Seismic architecture and seismic geomorphology of heterozoan carbonates: Eocene-Oligocene,Browse Basin,Northwest Shelf,Australia

Seismic characterization of Eocene-Oligocene heterozoan carbonate strata from the Browse Basin, Northwest Shelf of Australia, defines marked progradation of nearly 10 km. Stratal terminations and stacking subdivide the succession into mappable seismic units. Stratal architecture and seismic geomorphology varies systematically through the succession.Individual surfaces, discerned by toplap, onlap, and truncation, outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18°. Sigmoidal clinoforms can include aggradation in excess of ∼200 m, prograde more than 500 m, and have slopes characterized by inclined, wavy to discontinuous reflectors that represent ubiquitous gullies and channels. In contrast, the overlying tangential oblique clinoforms include downstepped shelf margins, limited on-shelf aggradation (<100 m) and toplap, subdued progradation (<500 m), and continuous parallel inclined reflectors on the slope. Wedges of basinally restricted reflectors at toe of slope onlap surfaces of pronounced erosional truncation or syndepositional structural modification. The succession includes repeated patterns of seismic units that onlap, aggrade, and prograde, interpreted to represent sequence sets and composite sequences.The associations of shelf aggradation, shelf-margin progradation, and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation. This distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded, perhaps enhanced by downwelling. Comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture, interpreted to be driven by distinct characteristics of heterozoan sedimentary systems.     

Outcrop and seismic expression of stratigraphic patterns driven by accommodation and sediment supply turnarounds: Implications on the meaning and variability of unconformities in syn-orogenic basins

Tectonically-complex settings present accommodation and sediment supply changes with patterns and rates for which the current sequence stratigraphy paradigms are not designed. In the Tertiary Piedmont Basin (TPB) and Peri-Adriatic Basin (PAB), outcrop and seismic examples demonstrate that the observed stratal and stacking patterns cannot be entirely explained using conventional sequence-stratigraphic models. Therefore, it is of paramount importance to use a model-independent more comprehensive approach encompassing advanced sequence-stratigraphic concepts combined with process changes, while being able to consider the morphostructural complexity that characterizes these margins and their changes induced by basin reshaping.Abrupt relative sea level falls generated by uplift or basin inversion may exceed several hundreds of meters, resulting in wedge-margin progressive unconformities characterized by subaerial and subaqueous erosional truncation. A progressive increase in sediment supply occurs, expressed by increasing volume and size of mass-transport complexes overlain by forced-regressive deltas, as the maximum sediment supply is delayed until after the main uplift. Different accommodation/sediment supply ratios may also occur at the same time along different margins of the same basin, generating a diachronism in the T-R or R-T cycles, adding further complexity to the variability produced by autogenesis.On clastic shelf margins characterized by an increasing rate of relative sea level rise, such as in case of increasing rollback velocities and related flexural tilting, or following an orogenic collapse, sediment supply may not keep pace with increasing accommodation so that initially retrogradation and basinward condensation occur, marked by omission surfaces. However, when the rate of subsidence increases, the succession is punctuated by multiple subaqueous erosional unconformities marking phases of basinward tilting leading to the oversteepening of basin margins and abrupt deepening. The downwarping usually produces large-scale subaqueous erosional surfaces passing laterally into paraconformities, so hinged-margin drowning unconformities affecting clastic shelves occur, associated with regional stratigraphic gaps.The re-establishment of the slope equilibrium profile implies high volume of sediments eroded from drowned deltas and shelves, feeding turbidites deposited at the toe of above-grade slopes. These turbidites can be therefore considered as high accommodation-high sediment supply systems. This suggests that turbidites are delivered basinward not only due to bypass at sequence boundaries or during the highstand progradation of supply-driven deltas, but also due to abrupt accommodation creation on hinged-shelf margin wedges.The great variability of tectonically-driven unconformities generated under either decreasing or increasing accommodation suggests that the features described in the TPB and the PAB are probably not uncommon, controlled by linked dynamic turnarounds of accommodation, sediment supply and stratigraphy taking place throughout the development of basin reorganizations.     

