Shallow marine sand bar sequences: an example from the late Precambrian of North Norway

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.     

Gravel-topped offshore bar sequences in the Lower Carboniferous of southern Ireland

Within the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave-produced structures; (b) sandstones dominated by swaley cross-stratification (SCS), parallel lamination, and rare hummocky cross-stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening-upward units of SCS pebbly sandstone capped by top-surface granule and pebble lags. The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow-marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm-generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long-term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel-bearing turbidity currents into muddy interbar trough areas.     

Eocene tidal sedimentation in the Anambra basin,Southern Nigeria

The Eocene Nanka Formation of the Anambra basin in southern Nigeria consists, in its type area, of four sand subunits each 50–90 m thick, separated by three gypsiferous glauconitic shales each about 2.5 m thick. The sand subunits are unconsolidated, uncemented quartzarenites, planar and trough cross-stratified, flaser and lenticular bedded, and burrowed (Ophiomorpha and Skolithos). Texturally the sands are medium to coarse, moderately sorted, positively skewed and leptokurtic. Two parallel belts, each with a distinctive paleocurrent pattern, are identified: one lying along the present eastern and northern margin of the formation, characterised by a shore-normal paleocurrent pattern; and the other lying on the west, distinguished by a shore-parallel orientation.An integrated analysis of stratigraphic, petrographic, textural, and structural data of the formation enabled reconstruction of an environmental model for the sand body. The model depicts sedimentation in a tidally influenced marine shoreline environment in which an intertidal and a subtidal zone can be delineated. Each zone is characterised by an assemblage of several sedimentary features. Interlaminations of shale and thin sand lenses, gypsiferous and glauconitic shale beds, flaser and lenticular bedding, herringbone structures, and shore-normal paleocurrent pattern are among the features that delineate the intertidal facies. The abundantly cross-stratified, coarse elastic sand-bars with rapid fining-upward sequences, burrowed pebbly horizons overlain by fine sand, and shoreparallel paleocurrent are the distinguishing features of the subtidal facies. The sandflat facies of the intertidal zone and the sandbar facies of the subtidal zone are, however, inseparable in most cases.     

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.     

Neoproterozoic siliciclastic sedimentation around Kampa-Tenpa area,Pranhita Godavari Graben (PGG), Central India: Facies analysis,petrography and depositional environment

The Pranhita-Godavari Graben (PGG) represents a major lineament in south Indian Peninsular cratonic province, which preserves a thick column of sediments. In geological time scale, these sedimentary units correspond to Mesoproterozoic to Mesozoic period. The Mesoproterozoic sedimentation has been confined mainly to southern part of the PGG, while Neoproterozoic sediments are exposed at northern tip of the graben. In the area of investigation siliciclastic sedimentation units are exposed, wherein five major lithofacies have been marked out. These lithofacies are i) breccia (Br), ii) large scale trough cross-bedded sandstone (Ls), iii) small-scale trough cross-bedded sandstone (Sss), iv) horizontal bedded sandstone (Hs) and v) ripple laminated sandstone (Rs). The amalgamation of these lithofacies strongly divulges that the lower part of the succession was deposited in braided-streams, whereas the upper part was deposited in erg environment. The unimodal paleocurrent is evident in lower part of the succession while bimodal paleocurrent is noticed from sandstones at upper part of the succession. In general, the sandstones exhibit northwesterly paleocurrent direction. The petrographic studies point out that the sandstones are arenites and were deposited in interior type of continental block provenance under semi-arid to hot humid palaeoclimate. The high percentage of floating grains and the low percentage of interpenetrative contacts as well as the low value of contact index for these sandstones divulge that the grains did not suffer much compaction thereby pointing to shallow burial. The sub-angular zircon and tourmaline suggest that these minerals have travelled short distance from the source rocks. The high percentage of garnet and presence of epidote, staurolite, sillimanite, zoisite, amphibole and kyanite indicate that the sediments were derived from the nearby metamorphic source rocks.     

