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1.
Summary A heretofore undocumented example of skeletal mounds formed by the dasycladacean alga Anthracoporella spectabilis is described from mixed carbonate-clastic cycles (Auernig cyclothems) of the Late Carboniferous (Gzhelian) Auernig Group
of the central Carnic Alps in southern Austria.
The massive mound facies forms biostromal reef mounds that are up to several m thick and extend laterally over more than 100
m. The mound facies is developed in the middle of bedded limestones, which are up to 16 m thick. These limestones formed during
relative sea-level highstands when clastic influx was near zero.
The mound facies is characterized by well developed baffler and binder guilds and does not show any horizontal or vertical
zonation. Within the massive mound facies Anthracoporella is frequently found in growth position forming bafflestones and wackestones composed of abundant Anthracoporella skeletons which toppled in situ or drifted slightly. Anthracoporella grew in such profusion that it dominated the available sea bottom living space, forming ‘algal meadows’ which acted as efficient
sediment producers and bafflers. Because Anthracoporella could not provide a substantial reef framework, and could not withstand high water turbulence, the biostromal skeletal mounds
accumulated in shallow, quiet water below the active wave base in water depths less than 30 m.
The massive mound facies is under- and overlain by, and laterally grades into bedded, fossiliferous limestones of the intermound
facies, composed mainly of different types of wackestones and packstones. Individual beds contain Anthracoporella and Archaeolithophyllum missouriense in growth position, forming “micromounds’.
Two stages of mound formation are recognized: (1) the stabilization stage when bioclastic wackestones accumulated, and (2)
the skeletal mound stage when the sea-bottom was colonized by Anthracoporella and other members of the baffler and binder guilds, forming Anthracoporella bafflestones and wackestones of the mound facies.
A slight drop in sea-level led to the termination of the mound growth and accumulation of organic debris, particularly calcareous
algae, fusulinids, crinoids and bryozoans, forming well bedded limestones, which overlie the mound facies 相似文献
2.
Fossiliferous mounds of carbonate mud are a distinctive facies in the middle Chazy Group (Crown Point Formation) at Isle La Motte, Lake Champlain. The mounds are surrounded by bedded calcarenite of spar-cemented pelmatozoan debris. Channels which cut into the mounds during mound growth are filled with the same calcarenite. The mud-free intermound rocks and the mound biota suggest agitated, normal marine shallow-water environments. The principal lime-secreting organisms within the mounds are stromatoporoids, calcareous algae, tabulate corals, sponges, and bryozoans. Each mound is dominated in terms of biomass by one of three groups: stromatoporoids, calcareous algae, and bryozoans. Most of the mound biota first appear at the base of the Crown Point Formation. In the lower Crown Point Formation the organisms increase in number and species. Both changes in the biota are related to periods of shallowing of the Chazy sea which are also reflected in the character of the carbonate sands. 相似文献
3.
Summary Following the demise of the stromatoporoid-coral reef community in Late Frasnian time, Lower Carboniferous carbonate shelf
profiles possessed a ramp geometry, with major organic buildups represented by mud-rich mounds. Microfacies petrography of
the exceptionally well-preserved Upper Viséan (Lower Carboniferous) carbonate ramp of the Béchar Basin, Algerian Sahara, may
well contribute significantly to our understanding of the paleoecological zonation of Carboniferous non-rimmed platforms,
and of the still enigmatic mounds commonly referred to as Waulsortian banks or mounds.
Facies are grouped into two broad groups: (a) a mound facies group which comprises sponge wackestone-bafflestone, sponge-fenestellid
bafflestone-wackestone, crinoid wackestone-packstone, and bedded flanks of intraclastic wackestone-packstone, all four facies
composing the actual mud-rich mounds, and (b) a supramound facies group composed of coral-microbial framestone, crinoid packstone-grainstone,
algal-foraminiferal grainstone and oolite grainstone.
