Cyclicity in early Permian fluviatile Gondwana coal measures: An example from Giridih and Saharjuri basins,Bihar, India

The Karharbari and Barakar coal measures of Giridih and Saharjuri basins of Bihar, eastern India, comprise an interbedded assemblage of sandstone, shale and coal in variable abundance. The lithofacies composition records a progressive decrease in sandstone and enrichment of shale and coal from Karharbari up to Barakar. Application of first-order embedded Markov-chain statistics to subsurface data of Karharbari (52 borehole logs) and Barakar (10 borehole logs) reveals that deposition in both the coal measures followed a Markovian mechanism with variable probability, to yield a sequence of upward transition from sandstone through shale to coal. The repetitive fining-upward cycles are asymmetrical, i.e. sandstone → shale → coal → sandstone in the case of Karharbari, but symmetrical as sandstone → shale → coal → shale in Barakar.The abundance of sandstone and the asymmetrical nature of Karharbari cycles are attributed to abrupt shifting of channel bars in low-sinuosity anabranching streams. By contrast, the subequal amount of sandstone, shale and coal forming symmetrical cycles in the overlying Barakar Formation is due perhaps to a slow and gradual shift of the stream channels over and across the adjacent subenvironments of the flood plain.     

Upper flow regime sheets, lenses and scour fills: Extending the range of architectural elements for fluvial sediment bodies

Fluvial strata dominated internally by sedimentary structures of interpreted upper flow regime origin are moderately common in the rock record, yet their abundance is not appreciated and many examples may go unnoticed. A spectrum of sedimentary structures is recognised, all of which occur over a wide range of scale: 1. cross-bedding with humpback, sigmoidal and ultimately low-angle cross-sectional foreset geometries (interpreted as recording the transition from dune to upper plane bed bedform stability field), 2. planar/flat lamination with parting lineation, characteristic of the upper plane bed phase, 3. flat and low-angle lamination with minor convex-upward elements, characteristic of the transition from upper plane bed to antidune stability fields, 4. convex-upward bedforms, down- and up-palaeocurrent-dipping, low-angle cross-bedding and symmetrical drapes, interpreted as the product of antidunes, and 5. backsets terminating updip against an upstream-dipping erosion surface, interpreted as recording chute and pool conditions. In some fluvial successions, the entirety or substantial portions of channel sandstone bodies may be made up of such structures. These Upper Flow Regime Sheets, Lenses and Scour Fills (UFR) are defined herein as an extension of Miall's [Miall, A.D., 1985. Architectural-element analysis: a new method of facies analysis applied to fluvial deposits. Earth Sci. Rev. 22: 261–308.] Laminated Sand Sheets architectural element. Given the conditions that favour preservation of upper flow regime structures (rapid changes in flow strength), it is suggested that the presence of UFR elements in ancient fluvial successions may indicate sediment accumulation under the influence of a strongly seasonal palaeoclimate that involves a pronounced seasonal peak in precipitation and runoff.     

新西兰TARANAKI盆地PAKAWAU组和KAPUNI组中生物成因的沉积构造及其环境意义

新西兰taranaki盆地中的Pakawau组和Kapuni组属于晚白垩世-始新世。它们为-套含煤层的陆相-海陆交互相层系,该层系中生物扰动和虫孔十分发育,研究表明生物构造的发育程度和虫孔大小是判别古水体底部沉积物中氧溶量的有效标志。     

论震积作用和震积岩

地震是一种灾害性的地质事件。地震及其引发的海啸、重力流可以形成震积岩、海啸岩和震浊积岩。在地震过程中,震积作用可以形成特殊的构造标志(地震断裂层、地裂缝、微同沉积断裂、微褶皱纹理、重力断层等)、沉积和成岩标志(砂岩脉、泥岩脉、泥晶脉,即磨牙(molar tooth)构造、枕状层、砂火山、泥火山、同沉积变形构造、丘状层理等)。地震沉积组合包括原地震积岩(A,包括震裂岩、震褶岩、自碎屑角砾岩、内碎屑副角砾岩等)、海啸岩(B,由具丘状层理、平行层理的碎屑岩或内碎屑灰岩组成)、震浊积岩(C,由具块状层理、递变层理或玻马序列的粗碎屑岩或碎屑灰岩组成)及背景沉积(D)4个单元,它们组合成A-B-C-D、A-B-D、A-C-D、A-D、B-D、C-D等6种类型的沉积序列。     

