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 architecture and sequentiality of nearshore and 'shelf'sandbodies; Haystack Mountains Formation, Wyoming, USA

The Haystack Mountains Formation (Campanian, Mesaverde Group, US Western Interior Basin, Wyoming) contains a series of shallow-marine sandbodies, extending tens of kilometres out from a basin margin. The study succession (around 200 m thick) is composed of eight major sandstone tongues (Bolten Ranch, O'Brien Spring, Seminoe 1–2–3–4, Hatfield 1 and 2 members), each partially encased within marine shale intervals. The Formation is ‘sequential’at several scales. At the largest scale, the whole succession presents an aggradational to basinward-stepping stacking pattern of the sandstone tongues. At a lower level, each tongue (member) is characterized internally by two different types of lithosome: the first represents shoreface progradation with hummocky cross-strata passing up to swaley and trough cross-stratified sandstones. This lithosome is erosively truncated at its top in most cases, and has a general sheet-like geometry along strike, whereas down dip it displays a series of sharp-bounded clinothems. The latter sometimes indicate a downward as well as a basinward shift through time, as suggested by the occurrence of coarser and/or shallower facies at a lower level in the shoreface profile. The second type of lithosome is sheet- or wedge-like and sharply overlies the shoreface deposits. The lithosome consists of laterally widespread units of planar tabular to trough cross-bedded medium sandstones passing laterally (in a dip direction) into bioturbated sandstones. The lower part of this lithosome is progradational, becoming retrogradational into the overlying shales. The facies within the cross-bedded lithosome suggest a tidally dominated delta front to estuarine depositional setting. The two types of lithosome are not related genetically. The erosion surface separating the two lithosomes is a sequence boundary separating forced-regressive (relative sea-level fall) shoreface deposits from lowstand to transgressive (early relative sea-level rise), cross-bedded deposits. The uppermost part of the cross-stratified lithosome shows a landward-stepping of component parasequences and is abruptly blanketed by open-marine shales. The most widespread cross-bedded lithosomes are apparently best developed in the lowermost members of the Haystack Mountains Formation, i.e. in the aggradational part of the large-scale progradational succession. In the uppermost, highly progradational sandstone tongues, the shoaling-upward shoreface lithosome dominates, whereas the cross-bedded lithosome occurs in narrow, lensoid belts, or is absent. The middle portion of the succession shows intermediate characteristics. The vertical variation in geometry, thickness and progradational extent of successive cross-bedded lithosomes results from greater confinement of the incised nearshore systems both in space (landward direction) and in time (from the aggradation to the progradation architecture). The latter is a consequence of a decreasing rate of accommodation creation through time.     

Sedimentology and sequence stratigraphy of a transfer zone coarse-grained delta, Miocene Suez Rift, Egypt

This study focuses on Miocene sedimentation and stratigraphic evolution in a major transfer zone at the northern tip of the Thal Fault segment, Gulf of Suez. The succession generally shoals upwards from offshore mudstone containing pro-delta turbidites, into conglomeratic delta foresets and topsets, with sandstone-dominated shoreface facies coexisting laterally. Despite this upward shoaling, key stratal surfaces marking abrupt changes in relative sea-level allow the succession to be divided into four stratal units. The stacking pattern of the stratal units suggests an initial relative sea-level rise that generated a major marine flooding surface. A relative sea-level fall followed, resulting in widespread exposure and incision. During the ensuing relative sea-level rise a lowstand coarse-grained delta and coeval shoreface succession prograded several kilometres basinward. The stratigraphic development of the transfer zone delta is in marked contrast to that of aggradationally stacked deltas that occur near the centre of the Baba-Sidri fault segment, further south. At the transfer zone, low rates of subsidence and accommodation development coupled with a high sediment supply derived from a large fault tip drainage catchment have produced a strongly progradational delta subject to marked changes in relative sea-level. In the fault centre location, however, higher rates of accommodation development coupled with lower rates of sediment supply from footwall catchments have produced aggradationally stacked deltas. The results from this study have implications for sequence stratigraphic models and hydrocarbon exploration within extensional basins.     

Allogenic and autogenic controls on facies and stratigraphic architecture of the Lower Jurassic Mashabba Formation,Gebel Al-Maghara,North Sinai,Egypt

The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems.     

