Deltaic,mixed and turbidite sedimentation of ancient foreland basins

The marine fill of ancient foreland basins is primarily recorded by depositional systems consisting of facies and facies associations deposited by a variety of sediment gravity flows in shallow-marine, slope and basinal settings. Tectonism and climate were apparently the main factors controlling the sediment supply, accommodation and depositional style of these systems. In marginal deltaic systems, sedimentation is dominated by flood-generated hyperpycnal flows that build up impressive accumulations of graded sandstone beds in front of relatively small high-gradient fan-deltas and river deltas. During periods of tectonically forced lowstands of sealevel, these systems may commonly shift basinward to shelfal and slope regions. Instability along the edges of these lowstand deltas and sand-laden hyperpycnal flows generate immature and coarse-grained turbidite systems commonly confined within structural depressions and generally encased in distal delta-front and prodeltaic deposits. Because of the close vertical and lateral stratigraphic relations between deltaic and turbidite-like facies, these marginal systems are herein termed ‘mixed depositional systems’. They are very common in the fill of foreland basins and represent the natural link between deltaic and basinal turbidite sedimentation.Basinal turbidite systems form in deeper water elongate highly subsiding troughs (foredeeps) that developed in front of advancing thrust systems. The impressive volumes of sheet-sandstones that form the fill of these troughs suggest that basinal turbidite systems are likely to form following periods of dramatic tectonic uplift of adjacent orogenic wedges and related high-amplitude tectonically-forced sealevel lowstands. In such deep basinal settings, sediment flux to the sea is dramatically increased by newly formed sediment in fluvial drainage basins and the subaerial and submarine erosion of falling-sealevel deltaic deposits generated during the uplift. Turbidity currents are very likely to be mainly triggered by floods, via hyperpycnal flows and related sediment failures, but can fully develop only in large-scale erosional conduits after a phase of catastrophic acceleration and ensuing bulking produced by bed erosion. This process leads to deepening and widening of the conduits and the formation of large-volume highly efficient bipartite currents whose energy dissipation is substantially reduced by the narrow and elongate basin geometry. These currents can thus carry their sediment load over considerable distances down the basin axis.     

下刚果盆地深水沉积中新统层序划分及其控制因素

对于具有较宽陆架的下刚果盆地,当相对海平面下降时期,陆架区沉积中心向外陆架迁移,形成陆架边缘三角洲.其为深水区提供大量陆源碎屑物质,使低位体系域发育碎屑流沉积、浊流水道及前端扇体系;当相对海平面上升时期,沉积中心后退至内/中陆架,使深水区海侵十高位体系域以深海、半深海原地泥质沉积及泥质碎屑流沉积为主.深水层序以凝缩层段...     

Tectonism: the dominant factor in mid-Cretaceous deposition in the Jeanne d'Arc Basin, Grand Banks

The development of stratigraphic sequences has been demonstrated to be controlled by a set of factors including variations in subsidence, sediment input, eustatic sea level and physiography. Well and seismic data from the Jeanne d'Arc Basin, Grand Banks indicate that mid-Cretaceous tectonism controls at least three of these factors, namely subsidence, sediment input and physiography. North Atlantic rift tectonism was therefore the dominant factor in controlling the migration of coastal to shallow marine environments and the development of sequence stratigraphy in this basin during the mid-Cretaceous. The Avalon Formation respresents a mainly Barremian to Early Aptian regressive phase of clastic, marine to marginal marine sedimentation. This followed the deposition of a thick sequence of mainly marine limestones and shales of the Whiterose Formation above a mid-Valanginian sequence-bounding unconformity. The increased clastic input and northward progradation of coastal environments represented by the Avalon Formation occurred during uplift of a basement arch to the south with subsidence of the basin increasing to the north, accompanied by only relatively minor faulting. These features indicate that a period of epeirogenesis was initiated during the Barremian. Continuing uplift over an expanding area at the southern end of the basin is interpreted to have resulted in the development of an angular unconformity with incised valleys. This mid-Aptian unconformity defines the top of the Whiterose/Avalon sequence. Initiation of brittle fracturing of the sedimentary package and underlying basement (i.e. rifting) in mid-Aptian times resulted in rapid fault-controlled subsidence and fragmentation of the Jeanne d'Arc Basin. This great increase in subsidence rate caused retrogradation of coastal environments across the previously developed sequence-bounding unconformity, despite continuing high rates of sediment input from the uplifted basin margins. The transgressive, siliciclastic Ben Nevis Formation comprises two separate but related facies associations. A locally preserved basal association represents interfingering back-barrier environments and is herein defined as the Gambo Member. An upper, ubiquitous facies association comprises tidal-inlet channel, shoreface and lower shoreface/offshore transition sandstones. This upper facies association onlapped marine ravinement diastems above the laterally equivalent back-barrier facies. The rapid fault-controlled subsidence and high sediment input rate of this mid-Aptian to late Albian rift period resulted in the accumulation and preservation of very thick shoreface sandstones. The transgressive sandstones were buried by laterally equivalent offshore shales of the Nautilus Formation. Flooding of the basin margins induced by the onset of thermal subsidence in latest Albian or early Cenomanian times marks the top of the Ben Nevis/Nautilus syn-rift sequence.     

