Bioclastic Limestone Research Articles

Coastal dynamics of uplifted and emerged late Pleistocene near-shore coral patch reefs at fins (eastern coastal Oman, Gulf of Oman)

We investigated two coral patch reefs of probably Weichselian age (Wyns et al., 1992a) which have been uplifted above sea-level in the coastal setting of uplifted marine terraces adjacent to the Oman Mountains. Uplift is also manifested in raised beach conglomerates and a wave-cut notch. We document some of the main reef features in the apprehension that building/development will preclude the study of the reefs in the near future. The reefs formed above a hardground of bioclastic limestone (mudstone to wackestone), characterized by intense bioerosion and red algae. The red algae may occur so plentiful to form (very localized) bafflestones. The diagenetic history of the rocks includes a transition from a marine environment (first cement) to meteoric conditions (dissolution and second cement). The reefs are 1.5 m thick and display planar as well as gently ocean-dipping reef tops, suggesting exposure to the wave-cut activity and, thus, shallow and nearshore deposition. The reefs are dominated by one coral species, identified as Cyphastrea serailia. On the protected side of the reefs the corals developed wavy and columnar growth forms while on the ocean facing side irregularly shaped exoskeletons developed. Our GPS-based topographic survey revealed an elevation of the two reefs of 21.5 m ± 0.06 m above sea-level. Taking into account (1) the global Holocene sea-level rise of 120 m ± 20 m (Pirazzoli and Pluet, 1991), (2) formation of the reefs slightly below sea-level (∼1.5 m) as well as (3) their present position above sea-level, we quantify the total upward transfer of the reefs to be 143 m and the average uplift rate to be 6.8 or 4.6 mm/yr depending on the correct age (either ∼21,000 or ∼31,000 yr B.P.). As the causes of uplift we mainly consider isostatic rebound following the late Cretaceous formation of the Oman Mountains (Yuan et al., 2016). The terrace development went through the following stages: (1) The patch reefs formed in shallow water. (2) Initial, gentle uplift caused abrasion of the reef tops. (3) Uplift continued and beach conglomerate 1was deposited on top of the reefs. (4) Abrasion continued, creating an abrasion platform. Uplift and creation of borings in platform required time and an interval of relatively slow uplift and tectonically and climatically stable conditions. (5) Uplift took place, and a wave-cut notch formed below the reefs. (6) Notch formation, including the borings into the notch, required time and stable conditions and was followed by deposition of beach conglomerate 2. (7) Finally, emergence/exposure of the notch and conglomerate 2 above sea-level ensued due to pronounced uplift, possibly associated with faulting and earthquakes. Formation of the reefs and the abrasion platform can be correlated with MIS 3 (Marine Isotope Stage 3) and notch formation with MIS 1.

Platy corals from the Middle Triassic of Upper Silesia, Poland: Implications for photosymbiosis in the first scleractinians

Coral patch reefs from Middle Triassic (upper Pelsonian–lower Illyrian) strata in the Upper Silesia region of southern Poland (Germanic domain of the Peri-Tethys) are rare examples of the first scleractinian buildups. The shallowing-upward succession in the Tarnów Opolski quarry records a transition from sponge to coral patch reefs interbedded with bioclastic limestones. Coral pillarstones built by thin, branching Volzeia szulci are succeeded by platy Pamiroseris silesiaca constructing two platestone layers, each up to 50cm thick. Serial sections through platestones revealed flat to undulose growth form of P. silesiaca. The maximum observable dimension of the coral plates is 24cm wide (typically up to 12cm), while thickness of most plates is 1–1.5cm. Coral plates are interlayered with crinoidal wacke- to packstone and microbialites, which are locally important component of the platestone. Platy corals grew in a shallow, turbid-water environment with changing, but dominantly moderate hydrodynamics. Net sedimentation was low, as indicated by the epibionts encrusted to the undersides of the coral plates, and locally common microbial fabrics. Growth-interruptions of P. silesiaca record events of storm-induced sediment input and resuspension of carbonate mud. Based on the euphotic floor model, the flattened morphology of P. silesiaca is interpreted as an optimal growth form in a turbid, low-light environment. Platy scleractinian assemblages from Silesia are the oldest occurrences of this ecological coral group (mostly representing deeper, mesophotic habitats) and support the hypothesis that some of the earliest scleractinians had photosymbionts like modern zooxanthellae.

