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1.
A ～6 Ma Messinian (late Miocene) Bioherm Unit on the southern slope of the Sorbas Basin, SE Spain, contains numerous biotically diverse lensoid patch reefs that formed on a shelf to basin slope during a cycle of relative sea-level change. Halimeda reefs are the largest and most complex of the patch reefs and are divisible into core, cap, and flank facies. On the upper and midslope they are up to 40 m thick and 400 m long. They become smaller downslope. The core consists of jumbled Halimeda segments, released by spontaneous disaggregation of the alga. The segments were stabilized close to their sites of growth and rapidly lithified by micritic and peloidal microbial crusts. Residual cavities were further veneered by isopachous marine cements. Flank facies, consisting of bedded packstones to rudstones, form wedge-shaped units lateral to the mounds. Cap facies consist of bioclastic calcarenites/calcirudites and microbial carbonates. Synsedimentary lithification assisted rapid accretion and inhibited off-mound export of sediment. Allochthonous reef-derived blocks on the mid-slope reflect penecontemporaneous rigidity of the Halimeda bioherms. Proximal Porites coral frame patch reefs associated with calcarenites were located near the shelf margin during the initial lowstand stage. Halimeda segment reefs associated with calcarenites and silty marls developed on the midslope and bivalve-bryozoan-serpulid reefs formed on the lower slope in silty marls with occasional turbidites. During the transgressive stage, coral patch reefs near the shelfbreak were overgrown by Halimeda. During highstand progradation, cap facies spread basinward as a sheet connecting many of the midslope patch reefs. These ancient analogues differ from most modern Halimeda reefs in being discrete laterally restricted patch reefs, surrounded by marly sediment, and located on a slope. They are, however, broadly comparable in biota, thickness, and depositional depth. Intense early lithification by microbial crusts and marine cements is an important feature of these Messinian segment reefs. It has not been reported from modern examples.  相似文献   

2.
《Sedimentary Geology》2005,173(1-4):345-371
Facies and environmental setting of the Miocene coral reefs in the Late Cenozoic Antalya Basin are studied to contribute towards a better understanding of the time and space relationships of the reef development and the associated basin fill evolution in a tectonically active basin. The Antalya Basin is an extention–compression-related late post-orogenic basin that developed unconformably on a basement comprising a Mesozoic para-authocthonous carbonate platform overthrust by the Antalya Nappes and Alanya Massif metamorphics within the Isparta angle. The Late Cenozoic basin fill consists of thick Miocene to Recent clastic-dominated terrestrial and marine deposits with subordinate marine carbonates and extensive travertines. Late Miocene compressional deformation has resulted into three parts, referred as Aksu, Köprüçay and Manavgat sub-basins, bounded by north–south extending dextral Kırkkavak fault and the westward-verging Aksu thrust.Coralgal reefs are common within the Miocene sequences and are represented by coral assemblages closely similar to that of the circum-Mediterranean fauna. They occur as massive, small, isolated, patch reefs that developed in two contrasting depositional systems (progradational coastal alluvial fan and/or fan-delta conglomerates and transgressive shelf carbonates) during Early–Middle Miocene and Late Miocene. The Early–Middle Miocene reefs are represented by rich and high-diversity hermatypic corals, mainly comprising Tarbellastraea, Heliastraea, Favites, Favia, Acanthastraea, Porites, Caulastraea and Stylophora with occasional presence of solitary (ahermatypic) corals, Lithophyllia, Mussismilia and Leptomusso, locally reflecting relative changes in the bathymetry. Densely packed, massive, domal and hemispherical growth forms bounded by coralline algae and encrusting foraminifera Acervulina construct the reef framework. They occur in the fan-deltas and the transgressive open marine shelf carbonates of the Manavgat and the Köprüçay sub-basins. The Late Miocene reefs occur only in the Aksu sub-basin and are characterized by low-diversity hermatypic corals exclusively dominated by Porites and Tarbelastraea with minor Siderastraea, Favites and Platygyra. They developed on alluvial fan/fan-delta complexes and shallow marine shelf carbonates.The Miocene coral reef growth and development in the Antalya Basin are characterized by large- to small-scale, transgressive–regressive reefal cycles which are closely related to the complex interaction of sporadic influxes of coarse terrigeneous clastics derived from the tectonically active basin margins and the related sea-level fluctuations.  相似文献   

