鄂尔多斯盆地南部延安组水下分流河道型湖泊三角洲体系的沉积构成

采用过程沉积学和成因地层分析相结合的方法,在鄂尔多斯盆地南部延安组中识别出了一种新的湖泊三角洲沉积体系。它与普通河控型湖泊三角洲的主要区别是下三角洲平原部分不是由典型的分流河口坝和前三角洲沉积组成,而是由多分支的水下分流河道、水下决口扇和湖泊沉积组成。在边缘三角洲部分,时常受波浪作用的改造,并形成各种类型的湖滨沉积。本文比较详细地讨论了这类湖泊三角洲的沉积构成及其发育的边界条件。     

湖泊三角洲的地层模式和骨架砂体的特征

大型湖泊三角洲主要形成于淡水或微咸水湖的滨岸地带。湖泊三角洲的突出特征是以分流河道砂体为骨架,河口坝沉积不发育.笔者按地层特征将湖泊三角洲分为两种基本类型:一种是缺乏完整进积序列的薄的浅水台地型三角洲,另一种是进积序列较完整的巨厚的深水盆地型三角洲。前者发育在基底稳定.水深很浅的滨岸带,后者是三角洲推进到有巨厚泥质充填物的深盆地区的产物。在大型湖盆中,随着盆地的充填作用,三角洲体系可以在空间上从“浅水”型向“深水”型过渡,在时间上从“深水”型向“浅水”型连续地演变。     

Stratigraphic architecture of the fluvio-lacustrine deposits, Bathan Formation, Al-Rehaili area, North Jeddah, Saudi Arabia

A siliciclastic-dominated succession (～11 m thick) underlying Harrat Rahat, belonging to the Miocene–Pliocene Bathan Formation is recently exposed at Al-Rehaili area, North Jeddah, Saudi Arabia. It covers a wide spectrum of grain sizes varying from clay-rich mudstones to cobble grade conglomerate and consists of a variety of facies vary from fluvial to marginal and open lacustrine deposited in a half-graben basin formed along the eastern margin of the extensional Red Sea Basin. Field-based sedimentologic investigation enables to identify ten facies grouped into three facies associations (A–C). The depositional history is subdivided into two stages. The first stage represents deposition in gravel to sand-dominated fluvial system sourced from a southern source and grade northward into lacustrine delta and open lacustrine setting. The second stage on the other hand includes deposition of fluvial channels running in E–W direction with attached bank sand bar. Sequence stratigraphic interpretations of the lacustrine deposits enable to identify three unconformity-bounded sequences (SQ_1–3). The basal sequence is incomplete, consisting of three aggradationally to progradationally stacked delta plain and delta front parasequences. The second sequence is sharply and erosively overlying a red paleosol bed that defines the upper boundary of the first sequence. It includes two system tracts; upward-fining and deepening lacustrine offshore mudstones of the transgressive system tracts unconformably overlain by red paleosol of the regressive systems tracts. The top of this sequence is delineated at the sharp transgressive surface of erosion at the base of delta mouth bar deposits of sequence 3. Changes in the accommodation and sedimentation rates by basin subsidence under the influence of tectonics and sediment compaction and loading as well as climatic oscillation between semi-arid to arid conditions were the major controls on the fluvio-lacustrine sedimentation and their facies distribution. Tectonic reorganization of the drainage system resulted in the formation of E–W flowing fluvial streams in the second stage.     

Interaction between Downslope and Alongslope Processes on the Margins of Daihai Lake,North China: Implication for Deltaic Sedimentation Models of Lacustrine Rift Basin

Daihai Lake, a modern lacustrine rift basin, located in Inner Mongolia, North China, serves as an important modern analog for understanding deltaic depositional processes in an active rift setting. Two of the deltas (Yuanzigou delta and Bulianghe delta) on the margins of Daihai Lake were surveyed to compare and contrast stacking patterns using aerial photographs, field trenching and sediment sampling. Shallow cores and trench data collected from the margins of Daihai Lake indicate that a variety of depositional processes have been active since Daihai Lake formed. Two 3-D sedimentation models which employ chronostratigraphic correlation technique were generated. The chronostratigraphic sedimentation models predict and represent the architectures and sand-body continuity of sediments. Stratigraphical coincidence of the broad sheeted drifts and channel erosion suggests a coupling between downslope and alongslope processes. Distributary mouth bars are prevalent in the front of deltas on steeper slopes due to the dominance of down-slope flows. On the contrary, the along-slope currents favor the development of distal bar deposits with sheeted sandbodies on gentle depositional slopes. This study provides an insight into the architecture of complex sedimentary facies associated with highlighting key differences between downslope flows and alongslope currents. The distribution of sand within these deltas is of particular interests, with applications in understanding the architecture of hydrocarbon reservoirs formed in lacustrine rift basin.     