Sedimentology and stratigraphic development of the upper Nyalau Formation (Early Miocene), Sarawak,Malaysia: A mixed wave- and tide-influenced coastal system

This work presents the first detailed facies analysis of the upper Nyalau Formation exposed around Bintulu, Sarawak, Malaysia. The Lower Miocene Nyalau Formation exposures in NW Sarawak represent one of the closest sedimentological outcrop analogues to the age equivalent, hydrocarbon-bearing, offshore deposits of the Balingian Province. Nine types of facies associations are recognised in the Nyalau Formation, which form elements of larger-scale facies successions. Wave-dominated shoreface facies successions display coarsening upward trends from Offshore, into Lower Shoreface and Upper Shoreface Facies Associations. Fluvio-tidal channel facies successions consist of multi-storey stacks of Fluvial-Dominated, Tide-Influenced and Tide-Dominated Channel Facies Associations interbedded with minor Bay and Mangrove Facies Associations. Estuarine bay facies successions are composed of Tidal Bar and Bay Facies Associations with minor Mangrove Facies Associations. Tide-dominated delta facies successions coarsen upward from an Offshore into the Tidal Bar Facies Association. The Nyalau Formation is interpreted as a mixed wave- and tide-influenced coastal depositional system, with an offshore wave-dominated barrier shoreface being incised by laterally migrating tidal channels and offshore migrating tidal bars. Stratigraphic successions in the Nyalau Formation form repetitive high frequency, regressive–transgressive cycles bounded by flooding surfaces, consisting of a basal coarsening upward, wave-dominated shoreface facies succession (representing a prograding barrier shoreface and/or beach-strandplain) which is sharply overlain by fluvio-tidal channel, estuarine bay or tide-dominated delta facies successions (representing more inshore, tide-influenced coastal depositional environments). An erosion surface separates the underlying wave-dominated facies succession from overlying tidal facies successions in each regressive–transgressive cycle. These erosion surfaces are interpreted as unconformities formed when base level fall resulted in deep incision of barrier shorefaces. Inshore, fluvio-tidal successions above the unconformity display upward increase in marine influence and are interpreted as transgressive incised valley fills.     

A tidal-inlet complex in the Cretaceous epeiric sea of North America: Virgelle Member, Milk River Formation, southern Alberta, Canada

The upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing-on-Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave-dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non-marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal-inlet complex. Most inlet sections consist of an erosional base overlain by a shale-pebble conglomerate, followed by cross-bedded sandstones which become finer-grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re-activation surfaces and herringbone cross-beds. The sequence through the tidal-inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb-dominated, flood-dominated, or mixed-tidal influence. Ebb-dominated sections commonly contain lateral accretion surfaces whereas flood-dominated sections contain tidal-ramp deposits. Back-barrier lagoon deposits are dominated by sandstones of an extensive flood-tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb-tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave-induced longshore currents acting on the regionally wave-dominated shoreline.     

Allogenic controls on the evolution of storm to tidal shelf sequences in the Early Proterozoic Uncompahgre Group, southwest Colorado, USA

Dominantly coarse-grained, shallow-marine, metasedimentary rocks of the Early Proterozoic Uncompahgre Group (UG) record periods of shoaling and drowning on different temporal scales that are attributed to episodic long-term oscillations in relative sea-level with superimposed shorter duration excursions in relative sea-level. Long-term events are probably tectonic whereas short-term events are eustatic. The 2–5 km thick Uncompahgre Group consists of 250–600 m thick, dominantly coarse-grained quartzite units (Q1–Q4) and 200–300 m thick mudstone/pelite units (P1–P5). Five depositional systems comprise the Uncompahgre Group. The outer shelf system (OSS) is composed of Bouma-type beds and intercalated mudstones that are transitional vertically to parallel-laminated to wave-rippled sandstones and hummocky cross-stratified sandstones of the inner shelf system (ISS). Trough cross-stratified sandstones comprise the shoreface system (SHS). The tidal inner shelf/shoreface system (TIS/SHS) consists of a complex interlayering of cross-bedded sandstones, thin-bedded conglomerates, mudstones and rippled sandstones. Trough cross-bedded pebbly sandstones and thin- to thick-bedded conglomerates represent the alluvial system (ALLS). Depositional systems in the UG are associated in transgressive and highstand-systems tracts that make up four sequences (1 to 4). Sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities. The basal Q1–P1 unit (lower sequence 1) consists of ALLS to TIS/ SHS to ISS comprising a transgressive systems tract. A maximum marine incursion is reflected by deposition of OSS facies in stratigraphic units P1–P2. Shoaling in the transition from P2 to the uppermedial portion of Q2 (OSS—ISS—SHS to a thick TIS/SHS—ALLS) records the highstand systems tract of upper sequence 1. A subtle disconformity/paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract. The drowning to shoaling pattern is replicated in sequence 2 (upper Q2 to P3 to upper medial Q3); sequence 3 (upper Q3 to P4 to upper-medial Q4); and an incomplete sequence 4 (upper Q4 through P5). Thinner shoaling intervals of OSS—ISS—SHS in P3 and in lower Q2, Q3 and Q4 represent parasequences. Sequences of 10⁷ years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin. Parasequences record shorter duration temporal controls of c. 10⁴ to 10⁵ years related to eustatic oscillations. As a consequence of shoaling and aggradation/ progradation in the highstand systems tract, TIS/SHS and ALLS overlie and are temporally separated from OSS to ISS to SHS. This transition records filling of the basin to sea-level leading to a shelf geometry that was conducive to tidal amplification. A composite relative sea-level curve integrating long-term pulsatory subsidence and short-term eustasy best explains the stratigraphic evolution of the Uncompahgre Group.     

