Bioherms Research Articles

Sponge diversity and community composition in Irish bathyal coral reefs

Sponge diversity and community composition in bathyal cold water coral reefs (CWRs) were examined at 500-900 m depth on the southeastern slopes of Rockall Bank and the northwestern slope of Porcupine Bank, to the west of Ireland in 2004 and 2005 with boxcores. A total of 104 boxcore samples, supplemented with 10 trawl/dredge attempts, were analyzed for the presence and abundance of sponges, using microscopical examination of (sub)samples of collected coral branches, and semi-quantitative macroscopic examination. Approximate minimum size of identified and counted sponge individuals was 1 mm. Literature data were added to the Porcupine Bank results to compensate for a less intensive sampling program in that location. Species richness and abundance were determined at local (sample diversity, pooled-sample diversity, local reef diversity), between-reef (diversity of two reef areas at 15 km distance), and regional scales (diversity of three reef areas over a distance of 200 km). Abiotic and biotic parameters including depth, the presence and cover of live coral, dead coral and sand, local reef, and orientation towards the nearest reef mound summit, were included in a constrained ordination technique (RDA); a Monte Carlo forward selection procedure was used to obtain significant predictors of variation in composition. The results of this analysis were compared with unconstrained ordination (PCA) and cluster analysis. The presence of live coral, depth and the local reefs C1 and C3 proved to be significant predictors of variation in sponge composition. The PCA and cluster analysis confirmed these results. Sample species richness was consistently heterogeneous from zero species and individuals up to 57 species and 90 individuals per (boxcore) sample. Species richness of local reefs determined from pooled samples showed the three localities studied to have similar species richness, namely 105-122 species in each location. Species richness was highest in samples with relatively low live coral cover. As in the RDA, live coral presence and depth appeared to be responsible for most of the variation observed in the cluster results. Cluster analysis of Bray-Curtis dissimilarity values of the pooled samples of all three reef localities using presence/absence data of all available samples indicated that distance appeared to structure the composition of the sponge assemblages of the three reef mound areas, but much less so within and among local reefs. Bathyal reefs of the regions to the west of Ireland were found to have a combined sponge species richness of 191 species, exceeding the richness of individual reef mound areas by c. 38-45%. Sponge presence in CWRs is clearly structured and controlled by biotic and abiotic factors. In particular, live coral presence appears a significant predictor of CWR sponge composition and diversity.

Eco-Geomorphological zonation of The Bangaram Reef, Lakshadweep

Coral reefs, which are known for rich biological diversity and productivity, are being threatened throughout the world by various natural and anthropogenic activities. The present study concentrates on establishing methodology to zone the geo-morphological and ecological zones of the Bangaram reef (of atoll type), Lakshadweep islands, using remotely sensed data and adequately supported by field data. Classification system has been evolved to zone the reefs. Comparative studies have also been carried out using image processing techniques in order to establish the suitable technique for studying the Indian reefs. The IRS LISS III images representing three different tidal conditions of the period 1998-2000 were analyzed. The unsupervised classification of both the raw images as well as principal component images gave similar information. The classified product was subjected to contextual editing. Misclassification among various classes was found to increase with the increase in the depth of the water column present over the reef. The zones identified on the image are central deep lagoon, reef knolls (rising steeply from the central deep lagoon floor), reef edge, reef platform, coralline shelf, broad shallow sandy-bottomed lagoon, beach and two islets. The zonation has helped in getting information on the presence of live corals on the Bangaram reef. This kind of study is a valuable input in assessing the health of the reefs and its diversity.

Late Early Triassic microbial reefs of the western United States: a description and model for their deposition in the aftermath of the end-Permian mass extinction

Research in the western United States to reinvestigate and characterize the previously reported stromatolites of the latest Early Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation has shown that these microbial build-ups formed patch reef mounds. Detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1–2 m of topographic relief above the seafloor. A petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics, as well as voids with early marine cements and formed in the absence of in situ metazoans. These reef mounds formed during the aftermath of the end-Permian mass extinction that spanned the Early Triassic. Due to the devastation of marine metazoans at this extinction, the Early Triassic has often been deemed a reef gap because no metazoan reefs are found at this time. Whereas colonial metazoan reefs are globally absent from Lower Triassic strata, microbial build-ups similar to these described from the western United States have been found in south China, Armenia, southern Turkey and Greenland. A regional Early Permian to late Early Triassic unconformity exists in the study region. However, analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4–8 m.y. after the end-Permian mass extinction. A depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and/or CO 2-rich deep waters may have fostered their occurrence.

