Shelf Physiography Research Articles

Timescale dependent sedimentary record during the past 130 kyr from a tropical mixed siliciclastic–carbonate shelf edge and slope: Ashmore Trough (southern Gulf of Papua)

AbstractIn tropical and sub‐tropical mixed siliciclastic–carbonate depositional systems, fluvial input and in situ neritic carbonate interact over space and time. Despite being the subject of many studies, controls on partitioning of mixed sediments remains controversial. Mixed sedimentary records, from Ashmore Trough shelf edge and slopes (southern Gulf of Papua), are coupled with global sea‐level curves and anchored to Marine Isotope Stage stratigraphy to constrain models of sediment accumulation at two different timescales for the past 130 kyr: (i) 100 kyr scale for last glacial cycle; and (ii) millennial scale for last deglaciation. During the last glacial cycle, carbonate production and accumulation were primarily controlled by sea‐level fluctuations. Export of neritic carbonate to the slopes was initiated during re‐flooding of previously exposed reefs and continued during Marine Isotope Stage 5e and 1 interglacial sea‐level highs. Siliciclastic fluxes to the slope were controlled by interplay of sea level, shelf physiography and oceanic currents. Heterogeneous accumulation of siliciclastic mud on the slope, took place during Marine Isotope Stage 5d to Marine Isotope Stage 3 sea‐level fall. Siliciclastics reached adjacent depocentres during Marine Isotope Stage 2. Coralgal reef and oolitic–skeletal sand resumed at the shelf edge during the subsequent stepwise sea‐level rise of the last deglaciation. Contemporaneous, abrupt siliciclastic input from increased precipitation and fluvial discharge illustrates that climate controlled deglacial sedimentation. Siliciclastic input persisted until ca 8.5 ka. Carbonate accumulation waned at the shelf edge after ca 14 ka, whereas it increased on the slopes since ca 11.5 ka, when previously exposed reef and bank tops were re‐flooded. When comparing the last sea‐level cycle sedimentation patterns of the southern Gulf of Papua with other coeval mixed systems, sea level and shelf physiography emerge as primary controls on deposition at the 100 kyr scale. At the millennial scale, siliciclastic input was also controlled by climate change during the unstable atmospheric and oceanic conditions of the last deglaciation.

Clay mineralogy of Gulf of Papua Shelf and Pandora Trough deposits constrains sediment routing during the last sea‐level cycle

AbstractThe Gulf of Papua inner mid‐shelf clinothem and lowstand deposits in Pandora Trough record sediment source and routing through the last sea‐level cycle on 20 kyr cycles. Clay mineralogy tracked dispersal of sediment from the two types of rivers (wide versus narrow floodplains) to constrain the contributions of river systems to the Gulf of Papua clinothem and Pandora Trough deposits. Fly River sediment has higher illite:smectite than clays from the small mountainous rivers (Bamu, Turama, Kikori and Purari rivers) that drain regions with more limestones. X‐ray diffraction shows high illite:smectite proximal to the Fly River delta that decrease towards the north‐east. Downcore mineralogy of inner mid‐shelf cores reveals that the largest shifts in illite:smectite correspond to changes in sediment units. The relict clinothem emplaced on the Gulf of Papua shelf during Marine Isotope Stage 3 has lower illite:smectite than the Holocene clinothem that has been building since 2 ka and the Marine Isotope Stage 5a relict clinothem. In the inner mid‐shelf, downcore decreases in illite:smectite during Marine Isotope Stage 3 suggest that this region received less clay from the Fly River and more contributions from small mountainous rivers. During Marine Isotope Stage 3, the exposed physiography and narrower shelf in this region may have deflected Fly River sediment more south‐eastward, where it bypassed the inner shelf via the Kiwai, Purutu and Umuda valleys and was deposited in the Pandora Trough. The Fly River may have been more susceptible to valley incision because of its limited shelf accommodation and higher ratio of water to sediment discharge. Such bypass of the inner mid‐shelf by Fly River sediment during the Marine Isotope Stage 2 sea‐level lowstand is recorded in Pandora Trough deposits with high illite:smectite ratios. Inner mid‐shelf clinothems with compositional shifts on the order of 20 kyr may be influenced by shelf physiography, accommodation and the variable incision by small and large rivers.

