Shelf Ridge Research Articles

Morphological Characteristics of an Open Shelf Ridge at Rio Grande do Norte Continental Shelf, NE Brazil

Morphological Characteristics of an Open Shelf Ridge at Rio Grande do Norte Continental Shelf, NE Brazil

Geoacoustic Model of Erosional Shelf Ridges in the Mid-eastern Yellow Sea

Geoacoustic Model of Erosional Shelf Ridges in the Mid-eastern Yellow Sea

A new interpretation for the Pliensbachian Cook Formation (northern North Sea) as north–south-prograding tidal deltas and shelf ridges in the Early Jurassic Seaway: new model of linkage to the Norwegian Sea

Abstract The Lower Jurassic Cook Formation reservoir is a hydrocarbon-prolific unit that produces from several fields in the northern North Sea. For 40 years this formation has been interpreted as a westward-prograding deltaic unit sourced from Norway. Despite numerous discoveries, exploration targeting this unit has been hampered by well failures with lack of reservoir sand, discouraging companies from further exploration of this play. During a current re-evaluation of the process sedimentology of the Norwegian offshore basins, the Cook Formation is now interpreted as the middle to distal reaches of a very large, north-to-south-oriented delta system, variably confined within the Early Jurassic Seaway running from the Norwegian Sea into the northern North Sea. The Cook Formation is a subaqueous delta built southward during regression, whereas several internal transgressive phases produced sands that were reworked as north–south-oriented, shelf tidal ridges. The tidal ridges of the Cook Formation constitute some of the best reservoirs and are elongated with stacked, well-sorted, cross-bedded sandstone sets with mudstone drapes. Both the elongate tidal sand-ridges and intervening mudstone-rich, inter-ridge zones are proven by numerous well observations and illustrated by seismic amplitudes. In contrast to earlier eastern derivation models, these new results for the depositional system of the Cook Formation better explain the Cook well successes and failures in the northern North Sea. This work also strongly suggests that the tide-dominated subaqueous delta to transgressive-ridge system of the Cook Formation is spatially linked with the time-equivalent shorelines, subaerial tidal deltas and estuaries of the Tilje Formation in the Haltenbanken region to the north. The Tilje Formation deltas built into the Early Jurassic Seaway due to rift-initiation and rift-shoulder uplift, drained southwards and spilled eventually into the northern North Sea, becoming the entirely subaqueous Cook Formation. The relatively narrow seaway enhanced the tidal currents and suppressed wave activity, resulting in Cook subaqueous delta lobes and ridges without any delta-top facies. Overall, this elongate and extensive, Pliensbachian deltaic to estuarine system of the Early Jurassic Seaway off Norway competes in scale with some of Earth's largest present-day deltas.

Distribution and dynamics of U.S. continental shelf ridge sediment and morphology: A brief review

The U.S. continental shelf is an important sediment source for beach nourishment and restoration efforts that mitigate erosion, increase resilience to storm impacts, provide habitat, and support the economy. The continental shelf is the preferred source for borrow sediment to closely match the native grain size distribution and composition of the site being restored. However, spatiotemporal variability of continental shelf sediment ranges in size and composition, resulting from previous sea levels and contemporary variability such as normal shelf processes, storm waves, and anthropogenic activities. Understanding the nature and distribution of continental shelf ridge sediment changes over time should reduce costs of efforts required to explore offshore sediments for coastal restoration projects. This review examines the present state of knowledge on the availability, distribution, and characteristics of continental shelf sediment under normal conditions and potential variability after storms and dredging. Under normal conditions sediment on the shelf is easily located and characterized as potential borrow areas. However, storms can induce enough sediment transport to change the boundaries of sediment borrow areas and the location of known sand ridges. Dredging can also influence sediment infilling of the dredged borrow areas, which impact benthic infauna and introduce potential onshore impacts depending on the geometry and nearshore proximity of the excavation. The results of this summary have identified gaps in the present knowledge such as a need for additional sand searches in under-investigated regions, a better understanding of storm impacts on hydrodynamic-driven ridge migration and continuing to review best management practices when new research of dredging practices and impacts are presented. A brief review of the present state of knowledge on the distribution and dynamics of continental shelf sedimentology and morphology are presented here to aid in advancing the scientific and coastal management community’s knowledge of shelf sediments and dynamics as well as highlight potential future research needs.

