Delta Top Research Articles

Marsh-induced backwater: the influence of non-fluvial sedimentation on a delta's channel morphology and kinematics

Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta.

The age and depositional environments of the lower Karoo Moatize Coalfield of Mozambique: insights into the postglacial history of central Gondwana

The Moatize Coalfield belongs to a network of continental Karoo basins of central Mozambique, known as the Zambezi Basin. Palynological and sedimentological studies were performed on four coal exploration boreholes to determine the age, depositional settings, and overall geological evolution of its extensive coal deposits. Clastic formations recognised in this coalfield, in ascending order, are the Vúzi, Moatize and Matinde formations. Palynomorph assemblages indicate that the Moatize Coalfield succession ranges from Roadian (lower Guadalupian) to Changhsingian (upper Lopingian) in age. Two main depositional phases are identified, whose initiation and development are attributed to regional tectonic events and climate amelioration. The first phase formed towards the end of the deglacial period, characterised initially by fan deltas, represented by the upper Vúzi Formation, and the shift to lake–delta environments, represented by the lower part of the Moatize Formation. This phase took place from Roadian to the Wordian times. The lake–delta settings indicate a sediment aggradation trend with high subsidence rates in the lake basin, which, together with the associated post-glacial climate amelioration, led to the accumulation of coal deposits in swamps of the delta top and lake margins. The second depositional phase took place from Capitanian to Changhsingian times and related to fluvial environments initiated by uplift that reorganised the depocenter into alluvial plains characterised by bedload dominated rivers (braided) and overbank floodplains. Results obtained in this study provide critical information for the onset of the deglaciation events and the age of coal deposits in this part of Gondwana, important for wider stratigraphic correlation of these events in Africa and throughout the Gondwana.

The impact of clastic syn‐sedimentary compaction on fluvial‐dominated delta morphodynamics

AbstractIn natural deltaic settings, mixed hydrodynamic forcings and sediment properties are known to influence the preserved delta deposits. One process that has not received much attention yet is syn‐sedimentary compaction of clastic sediment on millennial‐scale delta evolution. To study how compaction interacts with delta morphodynamics and preserved sediment, a modelling approach is proposed. A 1D grain‐size dependent compaction model was implemented into Delft3D‐FLOW, which provides an opportunity to understand the underexplored connection between grain sizes supplied to the deltas and sediment compaction. The compaction model allows deposited sediment to decrease in volume due to the accumulation of newly deposited sediments above or the elapsed time. Differences in morphological trends are presented for scenarios defined by the composition of sediment supply (mud rich and sand rich) and the maximum allowed compaction rate in the model (0–10 mm year−1). The resultant deposits are classified into sub‐environments: delta top, delta front and pro delta. The delta top geometry (e.g. area increase, rugosity and aspect ratio), sediment distribution alongshore and across sub‐environments, and delta top accommodation (e.g. volume reduction and average water depth) are compared. The modelling results show that compaction of the underlying delta front and pro delta deposits increases the average water depth at the delta top, driving morphological variability observed in the mud‐rich and sand‐rich deltas. The morphological changes are more prominent in the mud‐rich deltas, which experience larger compaction‐induced volume reduction for the same scenario. Moreover, higher compaction rates further increase the delta top accommodation, resulting in more deposition and evenly distributed sediment at the delta top. This leads to a less significant area increase and a wider delta top with a smoother coastline. The presented modelling results bridge the knowledge gap on the influence of syn‐sedimentary compaction on long‐term delta morphodynamics and preserved sediment. These findings can be applied to unravel the controlling processes in ancient delta deposits and predict the evolution of modern systems under changing climates.