Structural controls on the morphology of an extremely narrow, low-accommodation, passive margin shelf (Eastern Brazil)

The continental shelf adjacent to the municipality of Salvador (CSS), state of Bahia, Brazil, is one of the narrowest in the world (8 km) although it is located in a passive margin. This shelf is also shallow, with low accommodation space and sediment input. During the Quaternary, the area was exposed most of the time to subaerial conditions. This long period of erosion, controlled by the structural and lithological framework of the Cretaceous Northern Camamu Sedimentary Basin, was highly influential on shelf physiography. The paleo relief that was generated, in turn, controlled sedimentation during the sea-level rise since the Last Glacial Maximum (LGM). The present study used high-resolution shallow seismic surveys to investigate the influence of the structural framework of the Northern Camamu Sedimentary Basin on the erosion of CSS during lowstands and on sediment accumulation during highstands. Five seismic units were identified, representing the Crystalline Basement, the Northern Camamu Sedimentary Basin, and the Quaternary Sediments. Bathymetric highs and lows on the shelf are associated, respectively, with antiforms and synforms present in the syn-rift strata of the sedimentary basin. Normal and transfer faults related to the installation of the sedimentary basin controlled the incision of valleys during periods of subaerial exposure. This structural framework still presents a morphological expression on the shelf due to an immensely reduced sediment input, which precluded the burial of these structures.

Stromatolite Provinces of Hamelin Pool: Physiographic Controls On Stromatolites and Associated Lithofacies

Recent studies recognized distinct stromatolite provinces in Hamelin Pool, Western Australia, each with morphologically distinct stromatolite structures paired with characteristic shelf physiography. In the present paper, we couple detailed lithofacies mapping with Hamelin Pool bathymetry and consider physiography as a control of sedimentation processes, including stromatolite development. Bathymetric transects, derived from a high-resolution bathymetry map with depths from 0 to 11 meters, allow calculation of slope gradients in the provinces. As in other settings, bathymetry is linked to energy regimes, which in turn appear to be coupled with variations in stromatolite morphologies and associated lithofacies as follows: (1) low-gradient ramps with low-energy settings are associated with sheet mats and elongate-clustered stromatolites that exhibit regular spatial patterns, possibly indicative of self-organization; (2) low gradients coupled with high-energy settings resulting from strong winds result in seif stromatolites with pronounced directional bands; (3) medium to steep gradients coupled with medium to high energy are associated with individual and merged stromatolites, often with thin basal necks; (4) headlands and promontories where the topography deflects currents are associated with elongate-nested stromatolites; and (5) medium- to high-energy slopes typically found at promontory edges and shelf margins are dominated by blocky pavement. Observations linking stromatolite morphology to physiography in a modern microbial system provide insight into the long-lived debate about biology versus environment in controlling stromatolite morphology. When physiography leads to a high-energy regime, environmental controls are the main factor determining stromatolite morphology. In contrast, when physiography promotes a low-energy environment, the response of biological communities becomes the main driver of macroscale stromatolite morphology.

Spatio-temporal patterns in the postglacial flooding of the Great Barrier Reef shelf, Australia

The shelf of the Great Barrier Reef (GBR) was progressively marine flooded from the last glaciation maximum (LGM) (ca 20 ka BP) until the last sea-level highstand (ca 6 ka BP), affecting the depositional evolution of the GBR margin and associated deposits. However, the physiographic variables related to this process have not been fully characterized, especially in relation to the sedimentary processes at the shelf margin. For this study, we used a bathymetric model of the entire shelf and a shelf margin sub-set, divided into 33 latitudinal zones. Postglacial marine flooding was simulated and flooded area (km2), flooding magnitude (km2 per sea-level increment), flooding rate (km2 ky−1) and coastline length (km) were estimated for each zone, from 130 m to 0 m below present sea level, representing the period from 20 ka to 6 ka BP. Our results show that the postglacial marine flooding did not occur uniformly and that some sub-regions (e.g. the southern-central GBR) had early and rapid flooding. Coastal complexity increased in the mid-postglacial, reaching maximum values at around 9 ka BP. This reflects a coastal landscape evolving from a linear, laterally connected coast to a complex coast dominated by estuaries and lagoons, partly returning to its initial linearity during highstand. Flooding trends and geological evidence make two depositional relationships apparent. Firstly, the timing and magnitude of the off-shelf sediment flux appears linked to the presence and orientation of a shelf-edge rim, and to the extension and morphology of the evolving drainage network. Secondly, the periods of shelf-edge reef development and demise seem to respond to the remobilisation, trapping or redirection of fine sediments. We propose a sedimentation model for the shelf margin and the slope driven by the interplay of sea-level rise and shelf physiography, and we highlight two fundamental processes: (1) the cross-shelf sediment transport related to coastline retreat under rising sea levels, and (2) the effectiveness of transient embayments in redirecting or trapping sediments. The quantifications provided in this study have implications in the estimation of Pleistocene carbonate budgets and the atmospheric carbon cycle, as well as for past human migrations.

