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1.
Platform carbonate sediments of Liassic age cropping out in the area of the Pigadi-Fokianos Gulf (SE of Leonidion, Peloponnesus)
have been investigated in order to determine their depositional environment. Facies analysis allowed the recognition of several
microfacies types and their cyclic stacking pattern. The carbonates were deposited in a restricted inner platform environment
(lagoon-peritidal domain) and are arranged into small-scale shallowing-upward cycles. Palaeosol horizons containing typical
pedogenic features are developed on the top of the peritidal facies or are directly superimposed on subtidal deposits, forming
diagenetic caps. This implies repeated sea-level fluctuations and periodic emersion episodes. The presence of orbitally forced
cyclicity though is mostly probable, cannot be clearly documented by the available data. The studied carbonates are comparable
with other coeval analogous peritidal cycles of the same age along the southern margin of the Tethys. 相似文献
2.
Sabrina Amodio 《Facies》2006,52(1):53-67
A high-resolution stratigraphic study, carried out on the carbonate platform strata of the San Lorenzello section (Matese
Mountains, southern Italy), Valanginian–Hauterivian in age, has allowed to: recognise lithofacies and their associations;
assign the lithofacies associations to specific environments and individuate early meteoric diagenetic modifications, recurring
at specific horizons. In this frame the vertical variation of benthic foram diversity has been analysed. On the whole, foraminiferal
genera diversity decreases from open to restricted marine environments. Moreover, a climatic control on carbonate sedimentation
is suggested by a Milankovitch cyclicity organised in elementary cycles, bundles and superbundles as well as by diagenetic
characteristics testifying that humid and arid conditions alternated during the Early Cretaceous times. The orbital cyclicity
is also documented by foraminiferal diversity changes, even if some discrepancies between the lithofacies and the diversity
locally occur. Therefore, the above diversity changes do not appear to provide sufficient information for the sequence-stratigraphic
interpretation of shallow-water carbonates. 相似文献
3.
Dr. Diethard Sanders 《Facies》1998,39(1):139-177
Summary The Turonian to Santonian terrestrial to neritic succession (Lower Gosau Subgroup) in the Northern Calcareous Alps of the
eastern part of the Tyrol, Austria, provides an example for deposition on a compartmentalized, narrow, microtidal to low-mesotidal,
wave-dominated, mixed siliciclastic-carbonate shelf. The shelf was situated in front of a mainland with a relatively high,
articulated relief, and underwent distinct changes in facies architecture mainly as a result of tectonism.
The investigated succession was deposited above a deeply incised, articulated truncation surface that formed when the Eo-Alpine
orogen, including the area of the future Northern Calcareous Alps, was uplifted and subaerially eroded. Distinct facies associations
were deposited from (1) alluvial fans and fan deltas, (2) rivers, (3) siliciclastic lagoonal to freshwater marsh environments,
(4) areally/temporally limited carbonate lagoons, (5) transgressive shores, (6) siliciclastic shelf environments, and (7)
an aggrading carbonate shelf. During the Turonian to Coniacian, the combination of high rates of both subsidence and sediment
accumulation, and a narrow shelf that was compartmentalized with respect to (a) morphology of the substratum, (b) fluviatile
input of siliciclastics and contemporaneous input of carbonate clasts from fan deltas, (c) deposition of shallow-water carbonates,
and (d) water energy and-depth gave rise to an exceptionally wide spectrum of facies as a distinguishing feature of the succession.
With the exception of facies association 7, which formed only once, depositional sequences in the Turonian to Coniacian interval
contain all of the facies associations 1 to 6. During Turonian to Coniacian times, the shelf was microtidal to low-mesotidal,
and was dominated by waves, storm waves and storm-induced currents. In vegetated marshes, schizohaline to freshwater marl
lakes existed. Transgressions occurred onto fan deltas and in association with estuaries, or in association with gravelly
to rocky shores. The transgressive successions, including successions deposited from transgressive rocky carbonate shores,
are overlain by regressive successions of shelf carbonates or shelf siliciclastics. Deposition of shallow-water carbonates
generally occurred within lagoons and over short intervals of time. A „catch-up” succession of shelf carbonates about 100
m thick accumulated only in an area protected from siliciclastic input.
In its preserved parts, the Turonian to Coniacian succession does not record deposition adjacent to major active faults. Lateral
changes in thickness result mainly from onlap onto the articulated basal truncation surface. Subsidence most probably was
controlled by major detachment faults outside the outcrop area, and/or was distributed over a wide area in association with
secondary faults above the major detachments.
