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1.
A.G. PLINT 《Sedimentology》1983,30(4):525-535
ABSTRACT At Hengistbury Head, Dorset, the Boscombe Sands (Middle Eocene, Bracklesham Formation) are of estuarine channel facies. A mud-filled channel is exposed, the banks and eastern flank of which have a black carbonaceous stain, the degraded remains of a bitumen. At the time of deposition, the bitumen rendered the sediment firm and it was extensively burrowed by a Thalassinoides -forming organism (crustacean). The bituminous sand on the eastern channel bank suffered brecciation and dilation as a result of liquefaction and flowage of the underlying sediments. This is thought to have been due to rapid expulsion of pore water, possibly as a result of seismic shock. The layers of bituminous sand below the surface were ruptured during water-escape, resulting in localized zones of rapid flow causing fluidization and the development of dewatering pipes up to 1.2 m long. The estuarine sediments were subsequently transgressed during which the bituminous sand was exposed on the seafloor, when it was eroded into a hummocky topography and heavily burrowed. Blocks of bituminous sand were reworked into the marine basal conglomerate, composed mainly of flints, demonstrating the remarkable strength of the bituminous cement.  相似文献   

2.
塔里木盆地轮南地区三叠系扇三角洲沉积与储集层研究   总被引：21，自引：2，他引：21  
顾家裕  何斌 《沉积学报》1994,12(2):54-62
轮南地区三叠纪时由于受轮台断裂影响,断裂两侧地形高差大,冲积扇沉积直接进入湖盆,形成扇三角洲沉积体群?扇三角洲沉积体明显地可划分为:扇三角洲平原亚相?扇三角洲前缘亚相和前扇三角洲亚相?扇三角洲前缘亚相中的水下分流河道沉积砂体是主要的油气储集体,储集砂体最发育地区位于桑塔木断垒带及以南地区?储集砂岩类型主要是矿物和结构成熟度较低的细─粗粒岩屑砂岩,石英含量15~60%,长石含量10~25%,岩屑含量35~75%?储集层的储集空间以次生溶蚀孔为主,其中包括粒间溶孔?粒内溶孔?超大孔隙?胶结物溶蚀孔?微孔隙,其次是原生孔和裂缝?轮南地区三叠系属深埋?高孔?高溶储层特征,埋深4200~5400m,其储集砂体,泥质含量低,小于5%?有机酸和无机酸对颗粒的溶蚀,扩大了储集空间,粘土矿物中绝大部分为斑点状高岭石,不易堵塞孔隙和喉道,低的地温梯度和短期的深埋等使三叠系储层具备高孔高渗的特点?孔隙度为15~28.54%,渗透率为10~4317.9×10-3um2?  相似文献   

3.
ABSTRACT The Upper Carboniferous deep‐water rocks of the Shannon Group were deposited in the extensional Shannon Basin of County Clare in western Ireland and are superbly exposed in sea cliffs along the Shannon estuary. Carboniferous limestone floors the basin, and the basin‐fill succession begins with the deep‐water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m thick). The type of turbidite system, scale of turbidite sandstone bodies and the overall character of the stratigraphic succession make the Ross Formation well suited as an analogue for sand‐rich turbidite plays in passive margin basins around the world. The lower 170 m of the Ross Formation contains tabular turbidites with no channels, with an overall tendency to become sandier upwards, although there are no small‐scale thickening‐ or thinning‐upward successions. The upper 290 m of the Ross Formation consists of turbidites, commonly arranged in thickening‐upward packages, and amalgamated turbidites that form channel fills that are individually up to 10 m thick. A few of the upper Ross channels have an initial lateral accretion phase with interbedded sandstone and mudstone deposits and a subsequent vertical aggradation phase with thick‐bedded amalgamated turbidites. This paper proposes that, as the channels filled, more and more turbidites spilled further and further overbank. Superb outcrops show that thickening‐upward packages developed when channels initially spilled muds and thin‐bedded turbidites up to 1 km overbank, followed by thick‐bedded amalgamated turbidites that spilled close to the channel margins. The palaeocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity. Stacks of channels and spillover packages 25–40 m thick may show significant palaeocurrent variability at the same stratigraphic interval but at different locations. This suggests that individual channels and spillover packages were stacked into channel‐spillover belts, and that the belts also followed a sinuous pattern. Reservoir elements of the Ross system include tabular turbidites, channel‐fill deposits, thickening‐upward packages that formed as spillover lobes and, on a larger scale, sinuous channel belts 2·5–5 km wide. The edges of the belts can be roughly defined where well‐packaged spillover deposits pass laterally into muddier, poorly packaged tabular turbidites. The low‐sinuosity channel belts are interpreted to pass downstream into unchannellized tabular turbidites, equivalent to lower Ross Formation facies.  相似文献   

