Shelf-margin Deltas Research Articles

Sedimentary evolution of the Nakdong River deposits on the Korea Strait shelf since the Last Glacial Maximum

We investigated the development of depositional systems and the evolution of the depositional sequence in the Korea Strait shelf deposits since the Last Glacial Maximum (LGM), integrating a dense grid of seismic data and multiple long-core datasets from the delta plain to offshore. This depositional sequence includes the lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST) over an incised valley sequence boundary. These tracts developed discrete depositional environments in both the bay and the open shelf area demarcated by the island arc. The evolution of these tracts can be delineated into five distinct phases based on the sea-level curve for the Korea Strait since the LGM. First, in the LST (the LGM lowstand; 25–17 kyr BP), fluvial deposits with upward-fining characteristics were observed in bedrock-incised valleys within the bay, while amalgamated fluvial deposits were identified in unconfined incised valleys over Pleistocene sequences on the open shelf. A shelf-margin delta was also detected along the continental shelf break. The TST is composed of three subunits: the lower, middle, and upper TSTs. In the lower TST (the post-LGM transgression; 17–13 kyr BP), estuarine deposits were occupied in the bay, while a broad sand sheet with a few moribund sand ridges was present on the open shelf. In the middle TST (the Younger Dryas (YD) event; 13–12 kyr BP), bay barrier islands were observed growing along the island arc, but a mid-shelf delta and a sand ridge developed on the open shelf. In the upper TST (the Early to Mid-Holocene transgression; 12–6 kyr BP), the fining-upward retrogradational bayhead delta–estuarine complex within a bay encircled by barrier islands developed toward the head of the estuary. Finally, in the HST (the Late Holocene highstand; 6–0 kyr BP), the coarsening-upward delta was prograding seaward, with its prodelta initially distributed off the river mouth and later as a mud belt along the eastern coast. The TST deposits, encompassing diverse sedimentary systems that make up a substantial portion of the depositional sequence on the shelf, were significantly influenced by a combination of topographical characteristics limiting the discharge of sediment from the river into the open shelf and the displacement of accommodation space with stepwise sea-level rise.

Seismic geomorphological analysis of submarine fan architecture in the Baiyun Sag, Pearl River Mouth Basin: Impact of second-order relative sea-level change

Whereas the effects of high-order relative sea level change (3rd-order and higher) on the formation and architecture of submarine fan systems are relatively well established, a lack of large-scale spatial and temporal data has resulted in the impacts of low-order relative sea level change (i.e., 2nd-order) eluding characterization. With the aim of revealing the impacts of 2nd-order relative sea level, this study applied regional-scale seismic geomorphology to examine 142 Miocene-aged (21–8.2 Ma) submarine fans in the Baiyun Sag of the Pearl River Mouth Basin (China), integrating a large 3D seismic volume (6000 km2), well logs and core data. By extracting seismic attributes calibrated to well data, architectural elements were interpreted, statistically correlated, and their evolution through time was assessed. The results were as follows: (1) architectural elements of submarine channels and lobes, as well as associated highstand deltas, shelf-margin deltas and incised valleys, were identified based on their seismic geomorphological features and location, revealing that submarine channels shift paleo-seaward from incised valleys and feed lobes; (2) there is a positive correlation between average width of submarine channels and average area of lobes, as well as between the average width of incised valleys and average width of submarine channels; (3) the size of architectural elements is inversely related to proportion of sand; and most importantly, (4) the size variation of architectural elements coincides with changes in 2nd-order relative sea level. Results also indicate that rapid tectonic subsidence played an important control on the size and sandiness of deep-water architectural elements. The findings of this study may inform reconstruction of submarine fan systems and prediction of their petroleum reservoir potential in other systems globally.

Structural characteristics and deep-water hydrocarbon accumulation model of the Scotian Basin, Eastern Canada

Commercial hydrocarbon reservoirs have been discovered in shallow-water areas of the Scotian Basin, Eastern Canada. However, knowledge about the structure and hydrocarbon accumulation characteristics of the basin is still insufficient, which constrains the oil and gas exploration in deep-water areas. Based on comprehensive data of magnetic anomalies, seismic survey, and drilling, this study determines the structure characteristics of the Scotian Basin and its hydrocarbon accumulation conditions in deep waters and evaluates the deep-water hydrocarbon exploration potential. The transform faults and basement structures in the northern basin control the sedimentary framework showing thick strata in east and thin strata in west of the basin. The bowl-shaped depression formed by thermal subsidence during the transitional phase and the confined environment (micro basins) caused by salt tectonics provide favorable conditions for the development of source rocks during the depression stage (also referred to as the depression period sequence) of the basin. The progradation of large shelf-margin deltas during the drift phase and steep continental slope provide favorable conditions for the deposition of slope-floor fans on continental margins of the basin. Moreover, the source-reservoir assemblage comprising the source rocks within the depression stage and the turbidite sandstones on the continental margin in the deep waters may form large deep-water turbidite sandstone reservoirs. This study will provide a valuable reference for the deep-water hydrocarbon exploration in the Scotian Basin.

