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Famennian microbial reef facies, Napier and Oscar Ranges, Canning Basin, western Australia 总被引：5，自引：0，他引：5
Following the Frasnian–Famennian mass extinction, which eliminated most skeletal reef-building fauna, the early Famennian reefs of the Canning Basin were constructed primarily by reef-framework microbial communities. In the Napier and Oscar Ranges, the Famennian reef complexes had high-energy, reef-flat depositional environments on a reef-rimmed platform that transitioned into low-energy, deep-water reefs growing in excess of 50 m below sea level. High-energy, reef-flat depositional environments contain doming fenestral stromatolites that grade into porous thrombolites and are associated with coarse-grained sandstones and grainstones. The reef-margin subfacies contains mounds of microdigitate thrombolites, which are more delicate than the reef-flat thrombolites and locally contain abundant red algae, Girvanella, renalcids and sediment-filled tubes. Within the thrombolites, the red algae are in upright growth positions, suggesting that the thrombolites are largely composed of carbonate that precipitated in situ. Reefal-slope environments are dominated by Wetheredella and Rothpletzella with locally abundant Girvanella, renalcids and Uralinella. In reefal-slope strata, delicate fans and microdigitate stromatolites of Wetheredella and Rothpletzella are often oriented horizontal or diagonal to bedding and are interpreted as syndepositionally toppled over. Most mesoscale microbial community structures contain several species of microbial fossils, and no single microbial species appears to have controlled the morphology of the community structure. Therefore, the depositional environment must have determined the distribution and morphology of the stromatolites, thrombolites and other microbial community structures. The adaptability of microbial communities to various reef environments allowed them to fill ecological niches opportunistically after the Frasnian–Famennian mass extinction. 相似文献
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Thrombolite fabrics and origins: Influences of diverse microbial and metazoan processes on Cambrian thrombolite variability in the Great Basin,California and Nevada 
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下载免费PDF全文 Thrombolites are a common component of carbonate buildups throughout the Phanerozoic. Although they are usually described as microbialites with an internally clotted texture, a wide range of thrombolite textures have been observed and attributed to diverse processes, leading to difficulty interpreting thrombolites as a group. Interpreting thrombolitic textures in terms of ancient ecosystems requires understanding of diverse processes, specifically those due to microbial growth and metazoan activity. Many of these processes are reflected in thrombolites in the Cambrian Carrara, Bonanza King, Highland Peak and Nopah formations, Great Basin, California, USA; they comprise eight thrombolite classes based on variable arrangements and combinations of depositional and diagenetic components. Four thrombolite classes (hemispherical microdigitate, bushy, coalescent columnar and massive fenestrated) contain distinct mesoscale microbial growth structures that can be distinguished from surrounding detrital sediments and diagenetic features. By contrast, mottled thrombolites have mesostructures that dominantly reflect post‐depositional processes, including bioturbation. Mottled thrombolites are not bioturbated stromatolites, but rather formed from disruption of an originally clotted growth structure. Three thrombolite classes (arborescent digitate, amoeboid and massive) contain more