireland

Abstract Dolomitized Tournaisian rocks occur widely in Ireland. They host the major Zn-Pb ore bodies, at Lisheen and Galmoy. Despite significant research and exploration into fluid flow related to both dolomitization and base-metal mineralization, the evolution of the porosity and permeability of the dolomites and precursor limestones remains poorly understood. Study areas, both within and outside the main mineralized zones, have been chosen in an effort to unravel the evolution of the porosity and permeability of the Tournaisian successions. In each area the earliest phase of dolomitization of the limestone lithosome occurred after all of the primary porosity had been occluded. Fluid inclusions from dolomite and authigenic quartz indicate relatively high temperatures (>105 °C) of formation of all phases of dolomite. The fluid-inclusion data demonstrate that there has been no re-equilibration due to later Variscan heating. Laterally restricted bodies of dolomite at Newcastle West and Subulter may have formed under hydrothermal or geothermal conditions. The geometry of the dolomite body at Newcastle West suggests that the dolomitizing fluids moved within a steeply dipping zone. The laterally extensive Regional Dolomite of SE Ireland, examined at Lisheen and in Co. Carlow, is likely to have had a geothermal origin and to have formed at depths of more than 2.5 km. The dolomites are moderately porous with the notable development of vugs; but their permeability is low. The vugs are not related to primary cavities within the limestone.

Microbial boundstone dominated carbonate slope (Upper Carboniferous, N Spain): Microfacies, lithofacies distribution and stratal geometry

The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

AbstractInterpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ18O = 0·9 ± 1·0‰,N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ18O = 2·1 ± 0·7‰,N = 48) than the hypersaline backreef dolomite (δ18O = 3·6 ± 0·9‰,N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.

Structure and composition of organic reefs and carbonate mud mounds: concepts and categories

