The Cretaceous Uhangri Formation, SW Korea: lacustrine margin facies

The Uhangri Formation forms part of the Cretaceous sedimentary sequence deposited in a series of inland basins in the south-western Korean Peninsula. It comprises an approximately 400-m-thick epiclastic sequence of conglomerate, (gravelly) sandstone, cherty mudstone and black shale. The entire sequence can be represented by 16 distinctive sedimentary facies organized into four facies associations. Facies association I is characterized by thick homogeneous brownish siltstone, wedge-shaped disorganized conglomerate and thinly interlayered gravelly sandstone units. The siltstone units were formed by large floods submerging the alluvial fan fringe (floodplain), whereas the conglomerate and gravelly sandstone units were deposited by sheetfloods and debris flows. Facies association II consists of stratified conglomerate — gravelly sandstone, laminated sandstone and sandstone/siltstone couplets which form fining-upward cycles. Some facies units are low-angle trough cross-bedded and show broad channel geometries. This association represents subaqueous delta lobes fed by high- and low-concentration turbidity currents in the distal delta realm. Facies association III is characterized, by wedged conglomerate and gravelly sandstone facies with interfingered massive sandstone bounded by scoured bases. It represents a delta front where distributary channels and mouth bars are dominant. Facies association IV consists of laterally continuous sequence of laminated black shale, crudely stratified sandstone and convoluted sandstone/cherty mudstone. This facies association is suggestive of depositional processes controlled by chemical equilibrium resulting from an interaction between density inflows and lake water. The cherty mudstone resulted from inorganic precipitation from siliceous solution provided by acidic volcanism. The Uhangri sequence generally shows a fining-upward trend with a transition from alluvial fan fringe, coarse-grained subaqueous delta, to shallow lake. The retrogradation was probably due to continuous subsidence related to continental rifting in the oblique-slip mobile zone.     

Early Pleistocene glacial outwash deposits in the eastern San Joaquin Valley, California: a model for humid-region alluvial fans

ABSTRACT The early Pleistocene Laguna and Turlock Lake Formations and China Hat and Arroyo Seco Gravels along the east side of the San Joaquin Valley, California, were deposited in alluvial fans and marginal lakes. Upward-coarsening sequences of silt-sand-gravel record westward progradation of glacial outwash fans from the Sierra Nevada into proglacial lakes in the San Joaquin Valley. Distinctive sedimentary features delineate lacustrine, prodelta, and delta-front facies within fan-margin deposits and lower, middle, and upper-fan facies within alluvial-fan deposits. The lacustrine facies consists of a few metres of thinly and evenly bedded, rhythmically laminated claystone and clayey siltstone in varved couplets. Draped lamination, sinusoidal lamination, and load and pillar structures occur in some beds. Siltstone and claystone grade upward to slightly thicker wavy beds of siltstone and very fine-grained unconsolidated sand deposited in a prodelta setting. Convolute laminae within deformed steeply dipping foreset beds suggest slumping on the prodelta slope. The prodelta facies grades up to the delta-front facies, which consists of burrowed and bioturbated cross-bedded fine sand. Deltaic deposits are 5–6 m thick. The lower-fan facies forms the base of the fan sequence and consists of several metres of irregularly bedded, laminated, oxidized siltstone and fine sand. The middle-fan facies consists of cross-bedded, medium-grained to gravelly sand-filled channels cut into the lower-fan facies. Interbedded lens-shaped siltstone beds 2 m thick and several metres across were deposited in abandoned channels. The upper-fan facies consists of moderately to strongly weathered clayey gravel and sand containing pebble imbrication and crude stratification. Argillization during post-depositional soil formation has blurred the distinction between mud-supported debris-flow deposits and clast-supported channel deposits, but both are present in this facies. The deposits described here demonstrate the need for additional fan models in order to incorporate the variety of deposits developed in alluvial fan sequences deposited in humid climates. In previous models based on arctic fans, debris flows, abandoned channels, or widespread siltstone beds are not present in fan sequences, nor are marginal lacustrine and deltaic deposits well represented.     

Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers

Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross-bedding, arranged in coarsening-upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross-bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies. Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension. In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model.     

Tectonic controls on spatio-temporal development of depositional systems and generation of fining-upward basin fills in a strike-slip setting: Kyokpori Formation (Cretaceous), south-west Korea

Abstract The Kyokpori Formation (Cretaceous), south‐west Korea, represents a small‐scale lacustrine strike‐slip basin and consists of an ≈ 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep‐gradient large‐scale delta slope; (IV) base of delta slope to prodelta; (V) small‐scale nested Gilbert‐type delta; (VI) small‐scale delta‐lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large‐scale steep‐sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening‐ to fining‐upward succession (up to 215 m thick) and the upper fining‐upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. Stage 1 is represented by thick coarse‐grained deposits in the lower succession that form subaqueous breccia talus and steep‐sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests a half‐graben structure for the basin, and the thick accumulation of coarse‐grained deposits most likely reflects rapid subsidence of the basin floor during the transtensional opening of the basin. Stage 2 is marked by sandy black shale deposits in the upper part of the lower succession. The black shale is readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large‐scale dip slip suppressing the lateral slip component on basin‐bounding faults. Stage 3 is characterized by gravelly delta‐lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip‐slip displacement on basin‐margin faults can be regarded as the prime control for determining stacking patterns of such basin fills. The resultant basinwide fining‐upward sequences deviate from the coarsening‐upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike‐slip basins controlled by the changes in relative sense and magnitude of fault movements at the basin margins.     

浙江桐庐晚奥陶世晚期沉积层序和沉积环境分析

文昌组上段顶部是一套潮汐层理非常发育的泥质砂岩或砂质泥岩,存在双向交错层理,层面有雨痕,应为潮坪沉积。潮坪沉积由小型层序构成,小型层序又是由砂、泥质单层组成。砂质单层底部通常为岩性突变面或侵蚀面,砂质纹层较厚,其中可见对称波痕或泥砾;向上砂质纹层变薄,过渡到泥质单层。砂质单层形成于暴风浪时期,泥质单层是风浪衰减后恢复正常的潮汐沉积。因此,小型层序从成因上说是一风暴层序。碎屑成份、砾石成份分析表明沉积物均来自华夏古陆的沉积岩和变质岩基底。物源一致,岩层产状变化不大,反映文昌组沉积环境稳定。岩性、粒度分析表明文昌组是一向上变细、由浅海高能环境向近岸低能环境过渡的沉积层序。文昌组下段为浅海砂岩沉积,上段顶部为潮坪沉积。二者之间是一套夹砾岩透镜体的泥质粉细砂岩,其沉积环境应介于浅海和滨岸之间,为水下岸坡沉积。砾岩层只是大的沉积旋回中出现的事件性水下冲积物。     

Depositional environments of some Pleistocene coastal terrace deposits,southwestern Oregon—case history of a progradational beach and dune sequence

Pleistocene coastal terrace deposits exposed in sea cliffs near Gold Beach, Oregon can be divided into four stratigraphic units: a basal gravelly unit and three overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit consists of gravel and sand in its lower part, sand, in part pebbly or cobbly, in its middle part, and mud and sand in its upper part. Black sand and transported pieces of wood are common in the middle part of the unit, and wood is common in the mud. This unit is interpreted as a progradational deposit including environments ranging from lower forebeach at the base to backbeach flats and streams at the top.The main sandy parts of the sandy units are made up of a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding, and an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures. Deciding between shallow marine and eolian interpretations of the sandy units proved exceptionally difficult until modern analogues were found in the fine details of the internal structures. Largely on the basis of such structural details, the crossbedded sand facies is interpreted as the product of small eolian dunes, and the irregularly bedded sand facies is interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols at the tops of the sandy units represent times of temporary stabilization of the dune field.     

