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.     

Facies and sequence stratigraphic analysis in an intracratonic, thermal-relaxation basin: the Early Proterozoic, Lower Quilalar Formation and Ballara Quartzite, Mount Isa Inlier, Australia

The Quilalar Formation and correlative Mary Kathleen Group in the Mount Isa Inlier, Australia, conformably overlie rift-related volcanics and sediments and non-conformably overlie basement rocks. They represent a thermal-relaxation phase of sedimentation between 1780 and 1740 Ma. Facies analysis of the lower siliciclastic member of the Quilalar Formation and the coeval Ballara Quartzite permits discrimination of depositional systems that were restricted areally to either N-S-trending marginal platform or central trough palaeogeographic settings. Four depositional systems, each consisting of several facies, are represented in the lower Quilalar Formation-Ballara Quartzite; these are categorized broadly as storm-dominated shelf (SDS), continental (C), tide-dominated shelf (TDS) and wave-dominated shoreline (WDS). SDS facies consist either of black pyritic mudstone intervals up to 10 m thick, or mudstone and sandstone associated in 6–12-m-thick, coarsening-upward parasequences. Black mudstones are interpreted as condensed sections that developed as a result of slow sedimentation in an outer-shelf setting starved of siliciclastic influx. Vertical transition of facies in parasequences reflects flooding followed by shoaling of different shelf subenvironments; the shoreface contains evidence of subaerial exposure. Continental facies consist of fining-upward parasequences of fluvial origin and tabular, 0·4–4-m-thick, aeolian parasequences. TDS facies are represented by stacked, tabular parasequences between 0·5 and 5 m thick. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal conditions. WDS facies are preserved in 0·5–3-m-thick, stacked, tabular parasequences. Vertical transition of facies reflects initial flooding with wave reworking of underlying arenites along a ravinement surface, followed by shoaling from lower shoreface to foreshore conditions. Parasequences are stacked in retrogradational and progradational parasequence sets. Retrogradational sets consist of thin SDS parasequences in the trough, and C, TDS and probably WDS parasequences on the platforms. Thick SDS parasequences in the trough, and TDS, subordinate C and probably WDS parasequences on the platforms make up progradational parasequence sets. Depositional systems are associated in systems tracts that make up 40–140-m-thick sequences bounded by type-2 sequence boundaries that are disconformities. Transgressive systems tracts consist of C, TDS and probably WDS depositional systems on the platforms and the SDS depositional system and suspension mudstone deposits in the trough. The transgressive systems tract is characterized by retrogradational parasequence sets and developed in response to accelerating rates of sea-level rise following lowstand. Condensed-section deposits in the trough, and the thickest TDS parasequences on the platforms reflect maximum rates of sea-level rise and define maximum flooding surfaces. Highstand systems tract deposits are progradational. Early highstand systems tracts are represented by TDS and probably WDS depositional systems on the platforms and suspension mudstone deposits in the trough and reflect decreasing rates of sea-level rise. Later highstand systems tracts consist of the progradational SDS depositional system in the trough and, possibly, thin continental facies on the platforms. This stage of sequence development is related to slow rates of sea-level rise, stillstand and slow rates of fall. Lowstand deposits of shelf-margin systems tracts are not recognized but may be represented by shoreface deposits at the top of progradational SDS parasequence sets.     

Mid-cycle condensed shellbeds from mid-Pleistocene cyclothems, New Zealand: implications for sequence architecture

Richly fossiliferous and disconformity-bounded facies successions, termed Mid-Cycle Condensed Shellbeds (MCS), occupy a mid-cycle position within depositional sequences in the Castlecliff section (mid-Pleistocene, Wanganui Basin, New Zealand). These shell-rich intervals (0.1–4.5 m thick) comprise the upper of two loci of shell accumulation in Castlecliff sequences. The lower disconformable contacts are sharp and variably burrowed, and are interpreted as submarine transgressive surfaces formed by storm or tidal current erosion at the feather-edge of contemporary transgressive systems tracts. Above (i.e. seaward) of this erosion surface, macrofossil remains (mainly bivalves and gastropods) accumulated, with little reworking, on the inner-shelf under conditions of reduced terrigenous sediment supply. The upper contacts are sharp transitions from shell-rich to relatively shell-poor lithofacies; parautochthonous shell accumulation was ‘quenched’by downlapping highstand systems tract shelf siltstones and muddy fine sandstones. Castlecliff MCS, together with the basal shell-rich part of overlying highstand systems tracts, occupy a stratigraphic position which corresponds to the condensed section that forms at the transgressive/highstand systems tract boundary in the sequence model of Haq et al. (1987). Palaeoenvironmental analysis indicates that Castlecliff MCS are substantially, if not entirely, transgressive deposits. This study therefore shows that the ‘condensation maximum’within a depositional sequence does not necessarily bracket the transgressive systems tract/highstand systems tract boundary.     

