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1.
Iuliana Lazăr Cristina E. Panaiotu Dan Grigore Michael R. Sandy Jörn Peckmann 《Facies》2011,57(4):627-647
An unusual fossil assemblage dominated by superabundant rhynchonellid brachiopods in a stromatactis mud-mound is recorded
from the Hăghimaş Mountains (Eastern Carpathians), Romania. The mound mainly consists of bioclastic wackestones to packstones
with a very rich macrofauna including crinoids, sponges, juvenile ammonites, and echinoids. The brachiopods represent a low-diversity
but high-abundance association, dominated by the rhynchonellids Lacunosella and Septaliphoria. The taphonomical features of the fossil assemblage indicate an autochthonous fauna, with successive generations of brachiopods
in life position and complete well-preserved individuals in different growth stage alongside an accessory population of crinoids
and sponges. Brachiopod-brachiopod endosymbiotic life strategy is documented for the first time from a post-Paleozoic brachiopod
assemblage. The mound reveals abundant stromatactis, filled by radiaxial fibrous or drusy calcite cement and internal polymud
sediments. This is the first Late Jurassic (Kimmeridgian) stromatactis mud-mound identified in the Eastern Carpathians. 相似文献
2.
Dr. Andrei V. Dronov 《Facies》1993,28(1):169-180
Summary Several Waulsortian-type mud mounds nearly 500 m thick and about 5 km long occur in the Middle Paleozoic carbonate section
of the Aktur nappe in the mountains on the right bank of Isfara river. These buildups form a well developed barrier system
that stretches along the South Ferganian carbonate platform margin and divides the carbonate complex into a fore-reef and
a back-reef part. The time of the mounds' most active growth was from the Late Silurian (Ludlow) to the Middle Devonian (Eifel).
Three main facies types can be recognized in the mud mounds: 1. micritic core facies, 2. sparitic flank facies and 3. loferitic
capping facies. The central massive or crudely bedded part of the mounds consists of white or light grey clotted micrite.
Macrofossils are rare. The sparitic flank facies in contrast consists of coarse and densely packed crinoidal wackestone-floatstones
with some brachiopod shell debris. Solitary rugose corals, tabulate corals, stromato-poroids and fragments of mollusks are
also abundant. The tops of the mounds are usually covered with loferitic pelmicrites or oolitic grainstone caps. Stromatactis-like
structures are very rare and poorly developed in the South Ferganian mud mounds. However, almostin all such mounds horizons
of calcitic breccias can be found.
In order to explain all the features found in the Fergana mounds an ‘atoll-like’ model has been proposed which starts the
evolution of the mud mounds with a small nucleus bioherm. The main stage of the evolution corresponds to an atoll-like structure
developing on the surface of shallow water platforms. White clotted micrite of the mound core facies is interpreted as a accumulation
of fine-grained sediment in an inner lagoon flanked by crinoidal bar deposits. The mound flank facies represents the atoll
rim deposits from where the carbonate mud is derived. The capping loferitic facies is considered as tidal flat deposit that
developed on top of the buildups during the last stage of its evolution. The knoll shape of the mounds is explained by the
retreat of the atoll flanking crinoidal bars back into the inner lagoon during the rise in sea level. Stromatactis-like structures
of small cavities filled with sparry calcite owe their existence to burrowing organisms. Calcitic breccias are interpreted
as paleokarst collapse breccias. They indicate that the tops of the mud mound became subaerially exposed. Other evidence for
a subaerial exposure can be seen in the occurrence of Variscian ‘black and white’ limestone gravel on the tops of some mud
mounds. According toWard et al. (1970) these sediments were produced above the sea level at the edge of hypersaline lakes situated on islands. 相似文献
3.
