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1.
In the Bavarian Alps (Germany), west of the Isar River, the abyssal deposits of the Lower Barremian to Upper Campanian Rhenodanubian Group consist of siliciclastic and calcareous turbidites alternating with hemipelagic non-calcareous mudstones. The up to 1500-m-thick succession, deposited in the Penninic Basin to the south of the European Plate, is characterized by a low mean sedimentation rate (c. 25 mm kyr−1) over 60 million years. Palaeocurrents and turbidite facies distribution patterns suggest that sedimentation occurred on a weakly inclined abyssal plain. The highest sedimentation rates (up to 240 mm kyr−1) were associated with the calcareous mud turbidites of the newly defined Röthenbach Subgroup, which includes the Piesenkopf, Kalkgraben and Hällritz formations (Middle Coniacian to Middle Campanian). These calcareous turbidites prograded from the west, and interfinger towards the east with red hemipelagic claystone. A high sea level presumably favoured pelagic carbonate production and accumulation on the shelves and on internal platforms in the western part of the basin, whereas siliciclastic shelves with steep slope angles have bordered the eastern part of the basin, where a dearth of turbidite sedimentation and increased Cretaceous oceanic red beds deposition occurred. In contrast to the eustatically-induced Middle Coniacian to Lower Campanian Cretaceous oceanic red beds (calcareous nannoplankton zones CC14 to CC18), red hemipelagites of Early Cenomanian age (upper part of calcareous nannoplankton zone CC9) and early Late Campanian age (upper part of zone CC21 and zone CC22) are interpreted as the result of regional tectonic activity.  相似文献   

2.
Río Fardes剖面位于西班牙南部Granada东北,构造上属于深水环境的Subbetic中带。该剖面主要由白垩纪Fardes组第Ⅱ段和第Ⅲ段(半)远洋沉积构成,并出现浊流沉积和混杂沉积。本次研究在Fardes组浊流层序内首次发现两段红色沉积。钙质超微化石表明红层的时间从Turonian早期(UC7 带)到Coniacian中期—晚期界线(UC10/?UC11带)。红层由mm级红色泥岩夹灰色、杂色、偶尔黑色泥岩和钙质泥岩组成。沉积学研究表明新发现的Turonian Coniacian远洋红色泥岩沉积形成于CCD面之下深水盆地环境,浊流和碎屑流沉积强烈地影响着(半)远洋环境的背景泥岩相,并成为红色沉积结束的原因。  相似文献   

3.
Syn-rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault-bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates. The most abundant redeposits are bio- and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high- and low-density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow-water carbonates and skeletal debris from nearby submarine highs. After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge-shaped with a horizontal upper boundary and consists of a thinning- and fining-upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin-bedded turbidites and hemipelagic limestones. The thick-bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick-bedded turbidites, thinning- and fining-upward cycles are common. The overlying thin-bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine-grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin-plain association developed comprising fine-grained and thin-bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.  相似文献   

4.
In the Jebilet Palaeozoic inlier, 20 km north of Marrakech, there are extensive exposures of Carboniferous flysch deposits. Although there are some structural complications due to over-riding nappes with associated chaotic breccias, one clearly unbroken succession from basin-plain turbidites to shallow-marine deposits can be examined. The succession is more than 2 km thick and is dated as Upper Viscan in the uppermost part.The lowermost unit of B- and C-based turbidites shows no sequential organisation and is interpreted as a typical basin-plain association. Above this are similar turbidites arranged in thickening-upward sequences that may represent outer-fan or base-of-slope deposits. Succeeding these are thin-bedded turbidites with interbedded units formed by mass movement that represent a slope deposit. The overlying lenticular-bedded facies resembles previously described overflow deposits of submarine-fan channels, but is here interpreted as comprising storm-generated deposits on the outer shelf/upper slope. These deposits are genetically linked with the overlying parallel-laminated sandstones with irregular-rippled tops for which a storm-surge origin is suggested. The upper part of the succession shows cross-bedded, oolitic, bioclastic, sandy limestones with bipolar current structures sandwiched between low-energy siltstones containing thin-graded silt/sand beds. These are collectively interpreted as shelf deposits that formed under different depths due to transgressive-regressive events.The sequence differs from many described in the literature in that there is an absence of most submarine-fan facies. Locally a NNE-SSW basin strike is proposed with a basin margin to the ESE, but there is at present little control on regional palaeogeography.  相似文献   