3.
M. T. HARRIS 《Sedimentology》1993,40(3):383-401
The Latemar reef buildup of the central Dolomites (northern Italy) provides a rare opportunity to examine an in-place Middle Triassic (Upper Anisian to Lower Ladinian) platform margin that is not strongly deformed or dolomitized. The margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms. Across this transition, the depositional profile relates directly to a consistent lateral facies pattern: (1) restricted-biota grainstone of the platform interior, (2) ‘Tubiphytes’-rich boundstone and (3) diverse-biota grainstone that grades into (4) foreslope breccia beds. The boundstone and diverse-biota grainstone facies comprise the platform margin. The boundstone facies consists of a framework of small (< 10 cm) skeletal remains (< 10% by volume) with associated biotic crusts, internal sediments and syndepositional cements. Crusts and cements constitute most of the rock volume and created the boundstone fabric. Biotic crusts exhibit gravity-defying geometries and range from a light grey, ‘structure grumeleuse’ rind to dark grey, micritic laminae. Both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits, indicating a syndepositional origin. The diverse-biota grainstone facies primarily consists of skeletal-peloidal grainstone with a diverse open marine biotic assemblage, in contrast to the restricted biota grainstones of the platform interior that have a low diversity, restricted marine biota. Metre scale hexacoral boundstone and centimetre-scale sponge boundstone and microbial boundstone occur as isolated patches (tens to hundreds of metres apart) within the diverse-biota grainstone facies. The depositional profile, facies zonation and biotic constituents all indicate that the Latemar buildup had a shallow water reef margin, in contrast to previous interpretations that these were upper slope reefs. The syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave-resistant reef fabric.  相似文献   

4.
The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontia initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework.Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous faunal transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks.  相似文献   

5.
Growth and submarine fossilization of algal cup reefs,Bermuda*   总被引：2，自引：0，他引：2  
Small charges of explosives were used to section cup-shaped reefs that occur on the margins of the Bermuda Platform. Study of these artificial outcrops, up to 10 m high, and the samples collected from them show how the reef-building community is rapidly converted to well-lithified reef rock in the marine development. The reefs, known locally as boilers and breakers, occur along the wave-swept south shore of the Bermuda Islands and around the northeast and northwest margins of the Platform. They are cup-shaped, up to 30 m in long dimensions, and rise up to the sea surface as much as 12 m above the surrounding sea floor. The reefs are built by an intergrowth of encrusting organisms, principally crustose coralline algae, an encrusting hydrozoan, Millepora sp., and an attached gastropod, Dendropoma irregulare. The growth framework of these algal cup reefs has extensive voids: large and intermediate-sized growth framework and shelter pores; borings of bivalves and sponges; and both intra- and inter-particle pores. A variety of vagile and sessile organisms (coelobites) inhabit these pores: an encrusting Foraminifera, Homotrema rubrum, is the most abundant attached coelobite; the tests of a variety of benthic Foraminifera and ostracods are common: branched coralline algae, barnacles, bivalves, ahermatypic corals, bryozoans, and burrowing crustaceans occur in varying abundance. Beginning millimetres below the living surface, internal sediments accumulate in the extensive voids. Coarse-grained skeletal sand derived from the surface of the reefs is characteristic of the larger voids; lime mud with the tests of planktonic Foraminifera and planktonic algae occurs generally in the smaller voids. Most specimens from the interior of the reefs show multiple generations of internal sediment that vary in grain size, composition, and colour. The sand-sized sediments are pumped into the voids by the frequent and intense wave action; the lime mud settles out in the smaller, less agitated pores. Cementation of internal sediments and surrounding growth frame begins centimetres below the living surface; it is so pervasive that marble-hard reef rock is developed within 1/2 m or less. The cement is principally high-magnesium calcite of micrite size, and subordinately acicular aragonite, but there are locally wide variations in crystal size and morphology. The occurrence of the cement within the reefs well below sea level, the isotope ratios of the cement crystals, the mineralogy, and the age inferred from radiocarbon age determinations of the growth frame all indicate that the cement is submarine and deposited from water of oceanic composition. The algal cup reefs of Bermuda demonstrate the reef-building ability of a community of encrusting organisms that form only crusts in the intertidal zone of the Mediterranean and Northern Brazil. The cup reefs of the northern margins of the Bermuda Platform are true reefs, not merely veneers covering eroded blocks of Pleistocene limestone. In their composition, location, and early diagenesis, the cup reefs closely resemble the algal or lithothamnion ridge of Pacific atolls. Synsedimentary cementation of internal sediments and growth frame makes a major contribution to the rigidity of these ocean-facing reefs and atoll rims. The assemblage of features that characterize the submarine fossilization of the cup reefs is widespread elsewhere in the modern seas: the floors of the Persian Gulf and parts of the Mediterranean; the margins of Pacific atolls; and the reefs off the north coast of Jamaica. This fossilization is characterized by reiterated generations of coelobites, internal sediments, and synsedimentary cements that can in time replace a major part of the original growth framework. Major variations in the sequence of these generations from pore to pore is the signature of this kind of fossilization. The same features of fossilization are described from reefs in the Devonian, Permian, and Triassic.  相似文献   