4.
The Grès de Champsaur turbidite system, deposited in a distal setting in the Alpine Foreland Basin of south‐eastern France, exhibits a repeated upsection alternation in sand body geometry between incised channels and sheet sands. The channels form symmetric lenticular erosional features, of width 900–1000 m (measured between the lateral limits of incision) and depth 65–115 m, and can be traced axially for up to 5 km. In each case, the channel fill is capped by a laterally persistent sandy sheet‐form interval, which lies upon a fine‐grained substrate beyond the channel margins. No intrachannel elements have been traced into the substrate sequence, suggesting that, before infill, the channels acted as open sea‐floor conduits of essentially the same dimensions as the preserved channel deposits. The channels are vertically stacked, although axial erosion juxtaposes younger channel axis deposits against the fill of older channels and their channel‐capping sheet sandstones to produce an apparently well‐connected composite sandstone body geometry. The predominant channel‐fill facies comprises coarse‐grained, amalgamated sandstones, which are commonly parallel‐ or cross‐stratified. Subsidiary facies of finer grained sandstone–mudstone couplets and clast‐bearing muddy debrites are commonly preserved as erosional remnants, suggesting a complex channel history of aggradation and erosion. The repeated cycles of channel incision, infill and transition to sheet sandstone development indicate repetitive incision and healing of the palaeo‐sea floor. A model is proposed that links incision to the development of relatively steep axial gradients (parallel to the mean dispersal direction) and the return to sheet‐form deposition to the re‐establishment of lower axial gradients, with the repetitive switch between incisional channels and sheet sandstones driven by changes in sediment input rate against a background of ongoing sea‐floor tilting.  相似文献   

5.
A. Guy Plint 《Sedimentology》2014,61(3):609-647
Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.  相似文献   

6.
ABSTRACT Usually well preserved fluidization pillars and sand filled fluidization pipes occur within submarine channel sands of the basal Uratanna Formation (Lower Cambrian) in the Adelaide Geosyncline of South Australia. The morphology of these structures reflects complex lateral and vertical movement of fluids during liquefaction and dewatering. Fluidization pipes acted as conduits for highly concentrated, upward directed fluid flow. The formation and maintenance of these pipes was dependent upon the development of a pipe wall composed of clay plugged fine sand. Formed during initial fluidization, this lining acted as a permeability barrier, confining and concentrating fluidized flow within the pipe. Each of the pipes is surrounded by a cylindrical fluidization halo in which leakage through the pipe lining produced partial fluidization of the surrounding sediment. Fine scale structures within these haloes indicate that fluids flowed radially and upward out of the fluidization pipes at an acute angle. These fluids merged with and influenced the orientation and size of adjacent fluidization pillars. The fluidization pipes of the Uratanna Formation may represent unusual preservation of the unstable fluid flow conditions that occur during incipient fluidization of sand beds.  相似文献   

7.
辽河西部凹陷双台子构造带E2s3发育重力流沉积,沉积相类型为湖底扇中扇和外扇,同沉积断层和古地貌是控制沉积的主要因素。论文描述了同沉积断层和古地貌的展布特征,并以此为基础,开展了沉积相主控因素的研究工作。建立了“沟道控砂,坡度控形,物源控供给”的沉积模式:辫状沟道沉积分布在受同沉积断层和古地貌的联合控制的古沟道内。古地貌坡度小,辫状沟道沉积向盆地中心搬运的距离长,沟道弯曲度大,沉积分异作用更加明显,盆地内主要以砂岩与泥岩互层为特征（双南）。古地形坡度大,辫状沟道沉积向盆地中心搬运距离短,沟道弯曲度小,沉积分异作用差,盆地内主要以厚层砂岩和薄层泥岩组合为特征（双台子）。双台子和双南具有不同的物源,两个地区在同一层序内具有不同的砂体叠置样式。双南和双台子沉积作用机制上的不同,导致了二者在沉积相带展布、砂体叠置样式等方面的差异。对储集体而言,双台子比双南具有更有利的沉积相带。  相似文献   

