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1.
Summary Late Arenigian biohermal reef mounds and biostromes within the shallow-marine platform facies of the upper San Juan Formation
of the Precordillera (Western Argentina) represent a new Early Ordovician reef type. The meter-sized reefs are dominated byZondarella communis n.g. n. sp. The new taxon is characterized by domical, bulbous and laminar morphotypes exhibiting growth layers and thin
horizontal and vertical as well as intermingled skeletal elements included within different sets. The fossil maybe compared
with stromatolites and stromatoporoids but an interpretation as primitive stromatoporoids is favoured. 相似文献
2.
Summary The Linnaean nameCryptozo?n proliferum
Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in
New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary
structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither
fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving
a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions
may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozo?n proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction
other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion
or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur
on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic
microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures
up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms.
Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the
sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they
form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake
Clifton and the type exposures ofCryptozo?n proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which
upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern
environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include
some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper
Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the
B?tsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites
depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has
demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for
understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples
of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences
from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological
growth was adjusted to contemporary sea level. 相似文献
3.
《Palaeoworld》2020,29(1):126-136
Permian–Triassic boundary microbialites (PTBMs) are organosedimentary carbonates formed immediately after the end-Permian mass extinction. All those reported PTBMs constrained by convincing conodont biozones are present stratigraphycally not higher than the Hindeodus parvus zone and most of them are dominated by thrombolites. This paper provides the first record of a brief, but spectacular development of stromatolite-dominated PTBMs within the basal Isarcicella isarcica conodont zone of the earliest Triassic from the Xikou section of South Qinling Block that was at the margin of the North China Block during the Permian–Triassic transition and was geographically separated from the major occurrence of post-extinction microbialites in the South China Block. This stromatolite cap overlies a 3.7-m-thick oolitic limestone and is composed of a lower 0.2-m-thick bed and an upper 0.5-m-thick bed, separated by a 0.2-m-thick greyish green siliciclastic mudstone. These two stromatolite beds mainly consist of columnar stromatolites with subordinate domal stromatolites. The intercolumn and interstitial spaces within the stromatolites are filled with oolitic grainstones. At the microscopic scale, laminoid structures in stromatolites comprise wavy, millimetric-domical and tangled laminae. The increased grain and fossil contents and/or bioturbation in the domical and tangled laminae indicate that the formation of these laminae is likely related to an increase in the populations and the disruptions by benthic metazoans, as well as an influx of sediment grains. The δ13Ccarb values fluctuate between 2‰ and 3‰ in the uppermost Permian strata; a distinct negative shift of 1.9‰ occurs at the topmost oolitic grainstone, just below the lower stromatolite bed, and the lowest value of −0.1‰ is located at the base of the upper stromatolite bed. The stratigraphic succession from stromatolites to thrombolites of the PTBMs may represent a transgressive succession and/or a transient ecosystem recovery immediately after the end-Permian mass extinction. The thrombolites-dominated PTBMs mainly developed in near-equator shallow marine geographic locations, and stromatolite-dominated PTBMs mainly developed at higher latitude settings, which probably indicates that a relatively lower diversity and abundance of marine benthic metazoans existed at higher latitudes after the end-Permian mass extinction. 相似文献
4.
Two Patagonian lakes studied here, Lago Sarmiento and Laguna Amarga, are located within the orographic rain shadow formed to the east of the Southern Patagonian Ice Field in the Andes Range. Major thrombolite colonies are present in Lago Sarmiento, whereas widespread stromatolites occur in Laguna Amarga. Based on the characterization of the hydrologic system of these two lakes, together with an estimation of the isotopic balance and an analysis of the equilibrium conditions between the water and biologically induced carbonates, it is concluded that the microbialites of Lago Sarmiento are better suited as paleotemperature indicators than those of Laguna Amarga. Lago Sarmiento thrombolites contain unique carbonate mineral species in which carbonate precipitation occurs close to isotopic equilibrium and where the variation in water temperature controls fractionation of the stable oxygen isotope.The results indicate that at 1215 cal yr Bp the level of the lake was at 85 m a.s.l with a temperature close to 9.3 °C, was at 82 m a.s.l. at 600 cal yr Bp with a temperature close to 8.5 °C. This coincides with the timing of the Northern Hemisphere Medieval Warming Period. At 183 cal yr Bp the level of the lake was at 80 m a.s.l with a cooler temperature close to 7.7 °C, representing a colder period coinciding with the timing of the Little Ice Age (LIA). An interesting outcome of this study is that it reinforces the idea that the δ13C signal in carbonate deposits can be an effective tool in distinguishing between inorganic and biologically induced precipitation. 相似文献
5.
Caecidotea williamsi Escobar-Briones & Alcocer (2002) is the first asellid described from a saline aquatic habitat in America, Alchichica crater-lake, Puebla, Central Mexico.