During Coniacian to Early Santonian times, both the older substratum and the overlying Turonian-Coniacian succession were
subaerially exposed, faulted and deeply eroded. The following Early Santonian transgression ensued with rocky carbonate shores
ahead of a sandy, narrow shoreface-inner shelf environment and a deeper shelf with intermittentlydysaerobic mud. The transgression
was associated with the influx of cooler and/or nutrient-rich waters, and heralds an overall deepening. Still during the Early
Santonian, the deepening was interrupted by another phase of subaerial exposure. Subsequently, a short phase of shelf deposition
was terminated by deepening into bathyal depths. 相似文献
4.
The Anisian succession of Nakhlak (in Central Iran) is characterized by a siliciclastic succession with minor carbonate units, with massive carbonate mounds up to 50?m thick in its upper part. The mounds, constrained in age to the late Bithynian (Ismidicus Zone) by ammonoids and conodonts, are characterized by a flat top and a lateral pinch-out marked by clinostratified slopes (about 15° in dip). Stratigraphic and microfacies analyses document an inner part of the mound characterized by massive microbial carbonates with open-space structures (stromatactis) filled with fine-grained internal sediments and marine cements. Isolated sponges (up to 5?cm), serpulids and bryozoans are present, which grew on the calcimicrobial limestone. A narrow bioclastic margin (mainly with crinoids and brachiopods) produces most of the slope facies (consisting of bioclastic grainstone and packstone, with intraclasts from the inner part of the mounds) which interfinger basinward with volcaniclastic sandstones. The demise of carbonate productivity is marked on the top of the carbonate mounds by a condensed surface, rich in ammonoids, glaucony grains, and articulated crinoids, documenting a rapid drowning. Paleolatitude data support deposition in a tropical setting, and sedimentological constraints indicate deposition close to the fair-weather wave base, within the photic zone. The late Bithynian Nakhlak carbonate mounds developed before the appearance (documented since the Pelsonian in different parts of the world) of scleractinians which, despite the favorable environmental conditions, are absent at Nakhlak. The Nakhlak mounds thus represent one of the last occurrences of the microbial factories (which developed after the Permo-Triassic extinction event and persisted for most of the Middle Triassic, but with a gradually increasing role played by scleractinians) before the first appearance of the Mesozoic corals. 相似文献
5.
Paleontological and biostratigraphical studies on carbonate platform succession from southwest Iran documented a great diversity of shallow-water benthic foraminifera during the Oligocene–Miocene. Larger foraminifera are the main means for the stratigraphic zonation of carbonate sediments. The distributions of larger benthic foraminifera in two outcrop sections (Abolhayat and Lali) in the Zagros Basin, Iran, are used to determine the age of the Asmari Formation. Four assemblage zones have been recognized by distribution of the larger benthic foraminifera in the study areas. Assemblage 3 (Aquitanian age) and 4 (Burdigalian age) have not been recognized in the Abolhayat section (Fars area), due to sea-level fall. The end Chattian sea-level fall restricted marine deposition in the Abolhayat section and Asmari Formation replaced laterally by the Gachsaran Formation. This suggests that the Miocene part of the formation as recognized in the Lali section (Khuzestan area) of the Zagros foreland basin is not present in the Abolhayat outcrop. The distribution of the Oligocene larger benthic foraminifera indicates that shallow marine carbonate sediments of the Asmari Formation at the study areas have been deposited in the photic zone of tropical to subtropical oceans. Based on analysis of larger benthic foraminiferal assemblages and microfacies features, three major depositional environments are identified. These include inner shelf, middle shelf and outer shelf. The inner shelf facies is characterized by wackestone–packstone, dominated by various taxa of imperforate foraminifera. The middle shelf is represented by packstone–grainstone to floatstone with a diverse assemblage of larger foraminifera with perforate wall. Basinwards is dominated by argillaceous wackestone characterized by planktonic foraminifera and large and flat nummulitidae and lepidocyclinidae. Planktonic foraminifera wackestone is the dominant facies in the outer shelf. 相似文献
6.