8.
Late Eocene time in the Bremer and western Eucla Basins of southern Western Australia was a period of terrigenous clastic and abundant, unusual, biosiliceous sponge sedimentation. The Pallinup Formation (revised) consists of five units; 1 and 2 are basal sandstones, 3 and 4 are variably spiculitic mudstones, whilst the uppermost unit is spiculite and spongolite, and formalised as the Fitzgerald Member (new). The Pallinup Formation, plus coeval spiculites in palaeovalleys and carbonates in the western Eucla Basin, accumulated during one large‐scale, transgressive‐regressive relative sea‐level cycle. Drowned, low‐gradient rivers supplied mud but little sand. Instead, sand was locally sourced via transgressive shoreface erosion of deeply weathered regolith. Regression terminated shoreface erosion, eliminated the sand source, and resulted in a river‐supplied, clay‐dominated shallow‐marine depositional system. The unit 2–3 sandstone‐mudstone transition, which would normally be interpreted as transgressive drowning, is in this case the result of regressive cessation of sand supply. The peak relative sea‐level (highstand) horizon thus lies within unit 2 sandstones, a facies that would usually be considered wholly transgressive, and no highstand systems tract was deposited. The maximum flooding and downlap surfaces are the same horizon and cap the transgressive systems tract. They formed coincidentally or subsequent to peak relative sea‐level, but prior to initiation of unit 3 mudstone deposition. Upper unit 2 plus unit 3 represent a condensed section systems tract, and unit 4 plus the Fitzgerald Member comprise a regressive systems tract.  相似文献   

9.
The Lower Triassic succession of Barles, Alpes de Haute Provence, France, comprises an unconformable quartz arenite sand body of 90m thickness. The succession may be informally divided into (i) lower channellized cross-bedded member overlain by (ii) an upper fining upward member. The lower member comprises vertically stacked, subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits. Subtidal channel fill deposits are dominated by varying scales of cross bedding. These scales vary systematically from the base to the top of the member, with large scale planar sets dominating the lowest channel sand body (sand body 1), medium scale planar and trough cross bedding characterizing sand bodies 2-4, the largest scale planar sets in the highest sand body (sand body 5). This upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces, and the proportion of preserved interchannel shoal deposits. The succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits. Tidal flat sequences are characterized by parallel laminated, wave and current rippled sandstones separated by bioturbated, fine grained siltstones and mudstones. The vertical variation in facies of the Lower Triassic succession suggests two main periods of deposition. The lower member is considered to preserve successively more seaward components of a transgressive estuarine complex. The overlying upper member records the seaward progradation of tidal channel, shoal and tidal flat environments. The unconformity bounded nature of the lower member, combined with its systematic variation in facies, suggests it may represent an incised valley-estuarine fill developed in response to an early Triassic relative sea level fall and subsequent rise. Succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and/or rate of relative sea level rise comparable with a progradational shoreline. It is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and rise.  相似文献   

10.
青岛灵山岛南背来石剖面发育了两层大型不规则砂脉。其主要特征如下：上层大型不规则砂脉呈分散团块状顺黄褐色凝灰质泥岩分布,断续延伸约60 m,宽数米;有17个侵入砂团块。砂团块多呈浅灰色,主要岩性为砂岩、砂砾岩;砂团块直径在1~2 m。砂团块的形态非常复杂,侧向极易尖灭或突然中止。边界与围岩多突变,某些地方与节理发育方向相同。下层大型不规则砂脉浅灰色,主要底部为砂岩、顶部为砂砾岩,呈透镜状顺层产于灰黑色薄层砂泥岩中,均一团块状和边缘含大量泥砾;厚18 m,宽度和高度不详;上层砂脉发育在浅湖中;下层大型不规则砂脉发育在三角洲前缘。与此同时还探讨了上层砂脉的形成的动力学机制,建立了顺层砂脉的形成深度与液化层的深度之间的关系式（h=0.29H）。该公式可以用来预测顺层的侵位深度和寻找砂脉,为含油气盆地砂脉储层预测提供理论依据和新方法。通过流化公式计算获得了形成上层砂脉的流体上侵速度为1.26 m/s。通过本文的论述,进一步为理解灵山岛地区下白垩统的沉积环境和沉积动力学特征提供了新的岩石学证据。  相似文献   

11.
Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km2) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year−1 for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.  相似文献   