The carbonate—clastic cycles of the East-Alpine Raibl group: Result of third-order sea-level fluctuations in the Carnian

The Carnian Raibl group of the Eastern Alps consists of three 50–100 m thick, alternating carbonate and clastic third-order cycles, each of which can be traced for hundreds of kilometers. Tectono-eustatic sea-level fluctuations of a few tens of metres, spanning a few millions of years, are the driving mechanism of this cyclicity. The carbonate intervals represent restricted marginal marine, tidal and evaporitic environments. The clastic intervals represent inner and outer shelf facies, and are related to the fluviatile “Schilfsandstein” of the Germanic facies belt. In the Raibl group, contrary to other carbonate/clastic depositional settings, relative sea-level lowstands are dominated by carbonate production, and highstands are dominated by clastic deposition.

Each of the three Raibl cycles corresponds to a type-2 sequence, containing shelf margin, transgressive and highstand systems tracts. During sea-level lowstands, deltaic point sources were near the shelf margin, allowing clastics to bypass the carbonate platform. This setting corresponds to a shelf margin systems tract. Transgressive and highstand systems tracts developed during the subsequent sea-level rise, as deltaic clastics were reworked and redistributed over the carbonate platform, and the deltas retrograded to the inner shelf. The highstand systems tracts are capped by a type 2 sequence boundary, which is conformable in the study area. The systems tracts can be further subdivided into shallowing upward subcycles, caused by fourth-order sea-level fluctuations, believed to represent Milankovitch rhythms.

The middle Raibl cycle is consistently thinner, and may represent a shorter termed, third-order sea-level fluctuation. Our data also corroborate a second-order transgressive trend for the Carnian.     

Depositional facies,environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline,northeast Sinai,Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.     

Facies analysis and high-resolution sequence stratigraphy of the Lower Eocene shallow marine Ametlla Formation, Spanish Pyrenees

The Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn-sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment-starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway. In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized. (1) At the regional scale, elements of two 3rd-order composite sequences (sensu Exxon) have been recognized. These include a 3rd-order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd-order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward-stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment-starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high-frequency sea-level fluctuations, and to downslope redistribution of sand. (2) At the subregional scale, detailed mapping indicates the presence of five 4th-order sequences. The 4th-order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea-level fall. Sharp-based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea-level. Where 4th-order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone-dominated transgressive systems tract and a siltstone-dominated highstand systems tract. (3) At the local scale, 2–9 5th-order parasequences are present within the 4th-order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonly stacked in a landward-stepping manner.     

Late Cretaceous paleoenvironmental evolution of the Tarfaya Atlantic coastal Basin,SW Morocco

Lithological evidence, benthic foraminiferal census counts, and X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by laminated organic-rich deposits, which contain impoverished benthic foraminiferal assemblages, reflecting impingement of the oxygen minimum zone on the shelf during a sea-level highstand. This highstand level is correlated to the global transgressive pulse above the sequence boundary Tu1. The appearance of low-oxygen tolerant benthic foraminiferal assemblages dominated by Gavelinella sp. in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. This interval is marked by major regressive events expressed by a series of erosional truncations associated with the prominent sequence boundaries Tu3 and/or Tu4. Dysoxic–anoxic conditions recorded in the upper Santonian of the Tarfaya Basin coincide with the eustatic sea-level rise prior to Sa3 sequence boundary. The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the seafloor (highest diversity and abundance of benthic foraminiferal assemblages). Stable isotope data from bulk carbonates are tentatively correlated to the English Chalk carbon isotope reference curve, in particular the Hitch Wood Event in the upper Turonian, the Navigation Event in the lower Coniacian, the Horseshoe Bay Event in the Santonian and the Santonian/Campanian Boundary Event.     

