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1.
Prof. Dr. Reinhold R. Leinfelder Dr. Manfred Krautter Dipl.-Geols. Ralf Laternser Martin Nose Dieter U. Schmid Günter Schweigert Dr. Winfried Werner Prof. Dr. Helmut Keupp Dipl.-Geols. Hartmut Brugger Regina Herrmann Dr. Ursula Rehfeld-Kiefer Prof. Dr. Johannes H. Schroeder Dipl.-Geol. Carsten Reinhold Profs. Dres. Roman Koch Arnold Zeiss Prof. Dr. Volker Schweizer Dipl.-Geols Heinrich Christmann Götz Menges Prof. Dr. Hanspeter Luterbacher Reinhold R. Leinfelder 《Facies》1994,31(1):1-56
Summary In order to elucidate the control of local, regional and global factors on occurrence, distribution and character of Jurassic
reefs, reefal settings of Mid and Late Jurassic age from southwestern Germany, Iberia and Romania were compared in terms of
their sedimentological (including diagenetic), palaeoecological, architectural, stratigraphic and sequential aspects. Upper
Jurassic reefs of southern Germany are dominated by siliceous sponge—microbial crust automicritic to allomicritic mounds.
During the Oxfordian these form small to large buildups, whereas during the Kimmeridgian they more frequently are but marginal
parts of large grain-dominated massive buildups. Diagenesis of sponge facies is largely governed by the original composition
and fabric of sediments. The latest Kimmeridgian and Tithonian spongiolite development is locally accompanied by coral facies,
forming large reefs on spongiolitic topographic elevations or, more frequently, small meadows and patch reefs within bioclastic
to oolitic shoal and apron sediments. New biostratigraphic results indicate a narrower time gap between Swabian and Franconian
coral development than previously thought. Palynostratigraphy and mineralostratigraphy partly allow good stratigraphic resolution
also in spongiolitic buildups, and even in dolomitised massive limestones.
Spongiolite development of the Bajocian and Oxfordian of eastern Spain shares many similarities. They are both dominated by
extensive biostromal development which is related to hardground formation during flooding events. The Upper Jurassic siliceous
sponge facies from Portugal is more localised, though more differentiated, comprising biostromal, mudmound and sponge-thrombolite
as well as frequent mixed coral-sponge facies. The Iberian Upper Jurassic coral facies includes a great variety of coral reef
and platform types, a pattern which together with the analysis of coral associations reflects the great variability of reefal
environments. Microbial reefs ranging from coralrich to siliceous sponge-bearing to pure thrombolites frequently developed
at different water depths. Reef corals even thrived within terrigeneous settings.
In eastern Romania, small coral reefs of various types as well as larger siliceous sponge-microbial crust mounds grew contemporaneously
during the Oxfordian, occupying different bathymetric positions on a homoclinal ramp.
Application of sequence stratigraphic concepts demonstrates that onset or, in other cases, maximum development of reef growth
is related to sea level rise (transgressions and early highstand) which caused a reduction in allochthonous sedimentation.
The connection of reef development with low background sedimentation is corroborated by the richness of reefs in encrusting
organisms, borers and microbial crusts. Microbial crusts and other automicrites can largely contribute to the formation of
reef rock during allosedimentary hiatuses. However, many reefs could cope with variable, though reduced, rates of background
sedimentation. This is reflected by differences in faunal diversities and the partial dominance of morphologically adapted
forms. Besides corals, some sponges and associated brachiopods show distinct morphologies reflecting sedimentation rate and
substrate consistency. Bathymetry is another important factor in the determination of reefal composition. Not only a generally
deeper position of siliceous sponge facies relative to coral facies, but also further bathymetric differentiation within both
facies groups is reflected by changes in the composition, diversity and, partly, morphology of sponges, corals, cementing
bivalves and microencrusters.
Criteria such as authigenic glauconite, dysaerobic epibentic bivalves,Chondrites burrows or framboidal pyrite in the surrounding sediments of many Upper Jurassic thrombolitic buildups suggest that oxygen
depletion excluded higher reefal metazoans in many of these reefs. Their position within shallowing-upwards successions and
associated fauna from aerated settings show that thrombolitic reefs occurred over a broad bathymetric area, from moderately
shallow to deep water. Increases in the alkalinity of sea water possibly enhanced calcification.
Reefs were much more common during the Late Jurassic than during the older parts of this period. Particularly the differences
between the Mid and Late Jurassic frequencies of reefs can be largely explained by a wider availability of suitable reef habitats
provided by the general sea level rise, rather than by an evolutionary radiation of reef biota. The scarcity of siliceous
sponge reefs on the tectonically more active southern Tethyan margin as well as in the Lusitanian Basin of west-central Portugal
reflects the scarcity of suitable mid to outer ramp niches. Coral reefs occurred in a larger variety of structural settings.
Upper Jurassic coral reefs partly grew in high latitudinal areas suggesting an equilibrated climate. This appears to be an
effect of the buffering capacity of high sea level. These feedback effects of high sea level also may have reduced oceanic
circulation particularly during flooding events of third and higher order, which gave rise to the development of black shales
and dysaerobic thrombolite reefs. Hence, the interplay of local, regional and global factors caused Jurassic reefs to be more
differentiated than modern ones, including near-actualistic coral reefs as well as non-actualistic sponge and microbial reefs. 相似文献
2.