First occurence of rudists (Bivalvia) from the Coniacian-Santonian limestones of the Saharan platform, southern Tunisia: Description, biostratigraphy and correlation

This study focuses on the description of rudists observed within the Berressef Formation from the Matmata area in the Saharan platform of southern Tunisia. The study of rudists is based on two measured stratigraphic sections and five observation points. It has revealed for the first time the presence of Coniacian-Santonian rudists. Two rudist associations were identified: 1) Biradiolites angulosus-Biradiolites martellii-Radiolites trigeri association (Coniacian) containing Durania cf. arnaudi, Bournonia cf. gardonica, Biradiolites sp., Hippurites sp. and Apricardia sp.; and 2) Radiolites dario-Bournonia fascicularis association (late Coniacian-Santonian) including Sauvagesia sp. and Radiolites sp. These associations have showed a remarkable similarity with those of central-south of Apennines, Italy. Rudists of the Berressef Formation are mostly elevators in growth position. They present cylindrical to elongated cylindrical shapes and they are organized in isolated individuals, bouquets and clusters. The rudist facies A is mostly represented by rudists in growth position. The facies B consisted of bioclastic limestones rich in rudists fragments, which is considered to have been deposited not far from their original palaeoenvironment.

Palaeoecologic Significance of the Callovian-Oxfordian Trace Fossils of Gangeshwar Dome, Southeast of Bhuj, Mainland Kachchh, India

The shallow marine deposits of the Late-Middle Jurassic (Callovian–Oxfordian) Jumara Formation of the Gangeshwar Dome of Mainland Kachchh, India, comprise a succession of ~247 m thick clastic sediments with few non-clastic bands and contain a diverse group of ichnofauna. The succession is subdivided into seven lithofacies, viz., laminated shale-siltstone facies (LSS), sheet sandstone facies (SS), herringbone sandstone facies (HS), bivalve sandstone facies (BS), bioclastic limestone facies (BL), intraformational conglomerate facies (IC) and oolitic limestone facies (OL). The ichnofaunal study shows 29 ichnospecies of 23 ichnogenera including Arenicolites, Bifungites, Bolonia, Chondrites, Didymaulichnus, Diplocraterion, Gyrochorte, Helminthopsis, Isopodichnus, Laevicyclus, Lockeia, Monocraterion, Taenidium, Ophiomorpha, Palaeophycus, Planolites, Phycodes, Protopalaeodictyon, Rhizocorallium, Skolithos, Thalassinoides, Tisoa, and Zoophycos. These trace fossils are distributed among nine ichnocoenose, characterized by Chondrites, Diplocraterion, Gyrochorte, Ophiomorpha, Rhizocorallium, Skolithos, Taenidium, Thalassinoides and Zoophycos. Their occurrence in the facies corresponds to their trophic and ethological properties. The colonisation of the opportunistic Diplocraterion and the Skolithos ichnocoenose shows a high density and marks foreshore/nearshore environmental conditions. The Gyrochorte, the Rhizocorallium, the Taenidium and the Thalassinoides ichnocoenose indicate the typically lower energy zone of the shoreface-offshore region. The Chondrites ichnocoenosis indicates fluctuation in bottom water oxygen while the Zoophycos ichnocoenosis typically exploited a calm water niche in the offshore region. These ichnocoenose recur throughout the sequence and belong to the Skolithos and the Cruziana ichnofacies which marked changes in energy gradient, substrate stability, water depth and mode of life of invertebrate organisms. The study of trace fossil assemblages with sediment characteristics gives a detailed and accurate picture of foreshore to offshore palaeoenvironmental conditions.

An improved method for permeability estimation of the bioclastic limestone reservoir based on NMR data

Permeability is an important parameter in formation evaluation since it controls the fluid transportation of porous rocks. However, it is challengeable to compute the permeability of bioclastic limestone reservoirs by conventional methods linking petrophysical and geophysical data, due to the complex pore distributions. A new method is presented to estimate the permeability based on laboratory and downhole nuclear magnetic resonance (NMR) measurements. We divide the pore space into four intervals by the inflection points between the pore radius and the transversal relaxation time. Relationships between permeability and percentages of different pore intervals are investigated to investigate influential factors on the fluid transportation. Furthermore, an empirical model, which takes into account of the pore size distributions, is presented to compute the permeability. 212 core samples in our case show that the accuracy of permeability calculation is improved from 0.542 (SDR model), 0.507 (TIM model), 0.455 (conventional porosity-permeability regressions) to 0.803. To enhance the precision of downhole application of the new model, we developed a fluid correction algorithm to construct the water spectrum of in-situ NMR data, aiming to eliminate the influence of oil on the magnetization. The result reveals that permeability is positively correlated with percentages of mega-pores and macro-pores, but negatively correlated with the percentage of micro-pores. Poor correlation is observed between permeability and the percentage of meso-pores. NMR magnetizations and T2 spectrums after the fluid correction agree well with laboratory results for samples saturated with water. Field application indicates that the improved method provides better performance than conventional models such as Schlumberger-Doll Research equation, Timur-Coates equation, and porosity-permeability regressions.

