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1.
This paper re-examines the Upper Miocene Upper Mount Messenger Formation, Taranaki Basin, to characterize its architecture and interpret its environmental evolution. Analysis of stratal architecture, lithofacies distributions, and paleotransport directions over the 250 m thick formation shows the outcrops provide a nearly dip parallel section displaying the lateral relationships between contemporaneous channel-levee and overbank depositional environments. At least five 30–40 m thick upward fining units are recognized in the north-central parts of the outcrop and are interpreted as large-scale overbank avulsion cycles. Each unit consists of thick- to medium-bedded predominantly planar laminated sandstone turbidites at the base that fine upward into thin- to very thin-bedded, planar laminated and ripple cross-laminated mud-rich turbidites. The units are traceable laterally over a distance exceeding 3 km where they are cut by channels that show basal mudstone draped by medium- to thin-bedded sandstone, and onlapped by thick-bedded planar laminated sandstone at the margin. The channels are separated by tapered packages of medium- to thin-bedded turbidites containing climbing-ripple cross-lamination interpreted as levees. The individual channel-levee and overbank avulsion cycles formed through four stages: 1) a channel avulsion spread sand into the overbank as an unconfined splay, 2) preferential scouring in one area of the splay led to development of a channel with small levees that prograded across the splay, 3) a deep incision followed by abandonment of the channel deposited a mud lining. Alternatively, the mud lining was formed during the first stage as the downdip portion of the channel was abandoned. 4) The channel filled at first by thick-bedded planar laminated and then by climbing-ripple cross-laminated sand. At this time, the growth of constructional levees progressively limited sand into the overbank. Ratios of Bouma division thicknesses calculated over a stratigraphic interval present a new method to distinguish deep-water depositional environments.  相似文献   

2.
The sedimentary infill history of the Madeira Abyssal Plain (MAP) is established from correlation of ODP Leg 157 drillsites (Sites 950–952) with an almost regular grid of 7000 km of intermediate-resolution seismic reflection profiles covering the central part of the abyssal plain. The most conspicuous seismic reflectors bounding the seismostratigraphic units have been identified and mapped. Correlation between seismic and borehole data using synthetic seismograms allows the lithological attribution and dating of the reflectors and seismostratigraphic units. Lateral mapping and correlation of seismic units also allows both the volumes and rates of accumulation of sediments within each seismostratigraphic unit and equivalent time periods of deposition to be determined. These calculations have been corrected for the effect of compaction, calculated at around 40% at the base of the drillholes. Three main turbidite types have been identified at the drillsites and their emplacement frequency has been calculated for each site and time period. Our results show that Cretaceous oceanic crust was draped with red pelagic clays, and the fracture-zone valleys were completely infilled and levelled in a geologically rather short time, probably during the latest Oligocene and Early Miocene, by organic-rich turbidites derived from the NW African continental margin. At 16 Ma, the topography was levelled enough to allow large turbidity current flows to cover the entire plain. During the Middle and Late Miocene (16–5.9 Ma), organic-rich turbidites were emplaced on the abyssal plain at a low rate of accumulation (12 m/my). In the uppermost Miocene–Early Pliocene (5.9–3.6 Ma), turbidite emplacement increased markedly in both frequency and accumulation rate (e.g., 26 m/my for organic-rich turbidites). During this time, period emplacement of volcanic-rich turbidites also increased in volume and frequency, a trend that continued into the Pliocene. Increased volcanic-rich turbidite emplacement correlates well with increased volcanic activity on the Canary Islands, and increased organic-rich turbidite emplacement may correlate with periods of erosion on the NW African continental margin. These erosional periods may be related to global cooling and falling sea level, intensification of bottom-water currents, and enhanced upwelling on the margin.  相似文献   