Sharp‐based shoreface successions reconsidered in three‐dimensions: A forward stratigraphic modelling perspective

AbstractSea‐level fall is commonly inferred to generate a sharp‐based shoreface succession that displays an abrupt vertical transition from heterolithic, lower shoreface to sandy, upper shoreface deposits across a marine erosion surface. Three‐dimensional, process physics‐based, coupled hydrodynamic‐morphodynamic models are constructed to compare bedding architecture and facies patterns of wave‐dominated delta deposits preserved during normal (static sea level) and forced (falling sea level) regression and then transgression during subsequent sea‐level rise. The models suggest that wave‐dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave‐scoured subaqueous delta top, which is laterally separated from a delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual‐clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion. Significant grain‐size contrasts between these vertically stacked clinoform deposits reflect differences in sediment‐transport directions and sorting under river and wave‐driven littoral currents along the coast, and cannot be related uniquely to sea‐level changes. The marine erosion surface under a sharp‐based shoreface deposit records abrupt facies shift across a kilometres‐wide, wave‐eroded surface and defines a discontinuity in the preserved vertical succession. The continuity of a regressive surface of marine erosion mapped over many tens to hundreds of kilometres across mid‐shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The results of these models suggest caution in inferring sea‐level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures.

Assessment of contamination of the estuarine area of the Northern Dvina River by calculating background concentrations (Fe, Mn, Zn, Cu, Cd, Pb, and Ni)

This publication is a continuation of research on the quantitation of the heavy metal contamination level (Fe, Mn, Zn, Cu, Cd, Pb, and Ni) in the ecosystem components of the estuarine area of the Northern Dvina River. To assess the contamination level with these metals in the ecosystem of the Northern Dvina estuarine area in the spot of the highest anthropogenic load (the area of Novodvinsk – Arkhangelsk – Severodvinsk urban agglomeration), we use the method of comparative analysis of the study object with the background plot – the river section above urban development. Using spatial mapping of the enrichment factor values for the aquatic ecosystem components, areas with abnormal excess of metal content in mollusc tissues, bottom sediments, and bottom water layer were determined. As established, the area adjacent to the city center and to an industrial zone on the left bank is subject to the highest anthropogenic load in the water area of the river delta top. The most likely sources of contamination with the considered heavy metals are the inflow of surface wastewater (meltwater and rain storm runoff), untreated municipal wastewater, and direct short-range atmospheric transport. The analysis of mollusc tissues proved to be a more informative and indicative approach to the assessment of the heavy metal contamination in water areas with complex hydrological and hydrochemical environmental gradients than the analysis of bottom water layer and bottom sediments.

Bayhead delta evolution in the context of late Quaternary and Holocene sea-level change, Richards Bay, South Africa.

Richards Bay is part of a back-barrier lagoon fronted by high coastal dunes on the NE, Indian Ocean coast of South Africa. In the early 1970s, a berm was constructed, dividing the original Mhlathuze Estuary into two separate systems; the Richards Bay Harbour and the new Mhlathuze Estuary. This study investigates the stratigraphic evolution of the incised valley system and bayhead delta in the Richards Bay Harbour segment. Seven seismic units (Units 1–7) were imaged. A single regionally developed sequence boundary (SB) along with two tidal ravinement surfaces (tRS1 and tRS2) were identified. Surface SB is associated with the LGM lowstand which developed when sea levels were ~ 130 m below present, until ~18,000 year BP. Cretaceous age siltstones (Unit 1) form the basement. Transgressive material overlying SB (Unit 2) reflects the filling of an incised valley located in the middle segment of a wave-dominated back-barrier system. It is overlain by a bayhead delta (Unit 3), the geometry and seismic signature of which indicate alternating periods of aggradation/progradation and backstepping. The behaviour is attributed to episodic jumps in sea-level, and is tentatively (on the basis of elevations in relation to the regional sea-level curve) linked to periods of rapidly rising sea-level (8.2 ka event and Meltwater Pulse (MWP)-1d). These intervals of rapidly rising sea-level, combined with relatively low gradient settings facilitated backstepping of the delta. Fills (Unit 4) occur within minor incisions along the delta top. These are interpreted as distributary channels that fed sediment to the seaward edge of the bayhead delta system. Elongated mounds on the seafloor (Unit 5) are interpreted as spoil from contemporary port dredging. Slump deposits (Unit 6) along the delta front are attributed to a combination of oversteepening of the delta by dredging, as well as deposition of modern sediments brought into the system by tidal currents. The system is capped by fine-grained, tidally redistributed and deposited sediments (Unit 7) which were possibly sourced from older organic material of an indeterminate source. This site is especially sensitive to episodic rates of sea-level change due to the relatively small Glacial Isostatic Adjustments (GIA) during the postglacial transgression and the flat antecedent gradients of both the subaerial unconformity and the overlying tidal ravinement.

