Shelf-edge Deltas Research Articles

A “natural sand plant” at the shelf edge in the low-energy Gulf of Lions, western Mediterranean Sea

Thick sand deposits at the edge of continental shelves, sometimes covered with large bedforms, are generally considered as relict features inherited from periods of low sea level, but alternative interpretations are possible, even in zones where modern oceanic processes are moderate. Detailed investigations in the western Gulf of Lions (western Mediterranean Sea) reveal the presence of remains of Marine Isotope Stage 4 (MIS 4) and MIS 2 shelf-edge deltas at the head of canyons. The sands were then “cannibalized” by strong rising sea-level currents, forming a 20-km-long, strike-oriented sand body covered by dunes. Presently, the migration of dunes is maintained by strong storm-driven currents. The seasonal reversal of circulation is at the origin of inversion of dune morphology, thus the sand body mimics a tidal sand bank with clockwise circulation of bedforms. In the long term, dunes migrate southward with a negative angle of climb, eroding into the underlying shelf-edge deltas. Acoustic Doppler current profiles and numerical simulations indicate that the sediment is sorted, once in suspension, and is flushed to the deep sea while sand bedload contributes to bedform migration. This relative enrichment of sand without external supply is referred to as a “natural sand plant,” by analogy with industrial sand plants where sand is washed and sieved. It forms a discontinuous sand belt, 5−20 m thick, more than 110 km along-strike, connecting multiple shelf-edge deltas. The geological importance of recognizing this recycling mechanism is indicated by the preservation of similarly large buried dunes formed during the deglaciation leading to interglacial MIS 7 at ca. 240 ka (Termination III).

River-fed basin-fill and hyperpycnites in an island-arc setting: The Silurian–Devonian Kekexiongkuduke Formation from Western Junggar, China

Sediments from land are primarily transported into basins through hyperpycnal flow, which plays a significant role in the process of basin filling. Various models of hyperpycnites have been proposed. This study examines a Silurian–Devonian basin-fill sequence in NW China to validate existing models and presents a visually compelling case demonstrating the process of basin filling through flood-dominated river discharge.The Utubulak Member of the Kekexiongkuduke Formation can be subdivided into three parts, representing basin floor, slope and shelf environments, collectively representing a regressive basin fill sequence. The lower part consists of flood-related turbidites contain or encompass material of terrestrial and marine origin, their sustained and pulsed velocity structures are similar to river discharge. These turbidites may result from basinal hyperpycnal flow or retrogressive failures of the shelf-edge (hyperpycnal) delta. The middle (slope deposits) contains bi-graded hyperpycnite bed interpreted commonly attributed to deposition by river floods exhibiting the classical waxing-waning configuration. Additionally, fine-grained deposits resulting from river-generated, dilute hyperpycnal or hypopycnal plumes are observed interbedded with the basin turbidites or slope background deposits. In the upper part, the presence of thick-bedded sandstones indicates that sustained hyperpycnal flow can transport sands to the shelf-edge/shelf, facilitating deep-water sand delivery. The vertical distribution of the various flood-related facies in basin floor, slope and shelf strata emphasize the complexity of hyperpycnites in different settings, and particularly highlighting challenges associated with their recognition in basin deposits.The investigated succession accumulated in an island-arc setting which provided a favorable condition for a river-fed turbidite system to develop. The narrow shelf facilitated transport of sediments by hyperpycnal flows directly into the basin, while erosion of high mountains promoted a high sediment supply for hyperpycnal flow. And a river-fed turbidite system commonly developed during the late stages of basin filling.

Coupling relationship and genetic mechanisms of shelf-edge delta and deep-water fan source-to-sink: A case study in Paleogene Zhuhai Formation in south subsag of Baiyun Sag, Pearl River Mouth Basin, China