The Açu Reef morphology, distribution, and inter reef sedimentation on the outer shelf of the NE Brazil equatorial margin

Submerged reefs, referred to as the Açu Reefs, have been newly observed on both sides of the Açu Incised Valley on the northeastern equatorial Brazilian outer shelf. This study aims to understand the roles of shelf physiography, its antecedent morphologies, and its inter reef sedimentation on the different development stages of the biogenic reef during last deglacial sea-level rise. The data sets consist of side-scan sonar imagery, one sparker seismic profile, 76 sediment samples, and underwater photography. Seven backscatter patterns (P1 to P7) were identified and associated with eleven sedimentary carbonate and siliciclastic facies. The inherited relief, the mouth of the paleo incised valley, and the interreef sediment distribution play major controls on the deglacial reef evolution. The reefs occur in a depth-limited 25–55 m water depth range and in a 6 km wide narrow zone of the outer shelf. The reefs crop out in a surface area over 100 km2 and occur as a series of NW-SE preferentially orientated ridges composed of three parallel ridge sets at 45, 35, and 25 m of water depth. The reefs form a series of individual, roughly linear ridges, tens of km in length, acting as barriers in addition to scattered reef mounds or knolls, averaging 4 m in height and grouped in small patches and aggregates. The reefs, currently limited at the transition between the photic and mesophotic zones, are thinly covered by red algae and scattered coral heads and sponges. Taking into account the established sea-level curves from the equatorial Brazilian northeastern shelf / Rochas Atoll and Barbados, the shelf physiography, and the shallow bedrock, the optimal conditions for reef development had to occur during a time interval (11–9 kyr BP) characterized by a slowdown of the outer shelf flooding, immediately following Meltwater Pulse-1B. This 2 kyr short interval provided unique conditions for remarkable reef backstepping into distinct parallel ridge sets. Furthermore, the Açu Reefs have trapped relict siliciclastic sediments within the three sets of reefs, west of the Açu Incised Valley and adjacent coasts. Lines evidence of easterly nearshore currents carried sediments from the old Açu Incised Valley and adjacent coasts. These incipiently drowned reefs influence the water circulation patterns of the modern shelf system, its carbonate sedimentation, and sediment transport. This study provides a new example of reef occurrence which might be more commonly observed on similar equatorial continental shelves.

New multi-scale perspectives on the stromatolites of Shark Bay, Western Australia.

A recent field-intensive program in Shark Bay, Western Australia provides new multi-scale perspectives on the world’s most extensive modern stromatolite system. Mapping revealed a unique geographic distribution of morphologically distinct stromatolite structures, many of them previously undocumented. These distinctive structures combined with characteristic shelf physiography define eight ‘Stromatolite Provinces’. Morphological and molecular studies of microbial mat composition resulted in a revised growth model where coccoid cyanobacteria predominate in mat communities forming lithified discrete stromatolite buildups. This contradicts traditional views that stromatolites with the best lamination in Hamelin Pool are formed by filamentous cyanobacterial mats. Finally, analysis of internal fabrics of stromatolites revealed pervasive precipitation of microcrystalline carbonate (i.e. micrite) in microbial mats forming framework and cement that may be analogous to the micritic microstructures typical of Precambrian stromatolites. These discoveries represent fundamental advances in our knowledge of the Shark Bay microbial system, laying a foundation for detailed studies of stromatolite morphogenesis that will advance our understanding of benthic ecosystems on the early Earth.