Sedimentary facies associations and sequence stratigraphy of source and reservoir rocks of the lacustrine Eocene Niubao Formation (Lunpola Basin,central Tibet)

The Eocene Niubao Formation of the Lunpola Basin, a large Cenozoic intermontane basin in central Tibet, is an important potential hydrocarbon source and reservoir unit. It represents ∼20 Myr of lacustrine sedimentation in a half-graben with a sharply fault-bounded northern margin and a low-angle flexural southern margin, resulting in a highly asymmetric distribution of depositional facies and sediment thicknesses along the N-S axis of the basin. An integrated investigation of well-logs, seismic data, cores and outcrops revealed three third-order sequences (SQ1 to SQ3), each representing a cycle of rising and falling lake levels yielding lowstand, transgressive, and highstand systems tracts. Lowstand systems tracts (LST) include delta and fan delta facies spread widely along the gentle southern margin and concentrated narrowly along the steep northern margin of the basin, with sublacustrine fan sand bodies extending into the basin center. Highstand systems tracts (HST) include expanded areas of basin-center shale deposition, with sublacustrine fans, deltas and fan deltas locally developed along the basin margins. Sequence development may reflect episodes of tectonic uplift and base-level changes. The southern margin of the basin exhibits two different structural styles that locally influenced sequence development, i.e., a multi-step fault belt in the south-central sector and a flexure belt in the southeastern sector. The sedimentary model and sequence stratigraphic framework developed in this study demonstrate that N2 (the middle member of Niubao Formation) exhibits superior hydrocarbon potential, characterized by thicker source rocks and a wider distribution of sand-body reservoirs, although N3 (the upper member of Niubao Formation) also has good potential. Fault-controlled lithologic traps are plentiful along the basin margins, representing attractive targets for future exploratory drilling for hydrocarbons.     

Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea

A tectonostratigraphic model for the evolution of rift basins has been built, involving three distinct stages of basin development separated by key unconformities or unconformity complexes. The architecture and signature of the sediment infill for each stage are discussed, with reference to the northern North Sea palaeorift system. The proto-rift stage describes the rift onset with either doming or flexural subsidence. In the case of early doming, a proto-rift unconformity separates this stage from the subsequent main rift stage. Active stretching and rotation of fault blocks during the rift stage is terminated by the development of the syn-rift unconformity. Where crustal separation is accomplished, a break-up unconformity commonly marks the boundary to the overlying thermal relaxation or post-rift stage. Tabular architectures, thickening across relatively steep faults, characterize the proto-rift stage. Syn-rift architectures are much more variable. Depending on the ability of the sediment supply to fill the waxing and waning accommodation created during rotation and subsidence, one-, two- or three-fold lithosome architectures are likely to develop. During the post-rift stage, an early phase with coarse clastic infilling of remnant rift topography often precedes late stage widening of the basin and filling with fine-grained sediments.     

Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-to-orthogonal-rifted Kwanza basins

We analyse tectonic and sedimentary field and subsurface data for the Angola onshore margin together with free-air gravity anomaly data for the offshore margin. This enables us to characterize the mode of syn-rift tectonism inherited from the Precambrian and its impact on the segmentation of the Angola margin. We illustrate that segmentation by the progressive transition from the Benguela transform-rifted margin segment to the oblique-rifted South Kwanza and orthogonal-rifted North Kwanza margin segments. The spatial variation in the intensity of post-rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post-rift succession of the coastal Benguela and Kwanza Basins: Upper Cretaceous to Cenozoic uplifted palaeodeltas, erosional unconformities, palaeovalleys, Quaternary marine terraces and perched Gilbert deltas. The onshore Benguela transform margin has a distinctive, mainly progradational stratigraphic architecture with long-term sedimentary gaps and high-elevation marine terraces resulting from moderate Upper Cretaceous–Cenozoic to major Quaternary uplifting (i.e. 775–1775 mm/ky or m/Ma). By contrast, repeated synchronous episodes of minor Cenozoic to Quaternary uplift occurred along the orthogonal-rifted North Kwanza segment with its Cenozoic aggradational architecture, short-term sedimentary gaps and low-elevation Pleistocene terraces. Margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep-sea fans. Along the low-lying North Kwanza margin, sedimentation of the broad Cenozoic to Pleistocene Kwanza submarine fan was probably governed by the width of the Kwanza interior palaeodrainage basin combined with the wet tropical Neogene climate. Along the high-rising Benguela margin, the small size of the Benguela deep-sea fan is related to the interplay between moderate continental sediment dispersal from long-lived small catchments and a warm, very arid Neogene climate. However, the driving forces behind the epeirogenic post-rift uplift of the Angola coastal bulge remain a matter of speculation.     

Post-Calabrian sequence stratigraphy of the northwestern Alboran Sea (southwestern Mediterranean)

The post-Calabrian sedimentary column of the northwestern Alboran Sea comprises three depositional sequences. The two older depositional sequences are defined by lowstand systems tracts (shelf-margin deltas, slope, base-of-slope, and basin deposits, and the Guadiaro channel-levee complex). In contrast, the most recent depositional sequence also includes transgressive (relict shelf facies) and high-stand (the Guadalmedina-Guadalhorce prodelta and hemipelagic facies) systems tracts. The stratigraphic architecture of these depositional sequences is controlled by the synchronism between high frequency sea-level changes, variations in sediment supply, and sedimentary processes. The configuration of the depositional sequences is variable and their distribution is complex, as a result of the relative importance played by sea-level changes and tectonism through the area.