Sedimentary facies of the Mesoproterozoic Srisailam Formation, Cuddapah basin, India: Implications for depositional environment and basin evolution

Sedimentary facies of the Mesoproterozoic (~1400−1327 Ma) Srisailam Formation, exposed in the Srisailam sub-basin in the northern part of the Cuddapah basin, India, were analyzed. It helped in identifying eighteen different facies that can be grouped into six facies associations (FA-I to VI) – alluvial fan/fan-delta, delta top, delta front, lacustrine, fluvial, and aeolian. Hinterland geology, integrated palaeomagnetic, geochemical and sedimentological observations, and inferred palaeohydrological condition and palaeoclimate indicate more control of tectonic regime on the evolution of the facies architecture. Analysis of vertico-lateral disposition of the identified facies suggests that the deposition of the Srisailam sediments was initiated in a number of fault-controlled isolated depocenters (riftogenic lakes). However, those isolated lakes were deep enough to allow the evolution of lacustrine and deltaic depositional regimes. Basin-wide lacustrine environment was established during subsequent rifting and regional peak subsidence (rift-climax) of the basin floor. Cessation of further basin subsidence facilitated progradation of fluvial sediments. During low-stage fluvial sediments were reworked locally by aeolian processes, and during protracted dry periods, aeolian regimes were established over considerable stretches of the Srisailam sub-basin.It is likely that the Srisailam half-graben, adjacent to the Nallamalai Fold Belt, was developed due to lithospheric doming and limited crustal stretching during a mantle upwelling event responsible for lamproite (alkaline) magmatism in the northern part of the Cuddapah basin. The evolution of the Srisailam rift-basin may be compared with the Cenozoic rift system of Europe, where rift development took place simultaneously with the orogenic activity in the Alps and Pyrenees. The style of sedimentation in the Srisailam sub-basin may be considered as a variant of the Newark-type lacustrine facies complex, and the tripartite basin filling model of half-graben lacustrine complexes may possibly be extended to the Mesoproterozoic era.

Contrasting facies patterns between river-dominated and symmetrical wave-dominated delta deposits

ABSTRACT Process-physics-based, coupled hydrodynamic–morphodynamic delta models are constructed to understand preserved facies heterogeneities that can influence subsurface fluid flow. Two deltaic systems are compared that differ only in the presence of waves: one river dominated and the other strongly influenced by longshore currents. To understand an entire preserved deltaic succession, the growth of multiple laterally adjacent delta lobes is modeled to define delta axial to marginal facies trends through an entire regressive–transgressive depositional succession. The goal is to refine a facies model for symmetrical wave-dominated deltas (where littoral drift diverges from the delta lobe apex). Because many factors change depositional processes on deltas, the description of the river-dominated example is included to provide a direct reference case from which to define the impact of waves on preserved facies patterns. Both systems display strong facies trends from delta axis to margin that continued into inter-deltaic areas. River-dominated delta regression preserved a dendritic branching of compensationally stacked bodies. Transgression, initiated by sea-level rise, backfilled the main channel and deposited levees and splays on the submerging delta top. Wave-dominated deltas developed dual clinoforms: a shoreface clinoform built as littoral drift carried sediment away from the river month and onshore, and a subaqueous delta-front clinoform composed of sediment accumulated below wave base. Although littoral drift in both directions away from the delta axis stabilized the position of the river at the shoreline, distributary-channel avulsions and lateral migration of river flows across the subaqueous delta top produced heterogeneities in both sets of clinoform deposits. Separation of shoreface and subaqueous delta-front clinoforms across a subaqueous delta top eroded to wave base produced a discontinuity in progradational vertical successions that appeared gradual in some locations but abrupt in others. Littoral drift flows away from adjacent deltas converged in inter-deltaic areas, producing shallow water longshore bars cut by wave-return-flow channels with associated terminal mouth bars. Transgression initiated by sea-level rise initially led to vertical aggradation of wave-reworked sheet sands on the subaqueous delta top and then retreating shoreface barrier sands as the subaerial delta top flooded. Pseudo inter-well flow tests responded to local heterogeneities in the preserved deposits. As expected, abandoned channels in the river-dominated case defined shoreline-perpendicular preferential flow paths and wave-dominated delta deposits are more locally homogeneous, but scenarios for development of more pronounced shore-parallel heterogeneity patterns for wave-influenced deltas are discussed. The results highlight the need to consider the dual clinoform nature of wave-dominated delta deposition for facies prediction and subsurface interpretation.