Postglacial sediment deposition along a mixed carbonate-siliciclastic margin: New constraints from the drowned shelf-edge reefs of the Great Barrier Reef, Australia

A seismic stratigraphy analysis was conducted at two sites, Hydrographers and Noggin passages, separated by about 540km on the shelf-edge of the central Great Barrier Reef (GBR), Australia. We used recently available seismic and bathymetry data and a new synthesis of downhole logs and lithological, petrophysical and radiometric data from cores recovered by the Integrated Ocean Drilling Program Expedition 325 (Great Barrier Reef Environmental Changes). We compared the stratigraphy of both sites, identifying a full depositional sequence with deposits from at least 28ka BP to ~7ka BP, bounded by two marine flooding surfaces. Within this sequence, each systems tract is represented by unique depositional features characteristic of the shelf-edge systems. Despite the broad environmental and geomorphic similarities between the two sites, differences in postglacial reef development were remarkable. These contrasts can be explained as a result of: (1) local antecedent substrate variations and (2) the interplay of shelf physiography with Late Quaternary sea level fluctuations, which favoured changes in biological production and sediment flux as the palaeo-shoreline evolved from linear to complex during intermediate sea levels. During these intermediate sea levels, the northern estuarine coast and its steep substrate at shelf-edge locations contrasted strongly with the protected palaeo-lagoons and the extensive, gentle marginal terraces found at the southern central GBR. This setting enhanced the regional differences in sediment transport and reef development through the last transgression. The conceptual model presented here provides a broader depositional framework and improves the understanding of the main processes controlling the spatial and temporal depositional patterns on the shelf-edge of mixed siliciclastic-carbonate margins.

Nature, origin and evolution of a Late Pleistocene incised valley‐fill, Sunda Shelf, Southeast Asia

AbstractUnderstanding the stratigraphic fill and reconstructing the palaeo‐hydrology of incised valleys can help to constrain those factors that controlled their origin, evolution and regional significance. This condition is addressed through the analysis of a large (up to 18 km wide by 80 m deep) and exceptionally well‐imaged Late Pleistocene incised valley from the Sunda Shelf (South China Sea) based on shallow three‐dimensional seismic data from a large (11 500 km2), ‘merge’ survey, supplemented with site survey data (boreholes and seismic). This approach has enabled the characterization of the planform geometry, cross‐sectional area and internal stratigraphic architecture, which together allow reconstruction of the palaeo‐hydrology. The valley‐fill displays five notable stratigraphic features: (i) it is considerably larger than other seismically resolvable channel forms and can be traced for at least 180 km along its length; (ii) it is located in the axial part of the Malay Basin; (iii) the youngest part of the valley‐fill is dominated by a large (600 m wide and 23 m deep), high‐sinuosity channel, with well‐developed lateral accretion surfaces; (iv) the immediately adjacent interfluves contain much smaller, dendritic channel systems, which resemble tributaries that drained into the larger incised valley system; and (v) a ca 16 m thick, shell‐bearing, Holocene clay caps the valley‐fill. The dimension, basin location and palaeo‐hydrology of this incised valley leads to the conclusion that it represents the trunk river, which flowed along the length of the Malay Basin; it connected the Gulf of Thailand in the north with the South China Sea in the south‐east. The length of the river system (>1200 km long) enables examination of the upstream to downstream controls on the evolution of the incised valley, including sea‐level, climate and tectonics. The valley size, orientation and palaeo‐hydrology suggest close interaction between the regional tectonic framework, low‐angle shelf physiography and a humid‐tropical climatic setting.