cryptic textures. All eight of the thrombolite classes in this study formed in similar Cambrian depositional environments (marine passive margin). Overall, this suite of thrombolites demonstrates that thrombolites are diverse, in both internal fabrics and origin, and that clotted and patchy microbialite fabrics form from a range of processes. The diversity of textures and their origins demonstrate that thrombolites should not be used to interpret a particular ecological, evolutionary or environmental shift without first identifying the microbial growth structure and distinguishing it from other depositional, post‐depositional and diagenetic components. Furthermore, thrombolites are fundamentally different from stromatolites and dendrolites in which the laminae and dendroids reflect a primary growth structure, because clotted textures in thrombolites do not always reflect a primary microbial growth structure. 相似文献
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塔里木盆地阿克苏地区寒武系第二统第三阶肖尔布拉克组主要由微生物白云岩组成,代表性的剖面在阿克苏市西南90 km处的苏盖特布拉克。由于成岩作用改造严重,对这套微生物碳酸盐岩的特征和形成环境的认识还存在很多分歧,制约了勘探工作。通过详细野外勘察和室内研究,将肖尔布拉克组微生物岩分为4种结构类型: 凝块结构、纹层结构、砂屑结构和骨架结构。首次对凝块结构和纹层结构进行了亚类型的划分,其中凝块结构可划分为蠕虫状、网状和斑点状3种亚类型,纹层结构可划分为致密纹层、短薄纹层、颗粒纹层以及单纹层、纹层组、复合纹层。肖Ⅰ段—肖Ⅲ段发育凝块石白云岩、凝块—层纹石白云岩和层纹石白云岩,肖Ⅳ段发育凝块石微生物丘,肖V段下部发育网状结构凝块石白云岩,肖V段上部发育砂屑白云岩和肾形菌骨架岩。白云石化作用、溶蚀作用、重结晶作用是改变肖尔布拉克组微生物岩结构最重要的成岩作用,且该组下部比上部遭受了更强的成岩作用改造;微生物岩结构对成岩作用改造的抵抗能力为: 砂屑结构>纹层结构>凝块结构。根据微生物岩结构,推测肖Ⅰ段—肖Ⅲ段形成于潮坪环境,肖Ⅳ段和肖V段下部形成于深水潮下环境,肖V段上部形成于浅水潮下环境。以上成果为认识塔里木盆地肖尔布拉克组微生物岩的平面分布规律和今后开展被成岩作用强烈改造的白云岩型微生物岩的研究提供了一个重要参考。 相似文献
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STEVE KERSHAW SYLVIE CRASQUIN MARIE‐BÉATRICE FOREL CARINE RANDON PIERRE‐YVES COLLIN ERDAL KOSUN SYLVAIN RICHOZ AYMON BAUD 《Sedimentology》2011,58(3):739-755
The Permian–Triassic Boundary sequence at Çürük Dag, near Antalya, Turkey, begins with a major erosion surface interpreted as being the Late Permian lowstand, on which lies ca 0·4 m of grainstone/packstone composed of ooids, peloids and bioclasts. Most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present. The erosion surface is smooth and shows no evidence of dissolution; the grainstone/packstone contains intraclasts of the underlying wackestone, proving erosion. Next are 15 m of microbialite comprised of interbedded stromatolites, thrombolites, plus beds of planar limestones with small‐scale erosion. The latter comprise a complex interlayering of stromatolitic, thrombolitic and peloidal fabrics and precipitated crystal fans, which form a hybrid of microbialite and inorganic carbonate, together with bioclastic debris and micrite. The Çürük Dag microbialite sequence is repetitious; the lower part is more complex, with abundant stromatolites and hybrid microbialites. Some of the stromatolites are themselves hybrids composed of peloids and crystal fans. In the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form. The domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones/packstones are deposited. Repetition of facies may be controlled by sea‐level change; a deepening‐up model is consistent with the evidence. Stromatolites (with abundant crystal fans) dominate in shallower water, deepening through hybrid microbialite and interlayered sediments to thrombolite, probably no more than a few tens of metres deep, followed by breaks and renewal of microbialite growth. An interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified Bairdioid ostracods in the microbialite, a group known to be open marine. However, other researchers have proposed low oxygen conditions for Permian–Triassic boundary facies globally, so work continues to confirm whether the Çürük Dag microbialite grew in dysoxic or normally oxygenated conditions. The principal stimulus for post‐extinction microbialites is likely to be carbonate supersaturation of the oceans. The microbialite sequence is overlain by a further 25 m of grainstone/packstone (without microbialite), followed by Early Triassic shales. Overall, microbialites form a thin aggradational sequence during an overall relative sea‐level rise, consistent with global eustatic rise following the Late Permian lowstand. 相似文献