No previous reports exist for asellid isopods from inland saline waters in America in spite of the extensive research undertaken
so far on the continent. Differing from other asellids, C. williamsi inhabits saline waters (i.e. 8.5 g l−1) dominated by sodium, magnesium, chloride, and bicarbonate ions and markedly alkaline (pH 9.0 ± 0.1). Water temperature ranges
from 14.5°C throughout the water column in winter and in the deep waters for the rest of the year, up to 20°C in the surface
waters in summer. C. williamsi occurs in a depth range of surface to 30 m (lake’s maximum depth is 64 m), below which an anoxic layer is found during 9
months of the year. Generally it lives cryptically in tufa crevices and at shallower depths many inhabit empty trichopteran
cases embedded within the tufa crevices. Some specimens are heavily covered by epizooic ciliates on the thoracic and abdominal
segments of the exoskeleton and the pleopods. Data are presented on the diet, possible predators, saprobity and trophism,
and regional distribution of C. williamsi. The continued survival of this unique isopod is threatened by anthropogenic desiccation of its habitat.
Guest Editor: John M. Melack
Saline Water and their Biota 相似文献
6.
Leuko S Goh F Allen MA Burns BP Walter MR Neilan BA 《Extremophiles : life under extreme conditions》2007,11(1):203-210
Hamelin Pool in Western Australia is one of the two major sites in the world with active marine stromatolite formation. Surrounded
by living smooth and pustular mats, these ancient laminated structures are associated with cyanobacterial communities. Recent
studies have identified a wide diversity of bacteria and archaea in this habitat. By understanding and evaluating the microbial
diversity of this environment we can obtain insights into the formation of early life on Earth, as stromatolites have been
dated in the geological record as far back as 3.5 billion years. Automated ribosomal intergenic spacer analysis (ARISA) patterns
were shown to be a useful method to genetically discriminate halophilic archaea within this environment. Patterns of known
halophilic archaea are consistent, by replicate analysis, and the halophilic strains isolated from stromatolites have novel
intergenic spacer profiles. ARISA–PCR, performed directly on extracted DNA from different sample sites, provided significant
insights into the extent of previous unknown diversity of halophilic archaea within this environment. Cloning and sequence
analysis of the spacer regions obtained from stromatolites confirmed the novel and broad diversity of halophilic archaea in
this environment. 相似文献
7.
E. G. Nisbet 《Journal of molecular evolution》1985,21(3):289-298
Summary Life on Earth may have begun about 4×109 years (4 Ga) ago. Plate tectonics probably operated in the early Archaean, with rapid spreading at mid-ocean ridges, a komatiitic (magnesium-rich) oceanic crust, active volcanic arcs and the development of extensional basins on continental crust. Shallow water environments would have been more restricted and probably shorter-lived than in later geological times; however, extensive shallow seas existed in the later phases of the development of extensional basins. Bacterial communities-presumably photosynthetic-have probably existed in such shallow-water settings and probably at shallow depths in the oceans for at least 3.5 Ga. Because the mid-ocean ridges were probably subaqueous, hydrothermal systems would have been very vigorous and would have offered suitable habitats for early chemo-autotrophic bacterial communities. Early life forms probably also occupied vesicles in lavas, pumice and volcanic breccias, and pores in soft sediments, living in the constant flux of fluid flushing through permeable strata. Other, similar habitats would have existed in volcanic island arcs and in extensional basins. 相似文献
8.
RUSSELL S. SHAPIRO RONALD R. WEST 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1999,32(2):131-139
Carboniferous to Permian marine stromatolites are widely dispersed across the Pangaean margins and embayments and are typified by the ‘Ottonosia-grade stromatolite’ (designated herein). This stromatolite type consists of a well-laminated oncoid or domical stromatoid that developed into branching, laminated columns in the upper reaches. To develop a model for the global pattern, we investigated Lower Permian stromatolites from Kansas (Howe Limestone Member, Red Eagle Limestone). Stromatoids from the Lyon County locality typify the Ottonosia-grade stromatolites. The laminae are sharp throughout the stromatoid and are defined by an increase in cornuspirid foraminfera and algal filaments. The upper zone of the stromatoid is composed of well-laminated branching and brecciated columns (‘pseudo-thrombolitic’). Coeval stromatolites from a new exposure at the Tuttle Creek Dam spillway possess a more massive mesostructure. These stromatolites are composed of a turbinate stromatoid or oncoid base and an overlying domical stromatoid, and are rimmed by smaller meandering columns. Only the basal stromatoid, oncoids, and upper columns are well laminated. In both localities, the microbial-constructing ecosystem is dominated by cornuspirids and calcifying filamentous algae (?Girvanella). The mesostructural differences of the stromatolites are due to different environments of formation. The Tuttle Creek stromatolites formed in a shallow-subtidal to intertidal open marine setting. The coeval Lyon County stromatolites formed in a semi-restricted, marginal marine environment such as a lagoon or supratidal zone. Based on this information and independent sedimentological data, we conclude that lagoonal or supratidal zones were common features in the late Paleozoic intracratonal zones of the Pangaean supercontinent and account for Ottonosia-grade stromatolites occurring in the Laurentian mid-continent, the Zechstein Basin, Japan, Brazil, and Tunisia. 相似文献
9.