Slope and basin-floor reservoirs from the Miocene and Pliocene of the Veracruz Basin,southeastern Mexico

A regional study of the Veracruz Basin provided an excellent view of long-term deepwater sedimentation patterns from an evolving foreland-type basin. The regional seismic and well-log data set allows for an accurate reconstruction of slope and basin-floor depositional patterns, lithologic compositions, and paleogradients from a continuous succession of bathyal strata that span the Miocene to the lower Pliocene. Variations in Miocene and Pliocene deepwater reservoirs can be linked to prevailing slope characteristics. The Miocene basin had a high-gradient, tectonically generated slope, and the Pliocene basin had a low-gradient constructional slope. The Miocene basin owes its steep margin to the tectonic stacking of early Tertiary, Laramide-age thrust sheets. The Miocene margin shed a mixture of coarse elastic sediments (sands, gravels, and cobbles) and fines (silts and clays) that were transported into the deep basin via turbidity currents and debris flows. Channelized deposits dominate the Miocene slope, and reservoirs occur in long-lasting basement-confined canyons and shorter-lived shallower erosional gulleys. Thick and areally-extensive basin-floor fans exist outboard of the strongly channelized Miocene slope. Fan distribution is strongly controlled by synsedimentary contractional anticlines and synclines. In contrast, the latest Miocene to early Pliocene basin development was dominated by a strongly prograding wedge of shelf and slope deposits that was induced by volcanogenic uplift and increased sediment supply. During this phase, turbidite reservoirs are limited to narrow and sinuous deepwater channels that reside at the toe of the constructional clinoforms and areally limited, thinner basinal fans.     

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.     

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.     

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.     

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.     

Pleistocene progradational growth pattern of the northern Catalonia continental shelf (northwestern Mediterranean)

The Pleistocene sedimentary growth pattern of the northern Catalonia continental shelf is characterized by the vertical stacking of seaward downlapping regressive deposits. These deposits are characterized by a progradational development, with oblique clinoforms of low angle in the middle continental shelf, that become more inclined seaward in the outer continental shelf and shelfbreak. Eustatic sea level fluctuations controlled the development of this sedimentary pattern, whereas sediment supply conditioned the nonuniform progradation along the continental shelf and subsidence due to both sediment loading and tectonics controlled its preservation through and along the continental shelf.     

Controls on the development of valleys,canyons, and unconfined channel–levee complexes on the Pleistocene Slope of East Kalimantan,Indonesia

Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope.North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment.     

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.     

Seismic stratigraphy and geomorphology of a tide or wave dominated shelf-edge delta (NW Australia): Process-based classification from 3D seismic attributes and implications for the prediction of deep-water sands

Shelf-edge deltas (SEDs) forming during periods of relative sea level fall and lowstand are generally efficient in transferring sediments to the slope and basins, and their identification in subsurface data is often considered a good indication of coeval development of slope and basin-floor turbidite reservoirs. This study investigates the seismic stratigraphic evolution of a forced-regressive and normal regressive shelf-edge delta (Bonaparte SED) that accumulated on the edge of the NW Australian margin during the late Quaternary. High resolution 2D and 3D reflection seismic data allow reconstruction of the main episodes of delta progradation and understanding of the extrinsic and intrinsic controls on their deposition. The lack of a significant turbidite system forming off the shelf-edge delta throughout the Quaternary is a striking feature of the Bonaparte SED. Instead, slope sedimentation is dominated by the accumulation of plume-derived mud belts and their reworking through mass-transport processes. Seismic geomorphology permits interpretation of the process regime of the youngest shelf-edge depocentre by applying a new process-based shallow-marine classification scheme to the 3D seismic attribute data. Results suggest either a tide or wave dominated delta with fluvial processes being of tertiary significance. A tide or wave-dominated, fluvial-affected shelf-edge delta classification is consistent with the paleogeographical reconstruction of the margin during the last glacial maximum (ca. 25 ka BP). The comparison of this mixed-process shelf-edge delta and starved slope system with a fluvial-dominated counterpart with significant sandy slope deposits emphasizes the potential of assessing the process regime of shelf-edge deltas as a rapid, first approach for predicting the presence or absence of coeval slope and basin-floor reservoirs.     

