Shelf-edge Trajectories Research Articles

Indication of Deep-Water Gravity Flow Types by Shelf-Edge Trajectory Migration Patterns: A Case Study of the Quaternary Qiongdongnan Basin, South China Sea

The shelf-edge trajectory is comprehensively controlled by tectonics, sediment supply, sea level, and climate fluctuations; its migration and evolution have a strong influence on what happens in the deep-water depositional system during the Quaternary. The shelf-edge trajectory pattern, sediment-budget partitioning into deep-water areas, and reservoir evaluations are focused topics in international geosciences. In this paper, the Qiongdongnan Basin (QDNB) in the northern South China Sea is taken as an example to study how shelf-edge trajectory migration patterns can influence the types of deep-water gravity flow which are triggered there. Through quantitatively delineating the Quaternary shelf-edge trajectory in the QDNB, four types of shelf-edge trajectory are identified, including low angle slow rising type, medium angle rising type, high angle sharp rising type, and retrogradation-slump type. A new sequence stratigraphic framework based on the migration pattern of shelf-edge trajectory is established. There are four (third-order) sequences in the Quaternary, and several systems tracts named lowstand systems tract (LST), transgressive systems tract (TST), and highstand system tract (HST) are identified. This study indicates that the type of deep-water gravity flow can be dominated by the shelf-edge trajectory migration patterns. When the shelf-edge trajectory angle (α) ranged between 0° and 4°, the continental canyons were mostly small-scaled and shallowly incised, with multiple large-scale sandy submarine fan deposits with few MTDs found in the deep-water area. When the angle (α) ranged from 4° < α < 35°, the size and incision depth of the continental slope canyons increased, relating to frequently interbedded sandy submarine fan deposits and MTDs. When angle (α) ranged from 35° < α < 90°, only a few deeply-incised canyons were present in the continental slope; in this condition, large-scaled and long-distance MTDs frequently developed, with fewer submarine fans deposits. When angle (α) ranged from 90° < α < 150°, the valley in the slope area was virtually undeveloped, sediments in the deep-sea plain area consisted mainly of large mass transport deposits, and submarine fan development was minimal. Since the Quaternary, the temperature has been decreasing, the sea level has shown a downward trend, and the East Asian winter monsoon has significantly enhanced, resulting in an overall increase in sediment supply in the study area. However, due to the numerous rivers and rich provenance systems in the west of Hainan Island, a growing continental shelf-edge slope has developed. In the eastern part of Hainan Island, due to fewer rivers, weak provenance sources, strong tectonic activity, and the subsidence center, a type of destructive shelf-edge slope has developed. The above results have certain theoretical significance for the study of shelf-edge systems and the prediction of deep-water gravity flow deposition type.

Seismic stratigraphy of the Cretaceous post-rift sedimentary interval in the Punta del Este Basin (offshore Uruguay) and its implications for deep-water reservoirs

The formation of the Punta del Este Basin is associated with the fragmentation of West Gondwana and consequently the opening of the South Atlantic Ocean during the Early Cretaceous. The basin comprises the Cretaceous depocentre of the Uruguayan continental margin (UCM). This study provides a seismic stratigraphic analysis of the Cretaceous post-rift sedimentary interval in the basin by defining its seismic facies, depositional sequences, shelf-edge trajectories and palaeophysiographical settings. All depositional sequences of the Cretaceous post-rift sedimentary interval represent intense basinward progradation marked by two depositional trends. The Lower Cretaceous sedimentary interval exhibits a dominant flat-to-descending shelf-edge trajectory with a migration of up to 37 km. Deposition occurred over an irregular physiography with proximal NW–SE and distal WSW–ENE trends controlled by the characteristics of the volcanic margin. The basin geometry has strong implications for understanding the deposition of a deep-water turbidite system in the UCM, with a strong analogy to the recent discovery of the Venus well in the Orange Basin, offshore Namibia. Towards the Late Cretaceous, the NW–SE depositional trend was controlled by a NE–SW slope break, indicating deposition over the south and north highs. A low-angle ascending shelf-edge trajectory was observed, with a migration of up to 16 km.

Did eustatic sea-level control deep-water systems at Milankovitch and timescales?: An answer from Quaternary Pearl River margin