Reefs in the Cannindah Limestone at Old Cannindah Homestead, Monto region, Queensland, are exceptional in Eastern Australian
Mississippian (Carboniferous) build-ups because of their largest dimension and differentiated microbial fabrics. Calcimicrobes
and microbial carbonates, which represent a marine reefal environment occupied by both corals and sponges, are particularly
abundant in the reef framework fabrics compared to other Mississippian build-ups in the world. They contributed significantly
to the rigidity of the reefs on a crinoidal bank setting. Metazoans and calcimicrobes coexisted and played different roles
in reef construction. Reef-building and cavity-dwelling microbes include Renalcis, Palaeomicrocodium, Girvanella, problematic Aphralysia, Ortonella, Shamovella-like, Rothpletzella-like, Wetheredella-like, and some problematic calcimicrobes, which occur in inter-corallite infillings of fasciculate rugose corals, in thrombolitic
textures, in or within deposits between microdigitate stromatolite and laminated microbialites, and in reef cavities. Some
reef intervals are entirely formed by Renalcis, Palaeomicrocodium, problematic calcimicrobes, and cement. Girvanella, as an encrusting calcimicrobe, generally bound bioclasts and micrite, or together with cement, formed boundstone. Microbial
carbonates, including thrombolites, microencrusters, microdigitate stromatolite, laminated and tabular microbialite, irregular
layers of self-encrusting vesicles, and microbial micrite, occur commonly in reef framestone and boundstone. The role of microbes
and relevant microbial carbonates in the Cannindah reef limestone highlighted a significant account of microbial facies complexes
associated with the Mississippian reefs. 相似文献
3.
Small reefal bioconstructions that developed in lagoonal settings are widespread in a few horizons of the Late Jurassic (Oxfordian) succession of the Korallenoolith Formation, exposed southwest of Hannover, Northwest Germany. Especially the florigemma-Bank Member, “sandwiched” between oolite shoal deposits, exposes a high variety of build-ups, ranging from coral thrombolite patch reefs, to biostromes and to coral meadows. The reefs show a distribution with gradual facies variations along an outcrop belt that extends about 30 km from the Wesergebirge in the NW to the Osterwald Mts in the SE.The patch reefs from the Deister Mts locality at the “Speckhals” are developed as coral-chaetetid-solenoporid-microbialite reefs and represent a reef type that was hitherto unknown so far north of its Tethyan counterparts. They are mainly built up by coral thickets that are preserved in situ up to 1.5 m in height and a few metres in diameter. They contain up to 20 coral species of different morphotypes but are chiefly composed of phaceloid Stylosmilia corallina and Goniocora socialis subordinately. The tightly branched Stylosmilia colonies are stabilized by their anastomosing growth. The coral branches are coated with microbial crusts and micro-encrusters reinforcing the coral framework. Encrusters and other biota within the thicket show a typical community replacement sequence: Lithocodium aggregatum, Koskinobullina socialis and Iberopora bodeuri are pioneer organisms, whereas the occurrence of non-rigid sponges represents the terminal growth stage. The latter are preserved in situ and seem to be characteristic so far poorly known constituents of the Late Jurassic cryptobiont reef dweller community. The distance and overall arrangement of branches seems to be the crucial factor for the manifestation of a (cryptic) habitat promoting such community replacement sequences. Widely spaced branches often lack any encrusting and/or other reef dwelling organisms, whereas tightly branched corals, as is St. corallina, stimulate such biota. Hence, such reefs are well suited for research on coelobites and community sequences of encrusting and cavity dwelling organisms. 相似文献
4.
Prof. Dr. Reinhold R. Leinfelder 《Facies》1992,26(1):11-34
Upper Jurassic reefs rich in microbial crusts generally appear in deeper (sponge—‘algal’ crust reefs) or in very shallow but
protected settings (coral or coral-coralline sponge meadows with ‘algal’ crusts). Upper Jurassic high-energy reefs (coral
reefs and coral-stromatoporoid reefs) normally lack major participation of microbial crusts but rather represent huge bioclastic
piles with only minor framestone patches preserved.
An exception to this rule is represented by the high-energy, coral-‘algal’ Ota Reef from the Kimmeridgian of the Lusitanian
Basin (Portugal). The narrow Ota Reef tract rims a small intra-basinal carbonate platform exhibiting perfect facies zonation
(from W to E: Reef tract, back reef sands, peritidal belt, low-energy shallow lagoon). The reef is dominated by massive corals
(Thamnasteria, Microsolena, Stylina). Complete preservation of coral framework is rare: like other Upper Jurassic high-energy reefs, the Ota Reef is very rich
in debris; however, this debris is largely stabilized by algal and microbial crusts, what contrasts the other examples and
gives the Ota Reef the appearance of a typical modern high-energy coral-melobesioid algal reef. Further similarities to modern
reefs are the likely existence of a spur-and-groove system, the perfect sheltering of inner platform areas and the occurrence
of small islands, as indicated by local blackenings and early vadose and karstic features. 相似文献
5.