The first δ18O and δ13C data from the Upper Jurassic of the Northern Calcareous Alps are presented. The interpretation of stable isotope ratios serves as an approach for paleoenvironmental and diagenetic studies of the Plassen carbonate platform, which cannot be obtained by paleontological methods and microfacies analyses alone. The studied part of the Plassen limestone is characterized by (1) lithoclast facies, also called ‘intraformational breccia’; the origin of lithoclasts was formerly unknown; (2) peloid facies; (3) bioclastic facies, composed of peloids, porostromate algae, green algae and red algae; and (4) oncoid facies. Two types of fracturing and four cement generations can be distinguished. Isotope ratios of the matrix, oncoids, three cement generations and whole rock samples revealed that (1) the studied section represents an open marine carbonate platform with high water circulation and high input of cool oceanic waters; (2) the platform was not affected by emersion and fresh water influence; normal marine conditions prevailed; (3) carbonate cements were precipitated in a closed diagenetic system, but burial diagenesis was absent; (4) both fabric-selective and non-fabric-selective fracturing occurred in a normal marine environment, affecting the formation of ‘intraformational breccias’. 相似文献
7.
Abstract Nonskeletal sedimentary carbonate rocks are an important component of the Precambrian geological record, but consensus on their origin is lacking. Phanerozoic carbonates are almost exclusively biogenic products of shelly fossils, but it has generally been assumed that carbonate rocks deposited before a shelly biota evolved in the marine environment formed by direct precipitation from supersaturated solution in seawater. However, there is no unequivocal empirical evidence that calcium carbonate or dolomite precipitates directly from modern seawater, and it has been suggested that kinetic inhibitors to carbonate precipitation, related to the low concentration and activity of the carbonate ion, cation hydration and ion complexing, are especially effective in saline waters. On the other hand, there is increasing evidence that these inhibitors can be overcome through microbial mediation. Bacteria have been implicated in calcium carbonate precipitation since the Archaean, and though best known in seas and lakes, microbial carbonates are also important in fluviatile, spring, cave, and soil environments. The mechanisms of microbial mineral precipitation appear diverse, but many bacteria exhibit an ability to change solution chemistry and control pH at the microscale, passively or actively, thereby creating the ambient conditions for both oversaturation of Ca2 + and CO3 2 ? ions, and removal of kinetic inhibitors. Bacteria dominated the ecosystems of Precambrian shallow marine environments, enhancing their potential involvement in widespread carbonate formation. Chemical precipitation of evaporite minerals is generally accepted, but the involvement of microbes may be significant and underestimated. This review evaluates current knowledge and attempts to define some of the many questions that await resolution. 相似文献
8.
Observations on the diagenesis of the Lower Triassic Sudair Formation,Abu Dhabi,United Arab Emirates
Summary The Lower Triassic Sudair Formation in the United Arab Emirates (U.A.E.) ranges in thickness from 178–297 m and comprises
three units consisting of interbedded limestone, argillaceous limestone, dolomite and anhydrite. The Lower Unit contains variable
energy shallow marine, slightly argillaceous mudstones and subordinate ooliticpeloidal packstones and grainstones with minor
dolomite and anhdrite. The Middle Unit consists of argillaceous and ferroan dolomite deposited in a lagoonal to supratidal
setting. The Upper Unit comprises argillaceous mudstones and dolomites at the base grading upward into argillaceous anhydrite
deposited in a restricted shallow marine to sabkha setting. These units represent the transition from a carbonate/evaporite
shelf with significant terrestrial input to an evaporitic platform defined by an overall shallowing-upward sequence.
Diagenesis in the Sudair includes extensive leaching of grain-supported carbonates, partial to complete dolomitization, evaportie
formation, clay nucleation, fracturing/pressure solution, late cementation by coarse calcite spar and saddle dolomite, and
hematite formation. These processes have had the cumulative effect of reducing the secondary porosity. Dolomitization occurred
in two stages: an earlier progression of rhombic-sucrosic-aphanocrystalline dolomite, and a later coarse crystalline and saddle
dolomite fracture fill. 相似文献
9.
In the Central Iran Basin, the mixed carbonate–siliciclastic deposits of the C member of the Qom Formation were deposited on a carbonate platform which is dominated by rhodalgal associations occurring in tropical–subtropical environment. The biogenic rhodalgal association is dominated by bryozoa, coralline red algae, bivalves and echinoids together with smaller amounts of photo-dependent biota including large benthic foraminifera and corals. The abundance of heterozoan association and the bloom of suspension-feeding organisms are the result of an increase in nutrient availability which has profound controlling effect on the biotic system. The low occurrence of symbiont-bearing benthic foraminifera and coral, typical of stable, oligotrophic condition, represents their low tolerance to unstable, nutrient-rich environment. In the investigated Oligocene–Miocene shallow marine carbonate succession, 10 different microfacies were distinguished through depositional texture and biotic components. The rock sequences investigated are referred to an open shelf carbonate platform in which the depositional environments range from outer shelf to inner shelf conditions. 相似文献
10.