Carbon-oxygen isotopes and rare earth elements as an exploration vector for Carlin-type gold deposits: A case study of the Shuiyindong gold deposit, Guizhou Province, SW China

The Shuiyindong gold deposit is a deeply concealed strata-bound Carlin-type deposit in southwestern Guizhou Province, China. The deposit lies on the eastern limb of the Huijiabao anticline with ores mainly along the anticline axis and hosted in bioclastic limestone, containing calcite veins, of the Permian Longtan Formation units. In this study, we measured carbon and oxygen isotopic ratios and rare earth element (REE) concentrations of the host rocks and calcite veins along a profile across the Shuiyindong deposit. Orebodies in the upper unit of the Longtan Formation have higher δ18O values (20.6–22.4‰) and lower δ13C values (−3.7 to −0.5‰) than the country rocks (δ18O: 18.8–21.4‰; δ13C: −0.7 to 0.8‰). However, there are no obvious trends of δ18O and δ13C values from the country rocks to the orebodies in the middle unit of the Longtan Formation. The spatial distribution of the calcite veins displays distinct halos of δ13C and δ18O values and REE concentrations. Calcite veins along the anticlinal axis and major reverse fault are enriched in Middle REE (Sm, Eu, Gd, and Tb) and 18O and depleted in 13C. Surficial veining calcite-filled fractures/faults that connect to deep concealed strata-bound gold mineralization systems can be vectors toward deep ores in southwestern Guizhou Province, China.

A mid-Tournaisian-late Viséan carbonate ramp reconstructed from nappes and olistolites in the southern Montagne Noire (France)

The mainly calcareous succession from the mid Tournaisian to the lower part of the upper Viséan in the Southern Montagne Noire Mont Peyroux nappes and Cabrières slices is sedimentologically revised. From the late Viséan, the Variscan orogeny affected the area, and the studied Mississippian part of the succession was considered as part (together with the Devonian) of a period of tectonic quiescence, later included as nappes and olistolites in a wildflysch. This study presents the facies from pelagic limestones, calciturbidites, bioclastic limestones and microbial limestones, and the relationship between platform and calciturbidites in the pre-orogenic context of the Montagne Noire basin during the Mississippian. The differences in the lithostratigraphy is analyzed by means of four tectonostratigraphical units (Mont Peyroux nappe, Roquessels band, Cabrières area and La Serre band) previously defined by other authors, that together with the deep-water Vieussan-Caragnas unit (not studied here due to the poverty in chronostratigraphical data) allow reconstructing the original carbonate platform. The latter seems to correspond to a ramp for the latest Tournaisian to early Viséan, and a distally-steppened ramp from the middle Viséan to the early late Viséan, including basin plain to slope as the deepest water environments, and middle ramp as the shallowest, except for the uppermost part of the succession, where the inner ramp is recognized. During this period, the shallowing tendency in the tectonostratigraphic units is well marked: Mont Peyroux nappe – Roquessels band – Cabrières area – La Serre band (from deeper to shallower), which differs from the shallowing trend defined by previous authors during the Devonian. Strong similarities are observed with the preserved Mississippian carbonates in the Mouthoumet Massif to the south, which might represent the shallowest water deposits of the ramp, not preversed in the Cabrières-La Serre areas.

Detrital provenance, depositional environment, and palaeogeographic implications of Lower Triassic marine sediments in central Tibet

