Significance of interdune deposits and bounding surfaces in aeolian dune sands

Bounding surfaces and interdune deposits provide keys for detailed interpretations of the development, shape, type, wavelength and angle of climb of aeolian bedforms, as well as overall sand sea conditions. Current alternate interpretations of bounding surfaces require very different, but testable models for sand sea deposition. Two perpendicular traverses of Jurassic Entrada Sandstone, Utah, reveal relations among cross-strata, first-order bounding surfaces, and horizontal strata. These field relations seem explicable only as the deposits of downwind-migrating, climbing, enclosed interdune basins (horizontal strata) and dune bodies consisting of superimposed smaller crescentic dunes (cross-stratified deposits). A 1.7 km traverse parallel to the palaeowind direction provides a time-transgressive view showing continuous cosets of cross-strata, first-order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree. Changes occur in the angle of climb, cross-strata structure, and interdune deposits; these reflect changes in depositional conditions through time. A 1.5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first-order bounding surfaces and interdune deposits forming flat, laterally discontinuous lenticular bodies. The distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins, such as those on Padre Island, Texas. Hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg. The presence of three orders of bounding surfaces indicates dune bodies consisting of smaller, super-imposed dunes. The geometry of first-order bounding surfaces is a reflection of the shape of the inter-dune basins. Second-order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type. Third-order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes. The presence of only two orders of bounding surfaces indicates simple dunes. Modern and Entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet, damp, and dry conditions. Interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions.     

冲积沉积结构单元分析法综述

结构单元分析就是结合古流数据对露头横剖面进行岩石相、界面和结构单元的划分,以揭示沉积体系的三维展布,恢复沉积体系的演化史,它是目前研究露头沉积地质的最佳方法之一。本文对结构单元分析法的原理及其在几类沉积体系中的应用作了介绍。在此基础之上,讨论了结构单元分析法存在的几个问题以及它与层序地层学的关系。     

Stokes surfaces and the effects of near-surface groundwater-table on Aeolian deposition

ABSTRACT Stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near-surface water-tables (Stokes, 1968). A water-table forms a downward limit of scour through the cohesion of damp or wet sand near water-table, and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand-air interface. Study of modern analogues reveals that Stokes surfaces exist in a variety of depositional settings, including a coastal offshore prograding sand sea (Jafurah, Saudi Arabia); a coastal onshore prograding sand sea (Guerrero Negro, Mexico) and a continental sand sea (White Sands, New Mexico, USA). These modern analogues indicate that our concept of Stokes surfaces must be broadened to include the following: (i) modern analogues for Stokes surfaces described here cover areas on the order of 25 km². These may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas; (ii) Stokes surfaces occupy a continuum in scale from local to extensive, and erosional surfaces of different magnitude may be stacked closely in the sediments; (iii) Stokes surfaces, although erosional in nature, are commonly associated with deposits both above and below the Stokes bounding surface which plainly reveal the influence of a near-surface groundwater control on wind sedimentation. Moreover, the erosional relief of the bounding surface itself (as well as other features) reveals the influence of a groundwater-table; (iv) Stokes surfaces may be diachronous, representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface; (v) Stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments, including interdunes, tidal flats, lagoons, beaches, lakes and non-aeolian sabkhas. Finally, modern examples from different depositional settings suggest that while most Stokes surfaces have many features in common (such as erosional ridges due to early cementation), there are some features which may, with further study, be revealed to be distinctive of an individual depositional setting.     

Preservation of meandering river channels in uniformly aggrading channel belts

Channel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here, the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The authors find that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation, which explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut‐off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction.     