The ability to recognize the former existence of microbes as well as the biological origin of marine precipitates, such as putative microbialites, is crucial for understanding the development and history of early life on Earth. Increasingly, such rocks hold keys to understanding the geochemical evolution of the oceans and linked Earth systems. Vital trace elements previously have received relatively little attention as clues to the origin of carbonate rocks, and low abundance transition elements in particular, have been difficult to analyse in carbonate matrices for technical reasons. We have used laser ablation–inductively coupled plasma–mass spectroscopy for the in situ measurement of a broad suite of vital transition metals in Late Devonian reefal limestones that contain coeval microbialite (the calcimicrobe Renalcis), stromatoporoid sponge skeleton, early marine cement, and later diagenetic cement. Comparative experiments conducted in two different ion extraction modes determined theoretical detection limits for transition elements on NIST reference material SRM 612. Analyses of NIST glasses SRM 614 and 616 demonstrate accuracy relative to previously published data. On that basis we have identified significant enrichment of the vital elements V, Sn, Cu and Zn within the Renalcis. The stromatoporoid skeleton by contrast is enriched only in V. Earliest cements, which also may have been mediated to some degree by microbial biofilms on the basis of their morphology, show a much smaller degree of enrichment, and later cements show no enrichment, with the exception of Zn, which is concentrated in the latest cement. Fine particulate carbonate sediments (micrite) show variable metal enrichments that are attributable to varying contributions from detrital siliciclastic contamination. Renalcis was also enriched above the sponge and cements in regards to Mn, Cd, Co, and possibly Cr, but at less robust levels. Molybdenum and Sb were found not to be enriched in the Renalcis, and Ni, although clearly very low in concentration, could not be evaluated owing to its high detection limit. We additionally were able to identify specific zones of contamination in Renalcis encountered as the laser drilled deeper into the carbonate. Time resolved analysis allows exclusion of such contaminants from integration into the results. Successful application of the new technique will now allow us to assess metal uptake in ancient carbonates with implications for interpreting the biogenicity of putative microbialites. 相似文献
11.
Reef to basin sediment transport using Halimeda as a sediment tracer,Grand Cayman Island,West Indies
Fragments of the calcareous green alga Halimeda form a large part of the sediment in the fringing reef system and adjacent deep marine environments of Grand Cayman Island, West Indies. Nine species combine to form three depth-related assemblages that are characteristic of the major reef-related environments (lagoonpatch reef, reef terraces, and deep reef). These modern plant assemblages form the basis of the use of Halimeda as a sediment tracer. Halimeda-based tracer studies of Holocene sediments indicate that only sediments containing deep reef species of Halimeda are presently being transported through the reef system by sediment creep and being deposited at the juncture of the upper and lower island slope. Sediments containing shallow reef Halimeda are retained within the reef and lithified by marine carbonate cements. Tracer studies of Pleistocene sediment indicate large amounts of reef-derived carbonate sand containing deep water Halimeda were produced during interglacial high stands of sea level. Much of this material was removed by turbidity currents moving out of the reef system to the island slope down submarine channels perpendicular to the reef trend. These channels may still be identified on bathymetric profiles, but are no longer receiving coarse reef debris and are veneered with a blanket of pelagic carbonate mud. 相似文献
12.