Tethyan microencrusters and microbial crusts, most of them previously unknown in Japanese Mesozoic biotas, are present in the uppermost Jurassic–lowermost Cretaceous Torinosu Limestone distributed in southwestern Japan. They construct reefal facies together with reef-forming metazoans. Bacinella irregularis and Lithocodium aggregatum are quantitatively most important, while subordinate constituents include Thaumatoporella parvovesiculifera, Koskinobullina socialis , Iberopora bodeuri , Girvanella sp. and Tubiphytes morronensis. They are especially common in the shallow-water reefal facies, but appear micritic in outcrops. Microencrusters and microbial crusts can only be recognized in thin sections, and they grow around the reef building metazoans and form bindstone. Each microencruster exhibits some specific spatial distribution associated with its paleoecology. Similarities with the taxonomic composition of the upper Jurassic Tethyan microencruster association imply that the community extended geographically at least to the Tethyan gateway where the Japanese Island Arc was located. 相似文献
6.
Microbialites and micro-encrusters in shallow coral bioherms (Middle to Late Oxfordian, Swiss Jura mountains) 总被引:9,自引:5,他引:4
Summary Benthic microbial crusts (microbialites or microbolites) are an important component of Middle to Upper Oxfordian shallow-water
coral bioherms in the Swiss Jura. They display stromatolitic (laminated), thrombolitic (clotted), and leiolitic (structureless)
fabrics, which are distributed heterogeneously throughout the studied sections. The bioherms can be subdivided into coral-microbialite
facies, microbialite-dominated facies, and sediment matrix.
Macroscopic and microscopic study reveals that microbialitic encrustations commonly occur in two layers. The first one is
directly in contact with the substrate and composed of leiolite (locally stromatolite) and a well-diversified micro-encruster
fauna; the second one fills the remaining porosity partly or completely with thrombolite and low-diversity micro-encrusters.
The growth of the first layer accompanies the growth of the coral reef and thus formed under the same environmental conditions.
The second layer is the result of a moving encrustation front filling the remaining porosity (micro- and macrocavities) inside
the reef, below the living surface. Both layers play an important role in early cementation. Phototrophic cyanobacteria probably
intervene in the formation of the first encrustation zone, whereas heterotrophic bacteria associated to acidic, Ca2+-binding macromolecules in biofilms are thought to contribute to the thrombolite inside the reef body. When coral growth cannot
take pace with microbialite development, the thrombolite from reaches the surface of the construction and finally covers the
reef. The result is a thick interval of thrombolite, which can be interpreted as being related to an ecological crisis in
coral-reef evolution.
A semi-quantitative analysis of the relative abundance of microbialite types and associated micro-encrusters permits to better
constrain the processes leading to a reef crisis. Four micro-encruster associations can be distinguished, and each follows
an evolutionary trend in the studied section:Terebella-Tubiphytes dominated,Serpula-Berenicea dominated,Litho-codium dominated, andBacinella dominated. These trends are interpreted to reflect changes in environmental conditions. Bioerosion generally is at its maximum
before and after abundant growth of microbialite.
According to microbialite-bioerosion relationships and shifts in micro-encruster associations, we propose that the evolution
towards a coral-reef crisis involves four main phases: (1) An oligotrophic to low mesotrophic phase when low water turbidity
and good oxygenation allow phototrophic metabolisms. This leads to a maximum of coral diversity and development of light-dependent
micro-encrusters. (2) A low-mesotrophic phase when increased turbidity and slack water circulation reduce the photic zone
and favor heterotrophic micro- and macrofauna. Bioerosion through bivalves increases. (3) A high-mesotrophic phase when environmental
conditions are so bad that only microbiatite can be produced. (4) A eutrophic phase when carbonate production is inhibited
by high nutrient input and clay flocculation as a result of increased terrestrial run-off.
The observed evolutionary trends are not directly linked to changes in bathymetry, but sea-level fluctuations played an important
role in opening and closing the depositional environments on the shallow platform. Climatic changes contributed in modulating
the influx of siliciclastics and nutrients, and the alkalinity of the water. Demise of coral reefs generally coincides with
low sea level and humid climate. Sea-level and climatic fluctuations and, consequently, the crises in reef growth are linked
to orbital cycles in the Milandkovitch frequency band. 相似文献
7.
Microbial contributions to reefal limestones are evident in eastern Australian Lower Devonian microbial frame/bindstones, red algal-microbial-stromatoporoid bindstones, and microbial-stromatoporoid bindstones. Varied microbialite textures, such as stromatolites, thrombolites, and leiolites, originated as accumulations and partial aggregations of calcimicrobes, peloids, and micrites, which also derived from microbial activities. In microbial frame/bindstones, calcimicrobes (e.g., Rothpletzella and Wetheredella) and dense micrite layers covered and bound underlying substrates. Stabilized substrates promoted the subsequent construction of layered, domal, and columnar frameworks, which were produced by combined accumulations and intermixed associations of calcimicrobes and micritic microbialites. Microbes flourished in the microbial-stromatoporoid bindstones and red algal-microbial-stromatoporoid bindstones during repeated growth interruptions of the framework-building skeletal organisms. Microbes bored into and eroded the skeletal frameworks to subsequently leave micritic envelopes, on which microbial and skeletal encrustations took place in turn. The importance of microbial colonization on the skeletal frameworks was first as subsidiary encrusters that helped to preserve them from erosion, and second as modifiers of the spaces suitable for succeeding encrusters. Partial aggregations of Renalcis filled in the interstices of the skeletal and microbial frameworks, thereby enhancing their rigidity.The microbial impacts on the genesis of reefal limestones are: (1) origination of components (calcimicrobes, peloids, and micrites); (2) formation of characteristic microbial textures; (3) main and subsidiary reef construction and encrustation; and (4) destruction of these components, textures, and structures, but also the protection of resultant constructions in turn. The Lower Devonian reefal limestones treated herein, surprisingly, preserve excellent records of a variety of microbial impacts. Similar effects may also have been common, although variable in preservation, in other ancient reefal deposits. 相似文献
8.