Response of a wave-dominated coastline and delta to antecedent conditioning and fluctuating rates of postglacial sea-level rise

In the context of rising sea levels, wave-dominated coasts and deltas are among the most threatened of coastal landforms. The study of submerged deltas, in tandem with other drowned wave-dominated shoreline sequences, can shed light on the long-term coastal behaviour of these threatened shorelines when subject to high-end scenarios of relative sea-level rise. Using ultra high-resolution seismic reflection data and gravity core samples, this paper investigates the post-glacial stratigraphic development and architecture of the shelf off southern Mozambique. We document several well-preserved beachrock/aeolianite palaeo-shoreline complexes from water depths of −100, −75 and −60 m, interpreted as representing the positions of former sea-level slowstands or stillstands. These overlie the regional subaerial unconformity associated with the last glacial maximum and are linked to two phases of deltaic deposition. Pro-deltaic deposits on the outer shelf were linked with the period of sea-level stability attributed to the −100 m shoreline, while proximal delta front deposits on the inner- to mid-shelf are linked to a slowstand evident in the local sea level record at −40 m. Well-developed lower delta plain facies overlie both shoreline and delta front facies and show evidence of strong channelisation during normal regression of the delta. These are overlain across the shelf by isolated and stranded pieces of the now relict shoreface, trapped to seaward by the shoreline complexes. The more proximal parts of the shelf are in turn overlain by fluvial-derived muddy facies of the current prodeltas of the area.The preservation of shoreline and delta deposits was enhanced by intervening rapid increments in sea-level rise associated with meltwater pulses 1A (−100 m) and 1B,(−60 m) though the preservation of the −75 shoreline feature, in spite of modest rates of Older Dryas base-level change, points toward other factors such as rapid cementation as dominant in this instance. The region's high sediment supply exerted a primary control on shelf preservation by dampening the erosive extent of wave-ravinement. Variations in antecedent gradient are responsible for much of the documented along-shelf and down-dip depositional variability: low-gradient settings promoted the development of broad coastal plains and strongly wave-dominated deltaic facies, while steeper areas of the shelf favoured the development of coastal landforms in relatively narrow complexes fronted by deep-water deltaic deposits. Steeper areas of the shelf also tend be characterised by an embayed along-coast profile that locally mitigates wave-energy, reinforcing the influence of inherited gradient.Once preserved, drowned coastal barriers introduced an additional geological control as post-glacial sediment became trapped between these features on the outer shelf. The dispersal of modern sediments is strongly influenced by the hydrodynamic regime Delagoa Bight, under which fluvial muds are transported northeastward along the shelf by a persistent inner-mid shelf counter-current. A considerable proportion of the modern sediment budget is lost off-shelf where the shelf narrows.This study shows that low antecedent gradients foster gentle coastal profiles predisposed to delta overstepping. Though local topographic knickpoints can aid in preserving the delta during submergence, they do not buffer the delta from the general effects of inundation which include drowning of the delta top. Once overstepped, the former delta can be overlain by distal deltaic sediments associated with the higher sea level and associated fluvial and oceanographic forcing quite different from earlier phases of delta construction.

Predicting river mouth location from delta front dip and clinoform dip in modern and ancient wave‐dominated deltas

