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1.
Pyrite replacement of mollusc shells from the Lower Oxford Clay (Jurassic) of England 总被引：1，自引：0，他引：1
IAN ST JOHN FISHER 《Sedimentology》1986,33(4):575-585
Replacement of originally aragonite mollusc shells by pyrite commonly occurs in the Lower Oxford Clay. Petrographic studies show the shells to have constituted complex microenvironments in the sediment. A range of replacement textures is found showing a variable amount of solution of the original aragonite. Three distinct textures were found in crushed pyrite-replaced ammonite shells from heavily pyritized concretions. (1) A texture reflecting the original shell structure due to the replacement of the organic shell-matrix by pyrite. (2) An ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction. (3) Euhedral crystals lining cracks and fractures in the shell. Three types of replacement are found in small gastropods and bivalves from shell bed, some of which may relate to those seen in the ammonites. (1) Replacement of organic shell-matrix by pyrite preserving good shell-microstructure. (2) Replacement showing outwardly good preservation of morphological features but inwardly only the gross structure, such as growth lines, is preserved. (3) Replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell. The replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell. Early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell, with little or no solution of the carbonate, this produces textures which faithfully mimic the original shell microstructure. It is thought that the lack of carbonate solution is due to a limited availability of iron, brought about by the less intensively reducing nature of the sediment. Later stages of replacement are promoted by the cracking and fracturing of the shell and are, generally, not as faithful to the original shell structure. This is due to the greater availability of iron as the sediment becomes more reducing with burial. 相似文献
2.
为了分析含气陆相页岩中石英矿物的成因、分布及其影响因素,以四川盆地自流井组含气页岩层段为主要研究对象,利用X射线衍射分析、普通薄片、扫描电镜、阴极发光、电子探针及元素地球化学分析测试手段,识别出陆源石英和成岩过程中形成的石英。陆源石英在阴极光下表现为棕色光,粒径较大。成岩过程中形成的石英在阴极发光下不发光,晶形较好,粒径较小。根据成岩作用的不同,将成岩过程中形成的石英细分为黏土矿物转化过程中形成的石英和硅质交代介壳边缘形成的石英。分析不同成因的石英分布特征发现,东岳庙段内石英矿物主要为陆源输入和黏土转化成因;马鞍山段内的石英主要为陆源石英;大安寨段内的石英则为硅质交代自生和陆源输入成因。在此基础上,总结了陆相页岩中不同成因石英的影响因素,包括沉积环境和地层压力。在沉积环境方面,随陆源物质输入的增加,陆源石英增加。同时,适合的湖水盐度,有利于介壳生物的生存,为硅质交代介壳形成石英提供物质基础。此外,地层的异常高压会抑制黏土转化,减少硅质析出,阻碍自生石英的沉淀。 相似文献
3.
Shell aragonite from ammonites collected in the Upper Cretaceous of West Greenland was investigated by means of macroscopic/microscopic visual evaluation, analyses of calcite/aragonite ratios, carbon and oxygen isotopic compositions and Sr and Mg concentrations of shell carbonate and of amino acid compositions of organic matrices. The results are: (1) Material visually classified as well preserved may have suffered diagenetic modifications of mineralogical and chemical composition. (2) Of the chemical and mineralogical parameters studied, amino acid composition, calcite/aragonite ratios and magnesium concentrations were found to be most sensitive to post-depositional modifications, while oxygen isotope composition and strontium concentrations showed detectable diagenetic modifications only after more pronounced alterations. (3) Based on the Mg/Ca ratios and calcite concentrations of the shell aragonite, a diagenetic classification has been proposed grouping the material into well preserved, moderately preserved and poorly preserved. (4) The chemical and mineralogical composition of the best preserved material suggests that the Upper Cretaceous ammonites had a shell composition similar to that of modern Nautilus and other aragonite-shelled molluscs. 相似文献
4.
We have modified our equations of motion for sinking Nautilusshells to a more general form suitable for application to fossil cephalopods. The new equations incorporate the effects of hydrodynamic stability and loss of buoyancy due either to the unrestricted entry of water into the shell during sinking, or to entry by diffusion across the wall of the siphuncular tube. With our new equations it is possible to calculate sinking velocity and pressure across the shell wall as a function of depth for shells of any size and shape. Our system provides a means of analyzing several aspects of the post-mortem history of cephalopod shells including vertical preservation. In the latter case, our equations enable us to find water depth from the geometry of a vertically preserved shell. We calculate the maximum water depth of the Hauptmuschelkalk beds (Triassic, south-central Germany) to have been about 3 m. Our method is unique in providing a way of obtaining numerical values for maximum water depth of ancient sediments and sedimentary environments. These equations also offer the possibility of examining the paleoecology and paleobiology of the living animals, especially with respect to swimming ability and vertical migration. 相似文献
5.