Depositional architecture, evolution and controlling factors of the Miocene submarine canyon system in the Pearl River Mouth Basin, northern South China Sea

Submarine canyon system in the Miocene Pearl River Mouth Basin, northern continental margin of the South China Sea, records the process of sediments transported from shelf margin to advanced abyssal plain and comprises important petroleum reservoirs in the basin. Based on integrated analysis of seismic, well logs and core data, the sequence framework, depositional architecture and controlling factors of the submarine canyon system are well documented. As a composite sequence bounded by regional unconformity, the Zhujiang Formation (CS1) can be divided into 5 sequences (CS1-1-CS1-5), and the lower CS1-2 and CS1-3 are the main strata that the submarine canyon system developed. The submarine canyon system can be divided into five segments (A-E) by their distinct geomorphology and depositional architectures. From Segment A to E, the channels in the submarine canyon system transform from “V” shaped single channel, “U” shaped single channel, “gull-wing” shaped channel-levee complex, “W” shaped branch channels to mound shaped distal composite channel and front splay deposits. This is accordantly consistent with sedimentary process of the channels transitional from erosional, erosional-aggradational, aggradational and then lateral migrational. The changing interaction of multi-source sediment supply, differential tectonic subsidence and relative sea level change are considered to be the major control on the development and evolution of the submarine canyon system. The differential tectonic subsidence since the Late Oligocene produced the adequate slope angle, sufficient accommodation and distinct basin geomorphology for the development and segmentation of the submarine canyon system. Slope-gully to slope fan deposits fed by the shelf margin delta systems from the Paleo-Pearl River and the northeastern Dongsha Uplift converged into the submarine canyon system, providing abundant sediments for its development. Relative sea level changes, especially the two regional regressions with abundant sediment supply have made great influence on the evolution of the submarine canyon system.

Spatial and temporal variations in depositional systems in the Kazusa Group: insights into the origins of deep-water massive sandstones in a Pleistocene forearc basin on the Boso Peninsula, Japan

A detailed chronostratigraphic framework established by the mapping of tephra key beds and application of oxygen isotopic data allows assessment of the synchroneity and diachroneity of depositional systems formed in coastal and deep-water environments. This framework also allows estimation of the timing of active delivery of coarse-grained sediments beyond the shelf margin in relation to relative sea-level changes. The depositional processes of deep-water massive sandstones (DWMSs) are still enigmatic; their formation is a result of active delivery of sands in association with the supply of organic carbon into deep-water environments. DWMSs are also important as reservoirs for hydrocarbon explorations. This study investigated the origins of DWMSs in the upper Umegase, Kokumoto, and Chonan formations (in ascending order) of the Pleistocene Kazusa Group on the Boso Peninsula, central Japan. Each formation contains several packets of DWMSs that are interpreted to have formed in response to the progradation of gravelly shelf-margin deltas or fan deltas during the falling and lowstand stages of relative sea-level changes controlled primarily by glacioeustasy. The development of DWMSs and associated sandstone beds is interpreted to have been induced by hyperpycnal flows, in association with sediment gravity flows that were initiated by breaching and/or collapse of sandy substrates on the shelf-margin deltas or fan deltas. The timings of the initial and final deposition of the packets vary within and between the formations, and are considered to have been controlled by the interaction between allogenic and autogenic processes operating in the gravelly shelf-margin deltas or fan deltas. A muddy horizon that contains the Lower–Middle Pleistocene Subseries boundary (the base of the Chibanian Stage) in the Kokumoto Formation is also underlain and overlain by the packets and represents a deposit formed in a condensed section in an upper slope environment. This depositional setting may have favored the development of the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene Subseries boundary in the formation.

The Cenozoic sedimentary environment and sedimentary filling evolution of Wan'an Basin in the western South China Sea

As one of the basins with extremely abundant oil and gas resources in South China Sea, the Wan'an Basin has attracted great attention from domestic and international geologists and hydrocarbon explorers. Currently, the sequence stratigraphic framework, distribution of sedimentary systems and sedimentary filling evolution of the Wan'an Basin are still not clear, consequently, it bring some difficulties for hydrocarbon exploration. Based on the palaeobios, well drilling, well logging, and seismic and other data, the sequence stratigraphic framework of the Wan'an Basin is established, and the sequence interfaces of T100, T60, T50, T40 and T30 in this study are identified. On this basis, the sedimentary environment and sedimentary filling evolution of the Wan'an Basin are systematically analyzed. The results show that the basin had experienced three tectonic evolution periods since Cenozoic, i.e., the Oligocene–Early Miocene extensional fault-depression period, Middle Miocene strike-slip reformation period and Late Miocene–Quaternary regional depression period. Thereinto, the provenance in the extensional fault-depression period (Oligocene–Early Miocene) was mainly from the western uplift of the basin, where the delta–shore-shallow lacustrine–bathylimnetic sedimentary system during Early Oligocene and delta–littoral-neritic sedimentary system during Late Oligocene–Early Miocene were developed respectively. The provenance during the strike-slip reformation period (Middle Miocene) was still mainly from the western uplift of the basin, where the delta–littoral-neritic–carbonate platform sedimentary system was developed, and the carbonate platform began to develop abundantly. The provenance during the regional depression period (Late Miocene–Quaternary) is mainly supplied by input of the western distal large rivers, the delta–littoral-shallow sea–carbonate platform–bathyal sea sedimentary system was developed in the Late Miocene, and the shelf-margin delta–continental slope–bathyal sea sedimentary system was developed during Pliocene-Quaternary. Therefore, the Wan'an Basin has a favorable hydrocarbon resource potential.