Dr. Jacek Matyszkiewicz 《Facies》1993,28(1):87-95
Summary The Middle Oxfordian strata in the southern part of the Cracow-Wielun Upland consist of platy and bedded limestones (‘normal
facies’), of massive limestones as well as locally of mass flow sediments. Massive limestones, prevailing in the Upper Oxfordian,
form commonly carbonate buildups, which are made up predominantly of cyanobacterial allochems and to a minor amount of siliceous
sponges.
Stromatactis can be best observed in the Mlynka quarry. They occurs in the uppermost part of slope sediments close to a cyanobacterial-sponge
buildup. The bedding-plane of the slope sediments is directly overlain by debris-flow and grain-flow sediments.
Fragments of a primary laminar framework rich in growth-cavities occur in the uppermost part of the slope sediments as precondition
for the formation of stromatactis. The stromatactis cavities were formed by internal reworking and erosion within these organic
growth cavities, caused by strong bottom currents due to mass transport from higher parts of the buildup. 相似文献
4.
Summary Lower Devonian mud mounds and stromatactis fabrics are exceptionally well exposed in quarry walls and industrially sawed blocks
in the Montagne Noire in southern France. Interlayered red biomicrites and white to grey sparitic calcites form mounds up
to 70 m high. The red biomicrites contain predominantly bryozoans, sponges and echinoderms.
The sparitic layers show typical features of stromatactis fabrics, as outlined byBathurst (1982). We recognize two types of stromatactis fabrics: (1) Stromatactis type A: exentsive cavity systems filled by multiple
cement generations, which are interpreted to be related to microbial mats, and (2) Stromatactis type B: smaller patches of
blocky spar which are mainly diagenetic in origin, but show characteristic features of stromatactis. Type A is far more important
in terms of rock volume. The cyclic interlayering of red biomicrites and sparitic layers is supposed to result from frequent
changes in the composition of the mound biota.
The bryozoan/sponge community was displaced by short term propagations of microbial mats during times of extremely low sedimentation.
Sedimentation and thus the biotic community was probably determined by high frequency (6th order) sea level changes. Despite
these changes, mound growth continued, because once established the ecological advantage over the surroundings was maintained
by both communities alternating with each other. The microbial mats and the cavities they left after their decay were important
for the stabilization of the mounds, the latter allowing for enormous quantities of dissolved carbonate to be transported
and precipitated. We anticipate a close interrelation between mound formation and stromatactis formation, and we believe that
it is not incidential that both, mud mounds and stromatactis, are mainly restricted to the same interval, namely the Paleozoic. 相似文献
5.
Adelaide Mastandrea Francesco Muto Claudio Neri Cesare A. Papazzoni Edoardo Perri Franco Russo 《Facies》2002,47(1):27-42
Summary The “Calcare di Mendicino” is a mixed carbonatesiliciclastic informal unit of Miocene (Late Tortonian-Early Messinian age),
that crops out extensively in the northwestern part of the Calabria. In the Scannelle quarry near Belsito (Cosenza), four
stratigraphic sections were studied to define the sedimentological and paleoecological setting. The carbonate body records
the development of a deep-water coral bank characterized by a low-diverse community of azooxanthellate scleractinian (Oculina andDendrophyllia) and stylasterine hydrozoans colonies. Two main stages of bioconstruction development can be distinguished: a thicket and
a bank stage. Among the biostromal dwellers the more common are bryozoans, echinoids, benthic foraminifers, gastropods, and
bivalves. A higher content of planktonic foraminifers occur in the thicket stage. The coral bank flourished within the aphotic
zone, with deep currents loaded with nutrients and siliciclastic sediments.
The upper part of the “Calcare di Mendicino” carbonate body has been affected by a pervasive dolomitization destroying almost
completely the sedimentary structures and the biofacies. The lower part, the main object of this paper, preserves the microfacies
but it experienced a widespread recrystallization obliterating the primary geochemical characteristics. The diagenetic history,
partly hidden, reveals three main stages: primary marine with isopachous fibrous cements, deep burial with cavities infilled
by sparry calcite, and meteoric-phreatic with dog-tooth cements. 相似文献
6.