Stratigraphic relationships between the Middle Devonian Shell Lake Member and the Upper Winnipegosis reef mounds, Saskatchewan Sub-basin

ABSTRACT The spatial distribution of the Shell Lake Member and its relationship with the Upper Winnipegosis reef mounds in the Saskatchewan Sub-basin record an important aspect of the transitional history of the Elk Point Basin from fully marine to desiccation. The Shell Lake Member in the study area is composed of anhydrite and laminated dolomite, and produces a good reflection on seismic profiles. The top of many Winnipegosis reef mounds gives a seismic reflection similar to the Shell Lake Member, leading to the interpretation in the literature that the Shell Lake Member is a continuous unit overlying the Winnipegosis reef mounds in the Sub-basin. Data presented in this paper demonstrate that the Shell Lake Member is not continuous because it terminates against the fully developed reef mounds. The anhydrite on top of the reef mounds previously interpreted as the Shell Lake Member is diagenetic anhydrite that occurs as replacement of the Winnipegosis carbonate at the top of reef mounds and as cement in the vugs. This interpretation is based on differences recorded in the depositional, lithological and diagenetic features of these two units. The formation of the diagenetic anhydrite of the Winnipegosis reef mounds is distinct from the primary depositional anhydrite in the Shell Lake Member based on the lithologic and diagenetic features, spatial distribution and stratigraphic relationship of the Winnipegosis carbonate and the Shell Lake Member in the study area. The spatial relationships of various lithologic units may serve as a predictor for the location of new Winnipegosis reef mounds and potash deposits. 1 Current address: Department of Geology, Brandon University, Brandon, MN R7A 6A9 End_Page 156------------------------

FOSSILIZATION OF AMMONITES AND SEDIMENTARY EVENTS IN DEEP ENVIRONMENTS OF CARBONATE PLATFORM (HIGHEST MIDDLE TO LOWEST UPPER OXFORDIAN, IBERIAN RANGE, SPAIN)

In the Ricla area (Zaragoza, Aragonese Branch of the Iberian Range), at the top of the Yatova Formation, grey-reddish wackestone limestones grade into yellow-green siliciclastic limestones of the Aldealpozo Formation. These changes of facies between the two successive formations are associated with syndepositional palaeoreliefs developed during the Late Oxfordian. The uppermost deposits of the Yatova Formation represent an Oxfordian condensed section, from the upper Bifurcatus Zone (Middle Oxfordian) and Hypselum Zone (Upper Oxfordian). These deposits are interpreted as developed in an open marine, moderately deep carbonate platform, showing uniform low-energy conditions with extremely reduced carbonate and terrigenous background sedimentation, and very low sedimentation rates. The low diversity of the benthic fauna, scarce development of sponge bioherms and ammonite populations inhabiting the platform are palaeobiological criteria which corroborate these palaeoenvironmental conditions. Ammonite assemblages are composed of Sub-Mediterranean taxa. Over 900 ammonite specimens have been collected from the upper Bifurcatus and Hypselum zones. Oppeliidae (45,2%) and Perisphinctidae (37,9 %) are dominant. Aspidoceratidae (14,3%) are common. Haploceratidae (2,2%) are scarce. Two phylloceratids and a lytoceratid have been found. Ammonoids are commonly preserved as concretionary calcareous internal moulds of reelaborated elements. Resedimented shells are scarce. The degree of packing of ammonite remains and the stratigraphical persistence display high values. Taphonomic features indicative of sedimentary starving in deep carbonate platform environments are: 1) high concentrations of reelaborated ammonites, 2) taphonic population of type two, 3) phragmocones completely filled with sediment, and 4) homogeneous concretionary internal moulds, bearing no signs of abrasion, bioerosion or dense encrusting by organisms (such as serpulids, bryozoans or oysters). In conclusion, the occurrence of these ammonite associations confirms the development of an advanced deepening phase, within a 3rd order deepening/shallowing cycle, in the Aragonese platform, during the late Bifurcatus to Hypselum zones.

