首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.  相似文献   

2.
Seventeen vibrocores from the inner part of Joseph Bonaparte Gulf off northwestern Australia penetrate a range of marine and marginal‐marine sediments deposited in the post‐glacial transgression and highstand. Ranging from gravelly sand to fine silt, these sediments contain a diverse fossil biota dominated by molluscs and bryozoans, but also including ostracods and foraminifers. Minor components include solitary corals, echinoids, soft coral and sponge spicules, wood debris and bone fragments. The biota can be divided into five major marine or marginal‐marine environments (intertidal, lagoonal, estuarine, strandline and shelf) and one terrestrial (riverine) environment. The intertidal environment contains four sub‐assemblages (mangroves, salt marsh, mud flat and sand flat) and the shelf environment six sub‐assemblages (hard substrate inner shelf, sandy substrate inner shelf, muddy substrate inner shelf, epiphytic, inshore and oceanic). The most useful organisms for palaeoenvironmental reconstruction are bryozoans for differentiating various shallow‐marine substrates, and foraminifers and ostracods for defining water depths, euryhaline, freshwater and oceanic influences. Palynomorphs were the only microfossils capable of providing control on terrestrial environments. The scarcity of marine plankton and the dominance of terrestrial palynomorphs in these marine sediments provides a salutary warning of the dangers of relying on plant microfossils alone when no independent environmental data are available to test the interpretation. The mollusc and bryozoan biota in the inner part of Joseph Bonaparte Gulf superficially resembles the bryomol assemblage of cool‐water shelves. This biotic assemblage is the result of turbidity rather than water temperature. The turbidity suppresses the photosynthetic, zooxanthellate and hermatypic organisms allowing molluscs, bryozoans and other apparently cool‐water biotic elements to dominate.  相似文献   

3.
《Earth》2009,92(1-4):27-76
In this study we reconstruct the evolution of the northern New England passive margin whose development has been influenced by Pleistocene glaciations. The morphology of the northern New England shelf is rather unique consisting of a inner lowland, the Gulf of Maine, with an average depth of 150 m and an area of 90,700 km2 and Georges Bank, a high whose crest is less than 40 m deep and has an area of 27,000 km2. The bank's northern slope, facing the Gulf of Maine, has a maximum relief of 377 m. On the seaward side of Georges Bank is the 2000 m high continental slope deeply cut by canyons. Two channels, Northeast and Great South Channels, east and west of Georges Bank, provide passageways from the Gulf of Maine to the open sea. This morphology was acquired by a combination of Tertiary fluvial erosion, Pleistocene glacial erosion/deposition and Pleistocene/Holocene marine processes. Fluvial/glacial erosion in the Gulf of Maine was so extensive as to expose basement, thus making it possible to map the various terranes making up this foundation. These terranes include the pre-Carboniferous Avalon and Meguma units, a Carboniferous–Permian rift basin formed by the oblique continental collision during the closure of the Paleozoic proto-Atlantic and a Late Triassic–Early Jurassic rift system created during the opening of the present Atlantic. Basement in the Gulf of Maine remained above sea level from the opening of the Atlantic 190 Ma (Early Jurassic) to the Eocene 55 Ma. That the Gulf of Maine remained a high for so long may have been due to igneous activity along the northwest-trending Boston–Ottawa Lineation extending from the vicinity of the St. Lawrence River, Canada to Gulf of Maine from Late Triassic to Early Cretaceous. The northwest-trending New England Seamounts south of Georges Bank may represent a seaward extension of this lineation. On Georges Bank, rising hundreds of meters above the Gulf of Maine, the basement exposed in the gulf is mantled by sediments thousands of meters thick. Included in these sediments are Early Jurassic- to earliest Cretaceous reefs along the continental slope and carbonates north of the reefs grading landward into continental sediments, Cretaceous–Cenozoic continental/marine terrigenous sediments and Pleistocene glacial deposits. The continental slope on the seaward flank of Georges Bank has a complex history of early to mid Mesozoic carbonate accretion, mid to late Mesozoic and Cenozoic calcareous/terrigenous sediments and canyon erosion, burial and exhumation going back to Early Cretaceous.  相似文献   

