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1.
《Journal of African Earth Sciences》2007,47(3):121-134
The Neoproterozoic Zerrissene Turbidite Complex of central-western Namibia comprises five turbiditic units. From the base to the top they are the Zebrapüts Formation (greywacke and pelite), Brandberg West Formation (marble and pelite), Brak River Formation (greywacke and pelite with dropstones), Gemsbok River Formation (marble and pelite) and Amis River Formation (greywacke and pelites with rare carbonates and quartz-wacke).In the Lower Ugab River valley, five siliciclastic facies were recognised in the Brak River Formation. These are massive and laminated sandstones, classical turbidites (thick- and thin-bedded), mudrock, rare conglomerate and breccia. For the carbonate Gemsbok River Formation four facies were identified including massive non-graded and graded calcarenite, fine grained evenly bedded blue marble and calcareous mudrock. Most of these facies are also present in the other siliciclastic units of the Zerrissene Turbidite Complex as observed in other areas.The vertical facies association of the siliciclastic Brak River Formation is interpreted as representing sheet sand lobe to lobe-fringe palaeoenvironment with the abandonment of siliciclastic deposition at the top of the succession. The vertical facies association of the carbonate Gemsbok Formation is interpreted as the slope apron succession overlain by periplatform facies, suggesting a carbonate slope sedimentation of a prograding depositional shelf margin.If the siliciclastic–carbonate paired succession would represent a lowstand relative sea-level and highstand relative sea-level, respectively, the entire turbidite succession of the Zerrissene Turbidite Complex can be interpreted as three depositional sequences including two paired siliciclastic–carbonate units (Zebrapüts-Brandberg West formations; Brak River–Gemsbok formations) and an incomplete succession without carbonate at the top (Amis River Formation). 相似文献
2.
Turbiditic and non-turbiditic mudstone of Cretaceous flysch sections of the East Alps and other basins 总被引：4，自引：0，他引：4
REINHARD HESSE 《Sedimentology》1975,22(3):387-416
Recognition of the occurrence and extent of hemipelagic and pelagic deposits in turbidite sequences is of considerable importance for environmental analysis (palaeodepth, circulation, distance from land, hemipelagic or pelagic versus turbidite sedimentation rates) of ancient basins. Differentiation between the finegrained parts (E-division) of turbidites and the (hemi-) pelagic layers (F-division of turbidite-pelagite alternations) is facilitated in basins where carbonate turbidites were deposited below the carbonate compensation depth (CCD) such as the Flysch Zone of the East Alps but may be difficult in other basins where less compositional contrast is developed between the fine-grained turbidites and hemipelagites. This difficulty pertains particularly in Palaeozoic and older basins. For Late Mesozoic-Cenozoic oceans with a relatively deep calcite compensation level three other types of turbidite basins may be distinguished for which differentiation becomes increasingly more difficult in the sequence from (1) to (3): (1) terrigenous turbidite basins above the CCD; (2) carbonate turbidite basins above the CCD; (3) terrigenous turbidite basins below the CCD. Criteria and methods useful for the differentiation between turbiditic and hemipelagic mudstone in the Upper Cretaceous of the Flysch Zone of the East Alps include calcium carbonate content, colour, sequential analysis, distribution of bioturbation, and microfaunal content. In modern turbidite basins clay mineral content, organic matter content, plant fragments, and grain-size (graded bedding, maximum grain diameter) have reportedly also been used as criteria (see Table 3). Deposition of muddy sediment by turbidity currents on weakly sloping sea bottoms such as the distal parts of deep-sea fans or abyssal plains is not only feasible but may lead to the accumulation of thick layers. Contrary to earlier speculation it can be explained by the hydrodynamic theory of turbidity currents, if temperature differences between the turbidity current and the ambient deep water as well as relatively high current velocities for the deposition of turbiditic muds (an order of magnitude higher on mud surfaces than commonly assumed) are taken into consideration. The former add to the capacity of turbidity currents to carry muddy sediment without creating a driving force on a low slope. 相似文献
3.
Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings. 相似文献
4.
High-frequency cyclicity (Milankovitch and millennial-scale) in slope-apron carbonates: Zechstein (Upper Permian), North-east England 总被引：2，自引：0，他引：2
The Upper Permian (Zechstein) slope carbonates in the Roker Formation (Zechstein 2nd‐cycle Carbonate) in North‐east England consist of turbidites interbedded with laminated lime‐mudstone. Studies of turbidite bed thickness and relative proportion of turbidites (percentage turbidites in 20 cm of section) reveal well‐developed cyclicities consisting of thinning‐upward and thickening‐upward packages of turbidite beds. These packages are on four scales, from less than a metre, up to 50 m in thickness. Assuming that the laminae of the hemipelagic background sediment are annual allows the durations of the cycles to be estimated. In addition, counting the number and thickness of turbidite beds in 20 cm of laminated lime‐mudstone, which is approximately equivalent to 1000 years (each lamina is 200 μm), gives the frequencies of the turbidite beds, the average thicknesses and the overall sedimentation rates through the succession for 1000 year time‐slots. Figures obtained are comparable with modern rates of deposition on carbonate slopes. The cyclicity present in the Roker Formation can be shown to include: Milankovitch‐band ca 100 kyr short‐eccentricity, ca 20 kyr precession and ca 10 kyr semi‐precession cycles and sub‐Milankovitch millennial‐scale cycles (0·7 to 4·3 kyr). Eccentricity and precession‐scale cycles are related to ‘highstand‐shedding’ and relative sea‐level change caused by Milankovitch‐band orbital forcing controlling carbonate productivity. The millennial‐scale cycles, which are quasi‐periodic, probably are produced by environmental changes controlled by solar forcing, i.e. variations in solar irradiance, or volcanic activity. Most probable here are fluctuations in carbonate productivity related to aridity–humidity and/or temperature changes. Precession and millennial‐scale cycles are defined most strongly in early transgressive and highstand parts of the larger‐scale short‐eccentricity cycles. The duration of the Roker Formation as a whole can be estimated from the thickness of the laminated lithotype as ca 0·3 Myr. 相似文献
5.
MAREE CORKERON 《Sedimentology》2007,54(4):871-903
The term ‘cap carbonate’ is commonly used to describe carbonate units associated with glacigenic deposits in Neoproterozoic successions. Attempts to use carbonate units as stratigraphic markers have been counfounded by inconsistent identification of ‘cap carbonates’ and a somewhat broad use of the term. Systematic sedimentological and geochemical analysis of carbonate rocks (mostly dolomite) associated with glacigenic deposits from the Neoproterozoic succession of the Kimberley region, north‐western Australia, shows that it is possible to characterize such units by their specific mineralogical, sedimentological, petrographic, geochemical and stratigraphic features. Hence, it is possible to differentiate true ‘cap carbonates’ from other carbonate units that are associated with glacigenic deposits. In the Kimberley successions two broad carbonate types are identified that reflect two stratigraphically distinct depositional realms. Carbonate rocks from the Egan Formation and Boonall Dolomite (the youngest carbonate units in the succession) are characterized by sedimentary components and features that are consistent with deposition on shallow platforms or shelves, analogous to Phanerozoic warm‐water carbonate platform deposits. In contrast, dolomite from the Walsh, Landrigan and Moonlight Valley Tillites preserves a suite of sedimentary and geochemical characteristics that are distinctly different from Phanerozoic‐like carbonate rocks; they are thin (ca 6 m), laterally persistent units of thinly laminated dolomicrite/dolomicrospar recording δ13C fluctuations from −1‰ to −5‰. These latter features are consistent with a ‘Marinoan‐style cap‐carbonate’ rock described from other Neoproterozoic successions. The similarity and broad distribution of these rocks in Australia, when considered within the context of genetic models suggesting a global oceanographic–atmospheric event, support their use as a lithostratigraphic marker horizon for the start of the Ediacaran Period at ca 635 Ma. 相似文献
6.
