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1.
Stromatolitic crusts on stick-like and platy Porites corals forming Messinian reefs in Almería played an important role in supporting and binding the brittle corals. The crusts were previously regarded as probable marine cements. However, their clotted, peloidal, and micritic fabrics are directly comparable with those of stromatolites. They accreted allochthonous grains even on vertical faces, and include bushy fabrics closely comparable with those produced by cyanobacterial calcification. They contain numerous fenestrae, exhibit rapid fabric variation, and locally form micro-columns and laminated domes. Their similarities to peloid micrite crusts in Recent reefs suggest that at least some of these Recent crusts are microbial in origin, even though they have widely been interpreted as marine cements. The sedimentary effects of crust development substantially affected both the morphology and relief of the reefs and the generation of reefal clasts. Binding of the reef-frame in the Pinnacle and Thicket zones in the lower and middle parts of the reef created a rigid margin which shed large (commonly up to 5 m) cuboidal blocks of coral-stromatolite frame. The blocks broke along planes of weakness provided by the vertical Porites sticks and were deposited on the Fore-Reef Slope. In the uppermost parts of the reefs crusts dominate the structure, constituting 80% or more of the rock. Veneers up to 15 cm thick encrust thin irregular Porites plates to create a solid Reef Crest Zone which has not been recognized before. The coral-stromatolite framework is associated with echinoids, crustose corallines and halimedacean algae which, together with the scleractinians, indicate normal marine salinity throughout reef growth.  相似文献   

2.
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   

3.
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   

4.

Elizabeth and Middleton Reefs are atoll-like structures that have developed on top of volcanic edifices and are close to the southern environmental limit of reef development in the southwest Pacific. Reef morphology and vertical accretion rates during the Holocene appear similar to those on other more tropical reefs. Sediment samples were collected from the lagoon of both reefs and around the flanks of Middleton Reef. A distinctly chlorozoan assemblage was observed with coral, molluscs, Halimeda, coralline algae and foraminifers being the dominant sediment constituents. Lagoon sediment samples show little variation within or between reefs, lacking the concentric zonation characteristic of larger atolls. Samples collected from the flanks of Middleton Reef, and subsurface material from vibrocores, differ compositionally from the surficial lagoon sand and were typically more tropical in character. A comparison of the sediment constituents from these reefs with those of samples from within a fringing reef and from the shelf around Lord Howe Island, further south, indicated regional patterns in sediment composition. Halimeda rapidly decreased in abundance with increased latitude, and appeared confined to deeper water, whereas coralline red algae increased significantly. The rapid change in these major sediment contributors is coincident with the general decrease in coral growth rates with latitude. This reinforces the notion that the latitudinal limit of reef development is constrained by factors other than coral growth alone.  相似文献   

5.
Jos  M. Martí  n  Juan C. Braga 《Sedimentary Geology》1994,90(3-4):257-268
The Messinian (Late Miocene) marine stratigraphic record of the Sorbas Basin (S.E. Spain) is well preserved and can be considered as being representative of the entire western Mediterranean. It exhibits a series of features relating to: (1) the composition, characteristics and evolution of coral reefs; (2) changes between temperate and subtropical climates; and (3) the extensive development of microbial carbonates (stromatolites and thrombolites) at the end of the Messinian. Each of these features has global significance.

Porites, which is the major and almost only coral component in reefs, is heavily encrusted with stromatolites. These reefs grew at the edge of the subtropical belt and were totally eliminated at the end of the Messinian because of global cooling.

Lowermost-Messinian carbonate sediments in the Sorbas Basin reflect a temperate climate, whereas those immediately above, which contain bioherms and coastal reefs, are subtropical. The shift from temperate to subtropical conditions during the early Messinian was accompanied by an important change in water circulation within the western Mediterranean. Temperate times were marked by cold surface Atlantic waters entering the Mediterranean, whereas subtropical times coincided with warm surface waters entering the western Mediterranean from the east. The subtropical waters were thermally stratified, which favoured the deposition of euxinic marls and diatomites at the centre of the basin. The upwelling of nutrient-rich water promoted stromatolite development within reefs and Halimeda growth on adjacent slopes.

