Bouma Sequences Research Articles

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

Abstract High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation, the Binhai area of Qikou Sag, providing an example of medium- and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin. By using data of cores, cast thin sections, scanning electron microscope and physical property tests, the sedimentary facies, physical properties and main control factors of the high-quality reservoirs were analyzed. The reservoirs are identified as deposits of slump-type sub-lacustrine fans, which are marked by muddy fragments, slump deformation structure and Bouma sequences in sandstones. They present mostly medium porosity and low permeability, and slightly medium porosity and high permeability. They have primary intergranular pores, intergranular and intragranular dissolution pores in feldspar and detritus grains, and structural microcracks as storage space. The main factors controlling the high quality reservoirs are as follows: (1) Favorable sedimentary microfacies of main and proximal distributary gravity flow channels. The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow, and the effect of deposition on reservoir properties decreases with the increase of depth. (2) Medium texture maturity. It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing, and was good for the formation of the primary intergranular pores. (3) High content of intermediate-acid volcanic rock detritus. The reservoir sandstone has high content of detritus of various components, especially intermediate-acid volcanic rock detritus, which is good for the formation of dissolution pores. (4) Organic acid corrosion. It was attributed to hydrocarbon maturity during mesodiagenetic A substage. (5) Early-forming and long lasting overpressure. A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone, and is responsible for the preservation of abundant primary pores. (6) Regional transtensional tectonic action. It resulted in the structural microcracks.

Bioturbation, sedimentation rates, and preservation of flood events in deltas

We examine the nature and number of event beds preserved in heterolithic prodelta and delta-front strata in five proximal-accumulation-dominated (PAD) North American delta systems including the Quaternary Gulf of Mexico and the Late Cretaceous Western Interior Seaway. We also explore the continuum between PAD, more distal deposits of mud belts, and thicker subaqueous-delta-clinoforms (SDC). Sediment accumulation rates in modern PAD and SDC systems are commonly on the order of several to severaltens of centimeters per year. They also show that several event beds may be created annually, with preservation depending on supply and reworking. The PAD heterolithics commonly comprise interbedded sand, silt and clay. These show normal and inverse grading and Bouma sequences, indicating deposition from turbidity currents and hyperpycnal flows that are interpreted as the products of river plumes generated during flood episodes. Re-equilibrated Rosselia socialis in stacked delta front turbidite bedsets in the Cretaceous examples show that floods may occur as a cluster of events over several months. Shelf areas adjacent to river mouths may experience high rates of sediment delivery from rivers, but oceanographic processes are instrumental in moving much of the silt and clay farther along the shelf. Direct, hyperpycnal delivery from a river mouth on a low gradient shelf (i.e., typically <0.05°) is rare, owing to the difficulty of maintaining a high suspended sediment concentration, but sedimentation rates are nevertheless high. Shelf facies show evidence for extensive and frequent remobilization during storms and other meteorological events. Storm-wave-enhanced sediment-gravity flows (WESGFS) may form under favorable conditions, such as strong waves, high suspended sediments, and convergent currents, and can be significant in building SDC and other shelf deposits. The proximal portions of PAD and SDC systems typically show low intensity and diversity of bioturbation (BI 0–1). The bioturbation styles encountered in these deposits record: 1) high sedimentation rates, which preclude wholesale biogenic reworking; 2) high-frequency river floods and storm reworking, expressed by reduced colonization windows; 3) soupy substrates, likely reflecting formation of a permanent fluid-mud lutocline; and 4) elevated turbidity near the sediment-water interface, which may cause further suppression of trace-making.

Age, depositional environment, and tectonic significance of an Early Neoproterozoic volcano‐sedimentary sequence in the eastern Beishan orogenic belt, southern Central Asian Orogenic Belt