Summary Anin situ Oxfordian patch reef from the Süntel hills (florigemma-Bank, Korallenoolith, NW-Germany) is described. It is composed of an autochthonous reef core overlain by a ‘parautochthonous’
biostrome. The exposed reefal area amounts to about 20 m in lateral and up to 4 m in vertical direction. Nearly all major
marine reefal fossil associations from the Tethyal realm are present.
In the reef core two facies can be distinguished: (1)Thamnasteria dendroidea thicket facies and (2) thrombolite facies.
The first facies is composed of a thin branched autochthonous coral thicket mainly constructed ofTh. dendroidea colonies with only a minor portion ofStylosmilia. Frequently, theTh. dendroidea branches laterally coalesce bridge-like forming a delicate initial framework which was subsequently reinforced by thick microbial
coatings, that make up approximately 80% of the rock volume. This facies is an excellent example for microbialite binding
in reefal architecture. Additionally, several generations of micromorphic and partly cryptic encrusting organisms settled
on theTh. dendroidea branches and microbialite crusts. They successively overgrow each other and fill the space between the coral branches in
the thicket forming a characteristic community replacement sequence.
Initial colonization of theThamnasteria dendroidea took place on an oncoidic/bioclastic hardground. During this early phase of reefal development, microbialites also played
an important role in stabilizing and binding the reef body.
The thrombolite facies (2) occupying nearly the same volume of the reef body as facies type (1) consists of a thrombolitic
microbialitic limestone which fills the interstice between the coral colonies. It shows a considerably lower faunal diversity
than theTh. dendroidea facies. Numerous cavities are interspersed in the thrombolithe and are almost completely filled with dolomitized allomicrite.
In contrast, microbialite and allomicrite adjacent to the reef core rarely reveal any dolomitized areas.
Above the reef core, mostly toppledSolenopora jurassica thalli occur together with a few massiveIsastrea colonies forming a parautochthnous biostrome. They are inhabited by a low diverse assemblage of encrusting organisms. Microbialites
are only rarely present in this biostromal unit.
The patch reef is developed within a lagoonal limemud facies both separated by a sharp interface. In contrast, continuous
facies transition exists between theSolenopora biostrome and adjacent deposits which are characterized by micritic to pelmicritic limestone sometimes with lenses of oncoids.
Debris derived from the patch reef is only sporadically intercalated in the reef surrounding lagoonal sediments. Gastropods,
bivalves, and dasycladalean algae dominate the lagoonal biota. Up-section following theSolenopora biostrome nerinean gastropods become the most abundant species amounting to a ‘Nerinea-bed’. This horizon moderately elevates above the patch reef indicating, that is arose above the surrounding sea floor forming
a relief.
The patch reef established on a secondary hardground probably released by a minor transgression and a nondepositional regime.
It grew up on a well-illuminated sea floor only a few meters below sea level. Only a low background sedimentation rate and
modest water circulation are assumed during reefal growth. These features characterize an open marine lagoon.
A subsequent shallowing upwards trend caused emergence of the early lithifiedflorigemma-Bank sediments. In the following erosional phase the reef core,Solenopora biostrome and ‘Nerinea-bed’ were sharply cut. Paleokarst phenomena (karst solution of the rocks, selective leaching of the aragonitic corals) truncate
the surface of theflorigemma-Bank. Released by a transgressive sea level, the paleokarst surface is densely inhabited by marine boring and encrusting
organisms (oysters, serpulids). Karst cavities are filled with an oncoid-bearing bioclastic limestone with a large portion
of siliciclastics. Theflorigemma-Bank is overlain by the reddish bioclastic sandstone of the ‘Zwischenfl?zregion’. 相似文献
9.