The uppermost Rhaetian Adnet reef is part of the Dachstein carbonate platform and is situated at the transition to the intrashelf Kössen Basin. Its diagenetic evolution is investigated focusing on dissolution cavities in the Tropfbruch quarry of Adnet (near Salzburg) stratigraphically situated immediately below the Triassic–Jurassic boundary. Sea-level changes due to global eustatic trends and regional tectonics are assumed to be the controlling factors in the development of a manifold diagenetic sequence characterized by phases of meteoric dissolution, marine and burial cementation, and internal sedimentation. Despite small-scale variations of the sequence, a superordinate pattern of diagenetic phases could be elaborated. Small-scale eustatic sea-level falls subordinate to a global regression trend caused subaerial exposures of the Adnet reef in the latest Rhaetian to earliest Hettangian. The result was karstification and meteoric dissolution of aragonitic coral skeletons (Retiophyllia) leading to the formation of biomoldic porosity. Coral septa which escaped dissolution were transformed into neomorphic calcite spar under meteoric–phreatic conditions. A first generation of dog-tooth cements precipitated sporadically on the altered coral skeletons. Eustatic sea-level rise in Early to Mid-Hettangian times caused a renewed flooding of the pore space of the Adnet reef by marine water and the influx of a first generation of internal sediments (IS I), derived from the karstified host rock of the Upper Rhaetian reef limestone. These internal sediments are overgrown by radiaxial-fibrous calcites (RFCs) whose oxygen-isotopic signature (δ18O = ?1.3 (±0.7)‰) indicates precipitation in deeper (colder) water (18–21°C) due to a first phase of drowning. An intermediate phase of eustatic sea-level lowstand in the Late Hettangian is expressed by dissolution and corrosion of RFCs. Rapid drowning of the Dachstein carbonate platform due to eustatic sea-level rise and tectonic movements took place in the Early Sinemurian and a second generation of internal sediments (IS II) derived from the Lower Sinemurian Adnet Formation is washed into the dissolution cavities. Where IS II is absent, RFCs are overgrown by a second generation of dog-tooth cements with a bright-luminescent outer rim indicating the transition to negative redox conditions in the pore water during shallow burial. Burial diagenesis is represented by blocky calcite cements which occlude the remaining pore space. Depleted oxygen-isotope values and significant Fe contents indicate precipitation under reducing redox conditions and elevated temperatures of 30–50°C at burial depths of 420–870 m. Locally, replacive saddle dolomite is the latest diagenetic phase in the Adnet reef indicating crystallization under hydrothermal influences related to compressional subduction regimes of the Penninic Ocean. 相似文献
13.
The Permian–Triassic boundary and basal Triassic shallow-marine successions were studied and correlated in sections of two
structural units in Hungary (Transdanubian Range and Bükk units). Core sections in the Transdanubian Range unit recovered
inner ramp deposits whereas outcrops in the Bükk unit expose deposits of the deeper ramp area of the western Tethys. The inner
ramp section (studied ca. 10 m in thickness) is characterized by a succession of dolomites overlain by bioclastic limestones,
peloidal grainstones (which recorded the biotic decline) and oolites with finely crystalline limestone interlayers. The deeper
ramp section (studied ca. 15 m in thickness) is characterized by a succession of bioclastic limestones and marlstones, mudstone
beds (recording the first biotic decline), the ‘boundary shales’ (recording the second biotic decline and the stable carbon
isotope marker), mudstones with wackestone laminae, and stromatolite boundstones. Accordingly, oolite formation and microbial
micrite precipitation represent carbonate sedimentary responses of end-Permian mass extinction on the carbonate shelf. In
both successions, mudstones predominate the upsection, suggesting a relative sea-level rise. The succession of the deep ramp
area exhibits a continuous sediment accumulation and the diagenesis here was influenced by marine and marine-derived pore
water. The δ13C curve shows a continuous change towards more negative values, starting in bioclastic limestones, followed by a sharp symmetric
negative peak at the second biotic decline that is a chemostratigraphic marker of the boundary event. Facies and microfacies
trend of the inner ramp carbonates in the Transdanubian Range unit exhibits close similarities to that found in many South
Alpine sections. Relict peloidal deposits, formed cemented submarine hardground substrate, indicate the extinction level.
Sedimentary and diagenetic features of the overlying oolite bedset revealed slightly different depositional environments in
the two studied Transdanubian Range unit sections. Petrography of the oolites highlighted shallow burial diagenetic alterations
which includes marine cementation, marine-burial replacement and dolomitization. A lack of the specific negative peak in the
δ13C values is most likely due to the multiple redeposition events of the sedimentary grains. This led to the conclusion that
the deeper ramp deposits (e.g., in Bükk unit) have greater potential for recognizing trends in processes, affecting the marine
environments and related to the end-Permian mass extinction, at the western Tethys. 相似文献
14.