Sharp-based, mixed carbonate–siliciclastic shallow-marine deposits (upper Miocene, Betic Cordillera, Spain): The record of ancient transgressive shelf ridges?

Isolated sharp-based sedimentary bodies in shelf settings can develop via the reworking of regressive deposits during transgressions. An example of these are shelf ridges, formed under a wide range of processes, and widely studied due to their high reservoir potential. However, there is still a lack of examples in mixed (carbonate–siliciclastic) successions. This study presents an outcrop example from the Upper Miocene of the Betic Cordillera (Spain), with the aim to propose a model for the development of transgressive sharp-based mixed carbonate–siliciclastic deposits, and to provide criteria to differentiate these from their regressive counterparts. The studied succession is ca. 300 m-thick, and shows a cyclic alternation of coarse and fine-grained mixed deposits. Depositional cycles start with siliciclastic-dominated offshore to offshore transition deposits, progressively replaced by lower shoreface deposits. These are abruptly truncated by sharp erosive contacts bioturbated by passively-infilled large burrows; their ichnological features allow assignation to the Glossifungites ichnofacies. These contacts are interpreted as ravinement surfaces. They are overlain by mixed carbonate–siliciclastic barforms, rich in skeletal fragments and extraclasts, and displaying large-scale cross bedding. These form several m-thick and hundreds of m-long depositional elements interpreted as mixed shelf ridges. These ridges formed in a fine-grained, shallow-water shelf, which occasionally received coarse siliciclastic sediment supply via gravity flows, but had a coeval offshore carbonate factory, which provided the skeletal fragments. The sharp-based, coarser-grained nature and lithological break at the base of these mixed carbonate–clastic deposits could lead to their misinterpretation as forced-regressive wedges. However, the nature of their lower contact, combined with the reworked offshore skeletal fragments, and their stacking pattern are consistent with these mixed units forming during transgression. Other studies in relatively time-equivalent deposits have demonstrated the existence of coeval regressive, coarser siliciclastic-dominated shoreline systems in relatively close localities. These evidence a complex basin configuration in the area during the upper Miocene, with the development of local depocentres and relatively narrow corridors or seaways in the Mediterranean–Atlantic connection, which could have favoured shelf reworking processes, but also promoted the development of diverse stacking patterns, reflecting the differential interaction between active tectonics and sedimentation across the region.

Long‐Term Morphodynamics of a Coupled Shelf‐Shoreline System Forced by Waves and Tides, a Model Approach

AbstractSand ridges, with length scales of several km, are prominent features of the seafloor landscape of many sandy continental shelves. Knowledge about the extent to which these ridges influence the large‐scale (i.e., decadal and kilometer scales) morphodynamic evolution of the adjacent shoreline and vice versa (shelf‐shoreline morphodynamic coupling) is limited. The present work aims at quantifying this coupling by using a coupled nonlinear shelf‐shoreline model forced by tides and different wave conditions. Model results show that the presence of sand ridges on the shelf creates longshore non‐uniform wave patterns, which act as a forcing template for the morphodynamic development of the shoreline. The shelf‐shoreline coupling primarily works one way, meaning that the morphodynamic evolution of the shelf affects the evolution of the shoreline. When wave propagation is predominantly aligned with the long axis of the shelf ridges, the forced shoreline undulations are so prominent, that they affect the shelf morphology (significant two‐way coupling). Moreover, for those waves, the longshore spacing of the ridges is strongly imprinted on the shoreline morphology. Weaker shoreline undulations develop for waves that propagate more across the ridges and the weakest for time‐varying wave conditions with large variability in their angles of propagation. Model results compare fairly well with observations. Physical mechanisms underlying the different morphodynamic responses of the coupled shelf‐shoreline system to different wave conditions are also given.

