共查询到20条相似文献，搜索用时 31 毫秒
1.
The Kathmandu and Banepa Basins, Central Nepal, are located in a large syncline of the Lesser Himalayas. The Older Kathmandu Lake evolved during the Pliocene and early Pleistocene; the Younger Kathmandu Lake, which is the focus of this study, is infilled with late Quaternary sediments. Three formations, arranged in stratigraphical order, the Kalimati, Gokarna and Thoka Formations formed during the infilling stage of this lacustrine basin. Structural and textural sedimentological analyses, a chemical survey across the basin and mineralogical investigations of fine‐grained sediments form the basis of this palaeogeographical study. The basin under investigation was covered by a perennial freshwater lake before 30 000 yr BP. The lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin. A fairly low gradient was favourable for the formation of diatomaceous earths, carbonaceous mudstones and siltstones, which were laid down in the centre of the lake and in small ponds. Towards the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. Crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones. The proximal parts of the alluvial–fluvial sedimentary wedge contain debris flows that interfinger with fine‐grained floodplain deposits. Three highstands of the water‐level (>30 000 yr BP, 28 000–19 000 yr BP, 11 000–4000 yr BP (?)) have been recognised in the sedimentary record of the younger Kathmandu Lake in the Late Quaternary. Second‐order water‐level fluctuations are assumed to be triggered by local processes (damming by tectonically induced landslides). First‐order water‐level fluctuations are the result of climatic changes. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
ALAN P. HEWARD 《Sedimentology》1978,25(4):451-488
Vertical sequence analysis within 1500-2500 m thick coarse-grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine-grained coal-bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid-fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1-25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross-bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine-grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm-1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt-stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine-grained, low-energy, regressive, coarsening upward sequences. Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid-fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm-20 m thick, occur in the distal fan and lacustrine environments. These alluvial fan deposits infill‘California-like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits. 相似文献
3.
4.
5.
The Triassic deposits of Cerro Puntudo in the San Juan province of western Argentina constitute the northernmost exposures of the northern portion of the nonmarine Cuyo rift basin, also known as the Las Peñas-Tamberías half-graben. The local column, with an exposed thickness of approximately 400 m, consists of abundant basal and topmost coarse alluvial fan conglomerates and breccias (facies associations I and II) and a relatively thin (approximately 50 m) intervening sequence of marginal, shallow lacustrine deposits characterized by stromatolitic (domal) limestones, tuffaceous mudstones, and fine-grained sandstones (facies association III). Subaerial exposure in the lacustrine deposits is evidenced by desiccation cracks and brecciation. A very thin (0–6 m), laterally, discontinuous succession of lacustrine deposits with similar characteristics is interbedded with the basal conglomerates. Laterally, this lacustrine interval was eroded by overlying conglomerates. The basal conglomerates commonly show crude normal grading, faint cross-bedding, and b-axis clast imbrication. The predominance of coarse deposits and paleocurrents from NW to SE, indicative of an axial flow pattern, suggest that these exposures correspond to the northern end of the Cuyo basin, which is characterized by a shallow, alluvial fan-encased, carbonate-rich lake margin. This lacustrine interval can be correlated with the thicker lacustrine section exposed to the south at Quebrada del Tigre and Ciénaga Larga along the border fault margin of a nonmarine half-graben. The correlation with these sections suggests that the Cerro Puntudo lacustrine deposits are the shallowest equivalent of more profundal, organic-rich lacustrine sediments exposed in the deepest segment of the border fault margin. These thickness and facies variations are the result of differential subsidence along the border fault margin between the low accommodation, fault tip end represented by the Cerro Puntudo section and the high accommodation, central segment located to the south. 相似文献
6.
在距罗布泊以西20 km的沙漠与戈壁交界处(Y=157339013,X=4516330)发现了完整的侏罗系,地层为早、中侏罗世苏康组砾岩、砂岩、粉砂岩、泥岩夹多层煤层(线),在粉砂岩中保存有大量的硅化木化石,化石主要为石松柏纲Coniferae sp．,Cupressinoxycon sp．。这为研究塔里木盆地侏罗纪植物群及古气候提供了重要的古生物化石资料。在塔里木盆地东北地区阿满坳陷东北向沙漠中发现的侏罗系,对确定侏罗系的分布范围具有一定的意义,而且也填补了罗布泊地区侏罗系的空白。同时,还发现侏罗系中多层煤层,这为塔里木盆地油气勘探研究提供了重要的材料。 相似文献
7.