As one of the most important forcing factors, sea level fluctuations exert a major influence on deep-water depositional processes, however, it is still not well understood how they control the evolution of the specific deep-water system over different timescales. For relatively longer (1 My) and shorter (100 Ky) timescales, we characterize deep-water sedimentary records on the Pearl River margin using seismic and borehole data, and then compare them with the contemporaneous sea-level curve to exam the varied roles sea levels have played in impacting the development of the deep-marine system. Results indicate that over both the 1 My-scale and the 100 Ky-scale, the studied deep-water system shows systematic variations that are strongly suggested to arise from the modulation of sea levels. Specifically, over the 1 My-scale, facies and depocenters of the deep-water system all show significant changes. Large fan lobes (consisting of both turbidites and mass-transport deposits) with more distal depocenters likely correspond with low-amplitude and high-frequency sea level fluctuations and slightly rising shelf edge trajectories, whereas smaller turbidite fan lobes with more proximal depocenters likely reflect high-amplitude and low-frequency sea level fluctuations and steeply rising shelf-edge trajectories. In contrast, within the 100 Ky-scale of a glacial-interglacial cycle, the composition of deep-water deposits also shows significant variations. Coarser-grained deposits with higher organic carbon (TOC) content and lower calcium carbonate (CaCO3) content are interpreted to reflect periods of glacial sea-level lowstand, whereas finer-grained sediments having lower TOC and higher CaCO3 content reflect interglacial sea-level highstand. Moreover, the spectrum characteristics of these constituent curves are very similar to those of contemporaneous sea levels, reflecting Milankovitch climatic forcings and further validating the tight coupling between sea-level stands and sediment compositions. These correlations between sedimentary records and sea level behaviors suggest that it was mainly a long-term change in the amplitude and frequency of eustatic cycles that controlled the overall architecture of the deep-water system, whereas it was shorter term, changing sea-level stands that played a role and impacted the deep-water sediment composition.

High-resolution sequence stratigraphy in continental lacustrine basin: A case of Eocene Shahejie formation in the Dongying Depression, Bohai Bay Basin

Approximate 500 m deltaic-lacustrine successions were formed within 2 Ma in the Liangjialou area of the Dongying Depression in the Bohai Bay Basin (eastern China). However, few studies on sequence stratigraphy were previously conducted in the east of Liangjialou. In the present study, we integrate high-resolution three-dimensional (3D) seismic data with well-logging data to study the sequence stratigraphy and the depositional system.Our study shows that: (1) Four systems tracts have been identified in the study interval, they are HST, FSST, LST and TST. The HST has an ascending shelf-edge trajectory and aggradation to progradation stacking pattern. FSST has a descending shelf-edge trajectory and progradation to degradation stacking pattern. LST has a low-angle ascending shelf-edge trajectory and progradation to aggradation stacking pattern. TST has a backstepping shelf-edge trajectory and retrogradation stacking pattern. (2) Five high-resolution sequences (equal to 4th-order sequence), HRS1∼HRS5, were further subdivided within the 3rd-order LST. In addition, the depositional systems of HRS1∼HRS5 are significantly variable in 3D space. The delta and deep-water deposits in the north are larger than those in the south. (3) The formation of the sequence is controlled by both tectonics and dry-wet climatic cycles. The tectonic subsidence and high sediment supply in overall wet climate play a major role in the formation of the 3rd-order sequence. The higher frequency dry-wet climatic cycles dominate the formation of internal systems tracts. We further divide the climate of study interval into three stages that are excellent correlated to the specific systems tracts: Stage 1 (evaporation < precipitation) corresponds to HST; Stage 2 (evaporation > precipitation) corresponds to FSST; Stage 3 (evaporation < precipitation) corresponds to LST. (4) The basin morphology and source location mainly control the lateral variability of the depositional system of LST. Low paleotopography, narrow shelf width and steep slope dip in the north cause a large accommodation, and promote the development of the deltaic and deep-water deposits. In contrast, a high relief, wide shelf width and gentle slope cause only minor accommodation in the south, and limit the development of the deltaic and deep-water deposits.

Quaternary evolution of the southeastern Korean continental shelf, East Sea: Paleo-incised valley and channel systems

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (<ca. 2.0 Ma) sedimentary section of the southeastern Korean continental shelf, East Sea. Seven sequences, S1 to S7 from bottom to top, were analyzed on the modern continental shelf. These sequences comprise a ~300 m thick sedimentary section containing, at minimum, one cycle of sea-level lowstand, transgression, and high stand deposits. The lowstand sections are composed mainly of laterally and vertically accreting fluvial channel deposits (i.e., point bars, channel lags) formed on an alluvial plain during a lowstand in sea level. The transgressive sections consist of confined, low-to moderate-amplitude continuous reflections interpreted as estuarine facies. The high stand sections, in contrast, are comprised of laterally extensive low amplitude reflections interpreted as open marine facies. Each of these sequences rests unconformably on its’ predecessor, particularly at the mid-southeastern edge of the shelf where angular, toplap, and onlapping relationships can be discerned.The sequence boundaries (SB1 to SB7) bounding these sequences are produced by fluvial incision (i.e., valleys and channels) during sea-level fall. SB4, SB5, and SB7 form the most prominent unconformities on the shelf and are incised by extensive paleo-incised valleys and channels. These valleys and distributary channels display simple U- or V-shaped cross sections or complex morphologies; they are ~1–8 km wide and ~10–60 m deep. Incised-valley fills reach a maximum thickness of over 80 m. The largest incised valley fills (~8 km wide, ~60 m thick) are found near the shelf break in the SE, indicating relative sea-level fall on this location (ca. 150 m lower than the present sea level). In the SE, the incised valleys are comprised dominantly of stacked lowstand successions (point bars and channel lags) and a lesser degree of transgressive successions (estuarine mud and levees) that are capped by subsequent high stand deposits (marine mud). The youngest channel above the valley was inferred to have been formed during the Last Glacial Maximum (LGM) on the shelf edge trajectory about 140 m lower than the present. These large incised valleys are thought to have developed through a complex history of repeated avulsion and reoccupation during the lowstand sea level. The most important factors regulating the valley morphology are the duration of sea-level fall, lithologic variations, climate (i.e., fluvial discharge), and the river gradient relative to the shelf gradient. The ancient drainage patterns are more-or-less northeast-southwest orientated, consistent with the trend of the present-day submerged fluvial channel. Superimposed upon this general pattern are the effects of tectonic activity (uplift, folds, and faults), which appear to have modified the courses of the rivers. Our results, therefore, provide a good example of how external factors (i.e., climate, tectonics, sediment supply, and paleogeomorphology) can determine the location and geometry of the valleys and the architecture of the valley-filling deposits of continental shelves.