Load Transfer during Magnetic Mucoperiosteal Distraction in Newborns with Complete Unilateral and Bilateral Orofacial Clefts: A Three-Dimensional Finite Element Analysis

The primary correction of congenital complete unilateral cleft lip and palate (UCLP) and bilateral cleft lip and palate (BCLP) is challenging due to inherent lack of palatal tissue and small extent of the palatal shelves at birth. The tissue deficiency affects the nasal mucosa, maxillary bone and palatal mucosa. This condition has driven the evolution of several surgical and non-surgical techniques for mitigating the inherent problem of anatomical deficits. These techniques share the common principle of altering the neighboring tissues around the defect area in order to form a functional seal between the oral and nasal cavity. However, there is currently no option for rectifying the tissue deficiency itself. Investigations have repeatedly shown that despite the structural tissue deficiency of the cleft, craniofacial growth proceeds normal if the clefts remain untreated, but the cleft remains wide. Conversely, craniofacial growth is reduced after surgical repair and the related alteration of the tissues. Therefore, numerous attempts have been made to change the surgical technique and timing so as to reduce the effects of surgical repairs on craniofacial growth, but they have been only minimally effective so far. We have determined whether the intrinsic structural soft and hard tissue deficiency can be ameliorated before surgical repair using the principles of periosteal distraction by means of magnetic traction. Two three-dimensional maxillary finite element models, with cleft patterns of UCLP and BCLP, respectively, were created from computed tomography slice data using dedicated image analysis software. A virtual dental magnet was positioned on either side of the cleft at the mucoperiosteal borders, and an incremental magnetic attraction force of up to 5 N was applied to simulate periosteal distraction. The stresses and strains in the periosteal tissue induced by the magnet were calculated using finite element analysis. For a 1 N attraction force the maximum strains did not exceed 1500 µstrain suggesting that adaptive remodeling will not take place for attraction forces lower than 1 N. At 5 N the regions subject to remodeling differed between the UCLP and BCLP models. Stresses and strains at the periosteum of the palatal shelf ridges in the absence of compressive forces at the alveolar borders were greater in the UCLP model than the BCLP model. The findings suggest that in newborns with UCLP and BCLP, periosteal distraction by means of a magnetic 5 N attraction force can promote the generation of soft and hard tissues along the cleft edges and rectify the tissue deficiency associated with the malformation.

What is the topset of a shelf‐margin prism?

AbstractThe siliciclastic topset of a continental margin, or a shelf‐margin prism in subsiding nonplate‐margin deepwater basins, is the flat‐lying upper part of the margin succession; it is coeval basinwards with deepwater slope clinoforms. Topsets develop by the aggradation of repeated, cross‐shelf, shoreline regressions and transgressions, thereby hosting the shelf portion of stacked, fourth‐order stratigraphic sequences. Sediment spreading downdip and along strike during the cross‐shelf transit of the sediment delivery system, as well as process regime changes of deltas and shorefaces (regressive) and of estuaries, barrier–lagoon systems and shelf ridges (transgressive) are highly variable over short distances, so that correlation within a single stratigraphic sequence is far more difficult than correlation of the cross‐shelf maximum flooding surface boundaries. Thickness of individual regressive–transgressive, fourth‐order sequences is given by shelf accommodation, typically <10 m in embayment or on the inner shelf and up to 200 m on outer shelf. Tectonic subsidence and compaction will enhance this thickness only if rates are very high compared to shelf‐transit time. In very high subsidence rate settings, the transgressive tracts are well preserved and often thickly developed. Topset sequences in an Icehouse climate setting tend to have a high proportion and greater landward development of marine vs nonmarine deposits, compared to Greenhouse sequences, because of the importance of eustatic rise of sea level in the former. Previous numerical experiments show that even for very wide shelves and irrespective of Icehouse or Greenhouse conditions, deltas rarely take more than 10 s of ky to reach their shelf edge, suggesting that it is fourth‐order (or higher) sequences that are the fundamental ones in sequence stratigraphy.