Calcareous algae are important bathymetric indicators and are used to delineate three bathymetric zones based on light penetration:
the aphotic zone, which contains no calcareous algae; the dysphotic zone, where there is little ambient light, and which is
characterized by the presence of red algae ( Fasciella, Ungdarella, Stacheia, Epistacheoides) and absence of green algae; and the euphotic zone, which receives the full spectrum of sunlight, and is characterized by
the occurrence of both green algae ( Koninckopora, Kamaenella, Kamaena, Palaeoberesella, Calcisphaera, Anthracoporellopsis, Issinella, Exvotarisella) and red algae.
Integration of algal zonation, distribution of the other biota, and recurrence of distinct assemblages, enable recognition
of seven depth-related benthic assemblages. Together with the physical properties of the facies, the benthic assemblages were
used to define seven bathymetric zones, from upper to lower ramp: (1) algal assemblage (upper ramp); (2) crinoid-ramose bryozoan
assemblage (mid-ramp); and (3) productid brachiopod assemblage, (4) colonial rugose coral-microbial encruster assemblage,
(5) crinoid-fenestellid assemblage, (6) sponge-fenestellid, and (7) sponge assemblage (lower ramp).
The vertical zonation of the mud-rich mounds and associated facies differ from that reported from the classical Upper Tournaisian-Lower
Viséan Waulsortian mound-bearing successions. 相似文献
4.
Summary In the area of Haidach (Northern Calcareous Alps, Austria), coral-rudist mounds, rudist biostromes, and bioclastic limestones
and marls constitute an Upper Cretaceous shelf succession approximately 100 meters thick. The succession is part of the mixed
siliciclasticcarbonate Gosau Group that was deposited at the northern margin of the Austroalpine microplate.
In its lower part, the carbonate succession at Haidach comprises two stratal packages that each consists, from bottom to top,
of a coral-rudist mound capped by a rudist biostrome which, in turn, is overlain by bioclastic limestones and, locally, marls.
The coral-rudist mounds consist mainly of floatstones. The coral assemblage is dominated by Fungiina, Astreoina, Heterocoeniina
and Agathelia asperella (stylinina). From the rudists, elevators ( Vaccinites spp., radiolitids) and recumbents ( Plagioptychus) are present. Calcareous sponges, sclerosponges, and octocorals are subordinate. The elevator rudists commonly are small;
they settled on branched corals, coral heads, on rudists, and on biolastic debris. The rudists, in turn, provided settlement
sites for corals. Predominantly plocoid and thamnasteroid coral growth forms indicate soft substrata and high sedimentation
rates. The mounds were episodically smothered by carbonate mud. Many corals and rudists are coated by thick and diverse encrustations
that indicate high nutrient level and/or turbid waters.
The coral-rudist mounds are capped by Vaccinites biostromes up to 5 m thick. The establishment of these biostromes may result from unfavourable environmental conditions for
corals, coupled with the potential of the elevator rudists for effective substrate colonization. The Vaccinites biostromes are locally topped by a thin radiolitid biostrome. The biostromes, in turn, are overlain by bioclastic limestones;
these are arranged in stratal packages that were deposited from carbonate sand bodies. Approximately midsection, an interval
of marls with abundant Phelopteria is present. These marls were deposited in a quiet lagoonal area where meadows of sea grass or algae, coupled with an elevated
nutrient level, triggered the mass occurrence of Phelopteria.
The upper part of the Haidach section consists of stratal packages that each is composed of a rudist biostrome overlain by
bioclastic wackestones to packstones with diverse smaller benthic foraminifera and calcareous green algae. The biostromes
are either built by radiolitids, Vaccinites, and Pleurocora, or consist exclusively of radiolitids (mainly Radiolites). Both the biostromes and the bioclastic limestones were deposited in a low-energy lagoonal environment that was punctuated
by high-energy events. In situ-rudist fabrics typically have a matrix of mudstone to rudistclastic wackestone; other biogens (incl. smaller benthic foraminifera)
are absent or very rare. The matrix of rudist fabrics that indicate episodic destruction by high-energy events contain a fossil
assemblage similar to the vertically associated bioclastic limestones. Substrata colonized by rudists thus were unfavourable
at least for smaller benthic foraminifera.