Deepwater to fluvio-deltaic stratigraphic evolution of a deglaciated depocenter: The early Permian Rio do Sul and Rio Bonito formations, southern Brazil

This paper describes facies relationships and stratigraphic architecture of early Permian deglacial deepwater to fluvio-deltaic deposits exposed in the eastern border of the Paraná Basin, southern Brazil. The studied succession is located in the Rio do Sul depocenter and comprises an upward transition between the topmost interval of the Itararé Group (Rio do Sul Formation) and the lowermost unit of the Rio Bonito Formation (Triunfo Member). Geological mapping and facies analysis allowed the recognition of the following facies associations, vertically stacked: thin and thick-bedded turbidites, hyperconcentrated and concentrated density-flow deposits (hyperpycnites), mass-transport deposits, fine-grained heterolithic beds of prodelta, delta-front, shoreface and delta-plain deposits, as well as fluvial beds resting on incised valleys. The stratigraphic analysis through the interpretation of the facies associations, stacking pattern and paleocurrent dispersal trends were enough to define the clinoform stratigraphic architecture, composing a progradational-aggradational pattern. The paleoflows for all facies associations show a main trend towards the south-southwest. In this context, hyperpycnal beds can represent a glacial source from the northeast, because of striated and faceted clasts immersed in the conglomeratic facies, whereas turbidites have been understood as the products of their dilution. Likewise, mass-transport deposits are related to the collapse of the shelf-margin delta, represented by allochthonous blocks composed of deltaic beds within the mass-flow diamictites. Thus, these mass-transport deposits do not evolve downslope to turbidity currents. This configuration was likely for building the relief of the slope system, probably a shelf-edge delta susceptible to mass failure. Paleo-ice flows towards SW recorded in the Rio do Sul Formation, genetic related to fluvio-deltaic deposits of Rio Bonito Formation, highlight the scenario of final stages of the Late Paleozoic Ice Age in the SW of Gondwana and support the previous researches which point to the presence of diachronous ice-spreading centers from paleohighs.

Stratigraphic architecture, shelf-edge delta and constraints on the development of the Late Oligocene to Early Miocene continental margin prism, the Pearl River Mouth Basin, northern South China Sea

The early-stage shelf-margin prism in the Pearl River Mouth Basin (PRMB), northern South China Sea, formed from the Late Oligocene to Early Miocene, comprises a set of shelf-edge delta and slope fan wedges of the Zhuhai Formation and records the development of the continental slope in response to the interaction of sea-level change, tectonic subsidence and sediment supply. Based on integrated analysis of seismic, well logging and core data, the stratigraphic architecture, depositional systems and controlling processes are documented. The Zhuhai Formation is a composite sequence bounded by regional unconformity, and can be further divided into five sequences (CS3-1-CS3-5). The depositional genetic units (transgression, ascending normal regression and descending forced regression) are identified within sequences based on variation in shelf edge trajectories. The shelf deltaic clinoforms are characterized by oblique-tangential reflections in seismic profiles with a thickness of 100–150 m. In contrast, the shelf-margin delta and slope clinoforms generally are 300–1200 m thick and mainly display as sigmoid or sigmoid-tangential seismic reflections. The delta deposits usually consist of thick beds of delta front deposits and prodelta slump and deformed sediments, and are associated with sandy slope fan systems. Eleven delta complexes are distinguished by tracking the distribution of the deltaic clinoforms in each sequence, and these deltaic deposits tend to extend along their strike direction which may be related to the reworking of strong waves and coastal currents. Slope fan systems consist mainly of turbidite channel fills, frontal splay and debris flow deposits, and generally develop along the slope in CS3-3-CS3-5, which may be triggered by steep shelf margin slope and fault activity. The stratigraphic pattern of the shelf margin prism and depositional evolution of the deltaic systems from the Late Oligocene to Early Miocene were controlled by the interaction of sea level changes, sediment supply and tectonic subsidence. The ascending trajectory from the CS3-1 (30 Ma) to the top of the CS3-5 indicates that the relative sea level continued to rise generally but the shelf margin prograded basinward rapidly due to the great amount of sediment supply and lower subsidence rate. Tectonic uplift of the source area and strengthened physical erosion during this period may be the reason for the large sediment supply.

