共查询到20条相似文献，搜索用时 31 毫秒
1.
为探讨陆相断陷湖盆陡坡带构造活动控制下水下粗碎屑岩沉积特征、搬运机制及其演化规律,以滦平断陷盆地陡坡带下白垩统西瓜园组为研究对象,采用无人机倾斜摄影、实测剖面、砾石定向性定量表征等技术方法,从沉积背景、岩相类型、沉积单元及相序特征等方面开展野外露头解剖工作.滦平盆地西瓜园组沉积时期,近岸水下扇沉积于构造沉降速率大、湖平面上升、深水、古地貌陡峭环境,洪水携带粗碎屑沉积沿陡坡带入湖,底部发育与地震活动相关的砾质碎屑流,伴随发育滑动—滑塌沉积,上部发育高密度浊流.随着沉积物不断供给,斜坡坡度逐渐减小;随着粗碎屑沉积搬运距离不断增加,砂砾质碎屑流中砾石表现出明显定向性,高密度浊流所占厚度比例增加;末端以低密度浊流为主.扇三角洲沉积于构造沉降速率相对较低、水深相对较浅、古地貌相对平缓的环境,发育相对成熟的供源体系,汇水系统长度较长,扇三角洲前缘粗碎屑岩由碎屑流向高密度浊流、牵引流、低密度浊流转换. 相似文献
2.
三角洲冲淤具有空间异质性,而沉积物的磁性特征可灵敏地反映物源、沉积动力特征以及早期成岩作用影响,是揭示空间异质性的有效方法。选取长江水下三角洲20~35 m水深的四个约2 m长的柱样进行磁学和碳、硫地球化学分析,结合粒度及年代资料,探讨了长江水下三角洲沉积物柱样磁性特征空间差异及其冲淤指示意义。研究结果显示,粒度表征的动力分选影响沉积物的磁性特征,这在年代较新（<350 a）的柱样中表现尤为明显,即细颗粒沉积物中富集亚铁磁性矿物,从而具有较高的磁化率。铁、碳、硫及硫同位素特征揭示了长江水下三角洲沉积物较弱的硫酸盐还原特征。沉积物年龄及沉积速率影响沉积物成岩改造强度,进而导致了磁性特征垂向变化的空间差异,整体上看,远离现代沉积中心的柱样,成岩改造特征更为明显。磁化率值的大小、成岩分带的完整性及其反映的铁还原带和硫酸盐还原带转换深度,一定程度上可以揭示三角洲的淤积和侵蚀特征。 相似文献
3.
《Sedimentology》2018,65(6):2117-2148
The origin of the fourth member of the Eocene Shahejie Formation in the northern steep slopes of the Minfeng Sub‐sag, Dongying Sag, China, was investigated by integrating core studies and flume tank depositional simulations. A non‐channelized depositional model is proposed in this paper for nearshore subaqueous fans in steep fault‐controlled slopes of lacustrine rift basins. The deposits of nearshore subaqueous fans along the base of steep border‐fault slopes of rift basins are typically composed of deep‐water coarse‐grained sediment gravity‐flow deposits directly sourced from adjacent footwalls. Sedimentation processes of nearshore subaqueous fans respond to tectonic activities of boundary faults and to seasonal rainfall. During tectonically active stages, subaqueous debris flows triggered by episodic movements of border‐faults dominate the sedimentation. During tectonically quiescent stages, hyperpycnal flows generated by seasonal rainfall‐generated floods, normal discharges of mountain‐derived rivers and deep‐lacustrine suspension sedimentation are commonly present. The results of a series of flume tank depositional simulations show that the sediments deposited by subaqueous debris flows are wedge‐shaped and non‐channelized, whereas the sediments deposited by hyperpycnal flows generated by sporadic floods from seasonal rainfall are characterized by non‐channelized, coarse‐grained lobate depositional bodies which switch laterally because of compensation sedimentation of hyperpycanal flows. The hyperpycnal‐flow‐deposited non‐channelized lobate depositional bodies can be divided into a main body and lateral edges. The main body can be further subdivided into a proximal part, middle part and frontal part. Normal mountain‐derived river‐discharge‐deposited sediments are characterized by thin‐bedded, fine‐grained sandstones and siltstones with a limited distribution range. Normal mountain‐derived river‐discharge‐deposited sediments and deep‐lacustrine mudstones are commonly eroded in the area close to boundary faults. A nearshore subaqueous fan can be divided into three segments: inner fan, middle fan and outer fan. The inner fan is composed of debrites and the proximal part of the main body. The middle fan consists of the middle part of the main body and lateral edges, normal mountain‐derived river‐discharge‐deposited fine‐grained sediments and deep‐lacustrine mudstones. The outer fan comprises the frontal part of the main body, lateral edges, and deep‐lacustrine mudstones. Based on the non‐channelized depositional model for nearshore subaqueous fans, criteria for stratigraphic subdivision and correlation are discussed and applied. 相似文献
4.