Basin margin sediment wedge build out of the Eastern Niger Delta: application of shelf-edge trajectory pattern studies

The understanding of how basin margin sediment wedge builds out causes shelf-edge migration with time is approached based on shelf-edge trajectory pattern analysis using a high-resolution mega-merge seismic data from the eastern Niger Delta, Nigeria. The study focuses on a seismic dip transect traversing the Greater Ughelli, Central Swamp, Coastal Swamp and the Shallow Offshore Depobelts of the Niger Delta. On the regional dip transects, shelf-edge sediments occur as clinoform-bearing wedges at and immediately updip of the shelf-slope break. The shelf edge is deeply buried (> 2–4 s, twt), around the Greater Ughelli and Central Swamps. But with changing structural style, sudden change of ascending shelf edge around the Central Swamp was observed. The huge listric growth fault in the Coastal Swamp; around Bonny area, once again cut the shelf edge into half, rotated it along the listric fault and buried it distally. Several depositional packages show low to moderate ascending shelf-edge trajectory with progradational to aggradational clinoform growth that is characterized by thin sand sheets across most of the shelf and upper slope, though few are also characterized by progradational clinoform growth with thick sand on the shelf, upper-tolower slope and basin floor. The deposition is usually on the Outer Shelf Terrace (OST) which is regressive in a flat and rising trajectory style. This study has demonstrated that accommodation and sediment flux are the dominant controls on how the study basin’s sediment wedge built out, whereby limited accommodation promotes sediments with significant shelf-edge advance and descending trajectories, while increasing accommodation promotes ascending trajectories and increased deposition on the outer shelf. The greater sediments on the Outer Shelf Terrace and the shelf margin than on the slope gives more hydrocarbon prospectivity search around the outer shelf and shelf margin.

Forced or normal regression signals in a lacustrine basin? Insights from 3D stratigraphic forward modeling in the SW Pannonian Basin

Abstract The overall geometry of sedimentary basins is driven by a complex interplay between subsidence, water-level variations, sedimentation rates and basin-margin topography. Seaward movement of the shoreline is driven by either high sediment input or by base-level drop. The influence of the specific driving mechanisms is often overwritten by subsequent burial and compaction effects or it is below the resolution of observational data. Our main aim is to describe and discuss the influence of these internal and external forcing factors and demonstrate their sedimentary response, particularly the related unconformities and clinoform geometries in a deep lacustrine setting. To this aim, 3D numerical simulations were performed using DionisosFlow stratigraphic forward modeling software. Models are constrained by seismic and well data from the Drava Basin part of the SW Pannonian Basin, Central Europe. A wide range of parameters, such as subsidence rates, lake-level curves and sedimentation rates were compared and analyzed. Post-rift subsidence and inversion of the basin have been simulated together with the main sedimentary transport routes. Our calculation infers that paleo-water depth could have reached ca. 1300 m in the deepest parts of the basin. Furthermore, our models show that differential compaction, subsidence and lateral variations of the sediment supply result in apparently descending shelf-edge trajectories and onlapping clinoform surfaces that are often misinterpreted as base-level drops. By analyzing the sedimentary response to different climatic variations, we argue that there is no direct indication of a major base-level drop in the Drava Basin, which was much larger than the seismic resolution, i.e. 40–50 m. Modeling also infers that autoretreat and autocyclic variations are more effective at low sediment supply and higher amplitude lake-level variations.