ABSTRACTWe present our new investigation into the depositional environment and provenance of the Yingshuiquan Formation in the central Qiangtang region of northern Tibet, in order to further our understanding of the environment of the Longmu Co–Shuanghu Palaeo–Tethys during the Early Triassic. The Yingshuiquan Formation is composed of oolitic limestone, calcareous sandstone, calcarenite, thin-bedded ribbon limestone, bioclastic limestone, and coarse oolite limestone that were deposited in a shallow-marine basin and contain abundant Lower Triassic conodont fossils (e.g. Hadrodontina anceps, Pachycladina sp., gen. et sp, Pachyclaina oblique, Hibbardelloides sp). We selected detrital zircons from four calcareous sandstone samples for U–Pb dating, yielding minimum age peaks of 263, 269, 275, and 280 Ma, respectively, and a minimum age of 249 Ma, based on several zircons around the same age. Analysis of the conodont biofacies and zircon LA-ICP-MS dating of calcareous sandstone indicates that the data is consistent with deposition in the Early Triassic. The Yingshuiquan Formation records Early Triassic shallow-water sediment in the Longmu Co–Shuanghu Palaeo–Tethys, and has a Southern Qiangtang and Northern Qiangtang terranes provenance. During the Early Triassic, the carbonate sediments of the Yingshuiquan Formation were deposited in an active environment around the Longmu Co–Shuanghu Palaeo–Tethys, which has became a residual sea basin.

A Permian-Triassic boundary microbialite deposit from the eastern Yangtze Platform (Jiangxi Province, South China): Geobiologic features, ecosystem composition and redox conditions

A Permian-Triassic (P-Tr) boundary microbialite (PTB-microbialite) deposit occurs near Xiushui on the southern margin of the eastern Yangtze Platform, South China. This 2-m-thick microbialite overlies uppermost Permian bioclastic limestone with a 5-cm-thick oolite-like grainstone layer at the contact. This PTB-microbialite consists of lower thrombolite (1.5-m thick) and an upper dendrolite (0.5-m thick). The thrombolite interval is characterized by a clotted texture of reddish sparitic patches embedded in a gray micritic matrix, whereas the dendrolite texture consists of brownish sparitic patches embedded within a yellow micritic matrix. Abundant calcified microbes occur in both the thrombolite and the dendrolite, including 1) columnar and clustered fabrics, which are especially rich in the thrombolite, assigned as products of Gakhumella or related microorganisms and 2) microspheroids, 40–60μm in diameter, with different interior structures. These calcimicrobes are considered to have played an important role in constructing the Xiushui PTB-microbialite. The demise of the microbialite was likely due to a sea-level fall during the early Griesbachian (lowermost Triassic), which resulted in a lowering of the wave base thereby destroying the microbialite frameworks and facilitating the deposition of winnowed skeletal grainstone. Metazoan community structures within the microbialite changed sharply across the end-Permian extinction horizon, which is placed at a sharp negative shift in δ13Ccarb. Quantitative analyses show that the Xiushui microbialite community is similar to that of Chongyang and Cili microbialites. This is probably because all of these microbialites grew along the margins of the Lower Yangtze region during the P-Tr transition. However, unlike other PTB-microbialites, the Xiushui ecosystem contains fairly abundant metazoans, which are dominated by ostracods with minor occurrences of foraminifers, microgastropods and microconchids. Both the ostracod assemblage and the pyrite framboid analyses indicate an upper dysoxic zone to a well-oxygenated condition for the Xiushui microbialite. This implies that some PTB-microbialite ecosystems developed in conditions that were not always harsh; instead, some were hospitable for some metazoans to survive immediately after the end-Permian crisis. Considering that the redox conditions indicated by PTB-microbialites are quite variable, we infer that oxygen levels probably were not the crucial factor affecting the growth of the PTB-microbialites.

Litho- and biostratigraphy, facies patterns and depositional sequences of the Cenomanian-Turonian deposits in the Ksour Mountains (Saharan Atlas, Algeria)

The Cenomanian-Turonian deposits exposed in the Ksour Mountains, western part of the Saharan Atlas (Algeria), document marine shelf environments that had been thriving on the North African passive margin, connected northwards to the Tethys Ocean, and fringed southwards by the Saharan craton. Their lithological, palaeontological, and sedimentological characteristics have been investigated to provide new insights into the biostratigraphy, palaeo-environmental evolution and sea-level changes in this western part of the Saharan Atlas. Three formations are recognized, from base to top of the studied succession: 1. The El Rhelida Formation comprising two informal units: the mixed siliciclastic-carbonate unit deposited under different flow regime conditions, from shoreline to backshore environments, and the limestone–claystone unit including coastal mudflat deposits prone to storm events. The early Cenomanian age of the El Rhelida Formation is supported by vertebrate assemblages. 2. The Mdaouer Formation comprising two units: the evaporitic unit dominated by claystone and evaporite alternations deposited on a flat coastal sabkha with occasional storms, and the marlstone–limestone unit formed in a peritidal-lagoonal environment. The Mdaouer Formation is of early-middle Cenomanian age. 3. The Rhoundjaïa Formation comprising three units: the lower limestone unit consisting of relatively homogeneous fossiliferous limestones; the middle marly unit composed of marlstone and bioclastic limestones, and the upper limestone unit consisting of carbonates showing vertical variations in faunal content and stratonomy. The Rhoundjaïa Formation was deposited in homoclinal ramp setting. Ammonite data indicate an early late Cenomanian to early Turonian age for this interval. Within an overall transgressive trend, the Cenomanian-Turonian deposits of the Ksour Mountains record three third-order depositional sequences bounded each by regional discontinuities.