The sequence boundaries are represented by polygenetic surfaces in the proximal margin, and by monogenetic surfaces in the distal margin and basin. Each polygenetic surface results from the interaction between the sequence boundary with the lowstand erosional truncation surface and the transgressive surface, both developed during the previous sea-level cycle. The monogenetic surfaces correspond to unconformities and their correlative conformities, formed during sea-level lowstands. This pattern of depositional sequences developed in the margin and basin of the northwestern Alboran Sea shows differences with the Exxon Sequence Stratigraphy Model as traditionally applied: sea-level change control is essentially recognized through lowstand systems tracts, and sequence boundary coincides with lowstand erosional truncation surface and transgressive surface, both developed during the previous sea-level cycle.     

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos,Santos and Pelotas basins)

The Campos, Santos and Pelotas basins have been investigated in terms of 2D seismo-stratigraphy and subsidence. The processes controlling accommodation space (e.g. eustacy, subsidence, sediment input) and the evolution of the three basins are discussed. Depositional seismic sequences in the syn-rift Barremian to the drift Holocene basin fill have been identified. In addition, the subsidence/uplift history has been numerically modeled including (i) sediment flux, (ii) sedimentary basin framework, (iii) relation to plate-tectonic reconfigurations, and (iv) mechanism of crustal extension. Although the initial rift development of the three basins is very similar, basin architecture, sedimentary infill and distribution differ considerably during the syn-rift sag to the drift basin stages. After widespread late Aptian–early Albian salt and carbonate deposition, shelf retrogradation dominated in the Campos Basin, whereas shelf progradation occurred in the Santos Basin. In the Tertiary, these basin fill styles were reversed: since the Paleogene, shelf progradation in the Campos Basin contrasts with overall retrogradation in the Santos Basin. In contrast, long-term Cretaceous–Paleogene shelf retrogradation and intense Neogene progradation characterize the Pelotas Basin. Its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation. These temporally and spatially varying successions were controlled by specific long-term subsidence/uplift trends. Onshore and offshore tectonism in the Campos and Santos basins affected the sediment flux history, distribution of the main depocenters and occurrence of hydrocarbon stratigraphic–structural traps. This is highlighted by the exhumation and erosion of the Serra do Mar, Serra da Mantiqueira and Ponta Grossa Arch in the hinterland, as well as salt tectonics in the offshore domain. The Pelotas Basin was less affected by changes in structural regimes until the Eocene, when the Andean orogeny caused uplift of the source areas. Flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic.     

MTD distribution on a ‘passive’ continental margin: The Espírito Santo Basin (SE Brazil) during the Palaeogene

Mass-wasting on the Brazilian margin during the Mid-Eocene/Oligocene resulted in the accumulation of recurrent Mass Transport Deposits (MTDs) offshore Espírito Santo, SE Brazil. In this paper, we use three-dimensional seismic data to characterize a succession with stacked MTDs (Abrolhos Formation), and to assess the distribution of undeformed stratigraphic packages (i.e. turbidites) with reservoir potential separating the interpreted MTDs. High-amplitude strata in less deformed areas of MTDs reflect their internal heterogeneity, as well as possible regions with a higher sand content. Separating MTDs, turbiditic intervals reach 100 ms Two-Way Travel Time (TWTT), with thicker areas coincident with the flanks of growing diapirs and areas of the basin where mass-wasting is less apparent. Turbiditic strata laterally grade into, or are eroded by MTDs, with transitional strata between MTDs and turbidites being also influenced by the presence of diapirs. MTDs show average thickness values ranging from 58 to 82 ms TWTT and constitute over 50% of Eocene-Oligocene strata along the basin slope. Low average accumulations of 58 ms TWTT in areas of high confinement imposed by diapirs suggest sediment accumulation upslope, and/or bypass into downslope areas. This character was induced by the high sediment input into the basin associated with coastal erosion and growth of the Abrolhos volcanic plateau. Our results suggest that significant amounts of sediment derive from the northwest, and were accumulated in the middle-slope region. Interpretations of (palaeo)-slope profiles led to the establishment of a model of margin progradation by deposition of MTDs, contrasting with the retrogressive erosional margins commonly associated with these settings.     

长江口水下三角洲泥质区近期沉积物粒度变化特征及其影响因素

河口水下三角洲是河流和海洋环境共同作用的产物,沉积地层中蕴藏了许多环境变化信息.在长江口水下三角洲泥质区采集了柱样SC09,首先利用放射性同位素210Pb确定了沉积柱样的平均沉积速率,其次对沉积柱样以0.2 cm间隔进行高分辨率取样,获得了沉积物粒度参数,然后提取了沉积物粒度敏感组分,并对其进行了经验模态分解(EMD)...     