鄂尔多斯盆地神木地区太原组是在北隆南倾的古地形背景下形成的以浅水三角洲为主的充填沉积。携带沉积物的河流进入海水后,由于河水与海水之间存在着较大的密度差异、侧向扩散较少,其三角洲前缘沿着海底继续向前快速推进,使水下分流河道延伸较远。研究区地形坡度平缓、水体浅,三角洲平原向前推进并进一步降低了地形坡度,从而减弱了携带沉积物的流体的动能,使得大部分沉积物在三角洲平原的分流河道中沉积下来。同时因水体浅,河口坝、席状砂等前缘沉积物常遭受进积的水下分流河道的冲刷和侵蚀而难以保存。研究区三角洲平原分流河道沉积极为发育,前三角洲相对不发育,三角洲前缘也以水下分流河道沉积为主。分流河道、水下分流河道常对下伏沉积物强烈冲刷,切割先期的沉积物乃至包括海相沉积物在内的深水沉积物。在三角洲废弃期,三角洲前缘沉积物常被潮汐作用改造。三角洲平原分流河道及三角洲前缘水下分流河道砂体呈带状分布,是天然气勘探的有利目标。 相似文献
5.
Ancient sand-rich submarine fans: depositional systems, models, identification, and analysis 总被引：1，自引：0，他引：1
Sand-rich submarine fans are radial or curved in plan view depending on the slope of the basin floor. They occur isolated or in coalescing systems. The fans' average lateral extent measures close to 25 km and their thickness usually less than 300 m. The thickness of outer fan sequences averages around 120 m and that of middle fan successions around 160 m. Rarely reported inner fan sequences have a maximum thickness of 80 m.
The formation of sand-rich fans is closely related to tectonic activity. Their sediment is coarse-grained and compositionally immature as indicated by significant feldspar content due to close provenance and rapid transport by short rivers with a steep gradient controlled by tectonism. Tectonic activity also provides for narrow shelves making the fans relatively insensitive to sealevel changes. Formation of sand-rich fans typically occurs in restricted continental basins. The tectonic settings are highly variable. Sand-rich fans typically receive their sediment through submarine canyons which intercept sand from longshore drift and/or are fed more or less directly by regional rivers.
The type of ancient fan system (radial, curved, isolated, coalescing) may be identified through paleocurrent map plots, facies map sketches, recognition of lateral thickness variations and sediment influx centers, as well as lateral bed correlations defining the minimum fan extent.
Important in distinguishing different environments of ancient fans are detailed measured sections, their comparison and correlation. Channelized inner fan and middle fan deposits may be distinguished from the unchannelized outer fan successions through bed correlation tests which reflect their different stratigraphic architectures and bedding patterns. Bedding in outer fan deposits (lobes) is relatively simple, parallel, and regular. The lateral bed continuity is relatively high. Channel fills, especially those of middle fan distributary channels, display a complicated bedding pattern with vertical and lateral random distribution of channel fills, axial erosion, and bed convergence towards the channel margins. Channel fills exhibit only linear bed continuity. Thus, the probability in carrying out local to regional scale lateral bed correlations is almost exclusively limited to outer fan deposits.