Modern sediment characteristics and accumulation rates from the delta front to prodelta of the Yellow River (Huanghe)

Since 1976, the main channel of the Yellow River (Huanghe) has been on the east side of the delta complex, and the river has prograded a broad new delta lobe in Laizhou Bay of the Bohai Sea. In 2012, extensive bathymetric and high-resolution seismic profiles were conducted and sediment cores were collected off the new delta lobe. This study examined delta sedimentation and morphology along a profile across the modern subaqueous Yellow River delta and into Laizhou Bay, by analyzing sediment radionuclides (¹³⁷Cs, ²¹⁰Pb and ⁷Be), sedimentary structure, grain-size composition, organic carbon content, and morphological changes between 1976 and 2012. The change in the bathymetric profile, longitudinal to the river’s course, reveals subaqueous delta progradation during this period. The subbottom boundary between the new delta lobe sediment and the older seafloor sediment (before the 1976 course shift) was identified in terms of lithology and radionuclide distributions, and recognized as a downlap surface in the seismic record. The accumulation rate of the new delta lobe sediment is estimated to be 5–18.6 cm year^–1 on the delta front slope, 2 cm year^–1 at the toe of the slope, and 1–2 cm year^–1 in the shelf areas of Laizhou Bay. Sediment facies also change offshore, from alternations of gray and brown sediment in the proximal area to gray bioturbated fine sediment in the distal area. Based on ⁷Be distribution, the shorter-term deposition rate was at least 20 cm year^–1 in the delta front.     

The flexural margin,the foredeep,and the orogenic margin of a northern Cordilleran foreland basin: Cretaceous tectonostratigraphy and detrital zircon provenance,northwestern Canada

Reconstructions of the Albian to Campanian foreland basin adjacent to the northern Canadian Cordillera are based on outcrop and well log correlations, seismic interpretation, and reconnaissance-level detrital zircon analysis. The succession is subdivided into two tectonostratigraphic units. First is an Albian tectonostratigraphic unit that was deposited on the flexural margin of a foreland basin. At the base is a shallow marine sandstone interval that was deposited during transgressive reworking of sediment from cratonic sources east of the basin that resulted in a dominant 2000–1800 Ma detrital zircon age fraction. Subsequent deposition in a west-facing muddy ramp setting was followed by east-to-west shoreface progradation into the basin.Near the Albian–Cenomanian boundary, regional uplift and exhumation resulted in an angular unconformity at the base of the Cenomanian–Campanian tectonostratigraphic unit. Renewed subsidence in the Cenomanian resulted in deposition of organic-rich, radioactive, black mudstone of the Slater River Formation in a foredeep setting. Cenomanian–Turonian time saw west-to-east progradation of a shoreface-shelf system from the orogenic margin of the foreland basin over the foredeep deposits. Detrital zircon age peaks of approximately 1300 Ma, 1000 Ma, and 400 Ma from a Turonian sample are consistent with recycling of Mississippian and older strata from the Cordillera west of the study area, and show that the orogen-attached depositional system delivered sediment from the orogen to the foreland basin. A near syndepositional detrital zircon age of ca. 93 Ma overlaps with known granitoid ages from the Cordillera. After the shelf system prograded across the study area, subsequent pulses of subsidence and uplift resulted in dramatic thickness variations across an older structural belt, the Keele Tectonic Zone, from the Turonian to the Campanian.The succession of depositional systems in the study area from flexural margin to foredeep to orogenic margin is attributed to coupled foreland propagation of the front of the Cordilleran orogen and the foreland basin. Propagation of crustal thickening and deformation toward the foreland is a typical feature of orogens and so the distal to proximal evolution of the foreland basin should also be considered as typical.     