Cambrian Series 3 lithistid sponge–microbial reefs in Shandong Province,North China: reef development after the disappearance of archaeocyaths 下载免费PDF全文
Natsuko Adachi Ayaka Kotani Yoichi Ezaki Jianbo Liu 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2015,48(3):405-416
The Cambrian Series 3 Zhangxia Formation in Shandong Province, North China, includes small‐scale lithistid sponge–microbial reefs. The lithistid sponges grew on oolitic and bioclastic sediments, which were stabilized by microbial activities. The relative abundances of microbial components (e.g. calcimicrobe Epiphyton and stromatolites) vary among the reefs. However, the microbial components commonly encrusted or bound the lithistid sponges, formed remarkable encrustations on the surfaces of the sponges. Epiphyton especially grew upward and downward. The lithistid sponges thus provided substrates for the attachment and development of microbes, and the microbes played essential roles as consolidators, by encrusting reef‐building sponges. Additionally, the lithistid sponges were prone to degradation via microbial activities and diagenetic processes, and were thus preserved as micritic bodies, showing faint spicular networks or abundant spicules. Such low preservation potential within the reef environment obscured the presence of the sponges and their widespread contribution as reef‐building organisms during the Cambrian. During the prolonged interval after the demise of archaeocyaths, purely microbial reefs, such as stromatolites and thrombolites have been considered to be the principal reef builders, in association with rare lithistid sponge–microbial associations. However, recent findings, including those from Shandong Province and Korea, suggest that the lithistid sponge‐bearing reefs were more extensive during the Epoch 3 to the Furongian than previously thought. These lithistid sponge–microbial reefs were precursors of the sponge–microbial reefs that dominated worldwide in the Early Ordovician. 相似文献
10.
Ocean acidification by atmospheric carbon dioxide has increased almost continuously since the last glacial maximum (LGM), 21 000 years ago. It is expected to impair tropical reef development, but effects on reefs at the present day and in the recent past have proved difficult to evaluate. We present evidence that acidification has already significantly reduced the formation of calcified bacterial crusts in tropical reefs. Unlike major reef builders such as coralline algae and corals that more closely control their calcification, bacterial calcification is very sensitive to ambient changes in carbonate chemistry. Bacterial crusts in reef cavities have declined in thickness over the past 14 000 years with largest reduction occurring 12 000–10 000 years ago. We interpret this as an early effect of deglacial ocean acidification on reef calcification and infer that similar crusts were likely to have been thicker when seawater carbonate saturation was increased during earlier glacial intervals, and thinner during interglacials. These changes in crust thickness could have substantially affected reef development over glacial cycles, as rigid crusts significantly strengthen framework and their reduction would have increased the susceptibility of reefs to biological and physical erosion. Bacterial crust decline reveals previously unrecognized millennial‐scale acidification effects on tropical reefs. This directs attention to the role of crusts in reef formation and the ability of bioinduced calcification to reflect changes in seawater chemistry. It also provides a long‐term context for assessing anticipated anthropogenic effects. 相似文献
11.
Summary Microbial reefs, together with stromatolitic mounds and ooid shoals, constitute massive limestones in Famennian platform marginal
strata in Guilin, in sharp contrast to the well-known coral-stromatoporoid reefs in the Givetian and Frasnian. Microbes played
a significant and important role as stabilizers in the Famennian carbonate deposits of Guilin. A reef at Zhaijiang was constructed
byEpiphyton andRenalcis, and is representative of such carbonate buildups. The reef is situated 10 km west of Guilin and corresponds to a microbe-dominated
platform margin carbonate complex. Organisms in the Zhaijiang microbial reef are low diversity and dominated by ostracods
and two genera of microbes,Epiphyton andRenalcis. Other microbial genera such asSphaerocodium andWetheredella occur in most of reef facies in Guilin, but their role as reef builder is doubtful because they occur only in minor amounts.
The same four genera occur in volumetrically significant amounts in the upper Devonian carbonate complexes of Alberta. Canada
and Western Australia. However.Epiphyton is more abundant in the Guilin reefs.
The Zhaijiang microbial reef developed above Famennian proximal slope faices, as suggested by reef architecture and paleogeographic
setting. The facies sequence of the microbial reef can be divided into three parts. The lower part is composed of medium-bedded
bioclastic grainstones with a few microbial framestone lithoclasts, representing a proximal slope facies. The middle part
consists of thin-bedded mudstone and shale with limestone lenses that are thought to be low stand deposits. In some cross
sections, mudstone and shale infilled tidal channels that developed in the bioclastic grainstones.Renalcis-Epiphyton framestone constitutes the upper part with massive stacking patterns. The reef is 35 m thick and over 50 m in width. Nine
litho- and biofacies are recognized. Zhaijiang reef provides an example of a binder guild-dominated buildup in the almost
vacant reef ecosystem of the Famennian and represents a characteristic kind of reef after the Frasnian/Famennian extinction. 相似文献
12.
13.
Extensive metazoan reefs from the Ediacaran Nama Group,Namibia: the rise of benthic suspension feeding 下载免费PDF全文
We describe new, ecologically complex reef types from the Ediacaran Nama Group, Namibia, dated at ~548 million years ago (Ma), where the earliest known skeletal metazoans, Cloudina riemkeae and Namacalathus, formed extensive reefs up to 20 m in height and width. C. riemkeae formed densely aggregating assemblages associated with microbialite and thrombolite, each from 30 to 100 mm high, which successively colonised former generations to create stacked laminar or columnar reef frameworks. C. riemkeae individuals show budding, multiple, radiating attachment sites and cementation between individuals. Isolated Namacalathus either intergrew with C. riemkeae or formed dense, monospecific aggregations succeeding C. riemkeae frameworks, providing a potential example of environmentally mediated ecological succession. Cloudina and Namacalathus also grow cryptically, either as pendent aggregations from laminar crypt ceilings in microbial framework reefs or as clusters associated with thrombolite attached to neptunian dyke walls. These reefs are notable for their size, exceeding that of the succeeding Lower Cambrian archaeocyath–microbial communities. The repeated colonisation shown by C. riemkeae of former assemblages implies philopatric larval aggregation to colonise limited favourable substrates. As such, not only were skeletal metazoans more important contributors to reef building in the Ediacaran, but there were also more variable reef types with more complex ecologies, than previously thought. Such an abundance of inferred suspension feeders with biomineralised skeletons indicates the efficient exploitation of new resources, more active carbon removal with a strengthened energy flow between planktic and benthic realms, and the rise of biological control over benthic carbonate production. These mark the prelude to the Cambrian Explosion and the modernisation of the global carbon cycle. 相似文献
14.