Cretaceous shallow-marine carbonate rocks of SW Slovenia were deposited in the northern part of the Adriatic Carbonate Platform. A 560-m-thick continuous Upper Cenomanian to Santonian carbonate succession has been studied near Hru?ica Village in Matarsko Podolje. With regard to lithological, sedimentological, and stratigraphical characteristics, the succession has been divided into nine lithostratigraphic units, mainly reflecting regressive and transgressive intervals of larger scale. During the latest Cenomanian and Early Turonian, hemipelagic limestones were deposited on top of shallow-marine lagoon and peritidal Upper Cenomanian deposits indicating relative sea-level rise. Subsequently, the deeper marine depositional setting was gradually filled by clinoform bioclastic sand bodies overlain by peritidal and shallow-marine low-energy mainly lagoonal lithofacies. Similar lithofacies of predominately inner ramp/shelf depositional settings prevail over the upper part (i.e., Coniacian to Santonian) of the succession. In the area, the Upper Cetaceous carbonate rocks are separated from the overlying Lower Eocene (Upper Paleocene?) carbonate sequence by regional unconformity denoted by distinct paleokarstic features. On the Adriatic Carbonate Platform the deeper marine carbonate setting, developed at the Cenomanian/Turonian boundary, is usually correlated with OAE2 and related eustatic sea-level rise. Similarly, subsequent reestablished shallow-marine conditions are related to Late Turonian long- and short-term sea-level fall. However, we are suggesting that deeper marine deposits were deposited in a tectonically induced intraplatform basin formed simultaneously with the uplift of the northern and northeastern marginal parts of the Adriatic Carbonate Platform. 相似文献
15.
In situ trace metal analysis of Neoarchaean – Ordovician shallow‐marine microbial‐carbonate‐hosted pyrites
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Pre‐Cambrian atmospheric and oceanic redox evolutions are expressed in the inventory of redox‐sensitive trace metals in marine sedimentary rocks. Most of the currently available information was derived from deep‐water sedimentary rocks (black shale/banded iron formation). Many of the studied trace metals (e.g. Mo, U, Ni and Co) are sensitive to the composition of the exposed land surface and prevailing weathering style, and their oceanic inventory ultimately depends on the terrestrial flux. The validity of claims for increased/decreased terrestrial fluxes has remained untested as far as the shallow‐marine environment is concerned. Here, the first systematic study of trace metal inventories of the shallow‐marine environment by analysis of microbial carbonate‐hosted pyrite, from ca. 2.65–0.52 Ga, is presented. A petrographic survey revealed a first‐order difference in preservation of early diagenetic pyrite. Microbial carbonates formed before the 2.4 Ga great oxygenation event (GOE) are much richer in pyrite and contain pyrite grains of greater morphological variability but lesser chemical substitution than samples deposited after the GOE. This disparity in pyrite abundance and morphology is mirrored by the qualitative degree of preservation of organic matter (largely as kerogen). Thus, it seems that in microbial carbonates, pyrite formation and preservation were related to presence and preservation of organic C. Several redox‐sensitive trace metals show interpretable temporal trends supporting earlier proposals derived from deep‐water sedimentary rocks. Most notably, the shallow‐water pyrite confirms a rise in the oceanic Mo inventory across the pre‐Cambrian–Cambrian boundary, implying the establishment of efficient deep‐ocean ventilation. The carbonate‐hosted pyrite also confirms the Neoarchaean and early Palaeoproterozoic ocean had higher Ni concentration, which can now more firmly be attributed to a greater proportion of magnesian volcanic rock on land rather than a stronger hydrothermal flux of Ni. Additionally, systematic trends are reported for Co, As, and Zn, relating to terrestrial flux and oceanic productivity. 相似文献
16.