非渠化海相碳酸盐岩滩储集地质体的构型研究

１９８５年Ｍｉａｌｌ创立于河流沉积体系的构型单元分析方法可通过旋回性分级特征运用于非渠化海相碳酸盐岩滩沉积体中，并且在构型界面和构型 与河流沉积体的划分可进行对比，所获得的非均质性储层地质模型表明，沉积体最小的均一性单元是由二级界面所限定的较小规模中层型，该构型规模以上的非均质性特征受构型界面级次和构型单元多样性的控制。     

Palaeosols of the Lower Jurassic Mishhor and Ardon Formations ('Laterite Derivative Facies'), Makhtesh Ramon, Israel

The Lower Jurassic sequence exposed at Makhtesh Ramon, in the south-western Negev of Israel, comprises cyclic fluviatile sediments derived almost exclusively from the reworking of an older laterite terrain. A succession of palaeosols, having the characteristics of modern, red-brown earths, has developed during periods of sub-aerial exposure upon the fine grained overbank sediments. A marked seasonal palaeoclimate with accompanying poor sub-surface drainage conditions, and some stagnation of ground water, is reflected by the distribution and mottling patterns of the iron minerals (with varying degrees of hydration), precipitation of illuviated carbonate and sulphate minerals and the development of soil microrelief. A complex history of pedogenesis and ground water alteration is shown by the rapid lateral facies change within this sequence, into one dominated by high-alumina flint clay and bauxite. Relict pedogenetic and sedimentological textural features of the combined palaeosol/fluviatile sequence, have been identified and mapped within the flint clay/bauxite lithologies. These features include irregular voids, produced by solution of pedogenic sulphate nodules, slickensided curviplanar surfaces of soil microrelief, and depositional bedding. The field relationships of these relicts indicate the overprinting of a younger, chemical weathering event giving rise to the high-alumina flint clay and bauxites. The geometric shape of the upper bounding surface of the ‘bauxite’ alteration zone, is irregular and has an ‘intrusive’ character with respect to the younger, overlying, unaltered sediments. This, together with abrupt changes in mottling patterns near the contact zone and subtle changes in mineralogy from a bauxitic assemblage at depth to one containing mixed-layer clay minerals and carbonates nearer the contact, leads to the conclusion that the later stages of chemical weathering occurred within a confined system, where ground water movements were induced by intake of underlying aquifers.     

Internal structure of mixed-sand-and-gravel beach deposits revealed using ground-penetrating radar

ABSTRACT Mixed‐sand‐and‐gravel beaches are a distinctive type of coarse‐clastic beach. Ground‐penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed‐beach deposits at Aldeburgh in Suffolk, south‐east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward‐dipping bounding surfaces that are gently onlapped by seaward‐dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach‐profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm‐wave activity. Beach‐ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm‐ridge, overtop and overwash deposits. Relict berm‐ridge deposits, separated by seaward‐dipping bounding surfaces, form the main depositional element beneath the beach‐ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm‐ridge deposits. Consequently, beach‐ridge development in this progradational, mixed‐beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.     

Between beds and sequences: stratigraphic organization at intermediate scales in the Quaternary of the Virginia coast, USA

ABSTRACT Stratigraphy presupposes a hierarchy of scales of spatial organization supplemented at the small‐scale end by sedimentological concepts (beds, bed sets and bed cosets) and, at larger spatial scales, by sequence‐stratigraphic concepts (systems tracts, parasequences, sequences). Between these two end‐members are intermediate‐scale bodies described as ‘lithofacies’, or simply ‘facies’. A more restricted concept, granulometric facies, can be described in terms of horizontal grain‐size gradients (‘facies change’) and cyclic vertical grain‐size gradients (‘stratification’). Assemblages of facies so defined (also called depositional systems) are not random, but occur in a limited suite of patterns. Such assemblages may be linked to two classes of bounding surfaces, a source diastem (the immediate source of the sediment) and a surface of closure (if preserved), between which is sandwiched a transgressive or regressive, basinward‐fining facies succession. Systems‐bounding surfaces are notably more continuous than internal (gradational) facies boundaries. By thus restricting the definition of a facies assemblage (depositional system), it is possible to describe the Quaternary of the Virginia coast with as few as 12 systems. Depositional systems in the Quaternary of the Virginia coast are allometric, in that any system can be derived from any other by plastic expansion of one or more facies relative to another, or by simple symmetry operations. Self‐similarity prevails across this intermediate scale of stratigraphic organization. Facies assemblages (depositional systems) consist of event beds, which themselves have erosional basal boundaries and internal successions of microfacies. At larger spatial scales, depositional systems are repeated, either autocyclic repetitions forced by processes within the basin of deposition or allocyclic repetitions, as ‘parasequences’ and high‐frequency sequences. In the Virginia Quaternary, systems architecture is compatible with sequence architecture and nests conformably within its framework, but analysis of systems architecture reveals rules beyond those governing sequence architecture.     