Sedimentological synthesis of permian fluviatile sediments of East Bokaro basin,Bihar, India

The early Permian Karharbari and Barakar formations of East Bokaro basin comprise the following lithofacies:Lithofacies A, consisting of monomictic cobble- and pebble-conglomerate including pebbly sandstone, with faint crossbeds; deposited mainly by high-velocity aqueous currents as channel-lag deposit or longitudinal bars.Lithofacies B, coarse to medium sandstone, profusely crossbedded; interpreted as channel facies formed by downcurrent migration of sand bars in low-sinuosity streams.Lithofacies C, mainly fine sandstone with interbedded siltstone, characterised by small-scale ripple-lamination; formed in a low-energy environment such as swale-fill and/or overbank deposits.Lithofacies D, including carbonaceous shale and coal, with lack-of-current structures, indicating quiet-water deposition; interpreted as backswamp and lacustrine deposits.The overall context of the Karharbari and Barakar assemblage with relative abundance of pebbly coarse sandstone in the former and fine clastics in the latter, the presence of fining-upward cycles, widespread development of tabular and trough crossbedding and sample to sample variation of foreset azimuths, all suggest a generally alluvial environment. Analysis of crossbedding dip azimuth and dimensional fabric suggest that the sediment milieu in either case consisted of streams flowing persistently from the south-southeast to north-northwest direction. It is inferred that the unidirectional system of streams flowing across the East Bokaro basin changes systematically in channel sinuosity through time.     

Tidal shelf sedimentation in the Neoproterozoic Chattisgarh succession of central India

The Neoproterozoic Kansapathar Sandstone of the Chattisgarh basin, a shallow marine shelf bar sequence, consists of mineralogically and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. The sediments were transported, reworked and deposited in subtidal environments by strong tidal currents of macrotidal regime as well as storms, and accumulated as discrete shoaling-upward features, separated from each other by muddy to low-energy sandy deposits. The sandbodies developed into shoaling up linear bars, often more than a kilometre in length, through accretion of thick cross-stratified units in transverse directions under the influence of ebb and flood tidal currents, as well as in longitudinal direction affected by southeasterly flowing along-shore currents. The aggrading upper surfaces of the bars experienced protracted reworking by strong oscillatory wave currents leading to extensive development of subaqueous 2D or 3D dunes mantled with lag pebble deposits at different points. With continued shoaling and progradation, the bars amalgamated into large sandstone sheets with the development of high energy beach deposits and coastal sand flats in the uppermost part of the sequence. The presence of rill marks, flat-topped ripples, wrinkle marks, desiccation cracks and adhesion warts point to intertidal conditions with intermittent exposure. The high energy sandstone bars overlie a thick mudstone-dominated shelf sequence across a sharp interface indicating rapid change in the sea-level, provenance, rate of sediment generation and sediment input, and circulation condition in the shelf. A quiet muddy shelf was replaced by a major sand-depositing environment with strong, open marine circulation. An interplay of tidal currents, oscillatory wave currents and storm currents generated a complex flow pattern that varied in time and space from bimodal-bipolar to strongly unimodal flows. Close parallelism of wave ripple crests, trend of linear bars and unidirectional flows suggest that the elongate bars were parallel to sub-parallel to the coastline, and were strongly influenced by along shore drift. The inferred coastline was broadly N-S. The large-scale structures in the bar sandstones, emplacement of vast amount of sand and migration of large bedforms under strong macrotidal currents collectively indicate that the Kansapathar shelf was intimately connected with an open ocean basin towards north-northwest.     

Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin,North‐eastern Spain

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.     

Depositional features of a late Weichselian outwash fan; central East Jylland,Denmark

Four major sedimentary facies are present in coarse-grained, ice-marginal deposits from central East Jylland, Denmark. Facies A and B are matrix-supported gravels deposited by subaerial sediment gravity flows as mudflows (facies A) and debris flows (facies B). Facies C consists of clast-supported, water-laid gravels and facies D are cross-bedded sand and granules. The facies can be grouped into three facies associations related to the supraglacial and proglacial environments: (1) the flow-till association is made up of alternating beds of remobilized glacial mixton (facies A) and well-sorted cross-bedded sand (facies D); (2) the outwash apron association resembles the sediments of alluvial fans in containing coarse-grained debris-flow deposits (facies B), water-laid gravel deposited by sheet floods (facies C) and cross-bedded sand and granules (facies D) from braided distributaries; (3) the valley sandur association comprises water-laid gravel (facies C) interpreted as sheet bars and longitudinal bars interbedded with cross-bedded sand and granules (facies D) deposited in channels between bars in a braided environment.The general coarsening-upward trend of the sedimentary sequences caused by the transition of bars and channel-dominated facies to debris-flow-dominated facies indicate an increasing proximality of the outwash deposits, picturing the advance and still stand of a large continental lowland ice-sheet. The depositional properties suggest that sedimentation was caused by melting along a relatively steep, active glacier margin as a first step towards the final vanishing of the Late Weichselian icesheet (the East Jylland ice) covering eastern Denmark.     