4.
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.  相似文献   

5.
The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.  相似文献   

6.
Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud‐mounds in northern Belize. They encompass areas of 20 km2 and 35 km2 in distal and proximal positions, respectively, on a wide and shallow‐water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening‐upward transgressive systems tract, condensed section and shallowing‐upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud‐mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea‐level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound‐core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound‐cores represents highstand systems tract sediments deposited in the last ≈ 4500 years during slow and decelerating sea‐level rise. Within these sections, there is an early phase of progradationally offlapping catch‐up deposition and a later (and current) phase of aggradational keep‐up deposition. The mound‐cores comprise stacked storm‐deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia‐colonized surfaces. Vertical stacking of these units imparts a quasi‐cyclic architecture to the section that superficially mimics metre‐scale parasequences in ancient rocks. The locations of the mud‐mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud‐mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand‐size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.  相似文献   

7.
The Ouémé River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km2 circular central basin, Lake Nokoué, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km2). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood‐tidal delta sand deposited in association with non‐preserved transgressive sand barriers. The change in estuary‐mouth sedimentation from a transgressive barrier‐inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross‐barrier tidal flux associated with barrier‐inlet systems. As barrier formation west of the Ouémé River led to an increasingly rectilinear shoreline, the longshore drift cell matured, ensuring voluminous eastward transport of sand from the Volta Delta in Ghana, the major purveyor of sand, to the Ouémé embayment, 200 km east. Concomitantly, the number of tidal inlets, and the tidal flux associated with a hitherto interlinked lagoonal system on this coast, diminished. Complete sealing of Lake Nokoué has produced a large, permanently closed estuary, where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east. Since 1885, tides have entered the estuary directly through an artificial outlet cut across the sand barrier. Although precluding the seaward loss of fluvial sediments, permanent estuary‐mouth closure has especially deprived the highstand estuary of marine sand, a potentially important component in estuarine infill on wave‐dominated coasts. In spite of a significant fluvial sediment supply, estuarine infill has been moderate, because of the size of the central basin. Estuarine closure has resulted in two co‐existing highstand sediment suites, with limited admixture, the marine‐derived, estuary‐mouth barrier and upland‐derived back‐barrier sediments. This situation differs from that of mature barrier estuaries characterized by active fluvial‐marine sediment mixing and facies interfingering.  相似文献   

8.

Holocene sediments from southern Spencer Gulf are cool‐water carbonate‐rich gravels and sands, dominated by molluscs and Bryozoa. Five sedimentary fades are recognized: (i) molluscan gravel; (ii) branching coralline‐algal gravel, associated with shallow partially protected environments; (iii) molluscan‐biyozoan sand; (iv) mixed bioclastic sand, representative of the deeper central region of the lower gulf; and (v) bryozoan gravel, an isolated fades developed in a semi‐protected micro‐environment. The southern gulf is characterized by complex oceanographic conditions together with variations in water depth and substrate. The sediments share the characteristics of both the southern shelf and upper Spencer Gulf. Grain‐size distribution and sedimentary facies are controlled by a combination of all the above processes. Past sea level fluctuations are recognized from sea floor strand‐line deposits. The relic component of the palimpsest sediments has eroded from the Pleistocene aeolianite dunes. The sediments, therefore, reflect both the modern marine and past environments.  相似文献   

