A depositional model for offshore deposits of the lower Blue Gate Member,Mancos Shale,Uinta Basin,Utah, USA          下载免费PDF全文

Lauren P. Birgenheier  Brendan Horton  Andrew D. McCauley  Cari L. Johnson  Angela Kennedy 《Sedimentology》2017,64(5):1402-1438

Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.  相似文献   

Architecture and formation of transgressive–regressive cycles in marginal marine strata of the John Henry Member,Straight Cliffs Formation,Upper Cretaceous of Southern Utah,USA     

JESSICA L. ALLEN  CARI L. JOHNSON 《Sedimentology》2011,58(6):1486-1513

Marginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive – regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.  相似文献   

Combined tide and wave influence on sedimentation patterns in the Upper Jurassic Swift Formation, south-eastern Alberta   总被引：1，自引：0，他引：1  

M. Molgat  & R. W. C. Arnott 《Sedimentology》2001,48(6):1353-1369

In south‐eastern Alberta, the Oxfordian Swift Formation comprises two unconformity‐bounded sequences. Sequence 1 consists of the shale member. It unconformably overlies Bathonian calcareous shale of the Rierdon Formation and consists dominantly of shale with uncommon hum‐ mocky cross‐stratified siltstone. The shale member comprises up to four parasequences that show a subtle upward‐fining stacking pattern, suggesting overall deepening in a storm‐dominated fully marine basin. Deposition of the shale member was terminated by a fall in relative sea‐level, which in turn initiated sequence 2. During lowstand, a network of north‐east‐ to south‐west‐trending meandering channels incised the top of sequence 1. Despite the preserved morphology of these channels, observations in core suggest that lowstand deposits are absent. These strata were most likely thoroughly reworked by ravinement processes associated with the ensuing transgression and are recorded only by a thin chert pebble lag. Also associated with transgression were significant changes in the physical and ecological conditions in the local Swift basin. These changes were related to modifications in basin configuration and the development of a low‐energy strait, which probably formed as a result of uplift of the Sweetgrass Arch. Within this newly configured basin, brackish‐water conditions prevailed, and sedimentation was dominated by suspension deposition of mud. Bed‐load sediment, composed mostly of very fine and fine sand, made up only a small part of the total sediment flux into the area and was transported principally by low‐energy combined flows with variable‐speed microtidal currents and a low‐energy oscillatory component related to storm waves. Sand was generally deposited in the form of small, discontinuous sand ridges that developed locally throughout the study area – it is these features that form the principal hydrocarbon reservoir bodies in the Swift Formation in the study area. After initiation, ridges built upwards and migrated laterally in response to transport and preferential local deposition of bed‐load sediment. Although common near the unconformity along the top of sequence 1, sand ridges also occur at several higher stratigraphic levels in sequence 2. These latter ridges, however, are not associated with regionally correlatable discontinuities but, instead, were initiated by local bed irregularities, possibly related to breaking internal waves, and were supplied with new sediment transported into the study area. The origin of these sand ridges is therefore not related to changes in relative sea‐level but, instead, to intrabasinal processes in a low‐energy strait.  相似文献   

Depositional processes on an ancient and modern muddy shelf, northern California     

ELANA L. LEITHOLD 《Sedimentology》1989,36(2):179-202

The modern Eel River shelf and analogous Pleistocene Rio Dell Formation in northern California provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence, and thereby enables further understanding of muddy-shelf processes. The modern shelf is the site of accumulation of a thick deposit of Holocene mud. Both large-scale sediment distribution patterns and small-scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation, even on a high-energy, ‘storm-dominated’ coast. The major factors in preservation of this flood ‘signature’ are the cohesive behaviour of fine-grained sediments and episodically rapid rates of sediment input. The Rio Dell Formation includes approximately 400 m of mostly fine-grained shelf deposits that accumulated offshore from a palaeo-Eel River mouth. The shelf sediments comprise four depositional sequences. Sequence 1 records progradation from outer to inner shelf depths. Facies trends closely resemble across-shelf trends on the modern shelf, suggesting that processes were similar. Detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf. Muddy facies of the Rio Dell sequence are characterized by bioturbated, clayey silts, interbedded with event layers of several types. Clay-rich silt layers are interpreted as flood deposits and physically stratified, coarse-silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms. Sediment-transport calculations and consideration of grain-size distributions of bioturbated sediments, which form the bulk of the Rio Dell sequence, suggest that these sediments are the result of biological homogenization of the fine-grained flood deposits and of the coarser-grained storm deposits. The results of this study in general indicate that fine-grained shelf deposits do preserve a distinguishable, if subtle, record of depositional processes.  相似文献   