Regional sedimentological variations in lower triassic fluvial conglomerates (budleigh salterton pebble beds), southwest england: Some implications for palaeogeography and basin evolution

The Budleigh Salterton Pebble Beds (BSPD) are a 20–30 m thick formation of conglomerates and subordinate sandstones which crop out along the western margin of the Wessex Basin. The formation has previously been interpreted as representing a major conduit for southerly-derived (Armorican) detritus and as signalling the start of early Triassic rifting. In this paper the role of the BSPB in the evolution of the Wessex Basin is reassessed. In the south of the outcrop, the lower portions of the BSPB are dominated by extensive (> 50m) sheets or narrow lenses of planar cross-bedded conglomerate (sets 1–3m thick). This reflects deposition from linguoid-shaped bars whose downstream margins were bounded by slipfaces. These accreted at anabranch confluences and as bank-attached bars in relatively confined and channelized gravel-bed streams. Most of the rest of the BSPB consists of couplets of horizontally-bedded conglomerate overlain by large-scale trough cross-bedded sandstone. This style of accretion represents deposition from relatively low-relief gravel bars and large sand dunes in substantial but poorly confined channels. Towards the top of the BSPB in the central parts of the outcrop, there is a change to thinner-bedded units with more lenticular and ribbon-like geometries. This represents deposition in smaller and more flashy streams. The BSPB was deposited either on a braidplain or by an antecedent ‘wet’ alluvial fan. In both settings, the BSPB streams were exotic, draining basins to the south of the Wessex Basin. This implies that the BSPB did not necessarily develop in response to differential subsidence and rifting within the Wessex Basin.     

Nagthat Formation: An example of a progradational,tide-dominated Proterozoic succession in Kumaun Lesser Himalaya,India

The Proterozoic Nagthat Formation of the Krol-belt succession, in the Nainital area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and conglomerate horizons. The association with spilitic lava flows, variable palaeocurrent trends and the restricted lateral extent of the Nagthat Formation within the Krol-belt succession imply an active role for tectonism in the basin of deposition. In the upward coarsening succession of the Nagthat Formation, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-grained quartzarenite (Lithofacies B), horizontally laminated, fine-grained quartzarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and matrix-supported conglomerate (Lithofacies F). The constituent lithofacies are repetitive in nature, forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars, barrier-beachface, tidal channels (inlets), intertidal–sandflat–mixedflat environments and, occasionally, in the form of gravity flows in subtidal channels. The general upward coarsening succession of the Nagthat Formation represents deposition in a progradational (regressive) barrier island system. The palaeocurrent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW–SE trending shoreline, in response to a dominating flood tidal current system. The palaeocurrent pattern shows higher variability in the upper shore-face deposits than in the tidalflat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S directions.     

Fluvial to marine transition in the Ordovician of Ireland–A humid-region fan-delta?

The Mweelrea Group (Llanvirnian) in the South Mayo Inlier, western Ireland, consists mainly of coarse shallow-water sandstones with minor slates, cobble conglomerates, and ignimbrite tuffs. The Glenummera Formation at the base (up to 600 m), dominantly well-cleaved slates with some slump features, was deposited in a slope to deep marine shelf environment. A coarsening upward sequence at its top reflects the interaction of fluvial and marine processes, coarse clastic sediments having prograded from the southeast. The rest of the Mweelrea Group (2100 m in the northwest) is dominated by coarse trough cross-bedded sandstone with little evidence for channelling. Three lenticular marine slates (Glendavock, Uggool, and Glenconnelly Formations), up to 200 m thick, wedge out to the southeast. A humid alluvial fan-fan-delta model can explain many features of the Mweelrea Group. Fans built out to the northwest into a relatively small basin with negligible tides and moderate wave energy. Marginal sediments were reworked by waves, occasional storms, and a burrowing fauna.     