The described succession was deposited on a gently inclined shelf segment, where coral-rudist mounds and hippuritid biostromes
were separated by a belt of bioclastic sand bodies from a lagoon with radiolitid biostromes. The mounds document that corals
and Late Cretaceous elevator rudists may co-occur in close association. On the scale of the entire succession, however, mainly
as a result of the wide ecologic range of the rudists relative to corals, the coral-dominated mounds and the rudist biostromes
are vertically separated. 相似文献
5.
Early Danian cool-water bryozoan mounds exposed in the coastal cliff Stevns Klint in Denmark were formed shortly after the Cretaceous–Tertiary mass extinction. They represent a relatively deep-water, highly diverse benthic ecosystem within the epeiric seaway that covered the Danish Basin. The mounds are 50–110 m long and reached a height of about 5–10 m above the seafloor; they are asymmetrical with a steep southern and a gentle northern flank, and were dominated by small suspension feeders. The benthic elements generally occur as fragments set in a carbonate mud matrix. The main skeletal contributors are delicate branching bryozoans with minor contributions of bryozoan sheets, and nodular/arborescent bryozoans. Locally abundant octocorals occur on the mound crests and upper parts of the steep flanks. Echinoids are present in minor amounts, but are locally abundant. Serpulids, crinoids, asteroids, brachiopods, bivalves, massive calcareous sponges, and benthic foraminifers are generally minor contributors to the benthic mound fauna. Influx of planktonic foraminifers, coccoliths and other planktonic organisms was high and was probably a major source of nutrient supply to the mainly suspension-feeding benthic fauna. The faunal association reflects a relatively low energy environment with a high, possibly seasonal influx of particulate nutrients. The best growth conditions with respect to nutrient influx were on the mound crest and upper steep flank reflected by the diverse and relatively largest benthic faunal elements. Periodic reworking and winnowing occurred across the entire mound structure but most prominent on the gentle northern flanks limiting the benthic growth and notably the colony density and size of delicate branching bryozoans. Vagile benthic faunas were also adapted to different areas on the mound. Irregular echinoids preferred the intermound areas within fine-grained wackestone–packstone facies where they ploughed through the sediment, whereas regular echinoids were epifaunal and preferred the upper parts of the mounds, possibly feeding mainly on bryozoans. Skeletons of both groups became concentrated at the toe of the steep flanks and in the intermound areas by physical reworking during major storms. Changes in faunal composition on the mound crests occurred rhythmically on both small and large scale during mound growth. Rhythmically recurring faunal assemblages reflect alternating hydrodynamic conditions on the seafloor with respect to nutrient influx and energy, which probably were linked to short-term seasonal and long-term climatic variations; the long-term alternation may be within the Milankovitch frequency band. Blooming events of bryozoan sheets resulted from relatively short periods with large amounts of available food and suitable substrate. Successful colonisation by octocorals on the other hand reflected longer-term favourable conditions on the mounds possibly associated with overall higher energy levels. A possible Pleistocene analogue to the bryozoan-dominated Danian mounds occurs at the shelf-slope break of the Great Australian Bight. Both of these cool-water mound systems deviate from most other biogenic mounds known from the fossil record in their non-cemented nature, regular geometry and a lack of core and flank facies. 相似文献
6.
Summary Liassic sponge mounds of the central High Atlas (Rich area, northern Morocco) have a stratigraphic range from the Lower/Upper
Sinemurian boundary interval up to the lower parts of the Lower Pliensbachian (Carixian). The base of Liassic sponge mounds
consists of a transgressive discontinuity, i.e., a condensed section of microbioclastic wackestones with firm- and hardgrounds,
ferruginous stromatolites, sponge spicules and ammonites. The top of Liassic sponge mounds is an irregular palaeorelief covered
by cherty marl-limestone rhythmites, namely hemipelagic spicular wackestones with radiolaria.