珠江口盆地白云凹陷陆坡区10.5 Ma以来的沉积体系

通过对珠江口盆地白云凹陷陆坡区10.5Ma以来的地震相分析,共识别出席状平行亚平行地震相、透镜状前积地震相、深切河谷地震相、帚状地震相和杂乱地震相,不同的地震相分别代表不同的沉积体系类型。综合所识别的地震相类型,分析了陆架边缘下切谷、浊积扇和陆架边缘三角洲3种主要的沉积体系及其配置关系。物源供给是影响陆坡区沉积体系发育的最重要因素,是沉积体系发育的物质基础,海平面变化和构造运动为沉积体系发育提供了可容纳空间,3种影响因素共同影响了陆坡区沉积体系的发育。     

Genesis of the Northern Adriatic Sea (Northern Italy) since early Pliocene

The Northern Adriatic Sea is a shallow and very flat shelf area located between the northern Apennines, the southern Alps and the Dinarides; its present day physiography is the result of the filling of a relatively deep Quaternary foredeep basin, developed due to the northeastward migration of the Apennine chain. Multichannel seismic profiles and well data have allowed documenting the stratigraphic architecture, the depositional systems and the physiographic evolution of the Northern Adriatic sea since early Pliocene time. In particular, three main depositional sequences bounded by regional unconformities were recognized. The Zanclean Sequence 1 documents first the drowning of late Messinian incised valleys and then the southward progradation of a shelf-slope system, which is inferred to be related to a tectonic phase of the Apenninic front. The Piacenzian-Gelasian Sequence 2 records a relatively rapid transgressive episode followed by minor southward progradation; the top of the sequence is associated with a major late Gelasian drowning event, linked to the NE-ward migration of the Apennine foredeep. The Calabrian to upper Pleistocene Sequence 3 testifies the infilling of accommodation previously created by the late Gelasian drowning event, and it initially accumulated in deep-water settings and then in shallow-water to continental settings. The upper part of Sequence 3, consisting of the paleo-Po deltaic system, is composed of seven high-frequency sequences inferred to record late Quaternary glacio-eustatic changes. These high-frequency sequences document the stepwise filling of the remaining accommodation, resulting in the development of the modern shelf.     

Impacts of sediment supply and local tectonics on clinoform distribution: the seismic stratigraphy of the mid Pleistocene-Holocene Indus Shelf

We present results from the first high-resolution seismic reflection survey of the inner Western Indus Shelf, and Indus Delta, Arabian Sea. The results show major regional differences in sedimentation across the shelf from east to west, as well as north to south, both since the Last Glacial Maximum (~20?ka) and over longer time scales. We identify 10 major regional reflectors, interpreted as representing sea level lowstands. Strong compressive folding is observed underlying a reflector we have called Horizon 6 in the north-western shelf, probably compression associated with the transpressional deformation of the Murray Ridge plate boundary. Downslope profiles show a series of well developed clinoforms, principally at the shelf edge, indicating significant preservation of large packages of sediment during lowstands. These clinoforms have developed close to zones of deformation, suggesting that subsidence is a factor in controlling sedimentation and consequently erosion of the Indus Shelf. These clinoforms fan out from dome features (tectonic anticlines) mostly located close to the modern shoreline.     

Vast amount of accommodation space controlled evolution of a continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge in the Bay of Oran (Western Mediterranean)

The Bay of Oran is part of the northern Algerian continental margin, located in the Western Mediterranean Sea between Europe and northern Africa. A regional terrace in ca. 320 m water depth described in earlier studies and a second deeper located one (∼1200 m water depth) provide an unusually vast amount of accommodation space for an observed prograding wedge. Seismo-stratigraphic interpretation of high-resolution reflection seismic data show different phases of mixed cool-water carbonate-siliciclastic deposition: (Ia) Initial aggradation with low dipping foreset deposition during early-Pliocene relative sea-level highstand. (Ib) Deposition transitions to progradation when aggradation reaches the base level. (IIa) Once progradation reaches the shelf break, terrace deposition is reduced to coarse fraction foreset deposits until it ceases entirely. (IIb) Finer sediments are bypassed and start to aggrade on the lower slope terrace until deposits reach the shelf terrace depth. (III) Due to accommodation space prolongation progradation recommences. Phase IIa and phase III deposits are separated by a hiatus. A drop in mean sea-level during the mid-Pleistocene will have caused the base level to fall below the upper strata, hence causing some reworking and redeposition. However, sea-level variations are not considered to be a main controlling factor of the depositional sequences. The evolution of this continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge is instead attributed to the controlling factor of this unusually vast amount of accommodation space. In closest proximity to the sea-floor, sparse recent sedimentation in form of 5–10 m thick sediment lobes can be observed in subbottom profiler data only. From a tectonic point of view, a prolongation of the Yusuf Fault into the survey area though expected by other authors could not be supported with the available dataset.     