Permian–Triassic boundary microbialites at Zuodeng Section, Guangxi Province, South China: Geobiology and palaeoceanographic implications

A previously unknown microbialite bed in the Permian–Triassic (P–Tr) boundary beds of Zuodeng section, Tiandong County, Guangxi, South China comprises a thin (5cm maximum thickness) stromatolite in the lower part and the remaining 6m is thrombolite. The Zuodeng microbialite has a pronounced irregular contact between the latest Permian bioclastic limestone and microbialite, as in other sites in the region. The stromatolite comprises low-relief columnar and broad domal geometries, containing faint laminations. The thrombolite displays an irregular mixture of sparitic dark coloured altered microbial fabric and light coloured interstitial sediment in polished blocks. Abundant microproblematic calcimicrobe structures identified here as Gakhumella are preserved in dark coloured laminated areas of the stromatolite and sparitic areas in thrombolites (i.e. the calcimicrobial part, not the interstitial sediment) and are orientated perpendicular to stromatolitic laminae. Each Gakhumella individual has densely arranged segments, which form a column- to fan-shaped structure. Single segments are arch-shaped and form a thin chamber between segments. Gakhumella individuals in the stromatolite and thrombolite are slightly different from each other, but are readily distinguished from the Gakhumella- and Renalcis-like fossils reported from other P–Tr boundary microbialites in having a smaller size, unbranching columns and densely arranged, arch-shaped segments. Renalcids usually possess a larger body size and branching, lobate outlines. Filament sheath aggregates are also observed in the stromatolite and they are all orientated in one direction. Both Gakhumella and filament sheath aggregates may be photosynthetic algae, which may have played an important role in constructing the Zuodeng microbialites. Other calcimicrobes in the Zuodeng microbialite are spheroids, of which a total of five morphological types are recognized from both stromatolite and thrombolite: (1) sparry calcite spheroid without outer sheaths, (2) a large sparry calcite nucleus coated with a thin sparry calcite sheath, (3) a large nucleus of micrite framboid aggregates rimmed by a thin sparry calcite sheath (bacterial clump-like spheroids), (4) a large nucleus of micrite framboid aggregates coated with a thin micritic sheath, and (5) a small sparry nuclei rimmed by coarse-grained, radiated euhedral rays. The irregular contact beneath the Zuodeng microbialites is interpreted as a subaerial exposure surface due to regional regression in South China. The demise of the Zuodeng microbialites may have been due to rapid rise in sea-level because they grew in relatively shallow marine conditions and are overlain by muddy limestones containing pelagic conodonts. Also siliciclastic content increases above the microbialite, suggesting a possible climate-related increase in weathering as the transgression progressed.

Chapter 11 The Archipelago Group: current understanding

Abstract The geology and stratigraphy of the Archipelago Group are reviewed and new data presented to improve understanding. Major lithofacies are bioclastic limestones, bioturbated mottled marlstones, micritic limestones, trace fossil-bearing mudstones, quartzose sandstones and reworked volcanic material of Mio-Pliocene and Pleistocene age. The succession overlies the Pre-Neogene submarine fan turbidites and tectonic mélanges of ophiolite derivation and Mithakhari rocks, and is overlain by Quaternary deposits of late Pleistocene–Holocene age. The Neogene sequences were deposited in wave- and current-agitated shallow-marine intertidal and relatively deeper-water subtidal, nearshore and offshore shelfal environments. The existing stratigraphic framework is examined and suggestions made for improvement.