鄂尔多斯盆地侏罗系直罗组砂岩发育特征

根据岩性旋回结构,在全盆地范围将直罗组划分为上、下两段。分段制图表明,直罗组上、下段地层厚度展布规律与全组厚度展布规律基本一致,均表现为西厚东薄;上段地层较下段厚。下段砂岩层数少、单层厚度大;而上段砂岩以层多、层薄为特点。直罗组砂岩总体成熟度较低,区域制图和碎屑组分等反映周缘存在多个源区。砂岩粒度分布表现为河道和分流河道等沉积特征。直罗组砂岩发育多种反映陆上河流沉积环境的层面和层理构造。综合分析认为,在今残留盆地范围内,直罗组下段以辫状河、曲流河沉积为主,东北部可见三角洲平原沉积。上段以辫状河、曲流河、三角洲和湖泊沉积为主。直罗组下段辫状河道砂岩具有良好的渗透性、连通性和成层性,是砂岩型铀矿和石油的良好储层。
     

湖南邵阳地区茅口期晚期重力流沉积的发现及意义*

湖南邵阳市邵阳大道两侧出露的龙潭组下段(中二叠世茅口期晚期)发育一套地质特征典型、沉积序列明显的重力流沉积。重力流沉积以块状砂岩、粉砂岩及泥岩互层为主,砂岩底面发育有重荷模、槽模、工具痕及冲刷充填构造,砂岩内部有块状层理、平行层理、包卷层理及粒序层理,互层的泥岩中见类似于古网迹的痕迹化石。可识别出砂质碎屑流、浊流与滑塌沉积。剖面下部含泥砾块状砂岩发育,剖面中上部以发育薄到中厚层砂岩、粉砂岩与泥岩互层为特点。根据重力流沉积物特征及其垂向序列特征,建立了重力流沉积模式,将海底扇划分出内扇、中扇与外扇。通过与附近的短陂桥矿区的龙潭组下段沉积特征进行对比研究,结合华南地区岩相古地理特征,认为位于华南盆地范围的邵阳地区,在中二叠世茅口期晚期(龙潭组下段)发育的重力流沉积,可能意味着华南盆地在扬子陆块和华夏陆块之间的深大断裂在中二叠世末期曾经发生拉张,形成裂谷盆地。     

可可西里盆地早渐新世雅西措群沉积环境分析及古气候意义

可可西里盆地是青藏高原腹地最大的第三纪沉积盆地,第三纪沉积地层包括早始新世 -早渐新世风火山群、早渐新世雅西措群、早中新世五道梁群。其中,雅西措群为紫红色、砖红色泥岩、含膏泥岩与紫红色粉砂岩、细砂岩韵律互层,夹白色石膏薄层和石膏结核层,沉积厚度为 6 70.0m,沉积环境主要由河流和湖泊环境组成,并以湖泊环境为主,古水流方向反映盆地沉积中心逐渐向东向北迁移。雅西措群中石膏层的突然大量出现发生在底部地层距今约32.0Ma,体现了渐新世最早期的全球变冷变干事件在青藏高原北部的记录。     

Sedimentation and water escape structures in some late Precambrian shallow marine sandstones from Finnmark, North Norway

Water escape structures are abundant in the Grønnes Formation, a tectonically undeformed, late Precambrian shallow marine sandstone deposit in North Norway. Trough cross-bedded sandstones of the current-dominated shallow marine environment were frequently liquefied, presumably due to recurring seismic shocks. Subsequent dewatering resulted in deformation of the cross-bedding and the formation of convolute lamination. A three-fold upward vertical sequence developed where liquefaction occurred below the sediment-water interface: convoluted bed → passively deformed bed → undeformed bed. The passively deformed bed resulted from differential subsidence of a relatively plastic bed above a liquefied bed. It is characterized by anticlinal ridges and sand volcanoes at the sites of vertical sediment extrusion, and synclinal troughs at the sites of lateral sediment movement. Liquefaction may have been induced by either tectonic (earthquake shocks) or non-tectonic (storm-induced microseisms) trigger mechanisms, or a combination of both. The restriction of such a high frequency of water escape structures to deposits immediately above a gentle regional unconformity lends support for a tectonic trigger mechanism.     