9.
The Rottnest Shelf is a narrow, wave-dominated open shelf on the passive continental margin of southwest Australia, adjacent to a hinterland of low relief and sluggish drainage. High physical energy, low nutrients in cool subtropical waters, and rapid postglacial transgression have limited carbonate productivity, restricted grain types, and reworked the transgressed surface to form only a thin ( < 1 m) blanket of carbonate and relict sediment, with little terrigenous influx. Subaerial weathering of the shelf during Late Pleistocene emergence was followed by postglacial drowning, erosional shoreface retreat, and generation of a transgressive lag deposit. Establishment of the warm temperate biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf disconformity surface and generation of hardground veneers and thin skeletal carbonate sheets. Linear topographic ridges of Pleistocene limestone partition the shelf into systems with varying physical energy, biota and sediment supply. The Holocene sediments are a shallowing-upward coastal sequence; wave-ripple cross-stratified grainstone (Inner Shelf); and bioturbated bryozoan grainstone to skeletal wackestone (Outer Shelf to Upper Continental Slope), characterised by seaward fining and increasing percentages of planktic carbonate sediment.

Given sufficient time, the Rottnest Shelf could recover from drowning, and form blanket-like skeletal carbonates. Thin ( < 1 m) lags overlying disconformities, which underlie shallowing-upward coastal and shelf sediments a few metres thick, will be generated by glacio-eustatic cycles of sedimentation (105 y duration). Thick (several tens of metres) sediment bodies, composed of wave-rippled to bioturbated skeletal carbonate sediment with a temperate biota, will be formed during longer term (1–10 My) sedimentation cycles. Such cycles have characterised passive margins during the Cenozoic. The Rottnest Shelf thus provides a facies model for temperate shelf sedimentation along passive continental margins.  相似文献   


10.
High-resolution seismic records obtained in the Rio Grande do Sul coastal zone, southern Brazil, revealed that prominent valleys and channels developed in the area before the installation of actual coastal plain. Landwards, the paleoincisions can be linked with the present courses of the main river dissecting the area. Oceanwards, they can be linked with related features previously recognized in the continental shelf and slope by means of seismic and morphostructural studies. Based mainly on seismic, core data and geologic reasoning, it can be inferred that the coastal valleys were incised during forced regression events into the coastal prism deposited during previous sea level highstand events of the Quaternary. Seismic data has revealed paleovalleys up to 10 km wide and, in some places, infilled with up to 40 m thick of sediments. The results indicated two distinct periods of cut-and-fill events in the Patos Lagoon area. The filling of the younger incision system is mainly Holocene and its onset is related to the last main regressive event of the Pleistocene, when the sea level fell about 130 m below the actual position. The older incision and filling event is related to the previous regressive–transgressive events of the Middle and Late Pleistocene. The fluvial discharge fed delta systems on the shelf edge during the sea level lowstands. The subsequent transgressions drowned the incised drainage, infilling it and closing the inlets formerly connecting the coastal river to the ocean. The incised features may have played a significant role on the basin-margin architecture, facies distribution and accommodation space during the multitude of up and down sea level events of the Quaternary.  相似文献   

11.
Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.  相似文献   

12.
  相似文献   

13.
The Lower Silurian siliciclastic Coralliferous Group is shown to have been deposited in an intra‐shelf position 10–15 km south of the palaeogeographic shelf‐break of the Welsh Basin. After a phase of thermal subsidence related to the development of the predominantly Llandovery Skomer Volcanic Group, the shelf basin was transgressed. This transgression was punctuated by an episode of tectonic uplift in southern Pembrokeshire, resulting in subaerial exposure of the shelf and a significant basinward shift in sedimentary environments. Erosion and sediment bypass ensued, with coarse‐grained low‐sinuosity fluvial channels transporting sediment to the northerly Welsh Basin, where significant submarine fans developed. During the early Telychian, renewed transgression took place, with lowstand gravels being ravined and reworked into parasequences of the transgressive systems tract. These thin, coarse‐grained parasequences record deposition within high‐energy wave‐dominated shoreface/inner shelf environments. Further coastal onlap resulted in the closing down of significant coarse‐grained sediment supply, with the remaining Coralliferous Group being dominated by wave‐influenced silts, mud‐shales and thin sandstones comprising the highstand systems tract. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