Seismic geomorphology of three types of deepwater fans and their relationship with slope morphology: Qiongdongnan Basin, northern South China Sea

Analyses of high-resolution seismic reflection, drilling, and well-log data were combined to identify and investigate deepwater fans developed in the eastern part of the Qiongdongnan Basin, South China Sea, during the Middle Miocene and their relationship to slope morphology. We identified three types of deepwater fans that developed on the northern continental margin of the South China Sea during the Middle Miocene: (1) those consisting of small canyons and depositional lobes; (2) those consisting of large canyons and depositional lobes; and (3) those consisting of distributary channels without depositional lobes. Sourced from deltaic sediments deposited on the northern continental shelf margin, the lithologies of the sediments in all fans are dominated by siltstone. The scale of development and the internal features are mainly controlled by the slope topography. The deepwater fans consisting of small canyons and depositional lobes developed on slopes with low gradient, caused by the continuous progradation of the shelf-edge towards the basin centre due to sediment transport in high-speed flows. The canyon heads developed in this setting are located below the shelf-break; these canyons are narrower, shallower, and shorter than those of the deepwater fans consisting of large canyons. Thus, the scale of the depositional lobes deposited by gravity flow through such canyons is smaller than that of deepwater fans with large depositional lobes. Deepwater fans consisting of large canyons and depositional lobes developed on steep narrow slopes, caused by rising shelf-edge trajectories due to the equilibrium between the sediment supply and subsidence rates. The canyon heads developed in this setting are located upon the shelf-break and extend toward the toe of the slope, incising the shelf-break; these canyons are wider, deeper, and longer than those of the deepwater fans consisting of small canyons. The scale of the depositional lobes deposited by gravity flows transported through such canyons is correspondingly larger than that of deepwater fans with small depositional lobes. Deepwater fans consisting of distributary channels without depositional lobes developed on gentler and wider slopes than slopes where small canyons and depositional lobes formed, owing to the low sediment supply. The deepwater distributary channels have a long transportation distance and wider distribution area, enabling the formation of large-scale deepwater fans. Depositional models for all three types of deepwater fans were postulated.

Coupling Relationship between Shelf-Edge Trajectories and Slope Morphology and Its Implications for Deep-Water Oil and Gas Exploration: A Case Study from the Passive Continental Margin, East Africa

Both the shelf-edge trajectories and slope morphology are indicative of deep-water sedimentation, but previous studies are relatively independent from each other in the two dimensions. An integrated investigation can enhance the understanding of deep-water sedimentary systems and enrich reservoir prediction methods. Based on the bathymetry data and seismic data published, this study identified ten slope areas at the continental margin of East Africa and classified the clinoforms into three types: concave-up, sigmoidal and planar. Combined with the distribution of main modern rivers in East Africa, nine modern source-to-sink systems were identified and the catchment area is positively correlated with the size of the shelf-edge delta. It is found that the slope morphology of East Africa is closely related to the geological setting, sediment supply and sediment transport pathway in submarine canyon of passive continental margin. When the sediment supply is stable, the concave-up slopes are dominated by the river-associated and shelf-incising canyons and the sigmoidal slopes are determined by the headless canyons. There exists a strong coupling relationship between the shelf-edge trajectories and slope morphology. In general, concave-up slopes correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories and high-quality reservoirs developed on the basin floor under the influence of river-associated and shelf-incising canyons which have bright prospects for oil and gas exploration. Additionally, sigmoidal slopes usually correspond to descending trend, flat and low-angle ascending trend shelf-edge trajectories at times of relative sea-level fall and the reservoirs mostly developed on the upper slope under the influence of headless canyons. Moreover, the planar slopes correspond to high-angle ascending trend trajectories which are hardly potential for exploration. The coupling model built in this study will provide an insight for oil and gas exploration in deep-water areas with limited data and low exploration degree.

Alternating of aggradation and progradation dominated clinothems and its implications for sediment delivery to deep lake: The Eocene Dongying Depression, Bohai Bay Basin, east China

Abstract Sediment routing in deep-water settings can be significantly different in lacustrine basins from their well-studied marine counterparts, which has been largely overlooked during the past decades. The seismically well-imaged lacustrine clinothems in the Eocene Dongying Depression provide a good opportunity to bridge the deep-water sediment partitioning with clinothem architecture and shelf-edge trajectories, which have been studied through the integration of 3D seismic data and well-logging data. In the study area, the Eocene delta-complexes are architecturally manifested as alternating aggradation dominated (AC) and progradation dominated clinothems (PC). The Eocene Dongying clinothem architecture has been documented in terms of shelf–edge trajectory angle (θsht), edge-to-toe distance (ETT), aggradation height (Ah), progradation distance (Pd) and maximum foreset angle (γmfa). Aggradation dominated clinothems are characterized by ascending shelf-edge trajectories with thick and widespread sand-prone bottomsets, suggesting higher efficiency of sediment delivery into the outlying lake floor. Progradation dominated clinothems, in contrast, are characterized by flat to slightly descending trajectories and generally by a lack of bottomsets. Alternations of rising and flat to slightly descending trajectories accompanied with different clinothem types probably indicate frequent relative lake level changes (on time-scales of 100s of kyr). Further analysis of seismic images and extensive well-logs suggest the distal aggradational bottomsets are generally consist of sub-lacustrine channel-lobe systems, whereas sediment slide, slump blocks occur in slope to slope toe areas of progradation dominated clinothems, which indicate varied processes of deep-water sediment transport associated with different clinothem types and shelf-edge trajectories. Lake-level, sediment supply, water salinity and density change induced by humid-arid climate cycle are suggested to be main factors controlling clinothems stacking pattern and sediment dispersal into deep-lacustrine. Humid climate are considered to be related with strongly aggradational clinothems, during which sediment can be directly transported to the distant bottomset areas due to increased sediment concentration and energy in river discharge. Whereas during dry periods, progradation dominated clinothems start to build under stillstand or falling lake level conditions, sediment generally accumulates at river mouth and collapses due to sediment overload or fault activity. Therefore, a relationship between deep-lacustrine depositional process and climatic signature was established which may provide new thoughts in both the study of deep-water sequences and deep-water exploration in lacustrine basins.