Lithofacies attributes, depositional system and diagenetic properties of the Permian Gharif Formation from Haushi–Huqf area, Central Oman

Over 70 m thick interbedded sandstone, siltstone and claystone of the upper member of the Gharif Formation are exposed in western Huqf area in Oman Interior Sedimentary Basin. The Gharif Formation, particularly its upper member hosts major hydrocarbon reservoir in the subsurface of the Oman Interior Sedimentary Basin. The upper member of the Gharif Formation is comprised of interbedded thick sandstone, siltstone, carbonaceous clays and intraformational conglomerates. The sandstone lithofacies, on average, constitute 10 m thick multistoreyed sequences, which are composed internally of 2–3 m thick and 100 s of metres across vertically and laterally amalgamated sandstone bodies. Two major types of sandstones (types 1 and 2) are identified on the basis of their lithofacies association and internal architecture. The type 1 sandstone constitutes the lower part of the member and is comprised of pebbly to coarse-grained, planar and trough cross-bedded sandstone, plane bedded sandstone and pebble lags at the base of major sandstone bodies. The cross-beds are, on average, 30 cm thick exhibiting a dominant paleoflow direction towards NW (280–300° N). It is interpreted to be deposited by low sinuosity braided streams. The type 2 sandstone constitutes the upper part of the member and is comprised of medium-grained sandstone, trough to low angle plane bedding associated with lateral accretion surfaces. It is commonly interbedded with carbonaceous clays. Silicified plant fragments are commonly distributed in the upper part of the sandstone. Interbedded clays and siltstones are red, mottled and extensively bioturbated due to root burrows. It is interpreted to be deposited by high sinuosity meandering streams. In the uppermost part of the section, several dark grey to black carbonaceous clay/coal beds with plant matter are interbedded with sandstone and red clay indicating development of swampy conditions during onset of the coastal setting in the uppermost part of the formation. About 30 cm thick bioclastic sandstone deposited by the marine coastal bars mark transition from the Gharif Formation to carbonate dominated Khuff Formation. The sandstone of the Gharif Formation is arkosic in composition. Very small amount of cement and negligible compaction of constituent grains in sandstone indicates shallow burial before uplift.     

Studies in fluviatile sedimentation: Bars,bar-complexes and sandstone sheets (low-sinuosity braided streams) in the brownstones (L. devonian), welsh borders

The fine to very coarse sandstones, gravelly sandstones and intraformational conglomerates of the mid to upper Brownstones are excellently exposed in large fresh road cuttings near Ross-on-Wye in the southern Welsh Borders. Detailed mapping of the cuttings reveals an hierarchically ordered system of mainly erosional bedding contacts which divide the beds into hierarchically structured packets. The smallest packets, involving cross-bedded or plane-bedded sediments or combinations of these, are consistent with deposition from strongly three-dimensional and often large, loosely periodic to non-repetitive bars. A locally developed facies of trough cross-bedded sandstones points to the infrequent occurrence of fields of three-dimensional dunes. The bar- and dune-related units are grouped into large complexes (related to the storeys of other workers), with an internal geometry consistent with lateral accretion (in places clearly symmetrical) combined with forward accretion on shoals (sand flats) within a braided channel, as in the South Saskatchewan River. In their turn, the complexes are combined into laterally extensive, conglomerate-floored sandstone sheets several metres thick. These seem to express the wandering of a braided channel across a mud-draped floodplain. To judge from the sedimentary structures and textures, the thickness of the lateral accretion deposits, and the size of the major scours, the bankfull discharge of the rivers was a few thousand cumecs each.     

Probable late Messinian tsunamiites near Monte Dei Corvi, Italy, and the Nijar Basin, Spain: expected architecture of offshore tsunami deposits