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.     

Sequence Stratigraphy and Composition of Late Quaternary Shelf-Margin Deltas,Northern Gulf of Mexico

High-resolution seismic profiles and foundation borings from the northwestern Gulf of Mexico record the physical attributes and depositional histories of several late Quaternary sequences that were deposited by wave-modified, river-dominated shelf margin deltas during successive periods of lowered sea level. Each progressively younger deltaic sequence is thinner and exhibits a systematic decrease in the abundance and concentration of sand, which is attributed to a shift in the axis of trunk streams and greater structural influence through time. Our study shows that (1) contemporaneous structural deformation controlled the thickness of each sequence, the oblique directions of delta progradation, the axes of major fluvial channels, and the geometries of delta lobes at the shelf margin, (2) sedimentation was rapid in response to rapid eustatic fluctuations and structural influence, (3) boundaries of these high-frequency sequences are the correlative conformities of updip fluvial incision, and coincide with downlap surfaces at the shelf margin, (4) the downlap surfaces are not true surfaces, but zones of parallel reflections that become progressively higher and younger in the direction of progradation, (5) the downlap zones are composed of marine muds that do not contain high concentrations of shell debris that would be expected in condensed sections, (6) possible paleosols capping the two oldest sequences are regressive surfaces of subaerial exposure that were preserved during transgressions, and (7) no incised valleys or submarine canyons breach the paleoshelf margin, even though incised drainages were present updip and sea-level curves indicate several periods of rapid fall. (Published in American Association of Petroleum Geologists Bulletin 80: 505 530.) R. A. Morton and J. R. Suter AAPG Bulletin, Vol. 80, No. 4, AAPG 1996, reprinted by permission of the American Association of Petroleum Geologists whose permission is required for future use.     

Seismic expressions of deep-shelf depositional and erosional morphologies,Miocene Utsira formation,North Sea Basin

The Middle–Late Miocene Utsira Formation of the North Sea Basin contains a fully preserved, regional marine sand deposit that records a stable paleogeographic setting of sand transport and accumulation within a deep, epeiric seaway which persisted for >8 Ma. The sediment dispersal system was defined by (1) input through a marginal prograding strandplain platform, coast-to-basin bypass, transport along a narrow strait, and accumulation in strait-mouth shoal complexes within a shelf sea; (2) a high-energy marine regime; (3) very low time-averaged rates of sediment supply and accumulation; and (4) consequent high sediment reworking ratio. Sand distribution and stratal architectures reflect regional along-strike sediment transport and local to sub-regional landward sediment transport. Plume-shaped, south-building, submarine sand shoals that formed along the recurved arc of the strandplain margin nourished the shoal system. Very low-angle sigmoid clinoforms and down-stepping, aggradational top sets are distinctive architectures of these strike-fed sand bodies. The combination of strong marine currents and slow but long-lived sand supply from the Shetland strandplain created regional, sandy shelf shoal depositional systems that individually covered 3,500 to 6,000 km² of the basin floor. Defining attributes of the shelf shoal systems include their location within the basin axis, abundance of autochthonous sediment, and sandy marine facies composition. Diagnostic depositional architectures include the along-strike-dipping sigmoidal clinoforms, poly-directional low-angle accretionary bedding at both regional and local scales, and mounded depositional topography. Erosional features include regional hummocky, low-relief shelf deflation surfaces, broad, elongate scours and sub-circular scour pits.