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.

Origin of Late Pleistocene Bryozoan Reef Mounds; Great Australian Bight

Bryozoan-rich biogenic mounds grew periodically on the prograding carbonate slope of the central Great Australian Bight throughout Pliocene-Pleistocene time. Cores from three ODP Leg 182 drill sites provide a record of mound growth during the last 300,000 years over a stratigraphic thickness of similar to 150 m. These mounds, the first such structures described from the modern ocean, grew between paleodepths of 100 and 240 m; we infer that the upper limit of growth was established by swell wave base, and the lower boundary was fixed by an oligotrophic water mass. Detailed chronostratigraphy, based on radiometric and U-series dating, benthic foraminifer stable-isotope stratigraphy, and planktonic foraminifer abundance ratios, confirms that buildups flourished during glacial lowstands (even-numbered marine isotope stages) but were largely moribund during interglacial highstands and are not extant today. Mound floatstones are compositionally a mixture of in situ bryozoans comprising 96 genera and characterized by fenestrate, flat robust branching, encrusting, nodular-arborescent, and delicate branching growth forms. The packstone matrix comprises autochthonous and allochthonous sand-size bryozoans, benthic and planktonic foraminifers, serpulids, coralline algae, sponge spicules, peloids, and variable glauconite and quartz grains, together with mud-size ostracods, tunicate spicules, bioeroded sponge chips, and coccoliths. Intermound, allochthonous packstone and local grainstone contain similar particles, but they are conspicuously worn, abraded, blackened, and bioeroded. An integrated model of mound accretion during sea-level lowstands begins with delicate branching bryozoan floatstone that increases in bryozoan abundance and diversity upward over a thickness of 5-10 m, culminating in thin intervals of grainstone characterized by reduced diversity and locally abraded fossils. Mound accumulation was relatively rapid (30-67 cm/ky) and locally punctuated by rudstones and firmgrounds. Intermound highstand deposition was comparatively slow (17-25 cm/ky) and typified by meter-scale, fining-upward packages of packstone and grainstone or burrowed packstone, with local firmgrounds overlain by characteristically abraded particles. Mound growth during glacial periods is interpreted to have resulted from increased nutrient supply and enhanced primary productivity. Such elevated trophic resources were both regional and local, and thought to be focused in this area by cessation of Leeuwin Current flow, together with northward movement of the subtropical convergence and related dynamic mixing.

Late Pleistocene bryozoan reef mounds of the Great Australian Bight: Isotope stratigraphy and benthic foraminiferal record

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2–8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2–4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial‐interglacial fluctuations is a distinct long‐term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial‐interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long‐term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.

Herbivorous fish assemblages and herbivory pressure on Laamu Atoll, Republic of Maldives

While herbivory is recognized as a fundamental process structuring coral reef communities, herbivore assemblages and processes are poorly described for reefs in the Indian Ocean region. We quantified herbivorous fish assemblage structure (abundance and diversity) in Laamu Atoll, Republic of Maldives, in four reef habitat types: faro reef flats, faro reef slopes, inner and outer atoll reef slopes (20 sites in total). Herbivorous fish assemblages, representing a total of 30 species, grouped strongly by habitat type, with the highest absolute abundance observed on faro reef flats and lowest abundance on inside atoll rim reef slopes. Removal of Thalassia seagrass blades by ambient herbivore assemblages was used in a bioassay to assess relative herbivory pressure among four habitat types (eight sites). Also, at one site a choice herbivory assay was performed to assess herbivore preference among four benthic plants across three depth zones. Relative herbivory, as indicated by Thalassia assays, was highest on inside atoll rim reef slopes and lowest on outside atoll rim reef slopes. Thalassia consumption did not correspond to overall herbivorous fish abundance, but corresponded more closely with parrotfish abundance. In the choice assays, herbivores showed strong preferences among plant types and consumption of most plant types was higher at mid-depth than in the shallow reef flat or deep reef knoll zones.