The measured sections will help further distinguish fan environments by revealing: (1) different facies associations in outer fan sequences (mainly B, C and D) and middle fan successions (mainly A, B, C, D, and channel margin facies); (2) greater average bed and layer thicknesses in middle fan as opposed to outer fan successions (“bed” and “layer” as used herein); (3) more frequent amalgamation surfaces in channel fills than in unchannelized outer fan deposits; (4) more frequent tabular amalgamation surfaces in outer fan sections; (5) more frequent nontabular amalgamation surfaces in channel fills; and (6) more frequent dish structures in middle fan than outer fan successions.
Rarely exposed fan valley fills may be identified by coarse conglomerates. Moreover, in proximity to fan valley fills, relatively mud-rich sediments may be observed that derive from the depositional system of the basin slope. 相似文献
6.
本文根据露头、岩心、测井、薄片鉴定等资料,对孙吴-嘉荫盆地白垩系淘淇河组水下重力流沉积特征进行了详细的研究,认为淘淇河组水下重力流沉积规模大,为湖底扇沉积,其可进一步划分为内扇、中扇、外扇3种亚相以及主沟道、主沟道外、辫状沟道、辫状沟道间、远源浊积岩5种微相类型。内扇以水下泥石流沉积为主,中扇以液化沉积物流和浊流沉积为主,外扇发育浊积岩。根据地震剖面推断,白垩系淘淇河组时期发生的断层活动是湖底扇沉积发育的主要原因。 相似文献
7.
南大港构造带下降盘沙河街组沙三段岩石类型为砾岩、砂岩、粉砂岩及泥岩组合。岩心中发育块状构造、交错层理、变形层理等重力流和牵引流沉积构造,炭屑、植物茎干、同生断层、泥质砂岩团块等现象也常见。粒度概率累计曲线为一段式和二段式,测井曲线可分为箱形、钟形、漏斗形和平直形4类。地震剖面上也可识别出水下扇的反射特征。综合岩性、粒度、电性和地震反射特征,明确了南大港构造带发育水下扇沉积,并可进一步分为内扇主水道、中扇分支水道、中扇水道间、水道前端朵叶体、水下天然堤及决口扇6种微相。自沙河街组沙三段沉积开始,南大港断层活动强烈,在五断块形成了远源水下扇;而在歧南9X1断块发育近源水下扇。两种类型的水下扇在物源、岩性、电性、地震反射特征和形态上都表现出不同特征。近源水下扇以扇形为主,规模小而数量多,以古隆起物源、沉积物粒度粗、电性整体箱形、地震反射杂乱振幅强为特征;远源水下扇以朵叶形为主,规模大而数量少,以三角洲前缘物源、沉积物粒度细、电性整体钟形、地震反射杂乱振幅弱为特征。 相似文献
8.
The fan-delta reservoir play has become an important exploration target within the Junggar Basin, especially in the Mabei area within the Mahu Sag, where a fan-delta oil and gas pool has been recently discovered. The sedimentary characteristics, distribution patterns and formation mechanisms of the fan-delta clastic bodies in Lower Triassic Baikouquan Formation (T1b) were studied using seismic, well log and core data accompanied by a flume tank experiment and a modern analogue depositional study. The T1b in the Mabei area is composed of a fan delta consisting of fan-delta plain (including debris flow deposits, sheetflood deposits, braided channel deposits, and floodplain deposits), fan-delta front (including subaqueous reworked sheetflood deposits and distal sheetflood deposits) and muddy lake deposits. The sheetflood deposits, characterised by moderately sorted conglomerates with brown matrix, form during the peak flooding. They are widespread and sheet-like, occupying the major portion of the fan-delta plain. The braided channel deposits are characterised by well-sorted and clast-supported conglomerates and pebbly sandstones, formed later during the falling flow stage. Owing to the decline in volume and velocity, and the formation of continuously braided fluid flow, the sediments of the flood period are reworked, generating the braided channels. Laterally, the braided channels occur as belts of clastic bodies surrounded by continuous sheetflood deposits. Vertically, the braided channels are interbedded between the sheetflood deposits. The subaqueous reworked sheetflood deposits are characterised by greyish-green, well-sorted and clast-supported conglomerates, whereas the distal sheetflood deposits are characterised by well-sorted sandstones, intercalated with mudstone. The subaqueous reworked sheetflood and distal sheetflood deposits are a distal partly subaqueous extension of the main sheetflood deposits, albeit reworked by basinal currents and waves. The distal sheetflood deposits form on distal fringes of the sheetfloods and are more thoroughly reworked by longshore and wave currents. The braided channel, subaqueous reworked sheetflood and distal sheetflood deposits can form high-porosity reservoirs. These findings challenge the common view and suggest that the channelised facies on the fans are not the main flood events; rather, the more extensive sheetfloods are the major flood events. 相似文献
9.
