Tide-dominated Shelf Research Articles

The Heuksan mud belt on the tide-dominated shelf of Korea: a supply-driven depositional system?

A 20–50-km-wide, approx. 200-km-long and 50-m-thick mud deposit around Heuksan Island, southwest Korea, extends from the mouth of the Geum River to the shelf off Jeju Island. The deposit formed in association with postglacial sea-level rise. However, the substantial thickness and rapid seaward progradation of the deposit over a period of only about 3–4 thousand years, as postulated by previous investigators, are difficult to reconcile with the rapid sea-level rise during the early to mid-Holocene. To resolve this discrepancy, the origin and stratigraphic evolution of the Heuksan mud belt were reexamined using high-resolution seismic profiles and three drill cores penetrating down to the basement. A sedimentary facies analysis with AMS 14C age control facilitated reconstruction of the postglacial evolution of the deposit. The seismic profiles reveal that the Heuksan mud deposit can be divided into three depositional events clearly defined by distinct spatiotemporal bounding surfaces. The lower unit contains retrograding tidal flat sequences formed during an earlier transgression commencing at ca.16.0 ka and terminating at 11.6 ka. The middle unit consists of rapidly prograding mud clinoforms deposited from 11–6.5 ka. This unit extends 80 km onto the distal shelf, and is characterized by sedimentation rates of 5.0 cm/year. From 6.5–6 ka, progradation was interrupted for a 500 year period during which the deposit retreated by 35 km due to a decrease in the sedimentation rate to 0.18 cm/year. The abrupt stratigraphic break was evidently triggered by a sudden change in sediment supply, resulting in rapid retreat of the unit despite continuing local sea-level rise. Renewed sedimentation commenced at about 6.0 ka, being initially characterized by clinoform progradation, followed by autoretreat from about 2.5 ka onward due to a decreasing sediment supply. The Heuksan mud belt may thus represent an example of a shelf depositional system driven predominantly by the rate of sediment supply, rather than by sea-level rise.

Late Quaternary sedimentary environmental evolution offshore of the Hangzhou Bay, East China—implications for sea level change and formation of Changjiang alongshore current

This study focuses on sedimentary environmental changes offshore of Hangzhou Bay, East China, since the Late Quaternary. AMS C-14 ages from core CJK10, lithologies, distribution of foraminifera, heavy minerals, and S and Cl elements show a fluvial terrace environment during similar to 23.2-11.0 cal ka BP; a littoral to tidal-flat environment during 11.0-10.2 cal ka BP; and a shallow marine environment with a relatively low sedimentation rate (0.1-0.22 cm/a) since 4.3 cal ka BP. High depositional rates (similar to 1.6 cm/a) from 10.9 to 10.2 cal ka BP resulted from sufficient accommodation space created by rapid sea level rise from -44 m to -33 m, from high sediment delivery by local rivers, and effective trapping of sediments by tidal-flat vegetation. The rate of sea level rise was variable; relatively high from 10.9 to 10.6 cal ka BP (2.1 cm/a), and lower since 10.6 cal ka BP (1.2 cm/a). The Changjiang alongshore current crossed the Hangzhou Bay to form the mud wedge on the inner shelf of the East China Sea later than 9.4 cal ka BP. The CJK10 site was a tide-dominated shelf environment and experienced erosion from approximately 9.4-9.2 cal ka BP to 4.3 cal ka BP. The depositional hiatus was caused by the Changjiang alongshore current, which was relatively weak during 9.4-7.5 cal ka BP and increased in strength during similar to 7.5-4 cal ka BP. From similar to 4.3 cal ka BP, a large amount of sediment from the Changjiang River was partly deposited on the continental shelf of Hangzhou Bay with some transported southward. Therefore, this study clarifies the history of Changjiang-derived sediment dispersal and deposition, although a detailed record of the changes in the Changjiang alongshore current since 4.3 cal ka BP is difficult to obtain because of the scarcity of evidence.