14.
Upper Carboniferous to Lower Permian sedimentary rocks extend along the periphery of the northern Sydney Basin, a sub‐basin of the Sydney‐Gunnedah‐Bowen Basin complex. The basin contains basal basalts and volcanic sediments deposited in a nascent rift zone. This rift zone was created through crustal thinning during trench rollback on the eastern edge of the New England Orogen. Thermal subsidence created accommodation for predominantly marine Dalwood Group sediments. Clastic sedimentation then occurred in the Maitland‐Cessnock‐Greta Coalfield and Cranky Corner Basin during the Early Permian. This occurred on a broad shelf undergoing renewed thermal subsidence on the margin of a rift flank of the Tamworth Belt of the southern New England Orogen. Braidplain fans prograded or aggraded in two depositional sequences. The first sequence commences near the top of the Farley Formation and includes part of the Greta Coal Measures, while the second sequence includes the majority of the Greta Coal Measures and basal Branxton Formation. Thin, areally restricted mires formed during interludes in a high sedimentation regime in the lowstand systems tracts. As base‐level rose, areally extensive mires developed on the transgressive surface of both sequences. A paludal to estuarine facies changed to a shallow‐marine facies as the braidplain was transgressed. The transgressive systems tracts continued to develop with rising relative sea‐level. Renewed uplift in the hinterland resulted in the erosion of part of the transgressive systems tract and all of the highstand systems tract of the lower sequence. In the upper sequence a reduction in relative sea‐level rise saw the development of a deltaic to nearshore shelf highstand systems tract. Extensional dynamics caused a fall in relative base‐level and the development of a sequence boundary in the Branxton Formation. Finally, renewed thermal subsidence created accommodation for the overlying, predominantly marine Maitland Group.  相似文献   

15.
Very-high-resolution seismic data acquired on the Rhône continental shelf were used to address the detailed morphology of Late Pleistocene and post-glacial units (from 18 000 yr BP to the present). Two groups of units can be distinguished. (1) Lower units that are mainly the product of variations in relative sea level. They comprise the last Pleistocene regressive deposits made of alluvial sheets (U0, U1) and, above, transgressive deposits that can be divided into: backstepping transgressive units (U3 and U4a), deposited during the landward retreat of the river mouth and transgressive units (U4b, U4c) laid down during the inundation of the shelf. A prograding littoral/lagoon system (U5/U6) indicating a fluctuation during the rise in sea level caps the transgressive units. This work has emphasized the complexity of these depositional environments, mainly related to a river system situated near the current Petit-Rhône. (2) Upper units that make up the recent Rhône delta and correspond to the current highstand systems tract (HST) developed since the stabilization of relative sea level. Seven prodeltaic lobes have been identified (U7–U13). Two of them are bilobate (U7 and U8) and date from a period when the delta was split as a result of the fluviatile system being progressively divided into several channels. Other prodeltaic lobes (U9, U10) are stacked in front of the distributary's outlet, recording several periods of outflow. The results show a strong correlation with studies on land. The distribution of recent prodeltaic lobes was constrained by canalization of the Rhône river to prevent the effects of climatic crises or other natural disasters.  相似文献   

16.
Suspended sediments (SS) from the Atchafalaya River (AR) and the Mississippi River and surficial sediment samples from seven shallow cross-shelf transects west of the AR in the northern Gulf of Mexico were examined using elemental (%OC, C/N), isotopic (δ13C, Δ14C), and terrigenous biomarker analyses. The organic matter (OM) delivered by the AR is isotopically enriched (∼−24.5‰) and relatively degraded, suggesting that soil-derived OM with a C4 signature is the predominant OM source for these SS. The shelf sediments display OC values that generally decrease seaward within each transect and westward, parallel to the coastline. A strong terrigenous C/N (29) signal is observed in sediments deposited close to the mouth of the river, but values along the remainder of the shelf fall within a narrow range (8-13), with no apparent offshore trends. Depleted stable carbon isotope (δ13C) values typical of C3 plant debris (−27‰) are found near the river mouth and become more enriched (−22 to −21‰) offshore. The spatial distribution of lignin in shelf sediments mirrors that of OC, with high lignin yields found inshore relative to that found offshore (water depth > 10 m).The isotopic and biomarker data indicate that at least two types of terrigenous OM are deposited within the study area. Relatively undegraded, C3 plant debris is deposited close to the mouth of the AR, whereas more degraded, isotopically enriched, soil-derived OM appears to be deposited along the remainder of the shelf. An important input from marine carbon is found at the stations offshore from the 10-m isobath. Quantification of the terrigenous component of sedimentary OM is complicated by the heterogeneous composition of the terrigenous end-member. A three-end-member mixing model is therefore required to more accurately evaluate the sources of OM deposited in the study area. The results of the mixing calculation indicate that terrigenous OM (soil-derived OM and vascular plant debris) accounts for ∼79% of the OM deposited as inshore sediments and 66% of OM deposited as offshore sediments. Importantly, the abundance of terrigenous OM is 40% higher in inshore sediments and nearly 85% higher in offshore sediments than indicated by a two-end-member mixing model. Such a result highlights the need to reevaluate the inputs and cycling of soil-derived OM in the coastal ocean.  相似文献   