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

Sedimentology and tectonic implications of Cretaceous fan-delta conglomerates, Queen Charlotte Islands, Canada

The Honna Formation, of Coniacian age, consists of several hundred metres of polymictic clast-supported conglomerate associated with sandstone and mudstone. Five conglomerate facies are recognized: ungraded beds; inverse graded beds; normal graded beds; inverse-to-normal graded beds; and parallel-stratified beds. These facies are interpreted as the deposits of subaqueous cohesionless debris flows and/or high-density turbidity currents. The depositional environment was a deep-water, gravelly fan that draped a fault-controlled, basin-margin slope. The fan is inferred to have passed upslope directly into an alluvial fan (unpreserved); hence, the name fan delta can be applied to the overall depositional system. This type of fan delta, of which the Brae oilfield in the North Sea is an example, is defined here as a deep-water fan delta. The lack of a shelf is in marked contrast to other types of fan delta. Three facies associations are recognized in the Honna Formation: subaqueous proximal-fan conglomerates, distal-fan turbiditic sandstones, and pro-fan/interfan mudstones with thin sandy turbidites. The proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km; such a long subaqueous gravel belt lacks a known modern analogue. The distal fan was an unchannelled sandy extension of the proximal gravel belt. It is postulated that the Honna Formation accumulated in a foreland basin which migrated westwards from the Coast Mountains where the Wrangellia-Alexander terrane was colliding with North America. In this model, the Honna fan delta was sourced by a (west-verging) thrust sheet whose sole-thrust was the Sandspit Fault immediately to the east. Deep-water fan deltas appear to develop preferentially when eustatic sea-level is relatively high, so that the‘feeder’ alluvial fan is small, and gravelly throughout. In petroleum exploration and field development, care should be taken to distinguish deep-water fan deltas from base-of-slope (canyon-fed) submarine fans, because the two systems differ significantly in terms of coarse-sediment distribution.     

Sedimentology of a lacustrine fan-delta system, Miocene Horse Camp Formation, Nevada, USA

A 1600-m-thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east-central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan-delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan-delta facies associations include: ungraded, matrix-and clast-supported conglomerate; normally graded, matrix- and clast-supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan-delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan-delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan-delta environment by debris flows, high- and low-density turbidity currents, and suspension fallout. The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High-energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore-directed currents produced uncommonly large (typically 1–2 m thick) trough cross-stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high-energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout. A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan-delta, subaqueous fan-delta and offshore lacustrine deposition. This model suggests that high-energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan-delta environment and promoting development of a well-defined nearshore lacustrine environment. Low-energy nearshore currents induced by shallow water along the     

高邮凹陷北斜坡戴二段辫状河三角洲沉积特征和演化

以岩心、录井、测井、分析化验和地震资料为基础,结合区域地质背景,对高邮凹陷北斜坡戴二段沉积相类型、展布及其演化规律进行研究。在总结、比较各类型三角洲特征的前提下,认定北斜坡戴二段主要发育辫状河三角洲相,局部发育曲流河三角洲和湖泊相。在此基础上,重点对戴二段辫状河三角洲各亚相、微相进行分析,划分出辫状河三角洲平原、辫状河三角洲前缘和前辫状河三角洲亚相,并针对各亚相对其包含的微相进行进一步划分。在综合考虑目标区域沉积背景和沉积特征的基础上,对沉积相展布和演化进行分析,并对辫状河三角洲形成机理进行了探讨。     

酒西盆地下白垩统下沟组重力流水下扇沉积

重力流水下扇由四种岩相组成:1.砾岩相,属水下碎屑流沉积;2.砾质泥岩相,是多成因的;3.砂岩相,系高密度浊流沉积;4.粉砂岩泥岩相,为低密度浊流和正常湖泊沉积。细分为十五种亚相。岩相的空间配置关系表明水下扇是由突变性洪水事件(和水下滑坡)-水下碎屑流-高密度浊流-低密度浊流的重力流系列形成。本文对重力流沉积从层序结构、沉积体形态、岩相变化和构造控制诸方面进行了探讨。     

Sedimentology and stratigraphy of a transgressive, muddy gravel beach: Waterside Beach, Bay of Fundy, Canada

Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.     