MIOCENE SCLERACTINIAN CORALS OF GAVDOS ISLAND, SOUTHERN GREECE: IMPLICATIONS FOR TECTONIC CONTROL AND SEA-LEVEL CHANGES

Low-diversity scleractinian patch reefs that have been developed, during Early to Middle Tortonian, in Gavdos island are studied, aiming in a better understanding of the time and space relationships of the reef development and the associated basin fill evolution. Gavdos island consists part of a tectonically active setting constituting the southernmost extension of the Hellenic arc (Eastern Mediterranean). A representative Tortonian section (Bo section), located in the northwest part of the island, which hosts a mass occurrence of hermatypic corals, is studied. Scleractinian samples were collected from the upper 12 m of the section, which is characterized by interchanges of marly limestone and compact marls. The reefs are represented by the coral species Heliastraea oligophylla, Porites maicientensis, Thegioastraea roasendai and Porites collegniana. The microfacies analysis showed that the mediumto-thin-bedded carbonates of the limestone-marl alternations comprise patch reefs (boundstones-framestones, SMF 16, sensu Wilson, 1975, FZ 7-8, sensu Flügel 1982) consisted of screractinian corals. Corals are associated with corallinacean algae. Patch reefs are associated by bioclastic packstones-floatstones, characterized by benthic foraminifera, calcareous algae, spines of echinoids, gastropods and molluscs. Reefs are laterally associated by a bioclastic limestone rich in planktonic forams, associated by detrital material (fragments of quartz, feldspars, quartzites and cherts). The studied reef facies have been dolomitized and cemented in the meteoric realm. In places pseudomorphs after evaporites have been observed, tending to occlude cavities resulted after dissolution. The depositional environment corresponds to a moderate to high-energy inner platform setting, experiencing open-ocean influences. The pattern of coralgal reef development during Miocene, in the tectonically controlled Gavdos island, represents a complex interaction of tectonic activity and global sea-level changes.

Windward and leeward margins of an Upper Ordovician carbonate platform in the Central Tarim Uplift, Xinjiang, northwestern China

In the Central Uplift of the Tarim Basin, northwestern China, the Katian (Upper Ordovician) limestones of the Lianglitag Formation extend from outcrops in the Bachu area to the subsurface in the Tazhong Oil Field (TZOF) area. Reefs and inner platform deposits are the dominant facies throughout the formation. The reef composition illustrates a palaeoecologic gradient from microbial-dominated buildups on the northwestern margin to metazoan-dominated reefs on the eastern and southern margins of the platform. Between these areas no outcrops are present. In the present study, data including litho- and biofacies logs and correlations from eight wells are presented, six of them of the region between Bachu and Tazhong, and two wells along the former platform margin from the TZOF. Calcimicrobial carbonates are common in core from the inner platform part. Benthic biota documented here resembles that of the microbial mounds and bioclastic limestones in the Bachu outcrop area, and thus is considered as the eastward extension of this realm. However, metazoan-dominated boundstones formed by corals, stromatoporoids, sponges together with calcareous algae are abundant along the southern platform margin, and are similar in composition to the reefs of TZOF in the easternmost part of the platform. The palaeoecologic differentiation between microbial- and metazoan-dominated reefs suggests a strong environmental gradient from a well-oxygenated windward eastern and southern side of the platform, which favoured the open ocean and which was dominated by metazoan reefs, and the inner platform as well as the northwestern, leeward margin of the platform with warmer, less-oxygenated water, which favoured the growth of microbial carbonates. Assuming the trade winds as cause for the contrasting facies on both sides of the platform a palaeo-position of Tarim south of the ITCZ seems probable because a position north of it would require a post-Ordovician clockwise rotation of Tarim of about 90° which is possible but less probable.

РАСПРЕДЕЛЕНИЕ МШАНОК В РАЗЛИЧНЫХ ФАЦИЯХ ЭМССКИХ ОТЛОЖЕНИЙ САЛАИРА

<p>The article analyzes the facial dissimilarity of bryozoans of the Emsian Stage near the town of Gyr’evsk. The bryozoans associations have been found in all types of facies, except the sand-mudstone one. In the littoral facies (bioclastic limestones with subordinate sand-mud-siltstones rock) the bryozoans are represented by the orders of <em>Trepostomida, Fenestellida, Cryptostomida, Cystoporida</em>, but treposomides predominate (63 % of the total number of species). After the littoral facies turn into the facies of the open lagoon (mudstones with subordinate limestones), only trepostomides are represented in the Emsian section, near the boundary, forming the Briozoan interbeds. Briozoan interbeds consist of the fragments of Neotrematopora salairiensis colonies. The restoration of species and genus diversity in new facies occurs relatively quickly. In comparison with trepostomides, representatives of other orders appear with a large time-lag. Under the conditions of an open lagoon, bryozoans are characterized mainly by branched bifoliate colonies. In the slope facies (bedded limestones), characterized by greater depths and low hydrodynamics, fennestellids dominate (58 % of the total number of species). Trepostomides and cystoporides in these facies are represented by species with thick-branched and massive branched colonies. When the facies of bedded limestones change to sandy-mudstones (the beginning of the regression), the fenestellids also form Briozoan interbeds near the boundary. In the lateral rows of the facies of a single stratigraphic unit, several facial associations of bryozoans are often found. The species of bryozoans characteristic of this stratigraphic level are present in all associations. The unity of the Emsian complex of bryozoans is expressed in the gradual change of their associations upwards the section (three Briozoanbiostratigraphic Zones have been <span>identified).</span></p>