The investigation of a mixed carbonate-siliciclastic shelf, NE Brazil: side-scan sonar imagery, underwater photography, and surface-sediment data

Side-scan sonar imagery and underwater photography, coupled with bathymetric and sedimentologic data, revealed noteworthy features and the interplay of multiple processes on the north-eastern Brazilian continental shelf. The high-resolution data cover a 500 km2 area of the shallow (~70 m) and narrow (~40 km) shelf, which comprises the submerged portion of the Potiguar Basin; the area includes the partially filled incised valley of the ancient Acu River.The seafloor imagery was integrated with 673 surface sediment samples to distinguish boundaries within the mixed siliciclastic-carbonate region. The sediment grain size increases from nearshore, muddy, siliciclastic sediments to biogenic, coarse sand and gravel on the outer shelf. Carbonate-rich sediment predominates, except on the eastern margin of the Acu Incised Valley (AIV), the nearshore zone and estuary mouths. The AIV is filled mainly with muddy sediments reworked from the latest transgressive deposits. Bedform configurations indicate bottom currents associated with tidal and wave-driven currents.Mixing processes are controlled by the interplay of high hydrodynamic energy regime and shelf physiography. Special attention was paid to the middle and outer shelves, where a large field of newly found reefs occurs between 20 and 50 m depths. The reef substrate is composed of limestones, probably belonging to the Guamare Formation.SCUBA dive inspections revealed benthic colonies (corals: Montastraea cavernosa, Porites astreoides, Siderastrea stellata; sponges: Scopalina ruetzleri, Callyspongia vaginalis, Ectyoplasia ferox, Ircinia sp., Spirastrella sp., Aiolochroia sp.) over fossil reefs. A preliminary examination of the foraminiferal species showed a Caribbean-type reef community, including Amphisorus hemprichii, Amphistegina gibbosa, Archaias angulatus, Heterostegina antillarum, Homotrema rubra, Peneroplis carinatus and P. proteus.

Chapter 22 Pacific margin, Canada shelf physiography: a complex history of glaciation, tectonism, oceanography and sea-level change

Abstract Canada's western continental shelf can be broadly divided into three geographical regions: Salish Sea, Pacific North Coast and Vancouver Island Shelf. Each region's physiography has been uniquely impacted by a history of glaciation, tectonism, oceanography and sea-level change. Retreat of the Fraser Glaciation started from the Pacific towards the mainland of British Columbia first in the north on Haida Gwaii (16 14 C ka BP) then outer Vancouver Island (14 14 C ka BP), and generally progressed east with ice persisting on the Fraser lowland and northern British Columbia mainland, and possibly the central interbasin coast, until after 10 14 C ka BP. Retreat on the Pacific North Coast was rapid, while downwasting and stagnation occurred in the Salish Sea. Sea levels responded to this deglacial history, with submergence of most of the Salish Sea by over 250 m before 12 14 C ka BP then falling to approximately 100 m where it remained until approximately 10 14 C ka BP, and emergence of the Vancouver Island Shelf with the development of a forebulge. In contrast, the Pacific North Coast crustal loading of the British Columbia mainland and development of a peripheral forebulge towards Haida Gwaii resulted in submergence of the mainland by 120 m and emergence of the western basin by more than 120 m, with regional variation throughout. Holocene sedimentation is primarily restricted to the coastal inlets and fjords, except for the southern Salish Sea off the Fraser River.

Climate and its impact on shelf physiography, ecology and human occupation with a focus on Canada's Pacific margin

Climate and its impact on shelf physiography, ecology and human occupation with a focus on Canada's Pacific margin

High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

Research Article| July 01, 2007 High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture Anthony C. Runkel; Anthony C. Runkel 1Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057, USA Search for other works by this author on: GSW Google Scholar James F. Miller; James F. Miller 2Department of Geography, Geology and Planning, Southwest Missouri State University, Springfield, Missouri 65804, USA Search for other works by this author on: GSW Google Scholar Robert M. McKay; Robert M. McKay 3Iowa Department of Natural Resources, Geological Survey Bureau, 109 Trowbridge Hall, Iowa City, Iowa 52242-1319, USA Search for other works by this author on: GSW Google Scholar Allison R. Palmer; Allison R. Palmer 4Institute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304, USA Search for other works by this author on: GSW Google Scholar John F. Taylor John F. Taylor 5Geoscience Department, Indiana University of Pennsylvania, 129 Weyandt Hall, Indiana, Pennsylvania 15705, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (7-8): 860–881. https://doi.org/10.1130/B26117.1 Article history received: 07 Oct 2006 rev-recd: 27 Feb 2007 accepted: 19 Mar 2007 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anthony C. Runkel, James F. Miller, Robert M. McKay, Allison R. Palmer, John F. Taylor; High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture. GSA Bulletin 2007;; 119 (7-8): 860–881. doi: https://doi.org/10.1130/B26117.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings.Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography. Special conditions of exceptionally slow subsidence rate, shallow bathymetry, and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings. These results suggest that if other cratonic interior successions are as anomalous as reported, a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture. The results also lead us to suggest that a nonvegetated lower Paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones, particularly their widespread yet thin geometry, and a scarcity of shale and siltstone. 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Geomorphology and age of the Oxygen isotope stage 2 (last lowstand) sequence boundary on the northwestern Gulf of Mexico continental shelf