12.
川西新场地区须家河组储层埋藏深度大、成岩作用复杂、致密化程度高,但在整体超致密背景下,局部仍发育较多的相对优质储层.相对优质储层在埋深较大的须二段,往往发育较多的原生孔隙,和相对次要的次生孔隙;而埋深较小的须四段储层,次生孔隙占绝对优势,原生孔隙发育较少.造成这一现象的主要原因是须二、须四段原生孔隙保存和次生孔隙发育机制上的差异.须二段储层中较多刚性颗粒的存在和较为发育的包膜绿泥石是原生孔隙得到较好保存的主要原因;而其较大的单砂层厚度和较少的泥岩发育则导致了有机酸性流体的注入量较少,长石溶蚀有限.须四段塑性岩屑含量明显较高,储层原生孔隙在压实作用下几乎消失殆尽,但较薄的单砂层厚度和较多泥岩的叠置发育使长石在有机酸性流体作用下得到了充分溶蚀;这是须四段储层次生孔隙相对发育、长石含量很低、同时还有自生高岭石沉淀的主要原因.  相似文献   

13.
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   

14.
The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo‐Miocene basin that contains numerous salt‐walled mini‐basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono‐stratigraphic study of the Emirhan mini‐basin and its 2·6 km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized – playa‐lake, braided stream and lacustrine – each corresponds to a relatively long‐lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape‐fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini‐basin records the remobilization of diapir‐derived detritus and the presence of evaporitic bodies interbedded within the mini‐basin, implying the growth of salt walls expressed at the surface as palaeo‐topographic highs. The mini‐basin also records the signature of a regional change in stratigraphic assemblage, passing from playa‐lake facies to large‐scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini‐basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir‐derived detritus intercalated within the otherwise clastic‐dominated succession.  相似文献   

15.
ABSTRACT During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back-arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely-layered and parallel-laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast- and matrix-supported features, floating large clasts and absence of traction structures. Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment-gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high- and low-concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite-containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan-deltas, mainly by unchannellized sediment-gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris-apron systems, which provide alternatives to submarine-fan models in many clastic basins with a line rather than point source.  相似文献   

16.
砂岩侵入体是由处于浅埋藏阶段、尚未固结的砂质沉积物发生液化并侵入到上覆盖层所形成的一类软沉积物变形,在北海盆地维京地堑渐新统地层中非常发育。为探讨砂岩侵入体的形态特征及诱发机制,通过高分辨率三维地震及测井资料,利用地震反射结构分析、地震相干切片等手段对砂岩侵入现象进行了识别;并结合多边形断层系统、流体充注与砂岩侵入之间的关联性,对砂岩侵入体的成因机制进行了分析。结果表明:在地震剖面上可识别的砂岩侵入体多呈V型或W型强振幅反射特征,其横向展布规模约1~2 km,垂向侵入高度约100~200 m;流体的大规模充注及多边形断层诱发盖层破裂是形成砂体内部超压并诱发其发生液化的关键因素。砂岩侵入体在形成之后可以作为流体运移通道,对强化流体的垂向运移具有重要意义;并且砂岩侵入体本身即可作为油气的有利储集体。因此识别并分析砂岩侵入体的成因机理,对盖层封闭性评价及油气勘探具有重要指导意义。  相似文献   

17.
Episodic, large‐volume pulses of volcaniclastic sediment and coseismic subsidence of the coast have influenced the development of a late Holocene delta at southern Puget Sound. Multibeam bathymetry, ground‐penetrating radar (GPR) and vibracores were used to investigate the morphologic and stratigraphic evolution of the Nisqually River delta. Two fluvial–deltaic facies are recognized on the basis of GPR data and sedimentary characteristics in cores, which suggest partial emplacement from sediment‐rich floods that originated on Mount Rainier. Facies S consists of stacked, sheet‐like deposits of andesitic sand up to 4 m thick that are continuous across the entire width of the delta. Flat‐lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries. Beds of massive, pumice‐ and charcoal‐rich sand overlie one of the buried surfaces. Organic‐rich material from that surface, beneath the massive sand, yielded a radiocarbon age that is time‐correlative with a series of known eruptive events that generated lahars in the upper Nisqually River valley. Facies CF consists of linear sandbodies or palaeochannels incised into facies S on the lower delta plain. Radiocarbon ages of wood fragments in the sandy channel‐fill deposits also correlate in time to lahar deposits in upstream areas. Intrusive, sand‐filled dikes and sills indicate liquefaction caused by post‐depositional ground shaking related to earthquakes. Continued progradation of the delta into Puget Sound is currently balanced by tidal‐current reworking, which redistributes sediment into large fields of ebb‐ and flood‐oriented bedforms.  相似文献   