The evolution of a gravity-driven system accompanied by diapirism under the control of the prograding West Luconia Deltas in the Kangxi Depression, Southern South China Sea

Gravitational collapse structures are commonly observed in shelf-margin deltas underlain by mobile shales. However, these structures are rarely accompanied by mud diapirs. This paper presents an updated study of the gravity-driven system in the West Luconia Deltas, a shelf-margin delta system, in the Kangxi Depression, southern South China Sea. Compared to the classical shale-detachment model, the syn-collapse deformation in the contractional domain in this study is accommodated mainly by thrust faults combined with mud diapirs rather than simply imbricated thrusts. Based on seismic interpretation and structural analysis, this gravity-driven system is divided into three domains, the extensional domain, the contractional domain and the transitional domain. All of these domains are intruded by mud diapirs. The quantitative analysis of the amounts of extension and contraction suggests that these structures mainly resulted from gravitational collapse rather than the tectonic compression. Quantification of the relative contributions of gravity spreading and gliding indicates that the gravitational collapse was mainly driven by gravity spreading. Two episodes of collapse are suggested by the analysis of the progradation of the West Luconia Deltas and the features of the syn-collapse structures. The first episode was minor and not accompanied by diapirism, whereas the second episode was major and accompanied by diapirism. The entire evolution of the GDS is divided into five stages: (1) the first episode of the gravitational collapse, lasted from the earliest Middle Miocene to the earliest Pliocene; (2) the deposition of an interval between the syn-collapse strata and the mobile shale, occurred in the Early Pliocene.; (3) the initiation of the second episode of the gravitational collapse, lasted from the Early Pliocene to the Late Pliocene; (4) the attenuation and basinward migration of the gravity-driven deformation, lasted from the Late Pliocene to the Early Pleistocene; and (5) the ending of the gravitational collapse, lasted from the Early Pleistocene to the present. The last four stages were accompanied by intensive diapirs which pierced the overlying strata and became targets for the hydrocarbon exploration.

Early Miocene sedimentary processes and their hydrocarbon implications in the Baiyun Sag of Pearl River Mouth Basin, northern south China sea

Abstract The shelf-slope zone has received considerable attention for hydrocarbon exploration due to its unique location, where the sediments are delivered to deep-water environment and deposited as hydrocarbon reservoirs that have been discovered around the world. However, it is realized that the sedimentary processes are very complex, and various depositional bodies develop in the shelf-slope zone. Constrained by 3D seismic, core and well log data, numerical simulations can be applied to better understand the sedimentary processes, and predict the distribution and characteristics of deposits as hydrocarbon reservoirs. Based on core-log-seismic integration, the Early Miocene shelf-margin delta and channelized submarine fan systems along the northern slope of Baiyun Sag in the Pearl River Mouth Basin are distinguished and they are mainly composed of debris, turbidites and slumps. The numerical model is built based on Early Miocene topography acquired by back-stripped well-tied seismic profiles, and the initial boundary conditions, including sediment concentration, velocity and, grain size, and so on, behaviors and structures of slope system are set according to the results of marine surveys around the world. The Computational Fluid Dynamic software ANSYS-FLUENT was applied for the modelling. As a results, the distributions and geometries of the simulated deposits are compared with the depositional bodies interpreted from seismic data. The hydrodynamic simulations of the two sedimentary systems indicate that depositional bodies converge to the downstream of the slope and the basin floor, and turn tractive current into turbidity current during the sedimentary processes, which result in obviously various sedimentary characteristics in different locations. The identification of sedimentary processes in the shelf-margin delta and channelized submarine fan systems has been beneficial for predicting reservoirs and exploring hydrocarbons in the Baiyun Sag, Pearl River Mouth Basin, which is also contributing to the understanding of sedimentary process and reservoir prediction in deep-water areas around the world.

Changes in inner- to outer-shelf delta architecture, Oligocene to Quaternary Pearl River shelf-margin prism, northern South China Sea