Prof. Dr. Franco Russo Dr. Adelaide Mastandrea Dr. Marco Stefani Prof. Dr. Claudio Neri 《Facies》2000,42(1):211-226
Summary This article deals with the discussion of the role of the syndepositional cementation for the growth of the Middle Triassic
pre-volcanic carbonate platforms of the Dolomites (Southern Alps, Northern Italy). The study is concentrated on the Marmolada
Buildup, which escaped the facies destroying dolomitization which affected many surrounding platforms. The investigations
took place within an almost isochronous uppermost Anisian palcogeographic transect, ranging from the platform-top to the margin
and the upper slope. Methods used include geological mapping, sedimentological and paleontological studies, evaluation of
the microfacies, as well as SEM and EDS epifluorescence analyses.
The well bedded platform-top succession consists of intra-bioclast calcarenites and calcirudites, interbedded with subordinate
boundstones, and organized in shallowing upward, meter scale depositional cycles, sometimes capped by subaerial surfaces.
The platform margin belt is rich in boundstones and lacks a primary framework formed by organisms; metazoan skeletons form
less then 5% of the rock volume. The outer margin and the uppermost slope are characterized by decimeter-scale boundstone
blocks, coated and linked to each other by huge amounts of radiaxial fibrous calcite cements, arranged in concentric crusts.
These cements (“evinospongiac”) represent the main component of the margin and upper slope facies.
Epifluorescence analyses suggest the existence of abundant organic residual matter associated not only with the bioclasts
and peloids, but also with the syndepositional cements. Organic matter likely played a significant role in carbonate cementation
and was a key factor for the early lithification of the platform as well as for the sediment production. Minor element microanalyses
reveal an uniform Mg content in different calcite types (2–4 Mole % MgCO3), independently from the primary nature of the components. Late diagenetic sparry calcites exhibit similar Mg values but
no iron. These data point to a homogenization of minor element distribution, probably associated with a slow but long-lasting
semi-closed fluid circulation, possibly related with the Neogene uplifting of the Dolomite Mountains. 相似文献
7.
Summary The Belgian Frasnian carbonate mounds occur in three stratigraphic levels in an overall backstepping succession. Petit-Mont
and Arche Members form the famous red and grey “marble” exploited for ornamental stone since Roman times. The evolution and
distribution of the facies in the mounds is thought to be associated with ecologic evolution and relative sea-level fluctuations.
Iron oxides exist in five forms in the Frasnian mounds; four are undoubtedly endobiotic organized structures: (1) microstromatolites
and associated forms (blisters, veils...), possibly organized in “endostromatolites”; (2) hematitic coccoids and (3) non dichotomic
filaments. The filaments resemble iron bacteria of theSphaerotilus-Leptothrix “group”; (4) networks of dichotomic filaments ascribable to fungi; (5) a red ferruginous pigment dispersed in the calcareous
matrix whose distribution is related to the mound facies type. The endobiotic forms developed during the edification of the
mounds, before cementation by fibrous calcite. The microbial precipitation of iron took place as long as the developing mounds
were bathed by water impoverished in oxygen. 相似文献
8.
Giovanna Della Porta Jeroen A. M. Kenter Juan R. Bahamonde Adrian Immenhauser Elisa Villa 《Facies》2003,49(1):175-207
Summary The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite
reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface,
Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates
and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate
platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up
to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution.
This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and
to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation
rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins.
From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated
through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid
and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the
slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures,
which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk
of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal
micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers.
From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone
with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number
from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture.
The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited
during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated
with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder,
nutrient-rich waters lifting the thermocline to depths of 80–200 m.
Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform-
and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock
falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation
of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich
lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles
are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the
angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result
of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35
m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed
to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due
to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During
the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems.