Tight coupling between the cyclicity of deep-water systems and rising-then-flat shelf-edge pairs along the submarine segment of the Qiongdongnan sediment-routing system

ABSTRACTOn the basis of shelf-edge (SE) trajectory analysis, the present study demonstrates the tight coupling between the cyclicity of deep-water systems and contemporaneous SE trajectories with a paired rising-then-flat trajectory tendency (termed “SE pairs”) along the submarine segment of the mid-Pleistocene Qiongdongnan sediment-routing system, contributing to a better understanding of how to predict internal architecture and stacking patterns of deep-water systems. At the outlying deep-water reaches of the mid-Pleistocene Qiongdongnan sediment-routing system, Qiongdongnan deep-water systems are shown to have grown in a cyclic fashion that is stratigraphically manifested as the underlying mass-transport deposits (MTDs) systematically capped by submarine channels or sheet-like turbidites (i.e., MTD-channel and MTD-turbidite cycles, respectively). At the SE staging areas of the mid-Pleistocene Qiongdongnan sediment-routing system, Qiongdongnan shelf edges (SEs) have grown in a paired rising-then-flat fashion. The lower stratigraphic fill level of Qiongdongnan deep-water sedimentation cycles correlates to rising SE trajectories, during which the far shoreline to SE proximity (i.e., the long shoreline to SE distance of tens of kilometers) coupled to positive shelf accommodation [represented by positive SE trajectory angles () of 4.38° to 10.45°] most likely promoted passive sediment-transport agents and resultant MTDs. The upper stratigraphic fill level of mid-Pleistocene Qiongdongnan sedimentation cycles, in contrast, corresponds to flat SE trajectories, during which the close shoreline to SE proximity (i.e., the short shoreline to SE distance of &lt; 5 km) coupled to negative shelf accommodation (represented by of –0.17° to –1.32°), in contrast, favored active sediment-transport agents and resultant submarine channels or sheet-like turbidites.

The effects of differential compaction on clinothem geometries and shelf-edge trajectories

AbstractThe geometry of basin-margin strata documents changes in water depth, slope steepness, and sedimentary facies distributions. Their stacking patterns are widely used to define shelf-edge trajectories, which reflect long-term variations in sediment supply and relative sea-level change. Here, we present a new method to reconstruct the geometries and trajectories of clinoform-bearing basin-margin successions. Our sequential decompaction technique explicitly accounts for downdip lithology variations, which are inherent to basin-margin stratigraphy. Our case studies show that preferential compaction of distal, fine-grained foresets and bottomsets results in a vertical extension of basin-margin strata and a basinward rotation of the original shelf-edge trajectory. We discuss the implications these effects have for sea-level reconstructions and for predicting the timing of sediment transfer to the basin floor.

Development of the Middle Triassic Kobbe Formation shelf-margin prism and transgressive-regressive cycles on the shelf (Hammerfest Basin, SW Barents Sea)

Abstract The development and evolution of the middle Triassic shelf-margin prism of the southwestern Barents Sea has been so far under-researched. In particular, no attention has been paid to the thick transgressive packages developed at the top of the clastic prism. This study focuses on the Anisian-Early Ladinian Kobbe Formation, and it is based on a seismic and well log dataset from the Hammerfest Basin and adjacent Loppa High, as well as on a process-based investigation of available cores. 2D seismic lines show the development of shelf margin clinoforms up to 300 m thick, and shelf-edge trajectory analysis shows intervals with flat trajectories and intervals with ascending trajectories. The Kobbe Formation shelf-margin prism grew through repeated cross-shelf (regressive-transgressive) transits of deltaic systems. Mapping of the shelf-edge locations shows that the shelf-margin prism prograded ca. 30–50 Km in a north-westward direction, and localized protrusions of the shelf edges at different times can indicate cross-shelf sediment supply fairways. The farthest basinward extent of the sedimentary prism appears to have been controlled, at least in part, by the topographic relief of the paleo-Loppa High, as the bottomsets pinch out against this synsedimentary structural high. On the shelf, during regressive phases, the shoreline prograded from the south and from the east-southeast. This work recognizes, for the first time, the presence of thick transgressive packages within the Kobbe Formation. The transgressive sequence at the top of the Kobbe Formation is up to 50 m thick, and it is interpreted as stacked estuarine valley fills close to the Troms-Finnmark fault complex. In the Goliat area, the presence of an expanded sedimentary sequence, compared to adjacent areas, suggests a tectonic control in this area. Tectonic activity may have also influenced the development of stacked estuarine valleys in the same location, therefore controlling sand storage on the shelf.