The coupling relationship between shelf-edge deltas and deep-water fan sand bodies is a hot and cutting-edge field of international sedimentology and deep-water oil and gas exploration. Based on the newly acquired high-resolution 3D seismic, logging and core data of Pearl River Mouth Basin (PRMB), this paper dissected the shelf-edge delta to deep-water fan (SEDDF) depositional system in the Oligocene Zhuhai Formation of Paleogene in south subsag of Baiyun Sag, and revealed the complex coupling relationship from the continental shelf edge to deep-water fan sedimentation and its genetic mechanisms. The results show that during the deposition of the fourth to first members of the Zhuhai Formation, the scale of the SEDDF depositional system in the study area showed a pattern of first increasing and then decreasing, with deep-water fan developed in the third to first members and the largest plane distribution scale developed in the late stage of the second member. Based on the development of SEDDF depositional system along the source direction, three types of coupling relationships are divided, namely, deltas that are linked downdip to fans, deltas that lack downdip fans and fans that lack updip coeval deltas, with different depositional characteristics and genetic mechanisms. (1) Deltas that are linked downdip to fans: with the development of shelf-edge deltas in the shelf area and deep-water fans in the downdip slope area, and the strong source supply and relative sea level decline are the two key factors which control the development of this type of source-to-sink (S2S). The development of channels on the continental shelf edge is conducive to the formation of this type of S2S system even with weak source supply and high sea level. (2) Deltas that lack downdip fans: with the development of shelf edge deltas in shelf area, while deep water fans are not developed in the downdip slope area. The lack of “sources” and “channels”, and fluid transformation are the three main reasons for the formation of this type of S2S system. (3) Fans that lack updip coeval deltas: with the development of deep-water fans in continental slope area and the absence of updip coeval shelf edge deltas, which is jointly controlled by the coupling of fluid transformation at the shelf edge and the “channels” in the continental slope area.

Use of resistivity and density borehole image logs to identify and distribute facies in the Pikka unit — A case study from the Nanushuk Formation, North Slope, Alaska

Over the past few decades, the North Slope of Alaska has yielded several major hydrocarbon discoveries in the deltaic topsets of the Brookian Nanushuk Formation. Together, the Nanushuk topsets and genetically related foreset and bottomset beds of the Torok Formation comprise part of a giant clinothem system that prograded across the Colville Foreland Basin during the lower Cretaceous (the Aptian through the Cenomanian). The Nanushuk Topset Play contains stratigraphically trapped hydrocarbons within multiple fairways trending roughly north to south along the basin’s extent. Inside the Pikka Unit, the Nanushuk Formation represents a shelf-edge delta and attached shoreface system, defined primarily from sedimentological interpretations of conventional cores integrated with 3D seismic reflection data and well-log motifs. In this study, we build a structural and stratigraphic framework for the Nanushuk reservoir across the Pikka Unit through integrated observations of the borehole image logs, seismic data, and core. In the described workflow, the boundaries delimiting dip zones interpreted in the wells are correlated to higher order stratigraphic cycles within the Nanushuk clinothem zone. These boundaries are observed in seismic profiles and provide the basis of the structural framework. In addition, core facies interpretations are calibrated to CT-scan data to establish each facies character and vertical distribution within the wells. In the absence of core data, core-calibrated borehole images are instrumental in guiding sedimentological and structural interpretations. This extrapolation is dependent on the image log quality and resolution. The calibrated facies interpretations are applied to wells without core data and guided by seismic amplitude interpretation to extrapolate reservoir presence and distribution across the area.

Gravity tectonics controls on reservoir-scale sandbodies: Insights from 3D seismic geomorphology of the canyons buried in the upper slope of the Eastern Niger delta basin

High-resolution 3D seismic data analysis was integrated with a calibrated well and biostratigraphy data to present the first overview of a buried Pleistocene canyon system on the upper slope of the eastern Niger Delta, the Galabor Canyon. Attribute maps of specific horizons allow documenting the changing morphologies and infill lithologies of two main branches of the canyon through two stages of activity separated by a reference horizon dated at 0.99 Ma based on well calibration. At the upper slope, growth faults dissect the canyon heads, the catchment of which encroaches a network of valleys incised on the outer shelf. The canyon fill is composed of muddy channels and mass-transport deposits, largely derived from the collapse of canyon walls and sand-rich bodies forming a tract sourced by shelf-edge deltas at the outlet of the incised valleys. High-density turbiditic processes likely control the distribution of sand bodies along the canyon, ranging from tributary fans on the upper slope to 6 km-wide meander belts on the middle slope. The sandy deposits accumulate in minibasins formed along the canyon path, downstream of the subsiding hanging wall of the growth faults and upstream of shale ridges that damp the flow in the canyon. These results show that canyons can be major targets for sand reservoir exploration on the upper slope of large muddy deltas.