6.
Bryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef-dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower-like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust-forming contributors, suggest that it could have been formed directly at the palaeo-sea surface, probably in times of minor but high-frequency sea-level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ13C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ13C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control.  相似文献   

7.
The Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow-mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes. The framework of these reefs was principally the product of micro-organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward-directed thalii of Renalcis formed a framework that trapped fine-grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis-bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundant Epiphyton, but may include archaeocyathans, and the microbial microfossils Girvanella and Renalcis as well as cryptomicrobial clotted micrite. In this type of reef, frame-building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth-framework cavities, as well as rare metre-sized caverns. Within these spaces, Epiphyton and, less commonly Renalcis, encrusted framework elements, fine-grained sediments accumulated, and pervasive sea-floor cements were precipitated. Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan-rich reefs, as well as deep-water stromatactis-bearing mud-mounds. The Epiphyton-Girvanella-archaeocyathan frameworks and stromatactis-bearing boundstones, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework styles of later Phanerozoic time.  相似文献   

8.
A common assumption in the geological analysis of modern reefs is that coral community zonation seen on the surface should also be found in cores from the reef interior. Such assumptions not only underestimate the impact of tropical storms on reef facies development, but have been difficult to test because of restrictions imposed by narrow‐diameter cores and poor recovery. That assumption is tested here using large‐diameter cores recovered from a range of common zones across three Campeche Bank reefs. It is found that cores from the reef‐front, crest, flat and rubble‐cay zones are similar in texture and coral composition, making it impossible to recognize coral assemblages that reflect the surface zonation. Taphonomic signatures imparted by variations in encrustation, bioerosion and cementation, however, produce distinct facies and delineate a clear depth zonation. Cores from the reef‐front zone (2–10 m depth) are characterized by sections of Acropora palmata cobble gravel interspersed with sections of in‐place (but truncated) A. palmata stumps. Upper surfaces of truncated colonies are intensely bioeroded by traces of Entobia isp. and Gastrochaenolites isp. and encrusted by mm‐thick crustose corallines before colony regeneration and, therefore, indicate punctuated growth resulting from a hurricane‐induced cycle of destruction and regeneration. Cores from the reef crest/flat (0–2 m depth) are also characterized by sections of hurricane‐derived A. palmata cobble‐gravels as well as in‐place A. palmata colonies. In contrast to the reef front, however, these cobble gravels are encrusted by cm‐thick crusts of intergrown coralline algae, low‐relief Homotrema and vermetids, bored by traces of Entobia isp. and Trypanites isp. and coated by a dense, peloidal, micrite cement. Cores from the inter‐ to supratidal rubble‐cay zone (+0–5 m) are only composed of A. palmata cobble gravels and, although clasts show evidence of subtidal encrustation and bioerosion, these always represent processes that occurred before deposition on the cay. Instead, these gravels are distinguished on the basis of their limited bioerosion and marine cements, which exhibit fabrics formed in the intertidal zone. These results confirm that hurricanes have a major influence on facies development in Campeche Bank reefs. Instead of reflecting the surface coral zonation, each facies records a distinctive, depth‐related set of taphonomic processes, which reflect colonization, alteration and stabilization following the production of new substrates by hurricanes.  相似文献   

9.
Drill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yr bp to the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized.  相似文献   