Inner shelf to shelf-margin delta and slope-fan deposits in the Cenozoic Pearl River Mouth Basin in the northern continental margin of the South China Sea comprise the most important petroleum reservoirs in the basin and their evolution reflects a changing interplay of tectonics, sea level change and sediment supply. Integrated analysis of 3D seismic, well logs and core data allows interpretation of depositional architecture and evolution of the delta systems as they responded to the major controls. Inner shelf-delta systems, sited on the landward reaches of the continental margin, are characterized by thickly stacked distributary channel, thin (20-40 m) but relatively coarse delta-front and thin, fine-grained, distal delta to prodelta deposits. In contrast, outer shelf to shelf-margin deltaic systems formed 200 m (merging outwards to 1000 m) high clinoforms with steep slopes imaged on 3D seismic profiles as S-shape or tangential progradational reflections. Slumps and debris flow as well as sandy turbidite slope-fan deposits are common on the slopes of the clinoforms. Basinward and landward, cross-shelf transits of the delta systems are evident, as they formed some 18–20 stacked, high-frequency transgressive-regressive deltaic cycles (0.8–3 Myr) and five megacycles (3 to 10 Myr). Development of the deltaic cycles is interpreted as the result of high-amplitude sea level falls and rises related to glacio-eustasy as well as to changing sediment supply. The large-scale transgressive-regressive evolution of the deltaic clastic wedge is likely to have been controlled mainly by long-term changes in tectonic subsidence rate and sediment supply. The shelf-edge deltaic regressions are likely to have developed during sea level fall, but also during periods of high sediment input enhanced by uplifted tectonic relief and catchment enlargement or at times by monsoonal intensification events.

Coastal barriers, northern Gulf - Last Eustatic Cycle; genetic categories and development contrasts. A review

A large body of chronologically well-constrained and detailed Quaternary data accumulated in the last decades from the coastal-shelf region of the northern Gulf of Mexico. Consequently, a preliminary synthesis of major aspects of the late Quaternary barrier and mainland coastal development on the northern Gulf of Mexico is timely. Several major factors account for the striking differences between barrier size and development styles in the three northern Gulf coast regions. Barriers are nearly continuous in the NW where they dominate by the greatest dimensions and continuity. Sand supply from shelf and mainland sources, continued at variable rates throughout six stages of the Last Eustatic Cycle (LEC). Differences in antecedent topography, in fluvial runoff volumes and relative sea level rise, including rapid overstepping-flooding events were critical in the regional distinctions. Leaving fewer and smaller relict fluvial delta behind, drowned shore and nearshore landforms on the shelf, only two large fluvial systems impacted the NE. The NW shelf and nearshore received abundant sediment supply from the Mississippi and lesser streams; indirectly by ravinement erosion of relict landforms and landward-directed cross-shelf sediment transfer. Sands originated mainly in relict deltas, fluvial, tidal channel, and inlet fill, as well as from submerged shore ridge remnants on the continental shelf. Shelf-margin deltas represented major secondary sources. Barrier formation categories and their respective importance in specific areas represent critical aspects of coastal development. The authorship of various formation concepts (e.g., Penck, Gilbert, McGee, Haage, Ganong, and Keilhack) often is still miscredited or remains unrecognized in the literature. Recognition of the stratigraphic and sedimentological diagnostic characteristics of the basic genetic barrier categories plays a key role in testing the validity of barrier evolution models.Closely-knit process-form relationships prevail between hydrodynamic (sea level change, storm, tidal current, overwash), sedimentary (shoal and island aggradation, rollover, and eolian accumulation) processes, and the resulting landform morphology. The most detailed accounts that deal with the development of the Alabama-Louisiana and Apalachicola barrier chains are particularly instructive. Almost all modern Gulf barriers with identifiable genetic background formed by shoal aggradation between 5.5 and 2.0 ka, during a marked deceleration in sea level rise. Concurrently, a sizable mainland strand plain complex extended the NE coastal plain. Tide- and storm-driven sediment transport to the large Mobile Bay ebb-delta, the focal role of composite east Dauphin Island in establishing the MS-LA island chain and the island-blocking role of Mississippi Delta advance were prime factors in a complex barrier history. Migration, paradoxically, may also occur by transgressive rollover concurrently with regressive landward progradation of island strand plains. Stream discharge and sand reworked from relict lithosomes on the shelf were insufficient in compensating for wave- and tide-related sediment loss. Gulf levels stayed below the present throughout the mid- and late Holocene.

Late Pleistocene Bryant Canyon turbidite system: Implications for Gulf of Mexico minibasin petroleum systems

The western ancestral Mississippi shelf-margin delta fed the Bryant Canyon turbidite system in the intraslope basin province of the northwestern Gulf of Mexico (GOM) during the penultimate glacial lowstand of sea level (130–160 year BP). The Bryant Canyon links a chain of 15 fill-and-spill minibasins on the continental slope. On the upper and lower continental slopes, minibasins are narrow (1–3 km), elongate (3–6 km), and follow salt ridges. On the middle slope, minibasins are larger (8–15 km) semicircular basins. Three main depositional facies are recognized based on seismic-facies interpretation: (1) ponded turbidites (T), (2) mass-transport deposits (MTDs), and (3) bypass channelized turbidites (C). Thick, intrabasinal, muddy MTD wedges sourced from high-relief internal walls of the minibasins alternate with and frequently cap externally derived deposits. Tabular extrabasinal MTD deposits originated from shelf-margin delta or canyon-wall failures upslope. The T and MTD facies deposits each make up approximately 40% of basin fill, and the C facies deposits comprise approximately 20%. The T facies deposits form perched lobes at canyon inlets into basins and ponded units on distal sides of basins. Channels in the C facies are similar in width (500–2000 m) and relief (20–100 ms) to channels in productive GOM subsurface minibasins. Syntectonic activity of salt diapirs typically began midway through filling of Bryant Canyon minibasins and then preferentially uplifted northern portions of basin deposits. Local salt-tectonic activity caused greater basin relief and thicker capping MTDs than in subsurface minibasins to the west (e.g., Brazos-Trinity Basin IV) or east (e.g., Mississippi Canyon). Bryant Canyon minibasins provide excellent modern analogs for subsurface Miocene to Pleistocene GOM chains of minibasins because of similar scales and depositional facies. The youngest Bryant T facies deposits and their overlying incised, thick, channel deposits contain the most sand-prone facies and suggest the best potential for petroleum reservoirs in subsurface minibasins.