Lastly, microbial carbonates (stromatolites and thrombolites) attained giant dimensions during the late Messinian, which can be regarded as a measure of their success in occupying a variety of ecological niches. This abundance of available habitats is believed to have resulted from the Messinian “salinity crisis”, which was followed by a re-colonization of the western Mediterranean. In this context stromatolite proliferation was due to opportunism of microbial communities in colonizing the new environments, rather than to a complete absence of other competitive biota. We do not believe that hypersaline conditions were a causal factor in stromatolite development because of the normal-marine biota associated with them.  相似文献   


6.
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   

7.
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   

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.
Bottom sediments from the sea floor west of Barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits. Individual biogenic sediment grains and the cemented aggregates, nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms. On the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies: (1) a proximal slope facies down to a depth of about 140 m, rich in mollusc fragments, benthic foraminifera and bryozoans; (2) an upper distal slope facies between about 140 m and about 215 m, rich in benthic foraminifera, molluses and crustaceans; and (3) a lower distal slope facies from about 215 m to at least 300 m, dominated by molluscs, especially pelecypods, with subordinate scleractinians and tubes of the polychaete Lygdamus asteriformis. The appearance and quantitative importance of the cemented aggregates is also related to these facles. In the proximal slope facies, only relatively few irregular and very porous nodules are found, whereas in the lower distal slope facies, aggregates are very common. Most aggregates are crust-like with a smooth upper surface and a more or less irregular, knobby lower surface. The crusts are massive compared with the nodules of the proximal slope facies and, unlike the latter, the lower surfaces and walls of larger cavities are usually coated with Fe and/or Mn oxides. In the upper distal slope facies a gradual transition between the two types of aggregates is found. Petrographical and morphological evidence, together with carbon and oxygen isotopic data, indicates that the nodules and crusts were formed in situ by submarine lithification processes. Radiocarbon dating of two bulk samples suggests that the cementation took place during late Pleistocene and/or early Holocene.  相似文献   

10.
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   

11.
Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.  相似文献   

12.
南沙群岛中央水道及南华水道两侧的珊瑚礁,大部分为环礁,分属开放型、半开放型、准封闭型、封闭型和台礁化型,反映了环礁向灰砂岛演变的不同阶段。每个环礁,从礁前斜坡向礁坪至潟湖,可相应划分出3种沉积相和细分9种沉积带。礁顶是全新世中期以来形成发育的。  相似文献   

13.
The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the highly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Shoals, a shore‐attached ‘turbid‐zone’ coral reef, and at Phillips Reef, a fringing reef located 20 km offshore, have been studied to document the mechanisms controlling turbidity. At each reef, waves, currents and near‐bed turbidity were measured for a period of ≈1 month. Bed sediments were sampled at 135 sites. On the inner shelf, muddy sands are widespread, with admixed terrigenous and carbonate gravel components close to the reefs and islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of inner‐shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fascicularis, up to >50% coral cover) and is broadly similar to that at Phillips Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef flats consist of mixed terrigenous and calcareous gravels and sands, with intermixed silts and clays, whereas the reef slope is dominated by gravelly quartz sands. The main turbidity‐generating process is wave‐driven resuspension, and turbidity ranges up to 175 nephelometric turbidity units (NTU). In contrast, at Phillips Reef, turbidity is <15 NTU and varies little. At Paluma Shoals, turbidity of >40 NTU probably occurs for a total of >40 days each year, and relatively little time is spent at intermediate turbidities (15–50 NTU). The extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotrophy) in response to different levels of turbidity. Sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 000 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of <3 BTF. As elsewhere along the nearshore zone of the central GBR, dry‐season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments. The location of modern nearshore coral reefs is controlled by the presence of suitable substrates, which in Halifax Bay are Pleistocene and early Holocene coarse‐grained (and relatively stable) alluvial deposits.  相似文献   

14.
S.J. MAZZULLO 《Sedimentology》2006,53(5):1015-1047
Lithostratigraphy, depositional facies architecture, and diagenesis of upper Pliocene to Holocene carbonates in northern Belize are evaluated based on a ca 290 m, continuous section of samples from a well drilled on Ambergris Caye that can be linked directly to outcrops of Pleistocene limestone, and of overlying Holocene sediments. Upper Pliocene outer‐ramp deposits are overlain unconformably by Pleistocene and Holocene reef‐rimmed platforms devoid of lowstand siliciclastics. Tectonism controlled the location of the oldest Pleistocene platform margin and coralgal barrier reef, and periodically affected deposition in the Holocene. A shallow, flat‐topped, mostly aggradational platform was maintained in the Holocene by alternating periods of highstand barrier‐reef growth and lowstand karstification, differential subsidence, and the low magnitude of accommodation space increases during highstands. Facies in Pleistocene rocks to the lee of the barrier reef include: (i) outer‐shelf coralgal sands with scattered coral patch reefs; (ii) a shoal–water transition zone comprising nearshore skeletal and oolitic sands amidst scattered islands and tidal flats; and (iii) micritic inner‐shelf deposits. Four glacio‐eustatically forced sequences are recognized in the Pleistocene section, and component subtidal cycles probably include forced cycles and autocycles. Excluding oolites, Holocene facies are similar to those in the Pleistocene and include mud‐mounds, foraminiferal sand shoals in the inner shelf, and within the interiors of Ambergris and surrounding cayes, mangrove swamps, shallow lagoons, and tidal and sea‐marginal flats. Meteoric diagenesis of Pliocene and Pleistocene rocks is indicated by variable degrees of mineralogic stabilization, generally depleted whole‐rock δ18O and δ13C values, and meniscus and whisker‐crystal cements. Differences in the mineralogy and geochemistry of the Pliocene and Pleistocene rocks are attributed to variable extent of meteoric alteration. Dolomitization in the Pliocene carbonates may have begun syndepositionally and continued into the marine shallow‐burial environment. Positive dolomite δ18O and δ13C values suggest precipitation from circulating, near‐normal marine fluids that probably were modified somewhat by methanogenesis. Sedimentologic and diagenetic attributes of the Pliocene–Pleistocene rocks in the study area are similar to those in the Bahamas with which they share a common history of sea‐level fluctuations and climate change.  相似文献   