湖相重力流沉积特征及沉积模式

应用深水沉积学和地震沉积学的相关理论,通过岩心观察描述、钻测井资料分析及平面沉积相编图,对下刚果盆地A区块白垩系Pointe Indienne组深水重力流的类型、沉积特征、垂向沉积组合及沉积模式进行了探讨分析,指出该地区发育砂质碎屑流、泥质碎屑流、浊流及与重力流形成过程相关的滑动—滑塌沉积,并总结了该深水重力流的沉积模式。结果表明:砂质碎屑流沉积以块状层理细砂岩为主,含大型漂浮泥砾和泥岩撕裂屑;泥质碎屑流沉积以泥级碎屑为主,含有少量的暗色泥岩碎屑和砂质团块,见“泥包砾”结构;浊流沉积以发育完整或不完整的鲍马序列为特征;滑动—滑塌沉积具有明显的剪切滑移面,可见旋转火焰构造、砂岩扭曲杂乱分布及褶皱变形层;纵向上可识别出4种类型的重力流沉积垂向组合,以多期砂质碎屑流沉积叠置和砂质碎屑流沉积与浊流沉积叠置最为常见;研究区深水重力流沉积可分为上部扇、中部扇和外部扇3部分,上部扇以主水道沉积为主;中部扇以辫状水道和溢岸沉积为主,砂体厚度较大;外部扇以朵叶体沉积和薄层浊积岩为主,砂体厚度相对较薄。     

Lithofacies and origin of late Quaternary mass transport deposits in submarine canyons, central Scotian Slope, Canada

Mass transport deposits, up to 3·9 m thick, have been identified from piston cores collected from canyon floors and inter-canyon ridges on the central Scotian Slope. These deposits are characterized by four distinct mass-transport facies – folded mud, dipping stratified mud, various types of mud-clast conglomerate, and diamicton. Commonly, the folded and stratified mud facies are overlain by mud-clast conglomerate, followed by diamicton and then by turbidity current deposits of well-sorted sand. Stratified and folded mud facies were sourced from canyon walls. Overconsolidation in clasts in some mud-clast conglomerates indicates that the source sediment was buried 12–33 m, much deeper than the present cored depth, implying a source in canyon heads and canyon walls. The known stratigraphic framework for the region and new radiocarbon dating suggests that there were four or five episodes of sediment failure within the past 17 ka, most of which are found in more than one canyon system. The most likely mechanism for triggering occasional, synchronous failures in separate canyons is seismic ground shaking. The facies sequence is interpreted as resulting from local slides being overlain by mud-clast conglomerate deposits derived from failures farther upslope and finally by coarser-grained deposits resulting from retrogressive failure re-mobilizing upper slope sediments to form debrisflows and turbidity currents.     

新疆塔尔朗沟剖面中二叠统扇三角洲混合沉积特征及模式

在精细测量和详细观察新疆塔尔朗沟剖面二叠系露头的基础上,系统研究了中二叠统大河沿组与塔尔朗组扇三角洲相内陆源碎屑与碳酸盐组分的混合沉积特征。结果表明:大河沿组以扇三角洲平原分流河道沉积为主,其下部主要发育棕红色含粗砾中砾岩,碳酸盐组分不发育;上部主要发育棕红色含中砾细砾岩,夹若干层条带状分布的砾质泥晶灰岩。塔尔朗组底部自下而上发育扇三角洲前缘水下分流河道与水下分流间湾。水下分流河道为底凸顶平的砂砾岩体,下部发育灰绿色含中砾细砾岩夹含砾泥晶灰岩,灰岩呈断续条带状分布,横向延伸距离较小;上部发育灰绿色细砾岩夹含砾泥晶灰岩,灰岩在横向上的延伸距离较大。水下分流间湾以灰绿色泥岩和层状含砂泥晶灰岩为主,灰岩在横向上分布较稳定。结合前人在研究区中二叠世古气候、沉积环境等方面的认识,进一步建立了扇三角洲混合沉积模式。     