《Proceedings of the Geologists' Association. Geologists' Association》2023,134(3):322-328
Locally within the Charmouth Mudstone Formation (Sinemurian, Lower Jurassic) of the Dorset coast fatally bitten ammonites with ventral bite marks are common. Ventral bite marks occur at the rear of the body-chamber. The predator probably severed attachment muscles, thus removing its prey more easily. We report the first ventrally bitten fossil nautiloid, which is unusual in that the large bite mark is entirely within the phragmocone. The nautiloid is juvenile with a very short body-chamber extending 46° back from the apparently undamaged aperture. The whorl cross-section is nearly circular, giving the predator little to grip when biting. Nautiloid shells are also thicker than ammonite shells. Despite these difficulties, the predator bit repeatedly and damaged both the shell and septa. Five rounded bites occur on the left side, separated by sharp points. This suggests the predator was probably another cephalopod with suckered arms and powerful jaws. The centre of the bite is 160.5° back from the aperture, close to the mean value for ammonites (184.5°), suggesting the predator located the position of the first bite opposite the victim's aperture. Thus, we think the predator attacked an incomplete nautiloid shell which explains why the damage affects only the phragmocone. 相似文献
6.
Marie Grace Pamela G. Faylona Claire E. Lazareth Anne‐Marie Smah Sandrine Caquineau Hugues Boucher Wilfredo P. Ronquillo 《Geoarchaeology》2011,26(6):888-901
Tridacnidae shells, a valuable archive of past environments, are common in the Balobok Rockshelter archaeological site on Sanga‐Sanga Island in the south Philippines. This site was occupied during the mid‐Holocene (ca. 5000–8800 14C yr B.P.), a period of Neolithic cultural expansion in the Philippines. This paper focuses on the preservation of two shell specimens, Hippopus hippopus and Tridacna maxima, unearthed from two mid‐Holocene layers within the rockshelter. The shells' mineralogy and microstructure (prismatic and crossed‐lamellar) were studied using micro‐Fourier transform infrared spectroscopy and scanning electron microscopy to determine if the samples were suitable as paleoenvironmental records. Both shells are still aragonitic but aragonite crystals of both microstructure types are partly dissolved. This dissolution, characteristic of meteoric water alteration, precludes their utility in paleoenvironmental geochemical studies. Nevertheless, these shells are abundant in archaeological sites in the region and may be better preserved in other depositional contexts; more studies on Philippine Tridacnidae shell diagenesis are needed. © 2011 Wiley Periodicals, Inc. 相似文献
7.
A pyroxene-amphibole lamprophyre of Permian age intersecting Precambrian gneiss, etc., contains domains of pyroxenite breccia and of closely packed single- and multi-shelled orbicules. The multi-shelled orbicules have inner medium-grained, radial and outer aphanitic, laminated shells. The outer shells are explained as having formed by direct precipitation from the magma. The inner shells most probably underwent a recrystallization at magmatic temperatures, their original composition being like that of the outer shells. Shells of single-shelled orbicules are similar to the inner shell of the multi-shelled ones and contain relicts of aphanitic, laminated material.The cores, composed of Permian pyroxenite or more rarely Precambrian pegmatite quartz, were originally formed by volcanic fragmentation of the country rock. 相似文献
8.
A. B. Edwards 《International Journal of Earth Sciences》1959,47(2):668-682
Summary Recent exploration in northern Australia has discovered three shallow water oolitic iron formations, one of Permian age, and two of Upper Proterozoic age. The Permian formation resembles the oxidized oolitic chamositic formations of south-eastern England, but contains more detrital quartz and felspar.The two Precambrian iron formations, consist, below the zone of weathering, of oolites of hematite and of a chamosite-like substance, in a siderite cement, together with well-rounded quartz grains, and are interlayered with sandstones with a chamosite-like cement. Within the zone of weathering the siderite and chamosite are more or less completely replaced by quartz, but with preservation in detail of original textures.Published by permission of the Commonwealth Scientific and Industrial Research Organization (CSIRO). 相似文献
9.
Elena Dunca Harry Mutvei Peter Göransson Carl-Magnus Mörth Bernd R. Schöne Martin J. Whitehouse Mikael Elfman Susanne P. Baden 《International Journal of Earth Sciences》2009,98(1):3-17
Shells of Arctica islandica collected between 1884 and 2004 from Öresund, Kattegat and Skagerrak (Swedish West Coast) were used to monitor local climate variations and the influence of human activities on the local environment. For this purpose, we analysed the growth, structure and chemical composition of these shells and compared them with shells collected from Kiel Bay, Norway and Iceland. The growth rate of the studied shells registers an NAO periodicity of ca 8 years. However, the observed signal is weak because of other environmental interferences that are either of natural or anthropogenic origin. For example, the oxygen isotope ratios show temperature fluctuation, but also the influx of low salinity water. Higher contents of S, N, Cu, Zn, As, Cd and P in shell portions formed during the last century are related to human activities such as mining and industrial development. Our study indicates that in order to use Arctica shells as archives of climate change it is necessary to study the full range of environmental data that is recorded in the shells by using a multi element and isotope approach in combination with different analytical techniques including investigation of growth rates and shell structure. 相似文献
10.