The effects of differential subsidence and coastal topography on high-order transgressive-regressive cycles: Pliocene nearshore deposits of the Val d'Orcia Basin, Northern Apennines, Italy

This study focuses on the lowstand and early transgressive systems tracts of a basin-fill sequence of lower Pliocene nearshore deposits in the Val d'Orcia Basin of the Northern Apennines, Italy. The basin at that time was a semi-enclosed marine embayment, and, in the study area, its margin was subject to highly variable subsidence along the depositional strike, attributed to a decrease in tectonic displacement. The nearshore succession in the more rapidly-subsiding segment of the basin is around 20 m thick, comprising three storeys of laterally-stacked Gilbert-type delta lobes overlain by a shoal-water delta, whereas the nearshore succession in the adjacent, more slowly-subsiding segment, is up to 9 m thick. This succession is characterised by alternation of shoreface and offshore deposits, moderately wave-worked and covered by shoal-water deltaic facies.These coeval nearshore successions consist of several transgressive-regressive cyclothems. The development and lateral variation of the cyclothems was controlled by the local subsidence rate and coastal topographic gradient. Some of the cyclothems are considered to be higher-order sequences and others to be parasequences, with the former passing laterally into the latter in the area where the sea-level fall was countered by fast local subsidence. Some of the bounding surfaces are of limited lateral extent, with two parasequences passing laterally into a single one.Coastal topography controlled particularly the thickness of transgressive deposits. In the low-gradient setting of a delta plain, the relative sea-level rises caused major landward shifts of the shoreline and reduced fluvial sediment supply, with the formation of a transgressive lag in sediment-starved conditions. In the high-gradient coastal setting of the non-deltaic zone, the shoreline shift was minimal and had relatively little impact on local sediment supply, which promoted an accretionary transgression.At the end of the lowstand stage, the rate of sediment accumulation in the non-deltaic nearshore zone was lower, allowing the onset of subsequent transgression to be recorded considerably earlier than in the deltaic nearshore zone. This diachroneity suggests that facies criteria alone may not necessarily be a reliable basis for the recognition of systems tract boundaries.     

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.     

Intra-shoreface erosion in response to rapid sea-level fall: depositional record of a tectonically uplifted strand plain, Pacific coast of Japan

This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2·3 to 2·6 and 1·8 to 2·0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.     

The Lower Permian Wasp Head Formation,Sydney Basin: high‐latitude,shallow marine sedimentation following the late Asselian to early Sakmarian glacial event in eastern Australia

The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions.     