In the Rich area, section Foum Tillicht, the sponge mound succession has a total thickness of about 250 meters. Within this
succession we distinguished between three mound intervals. The lower mound interval shows only small, meter-scale sponge mounds
consisting of boundstones with lyssakine sponges, commensalic Terebella and the problematicum Radiomura. This interval forms a shallowing-upward sequence culminating in a bedded facies with Tubiphytes, calcareous algae ( Palaeodasycladus), sponge lithoclasts, coated grains, and thin rims of marine cement. The middle mound interval is aggradational with decametric
mounds and distinct thrombolitic textures and reefal cavities. The mound assemblage here consists of hexactinellid sponges,
lithistid demosponges, non-rigid demosponges, Radiomura, Serpula (Dorsoserpula), Terebella, encrusting bryozoa, and minor contributions by calcareous sponges, and excavating sponges (type Aka). Thrombolites are dendrolitic and may reach sizes of several tens of centimeters, similar to the maximum size of siliceous
sponges. The upper mound interval appears retrogradational and geometries change upsection from mound shapes to flat lenses
and level-bottom, biostromal sponge banks. The biotic assemblage is similar to that of the middle mound interval and there
is no difference between mound and bank communities. The demise of sponge mounds is successive from regional spread in the
Sinemurian to more localised spots in the Lower Pliensbachian. This reduction correlates with an increasing influence of pelagic
conditions.
At Foum Tillicht, sponge mounds lack any photic contribution and there is virtually no differentiation into subcommunities
between mound surface and cavity dwelling organisms. There is some evidence that the heterotrophic food web of mound communities
was sourced by oxygen minimum zone edge effects, namely microbial recycling of essential elements such as N and P. Basin geometry
suggests a waterdepth of several 100's of meters, well below the photic zone and possibly only controlled by the depth range
of the oxygen minimum zone. Palaeoceanographic conditions of well-stratified deeper water masses diminished gradually during
widespread transgression across the Sinemurian to Pliensbachian boundary culminating in the Lower Pliensbachian ibex ammonite zone. 相似文献
7.
Summary The Calcare di Nago is a carbonate unit of Middle-Late Eocene (Bartonian and Priabonian) age which is well exposed at the
north-eastern end of Lake Garda, on the western margin of the Lessini Shelf (Southern Alps). This unit is highly fossiliferous
as far as the coralline red algae and large foraminifera are concerned. Corals, bryozoans, echinoderms, and molluscs are also
present. The present study deals with the relationships among the coralline taxa, the coralline growth-forms, and their facies
development in the Priabonian part of the type section of the Calcare di Nago. The taxonomic investigation led to the identification
of 15 coralline red algal species belonging to 7 non-geniculate and 2 geniculate genera. One species of Peyssonneliacean (red
alga) and one of Halimedacean (green alga) were also recognized. The quantitative and qualitative analyses based on coralline
red algae and large foraminifera enabled five facies to be distinguished: Algal crust-branch rudstone, Algal/ Discocyclina packstone, Coralalgal boundstone, Rhodolith mound wacke/packstone, and Rhodolith pavement. According to the coralline assemblages,
coralline growth-forms, and large foraminiferal associations, the five facies reflect solid and soft substrate types. Some
of these facies are dominated by in situ rhodoliths, others by reworked algal debris. In the architecture of an interpreted prograding carbonate ramp, shallow water
facies are dominated by members of the subfamily Mastophoroideae, while deeper water facies are dominated by those of the
subfamily Melobesioideae and family Sporolithaceae. There is a significant increase both in size and in constructional voids
of the rhodoliths with depth. A concomitant decrease in algal species diversity with depth has been also recognized. Large Discocyclina assemblages are localized across the inner and mid ramp boundary. Pellatispira and Biplanispira are present only in the uppermost mid-ramp. Nummulites, Assilina, and Spiroclypeus are dominant together with small orthophragminids both in the mid- and uppermost outer ramp facies. 相似文献
8.