Stratigraphical architecture at the muddy margin of the Cretaceous Western Interior Seaway, southern Utah     

ELANA L. LEITHOLD 《Sedimentology》1994,41(3):521-542

The Tropic Shale and correlative Tununk Shale Member of the Mancos Shale accumulated during Cenomanian-Turonian time, within prodeltaic environments near the western margin of the Western Interior Seaway of North America. Stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick, fine grained succession. The Tropic and Tununk shales were deposited during the Greenhorn second-order sea level cycle, over a time span of about 2–5 million years. In southern Utah, six depositional sequences are superposed upon the record of this long term sea level change. The sequences developed during third-order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences, arranged in retrogradational, aggradational and progradational parasequence sets. The Tropic Shale and Tununk Shale Member accumulated just basinward of the axis of maximum subsidence of a foreland basin. Stratal geometries and facies distribution patterns in the succession indicate that in southern Utah the Greenhorn cycle was tectonically controlled. During the Greenhorn transgression and highstand, rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area, in the more proximal foreland. At this time, hemipelagic facies accumulated at relatively slow rates in southern Utah and type 2 sequences developed during third-order sea level cycles. In contrast, during the Greenhorn regression rates of thrust-induced subsidence in the proximal foreland basin evidently slowed, and deltaic clinoforms prograded across the study area. At least one forced regression occurred in southern Utah at this time, and type 1 sequences developed. The formation of type 1 sequence boundaries in the upper part of the Tropic Shale and Tununk Shale Member points to episodes of base level fall and indicates that the six third-order sea level cycles recorded in the succession were not the result of changes in sediment supply alone. The third-order cycles may have been a consequence of episodic tectonism. The timing of these cycles, however, suggests that development of sequences and parasequences in the Tropic Shale and Tununk Shale Member may have been related to orbital forcing in the Milankovitch band. Glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency sea level changes.  相似文献   

Expanding the spectrum of shallow‐marine,mixed carbonate–siliciclastic systems: Processes,facies distribution and depositional controls of a siliciclastic‐dominated example     

《Sedimentology》2018,65(5):1558-1589

Most of the present knowledge of shallow‐marine, mixed carbonate–siliciclastic systems relies on examples from the carbonate‐dominated end of the carbonate–siliciclastic spectrum. This contribution provides a detailed reconstruction of a siliciclastic‐dominated mixed system (Pilmatué Member of the Agrio Formation, Neuquén Basin, Argentina) that explores the variability of depositional models and resulting stratigraphic units within these systems. The Pilmatué Member regressive system comprises a storm‐dominated, shoreface to basinal setting with three subparallel zones: a distal mixed zone, a middle siliciclastic zone and a proximal mixed zone. In the latter, a significant proportion of ooids and bioclasts were mixed with terrigenous sediment, supplied mostly via along‐shore currents. Storm‐generated flows were the primary processes exporting fine sand and mud to the middle zone, but were ineffective to remove coarser sediment. The distal zone received low volumes of siliciclastic mud, which mixed with planktonic‐derived carbonate material. Successive events of shoreline progradation and retrogradation of the Pilmatué system generated up to 17 parasequences, which are bounded by shell beds associated with transgressive surfaces. The facies distribution and resulting genetic units of this siliciclastic‐dominated mixed system are markedly different to the ones observed in present and ancient carbonate‐dominated mixed systems, but they show strong similarities with the products of storm‐dominated, pure siliciclastic shoreface–shelf systems. Basin‐scale depositional controls, such as arid climatic conditions and shallow epeiric seas might aid in the development of mixed systems across the full spectrum (i.e. from carbonate‐dominated to siliciclastic‐dominated end members), but the interplay of processes supplying sand to the system, as well as processes transporting sediment across the marine environment, are key controls in shaping the tridimensional facies distribution and the genetic units of siliciclastic‐dominated mixed systems. Thus, the identification of different combinations of basin‐scale factors and depositional processes is key for a better prediction of conventional and unconventional reservoirs within mixed, carbonate–siliciclastic successions worldwide.  相似文献   

Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia   总被引：1，自引：0，他引：1  

NOEL P. JAMES  YVONNE BONE†  CHRISTOPHER C. VON DER  BORCH‡ VICTOR A. GOSTIN† 《Sedimentology》1992,39(5):877-903

The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.  相似文献   

Estimating palaeobathymetry of wave-graded continental shelves from sediment texture     

G. B. DUNBAR  P. J. BARRETT 《Sedimentology》2005,52(2):253-269

The concept of the wave-graded continental shelf, with sea floor sediment coarsening from offshore mud to shoreface sand, has been well known from the time of Johnson (1919) . Although most of the modern shelf shows textures unrelated to water depth on account of relict features or sediment starvation, the geological record is more likely to preserve sites where sediment is being fed to a subsiding inner-shelf. These consistently show the landward-coarsening pattern of the wave-graded shelf, recording past water depth history in accumulated sea floor sediment. The landward-coarsening pattern is driven primarily by wave-induced bed shear stress, which increases shoreward exponentially, although it also varies from place to place with wave climate, and can be influenced by sediment concentration and currents. In this study, the relationship between bed shear stress, sediment texture and water depth has been investigated by comparing per cent mud and wave climate data from shore-normal transects of three modern wave-graded coastal settings: Wellington Harbour (low energy) and the Manawatu coast (moderate energy) in New Zealand, and Monterey Bay in California (moderate–high energy). Samples from all three locations show a progressive change from poorly sorted mud offshore to well-sorted fine sand nearshore, with the sand–mud transition ranging from 3 m (low energy) to 50 m (moderate–high energy), reflecting differences in average bed shear. Repeat measurements of per cent mud on seasonal, annual and decadal time scales along a Manawatu coastal transect showed no measurable change, demonstrating equilibrium between sediment supply, wave energy, water depth and sediment texture. A simple model based on the relationship between wave climate, shear stress and per cent mud, and using data and conditions from the modern Manawatu coast, is applied to two mid-Pliocene cyclothems exposed 50 km inland, giving results comparable with estimates from foraminifera and the deep-sea isotope record. Per cent mud offers more detail for palaeobathymetric trends in shallow water shelf strata than other proxy depth recorders, although it is limited to depths above wave base and requires an independent estimate of wave climate if depths are to be quantified.  相似文献   

Cyclic sedimentation patterns in Lower Lias mudstones of Yorkshire (GB)     

F.S.P. van  Buchem I.N. McCave 《地学学报》1989,1(5):461-467

Distinct stratification patterns in Lower Liassic mudstones are due to the regular occurrence of shell beds, silty/sandy layers and concretionary horizons. The mudstones formed in four depositional environments: (1) a storm dominated shallow marine system; (2) a hemipelagic system; (3) a shallow marine system influenced by climatic changes at astronomical frequencies; and (4) a pro-deltaic offshore transition/shallow marine system. The tectonic setting played an important role in determining the facies sequence, whereas eustatic sea-level changes were of lesser influence.  相似文献   

Bed thickness characteristics of inner-shelf storm deposits associated with a transgressive to regressive Holocene wave-dominated shelf, Sendai coastal plain, Japan     

TORU TAMURA  FUJIO MASUDA† 《Sedimentology》2005,52(6):1375-1395

Holocene inner-shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north-eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave-dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) ¹⁴C ages and the origin of bed thickness variations in the inner-shelf storm deposits is explored. The Holocene succession is <30 m thick and overlies latest Pleistocene to early Holocene non-marine deposits above a transgressive ravinement surface. It comprises transgressive ravinement and inner-shelf deposits, and regressive inner shelf, shoreface, and coastal plain deposits. The inner-shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds. The average preservation interval of a single storm bed is shortest during the transgression (5·7–20·6 years), and then increases to a maximum during the early regression (83·3–250·0 years), decreasing to 7·7–31·3 years with shoreline progradation. Average accumulation rates decreased during the transgression and then increased during the regression, but the sand/mud ratio varies little, reflecting inefficient sediment segregation downdip on the inner shelf. The vertical pattern of sand-layer thicknesses also shows no relationship to position within the cycle, although small-scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and/or random storm processes. The average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low-magnitude storms during this period of low accumulation rates and extensive reworking. This preservation bias and the nature of the Sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits.  相似文献   

Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina     

AILEEN WOJTAL DUC  ROBERT S. TYE† 《Sedimentology》1987,34(2):237-251

Analysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation-controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud-rich central backbarrier sequence consisting of low marsh overlying fine-grained, tidal flat/lagoonal mud; (2) a sandy beach-ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island. Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine-grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell-rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded. Barrier progradation and sediment bar-bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach-ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining-upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization. This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand-rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea-level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.  相似文献   

Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man     

J. A. D. DICKSON  C. BARBER 《Sedimentology》1976,23(2):189-211

Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz. The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore-waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater. The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods. A four-fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions. Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.  相似文献   

A 6000 year tropical cyclone record from Western Australia     

Jonathan Nott 《Quaternary Science Reviews》2011,30(5-6):713-722

This study provides the first long-term tropical cyclone record from the Indian Ocean region. Multiple shore parallel ridges composed entirely of one species of marine cockle shell (Fragum eragatum) standing between 3 and 6 m above mean sea level occur at Hamelin Pool, Shark Bay, Western Australia. The ridges record a tropical cyclone history between approximately 500 cal BP and 6000–7000 cal BP. Numerical storm surge and shallow water wave modelling techniques have been applied to determine the intensity (central pressure with uncertainty margins) of the storms responsible for deposition of the ridges, which has occurred approximately every 190–270 years. The ridges also record a 1700 year gap in tropical cyclone activity, between approximately 5400 cal BP and 3700 cal BP, where ridges deposited prior to this time were buried by a substantial deposit of aeolian fine-grained terrestrial sediment. The presence of this sedimentary unit suggests that this 1700 year period was characterised by a very dry climate; possibly the driest phase experienced in this region since the mid-Holocene. The absence of tropical cyclones at this time and the occurrence of this mega-drought may be linked.  相似文献   

The upper Viséan–Serpukhovian in the type area for the Serpukhovian Stage (Moscow Basin,Russia): Part 2. bulk geochemistry and magnetic susceptibility     

Pavel B. Kabanov  Andrey O. Alekseev  Tikhon Zaitsev 《Geological Journal》2016,51(2):195-211

This study is the summary analysis of bulk XRF geochemistry (233 samples from three sections) of the Oka and Zaborie groups of the type Serpukhovian succession in the Moscow Basin. The siliciclastic wedges in the limestone‐dominated Oka Group are two to three times enriched in Fe, Ti, and Zr compared to Clarke values. Bulk iron strongly correlates with magnetic susceptibility. Iron tends to form ferruginized horizons (original siderites) in finer grained siliciclastic beds associated with coal seams. These beds also tend to be enriched in Cu, Ni, Pb, Zn, and other trace metals (metal enrichment horizons or MEHs). MEHs formed in ponded conditions of coastal low‐pH marshlands vegetated by mangrove‐like lycopsid bushes. Well‐drained environments of palaeokarst formation and alkaline everglades (Akulshino palustrine event) on the other hand did not accumulate Fe and trace metals. The thin shale seam (found close to the Viséan–Serpukhovian boundary in Polotnyanyi Zavod) has unusually high Rb and Sr values, which may contain volcanigenic material useful for absolute dating. The Gurovo Formation (Steshevian Substage of the Serpukhovian) is less enriched in Fe and Ti. In the Gurovo Formation, the transition from the lower montmorillonitic shale (Glazechnya Member) to the upper palygorskitic shale (Dashkovka Member) is expressed by a five‐fold increase in background MgO values, which indicates progressive shoaling and climatic aridization. Phosphorus remains close to 0% in the Oka Group and tends to increase in the Zaborie Group, in agreement with a dramatic increase of conodont numbers and other signatures of a lower Serpukhovian marine transgression. The lower half of the Glazechnya Member exhibits fluctuating enrichment in Fe, Cu, Ni, Pb, Zn, V, Cr, and Co. These fluctuations are mostly inverse to fluctuations of Mn. This pattern has been interpreted as a signature of seafloor oxygen deficiency, where Mn‐rich samples record oxygen‐poor environments (redox barrier level with the sediment surface) and Mn‐poor samples enriched in Fe and trace metals record transitions to anoxic setting. This interval is interpreted as the Lower Serpukhovian highstand. Enrichment in Fe, Ti, and Zr of Oka siliciclastic units of Polotnyanyi Zavod indicates provenance from the ore‐rich Voronezh Land, south of the Moscow Basin. The westerly flux regarded as a possible provenance in previous palaeogeographic reconstructions is discarded for the studied sections. The Gurovo Shale is also linked to the Voronezh province, although Fe, Ti, and Zr concentrations are lower than in the Oka shales. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Physical criteria for distinguishing sandy tsunami and storm deposits using modern examples   总被引：4，自引：0，他引：4  