Three distinct, 30- to 80-cm-thick, graded, multilayered, coarse-grained sandstone layers, intercalated in the late Messinian mudstones of the Colombacci formation in Lago Mare facies of the Trave section are interpreted as tsunamiites (Ts1?CTs3). Each of these layers is sheet-like and could be followed along strike over several tens of meters. The lower two layers (Ts1?CTs2) occur in the lower part of the Colombacci formation and the third (Ts3) just below a conspicuous white ??colombacci?? limestone near the top of the formation. The three sandstone layers represent unique sedimentary events within the 120-m-thick San Donato-Colombacci mudstones, which contain many thin, fine-grained, possibly storm-related turbidites. Each of the three clastic layers is overall graded and strongly cross-bedded. A single layer consists of a stack of several graded sublayers that are eroded into the underlying mudstones and into each other. Absence of hummocky cross-stratification (HCS) indicates that the layers are not produced during a large, catastrophic storm event. Current ripples such as dm-sized trough cross-beds suggest strong, prolonged, unidirectional currents, capable of carrying coarse conglomeratic sands. Climbing ripples in middle-fine sand units indicate a high suspension load settling under waning current strength. Each of the Ts1?CTs3 beds satisfies a combination of criteria, described in this paper, that allow interpretation as a tsunamiite in an offshore environment. Tsunamiite Ts2 is underlain by a 15-cm-thick meshwork of synsedimentary fissures, filled with coarse sand. Ground movements induced by strong earthquakes probably caused these crevasses. The uniqueness of each layer, the erosion of the base of each of the sublayers into underlying mudstones and previously deposited sublayer and the consistent stacking of graded sandstone beds within each of the three layers, underlain by earthquake-produced fissures, strongly point to deposition by traction currents produced by the surges of a large tsunami event, triggered by a large vertical fault movements. Vertical fault displacements most likely occurred along the thrust faults like the Sibilline thrust at the SW of the Laga foreland basin, which were active at late Messinian times. A series of cyclic graded turbidites, underlain by seismically induced sand-filled fissures in the Late Messinian Feos formation in SE Spain, are interpreted as tsunamiites produced by a tsunami or seiche.     

塔里木盆地大宛齐油田康村组储层特征及评价

在地层对比的基础上,深入研究了大宛齐油田康村组储层的岩石学特征、储集砂体成因类型及其特征、储层孔隙类型、孔隙结构特征及储层分布规律.大宛齐油田康村组属于低弯度河进入极浅水湖泊形成的三角洲沉积体系,决定了它主要的储集砂体成因类型为三角洲砂体,其中又可细分为三角洲平原分流河道砂体、三角洲前缘水下分流河道砂体和河口砂坝砂体,主要储集空间类型是剩余原生粒间孔隙,其次是粒间溶孔和微孔隙.分流河道砂体以粒度较粗的中粗砂岩、含砾砂岩和细砂岩为主,发育大孔隙,粗喉道,孔隙度大,渗透率高,储集性能好;河口砂坝砂体,粒度相对较细,以细-粉砂岩为主,孔隙度相对较低,孔径也相对较小.纵向上,康村组中下部砂体较发育,上部次之,以砂泥互层形式叠置,但总体上砂岩含量不高,一般在15%~30%之间;康村组储层横向分布受物源等因素影响,呈现出东北部厚、西北部薄的特点.储层物性总的分布规律是由北东往南西方向,孔隙度减小,由南东往北西方向渗透率变差.      

Sedimentary facies analysis and seismic interpretation of deep-water reservoirs, “PAC” field,offshore depobelt,Niger Delta basin

A high-resolution study involving the integration of cores, wireline logs and reflection seismic dataset was used to unravel the facies assemblages, reservoir quality and hydrocarbon potential of E1 and H9 reservoirs in the “PAC” field, Offshore Niger Delta. Facies analysis of the cored PAC-14 well shows six lithofacies: massive mudstone, parallel-laminated mudstones with sideritic bands, fine-grained parallel-laminated sandstone, medium-grained parallel-laminated sandstone, fine-grained ripple laminated sandstone, and coarse massive sandstone. These lithofacies were grouped into five facies associations: channel story axis (CSA), channel story margin (CSM), inter-channel thin beds (ICTB), mud-rich thin beds (MRTB) and injectites (INJ). These facies associations are typical of a confined channel and basin floor fan deposit of a deep-water turbidite depositional environment. Reservoir unit E1 was dominated by the facies association CSA while the H9 reservoir consists mainly of the facies associations MRTB and INJ. The observed sand injectites are suggested to have formed due to overloading of compacted sands leading to upward remobilization of sand into the overlying shale. Seismic horizons and faults were mapped to understand the structures, trend and reflections within the study area. Results from petrophysical volumetrics estimated the net hydrocarbon pore volume for the E1 and H9 reservoirs as 456 MMBBL and 378 MMBBL, respectively. Exploration for deep-water reservoir mainly targets high amplitude and bright seismic reflectors and ignores the low amplitude reflectors (dim loops) which are typically interpreted as non-reservoir units. However, this study shows a low-amplitude reflector containing medium-grained sand injectite unit, which has high porosity (31.7%) and permeability (4472 mD) values, and contains recoverable hydrocarbon.     