Stratigraphic evolution of the nearshore to fluvial plain of the Upper Cuyo Group,Neuquén,Argentina

The upper portion of the Cuyo Group in the Zapala region, south‐eastern Neuquén Basin (Western Argentina), encompasses marine and transitional deposits (Lajas Formation) overlain by alluvial rocks (Challacó Formation). The Challacó Formation is covered by the Mendoza Group above a second‐order sequence boundary. The present study presents the stratigraphic framework and palaeophysiographic evolution of this Bajocian to Eo‐Calovian interval. The studied succession comprises the following genetic facies associations: (i) offshore and lower shoreface–offshore transition; (ii) lower shoreface; (iii) upper shoreface; iv) intertidal–subtidal; (v) supratidal–intertidal; (vi) braided fluvial to delta plain; (vii) meandering river; and (viii) braided river. The stratigraphic framework embraces four third‐order depositional sequences (C1 to C4) whose boundaries are characterized by the abrupt superposition of proximal over distal facies associations. Sequences C1 to C3 comprise mostly littoral deposits and display well‐defined, small‐scale transgressive–regressive cycles associated with fourth‐order depositional sequences. Such high‐frequency cycles are usually bounded by ravinement surfaces associated with transgressive lags. At last, the depositional sequence C4 delineates an important tectonic reorganization probably associated with an uplift of the Huincul Ridge. This is suggested by an inversion of the transport trend, north‐westward during the deposition of C1 to C3 depositional sequences (Lajas Formation) to a south‐west trend during the deposition of the braided fluvial strata related to the C4 depositional sequence (Challacó Formation).     

Physical stratigraphy and sedimentology of the Late Pleistocene-Holocene Tiber Delta depositional sequence

The Tiber Delta lies on the passive continental margin of central western Italy. The Tiber Delta is a Late Pleistocene—Holocene sedimentary succession and constitutes an ‘incomplete’high-frequency depositional sequence (4th or 5th order), which developed from the last post-glacial rise in sea level to the present stillstand. It is bound by a type 1 basal unconformity which cuts Pleistocene deposits. Five depositional systems have been identified: (1) coastal barrier—lagoon; (2) shelf-transition; (3) fluvial braided; (4) Tiber lagoonal deltas (T1, T2, T3); and (5) Tiber wave-dominated delta. The first four systems constitute the transgressive system tract (TST) which is characterized by a retrogradational parasequences set, while the fifth system represents the highstand system tract (HST) characterized by only one parasequence (progradational). The evolution of these depositional systems has been mostly controlled by sea-level rise, which was not continuous, but punctuated by eight minor stillstands and rapid rises. This mechanism of sea-level change led to the formation of parasequences that developed within the depositional systems. From a palaeogeographical point of view the present Tiber Delta area was mainly characterized by a coastal barrier, lagoon and Tiber lagoonal deltas (T1, T2, T3), which migrated landwards during the sea-level rise. Throughout the time of the present stillstand, the T3 lagoonal delta prograded rapidly into the lagoon, reaching the coastal barrier, and the lagoon was replaced first by marshes and then by the alluvial plain. Finally, the Tiber River overcame the littoral barrier supplying the coast, giving rise to the present wave-dominated delta.     

The role of shelf morphology on storm‐bed variability and stratigraphic architecture,Lower Cretaceous,Svalbard

The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.     

层序界面对砂岩成岩作用及储层质量的影响——以鄂尔多斯盆地延河露头上三叠统延长组为例

砂岩成岩作用及储层质量是油气勘探的重要研究内容,不同地区砂岩成岩作用及储层质量的主要控制因素各不相同。本文以鄂尔多斯盆地东部延河露头剖面延长组长7上部—长4+5中部三角洲前缘—三角洲平原沉积为例,重点讨论三级层序界面（长16底）上下不同基准面半旋回中砂岩自生矿物、储集空间类型、储集性能和含油性差异及造成这些差异的原因。指出层序界面之下的下降半旋回中浊沸石和早期钙质砂岩结核的发育阻止了绿泥石等粘土矿物及自生石英和自生钠长石的发育,而界面之上的上升半旋回中因凝灰质不足以及地层温度较低造成浊沸石发育有限,加之受湖水影响较小而缺乏早期方解石胶结物,因而绿泥石等自生粘土矿物及自生石英和自生钠长石较为发育,并保留了一部分原生孔隙。层序界面及紧邻其上大规模发育的长16砂体成为流体运移的优势通道,是界面之上浊沸石、方解石胶结物及长石颗粒被酸性流体溶蚀形成次生孔隙的重要因素,也是石油的重要运移通道和储集体。     

Sedimentary facies and sequence stratigraphy of the Silurian at Tabei uplift,Tarim Basin,China