Facies analysis and sequence stratigraphy of a Miocene warm-temperate carbonate ramp,Montagna della Maiella,Italy

The uppermost Oligocene/Lower Miocene to Upper Miocene ramp carbonates from Montagna della Maiella (Italy) form a supersequence bounded by deeply incised truncation surfaces. This supersequence is subdivided into four sequences. Each sequence is composed of skeletal limestones in its lower part and marly limestones in its upper part. The lower parts of the sequences are foramol limestones, which suggest deposition in the warm-temperate climate zone. Changes in climate, oceanography and relative sea level combined to control sedimentation in the four sequences. In the lower parts of the two older sequences, the skeletal sands built dunes, suggesting high-energy conditions. The dominant skeletal grains in the oldest sequence are larger foraminifers and in the next sequence they are bryozoans; this change reflects cooling around the time of the Aquitanian/Burdigalian boundary. In the lower parts of the two younger sequences, of Middle and Late Miocene age, sediment sheets with red-algal–bryozoan oncoids suggest deposition under calmer conditions. Transgressive and highstand systems tracts are recognized in all sequences; a shelf margin systems tract may be exposed in the second oldest sequence. In contrast to the situation that exists when warm-water carbonates are deposited, sedimentation of the foramol limestones on this isolated ramp was unable to balance accommodation during sea-level rise; this led to hemipelagic sedimentation during sea-level highstands. Conglomerates resulted from reworking along flooding surfaces.     

Tectonic control on third‐order sequences in a siliciclastic ramp‐style basin: An example from the Roper Superbasin (Mesoproterozoic), northern Australia

TThe Roper Group is a cyclic, predominantly marine, siliciclastic succession of Calymmian (Early Mesoproterozoic) age. It has a distribution of at least 145 000 km² and a maximum known thickness of ～5000 m. In the Roper River district the middle part of the Roper Group (～1300 m thick) is characterised by the cyclical alternation of mudstone and sandstone units, and can be divided into six third‐order depositional sequences. A typical sequence is broadly progradational in aspect, and comprises a lower, mudstone‐rich, storm‐dominated shelf succession (up to 330 m thick), and a sequence‐capping unit dominated by tidal‐platform cross‐bedded sandstone (up to 80 m thick); both are interpreted as highstand systems tracts. Transgressive strata are poorly represented but where present are characterised by paralic to fluvial redbed assemblages that include ooidal ironstone. Roper Group sequences lack a distinct condensed section and sequence boundaries are mostly conformable. Erosional contacts separate mud‐rich shelf facies from sequence‐capping sandstones. We infer that these erosion surfaces were generated by episodic flexural tectonism, which also generated the accommodation and sediment supply for Roper sequences.     

Tidal shelf sedimentation in the Neoproterozoic Chattisgarh succession of central India

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

Deposition and preservation of fluvio-tidal shallow-marine sandstones: A re-evaluation of the Neoproterozoic Jura Quartzite (western Scotland)

The 2 to 5 km thick, sandstone-dominated (>90%) Jura Quartzite is an extreme example of a mature Neoproterozoic sandstone, previously interpreted as a tide-influenced shelf deposit and herein re-interpreted within a fluvio-tidal deltaic depositional model. Three issues are addressed: (i) evidence for the re-interpretation from tidal shelf to tidal delta; (ii) reasons for vertical facies uniformity; and (iii) sand supply mechanisms to form thick tidal-shelf sandstones. The predominant facies (compound cross-bedded, coarse-grained sandstones) represents the lower parts of metres to tens of metres high, transverse fluvio-tidal bedforms with superimposed smaller bedforms. Ubiquitous erosional surfaces, some with granule–pebble lags, record erosion of the upper parts of those bedforms. There was selective preservation of the higher energy, topographically-lower, parts of channel-bar systems. Strongly asymmetrical, bimodal, palaeocurrents are interpreted as due to associated selective preservation of fluvially-enhanced ebb tidal currents. Finer-grained facies are scarce, due largely to suspended sediment bypass. They record deposition in lower-energy environments, including channel mouth bars, between and down depositional-dip of higher energy fluvio-ebb tidal bars. The lack of wave-formed sedimentary structures and low continuity of mudstone and sandstone interbeds, support deposition in a non-shelf setting. Hence, a sand-rich, fluvial–tidal, current-dominated, largely sub-tidal, delta setting is proposed. This new interpretation avoids the problem of transporting large amounts of coarse sand to a shelf. Facies uniformity and vertical stacking are likely due to sediment oversupply and bypass rather than balanced sediment supply and subsidence rates. However, facies evidence of relative sea level changes is difficult to recognise, which is attributed to: (i) the areally extensive and polygenetic nature of the preserved facies, and (ii) a large stored sediment buffer that dampened response to relative sea-level and/or sediment supply changes. Consideration of preservation bias towards high-energy deposits may be more generally relevant, especially to thick Neoproterozoic and Lower Palaeozoic marine sandstones.     