The Cambrian Araba Formation in northeastern Egypt: Facies and depositional environments

The Cambrian Araba Formation exposed in Gabal El Zeit and in the Sinai Peninsula unconformably overlies Precambrian basement rocks, and is in turn overlain unconformably by the Ordovician Naqus formation. The Araba Formation has been subdivided into three informal lithologic units: lower, middle and upper, from field observations. Seven sedimentary facies assignable to three facies associations (fluvial, tidal and coastal to open marine) are recognized within the Araba Formation. The lower unit comprises two main facies: matrix-supported conglomerate (facies-A) in the south (Gabal Araba and Wadi Feiran) and interbeds of granulestone and sandstone (facies-B) in the north (Gabal Dhalal and Taba) deposited in fluvial conditions. The middle unit includes four facies: cross-bedded sandstone (facies-C), thin laminated sandstone (facies-D), burrowed-massive sandstone (facies-E) and rhythmically bedded sandstone–mudstone (facies-F) deposited mainly under tidal conditions in the coastal zone. The upper unit consists mainly of interbeds of burrowed mudstone-siltstone, with Skolithos, probably deposited in open marine environment. The sequence records the southward transgression of the Tethys Ocean over a horst and graben system developed in the late Precambrian on the northern margin of the Arabo-Nubian continent.     

Application of Markov chain and entropy analysis to lithologic succession — an example from the early Permian Barakar Formation,Bellampalli coalfield,Andhra Pradesh,India

A statistical approach by a modified Markov process model and entropy function is used to prove that the early Permian Barakar Formation of the Bellampalli coalfield developed distinct cyclicities during deposition. From results, the transition path of lithological states typical for the Bellampalli basin is as: coarse to medium-grained sandstone → interbedded fine-grained sandstone/shale → shale → coal and again shale. The majority of cycles are symmetrical but asymmetrical cycles are present as well. The chi-square stationarity test implies that these cycles are stationary in space and time. The cycles are interpreted in terms of in-channel, point bar and overbank facies association in a fluvial system. The randomness in the occurrence of facies within a cycle is evaluated in terms of entropy, which can be calculated from the Markov matrices. Two types of entropies are calculated for every facies state; entropy after deposition E(post) and entropy before deposition E(pre), which together form entropy set; the entropy for the whole system is also calculated. These values are plotted and compared with Hattori’s idealized plots, which indicate that the sequence is essentially a symmetrical cycle (type-B of Hattroi). The symmetrical cyclical deposition of early Permian Barakar Formation is explained by the lateral migration of stream channels in response to varying discharge and rate of deposition across the alluvial plain. In addition, the fining upward cycles in the upper part enclosing thick beds of fine clastics, as well as coal may represent differential subsidence of depositional basin.     

Depositional Record of Tidal-Flat Sedimentation in the Permian Coal Measures of Central India: Barakar Formation, Mohpani Coalfield, Satpura Gondwana Basin

The Permian Barakar Formation in the Mohpani coalfield, Satpura Gondwana basin, is composed of three broad lithologies that occur repetitively and are iterdigitated: (1) several metres thick coarse- to medium-grained sandstone bodies with scoured bases, (2) 5-20 m thick medium- to fine-grained sandstone bodies and (3) 5-20 m thick mudstone-dominated packages with variable proportions of centimetre- to decimetre-scale, fine- to medium-grained sandstone, carbonaceous shale and coal. The Barakar strata were previously interpreted as deposits of braided rivers and associated inter-channel flood basin in a continental setting. However, this study recognizes signatures of tidal current from the mudstone-dominated packages implying marine influence during Barakar sedimentation.

The mudstone-dominated sediment bodies are the focus of this paper and comprise of three lithofacies that bear imprints of tidal processes during Barakar sedimentation: (1) heterolith, (2) sandstone, and (3) coal-carbonaceous shale, which alternate with one another within individual bodies. The heterolithic facies show interlayering of sandstone and claystone resembling flaser, wavy and lenticular bedding, as well as pinstripe stratification. Successive sandstone-mudstone couplets indicate periodic waxing and waning of flows. Within individual heterolithic packages, the sandstone:claystone ratio along with the bedding style, varies cyclically upwards giving rise to alternate sandstone-dominated and claystone-dominated intervals suggesting tidal velocity fluctuation reflective of spring-neap lunar cycle. Thickness plots of successive sand-mud couplets also reveal cyclic variation with a conspicuous periodicity of around 12 couplets per cycle, which corroborates the spring-neap-spring (or neap-spring-neap) lunar cycle. Presence of abundant desiccation cracks indicates periodic emergence and points towards an intertidal setting. The sandstone facies is characterized by a variety of wave-generated features such as bundled and chevron upbuilding of lamina, bi-directional foreset orientations, offshooting and draping laminae, scour-and-drape feature, swollen lens-like geometries suggesting their emplacement under storm-induced combined-flow on the tidal-flat. The coal-carbonaceous shale facies represent supratidal marsh environment.     