15.
The Neuquén back-arc basin is located on the west margin of the South American platform between latitudes 36° and 40° S. The basin is famous for its continuous sedimentary record from the Late Triassic to Cenozoic comprising continental and marine clastic, carbonate, and evaporitic deposits up to 2.600 m in thickness.The stratigraphical and paleontological studies of the outcrops of the La Manga Formation, Argentina, located near the Bardas Blancas region, Mendoza province (35° S and 69° O) allow the reconstruction of the sedimentary environments of an Oxfordian carbonate ramp, where outer ramp, middle ramp, inner ramp (oolitic shoal), inner ramp margin (patch reef) lagoon and paleokarst were differentiated. The reefs consist of back reef facies and in situ framework of coral boundstones that was formed at the top of shallowing-upward succession.Coral reefs were analyzed by defining coral colonies shapes, paleontological content, coral diversity and taphonomy studies. In some studied sections abundant fragments of gryphaeids, encrusting bryozoans, and isolated sponges provided a suitable substrate for coral colonization; however, other sections show an increase in the proportions of ooids, peloidal and coral intraclasts.The core reef facies is composed of white-grey unstratified and low diversity scleractinian coral limestone dominated by robust and thinly branching corals with cerioid–phocoid growths and massive coral colonies with meandroid–thamnasteroid growth forms.The assemblage is characterized by Actinastraea sp., Australoseris sp., Thamnasteria sp. and Garateastrea sp. Internal facies organization and different types of coral colonies allow to recognize the development of varying framework as well as intercolony areas. A superstratal growth fabric characterizes the coral assemblage. On the basis of coral growth fabric (branche and domal types), the reef of La Manga Formation is considered a typical mixstones. The intercolony areas consist of biomicrites and biomicrorudites containing abundant coral fragments, parautochthonous gryphaeids and another bivalves (Ctenostreon sp.), gastropods (Harpagodes sp., Natica sp.), echinoderms test and spines (Plegiocidaris sp.), miliolids, Cayeuxia sp., Acicularia sp., Salpingoporella sp., intraclasts, ooids, peloids and coated grains.The domal growth forms are probably more protected against biological and physical destruction, meanwhile delicate branching growth forms with very open and fragile framework were more affected and fragmented due to wave action and bioerosion.The reef fabric shows different intervals of truncation as consequence of erosion resulting from coral destruction by storm waves or currents. The maximum flooding surface separates oolitic shoal facies below from the aggradational and progradational coralline limestones facies above. Subsequent sea-level fall and karstification (148 Ma) affected reef and oolitic facies.  相似文献   

16.
Marine microbial communities recorded in the Moroccan Anti‐Atlas were unaffected across the Neoproterozoic–Cambrian transition. A stromatolite‐dominated consortium was replaced at the beginning of the Atdabanian (ca 20 Myr after the Neoproterozoic–Cambrian boundary) by shelly metazoan and thromboid consortia, which contain the oldest biostratigraphically significant fossils of the Moroccan Cambrian. The associated collapse of microbial mat (stromatolitic) growth appears to coincide with a change from pre‐Atdabanian shallow‐water restricted conditions into Atdabanian deeper, open‐sea conditions. It is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors, promoting shelly metazoan immigration into the region. The Tiout/Amouslek lithostratigraphic contact in the early Atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures, and synsedimentary microfaults and cracks recorded in the underlying Tiout Member. Concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic–heterotrophic microbial community composed of stromatolitic crusts, Renalcis–Epiphyton–Girvanella intergrowths, and Kundatia thalli. In the overlying Amouslek Formation, archaeocyathan–thromboid reefs were constrained by substrate stability, water depth and subsidence rate. Four reef geometries are distinguished: (i) patch reefs surrounded by shales, (ii) bioherms in which flank beds intercalate laterally with carbonate and shale inter‐reef sediments, (iii) biostromes or low‐relief structures formed as a result of lateral accretion of patch reefs, and (iv) kalyptrate complexes that nucleated because of a marked tendency for aggregation, and in which patch reefs and bioherms occur stacked together bounded by clay–marl–silt seams.  相似文献   