Anisian (middle triassic) buildups of the Northern Dolomites (Italy): The recovery of reef communities after the permian/triassic crisis 总被引:1,自引:0,他引:1
Dr. Baba Senowbari-Daryan Dipl. Geol. Rainer Zühlke Prof. Dr. Thilo Bechstädt Prof. Dr. Erik Flügel 《Facies》1993,28(1):181-256
Summary After the end-Permian crisis and a global ‘reef gap’ in the early Triassic, reefs appeared again during the early Middle Triassic.
Records of Anisian reefs are rare in the Tethys as well as in non-Tethyan regions. Most Anisian reefs are known from the western
part of the Tethys but there are only very few studies focused on biota, facies types and the paleogeographical situation
of these reefs. From the eastern part of the Tethys, Anisian reefs, reefal buildups or potential reef-building organisms have
been reported from different regions of southern China. Most of the Anisian reefs known from western and central Europe as
well as from southern China seem to be of middle and late Pelsonian age.
The study area is situated in the northern Dolomites (South Tyrol, Italy) southeast of Bruneck (Brunico). It comprises the
area between Olang (Valdaora) and Prags (Braies). The study is based on detailed investigations of the regional geology, stratigraphy
and lithofacies (R. Zühlke, T. Bechst?dt) as well as on a comprehensive inventory of Anisian reef organisms (B. Senowbari-Daryan,
E. Flügel). These data are used in the discussion of the controls on the recovery of reefs during the early Middle Triassic.
Most late Anisian reef carbonates studied are represented by allochthonous talus reef blocks of cubicmeter size. Small biostromal
autochthonous mounds are extremely rare (Piz da Peres). The reef mounds as well as most of the reef blocks occur within the
middle to late Pelsonian Recoaro Formation. They were formed on the middle reaches of carbonate ramps in subtidal depths,
slightly above the storm wave base with only moderate water energy. Most lithotypes observed in the reef blocks correspond
to sponge and/or algal bafflestones. Low-growing sessile organisms (Olangocoelia (sponge, alga?), sphinctozoan sponges, bryozoans, soleno-poracean algae, corals) and encrusting epibionts (sponges, porostromate
algae, cyanophycean crusts, foraminifera, worms, microproblematica) created low cm-sized biogenic structures (bioconstructions)
which baffled and bound sediment. Organic framework was only of minor importance; it is restricted to theOlangocoelia lithotype. Framework porosity was small in these reef mounds. Submarine carbonate cements, therefore, are only of minor importance
s compared with Permian or Ladinian reefs. The relatively high number of lithotypes encountered in the reef blocks indicates
a high biofacies diversity.
Regarding the relative frequency, the diverse biota consist in descending order ofOlangocoelia, sponges (sphinctozoans, inozoans, siliceous sponges), bryozoans, porostromate algae and worm tubes. The sphinctozoans are
characterized by small, mostly incrusting forms. The numerical diversity (species richness) is low compared with late Permian
or Ladinian and late Triassic sphinctozoan faunas occurring within reefs.
Following the sponges, monospecific bryozoans (Reptonoditrypa cautica
Sch?fer & Fois) are the most common organisms in the reef limestones. Porostromate algae were restricted to areas within the bioconstructions
not inhabited by sponges. The low-diverse corals had no importance in the construction of an organic framework.
Surprisingly, microbial crusts are rare or even lacking in the investigated Anisian bioconstructions. This is in contrast
to late Permian and Ladinian as well as Carnian reefs which are characterized by the abundance of specific organic crusts.
The same comes true for‘Tubiphytes’ which is a common constituent in Permian, Ladinian and Carnian reef carbonates but is very rare in the Anisian of the Olang
Dolomites. Instead of‘Tubiphytes’ different kinds of worm tubes (spirorbid tubes, Mg-calcitic tubes and agglutinated tubes) were of importance as epifaunal
elements. Macrobial encrustations consisting of characteristic successions of sponges, bryozoans, algae, worm tubes and microproblematica
seem to be of greater quantitative importance than in Ladinian reefs.
Destruction of organic skeletons (predominantly of bryozoans) by macroborers (cirripedia?) is a common feature.
The Anisian reef organisms are distinctly different from late Permian and from most Ladinian reef-builders. No Permian Lazarus
taxa have been found.
