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1.
Summary Early Silurian reef reconstruction on the Yangtze Platform, in the northern part of the South China Block, is preceded by
a combination of regional and global processes. During most of Ashgill time (Late Ordovician), the area was dominated by Wufeng
Formation deep water graptolitic black shales. Reefs largely disappeard in the middle of the Ashgill Stage, from the northwestern
margin of Cathaysian Land (southeastern South China Block), in advance of the Late Ordovician glaciation and mass extinction,
due to regional sea-level changes and regional uplift, unrelated to the mass extinction itselt. Late Ordovician microbial
mudmound occurrence is also found in the western margin of the Yangtze Platform, its age corresponding to theDicellograptus complexus graptolite biozone of pre-extinction time. On the Yangtze Platform, a thin, non-reef-bearing carbonate, the Kuanyinchiao
Formation (=Nancheng Formation in some sites), thickness generally no more than 1m, occurs near several landmasses as a result
of Hirnantian regression. Reappearance of the earliest Silurian carbonates consisting of rare skeletal lenses in the upper
part of Lungmachi Formation, are correlated to theacensus graptolite biozone, early Rhuddanian of Shiqian, northeastern Guizhou, near Qianzhong Land. Carbonate sediments gradually
developed into beds rich in brachiopods and crinoids in the lower part of Xiangshuyuan Formation, middle Rhuddanian. In the
middle part of Xiangshuyan Formation, biostromes, containing abundant and high diversity benthic faunas such as corals, crinoids
and brachiopods, show beginnings of reconstruction of reef facies. Substantial reef recovery occurred in the upper part of
Xiangshuyuan Formation, lower Aeronian, as small patch reefs and biostromes. During the late Aeronian, carbonate sediments,
especially reefs and reef-related facies, expanded on the upper Yangtze Platform, and radiation of reefs occurred in Ningqiang
Formation, upper Telychian. The long period of reef recovery, taking several million years, remains difficult to explain,
because redistribution of any refugia faunas would be expected to take place soon after the extinction. Reefs and reef-related
facies subsequently declined after Telychian time due to regional uplift of the major portion of the Yangtze Platform. Carbonate
facies are therefore uncommon in South China during the rest of Silurian time. 相似文献
2.
Paleoecologic,temporal, and spatial analysis of early Silurian Reefs of the Chicotte Formation,Anticosti Island,Quebec, Canada 总被引:1,自引:0,他引:1
Summary Reefs of the Lower Silurian Chicotte Formation are the largest and most faunally diverse known on Anticosti Island, Quebec.
They reach up to 25 m in thickness and 250 m in diameter and are present predominantly at two intervals, forming a lower and
upper reef cluster. Remnants of bioherms are represented on the present-day wave-cut terrace as 60 to 100 m diameter, subcircular
erosional depressions known as Philip structures or as outcrop.
The bioherms were relatively low structures, with approximately 3 to 5 m maximum synoptic relief, some of which developed
on hardgrounds and possible paleokarst surfaces of crinoidal wackestone and packstone. Dominant skeletal framework builders
and sediment producers within all of the reefs are laminar to low domical stromatoporoids, colonial cerioid and fasciculate
rugose corals, colonial tabulate corals, and cryptostome bryozoans. Vertical zonation of reef biota is evident within well-exposed
reefs of the lower reef cluster. Three to four stages are recognizable:1) a low-diversity tabulate coral-dominatedpioneering community including large tabulate coral colonies (halysitids and favositids), and few stromatoporoids (clathrodictyids, ecclimadictyids),
fasciculate rugosans, large generally monotypic stalked crinoids, and shelly benthos (brachiopods, few ostracodes and trilobites);2) an intermediate- to high-diversity, mixed tabulate coral-stromatoporoid-dominatedreef-core community;3) a slightly lower diversity stromatoporoid-tabulate coral-dominatedclimax community with laminar coenitids and alveolitids; and,4) in a few localities, a capping, low-diversity tabulatecoral-dominated (alveolitid and coenitid), and stromatoporoid-bearing
community comprising laminar forms.