Sequence architecture and depositional evolution of the Late Miocene to quaternary northeastern shelf margin of the South China Sea

Based on an integrated analysis of seismic, well logging and paleontological data, the sequence architecture and depositional evolution of the northeastern shelf margin of the South China Sea since Late Miocene are documented. The slope deposits of the Late Miocene to Quaternary can be divided into two composite sequences (CS1 and CS2) bounded by regional unconformities with time spans of 3–7 Ma, and eight sequences defined by local unconformities or discontinuities with time spans of 0.8–2.3 Ma. Unconformities within CS1 feature shelf-edge channel erosion, while in CS2 they form truncations at the top of the shelf margin as prograding complexes and onlap contacts against the slope.Depositional systems recognized in the slope section include unidirectionally migrating slope channels, slope fans or aprons, shelf-edge deltas and large-scale slope clinoforms. CS1 (Late Miocene to Pliocene) is characterized by development of a series of shelf-margin channels and associated slope fan aprons. The shelf-margin channels, oriented mostly NW-SE, migrate unidirectionally northeastwards and intensively eroded almost the entire shelf-slope zone. Two types of channels have been identified: (1) broad, shallow and unconfined or partly confined outer-shelf to shelf-break channels; and (2) deeply incised and confined unidirectionally migrating slope channels. They might be formed by gravity flow erosion as bypassing channels and filled mostly with along-slope current deposits. Along the base of the shelf slope, a series of small-scale slope fans or fan aprons are identified, including three depositional paleo-geomorphological elements: (1) broad or U-shaped, unconfined erosional-depositional channels; (2) frontal splays-lobes; and (3) non-channelized sheets. CS2 (Quaternary) consists mainly of a set of high-angle clinoforms, shelf-margin deltas and lower slope unidirectionally migrating channels.The relative sea level changes reflected in the sequence architecture of the study area are basically consistent with Haq's global sea level curve, but the development of regional unconformities were apparently enhanced by tectonic uplift. The development of high-angle (thick) clinoforms in the Quaternary may be attributed to a high sediment supply rate and rapid tectonic subsidence. The formation of the unidirectionally migrating channels appears to have resulted from the combined effects of the northeastward South China Sea Warm Current (SCSWC) and downslope gravity flow. The formation of the slope channels in the outer-shelf to shelf-break zone may be predominately controlled by bottom current, whereas those developed along the middle to lower slope zone may be dominated by gravity flow.

Living On the Edge: Evaluating the Impact of Stress Factors On Animal–Sediment Interactions In Subenvironments of A Shelf-Margin Delta, the Mayaro Formation, Trinidad

Abstract Integrating sedimentological and ichnological characteristics of the Gelasian Mayaro Formation along the SE coast of Trinidad allows recognition of a river-dominated and wave-influenced shelf-edge deltaic succession of the paleo–Orinoco River developed under strong slope instability. Four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf (i.e., at the shelf margin) have been identified. Extreme paleoenvironmental conditions make the succession rarely and sporadically colonized. Ichnologic evidence suggests that shelf-edge deltas are among the most stressful marine environments, due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment. Within any particular subenvironment, the relative dominance of the fluvial feeder system, the action of waves (and rare influence of tides), and slope instability determine the combinations and ranking of stress factors. River-dominated shelf-edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity (e.g., salinity changes, high sedimentation and erosion rates, flocculation of mud, seasonal variations of fluvial flux), whereas subenvironments dominated by waves, storms, and oceanic swells away from the axial feeder(s) are mostly influenced by barform morphodynamics and its effects on local physiography. The paleo–Orinoco delta is represented not only by the delta lobe(s) developed at the shelf edge but by delta lobe(s) formed on the outer shelf as well. Outer shelf deltaic subenvironments manifest typical ichno-sedimentological signatures of a “normal” (wave-influenced in this case) inner-shelf delta. Being susceptible to extensive gravitational instabilities, the delta system suffered from canyonization (and smaller gullying) and subsequent filling (N.B. Hereafter we use the term canyon for both canyons and smaller gullies). The shelf-edge delta system was also associated with “shelf-attached” mass-transport systems. Such depositional subenvironments dominated by slope instability and sediment-gravity-flows appear to be the most unconducive for benthic colonization, resulting in almost complete absence of bioturbation. Characterization of the stress factors, as the functions of parameters arising from specific sedimentary and/or oceanographic processes, chemical conditions, and preservational constraints specific to each subenvironment, leads to the construction of a comprehensive ichnological and depositional analog model for shelf-edge deltas in general, and for the accommodation-driven low-latitude shelf-edge deltas at an active oblique foreland setting in particular.