Exploring the aftermath of the Cambrian explosion: The evolutionary significance of marginal- to shallow-marine ichnofaunas of Jordan

The middle Cambrian Hanneh Member of the Burj Formation and the middle to upper Cambrian Umm Ishrin Formation of the Dead Sea area, Jordan, contain well-preserved and abundant ichnofaunas. Trace fossils are present in a wide variety of depositional environments, from tide-dominated shelf to prodelta, delta front and interdistributary-bay tidal flats. Twelve trace fossil assemblages have been identified within a number of sequence-stratigraphic settings, namely early transgressive subtidal dune and bar complex, late transgressive isolated dune patches and dune-field abandonment, highstand deltaic progradation, and forced-regressive deltaic progradation. These ichnofaunas provide information that helps us to understand the nature of Cambrian ecosystems in the aftermath of the Cambrian radiation. The Jordan ichnofaunas display evidence of firm ground conditions at or near the sea bottom. These include (1) well-preserved scratch marks in most Cruziana and Rusophycus ichnospecies, (2) adhering Gyrolithes polonicus burrows, and (3) unlined passively filled Diplocraterion isp. penetrating into underlying mudstone. Whereas the Diplocraterion isp. occurrences are a classic example of the Glossifungites ichnofacies linked to erosional exhumation, the firmground G. polonicus is associated to a maximum flooding surface with no erosion involved. The trace fossil suites dominated by trilobite structures are firmground examples of the Cruziana ichnofacies, raising issues with respect to the substrate affinities of this ichnofacies, which is typically considered indicative of softgrounds. Our study underscores evolutionary controls on this ichnofacies, suggesting that lower Paleozoic occurrences, which are dominated by trilobite trails and trackways, reflect firm substrates and younger ones represent the typical softground examples. The occurrence of abundant bilobate trace fossils, scratch marks and trackways in interdistributary-bay tidal flats within a braidplain delta complex in the Umm Ishrin Formation provides evidence that arthropods were able to foray into marginal-marine brackish-water settings during the Cambrian. Analysis of the tiering structure indicates relatively simple communities and limited use of the infaunal ecospace. Low-energy fully marine and marginal-marine ichnofabrics are dominated by shallow-tier deposit- and detritus-feeding trace fossils. Deep-tier structures of detritus feeders are present in ichnofabrics from fully marine low-energy settings, whereas deep-tier trace fossils of suspension feeders are dominant in high-energy subtidal zones.

The ‘Rich’ group of the Drâa Basin (Lower Devonian, Anti-Atlas, Morocco): an integrated sedimentary and tectonic approach

Abstract In this study, based on a sedimentary analysis, an interpretation in terms of sequence stratigraphy is proposed for the ‘Rich’ group of the Early Devonian Drâa Basin (of Pragian to late Eifelian age). This group is composed of four depositional sequences (third order) containing a transgressive systems tract and a highstand systems tract, both deposited on a storm- and tide-dominated shelf. The depositional sequences are related to global sea-level changes but their geometric architecture is controlled by tectonic subsidence and by sediment supply. The basin forms an ENE–WSW channel with two internal high blocks acting on the depocentre shifts. The channel is surrounded by high points and by marine distal condensed series, and bordered by inherited various regional faulted axes. The regional setting corresponds to an extensional passive margin on the northern edge of the West African craton.

Construction of a Palaeogene tide-dominated shelf: influence of Top Chalk topography and sediment supply (Wessex Basin, UK)

Palaeogene siliciclastic sediments (Reading and London Clay Formations) in the Wessex Basin record an upward change through four tidally influenced deposition systems. The lowest (Reading Formation) was deposited in estuarine and coastal plain environments. The overlying London Clay records discrete upward changes from deltaic, to mixed tidal flat, to tidal sandsheet (‘Bagshot Sands’). Erosional topography on the Top Chalk surface had an important control on deposition: (1) topographic lows controlled the initial sites of marine flooding and sedimentation; (2) lows would have acted as conduits for sediment delivery; (3) lows created additional accommodation by the compaction of thick mud fills; (4) lows may have formed preferential sites for delta growth. As topography was reduced by sediment infill, depositional units tended toward more sheet-like geometries. Heavy minerals reveal that an upward progradational/aggradational trend and the development of more sand-rich facies are related to a change in provenance toward sand-rich Cornubian sources.

Depositional patterns of sand ridges in tide-dominated shallow water environments: Yellow Sea Coast and South Sea of Korea

The continental shelf of the Yellow Sea and the Korean South Sea is a tide-dominated shelf. A number of sand ridges occur in this area which form bathymetric highs. Surveyed were one large sand ridge (Jangan Satoe) along the western (Yellow Sea) coast and a group of mid-shelf sand ridges in the South Sea, to delineate their depositional pattern and probable origin in tide-dominated shallow water environments. Analyses of sediment samples, side-scan sonographs, and seismic profiles reveal that these ridges show regionally different characteristics in morphology, bedforms, and sediment. The sand ridge in the Yellow Sea shows a composite form of various bedforms including sand waves and megaripples. This ridge is interpreted to be in an accretional active stage, maintained by strong tidal currents. In contrast, the sand ridges on the mid-shelf of the South Sea show rounded, single forms without bedforms on them. They are interpreted to represent moribund-type sand ridges in the shelf environment, which were formed near the shoreline during the Holocene sea-level transgression. These ridges developed off the paleo-Seomjin River during a period of a stillstand or slow sea-level rise approximately between 10,000 and 7000 yrs B.P.