Carbonate pond deposits related to semi-arid alluvial systems: examples from the Tertiary Madrid Basin, Spain 总被引：5，自引：1，他引：5
Carbonate pond deposits occur associated with alluvial sediments in Miocene sequences of the Madrid Basin, central Spain. The ponds developed near the basin margins, either in floodplain environments (north) or mud-flat settings (south). Three main facies assemblages are recognized: (1) floodplain/mud-flat, (2) palaeosols and (3) pond deposits. In the northern part of the basin, ponds developed on the floodplain of terminal fluvial systems. The floodplain facies are typically red mudstones with interbedded sandstones and siltstones. Palaeosols associated with the ponds show a pedofacies relationship, the maturity of soils increasing with distance from the main channel. Carbonate pond deposits consist mainly of limestones, which display typical ‘palustrine’features. The formation and further accumulation of carbonate in the ponds took place in periods of reduced clastic sediment input and it is suggested that recharge into the pond areas was mainly from groundwater. In the south, ponds developed on mud-flats located between sheet-flood-dominated alluvial fans and evaporite lakes. Mud-flat facies consist of red mudstone that exhibits evidence of progressive soil development near both edges and beneath the carbonate pond lenses. Carbonate in the ponds is mainly dolomite and comprises two subfacies, mottled and laminated dolomicrites. This mineralogy, together with the presence of gypsum crusts below and in the lower part of the carbonate body, suggests higher evaporation rates and/or more saline waters filling the ponds in this part of the basin. In spite of differences in depositional setting and, to some extent, climatic conditions between the two areas of the basin, both facies associations and the sequential arrangement of the ponds show strong similarities that allow the proposal of a facies model for carbonate pond deposits related to semi-arid alluvial systems. The sequences recognized from the pond deposits record a set of facies clearly different to those forming in swampy lakes associated with many permanent fluvial systems developed in more humid climates. 相似文献
8.
C. M. BELL 《Sedimentology》1989,36(4):651-663
The Codocedo Limestone Member is a thin but laterally persistent lacustrine sequence within the red beds of the Upper Jurassic-Lower Cretaceous Quebrada Monardes Formation, in the Atacama region of northern Chile. The thick succession of clastic terrigenous sediments of the Quebrada Monardes Formation was deposited in an arid to semi-arid environment. Sedimentary facies are indicative of deposition of aeolian dunes, alluvial fans and braided streams, playa-lake mudflats, and saline lakes and coastal lagoons. The strata accumulated in a N-S elongated extensional back-arc basin on the landward side of an active volcanic arc. The 3 m thick Codocedo Limestone Member marks striking facies changes within the Quebrada Monardes Formation. It is underlain by a thick sequence of conglomerates and sandstones, deposited on alluvial fans. The limestone itself is characterized by evaporite minerals and laterally continuous laminations, indicative of deposition by vertical accretion in a perennial saline lake. The overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa-lake mudflats. The limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession. The lake was possibly formed quite suddenly, for example by damming of the basin by a lava flow. Sedimentation in the perennial lake was predominantly cyclical. Seasonal planktonic algal blooms produced millimetre-scale laminations. Interbedded with these laminites are centimetre-scale beds of evaporitic gypsum, anhydrite and minor halite. The evaporite minerals have been largely replaced by calcite, chalcedony and quartz. The centimetre-scale cycles may have resulted from periodic freshwater input into the lake. After a period of about 3000 yr the lake dried up, to be replaced by extensive playa-lake mudflats. The Codocedo Limestone Member possibly formed a plane of detachment during an early Tertiary phase of E-W directed regional compression. The limestones and evaporites were folded and extensively brecciated. This deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement by calcite and quartz. 相似文献
9.