Sedimentary characteristics and controlling factors of a Pliocene shelf-edge delta in the Papuan Basin

The Pliocene shelf-edge delta (SED) in the Papuan Basin was formed by deposition of the Pliocene Orubadi Formation. On the basis of a comprehensive analysis of seismic, wireline and mud logging, and paleontological data, the stratigraphic framework, depositional system and controlling factors on the SED development were established. The Orubadi Formation includes a parallel unconformity. Internally, two third-order sequences (SQ1/SQ2) are identified, and system tracts within the sequence layers are determined by the onlaps. By tracing changes in the shoreline trajectories within the sequence, the SED was divided into six phases in the study area, with seaward advances of ∼1543–5400 m during a single phase. The thickness of the foreset is ∼309–887 m, exhibiting sigmoid or sigmoid-tangential seismic reflections. The stratigraphic patterns and sedimentary system evolution of the Orubadi Formation are determined by the interplay between sea-level changes, sediment supply, tectonic evolution and paleogeomorphology. The Coral Sea spreading provided accommodation for the SED. The Miocene–Pliocene uplift and the middle Miocene arc–continent collision in northern PNG, which caused uplift and erosion of mountains in the late Miocene to Pliocene, is the major source of sediments. The large sediment supply and sea-level changes from SQ1–SQ2 indicate an overall relative sea-level fall resulted in rapid shelf margin advancement into the basin, with the Miocene carbonate platform edge providing the necessary slope conditions for material transport. This abundant sediment supply led to the significant development of the SED. This study provides robust insights for deep-water oil and gas exploration in the Papuan Basin. KEY POINTS The Pliocene shelf-edge delta (SED) in the Papuan Basin was formed in a compression setting with arc–continent collision. In the Orubadi Formation, the overall trend of sea-level change is decreasing, and the delta is divided into six stages. The Pliocene SED is controlled by the interplay between sea-level changes, sediment supply, tectonic evolution and paleogeomorphological.

Late Pleistocene-Holocene age and stratigraphy of the Currituck Slide Complex, U.S. mid-Atlantic continental slope: Implications for landslide triggering

Considerable effort has been made to link submarine slope failures to changes in local and global-scale environmental conditions, in order to assess landslide hazard probability. Here we provide the first radiocarbon dates of hemipelagic sediment overlying mass transport deposits and inferred failure surfaces of the Currituck Slide Complex (CSC), a prominent landslide scar on the U.S. mid-Atlantic continental slope. The dates, taken from both the upper and lower scars of the complex, constrain the age of the last major failure event to 13,835 and 16,020 years BP. Time correlation of the hemipelagic sediments across the landslide scar and proximal deposit suggests a single failure of both the upper and lower parts of this 160 km3 volume. A higher rate of sediment supply from the periglacial Appalachian Mountains and from glacial melt-water pulses, with an exposed continental shelf at that time, may have enlarged a shelf-edge delta at the site of the CSC, which may have facilitated or triggered failure. A smaller landslide at the southern edge of the complex with a less well-defined geomorphologic footprint is dated at 5500 BP and possibly represents the reshaping of the seafloor around the CSC triggered by low-frequency earthquakes on the nearby continental margin.

An integrated clinoform trajectory and sequence stratigraphic model for the Labrador margin, offshore eastern Canada

The passive margin offshore Labrador began forming during rifting in the Early Cretaceous, with seafloor spreading starting in the Maastrichtian and ending near the Eocene–Oligocene boundary. A foundational understanding of the associated stratigraphy of this margin has been developed through previous studies of the lithostratigraphy, biostratigraphy, and seismic stratigraphy, but the significance of clinoforms, stratal boundaries, and transgressive and regressive events has never been fully explored. In this study, we identify a series of clinoform successions from the Late Cretaceous through Pleistocene and apply a second-order sequence stratigraphic model to refine the geological framework for the margin. We focus on the Hopedale Basin where there is sufficient well and reflection seismic data, with substantial new seismic data collected over the last decade. Seismic interpretation is combined with paleoenvironmental and biostratigraphic data from the wells to map clinothem packages along the margin including: seven shoreline, five subaqueous delta, one shelf-edge, and three shelf-edge deltas, with illustration of these on seismic profiles and paleogeographic maps representing several time slices. Trajectory analysis of these clinothems permits the delineation of eight second-order sequences, with systems tracts and sequence stratigraphic surfaces identified. Broader implications of our results include a revised lithostratigraphic column for the Labrador margin. We further demonstrate regional trends during rifting and opening in term of controlling factors on clinoform development through comparison with adjacent and conjugate margins. Tectonism dominated as a controlling factor in the Cretaceous and dwindled near the end of the Paleocene. Eustasy and climate also influenced clinoform development in the Late Cretaceous, but became major factors during the Cenozoic, enhancing relative sea level fall and sediment supply, with similarities to other clinoform successions developed during Greenhouse and Icehouse conditions. Impacts to petroleum prospectivity in the region are also discussed, and our overall seismic and stratigraphic framework provide a basis for future study.