The 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium- to fine-grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone-shale; (LF2) laminated-homogenous mudstone; (LF3) calcareous grainstone-shale; (LF4) laminated mudstone-grainstone; and (LF5) laminated calcisiltite-grainstone. The sequence reflects deposition on a low-energy, muddy, carbonate to argillaceous ramp subject to short-lived, episodic high-energy storms. These events produced fining-upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium-grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low-energy, fairweather mudstones and calcareous shales. Deposition progressed from a carbonate mud-dominated ramp in the Becscie Formation to an argillaceous mud-dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south-east into the Iapetus Ocean and shallowed towards a SW—NE-orientated shoreline to the north-west. Vertical tempestite proximality trends and lithofacies changes identify third-order eustatic sea-level changes. After an initial deepening at the base of the formation, a shallowing-deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods. 相似文献
7.
8.
Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.Editorial handling: N. White 相似文献
9.
沉积于中中新世到上新世的浊积砂体是第三纪Niigata弧后盆地的主要油气储层之一 ,该盆地是日本列岛最富产石油的盆地。我们对这些浊积砂体的重矿物进行了详细和系统地分析。首先 ,作者展示了将重矿物分析和沉积作用分析相结合 ,从而得出有关沉积过程的全新观点的实例。沿着位于盆地东缘的上新世早期的浊积砂岩体的垂向和横向取样分析表明 ,砂体东半部重矿物的成分和西半部重矿物的成分不同。重矿物和沉积分析结果表明 ,发育在东半部的浅海陆架浊积砂岩和发育在西半部的深海水下浊积体在上新世早期是相互叠置的。其次 ,作者检验了用重矿物成分作为指标来表征某个特定浊积体的有效性。为此目的特选重位于Ni igata盆地中部的许多浊积砂岩分析研究 ,得出以下结论。 ( 1)基于主要重矿物 ,比如蛋白石、普通角闪石、紫苏辉石和普通辉石的定量综合分析 ,至少可将研究区的浊积砂岩划分为 6种类型 ;( 2 )一个组或段的浊积砂岩包括一种类型或紧密相关的两种类型 ;( 3)研究区不同的组或段通常由不同的重矿物组成构成 ,不同的物源或相同物源下不同的地质事件 (例如火山活动开始期 )可能是这种差异的原因。重矿物分析不仅可作为有价值的常规手段估计物源方向 ,而且可同其他的沉积资料结合来解析沉积过程 ,估计几个相邻 相似文献
10.
甘肃夏河麻隆沟二叠纪毛毛隆组沉积特征及环境分析 总被引：3，自引：2，他引：3
本文详细分析了该组沉积物的物理特征、地球化学特征、以及古生物特征,认为毛毛隆组是一套具有斜坡沉积特征的沉积物,是在沉积盆地不断裂陷的环境中形成的. 相似文献
11.
鄂尔多斯盆地延长组湖相黏土岩分类和沉积环境探讨 总被引：1，自引：0，他引：1
鄂尔多斯盆地三叠系延长组长7油层组沉积了区域分布的大套湖相黏土岩,它不但是鄂尔多斯盆地中生界最重要的烃源岩,同时也是目前致密油/页岩油勘探的主要领域.作者在对国内外黏土岩分类及其成因机制等系统调研基础上,通过对工区25口连续取芯井的岩芯描述和400余块典型黏土岩薄片观察,并充分应用X衍射、有机地球化学等分析测试资料,提出了湖相黏土岩的分类方案与基本特征,探讨了不同类型黏土岩形成的沉积环境.根据岩石组分、沉积构造等特征,鄂尔多斯盆地延长组长7油层组黏土岩主要发育5种类型:①块状泥岩;②粒序层理泥岩;③波状纹层页岩;④平直纹层页岩;⑤似块状页岩.该成因分类方案将黏土岩类型与沉积环境相结合.指出受三角洲前缘影响的浅湖区以块状泥岩为主;受湖流和波浪影响的浅湖—半深湖环境以波状纹层页岩为主;深湖静水区以平直纹层状页岩为主;深湖坳陷区以粒序层理泥岩为主;火山灰爆发时期以似块状页岩为主.粒序层理泥岩是致密油赋存的主要类型,有机碳平均含量为6%;似块状页岩是页岩油赋存的主要类型,有机碳平均含量为17.17%. 相似文献
12.