5.
The vertical and lateral stratigraphic relations of facies and facies associations, palaeocurrent directions, and geometry and internal organization of associated thick-bedded and coarse-grained bodies of sandstone provide the framework for distinguishing five thin-bedded turbidite facies in the Eocene Hecho Group, south-central Pyrenees, Spain. Each facies is characterized by a number of primary features which are palaeoenvironmental indicators by themselves. These features and their palaeoenvironmental significance are summarized below.
  • 1 The impressive regularity and lateral persistence of bedding and depositional structures, combined with the association of thin hemipelagic intercalations are typical characteristics of the basin plain thin-bedded turbidites. Lateral variations in bed thickness, internal structures, grain size, sand: shale ratio, and amounts of hemipelagic intercalations are present in these sediments, but take place so gradually that they cannot generally be recognized at the scale of even very large exposures. The basin plain facies has a remarkable character of uniformity over great distances and considerable stratigraphic thicknesses.
  • 2 Thickening-upward and/or symmetric cycles with individual thicknesses ranging from a few metres to a few tens of metres are typical of lobe-fringe thin-bedded turbidites. The sediments that comprise the cycles contain small but recognizable variations in bed thickness and sand: shale ratio. The diagnostic cyclic pattern can be detected in relatively small exposures. It should be noted that in absence of coarse-grained and thick-bedded sandstone of the depositional lobes the above cyclic pattern is diagnostic of fan-fringe areas.
  • 3 An extremely irregular bedding pattern with lensing, wedding, and amalgamation of individual beds over very short distances, sharp rippled tops of many beds, and internal depositional structures indicative of mainly tractional processes without substantial fallout, are typical and exclusive characteristics of channelmouth thin-bedded turbidites.
  • 4 Bundles of interbedded thin-bedded sandstone and mudstone as thick as a few metres that are separated in vertical sequences by mudstone units of roughly similar or greater thickness are typical of interchannel thin-bedded turbidites. The most diagnostic feature of this depositional environment is the presence of beds of sandstone filling broad shallow channels as probable crevasse-splays.
  • 5 Thin, thoroughly rippled sandstone beds with marked divergence of the bedding attitude characterize the channel-margin facies. The divergence or expansion in thickness, is consistently toward the channel axis. Small and shallow channels filled with thin-bedded deposits, interpreted here as crevasses cut into channel edges or levees during period of severe overbanking are also characteristic.
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6.
Here, we report that a lithostratigraphic unit that outcrops at Sararu village, 6 km northeast of Qumri village that had previously been assigned to the Baluti Formation is not Triassic in age and therefore can not be a correlative equivalent of the Baluti Formation. The outcropping unit at Sararu comprises intercalation of calcareous mudstones and limestones, and is indeed lithologically similar to the Baluti Formation (Late Triassic). The Baluti Formation (also known as the Baluti Shale) is known from a typical section found at the Gara Anticline and from many deep drilled oil exploration wells. It is generally composed of alternations of the shales, limestones, dolomites, and dolomitic limestones. It is underlain by the Kurra Chine Formation (Upper Triassic) and overlain by the Sarki Formation (Lower Jurassic). In this study, detailed field observations, an assessment of stratigraphic successions, studies of microfossils such as age-specific planktonic foraminifera (e.g., Globotruncana bulloides), and age-specific biomarkers (oleanane index and C28/C29 regular sterane index) reveal that the lithostratigraphic unit at Sararu village can not be a correlative equivalent of the Baluti Formation, and it is more likely from the Upper Cretaceous. There are a number of Upper Cretaceous formations found in this part of Kurdistan, but based on fossil-type and palaeoenvironmental associations, the Hadiena Formation, from the Upper Cretaceous, is considered as the most likely correlative equivalent to the calcareous mudstone and limestone succession found at Sararu village.  相似文献   