Sensitivity of marginal basins in recording global icehouse and regional tectonic controls on sedimentation. Example of the Cergowa Basin, (Oligocene) Outer Carpathians

Global sea-level fall during the late Eocene-Oligocene icehouse, in conjunction with the Alpine orogenesis, resulted in the epicontinental Paratethys Sea formed at the northern margin of the Tethys Ocean. Among the Paratethys sub-basins, the foreland Dukla Basin hosted dark dysoxic Menilite Shales. Within these the lenticular body of deep-marine Cergowa Beds was deposited. Due to its marginal position and orogenic activity, the Cergowa depocenter was sensitive to a broad range of influences reflected by: separation from and re-connection with the Tethys Ocean; reactions to global sea-level change; variations in calcareous nannoplankton palaeoecology and fluctuations in water chemistry; changing sediment gravity flow mechanisms; facies progradation and basin tectonics; palaeotransport direction; and changes in the calcite compensation depth. The proximal north-western part of the Cergowa depocenter consisted of two troughs evolving towards the south-east into an unconfined distal area. This is shown by palaeocurrent patterns changing down current from low-dispersion to fanning out distributions. Sedimentation during the succeeding NP23 nannoplankton zone was controlled by glacio-eustatic shallowing, enhanced by tectonic uplift, both resulting in isolation from the Tethys Ocean, forming a brackish environment affected by fluvial supply. Aggrading sand-rich proximal turbidite facies common at this stage include hyperpycnally-generated sustained turbidites that were sourced from shelf-edge delta, as well as hybrid event beds that reflect erosion of morphologically unbalanced slopes modified by fold-thrust tectonics. Nannoplankton of the succeeding NP24 zone documents normal salinity of an open sea that was reconnected with the Tethys Ocean, which is contrary to the global cooling and related eustatic sea-level lowstand during this time. At this stage, deposits of low-density turbidity currents predominate and form a succession prograding towards the south-east. Therefore, the oceanic reconnection was likely controlled by local subsidence, the rate of which exceeded the rate of deposition, especially in the south-east. The reconnection also influenced the calcite compensation depth level and resulted in an increase in coccolithophore productivity, recorded by laminated pelagic limestone interbeds within the youngest fining-upwards part of the Cergowa Beds succession. This fining-upwards sequence of low-density turbidites reflects retrogradation of the arenaceous material within the distributary system and a transition to mudstone-dominated disoxic facies of the Menilite Beds, being the most productive hydrocarbon and helium source rock unit in the Outer Carpathians. These processes and relationships show that due to the marginal position of the synorogenic basin, the Cergowa Beds record environmental sensitivity to extrabasinal and intrabasinal factors, and a broad range of global, regional, and local influences, thus providing insights applicable to research into the evolution of other basins placed in a similar context.

Shelf-edge deglacial reef establishment and subsequent partial demise: Response to distinct pulses of sea-level rise associated with environmental changes

The partial melting of Earth’s bi-polar ice sheets since the Last Glacial Maximum (LGM) has translated into a ~ 120 m amplitude stepwise sea-level rise punctuated by three major meltwater pulses that were tracked and recorded with some of the best accuracy by coral reefs. However, the initial meltwater pulse marking the end of the LGM, at 19 ka, is anchored in only two palaeo reefs (Barbados, Great Barrier Reef). Here, the authors present the analysis of a coralgal reef that thrived along the south-east Papua New Guinea Peninsula outer shelf during this initial pulse. In the cone of a piston core, a shallow Goniastrea retiformis coral colony was retrieved at 111 m below present sea-level and uranium/thorium dated to 19.4 ka BP. This colony had been buried beneath the debris of a proximal coralgal reef before its partial drowning at 14.5 ka BP. Seismic survey data suggest that the reef edifice was established directly on the eroded top of a lowstand shelf-edge delta, partially drowned and then back-stepped towards the south-east in response to three distinct deglacial sea-level pulses and stepwise increases of water column turbidity.