陕西银硐子-大西沟菱铁银多金属矿床热水沉积岩相特征及成因 总被引：13，自引：2，他引：11
陕西银硐子 -大西沟特大型菱铁银多金属矿床赋存于中泥盆世吉维特期之上的热水岩相中,其微相可划分为热水同生沉积 -沸腾交代微相、热水同生交代微相、热水同生沉积微相、热卤水渗滤交代微相、热水混合沉积微相。热水沉积岩富Na、Si、Fe、Mg、Mn、Ba、Cu、Pb、Zn、As、Ag、B等。在热水沉积盆地中,由富Ba的硫酸质热水、富Fe碳酸质热水以快速化学沉淀方式发生同生沉积成矿作用形成重晶石、菱铁矿层。银多金属矿层是由不同成分、性状、富Na、B、Si的铝硅酸质、硼酸质热水与前两类热水相互混合而形成热水混合体系,这种非平衡体系发生了剧烈地化学反应而使矿质发生骤沉。 相似文献
13.
Sediment in tectonically active, topographically restricted settings of the western Hellenic Arc, eastern Mediterranean, consists primarily of clayey silt and silty clay. Failure of metastable sediment temporarily stored on relatively steep slopes is triggered by earthquake tremors and eustatic oscillations. Redeposition of these materials by gravitative transport has resulted in markedly different lithofacies from site to site. Most piston cores include three Late Quaternary stratigraphic units that can be correlated with sections in other parts of the eastern Mediterranean; numerous radiocarbon-age determinations enhance the correlation. Seven fine-grained sediment types are identified in cores from eight distinct depositional environments. Some muds are closely related to specific environments (slump and debris flow deposits on slope and high-relief environments), or to time (well laminated mud during the latest Pleistocene-mid-Holocene), or to both (uniform and faintly laminated muds restricted to trench basins). Turbiditic and hemipelagic muds are common throughout the study area. Mud distribution patterns correlate closely with calculated sedimentation rates. We propose two depositional models for these sediments. The first emphasizes downslope transformations resulting in progressively reduced flow concentration during transport: from slump and debris flow–>turbidity current–>low density turbidity current or turbid layer mechanisms. The distal end-member deposits settling from low concentration flows are thick, rapidly emplaced, fine-grained uniform muds closely associated with faintly laminated muds. These were ponded in flat trench basin-plains. Planktonic and terrigenous fractions in the turbiditic, finely laminated and uniform muds record mixing of materials of gravitative and suspension origin during redeposition. This sequence prevails under conditions of minimal stratification of water masses, as characterized by the present Mediterranean. In the second model developed for conditions of well-developed water mass stratification, well laminated rather than uniform mud prevails as the end product of low concentration flows. These very finely laminated and graded muds record particle-by-particle settling from detached turbid layers concentrated along density interfaces; they include material from turbid layers complemented by the normal ‘rain’ of pelagic material. Stratification barriers resulted in region-wide distribution of such deposits, in both slope and trench environments. 相似文献
14.