Triassic platform-margin deltas in the western Barents Sea

The Early to Middle Triassic in the Barents Sea was dominated by prograding transgressive-regressive sequences. Internal clinoform geometries indicate that sediments were derived from the Baltic Shield in the south and the Uralian Mountains in the east and southeast. These systems were formed in a large, relatively shallow epicontinental basin, where modest variations in relative sea-level relocated the shoreline significantly. This study shows the development of strike elongated depositional wedges that thicken just basinward of the platform-edge. Seismic facies and time-thickness maps show the position and development of platform-margin delta complexes within each sequence. Seismic clinoforms and trajectory analysis show significant lateral variation from the axis of the delta complex to areas adjacent to the main delivery system. Frequent toplap geometries are observed in proximity to coarse-grained deposits, while aggradation of seismic clinoforms characterizes areas laterally to the platform-margin deltas. Complex shifts in depocenters are revealed by large-scale compensational stacking pattern and relict platform breaks. Locally, relict breaks are created due to pre-existing paleo-topography. Platform-margin deltas can be identified by careful mapping of clinoform geometries, clinoform angles and trajectories. However, seismic analysis of prograding clinoform units indicate that the shoreline and delta complexes commonly are positioned landward of the platform-edge. Deposition of platform-margin deltas is sometimes caused by locally increased sediment supply during slightly rising relative sea-level, and occasionally caused by a regional drop in relative sea-level with significant shelf bypass.Development, position, thickness and facies distribution of platform deltas and platform-margin deltas of very broad low-relief basins, like the Triassic of the epicontinental Barents Sea basin, are strongly sensitive to changes in relative sea-level due to rapid emergence and submergence of wide areas, and to changes in position of major rivers supplying sand to the delta systems. In this respect, the depositional model of the present study deviates from models of clinoform successions obtained from small and narrow basins or siliciclastic platforms with high coarse-clastic sediment supply.     

Sequence stratigraphy based on high-resolution seismic profiles in the late Pleistocene and Holocene deposits of the Venice area

The sequence-stratigraphic investigation by Very High-Resolution (VHR) seismic profiles allowed recognition of the detailed architecture of the late Pleistocene and Holocene succession of the Venice area. In this way deposits previously known by the analyses of scattered cores, mainly taken along the lagoon margin and the littoral strips, have been correlated at regional scale including the near offshore sector and the result has pointed out the lateral variability of the stratal architecture. Late Pleistocene deposits consist of an aggrading floodplain and fluvial channel fills accumulated during decreasing eustatic sea level, and they are coeval with offlapping forced regressive marine wedges in the Central Adriatic basin. The Holocene sequence is composed of three main seismic units separated by major stratal surfaces. Unit 1 (up to 9 m thick) is formed by channelized deposits separated by areas showing sub-horizontal and hummocky reflectors, and is bounded at the base by a surface that records prolonged conditions of subaerial exposure and at the top by a flatter surface resulting from erosion by marine processes. Deposits of Unit 1 are interpreted as estuarine and distributary channel fills, and back-barrier strata. Unit 2 is well distinguishable from Unit 1 only in the offshore area and at the barrier island bounding the Venice Lagoon, and is composed of a prograding marine wedge (up to 10 m thick) that interacts laterally with ebb tidal deltas. Unit 3 consists of a tidal channel complex and inlet deposits, which testify the evolution of the lagoon area. Tidal channels are entrenched in the lagoon mud flat (coeval with Units 1–2) and cut the Pleistocene–Holocene boundary in several places.Following current sequence-stratigraphic concepts, the Holocene sequence is composed of a paralic transgressive systems tract (TST) (Unit 1) overlying a sequence boundary (the Pleistocene–Holocene boundary) and overlain by a marine highstand systems tract (HST) (Unit 2) in seaward locations and by highstand lagoonal deposits landwards. TST and HST are separated by a downlap surface that is amalgamated with a wave ravinement surface in several places. Unit 3 is coeval with the upper part of Unit 2, and its development has been favoured by human interventions, which led to a transgression limited to the lagoon area.Local factors during the deposition, i.e. subsidence, sediment supply, physiography, and current/wave regimes, led to a significant lateral variability in the architecture of the Holocene sequence, as evidenced by the extreme thickness variation of the TST along both depositional strike and dip. The HST, instead, shows less pronounced strike variations in the stratal architecture. Also, present data clearly evidence that the human impact has a great relevance in influencing the late Holocene sedimentation.     

Geometry and architectural associations of co-genetic debrite–turbidite beds in basin-margin strata, Carboniferous Ross Sandstone (Ireland): Applications to reservoirs located on the margins of structurally confined submarine fans