Depositional Controls on Detrital Zircon Provenance: An Example From Upper Cretaceous Strata, Southern Patagonia

Understanding how depositional environments within a sedimentary system redistribute and sequester sediment is critical for interpreting basin-scale provenance trends. However, sedimentary source-to-sink models commonly examine temporal changes and do not consider how variation in sedimentation processes across a dispersal pathway may result in contrasting provenance signatures. In this paper, we demonstrate a down-paleoslope shift in detrital zircon provenance signatures correlated with shallow-marine lithofacies patterns from the Upper Cretaceous La Anita Formation and underlying continental slope lithofacies of the Alta Vista Formation (Magallanes-Austral Basin, southern Patagonia). New stratigraphic, sedimentologic, and lithofacies analysis results from the La Anita Formation suggest an upward shoaling succession, from a 1) storm-influenced shoreface, 2) fluvially dominated, wave-influenced delta, and a 3) high-energy, gravelly foreshore. Stratigraphic sections are paired with U-Pb detrital zircon sandstone samples (N = 20; n = 5,219), which provide both maximum depositional ages and provenance characteristics. While all samples contain abundant zircon derived from the Andean volcanic arc (ca. 145–75 Ma), the amount from both Jurassic distal volcanic massifs (ca. 188–162 Ma) and recycled orogenic sources exhumed during the advance of the Cretaceous fold-and-thrust belt (>200 Ma; 157–142 Ma) vary with changes in depositional environment. We argue that down-paleoslope, systematic enriching of local fold-and-thrust belt material within the La Anita Formation is reflective of progressive mixing of grains transported via shallow-marine processes, while distally enriched fluvio-deltaic transported zircons were sourced from large, regional catchments. This suggests that competition between transport processes across a shallow and marginal marine sequence of rocks affects the resulting provenance signatures recorded within a single stratigraphic succession. These data also detail the degree of sediment pathway connectivity between shallow-marine sources and deep-marine sinks. Detrital zircon results from muddy continental slope facies of the Alta Vista Formation are made up entirely locally derived material, while zircon results from deep-water, sand-rich channel facies of the Formation are indistinguishable from coeval fluvio-deltaic zircon signatures. This implies that continental shelf-to-slope connectivity in a sediment dispersal system, via submarine canyons or shelf-edge delta progradation, is necessary for detrital zircon distributions from the shallow-marine realm to propagate into the deeper marine.

Sedimentation of the Oligocene-Lower Miocene clinoforms of the Maikop formation in the Eastern and Central Pre-Caucasus region as a key criteria for reservoir exploration

Oligocene-Lower Miocene clinoforms of the Maikop formation are productive in the Eastern and Central Pre-Caucasus region. In spite of commercial discoveries and long exploration history, Maikop formation is poor characterized by borehole data in the deep basins such as Terek-Caspian trough. Furthermore, there are challenges of the detail stratigraphic subdivision in the deep basins, correlation, sedimentary conditions and criteria of reservoirs exploration. Based on seismic and borehole data analysis, sequent-stratigraphic framework, distribution area, progradation direction, seismic unconformities and thicknesses of the 17 Maikop sequences are established. Estimation of the clinoforms height allowed reconstructing paleobathymetry and paleogeography of the Pre-Caucasus region in the Oligocene – Early Miocene. Clinoforms M1-M7 prograded from the northeast, sea depth increased from 300 to 450 m. During clinoforms M8 sedimentation, new provenance area appeared on the west, sea depth increases up to 800 m. Clinoforms M12-M16 prograded from the north, sea depth decreases from 480 to 270 m. To the end of the M17 sequence deposition, starved deep basin was totally compensated. Three morphologic types of the clinoforms are identified. Type I represents by tangential clinoforms up to 75 m of height, that interpreted as subaqueous delta. Type II are the sigmoid shelf-edge delta clinoforms up to 800 m of height. Clinoforms of type III are low-angle wedges, confined to the slope and floor of the deep basin. Height of the wedges does not exceed 270 m. In the topset of the shelf-edge deltas, shallow-marine sandstones and structural traps predicted, while basin floor fans and stratigraphic traps expected in the bottomset. Subaqueous deltas are regarded to stratigraphic traps, while the wedges are predominantly mud-prone.