Defined here as ‘essentially in place calcareous deposits created by sessile organisms’, Organic Reefs are diverse and complex structures with a long geological history. Their classification has been the subject of fierce debate, often characterized by reliance on subjective features such as wave-resistance and qualitative attempts to discriminate between ‘first’ and ‘second class’ reefs. In contrast, emphasis is here placed on the objective characteristic of the type of sedimentary support, which largely determines the sedimentary composition of the deposit. Constructional and depositional processes result in three principal sedimentary components: matrix (M), essentially in place skeletons (S) and cavity/ cement (C), whose proportions can be represented on MSC triangular plots. Separately or together, these components also provide the structural support for the reef. On these compositional and structural bases, three main categories of Organic Reef are recognized: 1. Matrix-supported reefs (Agglutinated Microbial Reefs, Cluster Reefs, Segment Reefs), 2. Skeleton-supported reefs (Frame Reefs), 3. Cement-supported reefs (Cement Reefs). Agglutinated Microbial Reefs: possess laminated, clotted, or aphanitic fabrics created by microbial trapping of particulate sediment; in place skeletons and large primary cavities are rare; early cementation may provide added support; topographic relief is limited by the need for currents to provide sediment to accreting surfaces. Cluster Reefs: skeletal reefs in which essentially in place skeletons are adjacent, but not in contact, resulting in matrix support; characterized by relatively high matrix/skeleton ratios and low volumes of extra-skeletal early cement. Sediment trapping is an important corollary of skeletal growth and Cluster Reef organisms are tolerant of loose sediment. Absence of framework limits the topographic relief that Cluster Reefs can attain relative to spatial extent, and may permit bedding to develop within the reef. Close Cluster Reefs have skeletons up to 1 unit-distance apart. Spaced Cluster Reefs have skeletons more than 1, and up to 2 unit-distances apart; with increasing separation of skeletons they grade to level-bottom communities. Segment Reefs: matrix-supported reefs in which skeletons are adjacent, and may be in contact, but are mostly disarticulated and mainly parauthochtonous. Matrix abundance is high, and early cement relatively low. Moderate relief can develop in response to intense on-reef sediment production. Frame Reefs: skeletal reefs in which essentially in place skeletons (including calcified microbes) are in contact; characterized by relatively high skeleton/matrix ratio. Skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation. Simultaneously, by creating partly open shelter cavities, skeletal support may facilitate early cementation. Both relief and early lithification promote marginal talus formation. Skeletal shape and orientation distinguish: conical/stick-like, dendritic, domical, and laminar frames. Each of these may be open or filled. Open Frame Reefs: cavities remain open during the early stages of reef growth and are occupied by cryptic encrusters, early cements and internal sediment; exposed skeleton encourages endoliths. Filled Frame Reefs: inter-skeletal spaces penecontemporaneously occluded by surficial sediment during reef-growth. Cement Reefs: reefs created by cementation of essentially in place organisms. Cement provides strength and volume, mimicking skeletal growth, and can form on non-skeletal as well as skeletonized organisms. Non-skeletal Cement Reefs: created by synsedimentary cementation of essentially in place non-skeletal organisms. This converts a soft deposit with relatively poor preservation potential into a rigid lithified mass: e.g., Tufa Cement Reefs (phytoherms) in rivers and lakes and possibly Travertine Cement Reefs associated with hot springs. If the organisms are skeletal, synsedimentary cementation imparts extra strength and stability to what otherwise would be a Cluster or Frame Reef, and results in Skeleton–Cement Reefs. Cement Reefs exhibit complex relationships between cement, matrix and skeletons. Agglutinated Microbial, Cluster and Segment reefs tend to be structurally simple, have low primary relief, and may show bedding. Frame (including microbial Microframe) and Cement Reefs tend to be unbedded, structurally complex, and can have high relief. Carbonate Mud Mounds: carbonate mud-dominated deposits with topographic relief and few or no stromatolites, thrombolites or in place skeletons. Low Relief Carbonate Mud Mounds are typically thin. High Relief Carbonate Mud Mounds are thick, and internal bedding, slumping, stromatactis cavity systems, and steep marginal slopes may be common. Whereas Organic Reefs are biogenic, calcareous, and are created by essentially in place organisms, Carbonate Mud Mounds can be organic and/or inorganic in origin and it can be difficult to distinguish their origins.

Granite-hosted gold mineralization at Timbarra, northern New South Wales, Australia

The Timbarra gold deposits, located in the southern New England Fold Belt of New South Wales, Australia, represent an economically significant and distinctive member of the intrusion-related class of gold deposits. The five known deposits possess a total identified mineral resource of 16.8 Mt at 0.73 g/t gold, for a total of 396,800 contained ounces. The granites in the Timbarra region form a texturally complex, zoned pluton. The gold deposits are found within the Stanthorpe leucomonzogranite (242 to 238 Ma), which intrudes and forms a core to the more mafic, barren, Bungulla monzogranite (248 to 243 Ma). Gold is disseminated in the roof zone (upper 240 m) of a fractionated, magnetite- and ilmenite-bearing, I-type leucomonzogranite phase of the Stanthorpe body. The entire gold resource occurs in the areally extensive main leucomonzogranite pluton and is hosted by a medium- to coarse-grained granite. Disseminated ore is present in all five deposits, comprises >95% of the overall resource at Timbarra, and occurs predominantly as gently dipping, tabular to lenticular bodies that are conformably constrained beneath a fine-grained aplite carapace and internal aplite layers. The disseminated ore consists of gold-bearing muscovite–chlorite–carbonate alteration and infill of primary miarolitic cavities within massive leucomonzogranite or microgranite, and contains no discernable vein, joint, or fracture control at the outcrop or hand specimen scale. Structurally controlled mineralization forms the remaining 5% of the Timbarra resource, and comprises minor, low-density (0.02 to 0.25 per meter), vein-dikes and quartz–molybdenite veins emplaced along steeply dipping east-southeast, east-northeast, and north-northeast striking cooling joints. Both mineralization styles and alteration share a common paragenetic sequence of mineral precipitation. Quartz, perthitic K-feldspar, minor biotite, and albite are the earliest and most abundant infill minerals and commonly line primary cavities and vein-dikes. Subsequent minerals include coeval arsenopyrite, pyrite, fluorite, and molybdenite. The latest minerals include muscovite, chlorite, gold, calcite, silver–bismuth telluride, lead–bismuth telluride, and rare galena and chalcopyrite. The gold ore has a low total sulfide mineral concentration (≤1%). Ore contains elevated concentrations of Bi, Ag, Te, As, Mo, and Sb; gold is strongly correlated with Bi, Ag, and Te, but only weakly with Mo, As, and Sb. Gold grains are generally <1 to 50 µm in size, but rarer grains as large as 1 mm in diameter have been observed. Gold fineness ranges from 950 to 600, and varies both within and between individual grains for a given deposit. The moderately oxidized I-type host granite, low-sulfide (≤1%) ores, Au–Bi–Ag–Te geochemical signature, muscovite–chlorite–carbonate alteration assemblage, and low-salinity aqueous and carbonic fluids suggest that Timbarra is part of the newly recognized intrusion-related gold deposit class. Timbarra is distinguished from other intrusion-related gold deposits by the disseminated mineralization style within pervasively altered granite, forming gently dipping, tabular to lenticular ore zones.