兰坪中-新生代盆地位于青藏高原东南缘的三江造山带中段,受印度-欧亚大陆斜向碰撞相关构造的强烈影响,导致地层序列及时代归属难辨,区域地质与成矿作用认识存在诸多分歧。先期完成的区域地质调查发现,在金顶铅锌矿集区内常见的含石膏岩地层并非前人认为的古新统云龙组的一部分,而是角度不整合于前古新统之上的一套新地层,暂定名为中新统金顶砂泥岩。系统的剖面测量和详细的沉积学研究表明,金顶砂泥岩从下到上由洪积相砾岩、辫状河三角洲相粉-细砂岩夹泥岩和砾岩以及湖泊相粉砂岩夹泥岩组成,石膏层位于砾岩之上。辫状河三角洲相中频繁出现灰绿色、灰黑色粉砂质泥岩及黄铁矿结核层,可能是频繁的区域性挤压作用造成的逆冲-走滑断层使上三叠统基底岩系中古油气藏破裂、泄露所致;该套地层内广泛发育的同沉积逆断层(视正断层)、滑脱褶皱和液化砂脉等软沉积物变形构造,指示沉积盆地发育期间地壳发生了NEE-SWW向缩短。金顶砂泥岩是印度-欧亚大陆碰撞背景下近东西向挤压构造诱发并控制的周缘前陆盆地沉积,时代为中新世。这一认识为深入理解大陆斜向碰撞带构造演化及大规模金属成矿作用提供了新的资料。 相似文献
10.
大别山北缘早白垩世黑石渡组沉积体系研究 总被引：3，自引：0，他引：3
分布于大别山北缘晓天-磨子潭断裂以北和金寨-响洪甸-西汤池断裂以南呈东西向延伸的早白垩世黑石渡组,厚达千米,岩相主要有角砾岩、砾岩、砂岩、粉砂岩、泥岩等,包括冲积扇、河流、扇三角洲和湖泊沉积体系,其中河流沉积主要由洪泛平原和决口扇组成。黑石渡组属于向上变细层序,早期为冲积扇沉积,中期为洪泛平原、决口扇和扇三角洲沉积,晚期为深湖浊流沉积。冲积扇和湖相浊流沉积主要发育于盆地东段舒城晓天地区,洪泛平原和决口扇沉积主要发育于盆地西段霍山地区,扇三角洲在两地都比较发育。这是一个不对称的断陷盆地,晓天-磨子潭断裂可能对盆地的发育具有控制作用。 相似文献
11.
Ancient stream-dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi-arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream-dominated alluvial fan deposits because they developed during the Eocene-Oligocene temperate climatic regime in an active strike-slip orogen. The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike-slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream-dominated alluvial-fan systems. Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast-supported, boulder conglomerate and minor matrix(mud)-supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well-sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross-stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial-fan sedimentation was concentrated in three parts of the basin. The largest alluvial-fan system abutted the strike-slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal-faulted southern basin margin. Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream-dominated alluvial fan environments, but with several key differences. Middle-fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine-grained lithofacies and coal in the lower part, trough cross-stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward-coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple-laminated sandstone, to coarse grained trough cross-stratified sandstone. The upward-coarsening sequences are basinwide, facies independent, and probably represent progradation of stream-dominated alluvial-fan depositional systems. Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream-dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility. 相似文献
12.