Sequence stratigraphy from core and wireline log data: the Viking Formation, Albian, south central Alberta, Canada

An extensive core and wireline log database has allowed a regional (200 000 km 2) high resolution sequence stratigraphic analysis of the predominantly shallow marine, Viking Formation of south-western Alberta, Canada. The study has revealed new insights into: (i) the geometry and origin of sequence boundaries; (ii) temporal changes in shelf processes across key sequence stratigraphic surfaces; and (iii) the origin of the Viking Formation. Five regionally significant erosion surfaces (VE1-4 and VE3c) define six units, A-F, within the Viking Formation. Each unit is characterized by one or more of four facies associations: (i) tide-dominated shelf; (ii) storm-dominated shoreline-shelf; (iii) coastal plain; and (iv) estuarine. Sequence stratigraphic analysis of these units and their bounding surfaces defines five sequence boundaries and four sequences that correspond to units A, B and C, and D and E combined. Units A-C are lowstand deposits that are underlain by sequence boundaries and record stepped sea-level fall in a tidal shelf setting. Unit A is underlain by a correlative conformity, whereas units B and C are underlain by regionally extensive erosion surfaces that have a rolling topography and are considered to have been formed subaqueously by tidal scour. Unit D is an estuarine valley fill unit that records further sea-level fall. It is underlain by a laterally restricted erosion surface (VE3c) and capped by a regionally extensive transgressive surface (VE3). Unit E is a storm-dominated shoreline-shelf succession that forms a highstand systems tract to the VE3c sequence boundary. In turn unit E is capped by a stepped transgressive surface, coincident with a further sequence boundary. The potential for tidal currents to generate regional subaqueous erosion surfaces is great but not widely recognized. In tide-dominated environments, such surfaces are likely during relative sea-level fall and may be a common correlative surface to subaerial sequence boundaries. Similarly, changes in dominant shelf processes across key sequence stratigraphic surfaces may be more common than previously realized. Finally, the recognition of regionally extensive tidal deposits in units A-C is in marked contrast with several recent studies where the same successions have been interpreted as lowstand shoreline deposits.

Facies and sequence stratigraphic analysis in an intracratonic, thermal‐relaxation basin: the Early Proterozoic, Lower Quilalar Formation and Ballara Quartzite, Mount Isa Inlier, Australia

ABSTRACTThe Quilalar Formation and correlative Mary Kathleen Group in the Mount Isa Inlier, Australia, conformably overlie rift‐related volcanics and sediments and non‐conformably overlie basement rocks. They represent a thermal‐relaxation phase of sedimentation between 1780 and 1740 Ma. Facies analysis of the lower siliciclastic member of the Quilalar Formation and the coeval Ballara Quartzite permits discrimination of depositional systems that were restricted areally to either N‐S‐trending marginal platform or central trough palaeogeographic settings.Four depositional systems, each consisting of several facies, are represented in the lower Quilalar Formation‐Ballara Quartzite; these are categorized broadly as storm‐dominated shelf (SDS), continental (C), tide‐dominated shelf (TDS) and wave‐dominated shoreline (WDS). SDS facies consist either of black pyritic mudstone intervals up to 10 m thick, or mudstone and sandstone associated in 6–12‐m‐thick, coarsening‐upward parasequences. Black mudstones are interpreted as condensed sections that developed as a result of slow sedimentation in an outer‐shelf setting starved of siliciclastic influx. Vertical transition of facies in parasequences reflects flooding followed by shoaling of different shelf subenvironments; the shoreface contains evidence of subaerial exposure. Continental facies consist of fining‐upward parasequences of fluvial origin and tabular, 0·4–4‐m‐thick, aeolian parasequences. TDS facies are represented by stacked, tabular parasequences between 0·5 and 5 m thick. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal conditions. WDS facies are preserved in 0·5–3‐m‐thick, stacked, tabular parasequences. Vertical transition of facies reflects initial flooding with wave reworking of underlying arenites along a ravinement surface, followed by shoaling from lower shoreface to foreshore conditions.Parasequences are stacked in retrogradational and progradational parasequence sets. Retrogradational sets consist of thin SDS parasequences in the trough, and C, TDS and probably WDS parasequences on the platforms. Thick SDS parasequences in the trough, and TDS, subordinate C and probably WDS parasequences on the platforms make up progradational parasequence sets.Depositional systems are associated in systems tracts that make up 40–140‐m‐thick sequences bounded by type‐2 sequence boundaries that are disconformities. Transgressive systems tracts consist of C, TDS and probably WDS depositional systems on the platforms and the SDS depositional system and suspension mudstone deposits in the trough. The transgressive systems tract is characterized by retrogradational parasequence sets and developed in response to accelerating rates of sea‐level rise following lowstand. Condensed‐section deposits in the trough, and the thickest TDS parasequences on the platforms reflect maximum rates of sea‐level rise and define maximum flooding surfaces. Highstand systems tract deposits are progradational. Early highstand systems tracts are represented by TDS and probably WDS depositional systems on the platforms and suspension mudstone deposits in the trough and reflect decreasing rates of sea‐level rise. Later highstand systems tracts consist of the progradational SDS depositional system in the trough and, possibly, thin continental facies on the platforms. This stage of sequence development is related to slow rates of sea‐level rise, stillstand and slow rates of fall. Lowstand deposits of shelf‐margin systems tracts are not recognized but may be represented by shoreface deposits at the top of progradational SDS parasequence sets.