陕西省榆林地区延安组湖滨下三角洲平原沉积中的决口扇三角洲沉积及其意义

陕西榆林延安组湖滨下三角洲平原沉积中决口扇三角洲具如下特点:（1）发育在湖滨下三角洲平原的分流间地区;（2）平面上垂直主分流河道向外呈扇形或朵叶状;（3）剖面上呈透镜状夹在两个煤分层之间;（4）前三角洲沉积的厚度小,前缘中植物碎屑含量高;（5）发育时间短,一般不超过1万年。决口扇三角洲沉积体间的排列关系受控于压实沉降作用,不同的排列形式导致煤层出现叠瓦状分叉和环状分叉。     

Shoal‐water deltas in high‐accommodation settings: Insights from the lacustrine Valimi Formation (Gulf of Corinth,Greece)

The stratigraphic architecture of shoal‐water deltaic systems developed in low‐accommodation settings is relatively well‐known. In contrast, the features of shoal‐water deltas developed in high‐accommodation settings remain relatively poorly documented, especially when compared with the available data sets for Gilbert‐type deltaic systems developed in the same settings. The lacustrine Valimi Formation (Gulf of Corinth, Greece) provides an opportunity to investigate the facies assemblage and architectural style of shoal‐water deltaic systems developed in high‐accommodation settings. The studied interval accumulated during the Pliocene and Pleistocene and represents part of the early syn‐rift Gulf of Corinth succession. Six facies associations, each described in terms of depositional processes and geometries, have been identified and interpreted to represent a range of proximal to distal deltaic sub‐environments: delta plain, distributary channel, mouth‐bar, delta front, prodelta and open lake. The facies associations and their architectural elements reveal characteristics which are not common in traditionally described shoal‐water deltas. Of note, different facies arrangements are observed in the distributary channels in different sectors of the delta, passing from thick single‐storey channel fills embedded within delta‐plain fines in landward positions, to thin, amalgamated and multi‐storey channels closer to the river mouth. This study proposes a new depositional model for shoal‐water deltas in high‐accommodation settings documenting, for the first time, that shoal‐water delta deposits can form a substantial part of stratigraphic successions that accumulate in these settings. The proposed depositional model provides new criteria for the recognition and interpretation of these deposits; the results of this study have applied significance for reservoir characterization.     

Variable style of transition between Palaeogene fluvial fan and lacustrine systems, southern Pyrenean foreland, NE Spain

Two Palaeogene fluvial fan systems linked to the south‐Pyrenean margin are recognized in the eastern Ebro Basin: the Cardona–Súria and Solsona–Sanaüja fans. These had radii of 40 and 35 km and were 800 and 600 km² in area respectively. During the Priabonian to the Middle Rupelian, the fluvial fans built into a hydrologically closed foreland basin, and shallow lacustrine systems persisted in the basin centre. In the studied area, both fans are part of the same upward‐coarsening megasequence (up to 800 m thick), driven by hinterland drainage expansion and foreland propagation of Pyrenean thrusts. Fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan, channelized terminal lobe, non‐channelized terminal lobe, mudflat, deltaic, evaporitic playa‐lake and carbonate‐rich, shallow lacustrine environments. Lateral correlations define two styles of alluvial‐lacustrine transition. During low lake‐level stages, terminal lobes developed, whereas during lake highstands, fluvial‐dominated deltas and interdistributary bays were formed. Terminal lobe deposits are characterized by extensive (100–600 m wide) sheet‐like fine sandstone beds formed by sub‐aqueous, quasi‐steady, hyperpycnal turbidity currents. Sedimentary structures and trace fossils indicate rapid desiccation and sub‐aerial exposure of the lobe deposits. These deposits are arranged in coarsening–fining sequences (metres to tens of metres in thickness) controlled by a combination of tectonics, climatic oscillations and autocyclic sedimentary processes. The presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake‐level falls. Deltaic deposits form progradational coarsening‐upward sequences (several metres thick) characterized by channel and friction‐dominated mouth‐bar facies overlying white‐grey offshore lacustrine facies. Deltaic bar deposits are less extensive (50–300 m wide) than the terminal lobes and were also deposited by hyperpycnal currents, although they lack evidence of emergence. Sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays; whereas the terminal lobe sheets expand from a series of mid‐fan intersection points and coalesced to form a more continuous sandy fan fringe.     