Evolutionary history and controlling factors of the shelf breaks in the Pearl River Mouth Basin, northern South China Sea

The migration history of shelf breaks in the stratigraphy of a basin contains significant information about sediment-budget partitioning into the deep water area, which is thus important in predicting the distribution and quality of deepwater reservoirs. Previous studies primarily focused on the sedimentary-dominated shelf breaks (e.g., progradational-type), which are controlled by sediment supply and relative sea-level changes. The location and migration pattern of structure-controlled shelf breaks are comparatively less well documented.The evolutionary history of shelf breaks in the Pearl River Mouth Basin from 30 Ma to present is examined here. Shelf breaks are picked from 280 2-D depositional dip-oriented seismic profiles in the forced regressive systems tracts of 20 3rd-order sequences (SQ 1-20) in four super sequences (SSQ 1-4). Planview distribution maps indicate these shelf breaks can be divided into two groups of different character. Shelf breaks in SSQ 1 (30–23.8 Ma) have migrated progressively and considerably basinward (∼50 km) along with the progradation of shelf-margin deltas, which were propelled by a very strong sediment supply even under the background of relative sea-level rise. They are consequently classified as sedimentary-dominated types. In contrast, shelf breaks in SSQ 2-4 have stayed fairly close (∼20 km) to the boundary between the Panyu Low Uplift and the Baiyun Sag, which is identified as a tectonic hinge zone across which subsidence rate increased abruptly basinward. Shelf breaks were close to the hinge zone even in SQ 11 and 12, in which the depositional shoreline breaks were located ∼80 km further landward. They are classified as structure-controlled types. Our study indicates that through the evolutionary history of the Pearl River Mouth Basin, the locations and migration pattern of the shelf breaks were mainly sediment-supply controlled during 30–23.8 Ma but changed to structure-controlled (i.e., tectonic hinge zone) during 23.8 Ma-Present.

Seismic geomorphology and lithology of the early Miocene Pearl River Deepwater Fan System in the Pearl River Mouth Basin, northern South China Sea

Multi-phase deepwater fan systems have formed in the Baiyun Sag of the Pearl River Mouth Basin on the northern margin of the South China Sea since the beginning of the Miocene. Integrated analysis of 2D/3D seismic, well log, core, and biostratigraphic data shows that the early Miocene Pearl River deepwater fan deposits consist of four 3rd-order sequences, SQ23.8, SQ21, SQ17.5 and SQ16.5. Each of them is bounded by regional discontinuity surfaces (3rd-order sequence boundaries). Of particular interest are the sandy deepwater fan deposits in the HST of SQ23.8 and LST of SQ21, which are absent in SQ17.5 and SQ16.5. The SQ23.8 deepwater deposits are relatively thin sheets, densely spaced sandy debris flow channels and MTDs, whereas in SQ21 they manifest as thicker sheets and larger channels. In terms of lithofacies, SQ23.8 deepwater deposits are massive sandstones, in contrast to SQ21 deposits of massive sandstones, normally graded sandstones, inversely graded sandstones, and locally interbedded ripple laminated siltstones. It is interpreted that SQ23.8 deepwater sands result from homochronous slumping of the shelf margin delta front, while SQ21 deepwater sands result from the erosion and re-deposition of the ancient SQ23.8 highstand shelf margin delta.The Paleocene-Early Oligocene rift stage (57.5 Ma ∼ 32 Ma) was characterized by extensive development of half-grabens and normal faults. Since 32 Ma, the drift stage has been characterized by thermal subsidence and weak tectonic activity. The distributions of deepwater sands in SQ23.8 and SQ21 are interpreted to have been controlled by seabed topography that was inherited from buried rift-related tectonic features. We propose that the primary controlling factor for the development of the early Miocene Pearl River Deepwater Fan System was likely sediment supply of the Pearl River influenced by the Baiyun Event and the uplift of Qinghai-Tibet Plateau. The specific location of the sandy deepwater fan was due to tectonic influence on paleotopography of the shelf. The interpreted two depositional models can help predict the types of deepwater depositional elements are in the Baiyun Sag of the Pearl River Mouth Basin and where deepwater sands are likely to occur.