Amelioration of Early Silurian climates, following Late Ordovician glaciation, allowed gradual reestablishment of extensive
shallow-water reef growth, by mainly new and increasingly diverse genera and species of metazoans. Reef development within
the Chicotte Formation coincided with global, widespread development of latest Llandovery and earliest Wenlock reefs in subtropical
to tropical areas. Chicotte reefs have broad characteristics, in terms of overall biotic composition, vertical successions
recognized, and paleogeographic setting, similar to those of equivalent and slightly younger age from intracratonic settings
in Baltica (Gotland, Sweden and Estonia) and central and northern Laurentia (Midcontinent, U.S.A.; Hudson Bay, Canada; and
North Greenland, Denmark). 相似文献
3.
Coralline-algal frameworks from Atol das Rocas reefs were sampled along windward and leeward transects, and multivariate
statistical analysis was used to investigate the composition and community structure of the encrusting community. The following
components of the reefs were identified in slabbed and polished reef blocks sampled along each transect: encrusting coralline
algae dominated by Porolithon cf. pachydermum, vermetid gastropods, Homotrema rubrum, acervulinid foraminiferans, molluscs, corals, polychaete tubes, clionid sponge borings, polychaete/sipunculid borings, cemented
sediments, and growth cavities. Q-mode cluster analysis correctly classified 78% of all windward samples and 69% of all leeward
samples, and R-mode separated reef components adapted to high wave energy environments from those adapted to low wave energy.
Separate classification and ordination of samples from each transect indicate that seriation breakdown occurs in the windward
reef between 25 and 45 m from the reef front. In the leeward reef between 75 and 90 m from the leeward reef front, seriation
breakdown was not found to be significant. These results confirm that seriation (zonation) is best developed in the windward
reef, as has been documented in coral-dominated reefs.
Accepted: 28 February 2000 相似文献
4.
The history of Devonian-Carboniferous reef communities: Extinctions,effects, recovery 总被引:1,自引:0,他引:1
Dr. J. A. Fagerstrom 《Facies》1994,30(1):177-191
Summary Analysis of the taxonomic composition, diversity and guild structure of five “typical” reef and mud mound communities ranging
in age from Late Devonian-Early Carboniferous indicates that each of these aspects of community organization changed dramatically
in relation to three extinction events. These events include a major or mass extinction at the end of the Frasnian; reef communities
were also effected by less drastic end-Givetian and mid-late Famennian extinctions of reef-building higher taxa.
Peak Paleozoic generic diversities for reef-building stromatoporoids and rugose corals occurred in the Eifelian-Givetian;
reef-building calcareous algal taxa were longranging with peak diversity in the Devonian. These three higher taxa dominated
all reef-building guilds (Constructor, Binder, Baffler) in the Frasnian and formed fossil reef communities with balanced guild
structures. The extinction of nearly all reef-building stromatoporoids and rugose corals at the end of the Frasnian and the
survival of nearly all calcareous algac produced mid-late Famennian reef communities dominated by the Binder Guild. Despite
the survival of most calcareous algae and tabulate corals, the mid-late Famennian extinction of all remaining Paleozoic stromatoporoids
and nearly all shelf-dwelling Rugosa brought the already diminished Devonian reef-building to a halt. These Devonian extinctions
differ from mass extinctions by the absence of a statistically significant drop in taxonomic diversity and by their successional
and cumulative effects on reef communities.
Tournaisian mud mounds contain communities markedly different from the frame-building communities in Late Devonian and Visean
reefs. Mound-building biotas consist of an unusual association dominated by erect, weakly skeletonized members of the Baffler
Guild (chiefly fenestrate Bryozoa; Pelmatozoa) and laterally expanded, mud-binding algae/stromatolites and reptant Bryozoa.
The initial recovery to reefs with skeletal frameworks in the Visean was largely due to the re-appearance of new species of
abundant colonial rugose corals (Constructor Guild) and fenestrate Bryozoa.
This Frasnian-Visean evolution in the taxonomic composition and structure of the reef-building guilds is also expressed by
abrupt changes in biofacies and petrology of the reef limestones they produced. Thus, “typical” Frasnian reef limestones with
balanced guild structures are framestones-boundstones-bafflestones, Famennian reefs are predominantly boundstones, Tournaisian
mud mounds are bafflestones and Visean reefs are bafflestones-framestones. 相似文献
5.