Robert A. Morton  Guy Gelfenbaum  Bruce E. Jaffe 《Sedimentary Geology》2007,200(3-4):184-207

Modern subaerial sand beds deposited by major tsunamis and hurricanes were compared at trench, transect, and sub-regional spatial scales to evaluate which attributes are most useful for distinguishing the two types of deposits. Physical criteria that may be diagnostic include: sediment composition, textures and grading, types and organization of stratification, thickness, geometry, and landscape conformity.

Published reports of Pacific Ocean tsunami impacts and our field observations suggest that sandy tsunami deposits are generally < 25 cm thick, extend hundreds of meters inland from the beach, and fill microtopography but generally conform to the antecedent landscape. They commonly are a single homogeneous bed that is normally graded overall, or that consists of only a few thin layers. Mud intraclasts and mud laminae within the deposit are strong evidence of tsunami deposition. Twig orientation or other indicators of return flow during bed aggradation are also diagnostic of tsunami deposits. Sandy storm deposits tend to be > 30 cm thick, generally extend < 300 m from the beach, and will not advance beyond the antecedent macrotopography they are able to fill. They typically are composed of numerous subhorizontal planar laminae organized into multiple laminasets that are normally or inversely graded, they do not contain internal mud laminae and rarely contain mud intraclasts. Application of these distinguishing characteristics depends on their preservation potential and any deposit modifications that accompany burial.

The distinctions between tsunami and storm deposits are related to differences in the hydrodynamics and sediment-sorting processes during transport. Tsunami deposition results from a few high-velocity, long-period waves that entrain sediment from the shoreface, beach, and landward erosion zone. Tsunamis can have flow depths greater than 10 m, transport sediment primarily in suspension, and distribute the load over a broad region where sediment falls out of suspension when flow decelerates. In contrast, storm inundation generally is gradual and prolonged, consisting of many waves that erode beaches and dunes with no significant overland return flow until after the main flooding. Storm flow depths are commonly < 3 m, sediment is transported primarily as bed load by traction, and the load is deposited within a zone relatively close to the beach.  相似文献   

Asymmetrical deltas below wave base: Insights from the Fraser River Delta,Canada          下载免费PDF全文

Korhan Ayranci  Shahin E. Dashtgard 《Sedimentology》2016,63(3):761-779

The Fraser River Delta exhibits distinct asymmetry in the sedimentological and neoichnological characteristics of the updrift (south) and downdrift (north) sides of the main distributary channel in water depths below storm‐wave base. The asymmetry is the result of net northward tidal flow. Tides erode sediments across the updrift delta front, whereas the downdrift delta front is an area of net deposition. A submarine channel prevents sand eroded from the updrift delta front from reaching the downdrift delta. The updrift delta front and updrift upper prodelta are composed of sand or heterolithic sand and mud that show a low density of burrowing (Bioturbation Index 0 to 3) and are dominated by simple traces. The downdrift delta front and prodelta, and the updrift lower prodelta are composed of homogeneous muds with significantly higher bioturbation intensities (Bioturbation Index 3 to 6), and a more diverse suite of traces akin to Cruziana Ichnofacies. Using the Fraser River Delta as an archetype and comparing the Fraser to the Amazon River Delta, a preliminary model for deep‐water (below storm‐wave base: ca 20 m) asymmetrical deltas is proposed. Firstly, deep‐water asymmetrical deltas are recognized from sediments deposited below storm‐wave base. At these depths, tidal and ocean currents are more likely to impact sediment transport, but wave processes are less effective as a sediment transport mechanism. Sediments deposited below storm‐wave base in deep‐water asymmetrical deltas will display the following: (i) the updrift delta front will be coarser‐grained (for example, sand‐dominated or heterolithic sand and mud), than the downdrift delta front (for example, mud‐dominated); and (ii) the updrift delta front should show low‐diversity suites of simple burrows. Depending on sedimentation rates, the downdrift delta front and prodelta may show either high diversity suites of traces that are dominated by both complex and simple burrows (low sedimentation rates) or low density and diversity suites akin to the updrift delta front (high sedimentation rates).  相似文献   