Sequence stratigraphy division and comparison of the Silurian in Tarim Basin were a hot research field in oil industry and academia. However, basic geological problems limited the exploration needed for further research. In this paper, 21 lithofacies and 5 facies associations were identified based on the grain size of sediments, sedimentary characteristics, and bioturbation conditions: (1) fluvial-dominate delta front facies association; (2) tidal flat facies association; (3) tidal channel facies association; (4) offshore-transition facies association; (5) shoreface facies association. The seismic, outcrops, and logging data were involved to divide the Silurian (including upper Ordovician Tierekeawati Fm.) at Tabei uplift into five sedimentary sequences. SQ1 (Tierekeawati Fm.) is mainly characterized by tidal flat facies association, while delta front facies association locally develops; SQ2 (the lower Kepingtage Fm.) generally consists of offshore-transition facies association; SQ3 (the upper Kepingtage Fm.) is mainly characterized by shoreface and delta front facies association. For SQ4 (Tataaiertage Fm.), the transgressive system tract (TST) is dominated by shoreface facies association, while the fluvial-dominate delta facies association widely develops in highstand system tract (HST). SQ5 (Yimugantawu Fm.) is mainly characterized by tidal flat facies association. From SQ1 to SQ2, an overall sea level transgressive process is shown, while an overall sea level regressive process is found from SQ2 to SQ5. The results are consistent with the progradation and regression trends of large regions reflected by sequence framework pattern. As to SQ3 sequence, TST and HST sandstones are the main reservoir intervals in the Silurian. Hercynian movement led to the strong uplift and extensive erosion in the Silurian at Tabei and Tazhong uplift, and is favorable to the formation of strata erosion unconformable traps.     

Sedimentary response to Late Quaternary sea-level changes in the Romagna coastal plain (northern Italy)

Data from 17 continuously cored boreholes, 40–170 m deep, reveal the subsurface stratigraphy of the Romagna coastal plain. Sedimentological and microfaunal data allow the distinction of eight facies associations of Late Pleistocene–Holocene age, including 18 lithofacies and 16 faunal associations. Ten ¹⁴C dates provide the basis to establish a sequence stratigraphic framework for the succession corresponding to the upper part 35 ky BP of the last glacio-eustatic cycle. The eight facies associations can be grouped into lowstand, transgressive and highstand systems tracts. The upper part of the lowstand systems tract consists of alluvial plain deposits. These accumulated during the Late Pleistocene when the shoreline was ≈250 km south of its present-day position. A pronounced stratigraphic hiatus (between 25 and 8·8 ky BP) is invariably recorded at the upper boundary (transgressive surface) of these Pleistocene, indurated and locally pedogenized alluvial deposits. The succeeding postglacial history is represented by a well developed transgressive–regressive cycle. Transgressive deposits, interpreted to reflect the rapid landward migration of a barrier–lagoon system, include two wedge-shaped, paralic and marine units. These thicken in opposite directions and are separated by a ravinement surface. Above the transgressive deposits, the maximum flooding surface (MFS) marks the change from a transgressive barrier–lagoon complex to a prograding, wave-dominated delta system (early Po delta). The MFS can be traced landwards, where it constitutes the base of lagoonal deposits. An aggradational to progradational stacking pattern of upper delta plain (marsh), lower delta plain (lagoon/bay), and delta front (beach ridge) deposits reflects the progressive increase in the sediment supply/accommodation ratio during the following highstand. The alluvial deposits capping the sequence accumulated by the 13th century AD, in response to an avulsion event that caused abandonment of the former Po delta lobe and the northward migration of the Po River towards its present position.     

High-resolution sequence stratigraphy of a complex, incised valley succession, Cobequid Bay — Salmon River estuary, Bay of Fundy, Canada

The post-glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio-marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave-dominated estuarine environment under micro-mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun. The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial-estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments. These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical configurations.     

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

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

Paralic parasequences associated with Eocene sea-level oscillations in an active margin setting: Trihueco Formation of the Arauco Basin, Chile

The Eocene Trihueco Formation is one of the best exposed successions of the Arauco Basin in Chile. It represents a period of marine regression and transgression of second-order duration, during which barrier island complexes developed on a muddy shelf. The strata are arranged in classical shoaling-upward parasequences of shoreface and beach facies capped by coal-bearing, back-barrier lagoon deposits. These fourth-order cycles are superimposed upon third-order cycles which caused landward and seaward shifts of the coastal facies belts. The final, punctuated rise in sea level is represented by shelf mudrocks with transgressive incised shoreface sandstones. Relative sea-level oscillations as revealed in the stratigraphy of the Trihueco Formation show a reasonable correlation with published Eocene eustatic curves.