广西百色盆地百岗组的聚煤模式及其预测意义

本文研究广西百色盆地百岗组的聚煤古地理和古构造条件,提出该组的聚煤盆地模式,并讨论了地震相分析的初步应用。研究表明,可分出北缘盆缘断裂一侧的扇一扇三角洲、南缘滨浅湖-小型滨湖三角洲以及中部的浅-深湖等三个沉积相带。不对称的相配置受控于半地堑的构造格架。富煤带与盆内次级坳陷部位相吻合,并沿盆缘两侧的相带展布。地震相分析有助于圈定相带的分布,进而具有预测富煤带的意义。扇三角洲具向上变粗的三层结构,顶积层以含扇沉积为特征。其间湾为富煤中心的分布部位,而其砂体则是盆内重要的储集层。     

鄂尔多斯盆地东北缘神木地区浅湖三角洲沉积作用及煤聚集

鄂尔多斯盆地东北缘延安组含有大量的优质煤,属稳定大地构造条件下的浅湖三角州沉积物。三角洲体系有以下单元组成:(1)前三角洲;(2)三角洲前缘(水下分流河道相和河口坝相);(3)三角洲平原(分流河道相,天然堤、决口扇、沼泽和具决口三角洲的分流间湾);(4)废弃三角洲平原.通过详细的露头观察和岩心研究及沉积岩的地质填图工作,浅湖三角洲特征可归纳如下:(1)推进速度非常快、舌形体长而窄,间湾在其间发育,决口事件频繁发生;(2)前三角洲非常薄(0.2～0.5m),三角洲剖面几乎绝大部分由三角洲前缘和三角洲平原构成;(3)河口坝相由砂岩和泥岩互层组成;(4)水下分流河道砂岩厚度在0.5～3.5m范围内,被河口坝沉积物围绕;(5)在废弃三角洲平原中,由于极少沉积物带入沼泽,因此沉积了厚的低灰煤层。     

陕甘宁盆地晚三叠世的三角洲

引言陕甘宁盆地是一个矩形内陆构造盆地,晚三叠世盆地在左旋剪切力作用下形成后接受的第一套沉积物就是延长组,它是生储盖发育齐全配套的湖相沉积。横向上相带分布变化有序;纵向上湖进、成熟、湖退三大阶段发育明显。延长组的所有油层,均属低产低渗油层,勘探这套油层需要在沉积相研究的基础上,筛选出有利生、储油配置,即在低渗区中找相对高渗区,在低产区中找相对高产区。三角洲的研究是一个重要的方面。     

扇三角洲相与副层序关系的探讨

在湖泊中存在三种三角洲类型,即正常三角洲,辫状三角洲和扇三角洲。它们均由三角洲平原、三角洲前缘、前三角洲三部分所组成。陆相地层具有强烈的旋回性,只要将基准面看成海平面就完全可以运用层序地层学思路与方法研究陆相地层。扇三角洲相在断陷湖盆的陡坡一侧发育,通过与标准三角洲副层序模式对比,一个完整的扇三角洲序列可以划分出两个副层序。     

Progressive changes in sedimentary facies and stratal patterns along the strike-slip margin,northeastern Jinan Basin (Cretaceous), southwest Korea: implications for differential subsidence