Palaeoecology Of A Late Devonian Back Reef: Canning Basin, Western Australia

Back‐reef ecologies within the celebrated mixed carbonate‐siliciclastic Late Devonian (late Frasnian) Pillara Limestone of Windjana Gorge, in the Canning Basin, Western Australia, are re‐interpreted as being dominated by microbial communities. Proposed microbialites are expressed as weakly‐laminated, fenestral micrite, that show unsupported primary voids, peloidal textures, disseminated bioclastic debris, and traces of microfilaments. These grew as either extensive free‐standing mounds or columns, often intergrown with encrusting metazoans, or thick post‐mortem encrustations upon skeletal benthos. In some cases, microbial encrustations are inferred to have developed in protected cavities formed by progressive burial of the reef. The calcimicrobe Shuguria also shows a preferentially cryptic habit, encrusting either primary cavities formed by skeletal benthos, microbialite, or the ceilings of mm‐sized fenestrae within microbialite. A further calcimicrobe, Rothpletzella, formed columns up to 0.3 m high in areas enriched by very coarse siliciclastic sediment.Stromatoporoid sponges with a diverse range of morphologies also formed in situ growth fabrics. Monospecific thickets of closely‐aggregating dendroid stromatoporoid sponges (Stachyodes costulata), and platy‐laminar forms (?Hermatostroma spp.) were common, as were remarkably large stromatoporoids (Actinostroma spp.) that grew as isolated individuals up to 5 m in diameter. Such sponges showed impressive powers of regeneration from partial mortality, and individual clones may have been capable of substantial longevities of up to 500 years. Actinostroma spp. showed highly complex growth forms including platy‐multicolumnar (A. windjanicum), and a hitherto undescribed inferred whorl‐forming foliaceous morphology (Actinostroma sp.) reminiscent of the modern photosymbiotic coral Acropora palmata. These complex morphologies formed abundant primary cavities, previously thought to be only rarely developed in association with stromatoporoids.key words: Late Devonian, Canning Basin, reefs, palaeoecology, microbialite.

Holoprosencephaly at 10 weeks 2 days (CRL 33 mm).