Shallow-marine carbonate facies and facies models

Summary Shallow-marine carbonate sediments occur in three settings: platforms, shelves and ramps. The facies patterns and sequences in these settings are distinctive. However, one type of setting can develop into another through sedimentational or tectonic processes and, in the geologic record, intermediate cases are common. Five major depositional mechanisms affect carbonate sediments, giving predictable facies sequences: (1) tidal flat progradation, (2) shelf-marginal reef progradation, (3) vertical accretion of subtidal carbonates, (4) migration of carbonate sand bodies and (5) resedimentation processes, especially shoreface sands to deeper subtidal environments by storms and off-shelf transport by slumps, debris flows and turbidity currents. Carbonate platforms are regionally extensive environments of shallow subtidal and intertidal sedimentation. Storms are the most important source of energy, moving sediment on to shoreline tidal flats, reworking shoreface sands and transporting them into areas of deeper water. Progradation of tidal flats, producing shallowing upward sequences is the dominant depositional process on platforms. Two basic types of tidal flat are distinguished: an active type, typical of shorelines of low sediment production rates and high meteorologic tidal range, characterized by tidal channels which rework the flats producing grainstone lenses and beds and shell lags, and prominent storm layers; and a passive type in areas of lower meteorologic tidal range and higher sediment production rates, characterized by an absence of channel deposits, much fenestral and cryptalgal peloidal micrite, few storm layers and possibly extensive mixing-zone dolomite. Fluctuations in sea-level strongly affect platform sedimentation. Shelves are relatively narrow depositional environments, characterized by a distinct break of slope at the shelf margin. Reefs and carbonate sand bodies typify the turbulent shelf margin and give way to a shelf lagoon, bordered by tidal flats and/or a beach-barrier system along the shoreline. Marginal reef complexes show a fore-reef—reef core—back reef facies arrangement, where there were organisms capable of producing a solid framework. There have been seven such phases through the Phanerozoic. Reef mounds, equivalent to modern patch reefs, are very variable in faunal composition, size and shape. They occur at shelf margins, but also within shelf lagoons and on platforms and ramps. Four stages of development can be distinguished, from little-solid reef with much skeletal debris through to an evolved reef-lagoon-debris halo system. Shelf-marginal carbonate sand bodies consist of skeletal and oolite grainstones. Windward, leeward and tide-dominated shelf margins have different types of carbonate sand body, giving distinctive facies models. Ramps slope gently from intertidal to basinal depths, with no major change in gradient. Nearshore, inner ramp carbonate sands of beach-barrier-tidal delta complexes and subtidal shoals give way to muddy sands and sandy muds of the outer ramp. The major depositional processes are seaward progradation of the inner sand belt and storm transport of shoreface sand out to the deep ramp. Most shallow-marine carbonate facies are represented throughout the geologic record. However, variations do occur and these are most clearly seen in shelf-margin facies, through the evolutionary pattern of frame-building organisms causing the erratic development of barrier reef complexes. There have been significant variations in the mineralogy of carbonate skeletons, ooids and syn-sedimentary cements through time, reflecting fluctuations in seawater chemistry, but the effect of these is largely in terms of diagenesis rather than facies.