青岛灵山岛下白垩统湖泊风暴沉积与风暴作用

灵山岛背来石剖面发育了非常典型的湖泊风暴岩和风暴作用。非常典型的丘、洼状构造发育在火山岩底部的砂页岩中。丘状构造呈典型的丘状,规模不一,小者高数厘米,宽数十厘米;大者高2. 4m,宽12. 48m;具有典型的二元结构：底部为灰色或浅灰色的砂砾岩或砂岩;顶部为深灰色 黑色泥岩、页岩或薄层砂岩互层。砂砾岩分选磨圆均很差,砾石主要为片麻岩,直径多在数毫米到3~4cm,呈悬浮式胶结。砾岩底部为凹凸不平的侵蚀面,砾岩向丘状构造两翼变薄甚至尖灭,与下部侵蚀面呈典型的角度交切。丘状构造的上部的细粒层有四个要素：①上凸形态;②丘状交错层理;③下细上粗的二元结构;④底部具侵蚀面。洼状构造也具有典型的洼状,规模也不一,小者高数厘米,宽十余厘米;大者高2. 4m,宽18. 70m,具有下细上粗的（反)二元结构,但底部多为深灰 灰黑色、甚至黑色的泥岩、页岩或夹薄层砂岩,上部多为砂岩、砂砾岩或砾岩。完整的洼状构造也有四个要素：①下凹形态;②或有洼状交错层理;③下细上粗的（反)二元结构;④底部具侵蚀面。丘状构造与洼状构造在空间上相邻共生,但不是同时形成的,一个连续的完整序列是丘状构造形成在先,洼状构造形成在后,其间由一个粒度较细的薄层分隔开（风暴最高水位时的“静”水沉积)。基于实验和丘洼构造参数计算了风暴浪的相关参数。     

Stratigraphic architecture and depositional setting of the coarse‐grained Upper Cambrian Owen Conglomerate,West Coast Range,western Tasmania

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward‐fining successions of coarse‐grained siliciclastic rocks that exhibit a characteristic wedge‐shaped fill controlled by the basin‐margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group,while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse‐grained, alluvial‐slope channel successions, with prolonged channel bedload transport exhibited by the association of channel‐scour structures with upward‐fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large‐scale cross‐beds derived from accretion in low‐sinuosity, multiply active braided‐channel complexes. While the dipslope of the basin is predominantly drained by west‐directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west‐directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse‐grained alluvial‐slope deposits are replaced by coarse‐grained turbidites of thick submarine‐fan complexes. The middle conglomerate member comprises thickly bedded, coarse‐grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall‐derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward‐fining, coarse‐grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid‐delta and fine‐grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base‐level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates.     

东营凹陷北带古近系沙四段砂砾岩体沉积特征及沉积模式

综合岩心观察、薄片鉴定、砂砾岩厚度及百分含量等资料,对东营凹陷北带沙四段砂砾岩的沉积特征、成因类型及沉积演化模式进行了系统研究。结果表明,东营凹陷北带沙四段砂砾岩储层中块状层理、粒序层理、平行层理、叠覆冲刷构造、交错层理发育,同时变形构造、泥岩撕裂屑、重荷模和火焰构造常见。碎屑颗粒表现为无分选-中等分选、棱角状-次棱角状到次圆状的近物源沉积特征。岩相类型以块状层理砾岩相、递变层理砾岩相、递变层理砂质砾岩相、交错层理砾质砂岩相、平行层理砾质砂岩相、块状层理砾质砂岩相、块状砂岩相、平行层理砂岩相、块状层理砂岩相、变形层理砂岩相、泥质撕裂变形砂岩相、薄层透镜状或压扁层理砂岩相为主。综合古地貌、砂砾岩沉积特征及沉积机制等,认为东营凹陷北带沙四段主要发育近岸水下扇、湖底扇和扇三角洲等成因类型的砂砾岩扇体。在沉积演化上,沙四下亚段沉积时期,胜坨地区和民丰地区发育近岸水下扇沉积,利津地区发育扇三角洲沉积,扇体规模较小,横向连续性较差;沙四上亚段沉积时期,胜坨地区和民丰地区广泛发育近岸水下扇-湖底扇沉积,利津西部地区发育扇三角洲沉积,扇体规模较大,横向连片发育。     