3.
The deep lacustrine gravity-flow deposits are widely developed in the lower Triassic Yanchang Formation, southeast Ordos Basin, central China. Three lithofacies include massive fine-grained sandstone, banded sandstone, and massive oil shale and mudstone. The massive fine-grained sandstones have sharp upper contacts, mud clasts, boxed-shaped Gamma Ray (GR) log, but no grading and Bouma sequences. In contrast, the banded sandstones display different bedding characteristics, gradational upper contacts, and fine-upward. The massive, fine-grained sandstones recognized in this study are sandy debrites deposited by sandy debris flows, while the banded sandstones are turbidites deposited by turbidity currents not bottom currents. The sediment source for these deep gravity-flow sediments is a sand-rich delta system prograding at the basin margin. Fabric of the debrites in the sandy debris fields indicates initial formation from slope failure caused by the tectonic movement. As the sandy debris flows became diluted by water and clay, they became turbidity currents. The deep lacustrine depositional model is different from the traditional marine fan or turbidite fan models. There are no channels or wide lobate sand bodies. In the lower Triassic Yanchang Formation, layers within the sandy debrites have higher porosity (8–14%) and permeability (0.1–4 mD) than the turbidites with lower porosity (3–8%) and permeability (0.04–1 mD). Consequently, only the sandy debrites constitute potential petroleum reservoir intervals. Results of this study may serve as a model for hydrocarbon exploration and production for deep-lacustrine reservoirs from gravity-flow systems in similar lacustrine depositional environments.  相似文献   

4.
Sedimentation in the 9500 km2, 4100 m deep Hispaniòla—Caicos Basin is dominated by turbidity currents. Carbonate turbidites originate from the Bahama Islands, Great Inagua and Caicos at the north end of the basin. Mixed carbonate—non-carbonate flows come from Hispaniola and perhaps Cuba. Most flows originate on insular slopes rather than in shallow water. The relatively low CaCO3 content of hemipelagic sequences throughout the entire basin reveals that the influence of non-carbonate Hispaniola—Cuba sources is widespread.The basin was sampled with closely spaced piston cores. Sand-layer isopach and frequency maps reveal four or five major basin entry points for turbidity currents. Flow size is proportional to the size of source areas. Average volumes of flows originating from Hispaniola—Cuba, the largest source, are 109 m3. This compares to an average flow volume of 106 m3 for flows derived from the smallest source area, the Southeastern Caicos Bank. Measures of turbidity-current activity, such as thickness and frequency, change in a regular fashion away from each entry point. Average lutite thickness (combining hemipelagic and turbiditic lutite) are greatest near the basin entry points. On the abyssal plain occupying the south half of the basin, Bouma turbidite sedimentary structure sequences tend to be complete. However, on the Caicos Fan, the sedimentary structure sequences in turbidites are characterized by missing or repeated units. Six radiocarbon dates of two widespread presumed pelagic units in the basin yielded younger dates in stratigraphically older positions. The reversed dates are assumed to reflect storm erosion of older sediment on adjacent insular shelves.Consideration of a north—south reflection seismic profile over the basin indicates that the present sediment regimen has pertained through much of the Neogene. The coherence, convergence and termination of reflections in the seismic section are consistent with and tend to confirm conclusions based on the core study regarding the greater extent and volume of sediment deposits derived from the Hispaniola source area.  相似文献   