Exploitation of groundfish off the U.S. Pacific coast reached maximum levels during the 1990s, resulting in severe declines in at least nine species of groundfish. From 1988 to 1990, we used the 2-man submersible Delta to make 42 dives and run replicate visual belt transects at six stations ranging from 67-360 m in depth at Heceta Bank on the outer continental shelf of Oregon. We identified four major habitats and associated benthic macroinvertebrate and groundfish assemblages: (1) shallow rock ridges and large boulders (< 100 m deep) dominated by basket stars, juvenile rockfishes, yelloweye rockfish, and lingcod; (2) mid-depth small boulder-cobbles (100-150 m) dominated by crinoids, brittle stars, rosethorn, pygmy/Puget sound, and canary rockfishes; (3) deep cobble (150-200 m) dominated by crinoids, brittle stars and various small rockfish species, and (4) deep mud slope (> 200 m) dominated by fragile urchins, sea cucumbers, shortspine thornyhead, and flatfishes. Although substantial interannual variation in groundfish abundance among seafloor types was evident in the 12 most abundant and/or commercially important fish taxa sampled, high variance resulted in statistically significant differences among years only in juvenile rockfishes. These data provide a baseline for future comparisons exploring long-term change this continental-shelf ecosystem. 相似文献
6.
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
7.
Fish assemblages on artificial and natural reefs in the Flower Garden Banks National Marine Sanctuary, USA 总被引:5,自引:0,他引:5
Visual censusing was used to characterize fish assemblages on artificial and natural reefs located within the boundaries
of the Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico. Emphasis was placed on determining
spatial and temporal patterns in habitat utilization by fishes on an offshore artificial reef (Mobil Platform HI-A389A). Overall,
43 species were observed during diurnal surveys in the upper 24 m of the artificial reef. Midwater pelagic fishes (i.e., carangids
and scombrids) accounted for over 50% of all taxa enumerated on the artificial reef; however, these taxa were transient members
of the assemblage and were observed infrequently. Labrids, pomacentrids, and serranids were the dominant reef-dependent taxa.
Distinct trends in vertical, diel, and seasonal abundances were observed for juvenile and adult fishes. Of the three designated
depth zones (upper 1.5–9.0, middle 9.0–16.5; lower 16.5–24.0 m), abundance and species diversity were lowest in the upper
zone. Nocturnal counts were characterized by a marked reduction or complete absence of most species, due in part to twilight
cover-seeking and movement activities. Seasonal variation in community composition and species abundance (May versus September)
was primarily due to recruitment of juveniles (0-age fishes) to the artificial reef in late summer. Increases in total fish
abundance (all taxa combined) coincided with both increasing habitat rugosity and degree of fouling. Species richness on natural
coral reefs in the FGBNMS was higher than on the artificial reef. Unlike the artificial reef, fish assemblages on the natural
reefs were dominated by a single family (Pomacentridae) which accounted for over 50% of all individuals observed.
Accepted: 1 August 1996 相似文献
8.
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
9.
C. G. Messing D. L. Meyer U. E. Siebeck L. S. Jermiin D. I. Vaney G. W. Rouse 《Coral reefs (Online)》2006,25(1):164-168
A recent preliminary survey revealed that 12 species of unstalked crinoids occur on a gentle sandy slope (12–18 m depth) at Lizard Island, Great Barrier Reef, Australia; five of which are also found on coral reefs. The other seven appear to constitute a unique assemblage restricted to unconsolidated substrates, where most cling to algae or hide beneath rubble or sponges. Members of this assemblage exhibit all of the basic feeding postures found among reef-dwelling species. However, Comatula rotalaria, which lacks anchoring cirri and bears uniquely differentiated short and long arms, exhibits a posture different from other living crinoids. Quantitative transects reveal apparent depth-related differences in species composition: C. rotalaria dominated the 12 transects in 12–13 m (84% of 82 specimens), while Comatella nigra, Comatula cf. purpurea, Amphimetra cf. tessellata and Zygometra microdiscus accounted for 96% of 54 specimens observed along 12 transects in 16–17 m. 相似文献
10.
Facies models of a shallow-water carbonate ramp based on distribution of non-skeletal grains (Kimmeridgian,Spain) 总被引:2,自引:0,他引:2
The internal facies and sequence architecture of a Late Jurassic (Late Kimmeridgian) shallow carbonate ramp was reconstructed
after the analysis and correlation of 17 logs located south of Teruel (northeast Spain). The studied rocks are arranged in
five high-frequency sequences A–E (5–26 m thick) bounded by discontinuities traceable across the entire study area (20 × 25 km).
Facies analysis across these sequences resulted in the reconstruction of three sedimentary models showing the transition from
interior ramp environments (i.e., lagoon, backshoal, and shoal) to the progressively deeper foreshoal and offshore areas.