Outer-shelf conduit within growth-fault compartment Pliocene Orinoco Delta

Prominent outcrops of the Pliocene Orinoco Delta along the southeast coast of Trinidad Island display a well-known, very thick Icehouse succession of storm-wave dominated delta lobes, as part of a 10–12 km thick Neogene continental-margin prism. There is a previously discovered, steep incision or conduit (400 m wide) with south and north-bounding erosion surfaces, cutting deeply into the Mayaro Formation shelf-edge delta deposits. The ca. 100 m-thick infill of the conduit shows an initial muddy succession passing upwards to very coarse-grained debrites, rotated and slumped blocks, and structureless sandstones that pass upward to a stacked series of sandy to muddy channel fills. The channels show spectacular tidal rhythmites, probably developed on the point bars of subaqueous tidal channels. As one piece of the topset of the shelf-edge delta clinoform, this conduit displays features of the linkage hub between the landward on-shelf delta systems and the basinward slope deposits.New fieldwork and a re-examination of the incision-infill reveal new details and suggest three possible origins of the conduit: ① the first is a no river feeder, shelf-edge collapse scar, extending onto the upper slope and infilled by deeper to shallower deposits. This shelf-edge cutting canyon interpretation is supported by the presence of hardground ichnology on the steep walls of the incision and the abundance of chaotic blocks and debrites in the lower two-thirds of the incision infill. ② The second is a shelf channel or valley linking fluvial channels to on-shelf and outer shelf deltas. This river-connected, subaqueous shelf valley origin, is supported by the upward-coarsening nature of the infill succession and especially the occurrence of shallow-water tidal channels in the upper third of the infill. ③ The third is a canyon developed mainly during transgression or highstand with a collapsed canyon-head infill and an overlying muddy tidal-channel system that had only limited sediment bypass to the deepwater slope and basin floor. This case represents both the shallow-water and the deeper water features of the infill succession, and is supported by similar large shelf-edge collapse features seen elsewhere in the Cruse and Moruga Formations of the paleo-Orinoco margin. This case, unrelated to river supply, also benefits from slight rotational shelf-edge uplift to trigger collapse.The known Cedar Grove growth fault was active and located some 4 km to the west from the present coast on the southeast corner of Trinidad Island. The growth-fault trapped thickened outer shelf to shelf-edge delta-front and delta-plain sediments within the hanging-wall compartment as the fault stretched about 20 km along strike. All the shelf-edge aggradation and its partial collapse were likely triggered by growth-faulting. After some muddy backfilling and coarse-grained block collapse into the invaginated shelf-edge, this scalloped area attracted a belt of subaqueous tidal channels, thus healing the eroded topography.

Sedimentary facies characterization of forced regression in the Pearl River Mouth basin

Abstract The Miocene Zhujiang Formation is the key horizon for oil and gas exploration in the Pearl River Mouth basin of northern South China Sea. With the help of core observation, seismic attributes and various analytical data, the sedimentary facies marks, distribution of sedimentary facies and depositional model of forced regression in the Miocene Zhujiang Formation of the Pearl River Mouth basin, northern South China Sea, have been studied. Forced regressive deposits were formed during the period when relative sea level ranged from highstand to lowstand and the sediments were forced to undergo progradation so that five sets of foreset delta deposits are developed in turn. In the early stage of forced regression, the normal delta where the delta plain, delta front and prodelta are not absent mainly developed. In the later stage of forced regression, the shelf edge delta with only the delta front and the prodelta, the longshore bar along the shelf break and the turbidite fan in the deep water of the slope area were developed. The favorable reservoir of forced regressive deposits are located near the upper boundary of the falling stage systems tract and the basal surface of forced regression, and they are the sand bodies of shelf edge delta, longshore bar and turbidite fan. The research results may provide guidance for reservoir prediction.

Narrow is normal: Exploring the extent and significance of flooded marine shelves in icehouse, transitional, and greenhouse climate settings

Abstract Marine shelves are a ubiquitous feature of modern Earth, developed across a wide range of scales in many sedimentary basins and representing the flooded portion of basin-margin clinoform topsets. Analysis of 80 clinoforms from 10 basins spanning Cenozoic and Mesozoic icehouse, transitional, and greenhouse climate settings indicates that normalized mean greenhouse marine shelf width is 33% of normalized mean total measured clinoform topset length. The equivalent value for transitional settings is 43%, and 72% for icehouse marine shelves. These values demonstrate that greenhouse marine shelves were substantially narrower than icehouse equivalents, suggesting that narrower shelves with persistent shelf-edge deltas were a consequence of lower rates of accommodation change in greenhouse climate intervals that lacked the large ice sheets required to drive high-amplitude high-frequency glacio-eustasy. Because greenhouse climates have been the dominant mode through Earth history, narrow shelves have probably been the dominant form, and conceptual models based on modern relatively wide shelves may be poor predictors of paleogeography, sediment routing, and sediment partitioning throughout much of Earth history.