Dr. Eric Fookes 《Facies》1995,33(1):129-149
Summary This study consists of a sedimentological and diagenetical analysis of reef facies from the Upper Kimmeridgian (sensu gallico). The investigated deposits are situated in eastern France, about fifty kilometres west of the city of Geneva (Switzerland).
The reef complex is a fine example of vertical development and facies differentiation. It is subdivided into two distinct
sequences by a perforated hardground horizon and sand shoals. The onset of the first reef sequence is characterized by a pioneer
growth stage followed by up to 20 m of reef-core and-flank facies. Corals forming the reef-core are typically the ramose variety
ofCalamophylliopsis flabellum. The second reef sequence has a reef-core with an average thickness of about 5 m. Corals, however, display much more varied
morphologies, and in some areas massive rudist (Heterodiceras) build-ups occur. Development of the second reef sequence was seriously weakened by a storm which produced a 2 m thick accumulation
of coral rubble. A shallowing-upwards trend gradually leads to the formation of beach deposits, followed by a newly detected
black-pebble horizon.
Diagenesis is an important aspect of the reef complex. Especially noteworthy is the dolomitization of certain horizons. At
the base of the reef formation, the passage of the phreatic mixing zone provoked invasive dolomitization in large irregular
patches (probably deposits richer in Mg-calcite). Some of the beds above the black-pebble horizon, in particular a deposit
of accumulated microbial mats, are also dolomitized. In this case, dolomitization is stratiform and is interpreted as having
precipitated under conditions of evaporative pumping.
The sedimentary record clearly shows the imprint of eustasy. The reef complex was initiated during a transgressive cycle and
the hardground found between the two reef sequences is interpreted as a maximum flooding surface (mfs). At the top of the
sequence, the horizon overlain by the black-pebble conglomerate is believed to represent the new sequence boundary SB140.
Other significant features identified from the St. Germain-de-Joux deposits include the discovery of a new foraminifera,Troglotella incrustans, which is only marginally covered here but is the topic of another paper (Wernli & Fookes, 1992); the subdivision of the first coralligenous level defined byPelletier (1953) into two reef sequences; and a proposition to redefine the ‘Calcaires de la Semine’ (Bernier, 1984).
The investigations carried out in the past on the Kimmeridgian deposits in the area of St. Germain-de-Joux were mostly based
on stratigraphy and palaeontology. These reefs are among the finest known in the Jura Mountains, but no thorough study had
been made on their sedimentological aspects. The aim of this study is to fill this void and also to clarify the more confusing
aspects of local stratigraphy (paper based onFookes, 1991). 相似文献
10.
Microbial deposits in upper Miocene carbonates, Mallorca, Spain 总被引:1,自引:0,他引:1
The Santanyí Limestone, a 30-35-m thick upper Miocene limestone succession cropping out in Mallorca, contains abundant microbialite deposits, the shape, microstructure and texture of which was controlled by environmental factors: depth, energy and salinity. Three main types of microbialites are distinguished: (1) domed (DNOS) and stratiform, mostly undulate (UNOS) non-oolitic stromatolites, (2) undulate oolitic laminites (UOL) and (3) domed-oolitic thrombolites (DOTs). Based on lithofacies associations and occurrence of microbialite types, the Santanyí Limestone succession is subdivided into five stratigraphic units (I to V) separated by sharp surfaces. Within units II, III and V, the vertical evolution of microbialites was induced by changes in accommodation space/depth: (1) intertidal/very-shallow subtidal conditions at the base were induced by flooding over a wide area, (2) continued sea-level rise caused submergence to subtidal conditions, and (3) a significant bathymetric decrease created the sharp surface bounding these units.In units II and III, NOS accumulated in variable energy and depth conditions, as buildups with thick, somewhat discontinuous and mostly non-isopachous lamination, surrounded by oolitic grainstones with wave and current structures and oolitic intraclasts. In contrast, thin and generally regular and smooth lamination of NOS in unit V suggests, along with the absence of oolite grainstones and macrobiota, calm and restricted, maybe more saline, conditions.UOL, consisting of oolitic layers separated by thin micritic laminae, developed adjacent to NOS in units II and III and to DOT at the lower part of unit III, in shallow-water and low-energy conditions. Both ooids and micrite laminae have evidence for biogenesis. Micritized ooids containing microbial remains are common. Micritic laminae in UOL and the dark micritic laminae in NOS are thought to represent bacterially enhanced calcite precipitation and lithification during periods of low sedimentation.Oolitic thrombolites containing macrobiota are only present in unit III. They represent deeper and open-marine conditions affected by high-energy events, in which microbially mediated precipitation favoured microbialite accretion and lithification. 相似文献