7.
Eyles  & Eyles 《Sedimentology》2000,47(2):343-356
The intracratonic Canning Basin is Western Australia's largest sedimentary basin (>400 000 km2) and has experienced repeated episodes of Phanerozoic extension and subsidence, resulting in deposition of a number of first-order 'megasequences'. A major phase of basin extension and sedimentation (Grant Group) occurred in the Late Carboniferous/Early Permian when Australia lay at high palaeolatitudes. Facies analysis of 5000 m of drill core from 25 continuously cored wells in Grant Group strata on the fault-bounded Barbwire Terrace in the northern Canning Basin identified three facies associations (FAs). These record the predominance of fault-generated, subaqueous mass flow and sediment reworking. The lowest association (FA I; up to 355 m thick) rests unconformably on tilted older strata and consists of coarse-grained, subaqueously deposited, sediment gravity flow facies. These include fault-generated breccias, massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization. FA I is overlain abruptly by relatively fine-grained deposits of FA II (up to 140 m thick), which consist of laminated to thin-bedded mudstone and sandstone turbidites, recording an abrupt increase in relative water depths. In turn, these facies coarsen upwards and are transitional into shallow-water, swaley cross-stratified and rippled sandstones of FA III (up to 125 m thick). The overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment (FA I), a subsequent phase of rapid subsidence, increasing water depths and 'sediment underfilling' (FA II) and, finally, a regressive phase of shoreface progradation. The occurrence of rare striated clasts in FA I suggests reworking of glacial sediment, but no direct glacial influence on sedimentation can be identified.  相似文献   

8.
In the North Apennines of Italy, Upper Jurassic bedded chert stratigraphically overlies ophiolitic rocks and is overlain by Lower to Middle Cretaceous pelagic limestone and shale, and Upper Cretaceous flysch. The bedded chert, best exposed in East Liguria and on Elba, is typically 30–80 m thick, but occasionally reaches 150–200 m thickness. It consists of two main alternating lithologïes: siliceous mudstone (SM) and radiolarite (R). Chert sections commonly show characteristic stratigraphic changes. Lower cherts display a striking rhythmic alternation of R and ferruginous SM beds. In middle cherts, SM beds are much less ferruginous and shalier intercalations are locally present. In upper cherts, R beds are less frequent and SM beds are essentially non-ferruginous. R beds are generally 1–4 cm thick, and consist of 80–90% quartz, 5–15% clays and usually < 1% hematite. They are commonly parallel-laminated, and rarely size-graded. In size-graded beds, large radiolaria are more abundant near the bed base (commonly together with ophiolitic or SM clasts) and small radiolaria more abundant near the bed top. Sorting is poor throughout most R beds. R beds are interpreted as turbidites (cf. Nisbet & Price, 1974). Model calculations suggest that typical settling velocities of radiolaria during redeposition are < 1 cm sec?1, which is low and of restricted range relative to the 1–10 cm sec?1 settling velocities of clastic grains of comparable size range. Radiolaria therefore should have only a limited tendency to grade and sort during deposition from a turbulent current. SM beds are commonly 1–7 cm thick, although much thicker ones occur near the base of sections, and consist mainly of 50–70% quartz, 15–35% clays and 0–15% hematite. Microscopic clay-silica aggregates and highly corroded remnants of radiolaria are common. SM beds are interpreted as mainly ambient pelagic sediment which accumulated slowly in topographic lows, and which was modified by near-surface dissolution of biogenic silica. In SM beds which contain two texturally different layers, the lower one is interpreted as the top of the underlying radiolarian turbidite. North Apennine cherts represent the first sediment deposited on oceanic crust formed during the opening of the North Apennine part of the Tethys. The ophiolitic basement had a rugged topography which favoured the redeposition of siliceous sediment. Hematite and local Mn enrichments in SM beds in the lower chert sections represent hydrothermal precipitates inferred to have originated at a spreading axis. During seafloor spreading, accumulation of siliceous sediments progressively reduced the topography. Deposition of ophiolitic detritus within the sediments phased out during early chert sedimentation, and the hydrothermal contribution during early-middle chert sedimentation. As local basins filled, during late chert sedimentation, radiolarian turbidites became less frequent. The first limestones at the top of chert sections are calcareous ooze turbidites derived from above the CCD and deposited slightly below it. Gradual descent of the CCD to ocean floor depths at the end of the Jurassic (Bosellini & Winterer, 1975) led to the replacement of siliceous by carbonate sedimentation.  相似文献   