Michael Ed. Hohn Edward B. Nuhfer Robert J. Vinopal David S. Klanderman 《Mathematical Geology》1980,12(6):589-606
Classifying very fine-grained rocks through fabric elements provides information about depositional environments, but is subject to the biases of visual taxonomy. To evaluate the statistical significance of an empirical classification of very fine-grained rocks, samples from Devonian shales in four cored wells in West Virginia and Virginia were measured for 15 variables: quartz, illite, pyrite and expandable clays determined by X-ray diffraction; total sulfur, organic content, inorganic carbon, matrix density, bulk density, porosity, silt, as well as density, sonic travel time, resistivity, and -ray response measured from well logs. The four lithologic types comprised: (1) sharply banded shale, (2) thinly laminated shale, (3) lenticularly laminated shale, and (4) nonbanded shale. Univariate and multivariate analyses of variance showed that the lithologic classification reflects significant differences for the variables measured, difference that can be detected independently of stratigraphic effects. Little-known statistical methods found useful in this work included: the multivariate analysis of variance with more than one effect, simultaneous plotting of samples and variables on canonical variates, and the use of parametric ANOVA and MANOVA on ranked data. 相似文献
15.
Although the Permian–Triassic Semanggol Formation is widely distributed in northwestern Peninsula Malaysia and is made of various lithofacies, its sedimentology and possible relation with the Permian–Triassic boundary (PTB) were not considered before. In this study, detailed facies analysis was conducted for two sections of the Semanggol Formation at the Bukit Kukus and Baling areas, South Kedah to clarify its sedimentology and relation to the PTB. Four facies from the Permian part of the Semanggol Formation that were identified at the Bukit Kukus section include laminated black mudstone, interbedded mudstone and sandstone, volcanogenic sediments, and bedded chert. In Baling area, the Triassic part of the formation is classified into three members. The lower member comprises of claystone and bedded chert facies, while the middle member is composed of sandstone and claystone interbeds (rhythmite). On the other hand, the upper member is grouped into two main units. The lower unit is mainly claystone and includes two facies: the varve-like laminated silt and clay and massive black claystone. The upper unit is composed of various sandstone lithofacies ranging from hummocky cross stratified (HCS) sandstone to thinly laminated sandstone to burrowed sandstone facies. The HCS sandstones occur as two units of fine-grained poorly sorted sandstone with clay lenses as flaser structure and are separated by a hard iron crust. They also show coarse grains of lag deposits at their bases. The laminated black mudstone at the lowermost part of the Semanggol Formation represents a reducing and quite conditions, which is most probably below the fairweather wave base in offshore environment that changed upwards into a fining upward sequence of tide environment. Abundance of chert beds in the volcanogenic sediments suggests the deposition of tuffs and volcanic ashes in deep marine setting which continues to form the Permian pelagic bedded chert and claystone. The bedded chert in the lower member of the Triassic section suggests its formation in deep marine conditions. The rhythmic sandstone and claystone interbeds of the middle member are suggestive for its formation as a distal fan of a turbidite sequence. Lithology and primary sedimentary structure of the upper member suggest its deposition in environments range from deep marine represented by the varve-like laminated silt and clay to subtidal environment corresponds to the massive black claystone to coastal environment represented by the hummocky sandstone units and reaches the maximum regression at the hiatus surface. Another cycle of transgression can be indicated from the second hummocky unit with transgressive lag deposits that develops to relatively deeper conditions as indicated from the formation of relatively thick laminated sandstone and bioturbated massive sandstone facies that represent tidal and subtidal environment, respectively. Late Permian lithological variation from the radiolarian chert into early Triassic claystone probably resulted from a decrease in productivity of radiolarians and might represent a PTB in the Semanggol Formation. Volcanogenic sediments in the studied section can be used as an evidence for volcanic activities at the end of the Permian, which is probably connected to the nearby volcanic ash layers in the eastern China, the ultimate cause of the PTB in this area. Black mudstone in the Permian part of the studied section may be interrelated to the Latest Permian Anoxia that started to build in the deep ocean well before the event on shallow shelves. 相似文献
16.