Lower Triassic peritidal cyclic limestone: an example of anachronistic carbonate facies from the Great Bank of Guizhou, Nanpanjiang Basin, Guizhou province, South China

Most shallow marine sections described from the Lower Triassic formed in mixed carbonate-siliciclastic systems. Shallow marine limestones from an isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin, South China, provide a unique opportunity to examine a pure peritidal carbonate depositional system. The platform had a low-relief bank profile in the Early Triassic with oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and pelagics, debris-flow deposits and turbidites on gentle basin-margin slopes. Interior strata are 265 m thick beginning in the lowermost Triassic with Renalcis biostromes, followed by lime mudstone, oolite, and cyclic peritidal limestone. The cyclic limestone is Olenekian in age, 164 m thick, and contains up to 83 cycles (parasequences). Parasequences are 0.2–7.4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by Renalcis reef mounds or biostromes and capped by intertidal flaser-bedded ribbon rock. The skeletal packstone and reef mounds contain a low-diversity biota of cyanobacteria, echinoderms, bivalves, gastropods, lingulid brachiopods, spirorbids, and ostracodes. Ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae, scour surfaces, ripple cross lamination, lime-mud drapes, and minor prism cracks. Parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness, defining a third-order sequence boundary and several 4th-order parasequence sets that correlate between interior sections. Parasequence stacking patterns similar to those of Lower Palaeozoic green-house sequences and different from ice-house sequences. The Lower Triassic platform-interior parasequences represent an anachronistic facies in that they are strikingly similar to the common Renalcis-mound, ribbon rock parasequences of the Lower Palaeozoic and different from other Permo–Triassic parasequences (e.g. Capitan shelf parasequences or Loferites). Facies similarities such as Renalcis mounds and flaser-bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end-Permian extinction. Similarities in parasequence stacking patterns reflect low-amplitude, high-frequency sea-level fluctuations resulting from green-house conditions common to the Early Palaeozoic and Early Triassic.

Palaeoenvironment and palaeoecology of three Cretaceous snakes: Pachyophis, Pachyrhachis, and Dinilysia

The palaeoecology of three Late Cretaceous snakes is evaluated. Pachyophis woodwardi Nopcsa, 1923 and Pachyrhachis problematicus Haas, 1979, are Cenomanian in age and are found in carbonate rocks deposited in marine inter-reef basin environments of the European and African Tethys Sea. Dinilysia patagonica Woodward, 1901, Coniacian in age, is considered closely allied to living anilioid snakes, and is found in clastic rocks deposited in a terrestrial inter-dune basin environment in northern Patagonia, Argentina. All three snakes are known from well preserved and articulated specimens found in sediments where detailed sedimentological and taphonomic analyses are possible. Pachyophis and Pachyrhachis were laterally compressed, have pachyostotic ribs and vertebrae, and small, narrow heads. These two snakes are interpreted as aquatic predators living in and around the margins of reef mounds on a shallow water carbonate platform. Dinilysia was a large bodied snake with a relatively large head, and is interpreted here as a terrestrial predator that lived in a dry, interdune basin environment dominated by aeolian sedimentation. Sedimentary units preserve ichnological evidence of burrowing insects and rooting plants.

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Palaeoecological significance of calcimicrobial communities during ramp evolution: An example from the Lower Cambrian of Germany

The palaeoecology of calcimicrobial communities from the only Gondwana-related Lower Cambrian in Central Europe (eastern Germany, carbonates and siliciclastics) has been studied. Six morphological groups of calcimicrobes are described. Some of them show a significant ability for sediment stabilization and construction of biohermal reef mounds. Other types of calcimicrobes were more common in biostromal thickets. Some of them were capable of populating different environments, growing in different modes and on different substrates. AnEpiphyton-archaeocyathan reef mound is described, illustrating the importance of calcimicrobes for mound formation. The fossil communities together with a complex of sedimentary features allow a reconstruction of the depositional history of the environment. Based on comparison with similar Gondwanan Lower Cambrian successions (Sardinia/Italy, Spain) and facies development a model is proposed describing the sedimentary history. Three depositional stages are distinguished: (1) deep subtidal ramp, (2) shallow subtidal ramp, (3) shallow subtidal to intertidal mixed siliciclastic-carbonate ramp with migrating oolitic shoals. In spite of similarities with the deposits in southern Europe, some distinct differences exist with respect to the succession of facies, the completeness of the sections, the fossil spectrum, and the nature of the siliciclastic sediments. For the German Lower Cambrian, a facies development from a low energy deep environment to a high energy shallow environment (partly restricted and with some evaporites) can be reconstructed. As compared with Sardinia and Spain, the depositional environment of the eastern German Lower Cambrian successions was predominantly characterized by low-energy conditions.