Co-genetic debrite–turbidite beds are most commonly found in distal basin-plain settings and basin margins. This study documents the geometry, architectural association and paleogeographic occurrence of co-genetic debrite–turbidite beds in the Carboniferous Ross Sandstone with the goal of reducing uncertainty in the interpretation of subsurface data in similarly shaped basins where oil and gas is produced.The Ross Sandstone of western Ireland was deposited in a structurally confined submarine basin. Two outcrops contain co-genetic debrite–turbidite beds: Ballybunnion and Inishcorker. Both of the exposures contain strata deposited on the margin of the basin. An integrated dataset was used to characterize the stratigraphy of the Ballybunnion exposure. The exposure is divided into lower, middle, and upper units. The lower unit contains laminated shale with phosphate nodules, structureless siltstone, convolute bedding/slumps, locally contorted shale, and siltstone turbidites. The middle unit contains co-genetic debrite–turbidite beds, siltstone turbidites, and structureless siltstone. Each co-genetic debrite–turbidite bed contains evidence that fluid turbulence and matrix strength operated alternately and possibly simultaneously during deposition by a single sediment-gravity-flow event. The upper unit contains thin-bedded sandy turbidites, amalgamated sandy turbidites, siltstone turbidites, structureless siltstone, and laminated shale. A similar vertical facies pattern is found at Inishcorker.Co-genetic debrite–turbidite beds are only found at the basin-margin. We interpret these distinct beds to have originated as sand-rich, fully turbulent flows that eroded muddy strata on the slope as well as interbedded sandstone and mudstone in axial positions of the basin floor forming channels and associated megaflute erosional surfaces. This erosion caused the axially dispersing flows to laterally evolve to silt- and clay-rich flows suspended by both fluid turbulence and matrix strength due to a relative increase in clay proportions and associated turbulence suppression. The flows were efficient enough to bypass the basin center/floor, physically disconnecting their deposits from coeval lobes, resulting in deposition of co-genetic debrite–turbidite beds on the basin margin. The record of these bypassing flows in axial positions of the basin is erosional surfaces draped by thin siltstone beds with organic debris.A detailed cross-section through the Ross Sandstone reveals a wedge of low net-to-gross, poor reservoir-quality strata that physically separates sandy, basin-floor strata from the basin margin. The wedge of strata is referred to as the transition zone. The transition zone is composed of co-genetic debrite–turbidite beds, structureless siltstone, slumps, locally contorted shale, and laminated shale. Using data from the Ross Sandstone, two equations are defined that predict the size and shape of the transition zone. The equations use three variables (thickness of basin-margin strata, thickness of coeval strata on the basin floor, and angle of the basin margin) to solve for width (w) and trajectory of the basinward side of the low net-to-gross wedge (β). Beta is not a time line, but a facies boundary that separates sandy basin floor strata from silty basin-margin strata. The transition zone is interpreted to exist on lateral and distal margins of the structurally confined basin.Seismic examples from Gulf of Mexico minibasins reveal a wedge of low continuity, low amplitude seismic facies adjacent to the basin margin. Strata in this wedge are interpreted as transition-zone sediments, similar to those in the Ross Sandstone. Besides defining the size and shape of the transition zone, the variables “w” and “β” define two important drilling parameters. The variable “w” corresponds to the minimum distance a well bore should be positioned from the lateral basin margin to intersect sandy strata, and “β” corresponds to the deviation (from horizontal) of the well bore to follow the interface between sandy and low net-to-gross strata. Calculations reveal that “w” and “β” are related to the relative amount of draping, condensed strata on the margin and the angle of the basin margin. Basins with shallowly dipping margins and relatively high proportions of draping, clay-rich strata have wider transition zones compared to basins with steeply dipping margins with little draping strata. These concepts can reduce uncertainty when interpreting subsurface data in other structurally confined basins including those in Gulf of Mexico, offshore West Africa, and Brunei.     

Sequence stratigraphy of an argillaceous,deepwater basin-plain succession: Vischkuil Formation (Permian), Karoo Basin,South Africa

The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudstones, separated by packages of graded sandstone and siltstone turbidites, and volcanic ash beds, and is an argillaceous precursor to a 1 km thick sand-prone basin floor fan to shelf succession. The Vischkuil Formation provides an insight into the process by which regional sand supply is initiated and for testing sequence stratigraphic principles in a basin plain setting. Regionally mapped 1–2 m thick hemipelagic mudstone units are interpreted as condensed drapes that represent the starved basin plain equivalents of transgressive systems tracts and maximum flooding surface on the coeval shelf (now removed during later uplift). The section above each mudstone drape comprises siltstone turbidites interpreted as highstand systems tract deposits and a surface of regional extent, marked by an abrupt grain size shift to fine sandstone. These surfaces are interpreted as sequence boundaries, related to abrupt increases in flow volume and delivery of sand grade material to the basin-plain. The interpreted lowstand systems tract comprises sandstone-dominated turbidites and is overlain by another hemipelagic mudstone drape. The upper Vischkuil Formation is marked by three 20–45 m thick debrites, with intraformational sandstone clasts up to 20 cm in diameter that can be mapped over 3000 km². In each case, debrite emplacement resulted in widespread deformation of the immediately underlying 3–10 m of silty turbidites. A sequence boundary is interpreted at the base of each deformation/debrite package. Six depositional sequences are recognised and the interfered energy shift across each successive sequence boundary and LSTs include a larger volume of sandstone increases up section. The lower two sequences thin to the NW and show NW-directed palaeocurrents. The four overlying sequences show a polarity switch in palaeocurrent directions and thinning, to the E and SE. Sequence 6 is overlain sharply by the 300 m thick sandstone dominated Fan A of the Laingsburg Formation. The LST debrites may indicate gradual development of major routing conduits that subsequently fed Fan A. The polarity shift from westward flowing turbidity currents to an eastward prograding deepwater to shelf system represents establishment of a long term feeder system from the west. Sand supply to the Karoo basin floor was established in an incremental, stepwise manner. Given the early post-glacial setting in an icehouse climate, glacio-eustatic sea-level changes are considered to have been the main control on sequence development.     