Summary Late Permian bioclastic calcarenite beds of the middle Khuff Formation were sampled for their algal constituents near the
city of Unayzah, Quassim district in central Saudia Arabia. The algal flora includes two species of udoteacean algae ( Succodium difficile and Succodium sp.), the dasycladacean alga Mizzia velebitana and two species of red algae ( Gymnocodium bellerophontis, Permocalculus plumosus).
Other algal floral remains found forming isolated single layers which have generally been named algal microfacies. These include
a phylloid microfacies, an oncoid microfacies and algal stromatolites. The Khuff Formation is well-known for its accumulation
of non-associated gas and particularly for its oil accumulation in eastern Saudi Arabia and the Arabian Gulf states. 相似文献
9.
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
by Epiphyton and Renalcis, 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 and Renalcis. Other microbial genera such as Sphaerocodium and Wetheredella 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
10.
During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment. 相似文献
11.
塔里木板块塔中Ⅰ号坡折带附近上奥陶统良里塔格组取芯井段中可识别多种生物礁灰岩类型,包括珊瑚骨架/障积岩、海绵骨架/绑结岩、苔藓虫绑结岩、钙藻障积岩、钙质菌藻障积/绑结岩等礁灰岩类,藉此可归纳出珊瑚礁、珊瑚-钙藻礁、层孔虫礁、层孔虫-钙藻礁、珊瑚-层孔虫-钙藻礁、苔藓虫礁丘、钙藻礁丘、灰泥丘和微生物礁等生物建造单元。这些礁体的时空分布模式与古环境分异相关联,纵向上具有灰泥丘向珊瑚-层孔虫-钙藻礁至苔藓虫礁丘和钙藻礁的群落结构更替趋势;空间分布则向台地北缘,即I号坡折带延伸显示由低能带灰泥丘向高能带珊瑚-层孔虫-钙藻礁的相变,而且高能带珊瑚-层孔虫-钙藻主体礁和环其周缘相对低能带的钙藻礁丘、灰泥丘等在一定范围内构成造礁群落结构分异。 相似文献
12.
Summary An in 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 of Th. dendroidea colonies with only a minor portion of Stylosmilia. Frequently, the Th. 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 the Th. 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 the Thamnasteria 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 the Th. 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 toppled Solenopora jurassica thalli occur together with a few massive Isastrea 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 the Solenopora 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 the Solenopora 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 lithified florigemma-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 the florigemma-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. The florigemma-Bank is overlain by the reddish bioclastic sandstone of the ‘Zwischenfl?zregion’. 相似文献
13.
Slightly curved calcitic plates with marginal pores recalling an aciculariacean alga are common in Late Tithonian reefal platform
margin deposits of the Plassen Carbonate Platform of the Northern Calcareous Alps of Austria. Illustrated also from the Western
Carpathians, these forms were assigned to the genus Acicularia, e.g., Acicularia elongata Carozzi. It is demonstrated that these algal parts are not reproductive caps of polyphysacean algae (formerly known as acetabulariaceans),
but represent sections through scattered articles fragments of the dasycladalean alga Neoteutloporella socialis (Praturlon), more precisely the proximal parts of the laterals. This alga formed reefal bushes at the platform margin near-by
to coral-stromatoporoid patches. The characteristic aciculariacean algae recalling fragments occur in bioclastic packstones,
a facies adjacent to these dasycladalean algal microreefs. 相似文献
14.
Pennsylvanian phylloid algal reefs are widespread and well exposed in south Guizhou, China. Here we report on reefs ranging
from 2 to 8 m thickness and 30–50 m lateral extension. Algae, the main components, display a wide spectrum of growth forms,
but are commonly cyathiform (cup-shaped) and leaf-like (undulate plates). The algal reef facies is dominated by boundstone.