The sedimentology, petrography and mineralogy of the seemingly monotonous Late Triassic Mercia Mudstone Group across the Severn Basin and Bristol Channel region reveals a bipartite division of mudrock facies above and below the Arden Sandstone Formation. Frequently cryptic sedimentary and pedogenic features reveal diverse alluvial, aeolian, playa‐lacustrine and pedogenic processes operating at different times, locations and scales probably in response to climate change. The current study found no evidence of significant marine influence in either of the two mudrock facies associations that are described here. Blocky claystones dominate the lower Mercia Mudstone Group (Sidmouth Mudstone Formation), with pedogenic features such as slickensides, mottling, tubules and carbonate/sulphate nodules common and widespread. The claystones are of alluvial/lacustrine origin with subordinate sheet sandstones, themselves overprinted by pedogenic features, reflecting occasional high intensity rainfall events. These facies reflect a seasonal (wet/dry) semi‐arid climate favouring development of transformed/neoformed (smectite‐rich) clay minerals of intrabasinal origin. Massive to weakly stratified silty mudstones dominate the upper Mercia Mudstone Group (Branscombe Mudstone Formation). Commonly conchoidally weathered, locally gypsiferous, but with distinct sedimentary structures scarce, they alternate with subordinate blue‐grey laminated silty mudstones. Together they reflect fluctuating hydrological conditions within extensive saline mudflats and ephemeral playa lakes, with laminated facies deposited under subaqueous conditions during more humid phases whereas massive mudstones reflect modification through interstitial growth/dissolution of sulphates and deflation of surface sediments during drier episodes. These facies reflect increased aridity during deposition of the upper Mercia Mudstone Group compared with the lower Mercia Mudstone Group, favouring development of detrital/transformed (illite–chlorite) clay minerals of extrabasinal origin. The described facies associations and the sedimentary fabrics and structures that characterize them, occur widely in the Mercia Mudstone Group across the United Kingdom and comparable facies associations may be anticipated in other fine‐grained red bed successions. Recognition of these facies may aid palaeoenvironmental interpretation of such sequences on Earth and, potentially, on Mars also. 相似文献
13.
Kevin T. Pickering 《Sedimentary Geology》1982,33(2):79-110
The 3.2 km-thick late Precambrian Kongsfjord Formation Submarine Fan shows well-developed middle-fan facies-associations. Channel deposits are characterised by discrete packets of coarse-grained, medium to thick-bedded, amalgamated sandstone turbidites and other mass-flow deposits, generally 10 to 30 m thick. Individual beds, or packets of beds, wedge out and channel bases cut down by up to 11 m over a lateral distance of 150 m. Channel deposits often comprise a thinning-and-fining-upward sequence although they vary greatly in clarity. Interchannel deposits occur as packets, tens of centimetres to 25 m thick, of thin and very thin bedded Bouma Tcde siltstones and mudstones. Palaeocurrents within interchannel deposits commonly diverge from those of adjacent channel sandstones. Within the interchannel deposits, isolated beds or packets of beds occur that are both thicker bedded and coarser grained than the surrounding beds; these unusual deposits are sheet-like or fill small channels, and are interpreted as crevasse splays, lobes and channels. Packets, up to a few metres thick, of laterally discontinuous siltstone turbidites occur immediately above some of the channel sandstones, rarely below, and in some cases within interchannel deposits. These siltstones are thin to medium-bedded, show Bouma Tcd, with Tc often as climbing-ripple lamination, and commonly show soft-sediment deformation as slides, slumps, liquefaction and fluidisation structures. Palaeoflow within these packets, compared to adjacent channel sandstones, diverges by up to 90°, and in some cases channel sandstones are seen to pass laterally into these deposits with a swing in palaeocurrents from parallel to the inferred channel axis, to perpendicular to it. These deposits are thought to be levees. Channel-margin deposits are most distinctive, and they are recognised by extreme lateral wedging of channel sandstones, with concomitant thinning and fining of individual beds and their amalgamation towards the channel axis. Sliding and slumping of channel margin deposits is common. Throughout the Kongsfjord Formation Submarine Fan, channel sandstone palaeocurrents suggest a sediment-transport direction to the NE quadrant, although some channels funnelled sediment towards the southeast. 相似文献
14.
Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes 
下载免费PDF全文
下载免费PDF全文 Carys E. Bennett Timothy I. Kearsey Sarah J. Davies David Millward Jennifer A. Clack Timothy R. Smithson John E. A. Marshall 《Sedimentology》2016,63(6):1677-1700
Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems. 相似文献
15.