柴达木盆地处于古亚洲构造域和特提斯－喜马拉雅构造域的结合部，构造应力大而复杂，导致盆内地势起伏大，加上西南暖湿气流受喜马拉雅山系阻隔难以进入境内，盆内气候干旱，最终导致盆地内冲积扇极为发育。通过对大柴旦地区大头羊煤矿、鱼卡河、波门河和八里沟四个冲积扇的实地考察，共观测到3个亚相8个微相：扇根亚相沉积物最粗，分为古沟道、主水道和主水道间微相；扇中亚相沉积物偏细，成熟度增高，分为辫状水道、辫状水道间和纵坝微相；扇缘亚相沉积物最细，流体能量最低，分为水道径流和片流微相。不同沉积微相其沉积特征差异较大，认为古沟道、主水道和辫状水道微相具有较好的储集性能。勘探表明，冲积扇沉积与储层有着密切的关系，其内形成的油藏具有“自我保护”的能力；另外，冲积扇的形成很可能导致上覆地层形成扇背斜油藏，也可能导致下伏基岩形成基岩风化壳油藏。 相似文献
10.
Deposits of depletive high-density turbidity currents: a flume analogue of bed geometry, structure and texture 总被引：1，自引：0，他引：1
Flume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport. 相似文献
11.
Geochemical characteristics of fine and coarse fractions of sediments in the Yarlung Zangbo River Basin (southern Tibet,China) 总被引：1，自引：0，他引：1
Shisong Du Yongqiu Wu Lihua Tan Wenmin Huang Chengzhi Hao 《Environmental Earth Sciences》2018,77(9):337
Tracing sources of aeolian sediment is key to reconstructing earth surface processes in arid areas and interpreting the paleoenvironmental significance of aeolian sequences. However, the difference of geochemical characteristics between different fractions of sediments in the Yarlung Zangbo River Basin (YZRB) is still unclear, so we want to identify which fraction of sediments is more suitable for tracing sediment sources. Considering the long distance between different wide valleys in the YZRB, we wonder whether there is any difference between these wide valleys in terms of geochemical characteristics of sediments. Forty-three surface sediment samples in the YZRB are collected, and the grain-size distributions and the major-element composition for 37 samples and the trace and rare earth element composition for the coarse (75–500 μm) and fine (<?75 μm) fractions of all samples are determined. The results reveal the following: (1) the fine fractions of the deposits contain more environmental information, suggesting that the fine fractions cannot be directly compared between different climate zones for provenance identification and that appropriate coarse fractions, based on the grain-size distribution of the targeted sediments, are more ideal for tracing sediment sources; and (2) geochemical characteristics of various sediment types show spatial heterogeneity. The coarse and fine fractions of the loose sediment samples can be divided into two regional groups based on geochemical characteristics: the Maquanhe zone in the upper reach, the Xigaze, Shannan and Mainling zone in the middle reach, which is consistent with the geological background of the YZRB; (3) aeolian deposits in the YZRB are a local origin and predominantly derived from the adjacent loose sediments, and fluvial sediments in the upper reach contribute little to the aeolian sands in the middle reach. 相似文献
12.