10.
During Serravallian through Messinian time, marine carbonates flanked topographic highs that rimmed Neogene basins in the Western Mediterranean. Middle to upper Miocene carbonate strata in the Las Negras and Nijar areas (southeastern Spain) are 50-150 m thick and display 50-200 m of shelf-to-basin relief over 1-2 km. Detailed studies in those areas document the effects of relative sea-level change on sedimentation, biotic composition, and reef development. We identify three previously unrecognized, regionally correlatable depositional sequences (DS1, DS2, DS3) that occur between the underlying basement and the overlying Terminal Carbonate Complex. The lower depositional sequences (DS1, DS2) are mostly normal marine shelf (ramp) carbonates deposited on the flanks of basement highs. The basal part of DS2 locally contains some megabreccia reef blocks composed of Tarbellastraea and Porites. These blocks are the first evidence of reef growth in the area and represent a previously unrecognized period of reef development prior to the fringing reef development. The reef blocks probably formed as upslope patch reefs that were eroded and transported to distal slope locations. The upper sequence (DS3) is characterized by clinoform strata of a Porites-dominated fringing reef complex that prograded basinward in a downstepping style with successively younger reefs forming in a topographically lower and more basinward position as a result of a net sea-level drop. Regional correlation of Miocene shallow-marine strata between basins in Spain and elsewhere in the western Mediterranean is complicated because basins were semi-isolated from adjacent basins making physical correlation impossible. In addition, age-definitive biostratigraphic markers are poorly preserved in most of the Miocene shallow-water strata; basinal sediments that are more easily dated by microfossils do not typically interfinger with the shallow-marine strata in outcrop. Even where datable microfossils are found, resolution of dating is poor. Our studies in the Las Negras and Nijar areas illustrate the usefulness of integrating sedimentological, geometric and biotic data with locally derived relative sea-level (accommodation space) curves for correlation. The relative sea-level curves for each area show remarkable similarities in shape and magnitude of sea-level changes. These curves indicate several relative sea-level fluctuations during Miocene carbonate deposition prior to the major sea-level drop at the end of DS3 deposition that culminated in the exposure of the basin margin deposits and the deposition of evaporites in basinal areas during the Messinian. The depositional sequences in the Las Negras and Nijar areas may correlate with depositional sequences of similar age throughout the southern Cabo de Gata area, in Mallorca some 600 km to the northeast, and possibly in other Mediterranean locations. The widespread occurrence and possible correlation of the depositional sequences suggest regional processes such as eustacy or tectonism for their formation. The integration of sedimentological, palaeontological and sequence stratigraphic studies, and the construction of relative sea-level (accommodation space) curves may help in the interpretation of depositional histories of shallow-marine carbonate complexes and correlation of these strata between isolated areas. Other dating methods, in addition to microfossil dating, may allow for better age determination of the sequences and aid in identifying the importance of eustacy and tectonism in sequence development.  相似文献   

11.
The carbonate succession in the Lyalintsi section of the western Moesian Platform (western Bulgaria) displays a shallowing-upward trend. Growth of the Tithonian–Valanginian coral biostromes and low-relief bioherms was preceded by Oxfordian–Kimmeridgian sedimentation of fine-grained peloidal-bioclastic limestones and Saccocoma-bearing limestones on the homoclinal ramp and the carbonate platform slope. In the late Kimmeridgian, boundstones with very rare corals, but with easily recognisable biohermal morphology, were developed. The main components of this reef are encrusting microorganisms, microbial crusts and synsedimentary cements. Microencrusters Labes atramentosa, Crescentiella morronensis, Perturbatacrusta leini and Radiomura cautica, as well as thin crusts of calcified sponges (sclerosponges), are the main biotic components. Corals (almost exclusively microsolenids) are sparse, whereas photophilic microencrusters (e.g., “Lithocodium–Bacinella”), are absent, although they are common in the overlying shallow-water part of the Lyalintsi sequence. Microbialites and synsedimentary cements provided additional support for the reef framework. The framework, especially the biotic components, and the reefal facies position within the sedimentary succession, implies that the high-energy upper slope of the carbonate platform was the depositional setting of the microencruster-microbial-cement reef studied. Encrusting microorganisms, except for C. morronensis and sponges, are only known from the intra-Tethyan platforms. This study supports conclusion of studies of coeval Alpine reefs that the presence of the microencruster-microbial-cement framework provides insight into the palaeobathymetry, palaeogeography and tectonic configuration of the intra-Tethyan carbonate platforms.  相似文献   

12.
祝仲蓉 Marsh.  J 《沉积学报》1992,10(1):133-145
更新世以来,剧烈的构造运动已将巴布亚新几内亚合恩半岛东北海岸的晚第四纪珊瑚礁阶地抬升上千米.阶地中造礁珊瑚的成岩变化和成岩产物的组构特征反映了该礁的成岩历史,充分体现该区快速构造上升的影响.海水潜流带和淡水渗流带为上升礁的主要成岩环境.生物钻孔、生物碎屑填隙、珊瑚文石针粗化、珊瑚骨骼的溶解和新生变形转化,以及其不同矿物成分和组构的种种胶结物的胶结作用是造礁珊瑚经历的主要成岩作用.地球化学资料表明其成岩变化发生于开放的化学体系之中.  相似文献   