In strike-slip basins, proximal stratal patterns are a function of displacement on basin-bounding faults. In order to better understand factors that control changes in sedimentary facies and stratal patterns of the northeastern part of the Jinan Basin (Cretaceous), a strike-slip basin, we made a detailed analysis of sedimentary facies, depositional architecture and paleoflows. The sedimentary successions can be grouped into five facies associations representing five depositional environments: (1) facies association FA I (alluvial fan); (2) FA II (small-scale Gilbert-type delta); (3) FA III (large-scale, steep delta slope); (4) FA IV (base of large-scale, steep delta slope and prodelta); and (5) FA V (lacustrine plain). The successions are divided into two distinct sedimentary fills on the basis of facies associations, depositional architecture and paleocurrents: (1) marginal fill and (2) longitudinal fill. The marginal fill (ca. 3.2 km thick) is present along the strike-slip basin-bounding fault. The lower part of the marginal fill (ca. 1.3 km thick) consists of alluvial-fan deposits (FA I) along the bounding fault which are transitional northward to small-scale Gilbert-type delta (FA II) and lacustrine plain (FA V) deposits. The upper part of the marginal fill (ca. 1.9 km thick) contains large-scale, steep delta slope (FA III) and base of delta slope/prodelta (FA IV) deposits accompanied with a northward change in facies associations. In the marginal fill, the successive alluvial fan, small-scale Gilbert-type delta and large-scale, steep delta/prodelta deposits are overlapped (shingled) northeastward. The longitudinal fill (ca. 2 km thick) is characterized by eastward overlapped stacks of large-scale, steep delta slope (FA III) and base of delta slope/prodelta (FA IV) deposits with a westward progradation. The longitudinal fill was overstepped by the marginal fill. The northeastward shingled geometry of the marginal fill was most likely caused by sinistral strike-slip displacements on the basin-bounding fault. The slightly oblique (northward) progradation of the marginal fill was due to the northward basin-floor tilting. In the marginal fill, the progressive changes in facies and depositional architecture from the lower alluvial fan/small-scale Gilbert-type delta to the upper large-scale, steep delta/prodelta are suggestive of increase in basin subsidence along the strike-slip basin margin that was closely related to the variation in displacement on the basin-bounding fault. The sinistral strike-slip movements on the bounding fault also caused the eastward overlapping of the longitudinal fill.     

大型敞流坳陷湖盆浅水三角洲与湖盆中心砂体的形成与分布

大型敞流坳陷湖盆浅水三角洲及湖盆中心砂体已成为中国陆相盆地岩性油气藏最重要的勘探目标。通过对现代典型湖盆浅水三角洲的深入分析,结合我国中、新生代大型坳陷湖盆浅水三角洲及湖盆中心砂体的地质研究,取得以下主要进展与认识：①在分析浅水三角洲形成地质背景的基础上,按照供源体系、湖水深度及三角洲前缘倾斜坡度将湖盆三角洲分为9种成因结构类型;②建立了毯式浅水曲流河三角洲的沉积模式,指出末端分流河道及末端决口扇是浅水曲流河三角洲的典型微相类型;③分析了浅水三角洲砂体大面积分布的形成条件,指出敞流湖盆是湖盆中心浅水三角洲砂体发育的重要条件,敞流通道对湖盆中心砂体分布有重要控制作用;④湖盆中心发育河流、三角洲、湖流及密度底流砂体等牵引流成因砂体,也发育洪水浊积扇及滑塌浊积扇等重力流成因砂体;⑤大型浅水三角洲体系三级层序界面对岩性（成岩）圈闭的发育有重要控制作用,三角洲平原带层序界面上覆叠置砂体富集岩性油气藏,勘探潜力大。本文研究成果希对坳陷湖盆沉积学研究及岩性油气藏勘探能有推动作用。     

鄂尔多斯盆地上三叠统延长组长10油层组沉积特征及物源*

鄂尔多斯盆地上三叠统延长组为大型拗陷湖盆沉积,长10油层组为延长组初始期沉积物。但其物源特征、沉积体系划分、沉积相类型等基础问题尚不清楚,严重制约了全盆地长10油层组的勘探发现。针对这些问题,作者采用了轻重矿物、锆石测年等分析方法,明确了长10油层组主要发育东北、西北、西南三大物源,根据物源特征沉积体系划分为东北、西北和西南沉积体系。综合野外露头、岩心、钻井、测井等资料揭示了长10主要发育河流-三角洲沉积,其中,西北沉积体系主要发育辫状河沉积,东北沉积体系发育曲河流-三角洲沉积,西南沉积体系发育辫状河-三角洲沉积,沉积中心位于太白-宜君一带,为浅湖相沉积。     

川西前陆盆地上三叠统沉积特征

应用层序地层学方法将川西前陆盆地上三叠统划分为4个三级层序和12个体系域。在层序格架内,研究了不同时期的古地理背景、物源、沉积类型和沉积相展布,明确了上三叠统存在龙门山古陆、米仓山-大巴山古陆、康滇古陆、江南古陆4个物源,识别出上三叠统海湾、冲积扇、曲流河、辫状河、扇三角洲、辫状河三角洲、曲流河三角洲和湖泊8种沉积相类型。层序Ⅰ沉积时期,盆地处于海陆交互相向陆相转变的时期,整体具有填平补齐的沉积特征,发育海湾等沉积;层序Ⅱ沉积时期,川西前陆盆地成为统一的内陆湖盆,其湖侵期为须家河期最大的湖侵期,主要发育三角洲沉积和湖相沉积;层序Ⅲ沉积时期,米仓山-大巴山古陆提供充足的物源,在米仓山-大巴山两逆冲推覆带前缘广泛发育大型冲积扇、辫状河、辫状河三角洲沉积体系;层序Ⅳ沉积时期,主要受江南古陆的影响,发育三角洲沉积和湖泊沉积。整体上,川西前陆盆地沉积相带的展布格局受构造控制,具有平行构造带展布的特点。盆地西北部陡坡带发育粗粒的冲积扇、扇三角洲-湖底扇,西南部陡坡带发育辫状河三角洲、北部-东北部陡坡带发育辫状河三角洲,而在东南部缓坡带则发育远源的曲流河、曲流河三角洲。     

Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology,facies variability and large‐scale depositional architecture

This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice‐marginal deltas affected by rapid lake‐level change. The integration of digital elevation models, outcrop, borehole, ground‐penetrating radar and high‐resolution shear‐wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake‐level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair‐stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake‐level fall, fan‐shaped, lobate or more digitate tongue‐like delta morphologies developed. Deposits of the stair‐stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert‐type deltas and shoal‐water deltas. The sedimentary facies of the steep Gilbert‐type delta foresets include a wide range of gravity‐flow deposits. Delta deposits of the forced‐regressive phase are commonly dominated by coarse‐grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand‐rich Gilbert‐type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope‐failure events. Shoal‐water deltas formed at low water depths during both low rates of lake‐level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand‐rich delta bodies with a digitate, multi‐tongue morphology. In contrast, forced regressive gravelly shoal‐water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced‐regressive ice‐marginal deltas are mainly extensional features, including normal faults, small graben or half‐graben structures and shear‐deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial‐isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases.     

Morphologic and stratigraphic evolution of muddy ebb-tidal deltas along a subsiding coast: Barataria Bay, Mississippi River delta

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.     

Evaluating delta asymmetry using three‐dimensional facies architecture and ichnological analysis,Ferron ‘Notom Delta’, Capital Reef,Utah, USA

Delta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave‐influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along‐strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra‐parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6‐1 to Ps 6‐3. Four facies successions are recognized: (i) wave/storm‐dominated shoreface; (ii) river‐dominated delta front; (iii) wave/storm‐reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross‐strata within distributary‐mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6‐3 predominantly consists of river‐dominated delta‐front deposits, whereas Ps 6‐1 and Ps 6‐2 show an along‐strike facies change with shoreface deposits in the north, passing into heterolithic, river‐dominated delta‐front successions south to south‐eastward, and wave/storm‐reworked delta‐front deposits further to the south‐east. Trace fossil suites correspondingly show distinct along‐strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies‐crossing ichnogenera, reflecting a more stressed, river‐dominated environment. Further south‐eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm‐reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay‐fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.     

Interplay between tectonics and compaction in a rift‐margin,lacustrine delta system: Miocene of the Eger Graben,Czech Republic

The Early Miocene Bílina Palaeodelta consists of fluvio‐deltaic and lacustrine clastics deposited along the south‐eastern margin of the extensional Most Basin, part of the Eger Graben in north Bohemia (Czech Republic). The Bílina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault. Exposures up to ca 4·5 km long in the Bílina open‐cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small‐scale and basin‐scale stratal geometries to be evaluated. The Bílina Palaeodelta deposits include sand‐dominated, fluvial channel fills and heterolithic sheets interpreted as delta plain strata, sand‐dominated mouth‐bar wedges and heterolithic sheets of prodeltaic deposits, passing distally into lacustrine clays. The depositional environment is interpreted as a fluvial‐dominated, mixed‐load, lacustrine delta with a high degree of grain‐size segregation at the feeder‐channel mouths. On the largest temporal and spatial scales, variable tectonic subsidence controlled the overall advance and retreat of the delta system. The medium‐term transgressive‐regressive history was probably driven by episodes of increased subsidence rate. However, at this temporal scale, the architecture of the deltaic sequences (deltaic lobes and correlative lacustrine deposits) was strongly affected by: (i) compaction of underlying peat and clay which drove lateral offset stacking of medium‐term sequences; and (ii) growth of a fault‐propagation fold close to the active Bílina Fault. At the smallest scale, the geometries of individual mouth bars and groups of mouth bars (short‐term sequences) reflect the interaction among sediment loading, compaction and growth faulting that produced high‐frequency relative lake‐level fluctuations and created local accommodation at the delta front.