9.
The Upper Cretaceous part of the Great Valley Sequence provides a unique opportunity to study deep-marine sedimentation within an arc-trench gap. Facies analysis delineates submarine fan facies similar to those described from other ancient basins. Fan models and facies of Mutti and Ricci-Lucchi allow reconstruction of the following depositional environments: basin plain, outer fan, midfan, inner fan, and slope. Basin plain deposits are characterized by hemipelagic mudstone with randomly interbedded thin sandstone beds exhibiting distal turbidite characteristics. Outer fan deposits are characterized by regularly interbedded sandstone and mudstone, and commonly exhibit thickening-upward (negative) cycles that constitute depositional lobes. The sandstone occurs as proximal to distal turbidites without channeling. Midfan deposits are characterized by the predominance of coarse-grained, thick, channelized sandstone beds that commonly are amalgamated. Thinning-upward (positive) cycles and braided channelization also are common. Inner fan deposits are characterized by major channel-fill complexes (conglomerate, pebbly sandstone, and pebbly mudstone) enclosed in mudstone and siltstone. Positive cycles occur within these channel-fill complexes. Much of the fine-grained material consists of levee (overbank) deposits that are characterized by rhythmically interbedded thin mudstone and irregular sandstone beds with climbing and starved ripples. Slope deposits are characterized by mudstone with little interbedded sandstone; slumping and contortion of bedding is common. Progressions of fan facies associations can be described as retrogradational and progradational suites that correspond, respectively, to onlapping and offlapping relations in the basin. The paleoenvironments, fan facies associations, and tectonic setting of the Late Cretaceous fore-arc basin are similar to those of modern arc—trench systems.  相似文献   

10.
Recognition of the occurrence and extent of hemipelagic and pelagic deposits in turbidite sequences is of considerable importance for environmental analysis (palaeodepth, circulation, distance from land, hemipelagic or pelagic versus turbidite sedimentation rates) of ancient basins. Differentiation between the finegrained parts (E-division) of turbidites and the (hemi-) pelagic layers (F-division of turbidite-pelagite alternations) is facilitated in basins where carbonate turbidites were deposited below the carbonate compensation depth (CCD) such as the Flysch Zone of the East Alps but may be difficult in other basins where less compositional contrast is developed between the fine-grained turbidites and hemipelagites. This difficulty pertains particularly in Palaeozoic and older basins. For Late Mesozoic-Cenozoic oceans with a relatively deep calcite compensation level three other types of turbidite basins may be distinguished for which differentiation becomes increasingly more difficult in the sequence from (1) to (3): (1) terrigenous turbidite basins above the CCD; (2) carbonate turbidite basins above the CCD; (3) terrigenous turbidite basins below the CCD. Criteria and methods useful for the differentiation between turbiditic and hemipelagic mudstone in the Upper Cretaceous of the Flysch Zone of the East Alps include calcium carbonate content, colour, sequential analysis, distribution of bioturbation, and microfaunal content. In modern turbidite basins clay mineral content, organic matter content, plant fragments, and grain-size (graded bedding, maximum grain diameter) have reportedly also been used as criteria (see Table 3). Deposition of muddy sediment by turbidity currents on weakly sloping sea bottoms such as the distal parts of deep-sea fans or abyssal plains is not only feasible but may lead to the accumulation of thick layers. Contrary to earlier speculation it can be explained by the hydrodynamic theory of turbidity currents, if temperature differences between the turbidity current and the ambient deep water as well as relatively high current velocities for the deposition of turbiditic muds (an order of magnitude higher on mud surfaces than commonly assumed) are taken into consideration. The former add to the capacity of turbidity currents to carry muddy sediment without creating a driving force on a low slope.  相似文献   