The Beishan orogenic belt, located in the southern Central Asian Orogenic Belt, includes several discrete arc terranes. Here, we report on a newly discovered Early Neoproterozoic volcano‐sedimentary sequence in the eastern Beishan orogenic belt in order to constrain the composition and origin of the Shuangyingshan‐Huaniushan Terrane. The ~766‐m‐thick volcano‐sedimentary sequence is very low‐grade metamorphosed, and characterized by muddy and silty slate with the well‐developed parallel laminations and minor thinly‐bedded limestone in its lower and upper sections, and thick packages of sandstone‐conglomerate and basaltic rocks in the middle section. Lava flow units in the basalt are very common, and the sandstone‐conglomerate exhibits sedimentary structures typical of gravity currents, including erosional and slump features, and Bouma sequences. These features suggest that syn‐sedimentary volcanism occurred and the sequence formed in an active, deep marine environment. The basalt has a zircon U–Pb age of 901 ± 10 Ma, indicating that this sequence formed during the Early Neoproterozoic. Basalt samples have moderate SiO2 (45.33–49.18%), and high TiO2 (1.27–4.04%) contents, Mg# values of 25–48, and plot in the alkali basalt field in the Nb/Y versus Zr/TiO2 diagram, indicative of an evolved alkali basalt magma. They show inclined chondrite‐normalized REE patterns with (La/Yb)N ratios of 4.74–6.53, no pronounced negative Eu anomalies (Eu/Eu* = 0.87–1.22), no obvious negative Zr, Hf, Nb, and Ta anomalies in the primitive‐mantle‐normalized trace‐element diagram, consistent with oceanic island basalts. Considered together, these lithologies resemble those of an oceanic seamount. Our results, and the presence of the previously reported Gubaoquan granitic and mafic rocks (905–865 Ma) that formed in an active continental margin setting and underwent an amphibolite‐facies metamorphism, suggest that the Shuangyingshan‐Huaniushan Terrane may be an accreted terrane that consists of different lithologies with various origins that formed during the Neoproterozoic.

An integrated process‐based model of flutes and tool marks in deep‐water environments: Implications for palaeohydraulics, the Bouma sequence and hybrid event beds

AbstractFlutes and tool marks are commonly observed sedimentary structures on the bases of sandstones in deep‐water successions. These sole structures are universally used as palaeocurrent indicators but, in sharp contrast to most sedimentary structures, they are not used in palaeohydraulic reconstructions or to aid prediction of the spatial distribution of sediments. Since Kuenen's famous 1953 paper, flutes and tool marks in deep‐water systems have been linked to turbidity currents, as reflected in the standard Bouma sequence taught to generations of geologists. Yet, these structures present a series of unaddressed enigmas. Detailed field studies in the 1960s and early 1970s observed that flutes are typically associated with thicker, more proximal beds, whilst tools are generally prevalent in thinner, more distal, beds. Additionally, flutes and tool marks are rarely observed on the same surfaces, and flutes are seen to change downstream from larger wider parabolic to smaller narrower spindle‐shaped forms. No model has been proposed that explains these field‐based observations. This contribution undertakes a radical re‐examination of the formative flow conditions of flutes and tool marks, and demonstrates that they are the products of a wide range of sediment gravity flows, from turbulent flows, through transitional clay‐rich flows, to debris flows. Flutes are not solely the product of turbulent flows, but can continue to form in transitional flows. Grooves are shown to be formed bydebris flows, slumps and slides, not turbidity currents, and in many cases the debris flows are linked to the debritic component of hybrid flows. Discontinuous tool marks, including skim (bounce) marks, prod marks and skip marks, are shown to be formed by transitional mud‐rich flows. Consequently, the observed spatial distribution of flutes and tool marks can be explained by a progressive increase in flow cohesivity downstream. This model of flutes and tool marks dovetails with models of hybrid flows that predict such a longitudinal increase in flow cohesivity. However, some deposits show grooves preferentially associated with BoumaTAbeds, and these are likely formed by flows transforming from higher to lower cohesion, and are present in basins where hybrid beds are absent or rare. The recognition that sole structures may haveno genetic linkto the later overlying turbidity current deposits, and can be formed by a wide range of flow types, indicates that the existing pictorial description of the Bouma sequenceis incorrect. A modified Bouma sequence is proposed here that addresses these points. In utilizing the advances in fluid dynamics since Kuenen's pioneering research, this study demonstrates that it is possible to use flutes and tool marks to interpret flow type at the point of formation, the nature of flow transformations, and the mechanics of the basal layer. These advances suggest that it is then possible to predict the nature of deposit type down‐dip. This new understanding, in combination with further testing in outcrop of the proposed relationships between sole marks and palaeohydraulics, opens up a wealth of possibilities for improving the understanding of deep‐water clastic environments, with implications for developing more complete facies models, assessing subaqueous geohazards and the resilience of seafloor infrastructure, and advancing our understanding of deep‐water sediments as archives of palaeoenvironmental change.