Sedimentary Environment of the Early Ordovician in Danzhai, Guizhou

This study analyzes the variations in trace element contents and ratios, the distribution patterns of rare earth elements, and the sedimentary environment and its evolution in the Early Ordovician. In particular, the analysis is based on measured section of the Qingjiang Village located in Nangao Town, Danzhai County, Guizhou Province. The analysis also considers thin sections and the characteristics of C, O isotopes. The lower part of the Tongzi Formation has a simple lithology and mainly consists of light gray to gray thin-medium bedded muddy dolomite and dolomicrite with a few dolarenites and dolorudites. The upper part of this formation includes gray thin-medium bedded fine crystalline dolomite and gray massive bioclastic limestone. Only a fraction of gray massive fine crystalline limestone, sparry calcarenite, and calcirudite are on top of the upper part. The Honghuayuan Formation is generally composed of light gray to gray medium-thick bedded or massive bioclastic limestone, reef limestone, and calcarenite with a few sandstones at the bottom. The δ18OPDB values of 18 samples are less than -11‰; in addition, the Pr/Pr* and Ce/Ce* ratios are greater and less than 1, respectively. These data reflect a sedimentary environment. The lower part of the Tongzi Formation might have been formed in an evaporite platform, which was then transformed into a restricted one, which featured a dry climate, a shallow water depth, and an insufficient amount of oxygen. By contrast, the upper part of the formation was deposited in an open platform, which featured a humid climate and relatively increased water depth and reducibility. During the sedimentary period of the Honghuayuan Formation, the water energy further increased, the climate became humid, the water depth increased, and the reducibility increased. Correspondingly, the sedimentary environment, which originally comprised littoral facies that slowly developed into an open platform and into a platform marginal bank, gradually evolved into a platform marginal reef.

Multiple sulfur isotope records at the end-Guadalupian (Permian) at Chaotian, China: Implications for a role of bioturbation in the Phanerozoic sulfur cycle

A recent study on quadruple sulfur isotopes (32S, 33S, 34S, and 36S) of sedimentary pyrite suggested that the end-Guadalupian extinction was caused by shoaling of the sulfidic deep-water. This scenario is based on the assumption that sulfur isotopic compositions of pyrite from hosting sediments were controlled by benthos activities, thus by the redox conditions of the sedimentary environments. Nonetheless, the relationship between the sulfur isotope records and redox conditions, reconstructed from litho- and bio-facies, are poorly known. In order to examine the effect of bioturbation in sediments, quadruple sulfur isotopic compositions of sedimentary pyrite from the end-Guadalupian succession in Chaotian, South China, were analyzed. Black mudstones of deep-water facies immediately below the extinction horizon have consistently high Δ33S values of ca. +0.079‰, clearly suggesting a sulfate reduction in the anoxic water column. Our new data are consistent with the emergence of a sulfidic deep-water mass prior to the end-Guadalupian extinction; the upwelling of the toxic deep-water may have contributed to the extinction. In contrast, shallow-marine bioclastic limestones with burrows deposited under oxic conditions have negative Δ33S values. This anomalous isotopic signal indicates the mixing of two distinct types of pyrite; one generated during the sulfate reduction in an open system and the other in a closed system. We interpret that bioturbation supplied sulfate in the sediments and promoted sulfate reduction and in-situ sulfide precipitation within the sediments. The negative Δ33S values of oxic sediments in Chaotian are inconsistent with the previous model and demonstrate that the sedimentary sulfur cycle associated with bioturbation was more complicated than previously thought. Our study also implies that, more generally, the role of bioturbation in increasing seawater sulfate concentration in the Phanerozoic may have been overestimated in the previous studies, because bioturbation may have enhanced sulfide burial or sulfur output from the oceans.