New evidence of regressing and transgressing Jurassic siliciclastic coastlines within the Dhruma Formation in Northern Central Arabia, Saudi Arabia

The Middle Jurassic Dhruma carbonates have long been identified as a major hydrocarbon reservoir with massive proven reserves in Saudi Arabia and other Arabian Gulf countries. Regional studies indicate that these hydrocarbon-rich carbonates are replaced laterally by siliciclastic deposits in northern and southern central Arabia. However, none of those studies have clarified how these siliciclastic deposits were formed or how they can be correlated with their carbonate counterparts.Recently drilled shallow stratigraphic wells near the outcrops of the Dhruma Formation (approximately 250 km NW of Riyadh) provide an opportunity to analyze these siliciclastic sediments. Around 1500 m of cores from six shallow stratigraphic wells are described and interpreted, providing an understanding of the transgressive and regressive Middle Jurassic siliciclastic Dhruma shorelines. An unusual aspect of the Dhruma shorelines was the long-lived transgressions (greater than half the thickness of the succession) that initially created embayed estuarine coastlines. Rivers coming into the estuaries were backfilled and strongly impacted by tidal currents entering the embayments from the sea. Fluvio-tidal and tidal sandbars filled 11 m deep inner estuarine channels and were eventually overridden by muddy central basin deposits and then by the remnants of coastal barriers with tidal inlets. Higher frequency transgressions during the middle part of the Dhruma interval left an unusual record of stacked shelf ridges, until widespread shelf carbonates developed when the clastic supply system had back-stepped far landward. In the upper third of the Dhruma Formation, the clastic supply system was re-established, with river- and tide-influenced deltas regressing across the preexisting shelf area.Five thick transgressive-regressive sequences, in addition to a series of low-supply, high-frequency mid-Bajocian transgressive events, provide the details of the evolution of this early Middle Jurassic coastline in central Arabia. Given a likely total time span of 5–7 Ma (early Aalenian to Bathonian), the Dhruma siliciclastic sequences have a duration of ~1 Ma and an average thickness of 30–80 m. The existing Middle Jurassic paleogeography map in central Saudi Arabia has been enhanced, and a correlation has been made with the Dhruma reservoir-equivalent outcrops in southern central Arabia.

Lomonosov ridge and the Eastern Arctic Shelf as elements of an integrated lithospheric plate: Comparative analysis of wrench faults

The notions of deformations in the juncture area of the Eastern Arctic Shelf and Lomonosov Ridge are highly contradictory. It has been suggested that these geostructures were divided by a large right-lateral wrench fault of the transform type, which is known as the Khatanga–Lomonosov Fault. Data obtained by interpretation of the A7 profile have been compared with seismic sections crossing large-sized wrench faults in other sedimentary basins. The investigations have shown that on the A7 profile there are no structures typical of large-sized wrench faults. The Eastern Arctic Shelf and Lomonosov Ridge, which are located on the same lithospheric plate, form an integrated structure where the ridge is a natural continuation of the shelf.