Early Triassic calcimicrobial mounds and biostromes of the Nanpanjiang basin, south China

Stanley’s model (1988) proposed that a Permiantype reef fauna survived the Early Triassic reefless period in yet undiscovered refuges in the ancestral Pacific ocean and then reappeared as the builders of Middle Triassic reefs. Flugel (1994), in contrast, suggested that the Middle Triassic reefbuilding fauna are not holdovers from the Permian, but bear only a superficial resemblance to Permian fossils. Recent publications reflect a consensus that the Early Triassic lacked reefs and reef mounds although some authors acknowledged the possibility that reef-like structures may have existed in poorly explored areas such as south China and Russia (Senowbari-Daryan et al., 1993; Flugel, 1994). It is shown herein that Calcimicrobial framework mounds and biostromes existed in the Early Triassic in the Nanpanjiang basin of south China. Organic framework in this paper refers to a three dimensional, biologically-constructed network of calcified fossils that forms a rigid mass and supports an interconnected network of internal cavities that were open during accumulation. Although the calcimicrobial frameworks discussed herein are much smaller than frameworks of modern metazoan reefs, the term framestone is more appropriate for these fabrics than other reef-rock terms such as bindstone or bafflestone (Embry and Klovan, 1971). These calcimicrobial mounds and biostromes are important as the first organic framestones documented from the Early Triassic.

Carbonate platform drowning in a foreland setting; the Mid-Carboniferous platform in western Urals (Russia)

ABSTRACT The stratigraphic evolution through time of the mid-Carboniferous Russian Platform deposited on the distal margin of the West Uralian prograding foredeep basin can be observed in the approximately 3000-m-thick sedimentary cover that constitutes the upper Paleozoic foreland (mid-Carboniferous-Permian) of the Uralian fold belt. It is composed of wave-dominated, low-angle carbonate ramps of late Visean to early Moscovian age and carbonate platforms bordered by slope reef mounds from the Kasimovian to the late Artinskian-early Kungurian. Eastward, in the foredeep, the carbonate platforms pass into terrigenous deposits. A complete transgressive-progradational section at the transition between carbonate ramp and terrigenous deposits exhibits, from base to top, (1) deepening-upward carbonate ramp deposits, (2) a shaly condensed section with volcaniclastics, glauconite, and phosphorites of late Moscovian to early Asselian age (the transgressive part), and (3) a shallowing-upward section of turbidites ending in coarse-grained fan deltas, evaporites, and meandering-river redbeds by the earliest Triassic (the progradational section). This paper focuses on the uppermost part of the carbonate ramp deposits (Serpukhovian to early Moscovian). These ramp facies comprise peritidal channels and mud flats, wave- to storm-dominated shoals, and patch reefs that are overlain by channel-attached sand lobes of turbidites, black shales, and radiolarites of the condensed section. This large-scale deepening-upward trend from inner-ramp to outer-ramp environments reflects a change in the depositional profile, from a tide- and wave-dominated flat ramp to a wave- and storm-dominated, steepened ramp margin and then to foreslope turbidites. Correlation of sections, dated by fusulinid faunas in southern Urals, shows that the ramp evolved from emergent in the north (Perm-Solikamsk area), as evidenced by karstification, and deepened to the south (Belaya area) with emplacement of carbonaceous foreslope debris flows and turbidites, capped by deep-marine black shales and radiolarites. The time gap increases northward and reaches a span including the Siuransky pro parte, the Akavassky, the Askynbashky, and the Asatausky pro parte biohorizons. The mapping of these facies belts across all the Urals shows that carbonate ramps wedge out in the middle Urals and give rise to widely distributed, diversified, and deeper marine facies belts not only to the South, towards the Precaspian depression, but also to the North, towards the polar Urals. The drowning of these carbonate ramps and the cutoff in carbonate production on the Russian Platform during the mid-Carboniferous is explained by a succession of low-frequency flexure-relaxation cycles in the foreland associated with dilution by siliciclastics, outpouring of volcaniclastics, and changes in the nature and direction of the sediment input and high-frequency cycles mainly controlled by eustasy.