Bar development in a fluvial sandstone (Westphalian 'A'), Scotland

The fluvial sandstone beneath the Mill Coal in the Westphalian ‘A’ of Scotland erosively overlies a lake mudstone. Slightly erosive surfaces within the sandstone, traceable for over 200 m, are used to divide it into two types of major sedimentary units termed type A and type B. Type A sand units are approximately 200 m wide, up to 7 m thick, convex upward, and lenticular in all directions. The constituent cosets overlap to the ENE and dip mainly at 1–2° downcurrent (NNW), but locally at 10–15°. Where thickest, type A sand units display a vertical facies sequence commencing with trough cross-bedded and massive sandstone, overlain by a thick zone of ripple cross-lamination, a thin zone of trough cross-beds, and a variably eroded silt drape up to 0.4 m thick. Attenuated lateral margins are dominated by flat bedded sandstone with primary current lineation. Type A sand units are interpreted as deposits which were accreted on to a large fluvial bar during successive flood events. The bar is thought to have had a similar topographic significance to sand waves described from the Brahmaputra and slip face bounded bars observed in the South Saskatchewan river. Palaeocurrents measured from trough cross-bed sets 0.3–1.0 m thick within type B sand units indicate flow to the WSW, perpendicular to the palaeoflow direction measured from type A units. In sections perpendicular to the WSW flow direction type B units are lenticular, and in ENE-WSW trending sections they can be traced for over 80 m at a constant thickness. Type B sand units are interpreted as the product of low stage channels which flowed across bar fronts and tops. The sandstone described herein is interpreted as a braided-type river deposit but is atypical, because it is fine grained and has an internal structure dominated by ripple cross-lamination and upper phase plane beds. The palaeoriver is thought to have been of low sinuosity, 7–10 m deep, with a high suspended load and large rapidly fluctuating discharge. At low stage a braided-type flow pattern developed around submerged bars. The regime of the palaeoriver was probably controlled by the fine sediment grain size and humid tropical climate.     

Sheet-flow deposits of graded beds and mudstones on an alluvial sandflat-playa system: Upper Triassic Blomidon redbeds, St Mary's Bay, Nova Scotia

The 400 m of Blomidon redbeds accumulated in a semi-arid rift valley in the subtropics. At St Mary's Bay, these redbeds are 64% sandy mudstone (playa mudflats), 25% graded beds (sandflats at the toes of alluvial fans), 10% fissile claystone (playa lakes), and 1% channel sandstone (stream channels). Flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north-east as sheet flows on sandflats and playa mudflats. Deceleration of the sheet flows deposited graded beds 2–83 cm in thickness on the sandflats and thin layers of mud on the playas. Nine sequences, consisting of arrangements of six lithologies, compose 90% of the graded beds. In order of decreasing abundance, these are: ripple cross-laminated siltstone → horizontally laminated mudstone; fining-upward, ripple cross-laminated siltstone; ripple cross-laminated sandstone → horizontally laminated mudstone; cross-bedded sandstone horizontally laminated mudstone; ripple cross-laminated sandstone → ripple cross-laminated siltstone; rippledrift cross-laminated siltstone horizontally laminated mudstone; fining-upward, ripple-drift cross-laminated siltstone; cross-bedded sandstone → ripple cross-laminated siltstone; and cross-bedded sandstone → ripple cross-laminated siltstone → horizontally laminated mudstone. The sheet flows, perhaps up to 1 m in depth, had a high concentration of suspended load. Deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration. There are 32 thinning and fining-up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone. The cycles range in thickness from 1·3 to 13·3 m, averaging 4·6 m. Each cycle is initiated by avulsion to a new active channel network on a fan. Gradual abandonment of the channel network produces the thinning and fining-up cycle. The cycles are grouped in three 60–70 m fining-up megacycles. Upwards within each megacycle, the packages of sandy mudstone compose a progressively larger proportion of the cycles. Each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor. Subsequent erosion gradually lowered the relief to yield a fining-up megacycle.     