Architecture and recognition criteria of ancient shelf ridges; an example from Campanian Almond Formation in Hanna Basin, USA

AbstractShelf ridges are sedimentary bodies formed on the continental shelf due to transgressive reworking (tidal or storm) of lowstand deposits. Common on modern shelves, they are under‐represented in the geological record due to a lack of recognition criteria and facies model. This article proposes a new facies and architectural model for shelf ridges, linked to their inception–evolution–abandonment cycle and the process regime of the basin. The model is mainly based on new outcrop data and interpretations from three sandstone bodies of the Almond Formation, an overall transgressive interval during the infill of the Campanian Western Interior Seaway. Building from the case study, and ancient and modern examples, six characteristics are proposed for the recognition of ancient shelf ridges. Shelf ridges: (i) are encased between thick marine mudstone intervals; (ii) have a basal unconformity that erodes into marine muds or into the remnants of a previous shoreline; (iii) have a non‐erosional upper boundary that transitions into marine muds; (iv) are characterized by clean and well‐sorted sandstones, often cross‐bedded; (v) contain fully marine ichnofauna; and (vi) present compound architectures with large accretion surfaces and lower order structures. Although shelf ridges have been described in previous studies as generated exclusively by either tidal or storm currents, it is clear, from modern examples and the case study, that these two processes can be recorded and preserved in a single shelf ridge. The stratigraphy of these sandstone bodies is therefore much more complex than previously recognized, bearing the signature of changing tidal and storm intensity through time. Because they are developed during transgressions, shelf ridges are commonly subject to strong changes in process regime as sea‐level changes can easily affect the oceanographic conditions and the morphology of the basin. For this reason, shelf ridges can provide the best record of shelf process variability during transgressions.

Seismic geomorphology, architecture and genesis of Miocene shelf sand ridges in the Pearl River Mouth Basin, northern South China Sea

Multiple stages of large-scale shelf sand ridges, including the shoreface-attached and the offshore types, have developed in the Miocene successions on the mid-shelf region of the Pear River Mouth Basin, northern South China Sea. Utilizing a high-quality 3D seismic data set, accompanying 2D seismic profiles and well logs, the morphology, architecture and genesis of these shelf sand ridges have been systematically investigated in this study. The ridges are of very large scale, with the largest one having a maximum height of 64 m, a width of more than 20 km and a length of 37 km within the 3D survey area. Being mound-shaped, they also display obvious asymmetry character, with the ridge crest preferentially located on the SE side. Three main internal components, including the ridge front, central ridge and the ridge tail, have been recognized through careful anatomy analysis of the two most well-imaged ridges, each displaying distinct expressions on seismic amplitudes and geometries. In the plan view, most of the shelf sand ridges are generally NE–SW oriented and widening to the SW direction. Scouring features can also be clearly observed along the SW direction, including scour depressions and linear sandy remnants. On well logs, the shelf sand ridges are represented by an overall coarsening-upward pattern. Intervals with blocky sandstones are preferentially present on higher locations due to a differential winnowing process controlled by shelf topography.Plenty of evidence indicates that these ridges were primarily formed by the reworking of forced regressive or lowstand deltaic deposits under a persistent southwesterly flowing current during the subsequent transgression. This very current is a composite one, which is speculated to consist of winter oceanic current, SCSBK (South China Sea Branch of Kuroshio) intrusion onto the shelf and internal waves propagating from the Luzon Strait. Tidal currents might have contributed to the SE growth of the ridge. In response to the reglaciation of Antarctic ice-sheet and the closure of Pacific-Indian ocean seaway in the middle Miocene, the intensification of the North Pacific western boundary current was considered to have potential links to the initiation of the shelf sand ridges at ∼12 Ma. The development of shelf ridges was terminated and replaced by rapid deltaic progradation at ∼5.5 Ma.

Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high‐resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long‐term change in Pine Island Glacier. Corrugations and ploughmarks on a sub‐ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed‐shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding‐line configuration throughout the Holocene.