The Middle Albian Ondarroa turbidite system is a coarse grained, deep water unit which outcrops in the north-eastern part of the Basque-Cantabrian region, west of the Pyrenees. It is about 18 km long and 7 km wide, and shows an unusual ‘L’shape resulting from both a direct morphotectonic confinement and the presence of nearby shallow water carbonate buildups. Eight main facies have been distinguished within this turbidite system: (1) clast-supported conglomerates; (2) mud-supported conglomerates; (3) slump deposits; (4) normally graded pebbly sandstones; (5) cross stratified sandstones; (6) interbedded graded sandstones and mudstones; (7) interbedded non-graded sandstones and mudstones: and (8) mudstones. Inner system, middle system, outer system and basin plain divisions have been distinguished. The inner turbidite system is characterized by stacked channel fill conglomerates and lesser sandy turbidites and mudstones. The middle system consists of sandy and conglomeratic fining upwards sequences, normally several metres thick. The outer system has alternating non-channellized sandstones and mudstones, without any predictable vertical arrangement. The basin plain is characterized by mudstone-siltstone laminations and lesser, randomly occurring thin bedded sandy turbidites. Three main channel fills make up the inner turbidite system. Although all of them can be compared with the valley channel fills of the modern Mississippi Fan, and thus their bases can be interpreted as sequence boundaries, only the lowermost and the uppermost channel bases are documented as allocyclic boundaries. The Ondarroa turbidite system was deposited in an immature passive margin subjected to transtensional movements. It filled a composite pull apart depression with coarse clastics derived from a narrow platform to the north of the present outcrops which was invaded by fan deltas. A major pattern of sinistral strike-slip faulting linked to the opening of the Bay of Biscay is invoked to explain the Ondarroa turbidite system appearance and its tectonic confinement. 相似文献
16.
The Rotliegend beds of the middle of the Kusel Group to the middle of the Lebach Group in the Saar-Nahe Basin are characterized by sedimentary sequences which developed in streams, deltas, and lakes. The stream sequences consisting of cross-stratified sandstone units are attributed to braided fluvial environment. The lake-delta sequences usually show gradual transition from lacustrine mudstones to delta-front cross-stratified sandstones and to floodbasin, crevasse and overbank sandstones and mudstones. Loaded high-intensity flows occasionally result in deformation- and erosion-structures. Low-energy lacustrine sedimentation is evident from paper-shales and massive mudstones. The mudstones alternate with cross-laminated sandstones where influenced by terrigenous influx. 相似文献
17.
Sedimentological, pedological and palynological evidence reveal radically increasing onshore humidity during the Rhaetian marine invasion of central Europe along the north-eastern margin of the Central European Basin (southern Sweden). Pre-Rhaetian aridity favoured the formation of Carnian redbeds with calcrete, which were succeeded by Norian hematite-cemented conglomerates, arkoses, arkosic wackestones, and smectititic mudstones deposited on braidplains and in lakes. Superimposed autochthonous coals and gleysols indicate the Rhaetian onset of year-round humidity. Chemically mature sandstones, kaolinitic mudstones and luvisols also formed at this time, influenced by a permanent vegetation cover which lowered soil pH and strongly intensified chemical weathering. The Rhaetian deposits accumulated in floodplain lakes repeatedly subjected to sediment infill, plant colonization and palaeosol development. The humidity shift resembles that contemporaneously recorded in the North Sea region. 相似文献
18.
Fluvial and lacustrine palaeoenvironments of the Miocene Siwalik Group, Khaur area, northern Pakistan 总被引：1，自引：0，他引：1
MICHAEL J. ZALEHA 《Sedimentology》1997,44(2):349-368
The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records fluvial and lacustrine environments within the Himalayan foreland basin. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuousity (1·08–1·19), single-channel meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans’). River flow was dominantly toward the south-east and likely perennial. Palaeohydraulic reconstructions indicate that Chinji and Dhok Pathan rivers were small relative to Nagri rivers. Bankfull channel depths of Chinji and Dhok Pathan rivers were generally ≤ 15 m, and up to 33 m for Nagri rivers. Widths of channel segments (including single channels of meandering rivers and individual channels around braid bars) were 320–710 m for Chinji rivers, 320–1050 m for Nagri rivers, and 270–340 m for Dhok Pathan rivers. Mean channel bed slopes were on the order of 0·000056–0·00011. Bankfull discharges of channel segments for Chinji and Dhok Pathan rivers were generally 700–800 m3s?1, with full river discharges possibly up to 2400 m3s?1. Bankfull discharges of channel segments for Nagri rivers were generally 1800–3500 m3s?1, with discharges of some larger channel segments possibly on the order of 9000–32 000 m3s?1. Full river discharges of some of the largest Nagri braided rivers may have been twice these values. Thin (decimetres to a few metres) sandstones represent deposits of levees, crevasse channels and splays, floodplain channels, and large sheet floods. Laminated mudstones represent floodplain and lacustrine deposits. Lakes were both perennial and short-lived, and likely less than 10 m deep with maximum fetches on the order of a few tens of kilometres. Trace fossils and body fossils within all facies indicate the former existence of terrestrial vertebrates, molluscs (bivalves and gastropods), arthropods (including insects), worms, aquatic fauna (e.g. fish, turtles, crocodiles), trees, bushes, grasses, and aquatic flora. Palaeoenvironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions. 相似文献
19.