Coral-microbial reefs (meter-sized patch and pinnacle reefs) have a variable development throughout the sequences, mostly
in the foreshoal and offshore-proximal environments. The preferential occurrence and down-dip gradation of non-skeletal carbonate
grains has been evaluated across the three models: low-energy peloidal-dominated, intermittent high-energy oolitic-dominated
and high-energy oolitic–oncolitic dominated. The predominance of these non-skeletal grains in the shoal facies was mainly
controlled by the hydrodynamic conditions and spatial heterogeneity of terrigenous input. The models illustrate particular
cases of down-dip size-decrease of the resedimented grains (ooids, peloids, oncoids) due to storm-induced density flows. Offshore
coarsening of certain particles (intraclasts, oncoids) is locally observed in the mid-ramp areas favorable for microbial activity,
involving coral-microbial reef and oncoid development. The observed facies variations can be applicable to carbonate platforms
including similar non-skeletal components, where outcrop conditions make the recognition of their three-dimensional distribution
difficult. 相似文献
11.
12.
J. C. Brock M. Palaseanu-Lovejoy C. W. Wright A. Nayegandhi 《Coral reefs (Online)》2008,27(3):555-568
A portion of the northern Florida Keys reef tract was mapped with the NASA Experimental Advanced Airborne Research Lidar (EAARL)
and the morphology of patch reefs was related to variations in Holocene sea level. Following creation of a lidar digital elevation
model (DEM), geospatial analyses delineated morphologic attributes of 1,034 patch reefs (reef depth, basal area, height, volume,
and topographic complexity). Morphometric analysis revealed two morphologically different populations of patch reefs associated
with two distinct depth intervals above and below a water depth of 7.7 m. Compared to shallow reefs, the deep reefs were smaller
in area and volume and showed no trend in topographic complexity relative to water depth. Shallow reefs were more variable
in area and volume and became flatter and less topographically complex with decreasing water depth. The knoll-like morphology
of deep reefs was interpreted as consistent with steady and relatively rapidly rising early Holocene sea level that restricted
the lateral growth of reefs. The morphology of shallow “pancake-shaped” reefs at the highest platform elevations was interpreted
as consistent with fluctuating sea level during the late Holocene. Although the ultimate cause for the morphometric depth
trends remains open to interpretation, these interpretations are compatible with a recent eustatic sea-level curve that hindcasts
fluctuating late Holocene sea level. Thus it is suggested that the morphologic differences represent two stages of reef accretion
that occurred during different sea-level conditions. 相似文献
13.
Dr. Eberhard Gischler 《Facies》1994,31(1):243-254
Summary The chief mode of carbonate sedimentation on the Belizean atolls Glovers Reef, Lighthouse Reef and Turneffe Islands is the
accumulation of organically-derived particles. Variations in the distribution of the composition and grain-sizes of surface
sediments, collected along transects across the atolls, are environmentally controlled. Two major sediment types may be distinguished.
(1) Reef and fore reef sediments are dominated by fragments of coral, coralline algae andHalimeda. Mean grain-sizes range from 1–2 mm. (2) Back reef sediments contain more mollusk fragments, more fine-grained sediment (<125
μm) and appear to have fewerHalimeda fragments. In addition, sediments from inner platforms and shallow lagoonal parts of Glovers and Lighthouse Reefs comprise
non-skeletal grains, namely fecal pellets. Sediments from lagoonal patch reefs may contain up to 20% coral fragments. Mean
grain-sizes range from 0.1–1 mm and are finest on the inner platform and lagoon floor of the back reef environment. Within
the reef and fore reef environments, it is not possible to distinguish sub-environments on the basis of textural and compositional
differences of the sediments. Sediments from patch reefs contrast with those from back reef lagoons and inner platforms and
are similar in terms of grain-sizes and compositions to reef and fore reef surface sediments.
Non-skeletal grains forming in shallow parts of the back reef in Glovers and Lighthouse Reefs are interpreted to be indurated
by interstitial precipitation of calcium carbonate from warm, supersaturated water flushing the sediment. The lack of hardened
non-skeletal particles in the back reef sediments of Turneffe Islands is most probably due to the abundance of muddy, organic-rich
sediment in the well-protected lagoon. Fine sediment is less permeable and organic films prevent cement overgrowth on particles. 相似文献
14.