The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding
platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated
steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and
that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater. 相似文献
9.
Habitat fragmentation is a process involving splitting of continuous habitats into smaller, and more isolated habitat patches.
To assess the effects of small-scale habitat patchiness and isolation without the confounding effect of habitat loss on benthic
macrofauna, two field experiments were conducted in the Archipelago Sea, SW Finland. Using artificial seagrass units (ASUs)
we contrasted continuous patches (“C”) with fragmented patches (“F”) of the same combined area as the continuous patches.
The fragmentation treatment involved two isolation distances (0.5 and 3.0 m) between the ASUs (“F 0.5”) and (“F 3.0”). This
design was repeated in two consecutive experiments where the patch area was 0.25 and 0.0625 m2, respectively. Mobile epifauna were allowed to colonize patches for 12 days in both experiments. In both experiments, the
total epifaunal density was significantly higher in the “F 0.5” treatment than in the “C” treatment, and the three dominant
taxa showed positive or neutral responses to the habitat configuration. No fragmentation effect on the number of species was
detected in either of the experiments, but fragmentation had a negative effect on the epifaunal diversity (Shannon’s H′) in the experiment with the largest patch area. Epifaunal diversity was significantly lower in “F 3.0” treatment than in
“C” or “F 0.5” treatments in the first experiment, indicating stronger effect of isolation instead of fragmentation per se.
Edge effects were indirectly tested by comparing epifaunal densities with patch edge:area ratios. The results suggest that
edge effects may have a more important role than patch size for the total densities of epifaunal taxa, and that small, isolated
patches have equal or higher habitat value compared to larger fragments. 相似文献
10.
V. G. Kuznetsov E. A. Zhegallo L. M. Zhuravleva L. V. Zaytseva 《Paleontological Journal》2020,54(2):205-213
An ultramicroscopic study of tubiphytes—tubular formations, composed of pelitomorphic calcite from the Lower Permian Tra-Tau reef (Cis-Urals)—was carried out. A multilayer structure of the walls was recognized. The inner layer is an ultrathin layer of acicular calcite crystals, which often has aragonite habit; the next layer is a relatively thick one represented by a dense pelitomorphic mass, which is often encrusted with the calcite crystals. Between the first and second layers are traces of mineralized biofilms, “lace” secretions of mineralized glycocalyx. It is assumed the inner layer is the tubiphyte wall and the pelitomorphic one is the result of biochemogenic carbonate precipitation as a consequence of ability of epiphytic bacterial activity, which colonized the walls of an original organism. A relatively high C/Ca ratio in the glycocalyx relics, its consequent decrease from the acicular wall towards the pelitomorphic one, and finally to the purely chemogenic crystalline formations of crustified rims is an indirect proof of such a formation mechanism of tubiphytes. 相似文献
11.
Marine coccolithophorids (Haptophyceae) produce calcified scales “coccoliths” which are composed of CaCO3 and coccolith polysaccharides (CP) in the coccolith vesicles. CP was previously reported to be composed of uronic acids and
sulfated residues, etc. attached to the polymannose main chain. Although anionic polymers are generally known to play key
roles in biomineralization process, there is no experimental data how CP contributes to calcite crystal formation in the coccolithophorids.
CP used was isolated from the most abundant coccolithophorid, Emiliania huxleyi. CaCO3 crystallization experiment was performed on agar template layered onto a plastic plate that was dipped in the CaCO3 crystallization solution. The typical rhombohedral calcite crystals were formed in the absence of CP. CaCO3 crystals formed on the naked plastic plate were obviously changed to stick-like shapes when CP was present in the solution.