Stratigraphic evolution of Eocene clinoforms from northern Santos Basin,offshore Brazil: Evaluating controlling factors on shelf-margin growth and deep-water sedimentation

In order to assess the controlling factors on the evolution of a shelf margin and the timing of sediment transfer to deep waters, a seismic stratigraphic investigation was carried out in the Eocene interval of northern Santos Basin, offshore Brazil. The studied succession configures a complex of prograding slope clinoforms formed in a passive margin and encompasses five seismic facies and their respective depositional settings: shelf-margin deltas/shorefaces, oblique slope clinoforms, sigmoidal slope clinoforms, continental to shelfal deposits and mass-transport deposits. These are stratigraphically arranged as seven depositional sequences recording a total shelf-edge progradation of about 35 km and a progradation rate of 1,75 km/My. Two main types of sequences can be recognized, the first one (type A) being dominated by oblique slope clinoforms and shelf-margin deltas/shorefaces in which shelf-edge trajectories were essentially flat to descending and extensive sandy turbidites were deposited on the foreset to bottomset zones. Sequences of this type are dominated by forced-regressive units deposited during extensive periods of relative sea-level fall. Type B comprises an upper part represented by aggradational shelfal deposits and a lower part composed of mass-transport deposits and high-relief sigmoidal clinoforms with descending shelf-edge trajectory. Steep slump scars deeply cut the shelfal strata and constitutes the boundary between the two intervals observed in type B sequences. Sandy turbidites occur at the same frequency in both forced- and normal-regressive units but are more voluminous within forced-regressive clinoforms associated with shelf-margin deltas/shorefaces. Major slope failures and mass-transport deposits, by the other hand, occurred exclusively in type B sequences during the onset of sea-level fall and their volume are directly related to the thickness of the shelfal sediments formed during the pre-failure normal regressions.     

Depositional processes and stratigraphic architecture within a coarse-grained rift-margin turbidite system: The Wollaston Forland Group,east Greenland

The Wollaston Forland Basin, NE Greenland, is a half-graben with a Middle Jurassic to Lower Cretaceous basin-fill. In this outcrop study we investigate the facies, architectural elements, depositional environments and sediment delivery systems of the deep marine syn-rift succession. Coarse-grained sand and gravel, as well as large boulders, were emplaced by rock-falls, debris flows and turbulent flows sourced from the immediate footwall. The bulk of these sediments were point-sourced and accumulated in a system of coalescing fans that formed a clastic wedge along the boundary fault system. In addition, this clastic wedge was supplied by a sand-rich turbidite system that is interpreted to have entered the basin axially, possibly via a prominent relay ramp within the main fault system. The proximal part of the clastic wedge consists of a steeply dipping, conformable succession of thick-bedded deposits from gravity flows that transformed down-slope from laminar to turbulent flow behaviour. Pervasive scour-and-fill features are observed at the base of the depositional slope of the clastic wedge, c. 5 km into the basin. These scour-fills are interpreted to have formed from high-density turbulent flows that were forced to decelerate and likely became subject to a hydraulic jump, forming plunge pools at the base of slope. The distal part of the wedge represents a basin plain environment and is characterised by a series of crude fining upward successions that are interpreted to reflect changes in the rate of accommodation generation and sediment supply, following from periodic increases in fault activity. This study demonstrates how rift basin physiography directly influences the behaviour of gravity flows. Conceptual models for the stratigraphic response to periodic fault activity, and the transformation and deposition of coarse-grained gravity flows in a deep water basin with strong contrasts in slope gradients, are presented and discussed.     

The late-Holocene Gargano subaqueous delta,Adriatic shelf: Sediment pathways and supply fluctuations

The Gargano subaqueous delta formed on the eastern and southeastern sides of the Gargano promontory, in the western Adriatic. This subaqueous deposit represents the southernmost portion of the late-Holocene highstand systems tract (HST) growing along the western side of the Adriatic as an extensive wedge of deltaic and shallow-marine mud. The late-Holocene HST rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5.5 cal. kyr BP, at the end of the late-Pleistocene–Holocene sea-level rise. High-resolution seismic–stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5.5 and 3.7 cal. kyr BP over much of the basin. Above this unit, sediment accumulation rates increased to high values (as much as 1.5 cm yr⁻¹) reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics. The late-Holocene mud wedge, of which the Gargano subaqueous delta is a significant component, reaches up to 35 m in thickness and has a volume of ca 180 km³. The shore-parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources (with a combined modern delivery of 51.7×10⁶ t yr⁻¹ of mean suspended load). In sections perpendicular to the coast the late-Holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets (typically 0.5°). The Gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25–28 m, and accounts for about 1/7th of the total volume of the late-Holocene mud wedge, despite the absence of direct river supply to the Gargano area. In the area of maximum interaction between shore-parallel currents and basin morphology, progradation occurs onto a flat and barren bedrock outcrop in about 50–80 m water depth. The rapid transition from a thickness of 30 m of late-Holocene mud to nil is a good indication of the role of southward-flowing bottom-hugging shelf currents in causing the redistribution of sediment along the Adriatic inner shelf. Additional evidence of this regime comes from: (1) the most recent sigmoid (defined at seismic–stratigraphic scale) deposited since the onset of the Little Ice Age, showing a shore-parallel thickness distribution and a main depocentre to the southeast of the Gargano promontory; (2) the maximum values of sediment accumulation rates over the last century (documented by ²¹⁰Pb measurements) defining a narrow shore-parallel belt immediately seaward of the depocentre of the most recent sigmoid. The Gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones. The Gargano subaqueous delta differs from other documented late-Holocene subaqueous deltas because its growth reflects: (1) sediment transport dominated by bottom currents sub-parallel to the strike of the composing clinoforms; (2) a complex supply regime including the Po delta (350 km to the north) and several coalescing Apennine rivers acting as ‘line source’; (3) several alternating intervals of enhanced outbuilding and condensed deposition; and (4) an in-phase growth of the most recent sigmoid with the major progradation of the Po delta during the Little Ice Age.     