5.
To date, facies architecture models of submarine channel–levees have largely been derived from seismic data, isolated core data and limited field studies. We report field observations of an Upper Cretaceous submarine channel–levee complex within the Rosario Formation, Baja California, Mexico, which provide high-resolution data of lithofacies and ichnofacies distribution, and levee depositional thickness decay along transects perpendicular to the channel axis. Within the levee, both sandstone thickness and the overall proportion of sandstone decrease according to a power law away from the channel axis. Spatial variation in sedimentary structures away from the channel axis is predictable and provides an important link to the depositional flow regime. In channel-proximal locations, structureless sands, parallel lamination, overturned ripples, and ripple cross-lamination (including climbing ripple cross-lamination) are common; in channel-distal localities starved ripples are abundant. Sandstone bed thickness generally increases up stratigraphy within the levee succession, which is interpreted to indicate increasing turbidity current magnitude and/or contemporaneous channel floor aggradation reducing relative levee relief. However, in the most channel-proximal location sandstone bed thickness decreases with height; combined with evidence from both facies and palaeocurrent analysis this allows the position of the levee crest to be inferred. The thickest beds occur at higher levels with increasing distance from the channel axis, using this evidence we present a model for levee growth and migration of the crest.Quantitative analysis of ichnofacies distribution reveals that traces typical of the Cruziana and Skolithos ichnofacies are superimposed over the ‘normal’ background Nereites ichnofacies, forming a ‘bioturbation front’ which is indicative of proximity to the channel. By analogy with modern canyons and channels, the association of Cruziana and Skolithos ichnofacies with the channel may be attributed to oxygen and nutrient enrichment and possible turbidity current transport of organisms responsible for these ichnofacies.  相似文献   

6.
George Anastasakis   《Marine Geology》2007,240(1-4):113-135
Santorini volcano has been the largest source of volcaniclastic sediment in the Eastern Mediterranean during the late Quaternary. A dozen cores from the Cretan Basin, south of Santorini, have sampled two megabeds that consist of gravity emplaced volcaniclastic sequences. The uppermost megabed U consists of a succession of five (U5–U1) base cut out turbiditic units. Lower megabed A is a single turbiditic event. Only the uppermost U2 and U1 turbidites are separated from the underlying beds by hemipelagic marls. The texture and composition of the U and A megabeds closely match the texture and composition of the fine, vitric ash of the “Minoan” deposits on Santorini islands, dating from about 3500 yr BP. These megabeds are therefore attributed to rapid accumulation of separate gravity flows fed by the “Minoan” eruption, except for the upper U2 and U1 turbidites deposited from subsequent gravity flows transporting eroded volcaniclastic sediments. With the exception of the margin south of Santorini, dozens of cores retrieved around the margins of the Cretan Basin have a continuous late Quaternary succession that shows no evidence for massive sediment remobilization into the deeper basin, including the passage of the “Minoan” tsunami.

Extensive high-resolution 3.5 kHz records revealed the acoustic character, architecture and distribution of the U and A megabeds and four underlying late Quaternary volcanogenic megabeds in the Cretan Basin. The acoustic facies of megabeds are typical of megaturbidites and consist of an upper, transparent, lower velocity layer that corresponds to the fine-grained upper turbiditic silt and clay section and a lower, strongly reflective higher velocity section that corresponds to the lowest, coarser-grained base of the turbidite that is developed over a sharp erosional surface. Penetration of the high-resolution records reveals the existence of at least six megabeds. Correlation with core lithology and the physical properties of the various lithofacies, including down-core velocity profiles, has allowed us to determine the thickness and volumes of the upper four megabeds which are: U ≤ 9 m thick, volume 3.7 km3; A ≤ 25 m thick, volume 12.2 km3; B ≤ 22 m thick, volume 10.3 km3; C ≤ 15 m thick, volume 8 km3. These thick megabeds are the uppermost products of repeated explosive eruption of Santorini in the late Quaternary. Calculated sedimentation rates from and after the “Minoan” eruption are 9.4 m/1000 yr that rise to over 15.7 m/1000 yr if megabed B was also deposited during this eruption.  相似文献   


7.
Sedimentary facies of cores from the eastern side of the Rockall Bank and the Feni Drift (55°–58°N) have been studied. They confirm the existence of slumping which contributes only in small part to drift build-up.