11.
《International Geology Review》2012,54(16):2030-2059
Seismic and sequence stratigraphic analysis of deep-marine forearc basin fill (Great Valley Group) in the central Sacramento Basin, California, reveals eight third-order sequence boundaries within the Cenomanian to mid-Campanian second-order sequences. The third-order sequence boundaries are of two types: Bevelling Type, a relationship between underlying strata and onlapping high-density turbidites; and Entrenching Type, a significantly incised surface marked by deep channels and canyons carved during sediment bypass down-slope. Condensed sections of hemipelagic strata draping bathymetric highs and onlapped by turbidites form a third important type of sequence-bounding element, Onlapped Drapes. Five tectonic and sedimentary processes explain this stratigraphic architecture: (1) subduction-related tectonic tilting and deformation of the basin; (2) avulsion of principal loci of submarine fan sedimentation in response to basin tilting; (3) deep incision and sediment bypass; (4) erosive grading and bevelling of tectonically modified topography by sand-rich, high-density turbidite systems; and (5) background hemipelagic sedimentation. The basin-fill architecture supports a model of subduction-related flexure as the principal driver of forearc subsidence and uplift during the Late Cretaceous. Subduction-related tilting of the forearc and growth of the accretionary wedge largely controlled whether and where the Great Valley turbiditic sediments accumulated in the basin. Deeply incised surfaces of erosion, including submarine canyons and channels, indicate periods of turbidity current bypass to deeper parts of the forearc basin or the trench. Fluctuations in sediment supply likely also played an important role in evolution of basin fill, but effects of eustatic fluctuations were overwhelmed by the impact of basin tectonics and sediment supply and capture. Eventual filling and shoaling of the Great Valley forearc during early Campanian time, coupled with dramatically reduced subsidence, correlate with a change in plate convergence, presumed flat-slab subduction, cessation of Sierran arc volcanism, and onset of Laramide orogeny in the retroarc.  相似文献   

12.
Triassic–Jurassic sedimentary successions (Baluti and Sarki formations) in northern Iraq record a variety of environmental changes that may be related to global Triassic–Jurassic (Tr/J) boundary events. The diversity of some benthic fauna decreases through the transitional boundary beds. The coastal marine environment of the lower part of the Baluti Formation is followed by shallower tidal flat and supratidal marginal marine environments at the transitional boundary with the Jurassic‐age Sarki Formation. The alternating calcareous mudrocks and dolomitic limestones of the transitional succession are overlain by a succession of calcareous mudrocks and dolomicrites that form a dolocrete bed in the latest Triassic. The early Jurassic carbonates (lower part of Sarki Formation) were deposited in a shallow‐marine to lagoonal environment. Geochemical evidence supports this interpretation. TOC% increases towards the Tr/J boundary and the lower part of the Sarki Formation. This increase can be interpreted as resulting from the primary precipitation of dolocrete as palaeosol horizons. The variations in the oxygen isotope ratios mainly reflect the facies and diagenetic effects. Th/K ratio is generally constant and shows an increase in the calcareous mudrock beds of the upper part of the Baluti Formation, possibly related to the degradation of K‐bearing clay minerals. Low Th/U ratios are due to the depletion in thorium, typical of many marine carbonates rather than to an increase in authigenic uranium. This explanation is also corroborated by the presence of abundant fossils in some of the studied carbonates. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
The Upper Permian (Zechstein) slope carbonates in the Roker Formation (Zechstein 2nd‐cycle Carbonate) in North‐east England consist of turbidites interbedded with laminated lime‐mudstone. Studies of turbidite bed thickness and relative proportion of turbidites (percentage turbidites in 20 cm of section) reveal well‐developed cyclicities consisting of thinning‐upward and thickening‐upward packages of turbidite beds. These packages are on four scales, from less than a metre, up to 50 m in thickness. Assuming that the laminae of the hemipelagic background sediment are annual allows the durations of the cycles to be estimated. In addition, counting the number and thickness of turbidite beds in 20 cm of laminated lime‐mudstone, which is approximately equivalent to 1000 years (each lamina is 200 μm), gives the frequencies of the turbidite beds, the average thicknesses and the overall sedimentation rates through the succession for 1000 year time‐slots. Figures obtained are comparable with modern rates of deposition on carbonate slopes. The cyclicity present in the Roker Formation can be shown to include: Milankovitch‐band ca 100 kyr short‐eccentricity, ca 20 kyr precession and ca 10 kyr semi‐precession cycles and sub‐Milankovitch millennial‐scale cycles (0·7 to 4·3 kyr). Eccentricity and precession‐scale cycles are related to ‘highstand‐shedding’ and relative sea‐level change caused by Milankovitch‐band orbital forcing controlling carbonate productivity. The millennial‐scale cycles, which are quasi‐periodic, probably are produced by environmental changes controlled by solar forcing, i.e. variations in solar irradiance, or volcanic activity. Most probable here are fluctuations in carbonate productivity related to aridity–humidity and/or temperature changes. Precession and millennial‐scale cycles are defined most strongly in early transgressive and highstand parts of the larger‐scale short‐eccentricity cycles. The duration of the Roker Formation as a whole can be estimated from the thickness of the laminated lithotype as ca 0·3 Myr.  相似文献   