Type and evolution of landscapes of Nansha Islands

Landscapes of the Nansha Islands may be divided into five types: tropical marine organism-breeding landscape of reef knoll like Zengmu Shoal, Wan’an Bank and so on; tropical marine organism-breeding landscape of atoll including all emerged reefs and most submerged reefs; tropical evergreen arbor-bush forest and phosphorous lime soil landscape of limesand Islets like Taiping Islet, Nanwei islet and so on; tropical shallow sea marine organism-breeding landscape of southern continental shelf; tropical oceanic and deep-sea marine organism landscape in middle and north parts of the Nansha Islands area. These five landscape types may be also summed up as two categories, one is tropical sea landscape including those in shallow and deep sea, the other is tropical coral reef landscape including those of reef knoll, atoll and limesand islet. This paper outlines the evolutional model of landscapes of the Nansha Islands. The distribution and evolution of landscape types are related with palaeogeography and modern environmental conditions. The former shallow sea of the Nansha islands was coastal zone in the Late Pleistocene epoch. Deep sea is evolved from shallow sea due to long and slow subsidence of crust. Modern coral reefs develop on old reef top of the Late pleistocene epoch or on baserock of continental shelf in the Holocene due to the rise of sea level in postglacial. Limesand islet is in the peak of developmental stage.

Ahermatypic coral banks off mid-Norway; evidence for a link with seepage of light hydrocarbons

Large (up to 31-meter high) coral banks (or bioherms) occur on the continental shelf off mid-Norway at water depths between 220 and 310 meters. They are built up by the cold-water, ahermatypic, scleractinian coral Lophelia pertusa (L.). A 3-km-wide and 200-km-long traverse was mapped geophysically across a large part of the mid-Norway shelf. A total of 57 suspected individual banks were found. Although they occur in local clusters of up to 9 banks per km 2 , the mean density along the whole transect is only 0.09 suspected banks per km 2 , with the highest regional density (1.2 banks per km 2 ) occurring above subcropping presumed Paleocene bedrock. A detailed investigation employing an ROV (remotely operated vehicle) was conducted of a cluster consisting of 9 individual banks. Based on geophysical, visual, geochemical, radiocarbon, and other analyses, we conclude that at least some of the coral banks have been forming at the same locality for over 8,000 years, and that there is a strong correlation between coral-bank occurrence and relatively high values of light hydrocarbons (methane, ethane, propane, and n-butane) in near-surface sediments. To explain the structure and distribution of these coral banks, we propose a model where they form as a consequence of local fertilization that results from focused hydrocarbon micro-seepage of deep thermogenic hydrocarbons migrating to the surface along inclined, permeable sedimentary strata. A direct corollary of this model is that if and when the source of local fertilization is shut off, the bioherms die out. This possibly could be the reason why extinct bioherms are more common than live ones in some areas of the ocean.

Controls on facies architecture of a large Triassic carbonate platform; the Great Bank of Guizhou, Nanpanjiang Basin, South China

ABSTRACT The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Controls on Reservoir Quality of an Upper Jurassic Reef Mound in the Palmers Wood Field Area, Weald Basin, Southern England

An Upper Jurassic (Oxfordian) reef mound in the Palmers Wood field area of the Weald basin, southern England, clearly shows the relationship of facies, diagenesis, and porosity development to relative sea level changes. The coral-microbial reef mound was initiated over a drowned oolite shoal during a third-order marine transgression and exhibits changes in coral morphology from base to top as the reef mound caught up with sea level during the subsequent highstand. During the highstand, extensive encrustation of the reef mound took place by microbial, stromatolitic cements, with subsequent porosity loss. During the following lowstand of relative sea level, extensive leaching of the aragonitic corals took place, and a lowstand wedge accumulated down-ramp. Porosity was lost during burial-related cementation and compaction. Coral-microbial reef mounds of this type have moderate potential for porosity formation, unlike tighter, deeper ramp reefs in the Jurassic, which are less prone to subaerial leaching and have less potential for secondary porosity development because they are less rich in aragonitic components. Although the Upper Jurassic reef mound is only a minor contributor to reservoirs in the Palmers Wood field, it may provide a model for other Upper Jurassic reef plays.