These slumps occurred at different episodes during last Glacial and early Post-Glacial periods. They are sandwiched in between more important turbidites, and are found in places where the drift lies against the slope and so is acted upon by continental-rise sedimentary processes. The sea level rise periods with ice melting seems to favour gravity flow release.  相似文献   


8.
Mass-transport-deposits (MTDs) and hemipelagic mud interbedded with sandy turbidites are the main sedimentary facies in the Ulleung Basin, East Sea, offshore Korea. The MTDs show similar seismic reflection characteristics to gas-hydrate-bearing sediments such as regional seismic blanking (absence of internal reflectivity) and a polarity reversed base-reflection identical to the bottom-simulating reflector (BSR). Drilling in 2007 in the Ulleung Basin recovered sediments within the MTDs that exhibit elevated electrical resistivity and P-wave velocity, similar to gas hydrate-bearing sediments. In contrast, hemipelagic mud intercalated with sandy turbidites has much higher porosity and correspondingly lower electrical resistivity and P-wave velocity.At drill-site UBGH1-4 the bottom half of one prominent MTD unit shows two bands of parallel fractures on the resistivity log-images indicating a common dip-azimuth direction of about ∼230° (strike of ∼140°). This strike-direction is perpendicular to the seismically defined flow-path of the MTD to the north-east. At Site UBGH1-14, the log-data suggest two zones with preferred fracture orientations (top: ∼250°, bottom: ∼130°), indicating flow-directions to the north-east for the top zone, and north-west for the bottom zone. The fracture patterns may indicate post-depositional sedimentation that gave rise to a preferred fracturing possibly linked to dewatering pathways. Alternatively, fractures may be related to the formation of pressure-ridges common within MTD units.For the interval of observed MTD units, the resistivity and P-wave velocity log-data yield gas hydrate concentrations up to ∼10% at Site UBGH1-4 and ∼25% at Site UBGH1-14 calculated using traditional isotropic theories such as Archie's law or effective medium modeling. However, accounting for anisotropic effects in the calculation to honor observed fracture patterns, the gas hydrate concentration is overall reduced to less than 5%. In contrast, gas hydrate was recovered at Site UBGH1-4 near the base of gas hydrate stability zone (GHSZ). Log-data predict gas hydrate concentrations of 10–15% over an interval of 25 m above the base of GHSZ. The sediments of this interval are comprised of the hemipelagic mud and interbedded thin sandy turbidites, which did contain pore-filling gas hydrate as identified from pore-water freshening and core infra-red imaging. Seismically, this unit reveals a coherent parallel bedding character but has overall faint reflection amplitude. This gas-hydrate-bearing interval can be best mapped using a combination of regular seismic amplitude and seismic attributes such as Shale indicator, Parallel-bedding indicator, and Thin-bed indicator.  相似文献   

9.
 Seine Abyssal Plain sediments consist of turbidites interbedded with pelagic oozes, marls, and clays. The pelagics produce a high-resolution integrated stratigraphy combining oxygen isotope, carbonate curves, palaeotemperature analyses, and nannofossil acme zone stratigraphy. Individual turbidites could then be correlated. Isogram and isopach maps of the turbidites maximum median grain size and thickness, respectively, were used to identify turbidity current pathways and study their depositional processes. Nannofossil fingerprints were used to study possible sedimentary links between the Seine and Madeira abyssal plains. Received: 8 February 1996 / Revision received: 9 May 1996  相似文献   