Paleo-bathymetric controls on the stratigraphic architecture and reservoir development of confined fans in the Auger Basin: central Gulf of Mexico slope

With abundant well penetrations in proximal and distal settings and 3D seismic coverage, the Auger Basin is an ideal location to study the influence of basin setting and accommodation on the stratigraphic architecture of ancient turbidite systems. Pliocene-age turbidites at Macaroni Field were deposited in ponded accommodation in the distal portion of a salt-bounded intraslope basin, immediately inboard of a sediment spill point to the linked outboard basin. Deposits at Auger Field are contained within point-sourced submarine fans deposited in healed slope accommodation in the more proximal portion of the basin on the flank of a paleo-bathymetric ridge, immediately down depositional dip of a sediment spill point from an inboard basin. Both areas of the basin are distinct in terms of sediment dispersal patterns, rate of sediment fill, and preservation potential of reservoir/seal pairs, and while both fields contain sand-rich deposits and record vertical evolution from older sheet dominated- to younger channel dominated deposits over the Late Pliocene section, there are key differences in the nature in which the fill occurs. The ponded stratigraphic section at Macaroni Field records (1) an early mud-rich phase in which incoming flows are completely captured by confining topography, (2) a brief phase of diminished relief when high frequency fill/spill cycles occur, and ultimately (3) a phase of incision of the former basin sill and large-scale bypass to the outboard basin. Over the same period, the healed-slope section at Auger Field records a fill pattern consisting of alternating episodes of initial sand-rich sheet/lobe deposition followed by intervals of channelization. We acknowledge extra-basinal controls (eustacy, climate) on the timing, rate, and nature of sediment supply to the basin, but there is considerable evidence for paleo-bathymetric control on cyclical fill patterns observed at fourth and higher-order scales.     

Facies and sequential organisation of a mudstone-dominated slope and basin floor succession: the Gull Island Formation,Shannon Basin,Western Ireland

The lower part of the Carboniferous Shannon Basin of Western Ireland contains a deep-water succession which exceeds 1200 m in thickness that comprises five lithologically different units deposited within a confined, relatively narrow basin: (i) a calciclastic debris-flow and turbidite unit formed by resedimentation from nearby carbonate platforms, (ii) a siliciclastic black shale succession with former source potential which onlaps basin margins (Clare Shales), (iii) a sandstone-dominated turbidite formation, controlled by ponded accommodation and deposited axially in the basin (Ross Formation), (iv) a mudstone-rich turbidite-bearing succession, which onlaps basin margins (lower Gull Island Formation), and (v) a mudstone-dominated prograding slope succession (upper Gull Island Formation and lower Tullig Cyclothem), which grades transitionally upwards into deltaic deposits. The top unit records progradation at a time when basin differential subsidence had diminished significantly and local basin topography did not control deposition. The two upper mudstone-dominated units are different in terms of both sandstone content and their genetic significance within the overall basin-fill, and their potential relevance as reservoir analogues.The lower part of the Gull Island Formation contains three principal facies associations: (a) shallow turbidite channels and sheets representing channel margin and levee deposits, (b) mud-rich slumps, and (c) less than 1 m thick, rare, hemipelagic shales. More than 75% is deformed by soft-sediment deformation, but only to a smaller degree affecting sandstone units. The turbidites record transport to the ENE, along the axis of the basin, while the slumps were derived from an unstable northern slope and transported transversely into the basin towards the southeast. The distribution of turbidite sandstone and slumps is inversely proportional. Sandstones decrease in importance away from the basin axis as slumps increase in number and thickness. The lower part of the Gull Island Formation is interpreted to record progressive fill of a deep basin controlled by local, healed slope accommodation with onlap/sidelap of the basin margins. The instability resulted from a combination of fault-controlled differential subsidence between basin margin and basin axis, and high rates of sedimentation.The upper part of the Gull Island Formation is entirely dominated by mudstones, which grade upwards into siltstones. It contains rare, up to 15 m thick, isolated channels filled by turbidites, showing transport towards the east. The upper part records easterly progradation of a deep-water slope genetically tied to overlying deltaic deposits, and controlled by regional accommodation.The contrasts between the lower and upper parts of the Gull Island Formation show that onlapping/sidelapping turbidite successions have reservoir potential near basin axes, but that prograding deep-water slopes are less likely to have reservoir potential of significance. A suggested regional downlap surface between the two parts is a significant break and marker in terms of reservoir potential.     