Sedimentary Sequence, Evolution Model and Petroleum Geological Significance of Forced Regression: A Case Study of the Miocene Zhujiang Formation of the Pearl River Mouth Basin in the Northern South China Sea

Using 2D/3D seismic data and a large number of drilling and logging data and applying sequence stratigraphy, seismic sedimentology, and petroleum geology concepts, the characteristics of the sedimentary sequence of the forced regression have been analysed, the migration trajectory of the coastline have been reconstructed, the evolution model of the forced regression have been presented, and the significance for petroleum geology of the forced regressive sandbodies have been discussed. The falling stage systems tract (FSST) of the Zhujiang Formation present offlap high-angle oblique foreset reflection structure in the seismic profiles of the depositional trends and turbidite fan deposits with strong amplitude mound reflection structure are developed in the downdip direction of its front. The trajectory of migration of the shoreline shows a terraced downtrend in the direction of basin. The FSST is characterized by the shelf-edge delta without topset beds. The FSST was formed in the fall of relative sea-level. Five sets of foreset beds controlled by high-frequency relative eustatic were developed, therefore ordinal regressive overlap can be observed for the five sets of shelf-edge deltas in the depositional trends. The favourable reservoirs which were located close to the upper boundary of the falling stage systems tract and the basal surface of forced regression are sandbodies of the shelf-edge delta front and wave-dominated shoreface sands and the sandbodies of the turbidite fan. Those sandbodies favour the formation of lithologic oil–gas reservoirs by means of good trap sealing conditions, excellent oil–gas reserving performance, and effective oil source communication of fracture system.

Sequence stratigraphy and sedimentary characteristics of the shelf-edge delta and slope fan systems in the Late Oligocene, Baiyun Sag, Pearl River Mouth Basin, China

The Pearl River Mouth Basin in the South China Sea is one of the most important offshore petroliferous basins in China. Specifically, the shelf-edge delta and slope fan systems in the deepwater area of the Baiyun Sag have been regarded as the most important objects for hydrocarbon exploration, and the sequence stratigraphic structure, sedimentary evolution, and “source to sink” relationship between them is an important basis for predicting favorable areas for hydrocarbon exploration. In this study, the sequence stratigraphic structure and sedimentary system of the Late Oligocene Zhuhai Formation in the Baiyun Sag of the Pearl River Mouth Basin were studied based on a comprehensive dataset that integrated seismic, logging, and core data. A total of four third-order sequences were determined in the second-order composite sequence (CS3) according to nature of unconformities. Within each third-order sequence, the characteristics of different sedimentary systems were successfully identified. Within CS3-1 and CS3-2 the transgressive systems tracts (TST) are relatively thick, and the highstand systems tracts (HST) encompass mainly the outer-shelf deltaic deposits with thin foresets. As for CS3-3 and CS3-4, the lowstand systems tract (LST) and falling-stage systems tracts (FSST) are dominated by shelf-edge deltas and slope fans deposits, whereas the TST are very thin. The depositional evolution and “source to sink” relationship between the shelf-edge deltas and associated slope fans are revealed as follows: there are mainly three stages of shelf-edge deltas, and their sedimentary paleo-water depths were 794.9 m, 822.6 m, and 556.8 m, with progradation angles of 8.5°, 7.7°, and 5.7°, respectively; the thick delta front (200–500 m) and slump deposit are the main identification features of the shelf-edge delta. During the period of LST, although the progradation angle of the shelf-edge delta was the largest, there were few slope fans or only mud-rich slope fan deposits in the front of the delta due to the influence of relative sea-level rise, and during the period of FSST, the slump and redeposition of the delta front sand bodies provided the sand source for the formation of the slope fans, thus forming the sand-rich fans. Moreover, according to the root mean square amplitude slices and 3D seismic profile, it was found that the slope fans in this area are directly related to the development of the channels between the deltas and slope fans; generally, the larger and denser the channel, the larger the slope fan area.

Shelf-margin architecture and deposition variability across the mid-Pleistocene climate transition, northeastern South China Sea