深湖相泥岩的成因类型和组合演化 总被引：10，自引：4，他引：10
本文通过对渤海湾地区东营盆地沙三段的研究,区分了深湖相泥岩的岩相类型。根据各类岩相的结构、沉积构造和层序、有机地球化学、元素地球化学、矿物成分、生物组合等特征对深湖相泥岩的成因类型进行了划分,并讨论了各成因系列的沉积作用。在此基础上分析了湖盆发展不同阶段由于盆地地形、湖水性质、骨架沉积体系的改变及深湖相泥岩成因类型组合的演化特征。 相似文献
17.
Heterozoan carbonate deposition on a steep basement escarpment (Late Miocene,Almería,south‐east Spain) 
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下载免费PDF全文 Heterozoan temperate‐water carbonates mixed with varying amounts of terrigenous grains and muddy matrix (Azagador limestone) accumulated on and at the toe of an inherited escarpment during the late Tortonian–early Messinian (late Miocene) at the western margin of the Almería–Níjar Basin in south‐east Spain. The escarpment was the eastern end of an uplifting antiform created by compressive folding of Triassic rocks of the Betic basement. Channelized coralline‐algal/bryozoan rudstone to coarse‐grained packstone, together with matrix‐supported conglomerate, are the dominant lithofacies in the higher outcrops, comprising the deposits on the slope. These sediments mainly fill small canyon‐shaped, half‐graben depressions formed by normal faults active before, during and after carbonate sedimentation. Roughly bedded and roughly laminated coralline‐algal/bryozoan rudstone to coarse‐grained packstone are the main lithofacies forming an apron of four small (kilometre‐scale) lobes at the toe of the south‐eastern side of the escarpment (Almería area). Channelized and roughly bedded coralline‐algal/bryozoan rudstone to coarse‐grained packstone, conglomerates, packstone and sandy silt accumulated in a small channel‐lobe system at the toe of the north‐eastern side of the escarpment (Las Balsas area). Carbonate particles and terrigenous grains were sourced from shallow‐water settings and displaced downslope by sediment density flows that preferentially followed the canyon‐shaped depressions. Roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope, whereas the rest of the carbonate lithofacies were deposited by high‐density turbidite currents. The steep escarpment and related break‐in‐slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse‐grained, low‐transport efficiency skeletal‐dominated sediment in the apron lobes. Accelerated uplift of the basement caused a relative sea‐level fall resulting in the formation of outer‐ramp carbonates on the apron lobes, which were in turn overlain by lower Messinian coral reefs. The Almería example is the first known ‘base of slope’ apron within temperate‐water carbonate systems. 相似文献
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In Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro- and cryptocrystalline quartz, with local chalcedonic quartz (both length-fast and length-slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro- and cryptocrystalline quartz, length-slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg-rich environment. Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica-rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert. 相似文献
20.
The weight-percent values of four mineralogic variables (quartz, K feldspar, color index, and muscovite) for 10 sets of granitic rocks (20–50 samples in each set) from magmatic units of the Singhbhum granite were used for (1) computation of the Mahalanobis' generalized distance functions (D
2) between all pairs of the 10 sets, (2) testing significance of the difference between the multivariate means, and (3) computation of the linear discriminant functions between all possible pairs of the sets. The 10 data sets are for six magmatic units which belong to three successive but closely related phases of emplacement. The multivariate means for all sets are significantly different except for those between two of the sets of phase I. Cluster analysis on the basis of theD
2 values enables the 10 sets to be placed into four distinct groups. Group A includes two subgroups, one of which consists of two sets representing typical members of phase I; the other subgroup includes two sets which are typical of phase II. Group B includes two sets which are typical of phase III. The other four sets do not group with the typical representatives of the three phases, probably because of certain special conditions of their emplacement. A separate series ofD
2 computation from the same data, but excluding the color index, was unsuccessful in making the four aberrant sets group with the typical members of the respective phases. Efficient LDF's could be determined for discrimination between most pairs of the 10 sets of granite rocks. 相似文献