13.
In order to evaluate the geological record of climatic change in neritic carbonates, we studied Late Miocene rock outcrops in southern Spain. Six episodes of reef growth are documented (Burdigalian to Messinian) in Neogene basins of the Betic Cordillera, which were located close to the margin of the global reef belt. The reefs are characterized by various zooxanthellate corals which decrease in diversity with time, andHalimeda; the youngest reefs of the latest Messinian are characterized by the dominance of the genusPorites. Late Miocene coral reefs and reef-rimmed platforms alternate over time with non-reefal carbonate ramps characterized by skeletal calcirudites or with gypsum such as that formed during the Messinian salinity crisis. The calcirudites lack reef corals, calcified green algae and extensive marine cement, but exhibit skeletal components described from both modern and fossil nontropical carbonates. These include bryozoans, mollusks, foraminifers, echinoderms and minor balanids, as well as coralline algae of a bryomol association. The presence of some larger foraminifers indicates high temperatures, close to the lower temperature threshold of the reef assemblage. Sea level lowstands and highstands are documented by wedges of bryomol carbonate and chlorozoan patch reefs or prograding platforms. Thus, temperate climate depositional modes correspond to relatively low sea levels, and warm-water modes to high sea levels. The Neogene infill of the Agua Amarga and Sorbas basins documents two of these cycles. Other climate/sea-level cycles (including Messinian gypsum in the cool water depositional mode) are well established in adjacent Neogene basins in southern Spain. This type of composite sequence seems to occur only along the margin of the global reef belt and indicates an oscillatory latitudinal movement of the margin, which is associated with global climatic change. The analysis of turnover in neritic depositional carbonate systems may therefore be considered a sensitive tool for reconstructing climatic change from the fossil record. However, warm-water modes and temperate-water modes of carbonate sedimentation and diagenesis differ significantly. For this reason the interpretation of composite system sequences by sequence stratigraphy requires an extended concept. The particular type of mixed bryomolchlorozoan depositional sequence also bears some potential for drowning, because sea level rise may be faster than the net production rate of temperate carbonate systems.  相似文献   

14.
Neoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high-resolution geochemical data from well-preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south-west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high-Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length-slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox-sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre-scale spheroidal ankerite and sulphur-containing dolomite, and intergrown pyrite grains with U-shaped δ34S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite.  相似文献   

15.
Holocene reef development was investigated by coring on Britomart Reef, a mid-shelf reef, 23 km long and 8 km wide situated 120 km north of Townsville in the central Great Barrier Reef (GBR). Two holes were drilled, Britomart 1 on a lagoon patch reef, and Britomart 2 on the windward reef crest. The Holocene reef (25·5 m) is the thickest yet recorded in the GBR and overlies an uneven substrate of weathered Pleistocene limestone. Mineralogical and geochemical analyses show that magnesian calcite and aragonite were converted to low Mg-calcite below the Holocene-Pleistocene disconformity. Corals above the interface have 7500–8500 ppm Sr, but 1650–1500 ppm just below it, decreasing to 400–800 ppm downwards. The intermediate Sr values could be due to partial replacement of aragonite by calcite or higher original Sr content in the corals. Three units are recognized in the Holocene: (1) coral boundstone unit, (2) coral framestone unit, and (3) coral rudstone unit. The coral boundstone unit forms the top 5 m of both cores and is algal-bound coral rubble similar to the present reef top. The coral framestone unit is composed of massive head corals Diploastrea heliopora and Porites sp., and is currently forming in patch reefs situated in the lagoon and along the reef front. The coral rudstone unit comprises coral rudstone and floatstone with unabraded, and unbound, coral clasts in muddy matrix. This matrix may be up to 30% sponge chips. Radiocarbon dating indicates the reef grew more rapidly under the lagoon than under the reef front from 7000 to 5000 yr BP. The rate of reef growth matched existing estimates of sea-level rise, but lagged approximately 1000 years (5–10 m) behind it. Most of the reef mass accumulated between 8500 and 5000 yr BP as a mound of debris, perhaps stabilized by seagrasses or algae. Accretion of the reef top in a windward direction between 5000 and 3000 yr BP created the present, steep reef-front profile.  相似文献   