AbstractWave‐dominated deltas and strandplains make up the majority of the world’s depositional coastlines, provide an important record of sea‐level change and serve as hydrocarbon reservoirs worldwide. Satellite imagery forms a great source of data on the recent depositional history of modern deltaic systems. In the subsurface, three‐dimensional seismic and well data make the three‐dimensional assessment of large‐scale deltaic reservoir bodies possible but struggle to resolve internal heterogeneities away from wells. To bridge this gap in characterizing deltaic sedimentation, this study combines measurements from both the shallow, high‐resolution section of three‐dimensional seismic data of the Eocene Halibut Delta in the Outer Moray Firth, offshore Scotland, with information from Google Earth’s satellite imagery and digital elevation model on south‐east Brazilian river deltas (São Francisco, Jequitinhonha, Doce and Paraíba do Sul) to present a means of predicting the location of fluvial sediment input points with respect to clinoform geometry. The key measurement for this study is the delta front and clinoform dip which has been measured at multiple locations along strike of the coastline of the examined deltas. Dip decreases away from the inferred river mouth for all deltas by 50% within 7.2 km. The river mouth location was inferred from the position of palaeo‐channels visible on the delta top and coarse sediment recorded in grab samples offshore for the south‐east Brazilian deltas, and from imprints of palaeo‐channels on attribute maps for the Eocene Halibut Delta. In summary, this study found that delta front dip is steepest at the location of the river mouth and decreases, along with grain size, away from it. This suggests that high dip values correlate with the proximity to the channel mouth and can be used to predict fluvial channel facies in modern deltaic systems and subsurface reservoirs.

Growth‐faults from delta collapse – structural and sedimentological investigation of the Last Chance delta, Ferron Sandstone, Utah

AbstractInvestigations of syn‐sedimentary growth faults in the Last Chance delta (Ferron Sandstone, Utah, USA) show that fault‐bounded half‐grabens arrested high amounts of sand in the mouth bar and/or distributary channel areas. Fault‐controlled morphology causes changes in routing of the delta top to delta front drainage towards the long axis of half‐grabens. Faulting was spatially and temporally non‐systematic, and polyphase, with 3D cusp/listric fault geometries instigated by linkage of variously oriented segments. Hanging wall rollover folds consisting of wedge‐shaped syn‐kinematic sand attest to rapid <1‐m slip increments on faults followed by mild erosion along crests of fault blocks and sedimentary infill of adjacent accommodation. Triangle‐zones in prodelta to delta front muds are located underneath steeper faults and interconnected rotated fault‐flats. Their geometry is that of antiformal stack duplexes, in an arrangement of low‐angle‐to‐bedding normal faults at the base, replaced by folded thrusts upwards. These faults show a brittle, frictional flow deformation mechanism ascribed to early compaction of mud. For syn‐kinematic sand, there is a change from general granular/hydroplastic flow in shear zones to later brittle failure and cataclasis, a transition instigated by precipitation of calcite cement. Extensional faulting in the Last Chance delta was likely controlled by gravity driven collapse towards the delta slope and prodelta, as is commonly observed in collapsing deltas. The trigger and driving mechanism is envisioned as localized loads from sand deposited within distributary channels/mouth bars and fault‐controlled basins along the delta top. A regional tilt and especially displacement of compacted mud below sand bodies towards less compacted muds also contributed to the faulting.

Heavy metal flows in aquatic systems of the Don and Kuban river deltas

This paper presents the calculated heavy metal (Fe, Mn, Zn, Ni, Cu, Cr, Co, Cd, and Pb) flows in suspended and dissolved forms in the main navigable branches of the Don and Kuban river deltas during the low-water period of 2013–2014. This work is based on the data of field studies in which water and suspended matter samples were collected and the turbidity and water discharge in deltas were measured. A quantitative estimate of heavy metal inflows into the deltas of the Don and Kuban rivers is provided. Transformation of flows of suspended and dissolved metal forms from the delta top to the sea edge is discussed. The influence of localities (Rostov-on-Don, Temryuk) on the increase in heavy metal flows downstream is shown, and the heavy metal flows in the deltas of the Don and Kuban rivers are compared.

Subsurface structure of water–gas escape features revealed by ground-penetrating radar and electrical resistivity tomography, Glen Canyon National Recreation Area, Lake Powell delta, Utah, USA

Data gathered by electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to produce a three-dimensional image of subsurface soft-sediment deformation structures developed on the modern Lake Powell delta at Hite, Utah. ERT and GPR lines were run orthogonal across the crater. ERT resolved a low-resistivity layer up to 2m thick in the area near the vents within the crater. This low-resistivity layer thins toward the margins representing clays ejected from the vents. Below and adjacent to this layer is a high-resistivity layer that reflects delta top and pro-delta sands. The deepest zone recognized in the ERT profiles consists of a low-resistivity layer, clay deposits that accumulated during the maximum lake high stand. This clay zone is connected to the vent within the crater by a conduit that changes diameter vertically. GPR profiles recognized the presence of collapse features restricted to the proximity of the vent. The geometry of the model is consistent with those proposed for marine pockmarks that can be generated seismically or aseismically with the exception of subaerial exposure after the dome stage development.