PHILIP J. ASHWORTH GREGORY H. SAMBROOK SMITH JAMES L. BEST JOHN S. BRIDGE STUART N. LANE IAN. A. LUNT ARNOLD J. H. REESINK CHRISTOPHER J. SIMPSON ROBERT E. THOMAS 《Sedimentology》2011,58(7):1860-1883
The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture. 相似文献
13.
14.
以珠江口盆地白云凹陷钻井、测井、地震和古生物资料为基础,结合前人研究成果,系统的分析了白云凹陷晚渐新统到早中新统沉积相发育特征及凹陷沉积充填演化过程。研究结果表明,珠海组下部发育大型陆架边缘三角洲沉积,地震反射特征表明该三角洲发育三期;钻遇水下分支河道、支流间湾、河口坝及远砂坝微相,沉积物以富砂为特征,发育冲刷-充填构造、递变层理及较粗的水平潜穴等多种构造;珠海组上部为浅海相,沉积物以海相砂泥岩互层为主,此时期陆架坡折带位于白云凹陷南坡;珠江组沉积时期,海平面升降旋回频繁,陆架坡折带迁至凹陷北坡。随着古珠江流域扩大,沉积物输入量增加,在珠江组下部发育了深水扇,沉积物以细-粗砂岩为主,夹少量粉砂岩及深海泥岩,发育颗粒流、液化流、浊流及碎屑流等四种主要的重力流,钻遇内扇水道,中扇废弃水道及水道间漫溢沉积,外扇深海泥沉积;珠江组上部为半深海相,沉积物以深海泥岩为主。 相似文献
15.
北部湾盆地涠西南凹陷WZ油田古近系流沙港组一段是主要含油层系,实钻表明储集层成因复杂、厚度变化大、非均质性强,致使砂体分布预测困难。通过精细的岩心观察,结合各类测井、录井及岩心分析化验资料综合研究,认为流一段属陡坡型扇三角洲沉积,主要发育扇三角洲前缘亚相,其中夹杂有丰富的重力流沉积。扇三角洲前缘亚相主要由水下分流河道、河口坝、远砂坝和河道间沉积微相组成;重力流沉积包括重力滑塌沉积、碎屑流沉积、浊流沉积及液化流沉积,规模均较小。在深入的单井沉积微相分析和多井对比基础上,编制了流一段主力Ⅳ油组和Ⅱ油组的沉积微相平面图,展示出研究区总体沉积格局为稳定的扇三角洲前缘环境,物源充分,主要来自北部和西北部,因斜坡带较陡,在扇三角洲前缘至前扇三角洲环境发育丰富的各类重力流沉积。根据各沉积微相的特征与展布,建立了本区较深水陡坡型扇三角洲沉积模式。分析了不同微相砂体的物性与含油性,认为扇三角洲前缘水下分流河道砂体最好,其次是河口坝砂体,远砂坝和远端浊积砂体相对较差。提出了有利的挖潜区域和调整井位,实钻遇油层数十米,为油田增储上产发挥了重要作用。 相似文献
16.
17.
利用高分辨率三维地震资料分析了孟加拉扇上扇某区块的深水沉积体特征。发现了六种典型地震相:下切充填相、海鸥翼状相、楔形发散相、透明相、强振幅平行相和弱振幅平行相。下切充填相代表粗粒的水道充填沉积;海鸥翼状相代表水道-天然堤复合体;楔形发散相代表细粒的天然堤沉积;透明相代表块体搬运沉积;强振幅平行相代表朵叶体沉积;弱振幅平行相代表深海披覆沉积。通过对典型地震剖面特征和平面属性分布分析可知:上扇垂向上依次沉积了朵叶体、水道和堤岸、朵叶体、水道-天然堤复合体。 相似文献
18.
19.