10.
Isaac Channel 3 is a rare outcrop example of a perpendicular cut through a sinuous deep-water channel, and also where levee deposits formed on opposite sides of the channel are well exposed. Strata flanking the outer- and inner-bend margin of the channel show important differences in lithofacies, architecture and association with channel-fill strata. Proximal outer-bend levee deposits are sand-rich (N:G up to 0.68) and comprise medium- to thick-bedded, Ta-d turbidites interstratified with thinly-bedded, Tcd turbidites. The thicker-bedded deposits show lateral variation in grain size and thickness over hundreds of meters whereas thin-bedded strata thin and fine negligibly over similar distances. The distal outer-bend levee (up to 700 m laterally away from the channel) consists predominantly of thin-bedded turbidites interstratified with up to 5 m thick coarse-grained splay deposits. In contrast to the outer-bend, the inner-bend levee deposits are significantly more mud-rich (N:G as low as 0.15) and consist mostly of thin-bedded, Tcd turbidites with less common thicker-bedded, Ta-d turbidites. Lateral thinning and fining trends associated with these less common thicker-bedded deposits occur more rapidly than their outer-bend counterparts.Erosion associated with lateral migration of the channel axis produced a sharp contact along the outer-bend channel margin causing coarse-grained channel-fill deposits to be in erosional contact with levee deposits. This suggests that the crest of the outer-bend levee was elevated above the channel floor and produced a channel margin upon which channel-fill strata onlapped. Positive topography is interpreted to have developed by overspilling processes that deposited abundant sand on the outer-bend levee while the majority of the flow continued through the channel bend and bypassed to areas further downslope. In contrast, some thick-bedded, amalgamated channel-fill deposits in the axial channel area grade laterally over 140 m into thinly-bedded turbidites on the inner-bend levee. The lack of channel-fill on lap relationships implies that topography along the inner bend was sufficiently subtle that at least some flows were able to expand laterally and over the overbank area without becoming separated from the main throughgoing channel flow.Stratal relationships observed in Isaac Channel Complex 3 suggests three main episodes of channel-levee growth that were each initiated by a period of increased levee relief followed by channel filling and distal levee deposition. This consistent depositional history points to the regular variations, in both time and space, of sediment transport and deposition in a deep-marine sinuous channel-levee system.  相似文献   

11.
Using recently gathered onland structural and 2D/3D offshore seismic data in south and central Palawan (Philippines), this paper presents a new perspective in unraveling the Cenozoic tectonic history of the southeastern margin of the South China Sea. South and central Palawan are dominated by Mesozoic ophiolites (Palawan Ophiolite), distinct from the primarily continental composition of the north. These ophiolites are emplaced over syn-rift Eocene turbidites (Panas Formation) along thrust structures best preserved in the ophiolite–turbidite contact as well as within the ophiolites. Thrusting is sealed by Early Miocene (∼20 Ma) sediments of the Pagasa Formation (Isugod Formation onland), constraining the younger limit of ophiolite emplacement at end Late Oligocene (∼23 Ma). The onset of ophiolite emplacement at end Eocene is constrained by thrust-related metamorphism of the Eocene turbidites, and post-emplacement underthrusting of Late Oligocene – Early Miocene Nido Limestone. This carbonate underthrusting at end Early Miocene (∼16 Ma) is marked by the deformation of a seismic unit corresponding to the earliest members of the Early – Middle Miocene Pagasa Formation. Within this formation, a tectonic wedge was built within Middle Miocene (from ∼16 Ma to ∼12 Ma), forming a thrust-fold belt called the Pagasa Wedge. Wedge deformation is truncated by the regionally-observed Middle Miocene Unconformity (MMU ∼12 Ma). A localized, post-kinematic extension affects thrust-fold structures, the MMU, and Late Miocene to Early Pliocene carbonates (e.g. Tabon Limestone). This structural set-up suggests a continuous convergent regime affecting the southeastern margin of the South China Sea between end Eocene to end Middle Miocene. The ensuing structures including juxtaposed carbonates, turbidites and shallow marine clastics within thrust-fold belts have become ideal environments for hydrocarbon generation and accumulation. Best developed in the Northwest Borneo Trough area, the intensity of thrust-fold deformation decreases towards the northeast into offshore southwest Palawan.  相似文献   