EBSD analysis proved that the crystal is calcite of which c-axis was elongated. CP in the solution stimulated the formation of tabular crystals with flat edge in the agarose gel. SEM
and FIB-TEM observations showed that the calcite crystals were formed in the gel. The formation of crystals without flat edge
was stimulated when CP was preliminarily added in the gel. These observations suggest that CP has two functions: namely, one
is to elongate the calcite crystal along c-axis and another is to induce tabular calcite crystal formation in the agarose gel. Thus, CP may function for the formation
of highly elaborate species-specific structures of coccoliths in coccolithophorids. 相似文献
12.
Summary The calcareous housing tubes of fossil species of the polychaete wormsDiplochaetetes andDodecaceria from the Tertiary of Lower California (Mexico) are composed, as well as the tubes of recent species ofDodecaceria, by micritic, peloidal lamellae. The wall ofDiplochaetetes spp. consists of only a few, that ofDodecaceria spp. of numerous lamellae. They form a stromatolitic fabric with intercalated lenses of fibrous calcite/aragonite. Histological
investigation of the tubes ofDodecaceria showed that the tube formation is related to two processes. The initial process is weakly controlled by the worm itself (matrix
mediated). The worm produces acidic organic mucus substances which are enriched between the soft tissue and the tube wall.
The entire mucus inhibits the mineralization of the mucus for a certain time. The mineralization events of the mucus are responsible
for the stromatolitic microfabric. Within the spaces between the primary lamellae non-specific EPS (extracellular polymeric
substances)-rich mucus is enriched, which controls the formation of fibrous aragonitic crystals and peloidal fabrics. This
is a very characteristic organomineralization process which is not controlled directly by the organism. The entire process
is comparable with those seen in complex rigid modern microbialites from salt and alkaline lakes in Western Australia and
Nevada. 相似文献
13.
Calcification processes of siliceous sponges in Viséan Limestones (Counties Sligo and Leitrim,Northwestern Ireland) 总被引:1,自引:0,他引:1
Dr. Klaus Warnke 《Facies》1995,33(1):215-227
Summary In the Lower Carboniferous limestones and shales of the Benbulben Range, Counties Sligo and Leitrim, northwestern Ireland,
a suite of carbonate nodules, about 1 to 4 cm in diameter, has been sampled and investigated by thin sectioning. The nodules
consist of micritic, peloidal and fenestral fabrics. Many of them contain relics of desma bearing demosponges and hexactinellid
sponge skeletons. The nodules are interpreted as calcified siliceous sponges. Micrite and peloids have been formed via microbial
activity during the decay of the soft sponge tissue. The actual processes are deduced from Recent examples investigated at
Lizard Island, Autralia, byReitner (1993). The skeletal opal was dissolved very early. In places where the skeleton was already embedded in micrite the spicules
are preserved as molds cemented by granular ferroan calcite. The nodules were extensively inhabited by agglutinating polychaetes
and bored by sponges. Micrite clasts have been exported to the surrounding seafloor before the sponges were completely covered
by sediments. Fenestral fabrics represent primary sponge cavities, that may be enlarged due to volume reduction of the soft
tissue during calcification. Some originated from non-calcification of decaying tissue. The granular calcite cement, filling
the fenestral fabrics, contains relics of spicules and faintly visible peloids floating unsupportedly in the cement. These
peloids were probably produced in situ by calcification of organic mucilages that filled the cavities almost entirely. It
is evident that most diagenetic processes occurring within the sponges happened on the seafloor, most likely within the still
living individuals. Possibly the nodules represent a precursor stage of mud mound development. 相似文献
14.
Summary The shallow marine subtropical Northern Bay of Safaga is composed of a complex pattern of sedimentary facies that are generally
rich in molluscs. Thirteen divertaken bulk-samples from various sites (reef slopes, sand between coral patches, muddy sand,
mud, sandy seagrass, muddy seagrass, mangrove channel) at water depths ranging from shallow subtidal to 40m were investigated
with regard to their mollusc fauna >1mm, which was separated into fragments and whole individuals.