14.
Assemblages of benthic foraminifera from one clastic succession in the Afales Basin (Ithaki Island, western Greece) were investigated to reconstruct palaeoenvironmental conditions during the Oligocene. The section consists of alternating hemipelagic marls and detrital deposits, designated as flysch-like beds, attributed to biostratigraphic Zones P20 and P21. Planktic percentages are mostly high (66–80%). Benthic foraminiferal assemblages comprise calcareous and agglutinated taxa (up to 15%). The prevalence of epifaunal foraminifera indicates good ventilation of the bottom water resulting from basin morphology, which enabled the undisturbed flow of water throughout the basin. Palaeodepth estimates imply bathyal deposition, from about 800 to 1200 m deep. The benthic foraminiferal fauna is of high diversity along the section, as is expected in deep marine environments. The abundances of the most common foraminiferal taxa (Cibicidoides spp., Oridorsalis umbonatus, Gyroidinoides spp., Stilostomella spp., Nodosariidae, Nuttallides umbonifera) are quite variable and imply generally oligotrophic to mesotrophic environmental conditions with variable organic flux.  相似文献   

15.
The Silurian succession in the Tortworth Inlier includes strata belonging to the three main divisions of the System. The Upper Llandovery, which rests unconformably on Tremadoc beds, consists of about 700 ft. (213 m.) of fine-grained sandstones, mudstones and shales with two igneous bands. The Wenlock Series is represented by some 800 ft. (244 m.) of mudstones, siltstones and calcareous sandstones with impersistent limestone bands at various horizons. The Ludlow succession is incomplete, but at least 300 ft. (91 m.) of mudstones, siltstones and fine-grained sandstones are present, and they pass up conformably into the Downtonian.  相似文献   

16.
In the course of investigations in the North Caucasus, the Pamirs and the Soviet Northeast, geologists observed in sections certain beds with a peculiar fauna, underlain by deposits of the Norian stage and overlain by Upper Jurassic deposits. According to I. I. Tuchkov, in the Soviet Northeast these beds are unconditionally referred to the Rhaetian. In 1959 A. I. Afizky found numerous Upper Norian ammonites within these deposits on the Bolshoi Anyui River (lower course of the Kolyma River). As early as 1937 J. Fromaget mentioned findings of Norian ammonites in the supposedly “Rhaetian” beds in south-Asian sections. This author first put forward the Rhaetian stage problem, suggesting that it might be regarded as a zone of the Norian stage. Such a solution of the Rhaetian stage problem breaks with traditional stratigraphy. The present paper points out another way towards the solution of the above problem, taking into account both the numerous cases of findings of beds with a mixed Norian-Rhaetian fauna and the irregular distribution of ammonite zones in the Norian (six zones) and the Rhaetian (one zone) stages. By lowering the lower boundary of the Rhaetian stage and including the two upper zones of the Norian stage into the Rhaetian, the problem, of the latter may be solved without breaking with traditional stratigraphy.—Auth. Eriglish summ.  相似文献   

17.
Because oxygen deficient conditions enhance the preservation of depositional organic matter, analysis on paleooxygenation conditions of depositional environments becomes a routine work in evaluations of potential hydrocarbon source rocks. The article focuses on depositional and ecological features relating to oxygen deficient shelfal environments at the Shangsi (上寺) Section, as a part of multidiscipline collaboration to reevaluate the hydrocarbon potential of the Middle and Upper Permian, Guangyuan (广元), Northeast Sichuan (四川) Province. Icbnofabric Zoophycos, sepioilte-bearing limestones (SBL) were interpreted as indicators of dysaerobic environments. Laminated calcareous and/or siliceous mudstones with pelagic ammonites and radiolarians were believed to be the deposits of anaerobic environments. When rhythmic succession was considered, average strategy was adopted for the oxygenation explanation of a given intervaL The anaerobic condition in this Permian section was observed in the upper part of the latest Permian Dalong (大隆) Formation, in which siliceous mudstones with ammonite, radiolarians develop in association with lower U/Mo ratio, lower biomarker ratio of Pr/Ph, and the highest TOC content. The topmost Maokou (茅口) Formation, featured by thin-bedded calcareous and siliceous mudstones with ammonite, thin-shelled bivalves, and laminations would be deposits of the quasianaerobic condition. The middle part of the Members and Ⅲ of the Chihsia Formation is proposed to be dysaerobic condition as indicated by occurrences of SBL and ichnofabric features, with the Member Ⅲ being the severe dysaerobic condition.  相似文献   