B. CHARLOTTE SCHREIBER GERALD M. FRIEDMAN† ARVEDO DECIMA‡ EDWARD SCHREIBER 《Sedimentology》1976,23(6):729-760
In Sicily, Messinian evaporitic sedimentary deposits are developed under a wide variety of hypersaline conditions and in environments ranging from continental margin (subaerial), to basin-margin supratidal, to intertidal, to subtidal and out into the hypersaline basin proper. The actual water depth at the time of deposition is indeterminate; however, relative terms such as ‘wave base’ and ‘photic zone’ are utilized. The inter-fingering relationships of specific evaporitic facies having clear and recognizable physical characteristics are presented. These include sub-aerial deposits of nodular calcium sulphate formed displacively within clastic sediments; gypsiferous rudites, arenites and arenitic marls, all of which are reworked sediments and are mixed in varying degrees with other clastic materials (subaerial, supratidal, and intertidal to deep basinal deposits). Laminated calcium sulphate alternating with very thin carbonate interlaminae and having two different aspects; one being even and continuous and the other of a wavy, irregular appearance (subtidal, intertidal, and supratidal deposits). Nodular calcium sulphate beds, usually associated with wavy, irregular laminated beds (supratidal, sabkha deposits); very coarsely crystalline gypsum beds (selenite), associated with more even, laminated beds (subaqueous, intertidal to subtidal deposits); wavy anastomozing gypsum beds, composed of very fine, often broken crystals (subaqueous, current-swept deposits); halite having hopper and chevron structures (supratidal to intertidal); and halite, potash salts, etc. having continuous laminated structure (subaqueous, possibly basinal). Evidence for diagenetic changes is observed in the calcium sulphate deposits which apparently formed by tectonic stress and also by migrating hypersaline waters. These observations suggest that the common, massive form of alabastrine gypsum (or anhydrite, in the subsurface) may not always be ascribed to original depositional features, to syndiagenesis or to early diagenesis but may be the result of late diagenesis. 相似文献
20.
Upward-thickening patterns and lateral continuity of Permian sand-rich turbidite channel fills, Laingsburg Karoo, South Africa 总被引：1，自引：0，他引：1
Martin Grecula Stephen S. Flint Henry De V. Wickens† Stephen D. Johnson‡ 《Sedimentology》2003,50(5):831-853
Abstract Analysis of extensive exposures of the Permian Laingsburg Formation, Karoo basin, South Africa, have enabled a detailed reconstruction of the base of slope stratigraphy and palaeoenvironments in a deep-water system characterized by a very narrow grain-size range (fine sandstone). The deposits include an ≈ 4 km wide and 80 m thick channel complex, fringed by sandy sheet deposits that extend laterally for at least 6 km across depositional strike. Within the channel complex, individual channel fills are marked by shallow basal erosion surfaces draped by thin, parallel-stratified beds of very fine sandstone and siltstone, interpreted as flow tails to largely bypassing flows. These thin beds are overlain by 0·4 to 5 m thick beds of structureless, fine-grained sandstone that represent the majority of the channel fills. The basal packages may be partially to completely removed by localized scour in the axial zone of the channel complex but can be mapped laterally into overbank areas where they thicken and are dominated by rippled fine sandstones with intercalated siltstones. Axial confinement resulted from subtle topography on the basin floor, whereby the lower, dense parts of the initially erosive and bypassing flows were partially confined in the lows and the more dilute, slower moving upper parts of the flows deposited sheet-like successions across slightly elevated overbank areas. The narrow grain-size distribution prohibited the formation ofcoarse-grained residual bypass deposits during the initial phases of channel formation. With decreasing magnitude, later flows became more depositional, filling remaining axial depressions with thick-bedded structureless sandstone. The smaller volumes of late-stage sediment were more axially focused, producing local scour-and-fill features and starvation of the overbank areas. Resulting grain-size vertical profiles are complex. The basal flow tail packages and overlying massive deposits form a thickening and slightly coarsening-upward trend in the channel fills. The overbank deposits show a thinning- and fining-upward profile as a result of less bypass plus late-stage starvation of sand. Application of traditional deep-water facies models could therefore potentially lead to erroneous interpretations of the channel complex as a prograding lobe and the overbank sheets as channel-fills. 相似文献