From quantitative 3D seismic stratigraphy to sequence stratigraphy: Insights into the vertical and lateral variability of shelf-margin depositional systems at different stratigraphic orders

Abstract A major challenge in sequence stratigraphy is objectively identifying stratigraphic surfaces and sequences across multiple scales of observation. Identification is commonly dependent on the resolution of the data used (i.e., seismic vs. well data), its dimension (i.e., 1D vs. 2D vs. 3D) or the criteria chosen to select sequence boundaries. Through shelf-edge trajectory analysis, the clinothem (i.e., highest order seismic sequence identified on seismic data) can constitute the elementary building block of an observation-based and data-driven quantitative workflow to develop sequence stratigraphic frameworks across different orders and ranks of hierarchy. Here, we use high-quality 3D seismic data to interpret a Late Tithonian–Early Cretaceous shelf margin, the Lower Barrow Group (LBG), developed in the Northern Carnarvon Basin on the North West Shelf of Australia. Based on full-volume seismic interpretation techniques that integrate the 3D variability of the data when identifying seismic unconformities, a high-resolution seismic stratigraphic framework was built (73 interpreted clinothems with an average time duration of ~63,000 yrs). The computation of high-frequency shelf-edge trajectory angle (Tse) curves on selected seismic cross-sections is used to objectively pick sequence stratigraphic surfaces based on the accommodation succession method, thereby highlighting small changes in trajectory and proposing a method reproducible by interpreters based on the same quantitative data. Within the D. lobispinosum interval (142.3–140.9 Ma), the definition of stratigraphic sequences and composite stratigraphic sequences through this workflow is used to discriminate the controls at high and low temporal frequency on the vertical and lateral variability (which is here quantified) of this shelf-slope-basin system. The results show that the high-frequency interplay between short-term glacio-eustasy (i.e., Milankovitch eccentricity cycles of ~100,000 yrs) and sediment supply (locus of fluvial input along the margin) impacted the three-dimensional stratigraphic architecture of the LBG. In contrast, tectonic subsidence had a significant impact on the stratigraphic architecture of the LBG within the main depocentre at lower temporal frequency by overprinting the eustatic signal and accelerating/decelerating the rates of accommodation creation. However, identification of long-term glacio-eustatic Milankovitch cycles (~400,000 yrs) outside the main depocentre, where the rates of accommodation creation due to rift-related subsidence are moderate, also suggests low-frequency eustatic control. Therefore, the vertical and lateral variability of the LBG results from variations in sediment supply and subsidence regime under local (i.e., process regime, currents), regional (i.e., tectonics) and global (i.e., eustasy, climate) forcing parameters interplaying across timescales. In contrast to standard sequence stratigraphic workflows that are based on model-dependent choices to select sequence boundaries, quantitative 3D seismic stratigraphy constitutes an improved method to interpret 3D seismic data in shelf-margin depositional systems within a sequence stratigraphic framework, which provides an observation-based and model-independent tool allowing the definition of stratigraphic sequences with results that are reproducible across multiple stratigraphers. This work highlights the need for developing new sequence stratigraphic tools and methods that integrate the 4D variability of depositional systems and moves beyond the two-dimensionality inherent to current sequence stratigraphic methods. Quantitative 3D seismic stratigraphy represents a first step towards the creation of 3D sequence stratigraphic workflows that could improve the prediction of stratigraphic patterns and facies relationships (source, reservoir, seal distribution) across shelf margins.

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.