泥质对志留系礁滩生长的抑制作用:黔北桐梓韩家店组的例证 总被引:1,自引:0,他引:1
黔北桐梓的戴家沟剖面和狮溪剖面志留系兰多维列统特列奇阶下部的韩家店组出露完好,该组泥岩、粉砂岩中夹有厚度1—3m、直径4—7m的小型点礁。礁核相多具典型的障积格架岩特征,但生长时限短暂,群落分异度低,仅见床板珊瑚、单体四射珊瑚、苔藓虫和海百合茎,礁间为珊瑚、苔藓虫和海百合茎碎片堆积的滩相,伴生丰富的遗迹化石。在陆源碎屑快速沉积的背景下,浑浊海水频繁的富营养化过程限制了礁体纵横向生长和朝高分异度群落发展的可能性。 相似文献
15.
Igor C. S. Cruz Miguel Loiola Tiago Albuquerque Rodrigo Reis José de Anchieta C. C. Nunes James D. Reimer Masaru Mizuyama Ruy K. P. Kikuchi Joel C. Creed 《PloS one》2015,10(1)
Consequences of reef phase shifts on fish communities remain poorly understood. Studies on the causes, effects and consequences of phase shifts on reef fish communities have only been considered for coral-to-macroalgae shifts. Therefore, there is a large information gap regarding the consequences of novel phase shifts and how these kinds of phase shifts impact on fish assemblages. This study aimed to compare the fish assemblages on reefs under normal conditions (relatively high cover of corals) to those which have shifted to a dominance of the zoantharian Palythoa cf. variabilis on coral reefs in Todos os Santos Bay (TSB), Brazilian eastern coast. We examined eight reefs, where we estimated cover of corals and P. cf. variabilis and coral reef fish richness, abundance and body size. Fish richness differed significantly between normal reefs (48 species) and phase-shift reefs (38 species), a 20% reduction in species. However there was no difference in fish abundance between normal and phase shift reefs. One fish species, Chaetodon striatus, was significantly less abundant on normal reefs. The differences in fish assemblages between different reef phases was due to differences in trophic groups of fish; on normal reefs carnivorous fishes were more abundant, while on phase shift reefs mobile invertivores dominated. 相似文献
16.
Synopsis Research on eleven artificial reefs in Puget Sound, Washington examined the relative importance of reef-produced prey items
to recreationally important reef fish species assemblages. The colonization of potential prey items, and fish species assemblages
to ten artificial reefs were examined for the reefs first two to five years, and observations were conducted on an eleventh
reef during its forty-ninth productive year. Fish species became more abundant, or were seen more frequently on reef habitats
whose substrates had successionally developed from barnalces to algal mats. Fish species most affected by this successional
change foraged heavily on organisms which were associated with reef algae. Starfish and nudibranchs. who preyed on the barnacles,
were identified as the ‘keystone’ predators of these subtidal reef habitats. 相似文献
17.
Coral reefs in shallow-water environments (<30 m) are in decline due to local and global anthropogenic stresses. This has
led to renewed interest in the ‘deep reef refugia’ hypothesis (DRRH), which stipulates that deep reef areas (1) are protected
or dampened from disturbances that affect shallow reef areas and (2) can provide a viable reproductive source for shallow
reef areas following disturbance. Using the Caribbean as an example, the assumptions of this hypothesis were explored by reviewing
the literature for scleractinian corals—the reef framework builders on tropical reefs. Although there is evidence to support
that deep reefs (>30 m) can escape the direct effects of storm-induced waves and thermal bleaching events, deep reefs are
certainly not immune to disturbance. Additionally, the potential of deep reefs to provide propagules for shallow reef areas
seems limited to ‘depth-generalist’ coral species, which constitute only ~25% of the total coral biodiversity. Larval connectivity
between shallow and deep populations of these species may be further limited due to specific life history traits (e.g., brooding
reproductive strategy and vertical symbiont acquisition mode). This review exposes how little is known about deep reefs and
coral reproduction over depth. Hence, a series of urgent research priorities are proposed to determine the extent to which
deep reefs may act as a refuge in the face of global reef decline. 相似文献
18.
Summary Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata
are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence
of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian,
stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids,
receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef
occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef
area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China
with regard to the biotic composition. The reef is composed of fasciculate colonies ofSmithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The
species ofSmithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control:Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereasSmithiphyllum occidentale
Sorauf, 1972 andSmithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like
coral colonies baffled sediments. Algae and stromatoporoids (mainlyStachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods.