Storm deposits and storm-generated coarse carbonate breccias on a pelagic outer shelf (South-East Basin, France)   总被引：2，自引：0，他引：2  

Michel Seguret  Alexis Moussine-Pouchkine  Guilherme Raja Gabaglia  & Frederic Bouchette 《Sedimentology》2001,48(2):231-254

Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.  相似文献   

Intra‐parasequence architecture of an interpreted asymmetrical wave‐dominated delta     

KARL CHARVIN  GARY J. HAMPSON  KERRY L. GALLAGHER  RICHARD LABOURDETTE 《Sedimentology》2010,57(3):760-785

Although modern wave‐dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface‐shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave‐dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high‐resolution stratigraphic architecture and interpreted palaeo‐geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave‐dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south‐trending, wave‐dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave‐dominated shoreface‐shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial‐dominated delta‐front deposits, which have previously been assigned to a separate ‘stranded lowstand parasequence’ formed by a significant, allogenic change in relative sea‐level. High‐resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea‐level. Interpreted normal regressive shoreline trajectories within the wave‐dominated shoreface‐shelf deposits are marked by aggradational stacking of bedsets bounded by non‐depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave‐base scour. Fluvial‐dominated delta‐front deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave‐dominated shoreface‐shelf and fluvial‐dominated delta‐front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave‐dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.  相似文献   

Environmental changes in the Hirnantian (upper Ordovician) of the Prague Basin,Czechoslovakia     

P. J. Brenchley  P. &#x;torch 《Geological Journal》1989,24(3):165-181

Sedimentary sequences through the uppermost Ordovician (Hirnantian), Kosov Formation, are described. The Kosov Formation is a shale-dominated formation with two thin horizons of sandy diamictites near the base and two ‘flysch’ units separated by shales forming the main part of the formation, which is overlain abruptly by Silurian graptolitic shales. The diamictites are interpreted as of glaciomarine origin, probably deposited from coastal winter ice. The flysch units have many beds with hummocky cross-stratification and wave-ripples indicating deposition from storm generated currents above storm wave-base. The shales between the two flysch units have thin interbedded sandstones and siltstones with mainly unidirectional current structures, but also some wave-ripples, suggesting deposition around storm wave-base. The sequence shows clear evidence of two regressive phases, the second of which is the more pronounced, followed by rapid transgression near the Ordovician/Silurian boundary. The regressions and subsequent transgression are interpreted as being caused by Hirnantian glacioeustatic changes rather than being tectonic in origin. The importance of the glaciomarine diamictites is that they record the onset of cold climatic conditions in the region as being early Hirnantian. The disappearance of most of the fauna at about the same time might reflect the influence of low temperatures on faunal diversity, and could have significance for the end Ordovician extinction.  相似文献   

The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry   总被引：1，自引：0，他引：1  

MARK WILKINSON 《Sedimentology》1991,38(5):899-912

The sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05?3.28 mol% FeCo₃) formed during sulphate reduction (δ¹³C =?24 to ?32%o PDB) in marine porewaters (δ¹⁸O of cement from ?1 to ?4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre-scale clusters of crystals which acted as nuclei to the later, concretionary cements. The second generation of cements are more homogeneous ferroan calcites (mean 1?58% mol% FeCo₃) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ¹³C of cement from +1 to ?3%o PDB) into meteoric (δ¹⁸O of cement from ?6 to ?10%o PDB) or mixed marine meteoric waters (δ¹⁸O of cement from ?4 to ?6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth. Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment-water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.  相似文献