Abstract The sequence boundary associated with the last glacial-eustatic lowstand was mapped across the northwestern Gulf of Mexico continental shelf. The geomorphology of incised fluvial valleys varies widely across the shelf. These differences are due to differences in shelf physiography and the interval of the eustatic cycle the valleys were occupied. Incision begins during the falling limb of sea level and results in terraced valleys. Rivers that abandoned their valleys during the fall in sea level to cut new valleys during the lowstand generally have u-shaped profiles. Incised valleys connected to turbidite systems only occurred in two valleys (the Colorado and Rio Grande), but this may be because sea level did not fall below the shelf break during the last eustatic cycle. Some valleys deepen in an offshore direction, others become shallower. The timing of fluvial incision was constrained using radiocarbon dates so that incision can be tied directly to the sea-level curve for the last glacial-eustatic cycle. The results show that the fluvial incision occurred throughout the falling limb of sea level and lowstand; however, maximum incision occurred during the lowest position of sea level. The resulting surface has significant relief, extends across the shelf, and has time significance. The associated conformable surface, on the other hand, is much harder to recognize and occurs at different stratigraphic levels relative to different shelf-margin deltas.

The Effect of Sea Level, Climate, and Shelf Physiography on the Development of Incised-Valley Complexes: A Modern Example From the East China Sea

ABSTRACT Analysis of high-resolution seismic reflection and sonar data from the East China Sea (ECS) continental shelf reveals the presence of an extremely broad (> 330 km) and deep (maximum incision of 72 m) incised-valley complex whose development and subsequent fill is constrained to the end of the last glacial cycle ( 20,000 yr BP to present). Results from this study indicate that sea-level change, climate-controlled discharge and sediment supply, and shelf physiography were important factors in the development and subsequent fill of the ECS incised- valley complex. For example, wet climatic conditions during the initial Marine Oxygen Isotope Stage (MIS) 2 fall of sea level promoted fluvial erosion of the exposed shelf. However, the trend toward drier conditions during the MIS 2 fall of sea level slowed the overall development of the incised-valley complex to the point where only minor incision occurred during the maximum lowstand. Furthermore, the low-gradient morphology of the exposed outer shelf diminished the ability of fluvial systems to incise. Instead, fluvial systems migrated laterally, creating a shallow ( 300 km) incised-valley system on the outer shelf. During the late MIS 2 rise of sea level, accommodation was generated within the flooded reaches of the valley complex. However, arid climatic conditions that prevailed at this time resulted in minor sediment delivery to the transgressive shoreline via the incised-valley complex. Thus, the upper part of the drowned lowstand fluvial deposits were exposed at the sea floor and reworked into a valley-wide tidal-bar and tidal sheet complex. Wetter climatic conditions during middle to late MIS 1 resulted in abundant sediment delivery to the drowned parts of the valley complex (estuary) and buried lowstand fluvial deposits below the depth of tidal ravinement, where they remained undisturbed. Current sequence stratigraphic models can be applied to the late Quaternary stratigraphic secession of the ECS. However, these models are too simplistic because they rely largely on rates and directions of sea-level change to explain stratal architectures. Although the extent and rate of sea-level change is extremely important, this study shows that high-frequency climate change and shelf physiography also play important roles in the development of stratigraphic architectures. These factors are particularly important for the development of incised valleys, where they directly influence incised-valley morphology and facies distributions.