New taxa: Sphinctozoan sponges—Celyphia? minima n.sp.,Thaumastocoelia dolomitica n. sp.,Deningeria tenuireticulata n. sp.,Deningeria crassireticulata n. sp.,Anisothalamia minima n.g. n.sp., Inozoan sponges-Meandrostia triassica n.sp. Microproblematica-Anisocellula fecunda n.g. n.sp., Porostromate alga-Brandneria dolomitica n.g. n.sp.
Most of our data are in agreement with the model described byFois & Gaetani (1984) for the recovery of reef-building communities during the Ansian but the biotic diversity seems to be considerably
higher than previously assumed.
Anisian deposition and the formation of the reef mounds within the Pelsonian Recoaro Formation of the Dolomites were controlled
by the combined effects of synsedimentary tectonics and eustatic changes in sea-level. During several time intervals, especially
the early Anisian (northern and western Dolomites: tectonic uplift), the early Pelsonian (eastern Dolomites: drowning) and
the late Illyrian (wide parts of the Dolomites: uplift and drowning), the sedimentation was predominantly controlled by regionally
different tectonic subsidence rates. The amount of terrigenous clastic input associated with synsedimentary tectonics (tectonic
uplift of hinterlands) had a major influence on carbonate deposition and reef development. The re-appearance of reef environments
in the Olang Dolomites was controlled by a combination of regional and global factors (paleogeographic situation: development
of carbonate ramps; decreasing subsidence of horst blocks; reduced terrigenous input; moderate rise in sea-level). 相似文献
15.
Late Jurassic–Early Cretaceous ?tramberk-type reef limestones are known from some parts of the Southern Carpathians in Romania. The Upper Jurassic deposits mainly consist of massif reef limestones including a variety of microbialites associated with micro-encrusters. They played an important role in the formation and evolution of the reef frameworks and thus are of significant importance for deciphering the depositional environments. For our study, the most important encrusting organisms are Crescentiella morronensis, Koskinobullina socialis, Lithocodium aggregatum, Bacinella-type structures, Radiomura cautica, Perturbatacrusta leini, Coscinophragma sp., and crust-forming coralline sponges such as Calcistella. Based on microscopic observations, microbial contribution to reef construction is documented by the abundance of dense micrite, laminate structures, clotted, thrombolithic or peloidal microfabrics, constructive micritic cortices, biogenic encrustations and cement crusts, as well as by other types of microbial structures and crusts. Most of the investigated carbonate deposits can be classified as “coral-microbial-microencruster boundstones” which are characteristic for the Intra-Tethyan domain. Their paleogeographical significance is indicated by the presence of many features comparable with carbonate deposits of rimmed platform systems from the Northern Calcareous Alps or Central Apennines. Based on the distribution of the facies and facies associations within the carbonate sequences under study we can distinguish slope and external shelf margin environments. The microbial crusts, the encrusting micro-organisms, and in some cases the syndepositional cements have stabilized and bound the carbonates of the slope facies types. Subsequently, the stable substrate favored the installation of coral-microbial bioconstruction levels. 相似文献
16.
Intraspecific variation in an Ediacaran skeletal metazoan: Namacalathus from the Nama Group,Namibia 下载免费PDF全文
A. M. Penny R. A. Wood A. Yu. Zhuravlev A. Curtis F. Bowyer R. Tostevin 《Geobiology》2017,15(1):81-93
Namacalathus hermanastes is one of the oldest known skeletal metazoans, found in carbonate settings of the terminal Ediacaran (~550–541 million years ago [Ma]). The palaeoecology of this widespread, goblet‐shaped, benthic organism is poorly constrained yet critical for understanding the dynamics of the earliest metazoan communities. Analysis of in situ assemblages from the Nama Group, Namibia (~548–541 Ma), shows that Namacalathus exhibited size variation in response to differing water depths, hydrodynamic conditions and substrate types. In low‐energy, inner ramp environments, Namacalathus attains the largest average sizes but grew in transient, loosely aggregating, monospecific aggregations attached to microbial mats. In high‐energy mid‐ramp reefs, Namacalathus spatially segregated into different palaeoecological habitats with distinct size distributions. In outer ramp environments, individuals were small and formed patchy, dense, monospecific aggregations attached to thin microbial mats. Asexual budding is common in all settings. We infer that variations in size distribution in Namacalathus reflect differences in habitat heterogeneity and stability, including the longevity of mechanically stable substrates and oxic conditions. In the Nama Group, long‐lived skeletal metazoan communities developed within topographically heterogeneous mid‐ramp reefs, which provided diverse mechanically stable microbial substrates in persistently oxic waters, while inner and outer ramp communities were often ephemeral, developing during fleeting episodes of either oxia and/or substrate stability. We conclude that Namacalathus, which forms a component of these communities in the Nama Group, was a generalist that adapted to various palaeoecological habitats within a heterogeneous ecosystem landscape where favourable conditions persisted, and was also able to opportunistically colonise transiently hospitable environments. These early skeletal metazoans colonised previously unoccupied substrates in thrombolitic reefs and other microbial carbonate settings, and while they experienced relatively low levels of interspecific competition, they were nonetheless adapted to the diverse environments and highly dynamic redox conditions present in the terminal Ediacaran. 相似文献
17.