Insight into sequence stratigraphic and structural framework of the onshore Niger Delta Basin: Integrating well logs,biostratigraphy, and 3D seismic data

Stratigraphic and structural interpretative studies have been carried out within the paralic sequence of Agbada Formation in the Niger Delta Basin. The method of study involved integrating sequence stratigraphic techniques with well logs, biostratigraphy, and 3D regional seismic data obtained from several producing fields in the Coastal Swamp depobelt of the onshore Niger Delta Basin. This was targeted at establishing regional sequence stratigraphic and structural framework from well log correlation and seismic mapping so as to better define hydrocarbon leads. Well log correlation using stratigraphic bounding surfaces such as maximum flooding surfaces (MFSs) and sequence boundaries (SBs) indicates that the stratigraphic package was deposited within the Middle to Late Miocene (8.5 to 12.1 Ma) age. Recognized depositional sequences revealed differential thickening and thinning of the associated system tracts (lowstand system tract (LST), transgressive system tract (TST), and highstand system tract) (HST)). This observed stratal behavior has been attributed to the influence of syndepositional tectonic structures that characterize the Coastal Swamp depobelt, which lies within the extensional zone. Flattening of MFSs at various ages indicates observable basinward shift of the depositional center. The genetic units of LST and HST serve as good hydrocarbon reservoir whereas HST and TST constitutes mainly the source and seal packages rocks in the area. Structural interpretation revealed the occurrence of simple/faulted rollover anticline, collapsed crest, regional hanging wall and footwall, horst block, and subdetachment fault structural styles that constitute the key hydrocarbon entrapment mechanisms. Structural top maps show fault-dependent closures which dominate the extensional zone, which could form possible hydrocarbon leads and prospects that should be targeted during exploration studies for development.     

Geophysical diffraction tomography: New views on the shiqmim prehistoric subterranean village site (israel)

Shiqmim is a late prehistoric farming village in the Beersheva river valley in Israel's northern Negev desert dating to the late 5th-early 4th millennia B. C. Along with surface architecture, archaeologists discovered a sequence of large subterranean rooms interconnected by narrow tunnels. This discovery, found in the northern Negev, as well as similar subterranean features and dating back to the same period, has created considerable controversy as to the functions of this innovative architecture. To a large extent, this controversy results from conflicting hypotheses based on the small sample of subterranean Negev architectures. A fundamental task in the resolution of these problems is the location and investigation of similar features within this region. This is not a simple task because these subterranean features can be as much as 7 m deep in hard-packed soil and are usually completely filled with fine sediment as a result of depositional forces acting over the 6 millennium since their abandonment. To help quantify the spatial extent of subterranean features at Shiqmim and identify particularly promising locations for excavation, an emerging geophysical method, known as geophysical diffraction tomography, was applied. This method is based on the propagation of acoustic waves and employs the mathematical concepts of optical holography to reconstruct quantitative, high-resolution images of the subsurface environment. Two geophysical imaging field studies were performed at Shiqmim, and the results of these studies indicate that subterranean architecture is much more pervasive at this site than previously thought. The new geophysical studies help answer social questions and highlight the contribution of geoarchaeology to anthropological reconstructions of the past. This article discusses geophysical diffraction tomography and the first attempt to apply it in an archaeological context. © 1995 John Wiley & Sons, Inc.     

The sedimentology of river confluences

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.     

