Sedimentological and ichnological signatures of changes in wave,river and tidal influence along a Neogene tropical deltaic shoreline

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.     

Fluvial shoal water deltas: pre-vegetation sedimentation through the fluvial–marine transition,Lower Cambrian,English Channel region

Lower Palaeozoic fluvial systems tend to be more sand-prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro-delta and offshore carbonate bank environments. The basin fill is sandstone-dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse-grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north-west pass laterally south-east into deltaic and shallow marine pro-delta sediments. The delta front deposits show repetitively stacked, upward-coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially-dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth-bars and consistently delivered fine to medium-grained sand to the delta front. The study highlights an example of pre-vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.     

Aeolian to marine transition in Cambro—Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A.

Cratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet-like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re-study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non-marine deposition followed by varying degrees of marine reworking during transgressions. Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large-scale cross-stratified facies believed to represent aeolian ergs passes laterally into a planar-and-channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning-upward sets of trough cross-stratification, reactivation surfaces, low-index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse-sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non-marine deposits. In contrast is a burrowed and trough cross-stratified facies characterized by medium-scale cross-bedding alternating with bioturbated intervals and rare brachiopod or trilobite-mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non-marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense. It is suggested that the sheet-like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders-of-magnitude more important on pre-Devonian, vegetation-free landscapes than they have been since, not only helps to explain the sheet-like character but also the exceptional maturity of the older cratonic sandstones.     

Deltaic sedimentation in the Devonian of Western Libya

Middle and Upper Devonian deposits from the Aouinet Ouenine Formation in the southern Ghadames Basin of western Libya provide a well exposed example of a deltaic complex containing both progradational and transgressive facies. Progradational facies comprise both laterally accreting and incised distributary channels overlying prodelta deposits. Also present is a progradational beach environment showing build-up from an offshore shelf through nearshore shelf to shoreface and foreshore sub-environments. Over-lying these progradational facies are transgressive tidal-flat, washover-fan, foreshore and nearshore deposits.The characteristics and interrelationships of the different facies are explained by two sedimentation models: progradational facies existed contemporaneously during phases of active sediment supply whereas the transgressive facies existed contemporaneously during periods of diminished or absent detrital influx.     

Facies and sequence stratigraphy of a Late Barremian wave-dominated deltaic deposit, Agadir Basin, Morocco

The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface.     

Architecture and preservation in the fluvial to marine transition zone of a mixed-process humid-tropical delta: Middle Miocene Lambir Formation,Baram Delta Province,north-west Borneo

The interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed-process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, north-west Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed-energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active distributary channels are sandstone-rich, comprising fluvial-dominated and wave-dominated units. Successions in lateral areas, which lack active distributary channels, are mudstone-rich, comprising fluvial-dominated, tide-dominated and wave-dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well-developed turbidity maximum zones, a consequence of high suspended-sediment concentrations resulting from tropical weathering of a mudstone-rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial-dominated, tide-influenced subzone; (ii) a distal fluvial-dominated to wave-dominated subzone; and (iii) a conspicuously absent tide-dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics.     

Fluvial-aeolian interactions: Part II, ancient systems

An understanding of fluvial-aeolian deposition derived from modern case-examples in a previous study is applied to the Permian Cutler Formation and Cedar Mesa Sandstone on the Colorado Plateau. These formations supply an excellent three-dimensional exposure of intertonguing fluvial and aeolian strata. Four distinct facies associations form the bulk of the Cutler Formation and Cedar Mesa Sandstone: (1) aeolian dune deposits; (2) wet interdune deposits; (3) fluvial channel deposits; and (4) overbank-interdune deposits. In addition, two distinctive types of erosion surfaces are found within the Cutler Formation and Cedar Mesa Sandstone: pebble- to granule-rich erosion surfaces (aeolian deflation surfaces) and flood surfaces. Fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank-interdune deposits. Fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones. Overbank-interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones. The primary characteristics of ancient fluvial-aeolian deposition are overbank-interdune deposits and pronounced extensive erosion surfaces (flood surfaces), which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient.     