Fragments make up more than 88% of the total mollusc remains of the samples, and their proportions correspond to characteristics
of the sedimentary facies. The whole individuals were differentiated into 622 taxa. The most common taxon,Rissoina cerithiiformis, represented more than 5% of the total mollusc content in the samples. The main part of the fauna consists of micromolluscs,
including both small adults and juveniles. Based on the results of cluster-, correspondence-, and factor analyses the fauna
was grouped into several associations, each characterizing a sedimentary facies: (1) “Rhinoclavis sordidula—Corbula erythraeensis-Pseudominolia nedyma association” characterizes mud. (2) “Microcirce sp.—Leptomyaria sp. association” characterizes muddy sand. (3)”Smaragdia spp.-Perrinia stellata—Anachis exilis—assemblage” characterizes sandy seagrass. (4) “Crenella striatissima—Rastafaria calypso—Cardiates-assemblage” characterizes muddy seagrass. (5) “Glycymeris spp.-Parvicardium sueziensis-Diala spp.-assemblage” characterizes sand between coral patches. (6) “Rissoina spp.-Triphoridae —Ostreoidea-assemblage” characterizes reef slopes. (7) “Potamides conicus—Siphonaria sp. 2—assemblage” characterizes the mangrove.
The seagrass fauna is related to those of sand between coral patches and reef slopes with respect to gastropod assemblages,
numbers of taxa and diversity indices, and to the muddy sand fauna on the basis of bivalve assemblages and feeding strategies
of bivalves. The mangrove assemblage is related to those of sand between coral patches and the reef slope with respect to
taxonomic composition and feeding strategies of bivalves, but has a strong relationship to those of the fine-grained sediments
when considering diversity indices. Reef slope assemblages are closely related to that of sand between coral patches in all
respects, except life habits of bivalves, which distincly separates the reef slope facies from all others. 相似文献
15.
Prof.Dr. Franco Russo Dr. Claudio Neri Dr. Adelaide Mastandrea Dr. Alberto Baracca 《Facies》1997,36(1):25-36
Summary The sedimentological features and the microbiofacies of the Cassian platforms (Late Ladinian-Carnian) of the Dolomites can
be studied only on the basis of the socalled “Cipit boulders”, that are platform-derived olistoliths and clasts fed to the
basin and escaped to the extensive dolomitization affecting the buildups.
Our paper deals with the Cipit boulders occurring in the Punta Grohmann section (Wengen and S. Cassiano formations, Late Ladinian,
Archelaus and Regoledanus Zones). The dominant microfacies are represented by boundstone, consisting of nearly 60% of micritic
limestone occurring both as peloidal or aphanitic micrite, mostly organized into stromatolitic laminites of thrombolites.
The skeletal organism (Tubiphytes, skeletal cyanobacteria, sphinctozoan sponges, etc.) represent only a minor component of the rock (usually less than 10%).
Early cements are widespread and consist both of fan-shaped calcite (replacing former aragonite), bladed isopachous magnesian
calcite and radial-fibrous calcite (neomorphic after Mg-calcite). The carbonate platforms from which the olistoliths derive
were made up mainly of carbonate mud that underwent early lithification, as witnessed by the considerable amount of early
cements: therefore they may be regarded to as mudmounds, and more precisely as microbial mud-mounds, due to the clearly accretionary,
organic-controlled nature of most micrites. The micrites, subdivided into auto- and allomicrite on the basis of micromorphological
and fabric characteristics, have been tested for epifluorescence. The results confirm the organic control on the deposition
of automicrite, also in the cases in which a microbial influence is not obvious (i.e. aphanitic micrite without internal organization). 相似文献
16.