Isao Sarashina Yoshiki Kunitomo Minoru Iijima Satoshi Chiba Kazuyoshi Endo 《Organic Geochemistry》2008,39(12):1742
Organic molecules such as proteins can be preserved in certain fossils. The bulk properties of fossil proteins of both vertebrates and invertebrates have been studied for over half a century. Named proteins have so far been identified, however, only in vertebrate fossils, such as collagen from mammoth bones. Using immunological assays, we examined 1500 year old fossils of the extinct land snail Mandarina luhuana from the Bonin islands for the presence of dermatopontin, a molluscan shell matrix protein. First, we examined the shell microstructure and mineralogy of the fossil shells using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD) in order to estimate the extent of diagenetic alteration. The results suggest that the original microstructure and mineralogy of the shells are preserved. Antiserum raised against the Type-1 dermatopontin fragment of the living land snail Euhadra brandtii showed significant immunological reactivity with the extracts from the fossil shells of M. luhuana. Immunological binding curves drawn for the shell extracts of extant M. aureola and the extinct M. luhuana confirmed the presence of dermatopontin in the fossil shells and provided an estimate that about 75–98% of the original dermatopontin was lost from the M. luhuana fossils. This is the first report of a named protein being identified in invertebrate fossils. 相似文献
11.
12.
在上水桥金、萤石矿区的含矿硅化破碎带中,新发现一种围绕断层磨砾呈放射状生长的石英晶簇,因其像有刺、有壳、有仁的板栗,作者将其命名为板栗状石英晶簇。板栗状石英晶簇可能是硅质围绕悬浮于高比重热液中的断层磨砾生长形成,反映当时热液中可能含有高浓度的重金属化合物。 相似文献
13.
The peculiar taphonomy of the late Campanian ammonite Metaplacenticeras subtilistriatum is reconstructed on the basis of stratigraphic occurrence within the Hakobuchi Formation of northern Hokkaido (Japan), as well as of host rock and patterns of fragmentation. The species' taphonomy is closely linked to its streamlined, oxyconic shell with flat flanks and a smoothly tapered venter. Such shells were accumulated, together with numerous pebbles, by bottom currents (>0.25–0.5 m/s), in such a way that ventral portions of some shells fractured by collision with pebbles. Small holes and depressions in the external shell surface, presumably formed during compaction after final burial, are restricted to phragmocones. On this evidence, it is suggested that this species was nektobenthic, lived in a shallow environment (shoreface–inner shelf) where its streamlined shell allowed it to align with prevailing currents, similar to a wind vane. 相似文献
14.
A collection of subfossil specimens of Discinisca tenuis from Lüderlitz, Namibia was studied by means of the X-ray diffraction (XRD) technique. Using the whole-pattern fitting procedure refined by the authors, the unit cell dimensions, crystallite dimensions, and number of strains were detected. Post-mortality alterations in shell apatite are related to mineral component content ranging from 69 to 82% in subfossil shells, as compared to 68% in shells of living species. It has been established that the lattice parameter a decreases from 9.380 to 9.370 Å, while parameter c is less involved during the decomposition of organic matter. The post-mortality alteration is assumed to proceed by the subsequent recrystallization of shell apatite. Consequently, primarily elongated hexagonal prisms (150 × 370 Å) turn to smaller ones (120 × 260 Å), and the number of strains tends to increase. 相似文献
15.
Acanthoceratid ammonites from near Maceió, in the State of Alagoas, provide evidence of a mid-Cretaceous marine incursion into the present onshore part of the Alagoas Basin. The ammonites Pseudocalycoceras sp. cf. P. harpax (Stoliczka, 1864) and Kamerunoceras sp. are assigned a late Cenomanian age. The rock is a siliceous oolite that occurs as derived nodules and fragments in late Cenozoic continental sediments. It is interpreted as an originally calcareous oolite formed in a near-shore, high-energy environment. Absence of primary cement suggests that silicification took place early in diagenesis. The silicified, and therefore more resistant material from the original Cenomanian sequence was reworked and redeposited with the Cenozoic sediments. The age, indicated by the ammonites, suggests that the original deposition was related to the global late Cenomanian—early Turonian sea-level rise. 相似文献
16.