Integrated diagenesis and sequence stratigraphic study of tidal sandstones: the Adedia Formation (Cambro-Ordovician), Sinai, Egypt

This work examines the different effects meteoric versus marine diagenesis had on Cambro-Ordovician tidal sandstones during episodes of fluctuating sea level. The distribution of diagenetic fabrics was compared to a sequence stratigraphic framework. Initially, a rise in relative sea level (RSL) resulted in deposition of transgressive systems tract sands directly onto crystalline basement. These sandstones display evidence of limited cementation by marine, grain-fringing dogtooth-like and fibrous calcite. A fall in RSL resulted in the progradation of a tidal flat complex and deposition of highstand systems tract (HST) and lowstand systems tract (braided fluvial) sandstones. Contemporaneous meteoric-water flux into sands of all the systems tracts occurred. Sequence boundaries (SB) are marked by fluvial incision of tidal sands and by the development of palaeosols. Meteoric incursion during sea-level lowstands resulted in the dissolution and kaolinitization of feldspars, micas and mud intraclasts in all systems tracts, but is most extensive in HST sandstones below the SB. The effect of meteoric-water flux on the dissolution of marine calcite cements is poorly known. Mesogenetic alterations include intergranular pressure dissolution and formation of variable amounts of syntaxial quartz overgrowths in all systems tracts. Telogenetic alteration (i.e. weathering) in the sandstones includes the formation of goethite and calcite. Thus, the integration of diagenesis with sequence stratigraphy provides a useful tool with which to understand reservoir-quality distribution in sand-dominated, tidal sediments.     

云南思茅盆地二叠纪层序格架与生储盖研究

根据层序界面的特点、凝缩段的组成,思茅盆地二叠系可分为2个Ⅱ级层序、9个Ⅲ级层序.在此基础上,探讨了层序格架与油气生储盖的关系可分为2个二级层序界面为一构造侵蚀不整合层序界面,是极好的储集场所.此时西部盆地中沉积物已经变质,与生储盖关系不大;而东部地区以开阔碳酸盐台地为主,低位体系域、海侵体系域和高位体系域以形成储集层为主.第2个二级层序由海侵-高位体系域所构成.海侵体系域由龙潭组下部所组成,在普洱西部崖子以西为深水盆地（含斜坡）环境,以东为浅海环境,邻近东部古陆区为滨海环境.无论盆地或浅海,岩性以深灰-灰黑色泥岩为主,生烃性能极好.高位体系域可以分为早期高位体系域和晚期高位体系域.早期高位体系域由龙潭组上部层位组成,西为浅海相砂岩、火山岩等,可作储层;东为滨海平原,下部以深灰、灰黑色泥岩为主,是很好的生油岩,上部以砂岩为主夹火山岩,可作为储集层.晚期高位体系域西部为长兴组灰岩、白云质灰岩、白云岩,可作为储层之用.     