Peloids are rounded grains of micritic calcite whose origin has been attributed to various biological and abiotic mechanisms. To constrain abiotic parameters that favour the formation of peloids, we precipitated calcite crusts in the absence of microorganisms. Clotted opaque fabrics that formed during the initial stages of the experiment consisted of ~10 µm peloids, while compact clear sparitic crusts precipitated in subsequent stages. The increasing supersaturation of the solution in time is responsible for this morphological succession. Initially, peloids form by the radial growth of spar crystals around a small number of nuclei. As the supersaturation increases, more spar crystals nucleate and aggregate nonradially into compact crusts. Rounded clotted precipitates are a consequence of the growth in suspension and geopetal settling, and isopachous crusts grow in the absence of these processes. Although peloids are commonly assumed to have a microbial origin, our results show that very similar morphologies can be created by purely abiotic mechanisms. Thus, the biological origin of rounded micritic calcite grains in the rock record must be verified against the abiotic null‐hypothesis in each specific case. 相似文献
17.
Michael Hautmann 《Facies》2006,52(3):417-433
The Late Triassic-Early Jurassic change from aragonite- to calcite-facilitating conditions in the oceans, which was caused by a decrease of the Mg2+/Ca2+ ratio of seawater in combination with an increase of the partial pressure of carbon dioxide, also affected the shell mineralogy of epifaunal bivalves. In the “calcite sea” of the Jurassic and Cretaceous, the most diverse and abundant families of epifaunal bivalves had largely calcitic shells. Some of them, such as the Inoceramidae, acquired this shell mineralogy earlier in Earth's history but did not significantly diversify until the onset of “calcite sea” conditions. Others, however, replaced aragonite by calcite in their shell at the beginning of the Jurassic, as shown for the Ostreidae, Gryphaeidae, Pectinidae, Plicatulidae, and Buchiidae. In these families, replacement of aragonite by calcite took place in the middle and inner layer of the shell and was not associated with changes in morphology and life habit. It is therefore proposed that lower metabolic costs rather than higher resistance against dissolution or advantageous physical properties triggered the calcite expansion in their shells. This explanation fits well the observation that clades of thin-shelled bivalves were less affected by the change of seawater chemistry. Thick-shelled clades, by contrast, may suffer a severe decline in diversity until they adapt their shell mineralogy, as demonstrated by the Hippuritoida: The diversity of the Megalodontoidea, which failed to adapt their shell mineralogy to “calcite sea” conditions, dramatically decreased at the end of the Triassic, whereas their descendents became dominant carbonate producers during the Late Mesozoic after they acquired a calcitic outer shell layer in the Late Jurassic. These examples indicate that changes in the seawater chemistry and in the partial pressure of carbon dioxide are factors that influence the diversity of carbonate-secreting animals, and, as in the case of the decline of the Megalodontoidea, may contribute to mass extinctions. 相似文献
18.
Armando Blanco Marcella D’Elia Domenico Licchelli Vincenzo Orofino Sergio Fonti 《Origins of life and evolution of the biosphere》2006,36(5-6):621-622
The evidence of the water erosion on Mars is particularly interesting since present climatic conditions are such that liquid
water cannot exist at the surface. But, if water was present on the planet in the past, there may have been life, too. Since
the discovery of carbonates on Mars also may have very important implications on the possibility that life developed there,
we are studying minerals that can have biotic or abiotic origin: calcite (CaCO3) and aragonite, a metastable state of calcite.
We have analysed biomineral aragonite, in the form of recent sea shells, as well as crystals of mineral aragonite. Infrared
spectroscopy in the 2–25 μm wavelength range reveals that, after thermal processing, the biotic samples have a different spectral behaviour from the
abiotic ones. As a result, it is possible to distinguish abiotic mineral aragonite from aragonite of recent biological origin.
Obviously, if life existed in the past on the Red Planet, we could expect to find “ancient” biotic carbonates, which should
therefore be investigated, in order to search for a way of discriminating them from abiotic minerals.
For this reason, at the beginning we have considered samples of crushed fossil shells of aragonite composition. Afterwards,
in order to take into account that fossilization processes almost always produce a transformation of metastable form (aragonite)
into more stable form (calcite), we also studied samples of mineral calcite and different types of fossils completely transformed
into calcite. All these biotic fossil samples show the same spectral behaviour as the fresh biotic material after thermal
annealing at 485°C. Instead, the calcite behaves like abiotic aragonite.