Tectonic significance of syn-rift sediment packages across the Gabon-Cabinda continental margin

The tectonic development of a continental margin is recorded in the stratigraphic successions preserved along and across the margin in terms of stratal relationships (e.g., onlap, downlap, truncation), lithofacies, biostratigraphy, and paleo-water depths. By using these observations coupled to a kinematic and flexural model for the deformation of the lithosphere, we have elucidated the tectonic significance of the preserved stratigraphy that comprises the Gabon-Cabinda margin of west Africa. Two hinge zones, an Eastern and Atlantic, formed along the Gabon-Cabinda margin in response to three discrete extensional events occuring from Berriasian to Aptian time. The Eastern hinge zone demarcates the eastern limit of a broadly distributed Berriasian extension that resulted in the formation of deep anoxic, lacustrine systems as evidenced by the silts and shales of the Sialivakou and lower Djeno Formations and the regressive packages of the upper Djeno Formation. Approximately 1.5 to 2 km of asymmetric footwall uplift was induced across the Eastern hinge zone in response to the mechanical unloading of the lithosphere during this first phase of rifting. In contrast, the Atlantic hinge, located approximately 90 km west of the Eastern hinge, marks the eastern limit of a second phase of extension that began in the Hauterivian. Footwall uplift and rotation exposed earlier syn-rift and pre-rift sediments to at least wavebase causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon-Cabinda margins. We interpret the thickness variations of reworked clastic sediment of this age (e.g. the Melania Formation) between the hinge zones as indicative of variations in the degree of uplift and erosional truncation of the Atlantic hinge. For example, the absence of Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Cabinda and Gabon margins, the latter being characterized by significant thicknesses of Melania Formation (or equivalent). Material eroded from the Cabinda and Gabon Atlantic hinge zone may in part account for the thick wedge of sediment deposited seaward of the Gabon-Cabinda Atlantic hinge (the Erva Formation). Our modelling suggests that this wedge of reworked elastics represents deposition by along-axis gravity flows within a deep water (≈2 km) environment. A third and final phase of extension in the late Barremian-early Aptian was responsible for breaching the continental lithosphere to form the ocean/continent boundary and thus the installation of open marine conditions. Elsewhere, the environments will tend to be marginal marine to brackish, depending on the efficiency of the Atlantic hinge zone to act as a barrier to marine enchroachment. This third rift phase reactivated both the Eastern and Atlantic hinge0zones thereby creating accomodation for the Marnes Noires Formation (and equivalent) source rock deposition between the hinges and the Falcão source rock equivalent seaward of the Atlantic hinge. Two possible scenarios exist for the lateral distribution of the Marnes Noires Formation. If the reactivated rift flank topography across the Atlantic hinge was significant, then sedimentation would be restricted between the hinge zones within discrete lacustrine settings (e.g., Congo margin). Alternatively, if hinge zone uplift was relatively minor, then a coral-rimmed archipelago may have developed parallel to the margin with restricted communication across the Atlantic hinge zone (e.g., Cabinda margin). In this latter scenario, dilution of the Marnes Noires source rocks by terrigenous input from the eroding Atlantic hinge zone should be relatively minor thereby enhancing source rock quality. Furthermore, potential marine upwelling outboard of the Atlantic hinge zone is likely the cause for the production and accumulation of organic-rich material associated with the Falcão source rock of the Kwanza basin. By late Aptian time, the remaining accomodation between the hinge zones was partially filled by across- and along-axis prograding deltaic systems of the Argilles Vertes and Tchibota Formations. The progradation and interaction of the Argilles Vertes depositional lobes resulted in the formation of residual paleo-relief. Subsequent marine incursions and flooding of this paleo-relief led to the development of basal conglomerates (the Chela ‘lag’ unconformity) grading upward into fine-grained sands and evaporites. Consequently, an inverse relationship should exist betweeb evaporite thickness (in particular, the lower members) and the thickness of the underlying Argilles Vertes and Tchibota Formations. Variations in Loeme evaporite thickness is a consequence of stratigraphic and structural control with salt instability influencing local variability.Our modelling suggests the occurrence of two distinct evaporite sequences on the Congo margin, an earlier evaporite deposited seaward (west) of the Atlantic hinge during the second and third rift phases and the late Aptian Loeme Formation deposited between the hinge zones. An evaporite sequence seaward of the Atlantic hinge is inferred on the basis of extensive diapirs and salt tectonic structures observed in seismic data. In order to match the distribution and thickness of the observed post-salt stratigraphy across the basin, however, we require large paleowater depths west of the Atlantic hinge during the later Aptian. The existence of large paleowater depths precludes the formation of thick evaporite sequences within the outer basin. Consequently, we propose that the evaporites seaward of the Atlantic hinge were formed during the syn-rift development of the margin and are not contemporaneous with the post-rift Loeme salts deposited between the hinge zones. This double salt hypothesis is consistent with observations from the conjugate Brazilian margin.