Can ancient shelf sand ridges be mistaken for Gilbert-type deltas? Examples from the Vryheid Formation, Ecca Group, KwaZulu-Natal, South Africa

Medium scale clinoforms within sandstones of the Vryheid Formation, northern KwaZulu-Natal, South Africa have in the past been interpreted as the products of Gilbert-type or river-dominated deltaic deposition. Detailed sedimentary logging of these rocks reveals a tempestite association, overlain by a series of moderate to very large scale climbing bedforms. These are intersected at various stratigraphic levels by massive sandstone filled channels, some with subordinate rippled bases, which are orthogonally orientated to the largest scale bedding surfaces (clinoforms) of the climbing bedform association. The sedimentary architecture is dominated by the south-easterly, south-westerly and north-westerly dipping clinoforms, all orientated oblique to the east–west trending palaeo-coastline. Local dip direction reversals are commonplace. This is at odds with Gilbert-type or river-dominated deltaic settings where: (1) sediment progrades normal to the shoreline; (2) distributary channels do not intersect the underlying deltaic foresets orthogonally; and (3) wave action is typically limited. We subsequently interpret the succession as a shelf sand ridge that evolved on a subaqueous delta, fed by sediment exchange from the upper to lower shoreface via a series of rippled scour depressions. To foster such thick (∼30m) shelf ridge development, it is likely that these remained attached to the shoreface for a significant period of time. In light of these findings, caution should be adopted in applying a blanket deltaic interpretation of shallow marine deposits comprising similar medium scale clinoforms.

Salt diapir-influenced, shallow-marine sediment dispersal patterns: Insights from outcrop analogs

Unique outcrop exposures of two salt diapirs, a secondary salt weld, and associated syndiapiric strata in northeast Mexico offer an important perspective on salt-influenced petroleum reservoirs by allowing recognition and description of salt-related sandstone depocenters. Spectacular progressive unconformities and halokinetic sequences, coupled with laterally continuous exposures, permit accurate correlation and interpretation of syndiapiric units. Analysis of the syndiapiric Upper Cretaceous, Delgado Sandstone Member (Potrerillos Formation) delineates regional shoreline sediment dispersal locally impacted by diapiric relief and the distribution and internal character of salt diapir-proximal sandstone depocenters. Sequence-stratigraphic correlation defines striking relationships between highstand (HST) and transgressive systems tracts (TST), stratal thinning trends, and salt diapir relief. Transgressive systems tract and highstand systems tract strata show thinning and lithofacies shoaling trends toward diapirs; however, the latter is more pronounced in the HST and occurs at a greater distance from salt diapirs (within 1–2 km [0.6–1.2 mi]). Sandstone depocenters, roughly 0.5–1.0 km (0.3–0.6 mi) wide and 0.5–0.2 km (0.3–0.1 mi) thick, are present in both TST and HST strata and consist of sandier, shallower water facies. However, depocenters are better developed in TST strata as thicker stratigraphic sections on updip diapir margins. We propose that sandstone depocenters formed by preferential sediment reworking and shelf ridge development on landward diapir margins during marine transgression. Elevated diapir relief and higher subsidence rates adjacent to salt diapirs likely enhanced this process. Additionally, depocenters adjacent to El Papalote diapir are smaller and contain deeper water facies than the age-equivalent depocenters adjacent to El Gordo diapir, suggesting that it had higher, broader sea-floor relief.

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km2. Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/103 y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales’ largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/103 y, with the upper slope accumulating a lower 0.23 m/103 y since transgression. Transgression produced a diachronous (14–10 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic‐carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km2. Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/103 y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales' largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/103 y, with the upper slope accumulating a lower 0.23 m/103 y since transgression. Transgression produced a diachronous (10–14 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic–carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.