18.
The Cow Head Group is an Early Palaeozoic base-of-slope sediment apron composed of carbonate and shale. Whereas coarse-grained conglomerate and calcarenite are readily interpreted as debris-flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre- to centimetre-thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate-shale sequences (GCSS) deposited from waning carbonate turbidites and fall-out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin. The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments. These attributes characterize not only fine-grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.  相似文献   

19.
ABSTRACT Three transitional submarine fan environments are recognized in the late Precambrian, 3-2 km thick Kongsfjord Formation in NE Finnmark, North Norway, namely: (1) middle to outer fan; (2) fan lateral margin, and (3) fan to upper basin-slope deposits. Middle to outer fan deposits have a high proportion of sandstones, typically showing Bouma T bede with T a in the thicker beds. Deposition was mainly from sheet flows with rare shallow channels. Middle to outer fan deposits are an association of sandstone packets less than 10 m thick but commonly only a few metres thick, interpreted as channels or lobes. Interchannel and fan fringe deposits occur as discrete packets of beds between the thicker bedded and coarser grained channel or lobe deposits. Fan lateral margin deposits are recognized on the basis of their stratigraphic position adjacent to inner/middle fan deposits. They are characterized by: (a) a relatively high proportion of fine-grained sandstone/siltstone turbidites compared to other major fan environments; (b) relatively small channels oriented at various angles to the regional basin slope; (c) lobes associated with channels, and (d) abundant clastic dykes and other soft-sediment deformation. Fan lateral margin deposits are distinguished from the outer fan/basin plain successions on account of the very high proportion of siltstone turbidites comparable with middle fan inter-channel deposits. Fan to upper basin-slope deposits occur at the top of the formation as an alternation of sandstone turbidites, most of which are laterally discontinuous, and very thin-bedded upper basin-slope siltstones with slide deposits.  相似文献   

20.
Shelf‐edge deltas are a key depositional environment for accreting sediment onto shelf‐margin clinoforms. The Moruga Formation, part of the palaeo‐Orinoco shelf‐margin sedimentary prism of south‐east Trinidad, provides new insight into the incremental growth of a Pliocene, storm wave‐dominated shelf margin. Relatively little is known about the mechanisms of sand bypass from the shelf‐break area of margins, and in particular from storm wave‐dominated margins which are generally characterized by drifting of sand along strike until meeting a canyon or channel. The studied St. Hilaire Siltstone and Trinity Hill Sandstone succession is 260 m thick and demonstrates a continuous transition from gullied (with turbidites) uppermost slope upward to storm wave‐dominated delta front on the outermost shelf. The basal upper‐slope deposits are dominantly mass‐transport deposited blocks, as well as associated turbidites and debrites with common soft‐sediment‐deformed strata. The overlying uppermost slope succession exhibits a spectacular set of gullies, which are separated by abundant slump‐scar unconformities (tops of rotational slides), then filled with debris‐flow conglomerates and sandy turbidite beds with interbedded mudstones. The top of the study succession, on the outer‐shelf area, contains repeated upward‐coarsening, sandstone‐rich parasequences (2 to 15 m thick) with abundant hummocky and swaley cross‐stratification, clear evidence of storm‐swell and storm wave‐dominated conditions. The observations suggest reconstruction of the unstable shelf margin as follows: (i) the aggradational storm wave‐dominated, shelf‐edge delta front became unstable and collapsed down the slope; (ii) the excavated scars of the shelf margin became gullied, but gradually healed (aggraded) by repeated infilling by debris flows and turbidites, and then new gullying and further infilling; and (iii) a renewed storm wave‐dominated delta‐front prograded out across the healed outer shelf, re‐establishing the newly stabilized shelf margin. The Moruga Formation study, along with only a few others in the literature, confirms the sediment bypass ability of storm wave‐dominated reaches of shelf edges, despite river‐dominated deltas being, by far, the most efficient shelf‐edge regime for sediment bypass at the shelf break.  相似文献   

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