Quantitative stratigraphic analysis in a source-to-sink numerical framework

Abstract. The sedimentary architecture at continental margins reflects the interplay between the rate of change of accommodation creation (δA) and the rate of change of sediment supply (δS). Stratigraphic interpretation increasingly focuses on understanding the link between deposition patterns and changes in δA∕δS, with an attempt to reconstruct the contributing factors. Here, we use the landscape modelling code pyBadlands to (1) investigate the development of stratigraphic sequences in a source-to-sink context; (2) assess the respective performance of two well-established stratigraphic interpretation techniques: the trajectory analysis method and the accommodation succession method; and (3) propose quantitative stratigraphic interpretations based on those two techniques. In contrast to most stratigraphic forward models (SFMs), pyBadlands provides self-consistent sediment supply to basin margins as it simulates erosion, sediment transport and deposition in a source-to-sink context. We present a generic case of landscape evolution that takes into account periodic sea level variations and passive margin thermal subsidence over 30 million years, under uniform rainfall. A set of post-processing tools are provided to analyse the predicted stratigraphic architecture. We first reconstruct the temporal evolution of the depositional cycles and identify key stratigraphic surfaces based on observations of stratal geometries and facies relationships, which we use for comparison to stratigraphic interpretations. We then apply both the trajectory analysis and the accommodation succession methods to manually map key stratigraphic surfaces and define sequence units on the final model output. Finally, we calculate shoreline and shelf-edge trajectories, the temporal evolution of changes in relative sea level (proxy for δA) and sedimentation rate (proxy for δS) at the shoreline, and automatically produce stratigraphic interpretations. Our results suggest that the analysis of the presented model is more robust with the accommodation succession method than with the trajectory analysis method. Stratigraphic analysis based on manually extracted shoreline and shelf-edge trajectory requires calibrations of time-dependent processes such as thermal subsidence or additional constraints from stratal terminations to obtain reliable interpretations. The 3-D stratigraphic analysis of the presented model reveals small lateral variations of sequence formations. Our work provides an efficient and flexible quantitative sequence stratigraphic framework to evaluate the main drivers (climate, sea level and tectonics) controlling sedimentary architectures and investigate their respective roles in sedimentary basin development.

Shelf-margin architecture and shoreline processes at the shelf-edge: Controls on sediment partitioning and prediction of deep-water deposition style

The Lower Barrow Group (LBG; Latest Tithonian – Early Valanginian) is a shelf-margin that prograded during a late phase of rifting under various subsidence regimes and supply-dominated conditions. A 3D semi-automatic, full-volume seismic interpretation method allow identifying high-order clinothems presenting an estimated cyclicity of ~40,000 yrs, in which a quantitative analysis of the shelf-margin architecture and shorelines processes was conducted. Overall, three and four main types of hydrodynamic regimes and deep-water systems were identified, respectively. Falling to flat shelf-edge trajectories are associated with sediment bypass, whereas rising shelf-edge trajectories are linked with increasing sediment storage on the shelf. While fluvial to wave processes can be dominant in all A/S conditions, results show that fluvial-dominated coastlines are associated with steep high-angle slope clinoforms and short to longer run-out turbidites. Conversely, wave-dominated coastlines are linked to low-angle slope clinoforms and poor turbidite system development (occasional sheet sand and MTDs). The short and longer run-out turbidite systems present a tripartite architecture (canyon / slope valley; channel; lobes) which mostly appear as short-lived, vertically / laterally stacked elements fed my multiple small rivers forming linear ramp systems. Due to the shallow configuration of the margin (<500m), the presence of short slopes and overall high sand-to-mud ratio, the turbidite systems are smaller scale (<50 km) and probably shorter lived than most modern turbidite systems (100-1000 km). This study sheds new lights on the significant role of shelf-margin architecture (slope gradient, hydrodynamic regime) in predicting the deep-water sediment delivery behavior (sediment partitioning, type of deep-water system).

The Late Pleistocene Po River lowstand wedge in the Adriatic Sea: Controls on architecture variability and sediment partitioning

Abstract Although facies and stratal geometries of continental margin successions can be defined in detail based on subsurface and outcrop studies, most studies lack the high-resolution age control needed to constrain the time scale of formation of such successions and infer their external forcing mechanisms. Our work on the Po River Lowstand Wedge (PRLW) indicates that deposition rates are surprisingly high with the entire 350-m-thick succession being deposited in less than 17,000 years, and with individual clinothems recording time periods ranging from 400 to 4700 years. The PRLW preserves a high-resolution record of stacked, deltaic shelf-edge clinothems deposited during the Last Glacial Maximum (31.8–14.4 ky BP) in the Adriatic basin (Mediterranean Sea). We investigated clinothem internal geometry, stacking patterns, and facies distributions to infer the main controls on their growth by integrating seismic reflection data with seismic facies attributes and paleoenvironmental proxies. The stratigraphic framework of the shelf-edge clinothems was then related to major paleoenvironmental shifts during the last glacial cycle and driven by eustatic and climatic changes. Within the PRLW, we recognized three distinctive types of 100's-m-high shelf-edge clinothems, type A, type B and type C, each with diagnostic topset geometries, shelf-edge trajectories, and associated distal basin-fill deposits. These elemental clinothem types stack into two Clinothem Sets. Clinothem Set 1, with essentially flat to slightly descending shelf-edge trajectory, is composed of stacked types A and B clinothems, and records the direct influence of river flux leading to dysoxic conditions on the bottom of the basin. In particular, clinothem accumulation rates were as much as 200 km3/ky in some of the type B clinothems. Clinothem Set 2, showing ascending shelf-edge trajectory, records an aggradational stacking coupled with a retreat of the river-entry points with benthic fauna assemblages that reflect the influence of peaks in freshwater discharge. Whereas Clinothem Set 1 developed under perturbations of river supply linked to the multi-scale waxing and waning of glaciers during an interval dominated by eustatic fall, Clinothem Set 2 reflects the main thawing of glaciers concomitant to the first phase of the eustatic rise. From a sequence stratigraphic perspective, Clinothem Set 1 is interpreted as staked high-frequency sequences, while Clinothem Set 2 represents a stack of high-frequency parasequences. The high-resolution age control from boreholes and seismic data enabled us to relate stratal character to independently constrained environmental proxies: this revealed how the evolution of a shelf-edge system intricately convolves the influences of both global (eustacy) and regional (climate-driven supply fluctuations) controls, both at sub-Milankovitch scales. Finally, the thickness, geometry, and stacking patterns of the centennial to millennial clinothems of the PRLW vary in systematic ways resulting in geometries that closely resemble those of ancient shelf-edge systems, and offering the PRLW as a sub-modern analogue. Our observations also reinforce the focus of the classic sequence-stratigraphic approach on analyzing surfaces and their geometric relations and not on time duration or formation mechanisms.