18.
The Barataria barrier coast formed between two major distributaries of the Mississippi River delta: the Plaquemines deltaic headland to the east and the Lafourche deltaic headland to the west. Rapid relative sea‐level rise (1·03 cm year?1) and other erosional processes within Barataria Bay have led to substantial increases in the area of open water (> 775 km2 since 1956) and the attendant bay tidal prism. Historically, the increase in tidal discharge at inlets has produced larger channel cross‐sections and prograding ebb‐tidal deltas. For example, the ebb delta at Barataria Pass has built seaward > 2·2 km since the 1880s. Shoreline erosion and an increasing bay tidal prism also facilitated the formation of new inlets. Four major lithofacies characterize the Barataria coast ebb‐tidal deltas and associated sedimentary environments. These include a proximal delta facies composed of massive to laminated, fine grey‐brown to pale yellow sand and a distal delta facies consisting of thinly laminated, grey to pale yellow sand and silty sand with mud layers. The higher energy proximal delta deposits contain a greater percentage of sand (75–100%) compared with the distal delta sediments (60–80%). Associated sedimentary units include a nearshore facies consisting of horizontally laminated, fine to very fine grey sand with mud layers and an offshore facies that is composed of grey to dark grey, laminated sandy silt to silty clay. All facies coarsen upwards except the offshore facies, which fines upwards. An evolutionary model is presented for the stratigraphic development of the ebb‐tidal deltas in a regime of increasing tidal energy resulting from coastal land loss and tidal prism growth. Ebb‐tidal delta facies prograde over nearshore sediments, which interfinger with offshore facies. The seaward decrease in tidal current velocity of the ebb discharge produces a gradational contact between proximal and distal tidal delta facies. As the tidal discharge increases and the inlet grows in dimensions, the proximal and distal tidal delta facies prograde seawards. Owing to the relatively low gradient of the inner continental shelf, the ebb‐tidal delta lithosome is presently no more than 5 m thick and is generally only 2–3 m in thickness. The ebb delta sediment is sourced from deepening of the inlet and the associated channels and from the longshore sediment transport system. The final stage in the model envisages erosion and segmentation of the barrier chain, leading to a decrease in tidal discharge through the former major inlets. This process ultimately results in fine‐grained sedimentation seaward of the inlets and the encasement of the ebb‐tidal delta lithosome in mud. The ebb‐tidal deltas along the Barataria coast are distinguished from most other ebb deltas along sand‐rich coasts by their muddy content and lack of large‐scale stratification produced by channel cut‐and‐fills and bar migration.  相似文献   

19.
The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south‐east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross‐bedded dune sandstone dominated by grain flow and translatent wind‐ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind‐ripple, wavy and horizontal planar‐laminated strata resulting from accumulation on a range of dry, damp or wet substrate‐types in the flats and hollows between migrating dunes; and (iii) extensive, near‐flat lying wind‐rippled sandsheet strata. Fluvial facies comprise channel‐fill sandstones, lag conglomerates and finer‐grained overbank sheet‐flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south‐east, whereas south‐westerly flowing fluvial systems periodically punctuated the dune fields from the north‐east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo‐water‐table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid‐plains that fed sediment to a coastline that lay to the south‐west and which ran approximately north‐west to south‐east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low‐relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non‐marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north‐eastward directed marine transgression into the basin prior to the onset of exclusively non‐marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small‐scale autocyclic behaviour of competing, coeval depositional systems and larger‐scale allocyclic changes that record system response to longer‐term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea‐level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio‐eustatic driving mechanism.  相似文献   

20.
Identifying the driving mechanisms of soft‐sediment deformation in the geological record is the subject of debate. Thawing of ice‐rich clayey silt above permafrost was proved experimentally to be among the processes capable of triggering deformation. However, previous work has failed so far to reproduce similar structures in sand. This study investigates fluidization and intrusive ice formation from soil models in the laboratory. Experimental conditions reproduce the growth of ice‐cored mounds caused by pore water pressure increase during freeze‐back of sand in a permafrost context. Excess pore water pressure causes hydraulic fracturing and the development of water lenses beneath the freezing front. Later freezing of the water lenses generates intrusive ice. The main structures consist of sand dykes and sills formed when the increase in pore water pressure exceeds a critical threshold, and soft‐sediment deformations induced by subsidence during ice melt. The combination of processes has resulted in diapir‐like structures. The experimental structures are similar to those described in Pleistocene sites from France. These processes constitute a credible alternative to the seismic hypothesis evoked to explain soft‐sediment deformation structures in other European regions subjected to Pleistocene cold climates.  相似文献   

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