地震沉积学探讨

地震沉积学是用地震手段研究沉积岩及其形成过程的学科,其研究手段主要有90°相位转换和地层切片技术等。90°相位转换使地震相位具有了岩性地层意义,可以用于高频层序地层的解释;地层切片是指对某一层位内进行等比例内插切片之后用来研究各个等时地层单元的沉积体系（相）的平面展布。文章认为：①由于受地震分辨率的限制,地震沉积学目前主要应用于研究宏观的地层、岩石、沉积史和沉积体系,还没有达到全面研究沉积岩及其形成过程的程度,因此,目前的地震沉积学是利用地震手段结合井资料研究宏观的地层、岩石、沉积史和沉积体系的一门学科,它还需要进一步的发展才有可能继地震地层学、层序地层学之后真正成为一门研究沉积岩及其形成过程的新学科;②相位转换技术中转换的角度并不一定局限于90°,可以是其它角度的相位转换,这要根据层位标定的具体情况而定;③地层切片比时间切片和沿层切片更加合理,但是目前的地层切片技术还没有考虑地层的沉积速率随时间的变化,因此,地层切片还不是严格意义上等时的。     

A Fast Approximation for Seismic Inverse Modeling: Adaptive Spatial Resampling

Seismic inverse modeling, which transforms appropriately processed geophysical data into the physical properties of the Earth, is an essential process for reservoir characterization. This paper proposes a work flow based on a Markov chain Monte Carlo method consistent with geology, well-logs, seismic data, and rock-physics information. It uses direct sampling as a multiple-point geostatistical method for generating realizations from the prior distribution, and Metropolis sampling with adaptive spatial resampling to perform an approximate sampling from the posterior distribution, conditioned to the geophysical data. Because it can assess important uncertainties, sampling is a more general approach than just finding the most likely model. However, since rejection sampling requires a large number of evaluations for generating the posterior distribution, it is inefficient and not suitable for reservoir modeling. Metropolis sampling is able to perform an equivalent sampling by forming a Markov chain. The iterative spatial resampling algorithm perturbs realizations of a spatially dependent variable, while preserving its spatial structure by conditioning to subset points. However, in most practical applications, when the subset conditioning points are selected at random, it can get stuck for a very long time in a non-optimal local minimum. In this paper it is demonstrated that adaptive subset sampling improves the efficiency of iterative spatial resampling. Depending on the acceptance/rejection criteria, it is possible to obtain a chain of geostatistical realizations aimed at characterizing the posterior distribution with Metropolis sampling. The validity and applicability of the proposed method are illustrated by results for seismic lithofacies inversion on the Stanford VI synthetic test sets.     

Late Permian Sequence Stratigraphic Framework and Controlling Factors in Southern Yangtze Platform and Its Margin

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｃｏｎｃｅｐｔｓａｎｄｔｅｒｍｉｎｏｌｏｇｙｏｆｍｏｄｅｒｎｓｅｑｕｅｎｃｅｓｔｒａｔｉｇｒａｐｈｙｏｒｉｇｉｎａｔｅｄｆｒｏｍｓｅｉｓｍｉｃｓｔｒａｔｉｇｒａｐｈｙｌｉｎｋｅｄｔｏｐｅｔｒｏｌｅｕｍａｎｄｇａｓｅｘｐｌ...     

Facies analysis and sequence stratigraphy of neoproterozoic Platform deposits in Adrar of Mauritania, Taoudeni basin, West Africa

The Neoproterozoic and Palaeozoic Taoudeni basin forms the flat-lying and unmetamorphosed sedimentary cover of the West African Craton. In the western part of this basin, the Char Group and the lower part of the Atar Group make up a 400-m-thick Neoproterozoic siliciclastic succession which rests on the Palaeoproterozoic metamorphic and granitic basement. Five erosional bounding surfaces of regional extent have been identified in this succession. These surfaces separate five stratigraphic units with lithofacies associations ranging from fluvial to coastal and fluvial-, tide-, or wave-dominated shallow marine deposits. Owing to their regional extent and their position within the succession, the erosive bounding surfaces correspond to relative sea-level falls, and accordingly the five stratigraphic units they bound represent allocyclic transgressive–regressive depositional sequences (S₁–S₅). Changes in the nature of the deposits forming the transgressive–regressive cycles reflect landward or seaward shifts of the stacked sequences. These successive relative sea-level changes are related to the reactivation of basement faults and tilting during rifting of the Pan-Afro-Brasiliano supercontinent 1000 m.y. ago. The stromatolite bearing carbonate-shale sequences which form the rest of the Atar Group mark the onset of a quiet period of homogeneous subsidence contemporaneous with the Pan-African I oceanization 800–700 m.y. ago.     