17.
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   

18.
A superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea-level dynamics. In the first sea-level cycle, metre-scale reefs grew amongst mid-ramp calcarenites and outer-ramp shales during slow sea-level rise; a 25-m-thick oblate reef tract, separating mid-ramp and outer-ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea-level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea-level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high-standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea-level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst. These Proterozoic slope buildups are similar to Phanerozoic deep-water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well-developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand- and gravel-sized calcareous debris. Their responses to changes in sea-level were strikingly similar to those shown by their younger counterparts, and suggest that sequence-stratigraphic concepts derived from studies of Phanerozoic reefs can also be applied to the Proterozoic.  相似文献   

19.
Cold‐water coral ecosystems present common carbonate factories along the Atlantic continental margins, where they can form large reef structures. There is increasing knowledge on their ecology, molecular genetics, environmental controls and threats available. However, information on their carbo‐nate production and accumulation is still very limited, even though this information is essential for their evaluation as carbonate sinks. The aim of this study is to provide high‐resolution reef aggradation and carbonate accumulation rates for Norwegian cold‐water coral reefs from various settings (sunds, inner shelf and shelf margin). Furthermore, it introduces a new approach for the evaluation of the cold‐water coral preservation within cold‐water coral deposits by computed tomography analysis. This approach allows the differentiation of various kinds of cold‐water coral deposits by their macrofossil clast size and orientation signature. The obtained results suggest that preservation of cold‐water coral frameworks in living position is favoured by high reef aggradation rates, while preservation of coral rubble prevails by moderate aggradation rates. A high degree of macrofossil fragmentation indicates condensed intervals or unconformities. The observed aggradation rates with up to 1500 cm kyr?1 exhibit the highest rates from cold‐water coral reefs so far. Reef aggradation within the studied cores was restricted to the Early and Late Holocene. Available datings of Norwegian cold‐water corals support this age pattern for other fjords while, on the shelf, cold‐water coral ages are reported additionally from the early Middle Holocene. The obtained mean carbonate accumulation rates of up to 103 g cm?2 kyr?1 exceed previous estimates of cold‐water coral reefs by a factor of two to three and by almost one order of magnitude to adjacent sedimentary environments (shelf, slope and deep sea). Only fjord basins locally exhibit carbonate accumulation rates in the range of the cold‐water coral reefs. Furthermore, cold‐water coral reef carbonate accumulation rates are in the range of tropical reef carbonate accumulation rates. These results clearly suggest the importance of cold‐water coral reefs as local, maybe regional to global, carbonate sinks.  相似文献   

20.
The complex pattern of biological accretion, internal sedimentation, early lithification, and biological destruction, that characterizes modern reefs and many fossil reefs has been recognized in archaeocyathid-rich patch reefs of Lower Cambrian age in the Forteau Formation, southern Labrador. Patch reefs occur as isolated masses or complex associations of many discrete masses of archaeocyathid-rich limestone and skeletal lime sands, surrounded by well-bedded skeletal limestones and shales. Each reef is composed of many loafshaped mounds stacked on top of one another. The limestone of each mound comprises archaeocyathids and Renalcis or Renalcis-like structures in a matrix of argillaceous lime mud rich in sponge spicules, trilobite and salterellid skeletons. Numerous growth cavities roofed by pendant Renalcis-like organisms and Renalcis are partially to completely filled with geopetal sediment indicating that much of the matrix was deposited as internal sediment. Two stages of diagenetic alteration are recognized: (1) syn-depositional, which affected only the reefs, and (2) post-depositional, which affected both reefs and inter-reef sediments. On the sea floor reef sediments were pervasively cemented and fibrous carbonate was precipitated in intraskeletal and growth cavities. These limestones and cements as well as archaeocyathid skeletons, were subsequently bored by endolithic organisms. Later post-depositional subaerial diagenesis resulted first in dissolution of certain skeletons and precipitation of calcite cement above the water table, followed by extensive precipitation of pore-filling calcite below the water table. These carbonate reefs are similar in structure to the basal pioneer accumulations of much younger lower and middle Palaeozoic reefs. They did not develop into massive ‘ecologic’ reefs because archaeocyathids never developed the necessary large, massive, hemispherical skeletons. This occurrence indicates that reefs developed more or less coincident with, and not long after, the appearance of skeletal metazoans in the Lower Cambrian.  相似文献   

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