The shelf-margin prism of the Upper Pliocene to Pleistocene in the northeastern South China Sea has recorded significant signatures of glacio-eustatic cycles and paleo-climatic conditions. Integrated analysis of 3D seismic, well-logging, and borehole dating data in the study area provided a detailed depiction of the high-resolution sequence architecture and depositional evolution as they responded to the glacio-eustatic cycles and climate changes. The shelf-margin section of the Upper Pliocene to Pleistocene is comprised of a composite sequence bounded by a regional unconformity, which can be further divided into five sequences confined by subordinate unconformities. The two lower sequences (S1-S2) with similar and simple internal structures, are displayed as having roughly equal thicknesses (ca. 50 m) and gently seaward-dipping foresets. In contrast, the upper three sequences (S3-S5) are characterized by the development of high (200–600 m) and steep (2–5°) shelf-margin clinoforms, which can be further divided into fifteen clinothems. The sequences generally embody four systems tracts, showing different stratal stacking patterns, and the linking contemporaneous depositional systems including fluvial channels, deltaic and slope fan systems are documented. Based on the chronostratigraphic framework, comparisons have been made between the sequence cycles and the global oxygen isotopic records. The results suggest that the formation of different order sequences may have been in response to the glacio-eustatic cycles paced by the Milankovitch orbital cyclicity. The abrupt changes in the sequence and depositional architectures between S1-S2 and S3-S5 are assumed to be related to the significant increases in the amplitudes and durations of the glacio-eustatic fluctuations across the mid-Pleistocene climate transition (MPT). Intensively variable climatic conditions with strengthened monsoons during and after the MPT could have potentially led to an abundant sediment supply, which may have enhanced the development of shelf to shelf-edge delta systems with the large-scale prograding clinoforms.

Sedimentary characteristics and genetic mechanism of a deep-water channel system in the Zhujiang Formation of Baiyun Sag, Pearl River Mouth Basin

The external geometry, internal architecture and sedimentary evolution processes of the deep-water channel system in Zhujiang Formation in the Midwest of Baiyun Sag have been revealed in detail according to the integrated analysis of 3D seismic data and a large number of drilling and logging data sets. In addition, the controlling effect of sediment supply, sea level change, shelf break zone and palaeogeomorphology on the deep-water channel system was discussed. Finally, it is concluded that the development and distribution of the deep-water channel system is controlled by both provenance and landform. The deep-water channel system can be subdivided into different sections: the provenance area, the waterway and the sedimentary area. It presents the features of a ‘three segment channel’ which indicates that the fluid energy of the gravity flow changes from weak to strong and then weak along depositional inclination. Because the processes of internal filling and latter reformation which the channel experienced in different sections during different periods are complex, the inner filling characteristics of the channels are different, mainly high sand-shale ratio superimposed channel (sand-rich deposit) and low sand-shale ratio channel-levee complex (sand-lean deposit). It is clear that the development and spatial distribution characteristics of deep-water channel system in Zhujiang Formation are controlled by the shelf edge delta of Pearl River, forced regression, the shelf break zone and restricted landform collectively. The shelf edge delta at the lower member of Zhujiang Formation in the north of Baiyun Sag provides the large amount of clastic sediments for the development of deep-water channel system. Forced regression drives the continuous transportation of the sediments from shelf edge to deep-water channel system. The shelf break and the fault slope break provide favorable paths and topographic conditions for transportation of the sediments in the deep-water channel system. The fracture systems and the subaqueous low uplifts influenced the spatial distribution of the deep-water channel system together.

Shelf-edge delta and reef development on a mixed siliciclastic–carbonate margin, central Great Barrier Reef

ABSTRACT The Great Barrier Reef (GBR) is the world's largest extant mixed silicilastic–carbonate margin. Previous research on the Great Barrier Reef has suggested that the extensive barrier reef system may act as an impermeable barrier and limit the development of delta systems during lowstands, but sufficient geophysical data to support this hypothesis are lacking. We use dense sparker seismic and sub-bottom profiler data to better understand the structure of a large lobe-shaped feature (∼ 10 km × 10 km) on the shelf edge of the central GBR and the interactions between siliciclastic and carbonate sedimentary systems. Interpreted sparker seismic contains prograding clinoforms and suggest that the lobe-shaped feature was a river-dominated shelf-edge delta. A delta on the shelf edge implies that the presence of an exposed barrier reef was not a major impediment to deposition and that other adjacent lobe-shaped features are also deltaic deposits. The shelf-edge deltas were deposited onto a broad upper-slope terrace that allowed continued progradation and limited incision when sea level fell below the shelf edge. Delta foresets are commonly colonized by coral reefs, but the spatial and temporal relationship between reefs and some deltaic units remains unclear. The presence of multiple shelf-edge deltas that link to previously mapped Burdekin River paleo-channels indicates a complex history of sedimentation, with the Burdekin River delta migrating up to 100 km along the GBR margin during the late Quaternary. Regional bathymetric data suggest that large modern or recent shelf-edge deltas are rare on the GBR and that there was a broad range of sedimentary processes operating along the margin of the GBR during periods of low sea level.