12.
Gravity cores obtained from isolated seamounts located within, and rising up to 300 m from the sediment-filled Peru–Chile Trench off Southern Central Chile (36°S–39°S) contain numerous turbidite layers which are much coarser than the hemipelagic background sedimentation. The mineralogical composition of some of the beds indicates a mixed origin from various source terrains while the faunal assemblage of benthic foraminifera in one of the turbidite layers shows a mixed origin from upper shelfal to middle-lower bathyal depths which could indicate a multi-source origin and therefore indicate an earthquake triggering of the causing turbidity currents. The bathymetric setting and the grain size distribution of the sampled layers, together with swath echosounder and sediment echosounder data which monitor the distribution of turbidites on the elevated Nazca Plate allow some estimates on the flow direction, flow velocity and height of the causing turbidity currents. We discuss two alternative models of deposition, both of which imply high (175–450 m) turbidity currents and we suggest a channelized transport process as the general mode of turbidite deposition. Whether these turbidites are suspension fallout products of thick turbiditic flows or bedload deposits from sheet-like turbidity currents overwhelming elevated structures cannot be decided upon using our sedimentological data, but the specific morphology of the seamounts rather argues for the first option. Oxygen isotope stratigraphy of one of the cores indicates that the turbiditic sequences were deposited during the last Glacial period and during the following transition period and turbiditic deposition stopped during the Holocene. This climatic coupling seems to be dominant, while the occurrence of megathrust earthquakes provides a trigger mechanism. This seismic triggering takes effect only during times of very high sediment supply to the shelf and slope.  相似文献   

13.
以精度较高的现场测量Hamilton的原位测量所得数据为标准,对比研究了钱正明的直立同轴衰减测量法、刘强的垂直轴差距衰减测量法和我们课题组的同轴差距衰减测量法等3种研究海底沉积物声衰减的实验室测量方法。通过3种实验测量方法所得数据跟Hamilton原位测量所得数据进行对比,分析了以上实验室方法的优缺点,并分析了声衰减与样品长度和测量频率的关系。同轴差距衰减测量法与直立同轴衰减测量法相比,前者在消除测量仪器和耦合衰减误差方面,以及在实验样品长度方面更合理。同轴差距衰减测量法与垂直轴差距测量法相比,同轴差距衰减测量法在原理上保证了声波传播能量接收的同轴性和同指向性,其实验数据比利用相同实验仪器的垂直轴差距测量法的数据更准确。  相似文献   

14.
Peculiar pellicles have been recorded in the extant anoxic hypersaline basins of the Bannock area and the Western Strabo Trench (Eastern Mediterranean). Discrete layers of pellicles were found in 29 cores. They are oriented parallel to bedding planes or are folded within the sandy base of turbidites and within slumped sediments. In addition, similar pellicles occur at the surface of or interlocked within gypsum crystals recovered from the Bannock area.

The pellicles are 0.5–3 mm thick and dark greenish-grey in colour. They foliate into very small (<1 mm) undulating and anastomosing microlaminae entrapping abundant biogenic and inorganic particles. The amorphous organic matter of the pellicles is most probably produced by bacterial masses linked to mineralization processes of the organic debris. Anaerobic sulphate-reducing bacteria (Desulphovibrio) are abundant and the presence of methane bacteria is likely.

Detailed analyses with the light microscope and scanning electron microscope yielded differences and similarities concerning ultrastructure, content, abundance and morphotypes of the bacterial pellicles from the Bannock area and the Western Strabo Trench. A consistently well-developed ultrastructure comprising thin microlaminae characterizes the Bannock pellicles, whereas the ultrastructure of the Tyro and Poseidon pellicles is usually not organized. Siliceous microfossils are abundant in samples from both sites, but centrate diatoms prevail in the Bannock pellicles whereas pennate diatoms are overwhelmingly dominant in the Tyro and Poseidon pellicles. Calcareous micro- and nannofossils, together with pyrite framboids, are more abundant in samples from the Bannock area. Bacteria are extremely abundant in samples from the Bannock area and three different morphological types have been distinguished. Only one type of bacteria was commonly observed in pellicles from the Western Strabo Trench basins.

Pellicles form at the normal seawater/brine interface, as substantiated by transmittance profiles and bacteriological investigations. Therefore, the term “deep mid-water bacterial mats” is proposed herein for these peculiar layers.