Delta channel networks: 1. A graph‐theoretic approach for studying connectivity and steady state transport on deltaic surfaces

AbstractRiver deltas are intricate landscapes with complex channel networks that self‐organize to deliver water, sediment, and nutrients from the apex to the delta top and eventually to the coastal zone. The natural balance of material and energy fluxes, which maintains a stable hydrologic, geomorphologic, and ecological state of a river delta, is often disrupted by external perturbations causing topological and dynamical changes in the delta structure and function. A formal quantitative framework for studying delta channel network connectivity and transport dynamics and their response to change is lacking. Here we present such a framework based on spectral graph theory and demonstrate its value in computing delta's steady state fluxes and identifying upstream (contributing) and downstream (nourishment) areas and fluxes from any point in the network. We use this framework to construct vulnerability maps that quantify the relative change of sediment and water delivery to the shoreline outlets in response to possible perturbations in hundreds of upstream links. The framework is applied to the Wax Lake delta in the Louisiana coast of the U.S. and the Niger delta in West Africa. In a companion paper, we present a comprehensive suite of metrics that quantify topologic and dynamic complexity of delta channel networks and, via application to seven deltas in diverse environments, demonstrate their potential to reveal delta morphodynamics and relate to notions of vulnerability and robustness.

The “butterfly delta” system of Qiongzhou Strait: Morphology, seismic stratigraphy and sedimentation

Abstract Analysis of high resolution sub-bottom profiles, topographical data and sediment samples reveals that the tidal delta system of Qiongzhou Strait is morphologically a “butterfly delta” system consisting of Western Mouth Tidal Delta (WMTD) and Eastern Mouth Tidal Delta (EMTD). WMTD covers an area of about 11,000 km 2 , and is more than two times larger than EMTD (4000 km 2 ). WMTD is fan-shaped with a tributary branch extending southwards along Hainan Island, while EMTD is typically fan-shaped. Seismic architectures of WMTD and EMTD are both characterized by a thick progradational unit overlying a thin transgression unit. Both WMTD and EMTD start to develop at about 8.7 cal ka BP when Qiongzhou Strait completely opened, and have the same three sedimentary facies: tidal delta top, tidal delta slope and tidal delta toe. The formation of WMTD and EMTD is attributed to Qiongzhou Strait's hydrodynamic regime which is dominated by the strong east–west reciprocal-tidal currents and influenced by the year-round westward mean flow modified by the wind-driven seasonal currents.

Comparison between experimental and numerical stratigraphy emplaced by a prograding delta

Abstract. A one-dimensional model that is able to store the stratigraphy emplaced by a prograding delta is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e., a condition in which the stratigraphy emplaced by the delta front is entirely stored in the deposit. The migration of the delta front and the deposition on the delta top are modeled with total and grain-size-based mass conservation models. The vertical sorting on the delta front is modeled with a lee-face-sorting model as a function of the grain size distribution of the sediment deposited at the brinkpoint, i.e., at the downstream end of the delta top. Notwithstanding the errors associated with the grain-size-specific bedload transport formulation, the comparison between numerical and experimental results shows that the model is able to reasonably describe the progradation of the delta front, the frictional resistances on the delta top, and the overall grain size distribution of the delta top and delta front deposits. Further validation of the model in the case of variable base level is currently in progress to allow for future studies, at field and laboratory scale, on how the delta stratigraphy is affected by different changes of relative base level.

The role of climate variation in delta architecture: lessons from analogue modelling