Sedimentary cycles and facies architecture of aeolian–fluvial strata of the Upper Jurassic Guará Formation, southern Brazil

The Upper Jurassic Guará Formation comprises an 80–200 m thick continental succession exposed in the western portion of the Rio Grande do Sul State (Brazil). It comprises four distinct facies associations: (i) simple to locally composite crescentic aeolian dune sets, (ii) aeolian sand sheets, (iii) distal floodflows, and (iv) fluvial channels. The vertical stacking of the facies associations defines several 5–14 m thick wetting-upward cycles. Each cycle starts with aeolian dune sets followed by aeolian sand sheets deposits and culminating in either fluvial channels or distal flood strata. Within some cycles, aeolian sand sheets are absent and fluvial deposits rest directly above aeolian dune facies. The transitions from one facies association to another are abrupt and marked by erosive surfaces that delineate distinct episodes of sediment accumulation. The origin of both the wetting-upward cycles and the erosive surfaces was controlled by the ground-water table level, dry sand availability and aeolian and fluvial sediment transport capacity variations, related to climatic fluctuations between relatively arid and humid conditions. Preservation of the fluvial–aeolian deposits reflects an overall relative water table rise driven by subsidence.     

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.     

滇西洱源县军马场下三叠统青天堡组冲积扇-三角洲沉积体系研究

滇西洱源县位于扬子地块西南缘丽江拗陷盆地南侧。在详细野外剖面测量和室内分析的基础上,对研究区下三叠统青天堡组地层的宏观特征（颜色、原生沉积构造）、物质组分、岩石结构及古生物特征等进行了系统研究,划分出冲积扇、河流、三角洲3类沉积相,并进一步划分出河道充填、三角洲前缘和前三角洲3个沉积亚相及辨状河道、洪泛平原、水下分流河道、分流河道间、河口砂坝、席状砂6个沉积微相。研究表明,军马场下三叠统青天堡组垂向上表现为冲积扇—河流—三角洲的退积型沉积特征,表明在早三叠世青天堡时期盐源-丽江盆地发生了大规模海侵,盆地水体加深,军马场地区经历了由陆相到海相的沉积演化序列。     

Fluvial to tidal transition in proximal,mixed tide‐influenced and wave‐influenced deltaic deposits: Cretaceous lower Sego Sandstone,Utah, USA

Facies models for regressive, tide‐influenced deltaic systems are under‐represented in the literature compared with their fluvial‐dominated and wave‐dominated counterparts. Here, a facies model is presented of the mixed, tide‐influenced and wave‐influenced deltaic strata of the Sego Sandstone, which was deposited in the Western Interior Seaway of North America during the Late Cretaceous. Previous work on the Sego Sandstone has focused on the medial to distal parts of the outcrop belt where tides and waves interact. This study focuses on the proximal outcrop belt, in which fluvial and tidal processes interact. Five facies associations are recognized. Bioturbated mudstones (Facies Association 1) were deposited in an offshore environment and are gradationally overlain by hummocky cross‐stratified sandstones (Facies Association 2) deposited in a wave‐dominated lower shoreface environment. These facies associations are erosionally overlain by tide‐dominated cross‐bedded sandstones (Facies Association 4) interbedded with ripple cross‐laminated heterolithic sandstones (Facies Association 3) and channelized mudstones (Facies Association 5). Palaeocurrent directions derived from cross‐bedding indicate bidirectional currents which are flood‐dominated in the lower part of the studied interval and become increasingly ebb‐directed/fluvial‐directed upward. At the top of the succession, ebb‐dominated/fluvial‐dominated, high relief, narrow channel forms are present, which are interpreted as distributary channels. When distributary channels are abandoned they effectively become estuaries with landward sediment transport and fining trends. These estuaries have sandstones of Facies Association 4 at their mouth and fine landward through heterolithic sandstones of Facies Association 3 to channelized mudstones of Facies Association 5. Therefore, the complex distribution of relatively mud‐rich and sand‐rich deposits in the tide‐dominated part of the lower Sego Sandstone is attributed to the avulsion history of active fluvial distributaries, in response to a subtly expressed allogenic change in sediment supply and relative sea‐level controls and autocyclic delta lobe abandonment.     

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

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.     

准噶尔盆地玛湖凹陷八道湾组一段沉积微相特征 ——以TX1井区为例

通过岩石学、沉积学及测井等方法,对准噶尔盆地TX1井区侏罗系八道湾组一段进行详细研究,认为侏罗系八道湾组一段主要发育复成分砾岩、砾状砂岩及砂岩,煤夹层;沉积构造发育,兼具水上及水下沉积的特点,发育粒序层理、中大型交错层理及冲刷充填、植物枝干印模等沉积构造;测井上具箱型、漏斗型、指状、线性等响应.沉积相纵向上逐渐向盆地边...     