After bacterial mats grow at the interface, they are included within the sediments by following one of two different paths: (a) when pelagic sedimentation prevails, pellicles sink to the basin floor due to the increasing load of biogenic and inorganic detritus, and (b) under a tectonically active regime, turbidity currents and slumps can disrupt the interface and destroy the floating mats. In the latter case, fragments of pellicles are transported to the bottom within the gravity flows.  相似文献   


15.
Late Pleistocene to Holocene margin sedimentation on the Great Barrier Reef, a mixed carbonate-siliciclastic margin, has been explained by a transgressive shedding model. This model has challenged widely accepted sequence stratigraphic models in terms of the timing and type of sediment (i.e. carbonate vs. siliciclastic) deposited during sea-level oscillations. However, this model documents only hemipelagic sedimentation and the contribution of coarse-grained turbidite deposition, and the role of submarine canyons in this process, remain elusive on this archetypal margin. Here we present a new model of turbidite deposition for the last 60 ky in the north-eastern Australia margin. Using high-resolution bathymetry, 58 new and existing radiometric ages, and the composition of 81 turbidites from 15 piston cores, we found that the spatial and temporal variation of turbidites is controlled by the relationship between sea-level change and the variable physiography along the margin. Siliciclastic and mixed carbonate-siliciclastic turbidites were linked to canyons indenting the shelf-break and the well-developed shelf-edge reef barriers that stored sediment behind them. Turbidite deposition was sustained while the sea-level position allowed the connection and sediment bypassing through the inter-reef passages and canyons. Carbonate turbidites dominated in regions with more open conditions at the outer-shelf and where slope-confined canyons dominated or where canyons are generally less abundant. The turn-on and maintenance of carbonate production during sea-level fluctuations also influenced the timing of carbonate turbidite deposition. We show that a fundamental understanding of the variable physiography inherent to mixed carbonate-siliciclastic margins is essential to accurately interpret deep-water, coarse-grained deposition within a sequence stratigraphic context.  相似文献   

16.
使用序列化和不规则矩形划分的方法,开发了基于消息传递接口(MPI)的环流数值模式并行算法,并通过邻近点交换策略(NPES)进一步提升了计算效率。在HP C7000刀片系统上设计了两个实验,数值结果表明,使用了NPES的并行版本(PVN)的计算效率要高于原并行版本(PV)。在第二个实验中当进程数为100时PVN的计算效率仍可达到0.9以上,然而此时PV的计算效率已迅速下降至0.39。并用环流模式的PVN版本对一个高分辨率的区域进行了模拟,效果较好。该算法具备普适性,可用于其它环流模式的高效并行。  相似文献   

17.
This work discusses the synsedimentary structural control affecting the turbidites of the Marnoso-arenacea Formation (MAF) deposited in an elongate, NW-stretched foredeep basin formed in front of the growing Northern Apennines orogenic wedge. The stratigraphic succession of the MAF (about 4000 m thick) records the progressive closure of the Apennine foredeep basin due to the NE propagation of thrust fronts. In this setting, Langhian to Serravallian turbidites are overlain by Tortonian mixed turbidite deposits, i.e. sandstone-rich low-efficiency turbidites. The high-resolution stratigraphic framework of basin-plain turbidites has made it possible to identify five informal stratigraphic units (I, II, III, IV, V) mainly on the basis of the structural control highlighted by: 1) the presence of topographic highs and relative depocentres detected through a progressive flattening approach, and 2) the presence of thrust-related mass-transport complexes and the progressive appearance and disappearance of five bed types (Types 1, 2, 3, 4, 5) considered important to understand the interaction between flow efficiency and basin morphology. By contrast, the upper part of the MAF succession (Tortonian in age) is formed by more sandstone-rich systems characterized by beds whose origin is likely to depend, at least in part, upon flow decelerations related to topographic confinement due to the progressive closure of the foredeep. The vertical and lateral distribution of these types of beds is, therefore, useful for the reconstruction of the morphological evolution of structurally controlled basins; in the MAF example, this is mainly due to the progressive narrowing of the foredeep caused by the propagation of the main thrust fronts toward the foreland.  相似文献   