16.
It is a widely held concept that tropical coral reefs in shallower water with branched acroporid corals should accrete faster than those in deeper water dominated by massive corals. Results from a study of Holocene development of the largest Atlantic reef system, including paleo-waterdepth data, challenge these concepts. In Belize barrier and atoll reefs, reef accretion-rates range from 0.46 to 7.50 m/kyr, and average 3.03 m/kyr, as measured along 33 dated reef sections. Interestingly, accretion-rates increase with increasing paleo-waterdepth, and sections dominated by massive corals accumulated even slightly faster than those with branched acroporids. Published data from some other reef locations reveal no significant trends when plotting reef accretion-rate versus paleo-waterdepth, also indicating that the above-mentioned concepts should be questioned. Massive corals apparently are more resistant and accrete in lower disturbance conditions in slightly deeper water (5–10 m) and higher accomodation (space available for sediment deposition) as compared to shallow water (0–5 m) branched acroporids, which repeatedly get broken and leveled out during tropical cyclones.  相似文献   

17.
A review of the origin and setting of tepees and their associated fabrics   总被引：3，自引：0，他引：3  
Carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees. The types of tepees found in the geological record include: (1) Submarine tepees which form in shallow carbonate-saturated waters where fractured and bedded marine grainstones are bound by isopachous marine-phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements; (2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine-vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust. Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds. (3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid. In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.  相似文献   

18.
The coralgal framework within the outer reef margin of many Indo-Pacific reefs exhibits three main shallow-water communities, the environmental significance of which can be inferred by comparison with their modern counterparts. A community dominated by tabular Acropora gr. hyacinthus/cytherea with branching Pocillopora damicornis, P. eydouxi, Montipora digitata, occasional domal faviids and mm-thick crusts of the coralline algae Lithophyllum and Mesophyllum (mainly), typical of the 6 – 15 m paleodepth range; a community including robust-branching Acropora gr. danai/robusta, A. humilis, A. digitifera and subordinate Favia stelligera, Echinopora gemmacea, associated to vermetid gastropods and thick coralline crusts of Hydrolithon cf. onkodes and Neogoniolithon cf. fosliei flourishing in depths less than 6 m; in medium-to-high water-energy settings, a community composed of domal Porites cf. lutea and P. cf. lobata with occasional Acropora gr. danai/robusta and cm-thick crusts of coralline algae in sheltered habitats in depths less than 10 m.These biological assemblages allow us to determine relationships between reef growth and paleobathymetry and, consequently, to reconstruct regional relative sea-level curves. High water-energy reefal assemblages provide stronger evidence for reconstructing sea-level curves than low-energy buildups, because they have generally been controlled by a keep-up growth mode. Subsiding reef sites seem to be more reliable indicators of sea-level variations because they usually present expanded reef sequences.  相似文献   

19.
张俊明  彭克兴 《地质科学》1994,29(3):236-245
王家坪古杯礁丘是由不规则古杯和蓝绿藻Renalcis、Epiphyton、Cirvanella等组成的障积礁丘。可分为:孤立小型古杯泥丘和由丘状藻-古杯粘结岩叠置而成的点礁。以Renalcis为主的藻-古杯粘结岩与孤立小型古杯泥丘一样形成于风暴浪基面之下的低能陆架。以Epiphyton为主的藻-古杯粘结岩形成于较动荡的中-高能陆架浅滩。除造礁生物的沉积作用外,早期海底胶结作用和充填固化作用对古杯礁丘的形成亦起了十分重要的作用。  相似文献   

20.
We have used digital photography, image analysis and measurements in the field to determine the growth rates of Quaternary corals in the Wakatobi Marine National Park, Indonesia, and compared them to growth rates of similar corals in the same area. In the Quaternary deposits it was possible to measure the growth rates of two massive coral genera Porites and Favites. For each genus, the corals reworked from better‐illuminated upslope environments had higher growth rates than the in situ fossil corals. The calculated radial growth rates for the in situ Porites are slightly lower than, but of the same order of magnitude as, the modern Porites growing in 10 m water depth at Hoga (10.04 ± 3.34 mm yr?1 ± 1 s.d.; n = 3) and Kaledupa (15.26 ± 4.83 mm yr?1 ± 1 s.d.; n = 3). Sedimentation rates and underwater visibility are inferred to have been similar in the fossil site to that at the modern Kaledupa site. Decreasing light penetration due to increased water depth is inferred to have been a major influence on growth rates. The in situ massive corals with good growth banding are inferred to have grown in a comparable environment to modern Kaledupa and Hoga. The study highlights that it is possible to compare coral growth rates, and their influencing parameters, from modern and well‐preserved ancient examples. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

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