Ancient shelf ridges-A potentially significant component of the transgressive systems tract: Case study from offshore northwest Java

Detailed stratigraphic evaluation of three-dimensional (3-D) seis mic volumes calibrated with well-log and core data from the Mio cene section of the offshore northwest Java shelf reveals the exten sive presence of preserved shelf ridge deposits. These features are long linear bodies ranging from 0.3 to 2.0 km wide, more than 20 km long, and up to 17 m high. On close inspection, these features appear to be asymmetric, characteristically sharp-edged and thicker on one side and gradually thinning with an irregular edge on the other side. Possible sand waves, smaller in scale, are observed super imposed on these ridges and oriented oblique to the long axes of the ridges. The observed shelf ridge deposits tend to be sand prone and overlie ravinement surfaces. The ridges appear to be oriented parallel with the axes of broad paleoembayments associated with the structural fabric of the basin. In addition to shelf ridges, shelf ribbons, possibly less than 5 m thick and less than 100 m wide, are also imaged. Sand ridges are common on modern shelves but significantly less commonly recognized in the subsurface or in outcrop. The features observed here represent examples of preserved ancient shelf ridges. These ridges are thought to have formed as a result of erosion and subsequent reworking of sand-prone deltaic and/or coastal-plain deposits by shelf tidal currents, which became active immediately after shoreline transgression. These deposits appear to have migrated across the ancient sea floor and represent an important component of the transgressive systems tract. These transgressive systems tract deposits have significant exploration potential because they are commonly sand prone and tend to be encased in shelf mudstone seal facies. Depending on the degree to which sand is present in interridge locations, these linear sand bodies can comprise potential stratigraphic traps. (Begin page 76)

ABSTRACT: A Comparison of Late Quaternary Deltaic/Estuarine Depositional Systems from Wave- and Tide-Dominated Shelves

In the stratigraphic record, shelf sand bodies encased in mudstones are concurrently interpreted as shelf ridges or (lowstand) shorefaces. A comparison of Quaternary continental margins with high sediment supply and high subsidence rate (in the order 200300m/million year) demonstrate that both types of sand bodies can be preserved. The Rhone (NW Mediterranean Sea) and the Yang Tze (East China Sea) both exhibit large sand bodies, up to 30 m thick, 3-10 km wide and 10 to more than 100 km long. Their seismic internal structure displays clinoforms dipping at about 58. Despite a very similar morphology and internal structure, these sand bodies are interpreted as resulting from very different processes. On the wave-dominated Rhone deltaic margin, they correspond to forced-regressive shoreface sands, encased in prodeltaic muds. They parallel the paleobathymetric contour lines and they are encased in prodeltaic mud. The corresponding deltaic facies are not preserved. On the tide- and storm-dominated Yang Tze margin, sand is concentrated into large tidal ridges formed during post-glacial sea-level rises. They also are encased in prodeltaic muds, but deltaic/estuarine facies have also been preserved and their orientation is almost perpendicular to the general paleoshoreline. In the stratigraphic record, only three-dimensional exploration could allow to determine to which category such sand bodies belong. The Danube wave-dominated deltaic margin is a third category where most of shelf sediment consist of prodeltaic muds and deltaic heterogeneous deposits. Sand dunes from the last deglacial sea-level rise are observed, but they have no preserved equivalent in the Pleistocene.

Erosional shelf ridges in the mid-eastern Yellow Sea

In the mid-eastern Yellow Sea, closely spaced high-resolution seismic profiles and a 44-m-long sediment core (YSDP-104) were analyzed to reveal the internal structures and stratigraphy of the shelf ridges currently shaped by tidal currents. Three depositional sequences (sequences I, II and III in descending order) can be recognized. Sequence III, the substratum of the ridges, consists of coarse-grained sediments in the lower part (non-marine deposits) and tide-influenced muddy sediments in the upper part (probable transgressive to highstand systems tract). Sequence II represents internal ridge sediments, similar in character to sequence III, but is demarcated by an undulatory ridge topography. According to radiocarbon dating of marine muds, these sequences range in age from 47,000 to 28,000 years B.P., representing two cycles of short-term sea-level fluctuations during oxygen isotope stage 3. Sequence I consists mostly of late-Holocene transgressive sand veneer on the ridge surface. It also includes minor amounts of early-Holocene muddy sediments occasionally underlying the sand. Most of the ridges are presently undergoing erosion by tidal currents, forming widespread sand dunes on the entire surface.