Vanessa Moura dos Reis Cláudia Santiago Karez Rodrigo Mariath Fernando Coreixas de Moraes Rodrigo Tomazetto de Carvalho Poliana Silva Brasileiro Ricardo da Gama Bahia Tito Monteiro da Cruz Lotufo Laís Vieira Ramalho Rodrigo Le?o de Moura Ronaldo Bastos Francini-Filho Guilherme Henrique Pereira-Filho Fabiano Lopes Thompson Alex Cardoso Bastos Leonardo Tavares Salgado Gilberto Menezes Amado-Filho 《PloS one》2016,11(4)
The abundance of reef builders, non-builders and the calcium carbonate produced by communities established in Calcification Accretion Units (CAUs) were determined in three Abrolhos Bank shallow reefs during the period from 2012 to 2014. In addition, the seawater temperature, the irradiance, and the amount and composition of the sediments were determined. The inner and outer reef arcs were compared. CAUs located on the inner reef shelf were under the influence of terrigenous sediments. On the outer reefs, the sediments were composed primarily of marine biogenic carbonates. The mean carbonate production in shallow reefs of Abrolhos was 579 ± 98 g m-2 y-1. The builder community was dominated by crustose coralline algae, while the non-builder community was dominated by turf. A marine heat wave was detected during the summer of 2013–2014, and the number of consecutive days with a temperature above or below the summer mean was positively correlated with the turf cover increase. The mean carbonate production of the shallow reefs of Abrolhos Bank was greater than the estimated carbonate production measured for artificial structures on several other shallow reefs of the world. The calcimass was higher than the non-calcareous mass, suggesting that the Abrolhos reefs are still in a positive carbonate production balance. Given that marine heat waves produce an increase of turf cover on the shallow reefs of the Abrolhos, a decrease in the cover represented by reef builders and shifting carbonate production are expected in the near future. 相似文献
17.
Palaeozoic cold seep carbonates from Europe and North Africa—an integrated isotopic and geochemical approach 总被引:1,自引:0,他引:1
In this paper, we report the highest and lowest carbon isotope values known from Palaeozoic carbonate rocks. These unusual δ13C values (−50 to +23.5‰) are due to microbial methanogenesis and methanotrophy in Silurian to Carboniferous carbonates. Trace elements were used to decipher the primary mineralogy of the carbonate cements. Very high Sr values and low amounts of Mg, Fe and Mn point toward aragonite precursors, whereas high Fe and Mn values are indicative of primary calcites and allow reconstruction of the redox conditions. Four carbonate deposits are described from the Meseta and the Antiatlas of Morocco, the Pyrenees (France) and the Harz mountains (Germany). The highest δ13C values in concretion below the uppermost Silurian Spinatrypa Mound (Moroccan Meseta) give evidence, that CO2 was produced during methanogenesis. δ13C values between −10 and −32‰indicate that the formation of microbial carbonates and cements in the Middle Devonian Hollard Mound (Antiatlas) and in the Lower Carboniferous sediments of the Iberg (Harz) formed at thermogenetic methane or petroleum seeps. The Late Bashkirian carbonate mound of the High Pyrenees (Tantes Mound) is the first Palaeozoic carbonate with seepage fluids being dominated by biogenic methane. Matrix carbonates exhibit δ13C values as low as −34‰. In some parts, voids make up more than 50 vol% of the mound. They are filled with several generations of cement. The earliest void filling is isopachous fibrous cement, which represents former aragonite. Most negative δ13C values of −50‰were measured in these isopachous fibrous cements. The difference of 55‰in δ13C values between normal sediments and early aragonite cements can only be explained by the contribution of CO2 from anaerobic oxidation of biogenic methane in a cold seep setting. 相似文献
18.
Microbialites and global environmental change across the Permian-Triassic boundary: a synthesis 总被引:1,自引:0,他引:1
Kershaw S Crasquin S Li Y Collin PY Forel MB Mu X Baud A Wang Y Xie S Maurer F Guo L 《Geobiology》2012,10(1):25-47
Permian-Triassic boundary microbialites (PTBMs) are thin (0.05-15 m) carbonates formed after the end-Permian mass extinction. They comprise Renalcis-group calcimicrobes, microbially mediated micrite, presumed inorganic micrite, calcite cement (some may be microbially influenced) and shelly faunas. PTBMs are abundant in low-latitude shallow-marine carbonate shelves in central Tethyan continents but are rare in higher latitudes, likely inhibited by clastic supply on Pangaea margins. PTBMs occupied broadly similar environments to Late Permian reefs in Tethys, but extended into deeper waters. Late Permian reefs are also rich in microbes (and cements), so post-extinction seawater carbonate saturation was likely similar to the Late Permian. However, PTBMs lack widespread abundant inorganic carbonate cement fans, so a previous interpretation that anoxic bicarbonate-rich water upwelled to rapidly increase carbonate saturation of shallow seawater, post-extinction, is problematic. Preliminary pyrite framboid evidence shows anoxia in PTBM facies, but interbedded shelly faunas indicate oxygenated water, perhaps there was short-term pulsing of normally saturated anoxic water from the oxygen-minimum zone to surface waters. In Tethys, PTBMs show geographic variations: (i) in south China, PTBMs are mostly thrombolites in open shelf settings, largely recrystallised, with remnant structure of Renalcis-group calcimicrobes; (ii) in south Turkey, in shallow waters, stromatolites and thrombolites, lacking calcimicrobes, are interbedded, likely depth-controlled; and (iii) in the Middle East, especially Iran, stromatolites and thrombolites (calcimicrobes uncommon) occur in different sites on open shelves, where controls are unclear. Thus, PTBMs were under more complex control than previously portrayed, with local facies control playing a significant role in their structure and composition. 相似文献
19.