Impact of dynamic sedimentation on facies heterogeneities in Lower Cretaceous peritidal deposits of central east Oman

This study, based in the Haushi‐Huqf area of central east Oman, aims to characterize the controls on facies distribution and geometries of some of the best preserved examples of Lower Cretaceous tidal flat facies within the Tethyan epeiric platform. Field, petrographic and geochemical data were acquired from the Barremian–Aptian Jurf and Qishn formations that crop out in a 500 × 1000 m² butte, thus allowing for pseudo three‐dimensional quantitative data acquisition of the dimensions and spatial distributions of discontinuity surfaces and sedimentary bodies. The interpretation presented here suggests that the main processes impacting sedimentation in the Lower Cretaceous peritidal environment of the Haushi‐Huqf were transport and erosion processes related to storm waves and currents. The vertical evolution of the carbonate system is organized into six types of metre‐scale depositional sequences, from subtidal dominated sequences to supratidal‐capped sequences, which are bounded by regional discontinuity surfaces. At subaerial exposure and submarine erosion surfaces associated with a base level shift, sedimentary horizons along the entire depositional profile are cut by scours possibly created by storm events. Chemostratigraphy allows correlation between the Haushi‐Huqf and the age‐equivalent sections logged in the interior of the platform in Oman. The correlation suggests that the change from subtidal to intertidal depositional sequences during the late highstand is coeval with the development of rudist dominated shoals on the shelf. This study is the first to discuss the controls on Lower Cretaceous peritidal carbonate cyclicity of the Arabian epeiric platform. The results presented here also offer a unique quantitative dataset of the distribution and dimensions of peritidal carbonate shoals and storm scours in a regional sequence stratigraphic context.     

Correlation of hydraulic conductivity with stratigraphy in a fractured-dolomite aquifer, northeastern Wisconsin, USA

This study integrated surface and subsurface stratigraphic data with geophysical logs and hydrogeologic data in order to characterize the hydraulic properties of the Silurian dolomite in northeastern Wisconsin. Silurian stratigraphy consists of predictable alternations of characteristic facies associations. A vertical profile of hydraulic conductivity, obtained from short-interval packer tests in a core hole that penetrates a majority of the Silurian section, indicates that hydraulic conductivity ranges over five orders of magnitude (10^–1 to 10^–6 cm/s). Matrix conductivity is generally low and varies with texture; the finer-grained restricted-marine and transitional facies being less permeable than the coarser-grained open-marine facies. High-conductivity values are generally associated with bedding-plane fractures, and fracture frequency is greater in the restricted-marine facies. High-flow features in 16 wells were identified using fluid-temperature/resistivity and heat-pulse flowmeter logs. Natural-gamma logs were used to identify the stratigraphic position of flow features in each well and to correlate flow features to specific stratigraphic horizons. By combining stratigraphic, geophysical, and hydrogeologic data, 14 high-permeability zones within the Silurian aquifer have been identified and correlated in wells more than 16 km apart. These zones parallel bedding, appear most pronounced at contacts between contrasting lithologies, and are most abundant in restricted-marine lithologies. Electronic Publication     

Shallow-water gravity-flow deposits, Chapel Island Formation, southeast Newfoundland, Canada

A remarkable suite of shallow-water, gravity-flow deposits are found within very thinly-bedded siltstones and storm-generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non-graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine-grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore-fluid pressure dissipation support the feasibility of such transitions. Raft-bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm^-1 and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm^-2 and a depositional slope estimate of 0.77°. The lithologies and sedimentary structures in member 2 indicate an overall grain-size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure included in situ liquefaction (‘disturbed bedding’), sliding and slumping. Raft-bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross-stratified sandstone directly overlies slide deposits and raft-bearing beds, linking sediment failure to the cyclical wave loading associated with large storms. The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud-rich, marine deltas. Details of deltaic gravity-flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies primarily carried out with side-scan sonar.     

Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo-Tokyo Bay, Japan

Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.     

Architecture and formation of transgressive–regressive cycles in marginal marine strata of the John Henry Member,Straight Cliffs Formation,Upper Cretaceous of Southern Utah,USA

Marginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive – regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.