Algal thalli form a dense carpet whose framework pores are filled with marine cement and peloidal micrite. The peloidal matrix
is dense, partly laminated or clotted with irregular surfaces and often gravity defying. Algal reefs in Guizhou differ from
examples reported to date by the high biodiversity of organisms other than phylloids: e.g., the intergrowth of algae with
corals (some of which are twice the size of algal thalli) and numerous large brachiopods. This contrasts to previous views
that phylloid algal “meadows” dominated the actual seafloor, excluding other biota. Also, the pervasive marine cements (up
to 50%) including botryoidal cement are noteworthy. Algal reefs developed at platform margins, a depositional environment
similar to that of modern Halimeda mounds in Java, Australia and off Bahamas, and to that of time-equivalent examples reported from the Canadian Arctic Archipelago.
Whereas nutrients appear decisive in the growth of Halimeda reefs, algal reefs reported herein seemingly grew under conditions of low nutrient levels. Overall, algal reefs in Guizhou
challenge previous views on growth forms, diversity patterns, and depositional environments and add to the spectrum of these
partly puzzling biogenic structures. 相似文献
15.
Summary Several Waulsortian-type mud mounds nearly 500 m thick and about 5 km long occur in the Middle Paleozoic carbonate section
of the Aktur nappe in the mountains on the right bank of Isfara river. These buildups form a well developed barrier system
that stretches along the South Ferganian carbonate platform margin and divides the carbonate complex into a fore-reef and
a back-reef part. The time of the mounds' most active growth was from the Late Silurian (Ludlow) to the Middle Devonian (Eifel).
Three main facies types can be recognized in the mud mounds: 1. micritic core facies, 2. sparitic flank facies and 3. loferitic
capping facies. The central massive or crudely bedded part of the mounds consists of white or light grey clotted micrite.
Macrofossils are rare. The sparitic flank facies in contrast consists of coarse and densely packed crinoidal wackestone-floatstones
with some brachiopod shell debris. Solitary rugose corals, tabulate corals, stromato-poroids and fragments of mollusks are
also abundant. The tops of the mounds are usually covered with loferitic pelmicrites or oolitic grainstone caps. Stromatactis-like
structures are very rare and poorly developed in the South Ferganian mud mounds. However, almostin all such mounds horizons
of calcitic breccias can be found.
In order to explain all the features found in the Fergana mounds an ‘atoll-like’ model has been proposed which starts the
evolution of the mud mounds with a small nucleus bioherm. The main stage of the evolution corresponds to an atoll-like structure
developing on the surface of shallow water platforms. White clotted micrite of the mound core facies is interpreted as a accumulation
of fine-grained sediment in an inner lagoon flanked by crinoidal bar deposits. The mound flank facies represents the atoll
rim deposits from where the carbonate mud is derived. The capping loferitic facies is considered as tidal flat deposit that
developed on top of the buildups during the last stage of its evolution. The knoll shape of the mounds is explained by the
retreat of the atoll flanking crinoidal bars back into the inner lagoon during the rise in sea level. Stromatactis-like structures
of small cavities filled with sparry calcite owe their existence to burrowing organisms. Calcitic breccias are interpreted
as paleokarst collapse breccias. They indicate that the tops of the mud mound became subaerially exposed. Other evidence for
a subaerial exposure can be seen in the occurrence of Variscian ‘black and white’ limestone gravel on the tops of some mud
mounds. According to Ward et al. (1970) these sediments were produced above the sea level at the edge of hypersaline lakes situated on islands. 相似文献
16.
Summary The upper part of the Lower Pseudoschwagerina Limestone (Rattendorf Group), outcropping on the northwestern flank of Schulterkofel Mountain, Carnic Alps (Austria) is described
with special emphasis on fusulinid microfossils and facies. This fusulinid-rich section offers an ideal opportunity for biostratigraphy
in defining the Permo-Carboniferous boundary in this region.
The Lower Pseudoschwagerina Limestone is composed of shallow-marine limestones with intercalated thin siltstone and sandstone beds.
Fusulinid limestones are represented by two types of wackestones, both containing large quantities of smaller foraminifers.
Fusulinid grainstones are rare. Limestones rich in fusulinids were found only within the bedded limestone facies in beds both
below and especially above siliciclastic intercalations. This may indicate that the best living conditions for fusulinids
existed immediately before and especially after the climax of a regressive phase (sea-level lowstand). The fusulinid limestones
were deposited within a protected, shallow-marine shelf environment with normal salinity.