Dr. Enzo Insalaco 《Facies》1999,40(1):81-99
Summary This study documents the facies and fauna of Late Jurassic (Middle Oxfordian) coral reefs in England. Sedimentological and
palaeoecological analysis of these reefs distinguishes three generic reef types: (1) small reef patches and thickets associated
with siliciclastic deposits; (2) small reef patches and thickets associated with siliciclastic-free bioclastic grainstones
and packstones; and (3) biostromal units associated with deep water facies. The depositional environments of these reef types
are discussed. Two coral assemblages are identified: (1) the microsolenid assemblage; and (2) theThamnasteria, Isastraea, Fungiastraea andThecosmilia assemblage (Thamnasteria assemblage). TheThamnasteria assemblage developed in all shallow water environments in the study area, regardless of local environmental conditions. The
fauna is very eurytopic,r-selected and can tolerate significant environmental fluctuations on short temporal scales (sub-seasonal). The main control
on the development of the microsolenid assemblage was low light intensity, low background sedimentation rates and low hydrodynamic
energy levels. 相似文献
18.
THOMAS A. STEMANN KENNETH G. JOHNSON 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1992,25(3):231-241
Stemann, T. A. & Johnson, K. G. 1992 07 15: Coral assemblages, biofacies. and ecological zones in the mid-Holocene reef deposits of the Enriquillo Valley, Dominican Republic. A large, subaerially exposed mid-Holocene reef in the Enriquillo Valley (southwest Dominican Republic) provides an excellent opportunity to examine the relationship between reefal ecology and reefal deposits. Coral species richness and diversity in the Enriquillo reef are comparable to that found in the recent of the Caribbean, and ecological zonation comprised of a shallow-water branching coral zone and a deeper water mixed-coral zone is apparent. Similar zonation and diversity patterns have been recognized on living Caribbcan reefs with moderate wave exposure. Three statistically discrete biopdcies can be discriminated in the Enriquillo deposits using quadrat point-counting techniques commonly used to census modern reefs. They include a facies dominated by Acropora cervicornis, a low diversity assemblage with abundant, large colonies of Siderastrea siderea and Stephanocoenia intersepta, and a higher diversity assembbdge composed of various taxa including Montastraea spp., Colpophyllia spp., and Agaricia spp. Each facies can be recognized at scales of 1–3 m2, though in some cases they extend for more than 20 m2. In general, the A. cervicornis facies is spatially segregated from the other two biofacies. although neither the shallow nor the deep-water ecological zone is comprised of a single reef biofacies. Rather, the biofacies described here appear to represent distinct micro-environments resulting from ecological variation at a subzonal scale. Micro-environments of similar scale are most likely preserved in other reef deposits. Recognition of these subzonal biofacies may have important consequences for the stratigraphical and paleoccological interpretation of fossil reefs. Corals, biofacies, reef zonation, coral communities, fossil reefs. 相似文献
19.
M. R. Jones 《Coral reefs (Online)》1995,14(2):63-69
High resolution seismic data from Torres Strait off northern Australia provide the first insights into the development of the striking platform reefs of the region. In the southern part of Torres Strait, where the sea floor is shallow and featureless, the Torres Reefs have developed as long, narrow platforms parallel to very strong east-west flowing tidal currents. The reefs have expanded since the end of the post-glacial marine transgression (approximately 6000 years ago) through preferential growth at each end of their platforms. In these localized areas sheltered from the strong tidal currents, patch reefs have developed. Sediments swept from the reef margin and the inter-reef channels are deposited in these current lee areas at the ends of the reefs. The combination of the patch reefs and reefal sediments provides the essentials for extension of the platform reefs. The seismic data confirm that modern reef expansion is occurring without there being a high substrate for coral colonization. The evolutionary sequence from an uncolonized seafloor to a mature platform reef envisages a synchronous process of patch reef development and sediment accumulation. 相似文献
20.
Zaki A. Abdel‐Fattah Ehab M. Assal 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2016,49(3):398-412
Macroborings provide detailed information on the bioerosion, accretion and palaeoenvironment of both modern and fossil reefs. Dolomitized reefal carbonates in the Um Mahara Formation exhibit an outstanding example of spatially distributed, well‐preserved bioerosion structures in tropical to subtropical syn‐rift Miocene reefs. Ten ichnospecies belonging to five ichnogenera are identified; three belonging to the bivalve‐boring ichnogenus Gastrochaenolites, three attributed to the sponge‐boring ichnogenus Entobia, and four ichnospecies assigned to three worm‐boring ichnogenera Trypanites, Maeandropolydora and Caulostrepsis. The distribution of the reported borings is strongly linked to the palaeo‐reef zones. Two distinctive ichnological boring assemblages are recognized. The Gastrochaenolites‐dominated assemblage reflects shallower‐marine conditions, under water depths of a few metres, mostly in back‐reef to patch‐reef zones of a back‐reef lagoon. The Entobia‐dominated assemblage signifies relatively deeper marine conditions, mostly in reef core of the fringing Miocene reefs. These ichnological assemblages are attributed herein to the Entobia sub‐ichnofacies of the Trypanites ichnofacies. This ichnofacies indicates boring in hard carbonate substrates (such as corals, rhodoliths, carbonate cements and hardgrounds) during periods of non‐sedimentation or reduced sediment input. 相似文献