Using paper pulp poultices in the field and laboratory to analyse salt distribution in building limestones

We investigated the behaviour of salt solutions during the drying process in porous stone and the extraction of salts by paper pulp poultices (PPP). The investigations were carried out at a 300 year old boundary wall of the Worcester College in Oxford, UK, as well as under laboratory conditions. The laboratory blocks of porous, bioclastic limestone (Aflenzer stone, Austria) were cut into stone cubes and prisms (5 × 5 × 5 and 5 × 5 × 20 cm) and soaked first with ultrapure H2O and second with different concentrations (0.1, 0.5 and 1 mol l−1) of a saline solution of NaCl and Na2SO4 to determine their behaviour during the dehydration process under different conditions. After determining the capillary absorption coefficient and the capillary rise behaviour of the stone cubes, we sampled material by PPP and by drilling. The samples were immersed in ultrapure water and the salt ion content was measured via conductivity and ion chromatography (IC). The main aim of the laboratory analysis was to investigate the effectivity of the PPP in extracting salts from the stone and to derive a semi-quantitative calibration (PPP results vs. original salt content). The field work at the college boundary wall comprised sampling (PPP and stone surface material) at surfaces of different weathering features. Finally, the field work results were compared to the laboratory work. The lab experiment results show that approximately 24% of the salts within a 100% saturated stone cube moved into the poultice within the 1 h of application. At 75% and lower saturation degrees, only 14–11% of the salts moved into the respective poultices. For the field setting, the amount of anions extracted by PPP is 6–63% (mean 20.7%) of the amount found in the stone samples. The spatial distribution of salt ion concentrations, collected by poultices, corresponds with the ‘hot spots’ of weathering.

Lower Silurian stromatolites in shallow-marine environments of the South China Block (Guizhou Province, China) and their palaeoenvironmental significance

In northern Guizhou Province (Upper Yangtze Platform, South China Block) two types of reef communities developed in the Lower Silurian (upper Aeronian, Llandovery) Shihniulan Formation; they are calcimicrobial- and metazoan-dominated structures, and existed because of northward deepening of the shallow-marine ramp setting in which they grew. Stromatolitic communities are the focus of the present study and dominated the shallowest reef structures, while metazoan-dominated reefs, previously described in other papers, grew in the outer shelf portion of the ramp. Stromatolitic reefs occur in several sections (Daijiagou, Baishanxi, Jianba and Lianghekou), palaeogeographically close to Qianzhong Land. Within the stromatolite units, laminar sheets of microbial mats and columns are pronounced, with individual stromatolite thicknesses generally less than 1m. Some very small stromatolites are only centimeters in diameter and thickness. Stromatolitic units are cyanobacterial bindstones mostly associated with shales, siltstones and thin-bedded bioclastic limestones. Their growth was frequently punctuated by siliciclastic sediments, and their shallow-water nature is demonstrated by association with birds-eye structures, cross-stratified sediments, and Lingulella-bearing silts in intertidal or/and lagoonal environments. The stromatolites formed during a regression and erosion surfaces are common at the top of the Shihniulan Formation. The Tongzi Uplift, a short-duration expansion of Qianzhong Land, ended the deposition of the late Aeronian limestones.

Paleoenvironmental analysis of the ichnogenus Zoophycos in the Lower Devonian tempestite sediments of the Longmenshan area, Sichuan, China

The trace fossil Zoophycos is characteristic of the Lower Devonian in the Longmenshan area of southeastern China. Three distinct storm-generated or induced types of sedimentary sequences bearing Zoophycos have been identified in shallow marine deposits of the Bailiuping Formation and the Yangmaba Formation. The type A deposit is composed of siltstones with shell concentrations and wave ripples and small Zoophycos at the top. The type B deposit is characterized by fine-graded sandstone with long wavelength hummocky cross-stratification and black gray mudstone as well as the largest and densest Zoophycos at the top. The type C deposit has graded bioclastic limestone with short wavelength hummocky cross-stratification, muddy wackestone and large Zoophycos at the top. Based on the features of the types of Zoophycos, the distribution zone of the Zoophycos ichnogenus that preserved in tempestite sequence was built between the fair weather wave base and storm wave base in the Lower Devonian. This research suggests that the distribution patterns of Zoophycos formed in storm-induced deposits may not provide significant clues for the estimation of relative water depths.The hydrodynamic conditions and sedimentation rate are two important environmental factors of the development of the Zoophycos in the Lower Devonian shallow marine deposits where it was mainly affected by storms between the fair weather and storm wave bases. The abundance of Zoophycos at the top of locally frequent storm deposits could indicate a fairly opportunistic behavior and they were formed in the early HST deposit, which had a relatively stable environment with low energy and higher sedimentation rate.