Linking shelf delta to deep-marine deposition in reservoir dispersal of the upper Oligocene strata in the Baiyun Sag, the northern South China Sea

Linking shelf delta to deep-marine sediment in the Baiyun Sag (BYS) along the northern South China Sea has led to a great deal of interest, but it remains poorly known. In this paper, we focus on the detailed stratigraphic evaluation and reservoir dispersal of upper Oligocene strata (25.5 to 23.8 Ma, referred to as ZHSQ6), using three-dimensional (3-D) seismic volumes (approximately 2500 km2) calibrated with well-log and core data, to determine internal geometry and sedimentary evolution. Rock velocity estimates from well data show that sedimentary environments may be the most direct controlling factor of wave impedance, reflected polarity and the value of amplitude, which in turn predict the nature of the sedimentary system. Likewise, the analysis of sedimentary sequences reveals that lowstand systems tract and transgressive systems tract consist of inner-shelf deltas. Highstand systems tract, by contrast, contained mid-shelf deltas. A northeast-oriented advantaged longshore current influenced asymmetric sandy ridges with a mixed carbonate–siliciclastic system in the north controlled by paleo-uplift geomorphology. Muddy mass transport deposits (MTDs) and sheet sandstones are both widespread. During falling-stage systems tract, shelf-margin deltas provided coarse grain sediments directly onto the basin floor. Consequently, the volumes of dispersed thick deep-water stacked channels and sand-rich MTDs are significant. The existence of the eastern source clearly increased. Finally, the sediment model for the upper Oligocene Baiyun Sag has been established as an optimal component of source-to-sink systems. This study indicates that sedimentary evolution is controlled by source supply flux and duration, base-level behaviour, topographic gradient and growth faults. Delta front channels and longshore sandy ridges, together with the gully fill, represent the most favourable reservoirs for further hydrocarbon exploration.

Chapter 9 The Gulf of Cádiz continental shelves

Abstract The continental shelves fringing the Gulf of Cádiz are shallow-water environments of high scientific interest as they record the complex interplay between competing sedimentary, tectonic and oceanographic processes. These shelves receive the contributions of fluvial sources of varying significance, evolving laterally to shelf segments dominated by a vigorous oceanographic regime. In the long term, the Gulf of Cádiz margins show lateral physiographical compartmentalization related to the structural imprint. Recent approaches on the Iberian Shelf have focused on the study of shelf sedimentary processes and long-term evolutionary patterns through the application of high-resolution seismic-sequence stratigraphy concepts. The Late Quaternary record exhibits extensive regressive-lowstand shelf-margin deltas mostly comprising nested sequences disposed in a hierarchical pattern. This complex architecture has been attributed to the leading control of orbitally driven, climatic–eustatic changes. As a result, the Iberian Shelf constitutes an excellent location to study the interaction between sedimentary processes, climatic and sea-level changes. Recently collected data in the Moroccan Shelf allow a high-resolution seismic-sequence stratigraphy scheme to be established. Significant latitudinal changes on depositional sequence architecture are driven by differential accommodation led by the reactivation of deformational fronts during the Pleistocene. As a consequence, this southern shelf constitutes a good example of tectonic control on sequence stratigraphy.

Unusual occurrence and stratigraphic significance of the Glossifungites ichnofacies in a submarine paleo-canyon — Example from a Pliocene shelf-edge delta, Southeast Trinidad

Sedimentary rocks belonging to the upper-slope shelf-margin delta of the Paleo-Orinoco River are present at the southeast coastline of Trinidad in the Columbus Basin. The Pliocene Mayaro Formation, exposed as foreshore cliffs, represents the wave-influenced delta front and mouth bar of this system. These deposits consist of thick to very thick hummocky cross-stratified sandstone beds and thin-bedded to laminated heterolithic sediments. They also contain abundant soft-sediment deformation structures and sparse well-preserved softground burrows (e.g. Ophiomorpha nodosa). Towards the north-central part of the outcrop, the delta front deposits are cut across by a paleo-canyon filled with younger mud-dominated prodeltaic sediments. The rare exposure of the canyon-wall exhibits an unusual occurrence of Glossifungites ichnofacies. Contrastingly distinct from archetypal examples, this monospecific suite contains a low abundance of firmground Thalassinoides filled with mud rather than sand. The tracemakers burrowed into a firm medium-grained sandy substrate of the delta front, and the burrows were subsequently passively filled by the mud from the overlying prodeltaic sediments filling the canyon. The deep-tier firmground Thalassinoides suite crosscuts the pre-existing softground trace fossils. Integration of ichnologic, sedimentologic and sequence-stratigraphic datasets indicates that the older delta front sediments are separated from the prodeltaic deposits by distinct episodes of fluctuating relative sea-level controlled by the basin-bounding growth-fault activities and the development of the canyon. Whereas the entire shelf-margin megasequence might have been deposited through a regional scale sea-level lowstand, the local fluctuations in accommodation space resulting from the growth-fault movements and the incision of the canyon were responsible for the shifting positions of the depositional architectural elements of the shelf-edge delta.