The increasing relevance of sonoembryology in the management of pregnancies at risk of developmental anomalies has already been recognized in previous issues of this Journal 1–3. The latest paper from the Trondheim group, published in this issue provides, in addition, evidence of the significant role which 3-D reconstruction can play in the diagnostic process in the first trimester 4. We are now becoming familiar with features that remained unseen by earlier generations of equipment and operators and the Picture of the Month demonstrates this fact with impressive clarity. The image (Figure 1) demonstrates the three large brain cavities of the early fetus, namely the diencephalon, mesencephalon and rhombencephalon. Closer observation below the normal brain cavities reveals an abnormal smaller cystic area, the holosphere, which lies immediately superior to a finger-like projection of a proboscis. These two features strongly suggest the diagnosis of alobar holoprosencephaly. Towards the end of the sixth postmenstrual week growth and differentiation of the cranial end of the neural tube produces the three primary brain cavities, the forebrain or prosencephalon, the mid brain or mesencephalon and the hind brain or rhombencephalon. In normal embryological development the prosencephalon divides, during the sixth postmenstrual week, into two cavities, the telencephalon and the diencephalon. The cerebral hemispheres and the lateral ventricles are derivatives of the former while the thalami and the third ventricle derive from the latter. Complete failure of the telencephalon to divide during the seventh week 5 results in a single cavity or holosphere rather than two lateral ventricles typical of normal development. The holosphere is diagnostic of alobar holoprosencephaly. The less severe forms of holoprosencephaly, namely semi-lobar and lobar, arise from incomplete division of the telencephalon. The size of the holosphere varies considerably and may be small as in the case demonstrated here or may be far larger as typified by the cases reported at 10 weeks by both Wong and Turner 2,3. There is therefore no ‘typical’ appearance of alobar holoprosencephaly, based on the size of the holosphere, rather the diagnosis is made by failure to demonstrate both and separate lateral cerebral ventricles. Incomplete cleavage and development of the forebrain also results in the facial anomalies frequently associated with holoprosencephaly and as demonstrated by the Picture of the Month. The proboscis, which can be clearly seen in this sagittal view, commonly accompanies cyclopia where the eyes or their precursor anlagen are fused or partially fused into a midline structure within a single orbit. Coronal views of the fetal face would be required to confirm the presence of the cyclopia. All these features can be identified using 2-D imaging providing adequate resolution of the equipment and optimal presentation of the late embryo/early fetus. The advantage of the 3D techniques is that such appearances can be obtained and a diagnosis made when the fetus is not in such a position, thus reducing stress and anxiety to the parents and maximizing use of operator time for the clinical department. T. Chudleigh and S. Campbell

Novel reef fabrics from the Devonian Canning Basin, Western Australia

Large cement-filled cavities (0.2 to 1.5 m wide) are well developed in slope-margin sediments of the spectacular Upper Devonian (Frasnian) reefs of the Canning Basin, Western Australia, where they account for up to 50% of the primary porosity. These are here interpreted as primary reef framework cavities that formed beneath a variety of domal, tabular or laminar stromatoporoid sponges. Of particular note are those created by unusual, very thin (2 to 8 mm), laminar stromatoporoids (mainly Stachyodes australe), that formed arching, hollow domes up to 0.3 m in height and 1.5 m in diameter over the sediment surface to enclose flat-based cavities. The free undersurface of these stromatoporoids often supported a hitherto unrecognised cryptic community, dominated by pendent growth of the putative calcified cyanobacterium Renalcis, with rare intergrown lithistid sponges and spiny atrypid brachiopods. The uneven growth surface of the cryptos imparts an irregular, stromatactis-like texture to the upper surface of the remaining cavity, which is filled by early marine, finely banded, fibrous cements (mainly radiaxial calcite) interbedded with often multiple generations of geopetal sediment containing peloids and ostracod debris. This ecology yields the tabular stromatoporoid– Renalcis fabric described ubiquitously from the Canning Basin reef complex. Such unusual reef fabrics are a consequence of the ecology of shallow marine mid-Palaeozoic reefs which were quite unlike that of modern coral reefs. The frequent preservation of relatively delicate, in situ communities was due to (1) rapid and pervasive early cementation, (2) growth under non-energetic conditions, and (3) the relative insignificance of bioeroders associated with reefs at this time.