Facies model for a coarse‐grained,tide‐influenced delta: Gule Horn Formation (Early Jurassic), Jameson Land,Greenland

Tide‐dominated deltas have an inherently complex distribution of heterogeneities on several different scales and are less well‐understood than their wave‐dominated and river‐dominated counterparts. Depositional models of these environments are based on a small set of ancient examples and are, therefore, immature. The Early Jurassic Gule Horn Formation is particularly well‐exposed in extensive sea cliffs from which a 32 km long, 250 m high virtual outcrop model has been acquired using helicopter‐mounted light detection and ranging (LiDAR). This dataset, combined with a set of sedimentological logs, facilitates interpretation and measurement of depositional elements and tracing of stratigraphic surfaces over seismic‐scale distances. The aim of this article is to use this dataset to increase the understanding of depositional elements and lithologies in proximal, unconfined, tide‐dominated deltas from the delta plain to prodelta. Deposition occurred in a structurally controlled embayment, and immature sediments indicate proximity to the sediment source. The succession is tide dominated but contains evidence for strong fluvial influence and minor wave influence. Wave influence is more pronounced in transgressive intervals. Nine architectural elements have been identified, and their internal architecture and stratigraphical distribution has been investigated. The distal parts comprise prodelta, delta front and unconfined tidal bar deposits. The medial part is characterized by relatively narrow, amalgamated channel fills with fluid mud‐rich bases and sandier deposits upward, interpreted as distributary channels filled by tidal bars deposited near the turbidity maximum. The proximal parts of the studied system are dominated by sandy distributary channel and heterolithic tidal‐flat deposits. The sandbodies of the proximal tidal channels are several kilometres wide and wider than exposures in all cases. Parasequence boundaries are easily defined in the prodelta to delta‐front environments, but are difficult to trace into the more proximal deposits. This article illustrates the proximal to distal organization of facies in unconfined tide‐dominated deltas and shows how such environments react to relative sea‐level rise.     

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.     

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。     

胜利油田渤南洼陷古近系沙河街组沉积相

渤南洼陷古近系沙河街组为一套冲积－湖泊沉积，由曲流河三角洲相、辫状河三角洲相、扇三角洲相、水下扇相和湖泊相组成，可分出14个亚相，35个微相，其中辫状河三角洲最发育，其特征显著，明显有别于其它四种类型的三角洲。研究区辫状河三角洲由辫状河三角洲前缘及前三角洲构成，辫状河三角洲前缘的主体是由含砂岩及中粗粒砂岩组成的水下分流河道砂坝，三角洲中交错层理发育，其中尤以水道砂坝侧向迁移加积而形成的侧积交错层异常发育为标志。     

Depositional facies and the sequence stratigraphic control of a mixed-process influenced clastic wedge in the Cretaceous Western Interior Seaway: The Gallup System,New Mexico,USA

Depositional facies have been hypothesized to be linked to sequence stratigraphic positions. Also, shoreline systems are built by mixed processes, including rivers, storms, fair-weather waves and tides. Resolving the complexity of shoreline deposition requires detailed quantitative facies analysis with particular attention to heterolithic successions. In this study, 71 sections in a 130 km long outcrop belt of the Cretaceous Gallup Formation in the north-west of the San Juan Basin were measured. Five major facies associations were identified using sedimentological and iconological interpretations, including offshore shelf, non-deltaic shoreline sandstones, deltas, coastal bayline and fluvial. Each facies association also comprises subordinate facies. Depositional facies interpretations are placed in a high-resolution sequence stratigraphic framework that allows for reconstructions of the palaeogeography of individual parasequence sets that demonstrate temporal and spatial evolution of facies associations and depositional processes. The results show that the Gallup is a mixed-process-controlled depositional system with fair-weather and storm-wave dominance, river influence and tide-effect, contrasting with previous interpretations of a solely fair-weather wave-dominated environment. Depositional processes and the resultant facies change with sequence stratigraphic positions in response to relative sea-level changes – particular facies are only deposited in certain systems tracts. Distinction and transition between non-deltaic shorefaces and wave-dominated deltas have also been documented in this study. Non-deltaic shorefaces are characterized by homogeneous sandstones with a wide-range bioturbation index and the absence of mudstones. Wave-dominated deltas are subject to river influence and contain prodelta facies. This study shows the importance of detailed facies analysis with high-resolution sequence stratigraphic control using outcrops for documenting sedimentary processes of shallow marine shoreline systems.