On the significance of sea-level variations and shelf paleo-morphology in governing sedimentation in the southern South China Sea during the last deglaciation

Deglacial sedimentation on the outer Sunda Shelf, the shelf margin and slope shows a close correlation with the stages of flooding of the emerged shelf. It is mainly controlled by the relationship between shelf paleo-physiography and changes in sea level and sediment supply. Based on the sedimentological and geochemical variability at five sites along a transect across the outer shelf and the continental slope covering the last 20 000 years, four intervals of significant depositional changes are identified. The first reorganization in the sedimentation regime, which was established during sea-level lowstand, is associated with the drowning of the lower course of the North Sunda River between 16.5 and 14.5 ka. During and following the rapid rise in sea level at 14.5–14 ka, sedimentation on the slope is characterized by an accelerated decrease in the supply of terrigenous material, associated with the flooding of the middle part of the paleo-valley. This abrupt change in sedimentation is followed by a period of gradual decrease of terrigenous supply to the slope, when the surrounding plains of the river valley were flooded (14–8.5 ka). The final change occurred between ca. 8.5 and 6 ka, when the coastline reached its modern position, and modern sedimentary patterns are established. This change probably marks the end of reworking by transgression. Given the synchroneity of sedimentological changes along the transect across the Sunda Shelf and into the deep sea with the environmental shifts on the central shelf, sedimentation on the margin and slope appears to be mainly controlled by the interrelationship of sea level, shelf paleo-physiography and sediment supply. Moreover, the records display lateral changes through time associated with stages of the deglacial transgression, which mask any potential sedimentological variability due to changes in precipitation, vegetation or atmospheric circulation. This fact cautions the interpretation of near-shore sedimentary records as unequivocal recorders of local climate change (e.g. SE Asian monsoon), and highlights the need to include variations in shelf physiography and sea level in future paleoceanographic studies.

Surface facies and sediment dispersal patterns: southeastern Gulf of Cadiz, Spanish continental margin

Grain size of 294 samples and mineralogical analysis of 364 surface samples from the Spanish continental shelf and upper slope of the Gulf of Cadiz delineate four sediment facies that derive mainly from siliciclastic sources. The northern area is formed by the mud-rich prodelta of the Guadalquivir River, parallel to the coast and progradational towards the southeast. It exhibits a typical bulk mineral association of illite ≫ smectite > kaolinite + chlorite. In the north-central area, there is a smaller, coarser-grained prodelta in the Bay of Cadiz, linked to the Guadalete River, which offshore becomes connected to the Guadalquivir prodelta. Its mineral association is quartz ≫ illite > kaolinite + chlorite. In the southern area, where fluvial input is negligible, the characteristic coarse-grained facies generally corresponds to relict, Late Pleistocene reworked deposits. The typical mineral association is quartz > calcite > aragonite ≫ smectite–mixed layer clay minerals. The fourth area corresponds to the shelf break and upper slope offshore Cadiz Bay, where a westward grain-size coarsening differentiates the deposits of the last low sea-level stand and no typical mineral association exists. The mineralogy of the terrigenous deposits indicates a source area from Paleozoic sequences of the Sierra Morena to the north and the Subbetic Units and Gibraltar flysch to the east. The Neogene deposits of the Guadalquivir depression are an additional source of shelf sediments. The surface sediment distribution on the inner continental shelf is largely determined by the southeastern Atlantic water flow, the terrigenous input from rivers, and the shelf physiography. Sediment dispersal on the outer continental shelf and upper slope, in contrast, largely reflects the location of low sea-level stand, the development of palimpset deposits, and the location of the Mediterranean undercurrent over the margin.

Volcanic ash in surficial sediments of the Kodiak shelf — An indicator of sediment dispersal patterns

Surficial sediments of the Kodiak shelf, Gulf of Alaska, contain various amounts of volcanic ash whose physical properties indicate that it originated from the 1912 Katmai eruption. The distribution of ash is related to the shelf physiography and represents redistribution by oceanic circulation rather than the original depositional pattern from the volcanic event. The ash distribution can be used, in conjunction with the distribution of grain sizes, as an indicator of present-day sediment dispersal patterns on the shelf. No significant modern input of sediment is occurring on the Kodiak shelf, which is mostly covered by Pleistocene glacial deposits. Coarse-grained sediments on flat portions of shallow banks apparently are being winnowed, with the removed ash-rich fine material being deposited in shallow depressions on the banks and in three of the four major troughs that cut transversely across the shelf. The other major trough seems to be experiencing a relatively high-energy current regime, with little deposition of fine material.