Muddy lateral accretion and low stream power in a sub-recent confined channel belt, Rhine-Meuse delta, central Netherlands

The Hennisdijk fluvial system in the central Rhine-Meuse delta is an abandoned Rhine distributary that was active on a wide floodplain from 3800 to 3000 years BP . Cross-sectional geometry, lithological characteristics and planform patterns of the channel-belt deposits indicate lateral migration of the Hennisdijk palaeochannel. Channel-belt deposits are around 10 m thick and 200–400 m wide. A gravelly facies near the base of the channel-belt deposits represents channel-lag and lower point-bar deposits. The axis of the channel belt is dominated by a sandy facies (medium and coarse sand), showing an overall fining upward trend with multiple cycles. This facies is interpreted as lower and middle point-bar deposits. The sandy facies is capped by a muddy facies, which is 1–2 m thick near the axis of the channel belt and thickens to 5–6 m along the margins. It laterally interfingers with the sandy facies that occurs near the channel-belt axis, but it has sharp, erosive outer contacts marking the edges of the channel belt. The muddy facies comprises inclined heterolithic stratification (IHS) (fine/medium sand–mud couplets) in its upper part. The relatively thin muddy facies with IHS that occurs near the channel-belt axis is interpreted as upper point-bar deposits with lateral accretion surfaces, formed under marine influence. Along the margins of the channel belt the muddy facies consists of thick, fairly homogeneous, successions of mud with variable sand content, and fine sand. Based on facies geometry and position, this part of the muddy facies is interpreted as counterpoint deposits, formed along the upstream limb of the concave bank of a channel bend. Counterpoint accretion seems to have been associated with the confined nature of the channel belt, which was the result of low stream power (4·5–7·8 W m⁻², based on reconstructions of palaeodischarge and channel slope) and cohesive bank material, i.e. clayey floodbasin deposits with intercalated peat beds occurring next to the channel belt. In the literature, counterpoint accretion is mostly reported from alluvial valleys, where meandering is confined by limited floodplain width, whereas muddy lateral accretion surfaces are commonly reported from much wider marine-influenced floodplains. The present study shows juxtaposition of both forms of muddy channel deposits in a low-energy, wide coastal plain setting, where preservation potential is considerable.     

Sedimentary facies of the Nova Scotian upper and middle continental slope, offshore eastern Canada

A detailed survey of the upper and middle Nova Scotian continental slope at 42°50′N and 63°30′W indicates a complex morphology dominated by mass movements on various scales and an immature turbidity current channel. The range of sediment facies is diverse including hemipelagic and turbidite muds, turbidite sands and gravelly sandy muds of debris flow origin. Deformed units, interpreted as slump deposits are also observed. Several facies associations, related to discrete morphological environments, are recognized. Thick turbidite sand units with minor intervening mud beds are characteristic of the high-relief uppermost slope and channel margin. Thinner turbidite sands, deformed slump beds and various mud facies are associated with small-scale, hummocky mid-slope topography. Sand beds are more abundant in the depressions than on intervening hummocks indicating the preferred transport paths of small turbidity currents. At the lower end of the main turbidity current channel, frequent turbidite sand beds with relatively minor mud beds are deposited on a depositional lobe. In areas unaffected by mass movements, alternating bioturbated mud and sandy muds make up the core sequences. A local model of sedimentation is proposed for this area and illustrates that simple models of continental slope sedimentation only apply to a limited range of settings.     

山西晚古生代层序地层低水位期沉积特征探讨

华北陆表海层序地层有无中低水位期沉积,学术界观点不一。通过对华北(主要是山西境内)晚古生代含煤沉积建造特征与北美精典层序地层特征进行分析对比,认为山西晚古生代含煤沉积建造中存在中低水位期沉积。待征表现为:在北部近陆源区的一套巨厚砾石、含砾粗砂沉积,向南变为一套含砾粗砂岩、粗砂岩的河流相沉积,最南部变为三角洲相和泻湖—堡岛相含煤沉积。