Pseudoschwagerinid fusulinids appear in the upper part of the Lower Pseudoschwagerina Limestone, in samples SK 107d (undeterminable species) and SK 108, i.e. between 92 m and 93 m above the base of the section
within a bedded limestone immediately above the uppermost clastic intercalation.
The fusulinid fauna is represented by about 30 species belonging to only a few genera. Species of Triticites and Rugosofusulina dominate, whereas those of Daixina, Rugosochusenella and Pseudofusulina are rare. A characteristic feature of the fauna is the strong similarity with fusulinid faunas described from Russia as well
as from Middle and East Asia. Some of the described fusulinids are new for the Carnic Alps.
The first appearance of Pseudoschwagerina and Occidentoschwagerina (Occidentoschwagerina alpina Zone) in the upper part of the Lower Pseudoschwagerina Limestone in the Schulterkofel section defines the position of the Carboniferous-Permian boundary. 相似文献
17.
Summary Modern carbonate sedimentation in the Caribbean Sea commonly occurs on banks that are surrounded and isolated by deep oceanic
water. This depositional regime also occurred during the Tertiary, and many islands, such as Cayman Brac, have sequences that
evolved in such settings.
Cayman Brac is a small (about 39 km 2) island, located on the Cayman Ridge, that has an exposed Oligocene to Pliocene succession which encompasses three unconformity-bounded
formations. The upper Lower Oligocene Brac Formation is formed of Lepidocyclina limestones and sucrosis dolostones that locally contain numerous bivalves and gastropods. The overlying Lower to Middle Miocene
Cayman Formation is formed of pervasively dolomitized mudstones to grainstones that contain an abundant, diverse biota of
corals, gastropods, bivalves, foraminifera, and algae. Rhodolites are locally common. The Pliocene Pedro Castle Formation
is formed of limestones, dolostones, and dolomitic limestones that contain a biota which is similar to that in the Cayman
Formation. The unconformities between the formations represent substantial periods of time during which the previously deposited
carbonates were lithified and eroded to produce karst terrains.
All facies in the Brac, Cayman, and Pedro Castle formations on Cayman Brac developed on a bank that was no more than 20 km
long and 3 km wide. There is no evidence of reef development other than isolated thickets of Stylophora and/or Porites and no systematic stratigraphic or geographic changes in the facies patterns of the formations. Comparison with modern Caribbean
banks shows that the depositional regime was primarily controlled by water depth and energy levels. Limestones of the Brac
Formation probably accumulated in low-energy conditions in water less than 10 m deep. The overlying Cayman Formation contains
facies that formed in water 15 to 30 m deep with good cross-bank circulation. The Pedro Castle Formation formed in slightly
shallower water (5–25 m) and lower energy conditions. The disconformities between the packages correlate with world wide eustatic
drops in sea level. 相似文献
18.
The Late Paleozoic (early Kasimovian-late Artinskian) sedimentary sequence of the Carnic Alps (Austria/Italy) is composed of cyclic, shallow-marine, mixed siliciclastic-carbonate sedimentary rocks. It contains different types of skeletal mounds in different stratigraphic levels. The oldest mounds occur at the base of the Auernig Group, within a transgressive sequence of the basal Meledis Formation. These mounds are small and built by auloporid corals. Algal mounds are developed in the Auernig Formation of the Auernig Group, forming biostromes, and Lower Pseudoschwagerina Limestone of the Rattendorf Group forming biostromes and bioherms. The dominant mound-forming organism of these mounds is the dasycladacean alga Anthracoporella spectabilis. In mounds of the Auernig Formation subordinately the ancestral corallinacean alga Archaeolithophyllum missouriense is present, whereas in mounds of the Lower Pseudoschwagerina Limestone a few calcisponges and phylloid algae occur locally at the base and on top of some Anthracoporella mounds. Mounds of the Auernig Formation formed during relative sea level highstands whereas mounds of the Lower Pseudoschwagerina Limestone formed during transgression. The depositional environment was in the shallow marine, low-turbulence photic zone, just below the active wave base and lacking siliciclastic influx. The algal mounds of the Carnic Alps differ significantly from all other algal mounds in composition, structure, zonation and diagenesis; the formation of the mounds cannot be explained by the model proposed by Wilson (1975). The largest mounds occur in the Trogkofel Limestone, they are composed of Tubiphytes/Archaeolithoporella boundstone, which shows some similarities to the “ Tubiphytes thickets” of stage 2 of the massive Capitan reef complex of the Guadalupe Mountains of New Mexico/West Texas. 相似文献
19.