17.
Thick bay‐fill sequences that often culminate in strandplain development serve as important sedimentary archives of land–ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground‐penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes – notably a reduction in wave energy in response to coastal embayment infilling – in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5·8 ka, the 75 km2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi‐enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressive–regressive sequence, and backed by a highstand barrier‐island. The strandplain is immediately underlain by 5 to 16 m of seaward‐thickening, fluvially derived, Holocene‐age, basin‐fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand‐dominated to mud‐dominated; (ii) beachface slopes decrease from >11° to ca 7°; and (iii) progradation rates increase from 0·4 to 1·8 m yr?1. Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea‐level fall and sediment infilling and a decrease in onshore wave‐energy transport from 18 to 4 kW m?1. The combination of allogenic (sediment supply, falling relative sea‐level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records. These findings demonstrate the complexities in architecture styles of highstand and regressive systems tracts. Furthermore, this article highlights the diverse internal and external processes and feedbacks responsible for the development of these intricate marginal marine sedimentary systems.  相似文献   

18.
雪峰古陆边缘沅陵地区的上石炭统岩性特殊,以灰岩为主平白云质灰岩、砾屑灰岩与砾岩、砂岩相互成层,交替出现,因此,相对海平面升降变化在这种类型沉积中表现明显,利用层序地层的研究。上石炭统为１个三级层序,底部以Ｉ型层序与震旦系留茶坡组硅质岩接触;顶部仍以Ｉ型层序边界与下二叠统黔阳组成邻。包括低水位体系域、海进体系域和高水位体系域;依据准层序的叠是分为两个准层序组,即海进体系域和高水位体系域。  相似文献   

19.
High resolution seismic lines from the inner and mid-shelf of the Durban Bight reveal an unprecedented view of the seismic stratigraphy of the central KwaZulu-Natal uppermost continental margin. Seven units are recognised from the shelf on the basis of their stratal architecture and bounding unconformities. These comprise four incompletely preserved sequences consisting of deposits of the highstand systems tract (Unit B), falling stage systems tracts (Unit C), the transgressive systems tract (Units A, D and G) and lowstand systems tracts (early fill of the incised valleys and strike diachronous prograding reflectors of Unit A). Seismic facies recognised as incised valley fills correspond to the lowstand and transgressive systems tracts. When integrated with published accounts of onshore and offshore lithostratigraphy and local sea level curves, we recognise an Early Santonian transgression (Unit A to Unit B), superimposed by uplift-induced pulses of forced regression. A Late Campanian relative sea level fall (Unit C) followed. Sediments of the Tertiary period are not evident on the Durban Bight shelf except for isolated incised valley fills of Unit D lying within incised valleys of Late Pliocene age. Overlying these are two stages of Pleistocene shoreline deposits of indeterminate age. Erosion concurrent with relative sea level fall towards the last glacial maximum shoreline carved a third set of incised valleys within which sediments of the Late Pleistocene/Holocene have infilled.  相似文献   

20.
Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号