The reefs are composed from base to top of five lithofacies types: 1) cryptalgal micrite, 2) peloidal packstone, 3) stromatactis
limestone, 4) coral-bafflestone, and 5) pseudopeloidal packstone. The reef complex can be subdivided into back-reef subfacies,
reef flat and marginal subfacies, and marginal fore-slope subfacies. The Houshan coral-bafflestone reef is not a barrier reef
but a coral patch reef located near the inner margin of a carbonate platform. 相似文献
19.
Cambrian Series 3 lithistid sponge–microbial reefs in Shandong Province,North China: reef development after the disappearance of archaeocyaths
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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
20.
Summary The Silurian of Gotland is characterized by repeated changes in depositional facies development. The deposition of uniform
sequences of micritic limestones and marls was interrupted four times by the growth of reef complexes and the formation of
expanded carbonate platforms. Coinciding with these, often abrupt, facies changes extinction events occurred which predominantly
affected nektonic and planktonic organisms. Ratios of carbon- and oxygen-isotopes covary with the facies development. Periods
in which the deposition of limestonemarl alternations prevailed are characterized by relatively low C- and O-isotope values.
During periods of enhanced reef growth isotope values are high. For these changes,Bickert et al. (1997) assume climatic changes between humid “H-periods”, with estuarine circulation systems and cutrophic surface
waters with decreased salinity in marginal seas, and arid “A-periods”, with an antiestuarine circulation and oligotrophic,
stronger saline surface waters.
In order to separate local and regional influences on the isotopic development from the global trend, the interactions between
facies formation and isotope record have to be clarified. For this purpose, the patterns of isotope values in the upper part
of the Silurian sequence on Gotland (upper Wenlock —upper Ludlow) has been determined and stratigraphically correlated along
four transects through different facies areas. Facies formation during this time interval was investigated by differentiation
and mapping of twelve facies complexes in the southern part of Gotland. These include shelf areas, reef complexes with patch
reefs and biostromes, backreef facies, and marginal-marine deposits. The good correspondence between the carbon-isotope records
of the four transects suggests that local environmental conditions in the different facies areas did not influence the δ13C values. Therefore, a supra-regional or even global mechanism for the C-isotope variations is likely.
In contrast to carbon istopes, the oxygen-isotope values of the four transects generally show parallel trends, but higher
variabilities and in parts distinctly deviating developments with a trend to more negative values. These are interpreted as
an effect of local warming in small shallow-water areas which developed during arid periods in reef complexes and backreef
areas.
The boundaries between A-periods and H-periods, as defined by δ13C values, which are interpreted as isochrones, can be mapped. From the upper Homerian to the Pridolian six parastratigraphic
isotope zones are defined which only partly match the stratigraphic division ofHede (1942, 1960). The isotope stratigraphy results in an improved correlation between the shallow and marginal-marine areas in
the eastern part of Gotland and the uniform shelf areas at the west coast of the island.
Furthermore, a detailed relationship between the development of carbon and oxygen isotope ratios, the carbonate facies formation,
and the succession of palaeontological events could be observed. At the transition from H-periods to A-periods, major extinction
events occurred prior to the first increase of δ13C and δ18O values. Extinction events affected conodonts, graptolites, acritarchs, chitinozoans, and vertebrates and resulted in impoverished
nektonic and planktonic communities. The reef-building benthos was less affected. Parallel to a first slight increase of isotope
values, facies began to change, and reefs developed in suitable locations. The subsequent rapid increase of C- and O-isotope
values occurred contemporarily with strong facies changes and a short-term drop of sea-level. Oligotrophic conditions in the
later stages of A-periods led to strong reef growth and to an expansion of carbonate platforms.
The transitions from A-periods to H-periods were more gradual. The δ13C values decreased slowly, but reef growth continued. Later the reefs retreated and were covered by the prograding depositional
facies of shelf areas. The diversity of planktonic and nektonic communities increased again.
The close relationship between facies formation, palaeontological events and isotope records in the Silurian suggests common
steering mechanisms but gives no indication of the causes for the repeated extincion events related to H-period/A-period transitions.
Especially the observation, that strong extinctions occurred prior to changes of isotope values and facies, points to causes
that left no signals in the geological record. Hypothetical causes like collapse of trophical nets, anoxias, or cooling events
are not evident in the sediment record or do not fit into the regular succession of period transitions. 相似文献