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微生物岩的原始定义指底栖微生物主导形成的沉积体或岩石体。笔者对该概念进行了扩充,认为微生物岩除了包括叠层石、凝块石、纹层石、核形石、均一石之外,还应该包括微生物骨架岩、微生物粘结岩、非钙化浮游或漂浮微生物形成的模铸岩、矿化浮游或漂浮微生物形成的颗粒岩和泥粒岩。P-T界线地层微生物岩的特征是具有由较粗矿物晶体(主要是方解石,其次是白云石)组成的斑点状、树枝状、网状结构的灰岩。这3种结构分别称为斑点状体、树枝状体、网状体,在露头上呈暗色,在薄片中呈浅色,一般由无定形的亮晶充填体和其间的微亮晶组成。亮晶充填体是指无定形的孔洞被不同成岩期形成的矿物充填形成的结构体,因成岩作用各异,造成不同层位、不同地点的亮晶充填体内部的矿物类型和充填顺序存在差异;所有的亮晶充填体都不具有壁,故不是钙化化石。通过形态、大小和生态比较,以及形成演化分析,认为亮晶充填体的前身是漂浮蓝细菌微囊菌,胶鞘是微囊菌形成模铸化石的关键因素。亮晶充填体是表层水漂浮生活的微囊菌沉入海底后,被泥晶沉积物掩埋或者被早期海底胶结物胶结,在泥晶沉积物半固结或固结之后腐烂留下的孔洞被后期成岩作用形成的矿物充填形成的。P-T界线地层微生物岩段顶部遭受成岩作用程度高,树枝状体和网状体中的亮晶充填体的轮廓基本都被破坏,变成微亮晶和亮晶,以前被学者解释为凝块石;但斑点状体、树枝状体、网状体是成岩流体沿着亮晶充填体或其他化石丰富的地方运移形成较粗的晶体而造成的,并不符合凝块石的定义。同时,少数学者把树枝状体本身当成底栖生物,也是没有充分认识树枝状体的矿物组成而做的解释。该微生物岩段含有钙化的小球状化石和同心层状化石,但它们不是组成微生物岩的主体。 相似文献
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Tufa domes and towers are common around the margins of Winnemucca Dry Lake, Nevada, USA, a desiccated sub‐basin of pluvial Lake Lahontan. A 2·5 m diameter concentrically‐layered tufa mound from the southern end of the playa was sampled along its growth axis to determine timing, rate and geochemical conditions of tufa growth. A radiocarbon‐based age model indicates an 8200‐year tufa depositional record that begins near the end of the Last Glacial Maximum (ca 23 400 cal yr bp ) and concludes at the end of the most recent Lahontan highstand (ca 15 200 cal yr bp ). Petrography, stable isotopes and major and minor elemental compositions are used to evaluate the rate and timing of tufa growth in the context of the depositional environment. The deposit built radially outward from a central nucleation point, with six decimetre‐scale layers defined by variations in texture. Two distinct tufa types are observed: the inner section is composed of two layers of thinolite pseudomorphs after ikaite, with the innermost layer comprised of very small pseudomorphs (<0·25 cm) and an outer layer composed of larger, ca 3 cm long pseudomorphs, followed by a transitional layer where thinolite pseudomorphs grade into calcite fans. The outer section consists of three distinct layers of thrombolitic micrite with a branching mesofabric. The textural change occurred as lake levels began to rise towards the most recent Lahontan highstand interval and probably was prompted by warming of lake waters caused by increased groundwater flux during highstand lake levels. The Mg/Ca and Sr/Ca variations suggest a warming trend in the tufa growth environment and may also reflect increasing growth rates of tufa associated with increased fluxes of groundwater. This systematic study of tufa deposition indicates the importance of the hydrology of the lacustrine tufa system for reconstructing palaeoenvironmental records, and particularly the interaction of ground and surface waters. 相似文献
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鄂尔多斯盆地奥陶系马家沟组广泛发育岩溶角砾岩,结构特征复杂。近期研究发现,这些角砾岩的组构和分布特征难以能单纯地用经典的风化壳岩溶模式解释,各类角砾岩的成因机理值得进一步探究。文中基于大量岩心及薄片观察,对苏里格气田东区奥陶系马家沟组上组合不同层位角砾岩进行归纳和判识,明确其空间展布和序列结构特征,分析形成机理。研究发现: (1)根据成因与发育环境,苏格里气田东区奥陶系马家沟组上组合角砾岩可划分为5类,分别是近原地解离角砾岩(B1)、近地表堆积角砾岩(B2)、洞穴堆积角砾岩(B3)、洞顶(壁)碎裂角砾岩(B4)和坍塌角砾岩(B5),它们具有纵向多层叠置、横向可对比的“准层状”特征; (2)不同类型角砾岩的纵向配置和演化与周期性的向上变浅沉积序列和暴露溶蚀有关,共归纳为3种类型,即B1主发育型、B2-B3-B4组合发育型、B5主发育型; (3)准层状角砾岩形成于早成岩期,受控于古地貌起伏背景下的高频海平面变化,由暴露溶蚀透镜体横向迁移连片和纵向多旋回叠置而成。研究结果不仅有助于了解鄂尔多斯盆地马家沟组多样化的岩溶角砾岩特征与成因,也为白云岩早成岩期岩溶研究提供了新的素材。 相似文献