Bioclastic carbonate sedimentation on a high-latitude, tide-dominated shelf; Northeast Orkney Islands, Scotland

ABSTRACT Shell-sands and gravels cover much of the shallow Orkneys shelf at 59°N, accumulating locally into 30-m-high banks at rates up to 540 g/m2/yr (67 cm/l,000 yr). Overall, the Orkney shelf sedimentation rate is approximately 10 cm/l,000 yr, compared with 3 cm/1,000 yr for the entire Scottish continental shelf. Major sandbanks are located off headlands that produce circulation loops in the tidal flow. Regional sandwave orientations reveal a clockwise transport of sediment around the islands, probably resulting from storm-wave reinforcement of the tidal asymmetry combined with the net inflow of Atlantic water into the North Sea. Carbonate production is high from the Modiolus epifauna. Modiolus shell gravels, commonly containing Glycymeris, pass laterally into comminuted shell-sands. Within the euphotic zone (down to 40 m) dead shells are weakened by echinoid biting, algal boring, and limpet grazing. Boring by fungi and clionid sponges and grazing by chitons are common but not depth-restricted. Sediments contain 89-95% carbonate on the level bottom offshore, but 94-99% in sandwaves. Mean values for the main skeletal components are--bivalves 46%, barnacles 18%, bryozoans 11%, and serpulids 7%. Calcareous algal gravels occur in sheltered areas less than 20 m deep. Bryozoa typify lower-energy offshore environments, while more durable barnacle and serpulid debris is concentrated in sandwave fields. The sediments are dominantly calcitic and have a high preservation potential.

Devonian Storm- and Tide-Dominated Shelf Deposits, Parnaiba Basin, Brazil: ABSTRACT

Storm- and tide-dominated shelf deposits crop out in the Parnaiba basin, Picos region, State of Piaui, Brazil, belonging to the Pimenteiras and Cabecas Formations. The Pimenteiras is dominantly Middle Devonian whereas the Cabecas Formation is Late Devonian in age. The Pimenteiras Formation is organized into coarsening-upward cycles, with the top marked by bar-like sand bodies, bearing hummocky cross-stratification. Locally, channel-like depressions, carpeted with shale clasts, transect or are nested between adjacent bars. The overall environment is envisaged as a storm-dominated open shelf. The Cabecas Formation consists of a thick quartzarenite interval composed of apparently elongate bodies with radial accretionary wedges in a complex imbricated pattern. Double-tangential--or sigmoidal--bedding is the main sedimentary structure, followed by slumping and dewatering features and locally by lenticular-wavy-flaser bedding. This unit, in this part of the Parnaiba basin, can be interpreted as formed in a tide-dominated environment, probably of the shoal-retreat massif type, with subordinate tidal flats. In terms of vertical succession, the Pimenteiras and Cabecas Formations depict a cycle of relative rise of sea level (or of coastal onlap) with an increasing terrigenous influx toward the upper end of the cycle. The dominance of storm-wave action in the base of the cycle (Pimenteiras paracycles) points to a narrower shelf or steeper basinal slope. Later, the high terrigenous influx would decrease such a slope, preclude storm-wave action, and possibly amplify the tidal range--by shelf widening--in such a way that deposition would become tide-dominated. Oil shows have been reported in the Cabecas Formation, which presents remarkable reservoir characteristics, but apparently its high sand/shale ratio would not favor the formation of stratigraphic traps. Conversely, storm-dominated deposits appear to form excellent stratigraphic traps despite their poorer reservoir characteristics. End_of_Article - Last_Page 562------------

Relict Sediments on Continental Shelves: A Reconsideration

Relict sediments on shelves, originally defined as remnant from a different earlier are recognized by petrographic criteria (grain size, iron staining, etc.), fauna, and topography. Recent studies have revealed a second set of attributes which indicate that these deposits, although originating in an earlier environment, are dynamic systems which are undergoing modification in response to their present environment, especially the hydraulic regime, and are approaching a state of equilibrium with this environment. The modification may be simulated by means of a stochastic process model. A spectrum of modern shelf regimes and the resulting deposits is considered. The high-energy, tide-dominated shelf seas of western Europe have extensively reworked their Pleistocene and Holocene transgressive substrates, producing a constructional topography and regional textural gradients. Similar topography and textural gradients are reported from the tide-swept shoals and banks off northeastern North America...