Quantitative analysis of siliciclastic clinoforms: An example from the North Slope, Alaska

Abstract Seismically-imaged clinoforms, constrained by well data, provide a unique opportunity to evaluate sedimentary processes using geometric considerations. The Lower Cretaceous of the Alaskan North Slope is a well-known example of large-scale (600 - 2500 m depositional relief), constructional, siliciclastic clinoforms. This study has depth-converted and decompacted a regional seismic profile in this area to evaluate the relationships among depositional slope angles, shelf-edge trajectories, seismic facies, sediment volume ratios, graded/erosional profiles, and lithologies. Insights from this analysis refute several assumptions about clinoform formation, and could help establish a quantitative methodology for clinoform analysis. Few published examples have been decompacted to estimate original depositional relief. In this study, seismically-imaged slopes of about 1.5° - 3.5° have approximately doubled in magnitude after backstripping, owing to their fine-grained nature and deep burial history. These slopes exceed the largest published estimates of constructional (“graded”) margins. North Slope clinoforms that show more evidence of slope erosion and bypass (out-of-grade) have, on average, lower slope angles than graded clinoforms. This observation is contrary to the assumption that higher slope angles correlate with more extensive erosion. In this study, we hypothesized that erosional modification has lowered the clinoform slope. On average, erosionally-modified clinothems have lower angle shelf-edge margin trajectories and thinner topsets relative to foresets/bottomsets. Additionally, erosionally-modified clinoforms are characterized by higher amplitude seismic facies. Well calibration indicates that the higher amplitude response results from interbedded sandstones and shales; low amplitude seismic facies, associated with graded margins, are more mud-prone. Out-of-grade clinoforms likely facilitate additional bypass/deposition of sediment downslope via sediment gravity flows. Flatter shelf margin trajectories indicate sediment supply exceeds accommodation; this may drive greater slope erosion and bypass. A predictive model, employing these clinoform geometric criteria, can assist in delineating intervals of enhanced bypass/submarine fan development. However, wide applicability beyond this example would require additional calibration and further testing.

Shelf-edge trajectory and sediment dispersal in a lacustrine setting: A case study from Qinnan Depression, Bohai Bay Basin, China

The Oligocene succession of Dongying Formation in Qinnan depression in Bohai Bay Basin has preserved well-imaged lacustrine prograding shelf-edge scale clinoforms, providing a good opportunity to study the relationship between shelf-edge trajectory, stratal stacking patterns and sediment dispersal, and to investigate the effect of controlling factors on sediment dispersal shelf-edge trajectories in a lacustrine setting. 3D seismic volumes were used for defining seismic units, bounding surfaces, shelf-edge trajectories and sediment dispersal. Four seismic units have been subdivided, and four types of shelf-edge trajectories and the associated stratal stacking patterns have been recognized. Greater volume of sandstone in deep-water area tends to develop in seismic units with descending shelf-edge trajectory angles and tends be linked to lower shelf-edge trajectory angles. Differential tectonic subsidence rates along the shelf and along-strike variation in sediment supply were prime factors controlling lateral variability in trajectory, progradation and sediment dispersal. The evolution of shelf-edge trajectory can be subdivided into three stages. Local climate might play a major role in controlling lake-margin evolution during the first two stages, when dry hot weather foster basin floor fan deposition and descending trajectories, and humid cold weather hindered the development of basin floor fan and forms ascending trajectories. While differential tectonic subsidence could have the dominate role during the third stage, when climate signal can hardly be recognized and lateral variability in trajectory angles and sediment supply is most obvious.