AbstractSequence‐stratigraphic models for fourth to sixth order, glacio‐eustatic sequences based only on relative sea‐level variations result in simplified and potentially false interpretations. Glacio‐eustatic sea‐level variations form only one aspect of cyclic climate variation; other aspects, such as variations in fluvial water discharge, vegetation cover, weathering and sediment supply can lead to variable sediment yield, thus adding complexity to sequence‐stratigraphic patterns normally attributed to sea‐level variations. Analogue flume models show a significant impact of water discharge on the timing and character of sequence boundaries, and on changes in the relative importance of systems tracts, as expressed in sediment volumes. Four deltas, generated under the influence of an identical sea‐level curve, and affected by different water‐discharge cycles were generated in the Eurotank facility: (1) constant discharge; (2) high‐frequency discharge variations (HFD); (3) discharge leading sea level by a quarter phase; (4) discharge lagging sea level by a quarter phase. HFD shift the parasequence stacking pattern consistently but do not alter large‐scale delta architecture. Water‐discharge changes that lead sea‐level changes result in high sediment yield during sea‐level rise and in the poor development of maximum flooding surfaces. Delta‐front erosion during sea‐level fall is expressed by multiple, small channels related to upstream avulsions, and does not result in an incised valley that efficiently routs sediment to the shelf edge. When water‐discharge changes lag sea‐level changes, sediment yield is high during falling sea level and results in rapid progradation during forced regression. Erosion from incised valleys is strong on the proximal delta top and dissipates towards the delta front. The combination of high discharge and sea‐level fall provides the most efficient mode of valley incision and sediment transport to the shelf edge. During sea‐level rise, low water discharge results in sediment starvation and well‐developed maximum flooding surfaces. Water‐discharge variations thus alter sequence‐stratigraphic patterns and provide an alternative explanation to the amplitude of sea‐level fall for generating either type 1 or 2 erosional unconformities.

Detrital zircon data reveal the origin of Australia’s largest delta system

The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582 .

Stress deflections around salt diapirs in the Gulf of Mexico

Abstract Delta–deepwater fold–thrust belts are linked systems of extension and compression. Margin-parallel maximum horizontal stresses (extension) on the delta top are generated by gravitational collapse of accumulating sediment, and drive downdip margin-normal maximum horizontal stresses (compression) in the deepwater fold–thrust belt (or delta toe). This maximum horizontal stress rotation has been observed in a number of delta systems. Maximum horizontal stress orientations, determined from 32 petroleum wells in the Gulf of Mexico, are broadly margin-parallel on the delta top with a mean orientation of 060 and a standard deviation of 49°. However, several orientations show up to 60° deflection from the regional margin-parallel orientation. Three-dimensional (3D) seismic data from the Gulf of Mexico delta top demonstrate the presence of salt diapirs piercing the overlying deltaic sediments. These salt diapirs are adjacent to wells (within 500 m) that demonstrate deflected stress orientations. The maximum horizontal stresses are deflected to become parallel to the interface between the salt and sediment. Two cases are presented that account for the alignment of maximum horizontal stresses parallel to this interface: (1) the contrast between geomechanical properties of the deltaic sediments and adjacent salt diapirs; and (2) gravitational collapse of deltaic sediments down the flanks of salt diapirs.

The Hammerhead Delta—deepwater fold-thrust belt, Bight Basin, Australia: 2D kinematic and geomechanical reconstructions

The Hammerhead Delta—deepwater fold-thrust belt is located in the Ceduna Sub-basin of the Bight Basin, offshore southern Australia. It is a short lived gravity gliding system, Late Santonian-Maastrichtian in age. It exhibits a distinctive spoon shape in cross-section and detaches on a master horizon above Santonian marine shales of the Tiger Supersequence. Here, we have interpreted a large seismic dataset—including the recently acquired regional two-dimensional seismic dataset provided by Ion Geophysical—to constrain the regional structural geometry of the Hammerhead Delta—deepwater fold-thrust belt. Two structural restorations were completed to quantify the amount of extension and shortening in the system. These restorations were: a two-dimensional kinematic restoration, using 2D MOVE; and a two-dimensional geomechanical restoration, using Dynel 2D. By comparing results from the two techniques we demonstrate that the amount of observed extension in the delta top is nearly balanced by the shortening in the delta toe. The near balance (< 2 % excess extension) of the system is a unique result. Other passive margin systems demonstrate larger amounts of extension compared to shortening, due to the regional-scale pro-gradational nature of the systems. These results suggest that the balanced geometry of the Hammerhead Delta—deepwater fold-thrust belt is consistent with either a sudden decrease in sediment supply during the upper Maastrichtian, resulting in a cessation of prograding fault activity, or a loss of extension to the underlying Cenomanian growth faults or some combination thereof. Thus, the system failed to develop into an extensive passive margin delta—deepwater fold-thrust belt.