豫西陕渑煤田山西组煤层形成环境和聚煤特征

本文以宏观沉积学研究方法为主，辅助进行室内岩矿签定，编制沉积断面图和各种等值线图，在此基础上讨论了山西组煤层的沉积环境。并对煤层与聚煤沉积环境间的关系进行讨论。研究结果表明，泥炭沉积前和沉积期的沉积环境是影响煤层厚度和煤质的主要原因，泥炭沉积后的沉积环境仅在局部地区影响煤厚分布，而后期构造对煤厚影响程度较小。     

鄂尔多斯盆地乌审旗气田中二叠统下石盒子组盒8段下亚段滩坝砂体沉积特征

鄂尔多斯盆地乌审旗气田中二叠统下石盒子组盒8段下亚段中下部岩石为石英砂岩类砂岩,即“骆驼脖砂岩”,上部岩石为暗色粉砂质泥岩或碳质泥岩。“骆驼脖砂岩”广布于华北地台,在鄂尔多斯盆地呈“满盆砂”现象,是盆地的主要储层之一。以往学者多认为盒8段下亚段是河流相或三角洲相沉积。通过对众多取心井盒8段下亚段岩心的观察,结合对砂岩岩性、剖面结构、沉积构造、测井曲线特征及砂体空间展布规律的综合研究,提出了盒8段下亚段中下部是滩坝相沉积,上部是沼泽相或沼泽相为主夹小河道沉积。盒8段下亚段可发育一段或两段滩坝相石英砂岩类砂岩,其成分和结构成熟度较高,两段砂岩间夹若干米半深湖相的暗色粉砂质泥岩或沼泽相的碳质泥岩。发育＜10°的冲洗交错层理、互成180°相交的板状和楔状层系组,具丘状和洼状交错层理的风暴岩。自然伽马曲线呈箱形或带齿的箱形。众多厚度较大的砂岩组成的若干列近东西向展布的长条透镜体状砂体,大致平行于古岸线。     

季节性河流体系高流态沉积构造特征:以内蒙古岱海湖半滩子河为例*

高流态尤其是超临界流动的水动力学机制及其床沙底形演化的研究相较于次临界流动具有一定的差距。季节性河流以高流态为主要沉积搬运过程,为超临界流的形成与保存提供了有利条件,是研究超临界流沉积的重要载体。在季节性河流沉积体系研究进展调研基础上,明确了其基本定义、判别标准及沉积特征。通过对内蒙古岱海湖北部典型季节性冲积体系——半滩子河流发育的沉积构造进行研究表明:随着低流态向高流态演化,沙丘底形(Dune)逐步向上部平坦床沙底形(Upper plane bed)过渡,形成了低角度/S型交错层理;在高流态初期,形成了上部平坦床沙成因的平行层理;随着流动强度逐渐增大,流动机制演变为超临界流,平坦床沙逐渐向逆行沙丘(Antidune)过渡,形成了与平行层理伴生的逆行沙丘交错层理;当流动强度进一步增大,携带沉积物的流体发生较强的水力跳跃,形成了流槽与冲坑(Chute-and-Pool)。半滩子河流现代沉积中发育的高流态沉积与区域内气候变化具有明确的响应关系,表明河流沉积中广泛发育的高流态沉积构造指示了强烈季节性变化的气候特征。     

湖相重力流水道沉积特征与沉积模式:以鄂尔多斯盆地瑶曲铁路桥剖面三叠系延长组为例*

陆相深水重力流水道的类型细分及其沉积模式是制约其油气勘探开发的重要因素,但研究程度低。通过对鄂尔多斯盆地南缘瑶曲铁路桥剖面三叠系延长组实测、水道形态参数统计及岩相、粒度等分析,开展了湖相重力流水道的沉积特征、沉积过程及沉积模式研究。结果表明:(1)研究区内可识别出4期复合水道,主要为洪水重力流成因。根据其内部单一水道及单砂体形态特征、岩相组成,将其细分为沉积型和过渡型两类。(2)剖面下部2期复合水道为沉积型,以悬浮载荷成因岩相为主,常见块状净细砂岩、薄层泥岩岩相组合和鲍马序列岩相组合;上部2期复合水道为过渡型,岩相以底床载荷与悬浮载荷共存为特征,自下而上以交错层理细砂岩或叠瓦状泥砾细砂岩与含泥砾/泥岩撕裂屑块状细砂岩、平行层理粗粉砂岩及薄层泥岩的岩相组合为特征。(3)结合单一水道规模及其相互关系,建立了区内过渡、沉积型重力流水道的半定量沉积模式。过渡型水道内部侵蚀与沉积作用共存,单一水道宽度小、宽厚比低,呈透镜状,水道间切割性强,砂体横向稳定性较低,表现出不定向叠加、侧向拼接样式;沉积型水道内部由沉积作用主导,单一水道宽度较大、宽厚比较高,呈似板状—透镜状,砂体横向稳定性较高,表现出稳定的垂向加积样式。