Modern cool-water carbonate mounds topped by corals form an extended reef belt along the NW European continental margin at 200–1200 m water depth. An essential element of mound growth are hardgrounds which provide a stable substratum for mound-building invertebrate colonisation and stabilise the inclined mound flanks. Evaluating the degree of lithification and the slope stability against erosion represents an important task within the ESF programme MOUNDFORCE under the umbrella of EUROMARGINS. Sampling of hardgrounds during RV Meteor cruises M61-1 and -3 in 2004 by means of the IFM-GEOMAR TV-grab and the Bremen ROV QUEST focused on carbonate mounds of the Porcupine Seabight and northwestern Rockall Bank off Ireland. Lithified carbonates of mid-Pleistocene age were exhumed during the Holocene and are now exposed on the top and flanks of numerous carbonate mounds showing a patchy to dense colonisation by living corals and associated invertebrates. The sediments, composed of foraminiferal–nannoplankton oozes and admixed mound-derived invertebrate skeletons, range from partly lithified chalks to dense micritic limestones. These wackestones to packstones clearly differ from bacterially induced authigenic carbonate crusts typical of hydrocarbon seep settings by showing current-induced sedimentary structures, a non-luminescing matrix indicating oxic pore fluids, and a marine isotopic signature lacking any depleted carbon regime which is typical of anaerobic methane oxidation. The carbonate lithification is driven by carbonate ion diffusion from supersaturated seawater into the pore fluids in the studied areas. Vigorous bottom currents were the ultimate control not only of carbonate cementation by enhancing the diffusion process and supporting a pumping mechanism, but also of hardground formation and mound shaping by exhuming lithified carbonates and preventing fine-grained sediment accumulation at the downslope mound flanks. 相似文献
20.
Summary Calcareous red algal nodules growing on mobile substrates have been sampled from 28 to 117m off Fraser Island in southern
Queensland, eastern Australia. This is a subtropical, transitional area between the tropical Great Barrier Reef to the north
and temperate, cooler waters to the south. Red algal nodules are the most common components in bioclastic gravels that extend
from about 50 to 110 m and locally cover 40–50% of the seafloor. Variations in the overall character and floristic composition
of the nodules with depth can be observed. Algal nodules comprise algal covered pebbles/cobbles and rhodoliths in depths shallower
than 60 m whereas only rhodoliths occur in deeper settings. No changes in nodule shape occur but shallower algal nodules have
larger mean size with higher standard deviation than the deeper ones (39.2 vs. 30.5 mm and 20.5 vs. 6.3 mm s.d.). Living and
subrecent red algae in nodules shallower than 60 m are mainly Melobesioideae and peyssonneliaceans with minor Lithophylloideae
and Mastophoroideae. Most plants belong to a few species of the genera Phymatolithon and Lithothamnion. Below 68 m, rhodoliths are dominated by the family Sporolithaceae, melobesioids and peyssonneliaceans. Sporolithon is the main component below 80 m. Algal growth forms are mostly smooth encrusting to warty with no depth variation. Maximum
plant thickness, however, decreases with increasing depth. Thallus thickness in the deeper water samples is more than three
times smaller than in those from shallower waters. These data are important for understanding the paleoenvironmental context
of deposition of the abundant coralline algal limestones with similar algal nodules found in the geological record. 相似文献
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