An iron-bearing wave-dominated siliciclastic shelf: Facies analysis and palaeogeographic implications (Silurian-Lower Devonian Sierra Grande Formation,Southern Argentina)

The Sierra Grande Formation (Silurian-Early Devonian) consists of quartz arenites associated with clast supported conglomerates, mudstones, shales and ironstones. Eight sedimentary facies are recognized: cross-stratified and massive sandstone, plane bedded sandstone, ripple laminated sandstone, interstratified sandstone and mudstone, laminated mudstone and shale, oolitic ironstone, massive conglomerate and sheet conglomerate lags. These facies are interpreted as shallow marine deposits, ranging from foreshore to inner platform environments. Facies associations, based on vertical relationships among lithofacies, suggest several depositional zones: (a) beach to upper shoreface, with abundant plane bedded and massive bioturbated sandstones; (b) upper shoreface to breaker zone, characterized by multistorey cross-stratified and massive sandstone bodies interpreted as subtidal longshore-flow induced sand bars; (c) subtidal, nearshore tidal sand bars, consisting of upward fining sandstone sequences; (d) lower shoreface zone, dominated by ripple laminated sandstone, associated with cross-stratified and horizontal laminated sandstone, formed by translatory and oscillatory flows; and (e) transitional nearshore-offshore and inner platform zones, with heterolithic and pelitic successions, and oolitic ironstone horizons. Tidal currents, fair weather waves and storm events interacted during the deposition of the Sierra Grande Formation. However, the relevant features of the siliciclastics suggest that fair weather and storm waves were the most important mechanisms in sediment accumulation. The Silurian-Lower Devonian platform was part of a continental interior sag located between southern South America and southern Africa. The Sierra Grande Formation was deposited during a second order sea level rise, in which a shallow epeiric sea flooded a deeply weathered low relief continent.     

Late Neogene loess deposition in southern Tarim Basin: tectonic and palaeoenvironmental implications

Uplift of the Tibetan Plateau during the late Cainozoic resulted in a thick apron of molassic sediments along the northern piedmonts of the Kunlun and Altyn Mountains in the southern Tarim Basin. Early Neogene sediments are characterised by sandstone, siltstone and red mudstone, representing floodplain to distal alluvial fan environments. The Early Pliocene Artux Formation consists of medium-grained sandstone and sandy mudstone with thin layers of fine pebbly gritstone. The Late Pliocene to Early Pleistocene Xiyu Formation is dominated by pebble to boulder conglomerate typical of alluvial fan debris flow deposits. Sedimentological investigation, together with grain size and chemical analyses of siltstone bands intercalated with sandstone and conglomerate in the Xiyu and Artux Formations, point to an aeolian origin, suggesting desertic conditions in the Tarim Basin by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau inferred from the lithofacies change from fine-grained mudstone and sandstone to coarse clasts. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.     

松辽盆地西部斜坡英台地区姚二、三段辫状河三角洲沉积特征研究

松辽地区西部斜坡英台地区姚家组二、三段,是在干旱一半干旱、源近流短、东倾陡坡背景下,在洪积扇前,由辫状河入湖形成的辫状河三角洲沉积。可明显区分出狭窄的辫状河三角洲平原,河口坝不太发育的辫状河三角洲前缘和夹浊积砂的辫状河前三角洲三种沉积亚相。辫状河三角洲平原由灰色含砾砂岩,粗、中砂岩,少量细砂岩,粉砂岩夹紫红色泥岩组成。具有明显的正韵律为特征。辫状河三角洲前缘沉积为灰色-绿灰色中、粗砂岩、细砂岩、少量的含砾砂岩,成正韵律或反韵律夹在灰绿色、灰黑色泥质岩中。辫状河前三角洲沉积多为灰绿色、灰黑色泥质岩,可夹少量砂质浊积岩。上述沉积特征明显地区别于正常河流三角洲沉积和扇三角洲沉积。