18.
In order to define the nature and distribution of the organic matter (OM) preserved in the modern Ogooué deep sea turbidite system (Gabon), bulk geochemical techniques (Rock-Eval pyrolysis, elemental and isotopic analyses) and palynofacies were applied to three piston cores collected in the Cape Lopez Canyon and lobe and on the continental slope, north of the canyon.The hemipelagic sedimentation in the study area is characterized by high accumulations of well-preserved OM (∼2-3 wt. TOC %). Bulk geochemical and palynofacies analysis indicate both a marine and terrestrial origin of the OM. Contribution of the marine source is higher on the slope than in the canyon and lobe.OM accumulation in turbidites is strongly controlled by the combined influence of the Cape Lopez Canyon and littoral drift. In the canyon and lobe, turbidites show generally low TOC content (0.5 wt. %) and OM is oxidized. The origin of the OM is interpreted as both marine and terrestrial, with a higher contribution of continental source versus marine source. The low TOC contents are due to the large siliciclastic fraction transported by the littoral drift and diverted in the Cape Lopez Canyon during high energy processes (e.g. storms) which tend to dilute the OM in the turbidites. Transport by long-shore currents and/or turbiditic flows leads to oxidation of the OM.On the continental slope located north of the Cape Lopez Canyon, large amounts of OM are deposited in turbidites (up to 14 wt. %). The OM is predominantly derived from terrestrial land plants and has not been subjected to intense oxidation. These deposits are characterized by high hydrocarbon potential (up to 27 kg HC/t rock), indicating a good potential as gas-prone source rock. Because Cape Lopez Canyon captures a significant part of the sediment transported by the littoral drift, the siliciclastic sedimentary flux is reduced north of the canyon; OM is thus concentrated in the turbidites. Variation in TOC content within turbidite laminae can be explained by the burst and sweep deposition process affecting the boundary layer of the turbulent flow.This study confirms that gravity flows play a preponderant role in the accumulation and preservation of OM in deep water and that deep sea turbidite systems could be regarded as an environment where organic sedimentation occurs.  相似文献   

19.
Summary

Reids Rise is a remnant of a marine sand body that accumulated in the inner neritic zone of the continental shelf south‐west of Westport during the Last Interglacial. The sand body is notable for its spectacular sedimentary structures and consists of an upper unit that is characterised by trough cross‐lamination, locally forming mesoscopic ridges, and a lower unit that is characterised by horizontal lamination. The sand body is underlain by gravels of probable alluvial origin.

The trough cross‐laminated sets of the upper unit dip off near‐symmetrical ridges that probably formed as subaqueous bars. The dip azimuths of the cross‐laminae have a polymodal distribution with two bimodal maxima, one almost perpendicular to, and the second roughly parallel to the shoreline formed during the Last Interglacial. Magnitude of dip varies from horizontal to over 40°.

The sediment is clean, fine sand composed predominantly of angular grains. Sample to sample variation in grain size statistical parameters is small. The most notable feature is the extremely good sorting of the sand. Inclusive graphic standard deviation values range over 0.17–0.24? with 0.20? the modal value.

The mineralogy of the sand points to two main source areas, the Alpine metamorphic rocks to the south and south‐east, and the plutonic rocks of the Paparoa Range immediately east and south‐east of the study area.

The stratigraphic sequence is interpreted as being the product of deposition in a high energy, wave‐, tidal‐current‐, and longshore‐current‐dominated shallow neritic zone on an open coast during a transgressive cycle.  相似文献   

20.
Do the Apennines represent a vantage point for studying turbidites? How did research develop in this area? These questions are discussed briefly in the following pages from a historical and autobiographical perspective. Some items are emphasized: the shifting of depocentres, the presence of megabeds (basinwide events), the definition of ‘classical’ turbidites, the facies approach, the recognition and distinction of hyperpycnal flows  相似文献   

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