Mark Williams Melanie J. Leng Michael H. Stephenson Julian E. Andrews Ian P. Wilkinson David J. Siveter David J. Horne Jean M.C. Vannier 《Palaeogeography, Palaeoclimatology, Palaeoecology》2006,230(3-4):299-318
A study of the stable isotope composition (δ18O, δ13C) of biogenic (ostracod, mollusc) and authigenic carbonates in the Ballagan Formation, Lower Carboniferous of Scotland, coupled with evidence from sedimentology and associated fossil fauna and flora, supports the argument that this formation was deposited in a coastal flood plain setting, in brackish (0.5 < 30‰ NaCl) and hypersaline (> 40‰ NaCl) waters, but in the absence of persistent normal marine conditions. The oxygen isotope data from the Ballagan Formation divide into three clusters: a diagenetic field defined by low δ18O (< − 11‰ VPDB); an intermediary field (δ18O − 11‰ to − 9‰) composed of a mixture of known primary and secondary (diagenetic) carbonates; and samples within the range of − 9‰ to − 4‰ which, as far as we can ascertain, are largely unaltered. No samples give typical Early Carboniferous δ18O marine values. Average marine carbonates from Europe have δ18O between − 4‰ to − 3‰. The Ballagan Formation carbonates were probably deposited in evaporated freshwater and/or brackish water. This conclusion is supported by the presence of evaporites (gypsum, anhydrite, halite pseudomorphs) and common desiccation-cracked mudstone surfaces throughout the Ballagan Formation, suggesting conditions of fluctuating salinity in ephemeral bodies of water. The stable isotope data support the notion that the ostracod assemblages of the Ballagan Formation were colonising brackish water and hypersaline ecologies on a coastal flood plain during the Early Carboniferous, a stage of development that may have encouraged their colonisation of fully non-marine (limnetic) environments during the later Carboniferous. The ostracods include cytherellacean and kloedenellacean species known from marginal marine sites elsewhere, but probably tolerant of brackish water, podocopid species such as ‘Bythocypris’ aequalis that may have been adapted for brackish water settings on coastal flood plains (ephemeral lakes and lagoons), and paraparchitacean-dominated assemblages that may signal harsh (hypersaline or desiccating) environments. 相似文献
20.
N. Sabatino R. Neri A. Bellanca H.C. Jenkyns D. Masetti G. Scopelliti 《Palaeogeography, Palaeoclimatology, Palaeoecology》2011,299(1-2):97-109
Deposits with unusually high Mn contents sampled at Monte Mangart in the Julian Alps include organic-rich marlstone and black shale with interbedded manganoan and siliceous limestone, which were deposited during the early Toarcian Oceanic Anoxic Event. Mn enrichment during that period has been related to global sea-level change coincident with increasing subsidence rate. The formation of Fe–Mn nodules, marking a hardground at the base of the Monte Mangart section, seems to be triggered by release of Mn from remote hydrothermal vents into a region of relatively elevated submarine topography where oxidizing conditions prevailed. However, very high Mn contents in carbonate phases above the hardground imply an additional diagenetic source of this element in the lower part of this section. The whole stratigraphic sequence (ca 30 m) displays a transition from Mn-rich (up to 8.8%) sediments, in the lower part, to Mn-poor (less than 1.8%) sediments in the middle and upper parts. The drastic decrease in Mn content's up-section is accompanied by a clear decrease in the mean size of pyrite framboids, indicating more intense anoxia/euxinia in the water column. In the presence of Mn2+, conditions of high alkalinity induced precipitation of Mn carbonates during early diagenetic processes. Negative δ13Ccarb values coincident with high Mn contents indicate involvement of organic matter in the mineralization process. The striking similarity of Ce/Ce* and Mn profiles demonstrates that, consistent with redox-chemistry of Mn and Ce under anoxic conditions, Ce3+ and Mn2+ were mobilized and released into pore water where precipitation of Mn carbonates occurred. 相似文献