Furthermore, it is known that seashells and other biominerals are formed through an intimate association of inorganic materials
with organic macromolecules. The macromolecules control the nucleation, structure, morphology, crystal orientation and spatial
confinement of the inorganic phase: this differentiates biominerals from minerals. Analysing the aragonite or calcite fossils
with a Scanning Electron Microscope, we found that the fossilization process did not modify the structure of the biominerals
which maintain their microscopic characteristics. Looking at the morphology of fossil biominerals, it is evident that the
crystals are arranged in complex architectures compared with the compact structure of the mineral crystals.
In conclusion, the properties and structure of the biominerals are different from those of the minerals. The rapid increase
of the crystalline structure developed under biotic conditions makes these minerals less resistant to thermal treatments,
compared with samples of abiotic origin. This result holds both for recent shells as well as all fossil samples. The spectroscopic
behaviour of all analysed calcium carbonates of biotic origin is different from that of the abiotic one. Therefore, the infrared
spectroscopy is a valid technique to discern the origin of the samples and a powerful tool for analysing in-situ and “sample-return”
Mars missions specimens. Also Optical and Scanning Electron Microscopy can be useful to support this type of studies.
*Presented at: National Workshop on Astrobiology: Search for Life in the Solar System, Capri, Italy, 26 to 28 October, 2005 相似文献
19.
An emerging pattern is that population densities of generalist rodents are higher in small compared to large forest patches
in fragmented landscapes. We used genetically based measures of migration between patches to test two dispersal-based hypotheses
for this negative density-area relationship: (1) emigration rates from small patches should be relatively lower compared to
large patches (“inhibited dispersal hypothesis”), or (2) immigration rates should be higher into small than large patches
(“immigration hypothesis”). Neither hypothesis was supported using data on dispersal inferred from eight microsatellite loci
for 12 populations of Peromyscus leucopus in six small (1.3–2.7 ha) and six large (8–150 ha) forest patches. Emigration rates were not lower from and immigration rates
were not higher into small than large patches. In fact, contrary to both hypotheses, emigration rates were higher from populations
of P. leucopus in small compared to large patches. Based on a combination of genetic and field data, we speculate that higher reproduction
in smaller patches resulted in higher densities which led to higher emigration rates from those patches. Rates of reproduction
(presumably driven by better habitat conditions in smaller patches), rather than dispersal, seems to drive density differences
in forest patches. We conclude that smaller forest patches within an agricultural matrix act as a source of individuals, and
that migration rates are fairly high among forest patches regardless of size. 相似文献
20.
J. Scott Turner 《Swarm Intelligence》2011,5(1):19-43
Eusociality has evolved independently at least twice among the insects: among the Hymenoptera (ants and bees), and earlier
among the Isoptera (termites). Studies of swarm intelligence, and by inference, swarm cognition, have focused largely on the
bees and ants, while the termites have been relatively neglected. Yet, termites are among the world’s premier animal architects,
and this betokens a sophisticated swarm intelligence capability. In this article, I review new findings on the workings of
the mound of Macrotermes which clarify how these remarkable structures work, and how they come to be built. Swarm cognition in these termites is in
the form of “extended” cognition, whereby the swarm’s cognitive abilities arise both from interaction amongst the individual
agents within a swarm, and from the interaction of the swarm with the environment, mediated by the mound’s dynamic architecture.
The latter provides large scale “cognitive maps” which enable termite swarms to assess the functional state of their structure
and to guide repair efforts where necessary. The crucial role of the built environment in termite swarm cognition also points
to certain “swarm cognitive disorders”, where swarms can be pushed into anomalous activities by manipulating crucial structural
and functional attributes of the termite system of “extended cognition.” 相似文献