在扬子陆块核部黄陵地区发现具球状构造的花岗闪长岩。岩石产于新元古代斑状花岗闪长岩与石英闪长岩之间,出露面积极小。球状花岗闪长岩岩体由主岩、球状体及球间基质三部分组成,球状体呈圆形、椭圆形及不规则状,由球核和同心球壳组成,球体大小多为5~12cm。依据球壳及球核的差异,可大致分为具有多壳层同心结构、具单壳层结构、无明显壳层结构3种类型的球状体。单壳层球壳由角闪石、黑云母等暗色矿物含量高的暗色层组成,具放射状构造;多壳层球壳由含角闪石、黑云母等暗色矿物的暗色层和基本不含或含少量暗色矿物、以斜长石为主的浅色层组成韵律层,具梳状、放射状构造;球核成分可以是同源岩浆组分,也可以是异源捕虏体。单壳层球体,球核、球壳、基质的斜长石牌号依次为36、32、26,酸性程度不断增加,内部呈放射状生长,反映其可能由内向外经岩浆结晶作用形成。初步认为多壳层球体球壳富韵律的变化为富含气水热液的岩浆在动荡的环境下结晶形成,球壳中矿物结晶顺序可能由外向内,核部结晶时间相对更长。 相似文献
17.
J. D. HUDSON 《Sedimentology》1982,29(5):639-667
Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from ?55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near ?20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale. 相似文献
18.
宣龙铁矿的铁质鲕粒由各种不同的核心和微层状外壳组成。核心的组分主要是石英砂粒、赤铁矿内碎屑和凝块以及粒状菱铁矿等。外壳的纹层分别由显微结构不同的板状、片状、均匀和不均匀粒状泥晶赤铁矿以及少量菱铁矿组成。其中板伏、片状、均匀粒状赤铁矿和菱铁矿主要是以化学或生物化学方式在核心表层逐层沉积结果;不等粒状赤铁矿微层则是鲕粒滚动中对铁质颗粒粘附而成。菌藻类生长繁殖改变沉积环境的物化性质,对铁质沉积起着重要作用。 相似文献
19.
The most complete Hungarian Valanginian perisphinctid ammonite fauna of the Mecsek Mountains (South Hungary) consists of 14 species is reported. Thurmanniceras sp. aff. otopeta, Fuhriella michaelis, F. cf. hoheneggeri, Sarasinella cf. ambigua, Neocomites (Neocomites) subtenuis, Neocomites (Eristavites) platycostatus are reported for the first time from Hungary. The fauna comprises 23 ammonite species and is reported from Valanginian tuffaceous marl, alternating marl–limestone beds and loose limestone blocks. Two studied sections represent the Lower Valanginian Thurmanniceras pertransiens and partly the Busnardoites campylotoxus Zones while ammonites from loose blocks indicate the latter and possibly the Upper Valanginian Saynoceras verrucosum Zone. Variations in the faunal composition of the marl beds and the limestone blocks respectively, refer to the contrast between the ammonite ecological demands. Phylloceratid and lytoceratid ammonites prevail the marl beds, while olcostephanid and neocomitid ammonites prevail the shallower platform-like limestones. Sediment accumulation rate in that palaeo volcano related environment was high and bottom currents frequently washed together ammonite shells or filled their body chambers with smaller ammonite shell fragments. Due to the fast sedimentation the sections represent only partially the recognized ammonite zones therefore long-term or even infra-regional correlation was not possible. Palaeobiogeographically, the ammonite fauna has Mediterranean character and it shows close relationships to the Valanginian faunas of the Bakony Mts. (Hungary), Western Carpathians (Slovakia), Northern Calcareous Alps (Austria), and SE Spain. The presence of Fuhriella species is remarkable and enlarges our knowledge on the distribution of this enigmatic ammonite taxon adding new data to its Valanginian stratigraphic position and geographic distribution. 相似文献
20.
Metabolic effects on stable carbon isotopic composition of freshwater bivalve shell Corbicula fluminea 总被引：1，自引：0，他引：1
The stable isotopic composition of the bivalve shell has been widely used to reconstruct the pa-laeo-climate and palaeo-environment. The climatic and environmental significance of carbon isotopic composition of the bivalve shell is still in dispute, and incorporation of metabolic carbon can obscure carbon isotope records of dis-solved inorganic carbon. This study deals with freshwater bivalve, Corbicula fluminea aragonite shell. The results indicated that the δ13C values of bivalve shells deposited out of equilibrium with the host water and showed an onto-genic decrease, indicating that there are metabolic effects and more metabolic carbon is incorporated into larger shells. The proportion of metabolic carbon of shells varies between 19.8% and 26.8%. However, δ13CS can still be used as qualitative indicators of δ13CDIC and environmental processes that occurred during shell growth. 相似文献