M. Julleh Jalalur Rahman Peter Faupl M. Mustafa Alam 《International Journal of Earth Sciences》2009,98(8):1971-1980
The Bengal Basin, in the north-eastern part of the Indian subcontinent, contains a thick (± 22 km) early Cretaceous-Holocene
sedimentary succession. The Neogene succession in the Sylhet Trough of the basin reaches a thickness of more than 6 km of
which the Surma Group contains important sandstone reservoirs. Lithologically, the group consists of a succession of alternating
shales, siltstones, sandy shales and sandstones, with minor conglomerates. This research work is a sedimentological analysis
of the subsurface Neogene succession encountered in the petroleum exploration wells in the Sylhet Trough of the Bengal Basin.
Detailed lithologic logs of the cores, based on considering texture and sedimentary structure, permit a subdivision into eight
lithofacies, e.g., a shale-dominated facies, interbedded fine sandstones and mudstones, ripple-laminated sandstones, parallel-laminated
sandstones, massive sandstones, cross-bedded sandstones, cross-bedded sandstones with pebble/granule lag and conglomerates.
Characteristic sedimentary structures of the Surma Group, such as flaser-, wavy- and lenticular-bedding, bipolarity of ripple
cross-stratification, evenly laminated sand/silt-streaked shales, reactivation surfaces within cross-bedded sandstone sets,
mud-drapes on foreset laminae and herringbone cross-stratification as well as small-scale vertical sequences (several fining-upward
cycles) are diagnostic for tidal influence. On the basis of the lithofacies associations and prograding character of the deposits
revealed from the electrofacies associations, the Surma Group sediments have been interpreted as representing deposits of
tide-dominated deltaic depositional setting. 相似文献
20.
The late Permian to Triassic sediments of the Solway Basin consist of a layer-cake succession of mature, predominantly fine-grained red clastics laid down in semi-arid alluvial plain to arid sabkha and saline marginal marine or lacustrine environments. The Cumbrian Coastal Group consists of Basal Clastics and Eden Shales. The Basal Clastics are thin regolith deposits resting unconformably on all-underlying units and are composed of mixtures of angular local gravel and far-transported fine to very fine-grained sands deposited as basal lag. The Eden Shales are predominantly gypsiferous red silty mudstones, with thin very fine-grained sandstone beds, and with thick marine gypsum beds at the base, deposited at a saline lake margin. The overlying Triassic Sherwood Sandstone Group consists of the Annan and Kirklinton Sandstones. The Annan Sandstones are predominantly thick-bedded, multi-storied, fine-grained mature red quartz sandstones in which coarse sand is practically absent despite channels with clay pebbles up to 30 cm in diameter. The overlying, predominantly aeolian, Kirklinton Sandstones consist of festoon cross-bedded and parallel-laminated fine-grained sandstones, almost identical to the Annan Sandstones except that mica and clay are absent. The Stanwix Shales, located above, consist of interbedded red, blue and green mudstones, siltstones, and thin very fine-grained sandstones, with gypsum layers. Although the entire succession can plausibly be interpreted as deposited in a large desert basin opening into a hypersaline marine or lacustrine embayment to the southwest, the uniformly fine-grained nature of the succession is unusual, as is the absence of paleosols, and body and trace fossils. There is almost no coarse sand even in the river channel units, and it seems likely that the basin was not only extremely arid but supplied predominantly by wind rather than water. 相似文献