鄂尔多斯盆地奥陶系马家沟组层序地层格架研究——兼论陆表海沉积作用和早期成岩作用对相对海平面变化的响应

在论述马家沟组三级层序地层成因的基础上,重新厘定了鄂尔多斯盆地马家沟组层序地层格架。马家沟组马一段至马五段划分为4个三级层序,除了Sq1为Ⅰ型层序之外,Sq2、Sq3、Sq4都是Ⅱ型层序。在陆架内Sq1和Sq4由TST和HST构成;Sq2、Sq3由TST、HST和ISLST构成。TST以正常盐度浅海中形成的生物灰岩和泥晶灰岩为特征,HST以准同生期近地表的略为咸化海水成因的泥晶—细粉晶白云岩为特征,陆架内低水位体系域(ISLST)由陆架内盐湖形成的巨厚的蒸发岩系构成。ISLST和HST在空间分布上和成因上是密切相关的,ISLST置于Ⅱ型层序的顶部或者近顶部比较合理。传统的层序地层学将层序界面置于高水位体系域与低水位体系域之间,在此建议将层序界面置于低水位体系域与海侵体系域之间的海侵面。编制了层序3的海侵体系域、高水位体系域、低水位体系域全盆地层序古地理略图。     

Detached mud prism origin of highstand systems tracts from mid-Pleistocene sequences, Wanganui Basin, New Zealand

Marine siltstone successions, 1–20 m thick, form the upper part of sequences in the mid-Pleistocene Castlecliff section (≈ 0·98–0·35 Ma). The siltstones were deposited within a broad shelf embayment at and about glacioeustatic highstands and are interpreted as highstand systems tracts (HST). Shell-rich to relatively shell-poor contacts at the base of Castlecliff HST are interpreted as downlap surfaces, which mark the quenching of transgressive in situ biogenic accumulation (backlap shellbed). Nonetheless, the basal parts of Castlecliff HST successions are enriched in fossil content in the context of the highstand successions as a whole and represent downlap shellbeds. Castlecliff HST are truncated above by sequence-bounding ravinement surfaces, such that complete sandier-upward successions and subaerial exposure surfaces associated with sequence boundaries sensu stricto are never preserved. Modern highstand sedimentation in the Taranaki Bight offshore from Castlecliff is characterized by a mid-shelf mud depocentre and a coastal shore-connected sand prism, both of which are encroaching upon intervening shell-rich relict and palimpsest transgressive deposits. The mud depocentre is up to 9 m thick, and deposition is influenced by a gyre caused by bathymetric steering of storm-driven currents along the embayed coastline. Modern highstand deposition in the Taranaki Bight, in which the mud depocentre is in part detached from the contemporary shore-connected sand prism, is regarded as an analogue for the deposition of the preserved lower parts of Castlecliff HST. The inferred architecture of Castlecliff HST therefore need not refer to the shore-connected, progradational geometry predicted by traditional sequence models. The model proposed herein may have application to other shelf palaeo-embayments in which mid-shelf focusing of fine-grained sediment has resulted from coastal steering of currents.     

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.     

黔北-川东南志留系层序格架下的沉积体系演化特征及有利区带预测

黔北—川东南志留系受加里东运动抬升剥蚀仅残留中、下统地层,通过大量钻井及野外露头资料的沉积学研究,应用层序地层学方法,划分出8个三级层序,其中下志留统龙马溪组2个（SSQ1~SSQ2）,石牛栏组或小河坝组2个（SSQ3~SSQ4）,中下志留统韩家店组4个（SSQ5~SSQ8）,识别出三角洲、陆棚、碳酸盐台地3类沉积体系,在等时地层格架中探讨了各沉积体系特征及其平面展布。通过层序地层划分对比及有利沉积相带研究,指出了两类有利储集层:一类为台地浅滩相和台地前缘斜坡相礁滩型生物灰岩、颗粒灰岩储层,主要发育于石牛栏期三级层序SSQ3和SSQ4的高位体系域,纵向上分布于石牛栏组中、上部,横向上分布于研究区南部叙永—习水—观音桥一带;另一类为浅水陆棚碎屑岩储层,主要发育于小河坝期三级层序SSQ4高水位体系域,横向上分布于南川—石柱—利川等地。     

Facies development,depositional settings and sequence stratigraphy across the Ordovician–Silurian boundary: a new perspective from the Barrandian area of the Czech Republic

The Hirnantian and Llandovery sedimentary succession of the Barrandian area has been assigned to middle and outer clastic‐shelf depositional settings, respectively. Deposition was influenced by the remote Gondwanan glaciation and subsequent, long‐persisting, post‐glacial anoxia triggered by a current‐driven upwelling system. High‐resolution graptolite stratigraphy, based upon 19 formally defined biozones—largely interval zones—and five subzones, enabled a detailed correlation between 42 surface sections and boreholes, and enabled linking of the sedimentary record, graptoloid fauna dynamics, organic‐content fluctuations and spectral gamma‐ray curves. The Hirnantian and Llandovery succession has been subdivided into four biostratigraphically dated third‐order sequences (units 1–4). Time–spatial facies distribution recorded early and late Hirnantian glacio‐eustatic sea‐level lowstands separated by a remarkable mid‐Hirnantian rise in sea‐level. A major part of the post‐glacial sea‐level rise took place within the late Hirnantian. The highstand of Unit 2 is apparently at the base of the Silurian succession. Short‐term relative sea‐level drawdown and a third‐order sequence boundary followed in the early Rhuddanian upper acuminatus Zone. Early Aeronian and late Telychian sea‐level highstands and late Aeronian drawdown of likely eustatic origin belong to units 3 and 4. Sea‐level rise culminated in the late Telychian, which may also be considered as a highstand episode of a second‐order Hirnantian–early Silurian cycle. Facies and sequence‐stratigraphic analysis supports recent interpretations on nappe structures in the core part of the Ordovician–Middle Devonian Prague Synform of the Barrandian. Copyright © 2006 John Wiley & Sons, Ltd.     

四川龙门山甘溪泥盆纪生态地层、层序地层与海平面变化

龙门山甘溪剖面是我国泥盆系重要典型剖面之一,倍受中外同行关注。本文着重对生态地层、事件地层、层序地层进行研究,为研究全球海平面变化提供区域背景资料。龙门山甘溪剖面含十分丰富的底栖生物化石,从洛赫柯夫阶-弗拉斯阶自下而上可识别出24个腕足动物群落,另外还建立了若干礁复体群落和浮游群落
本文对以上群落的特征、性质、分异度、成分、底栖组合及其与沉积环境的关系作了分析和讨论,并识别了11个海进海退事件(生物的或非生物的)
本区泥盆系是加里东构造旋回后的第一个沉积盖层,属海平面主体上升和海侵同步条件下的旋回超覆地层,由砂质海岸环境向碳酸盐台地环境推进,构成了区内泥盆纪沉积层序序列的组合特征。根据海平面的变化及其相旋回的变迁,划分出6个三级沉积层序。6个沉积层序代表6次海平面的相对升降周期,大致相当于Vail,P.R.(1977)划分的三级地层旋回的海平面变化周期(延续时限1-12Ma),包括4个较大的海平面上升周期与2个较大的海平面下降周期,即洛赫柯夫期、布拉格晚期至埃姆斯早期,吉维中期及弗拉斯早、中期的海平面上升期与艾费尔期中晚期至法门期的海平面下隆期。
上述沉积层序特征和海平面变化,说明区内泥盆纪台缘断陷盆地的形成经历了由陆向海转化和盆地发展演化过程,反映了上扬子地台西缘陆架沉积发展的历史大致可划分为:盆地的雏形阶段(碎屑岩陆架的形成阶段),盆地的发展阶段(碎屑岩与碳酸盐岩混积陆架的形成阶段)和盆地形成与消亡不同性质的三个阶段。它们的形成与演化主要是构造断陷活动和龙门山海水不断向东侵进、古特提斯海北支向东扩展的结果,展示了由滨岸陆架转变为碳酸盐台地,由陆源碎屑充填转化为碳酸盐岩沉积的发展史。     

River‐dominated,shelf‐edge deltas: delivery of sand across the shelf break in the absence of slope incision

Shelf‐edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un‐incised, represent the main increment of shelf‐margin growth into the basin, for that period. The three‐dimensional complexity of shelf‐edge delta systems and along‐strike variability at the shelf edge in particular, remains understudied. The Permian–Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three‐dimensional exposure (>100 km²) and therefore a unique opportunity to evaluate shelf‐edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer‐shelf areas, parasequences are characterized by undeformed, river‐dominated, storm‐wave influenced delta mouth‐bar sandstones interbedded with packages showing evidence of syn‐depositional deformation. The amount and intensity of soft‐sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth‐bar packages transport material down slope. (ii) On the upper slope, mouth‐bar and delta‐front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer‐grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river‐dominated, shelf‐edge deltas transport large volumes of sand to the upper slope, even when major shelf‐edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un‐channellized low density turbidites is distributed evenly along strike.