Lithology, stratigraphy, chemostratigraphy, and depositional environment of the Mississippian Sycamore rock in the SCOOP and STACK area, Oklahoma, USA: Field, lab, and machine learning studies on outcrops and subsurface wells

Abstract This paper presents an interpreted depositional environment, developed 2nd order sequence stratigraphy framework, and detailed lithofacies identification from two Mississippian Sycamore outcrops (I-35 Sycamore and Speake Ranch) and subsurface wells in the SCOOP (South Central Oklahoma Oil Province). Then these rocks calibrate the rock properties with wireline log responses to identify the best landing zones. Qualitative and quantitative techniques of field, laboratory, and machine learning studies were conducted. For the field studies, we measured the complete 450 ft of the outcrop stratigraphic section, another separate 50 ft outcrop, examined the underlying Woodford Shale and overlying Caney Shale boundary contacts, documented sedimentary structures, constructed an outcrop gamma-ray profile, and developed a sequence stratigraphic framework. Laboratory studies included petrographic analyses, detailed X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). For machine learning studies, a principal component analysis (PCA), elbow method, and self-organizing map (SOM) were used to analyze the electrofacies and chemofacies from the outcrop and a subsurface uncored well. The outcrop hand-held gamma ray profile was obtained and correlated with subsurface wells. Five major outcrop lithofacies and chemofacies, within six stratigraphic units of alternating siltstone and shale strata, were identified from the wireline logs. A Maximum Flooding Surface (MFS), and two major 2nd order Sequence Boundaries (SB) were recognized at the outcrop and a nearby subsurface well. Bouma sequences and repetitive cycles of sedimentary structures indicated sediment gravity flow deposition on a marine slope setting. This study provides geologic insights to better understanding the depositional environment and the lithology of the Sycamore rocks. The bioturbated siliceous shale and/or the sandy siltstone can be potential target zones due to their reservoir quality, lithology, bed continuity, and brittleness. This information can be of direct benefit to the exploration and development programs of many companies in the SCOOP area, particularly in the Anadarko and Ardmore basins in Oklahoma.

A giant sandy sediment drift in early Silurian (Telychian) and its multiple sedimentological prosess

Abstract This study describes a regional-scale sedimentary feature in the Upper Yangtze region of China that is identified as a Telychian age (438 Ma) sediment sandy drift that is 500–800 km long, 100–150 km wide, and 10–20 m thick. Graptolite zones is applied to recognize Telychian sandstone beds and reconstruct an isochronal stratigraphic framework for the study area. It is possible to observe numerous traction structures in Telychian sandstone beds, including ripples, cross-bedding, cross-laminations, wavy-laminations and horizontal-laminations and Bouma sequence. The palaeocurrent direction is towards SSW (200°–240°), which is measured by sedimentary structures (cross-bedding, cross-laminations and ripples) in outcrops. Besides, the alignment of elongate particles in thin section, and the distribution characteristics of heavy minerals also indicate a SSW palaeocurrent direction. This direction is parallel to palaeo-coastline, which is a characteristic feature of bottom current. A multiple sedimentary processs is identified based on analysing facies of each sections. Contourite-dominated sequences are observed in channel-related seaway while turbidite sequences are observed in shelf near palaeo-coastline. Based on provenance analysis, the Telychian drift (channel reworked sandy drift) is the production of a mixed depositional system (first turbidite deposition and then bottom current deposition). The turbidite sand on the shelf provides material source for the sandy drift, and the deposit is transported from the shelf to the channel (parallel to ancient coastline). The provenance direction is identified by analyzing the distribution of heavy minerals. The unstable heavy minerals content is higher in the turbidites samples (near provenance region) than the channel reworked sandstone samples (far away from provenance region). This Telychian deposit is a rare example of an ancient channel reworked sandy drift.

Trigger mechanisms of gravity flow deposits in the Lower Cretaceous lacustrine rift basin of Lingshan Island, Eastern China

Gravity flows were commonly developed in subaqueous, steep slopes in lacustrine basins, which show distinct sedimentary records due to different flow behaviors and trigger mechanisms. Outcrops of the Lower Cretaceous deposits on the Lingshan Island are well exposed to illustrate flow transformations between turbidity currents and debris flows in shallow water and basin floor in a lacustrine basin. Outcrop sections were carefully measured to characterize sedimentary records of the gravity flows. Facies associations include laminated mudstone (FA1), deformed silty mudstone (FA2), massive very-fine sandstone with rip-up mud clasts (FA3), graded very-fine sandstone with (partial) Bouma sequences (FA4), massive gravelly sandstone with rip-up mud clasts (FA5), gravelly sandstones with partial Bouma sequences (FA6), and cross-stratified pebbly coarse-grained sandstones (FA7). Trigger mechanisms for the gravity flow include mainly plunging of hyperpycnal river discharges and earthquakes, which both induced failure of delta front/slope and the consequent debris flows/turbidity currents. Flow transformations occurred both at the shallow water and basin floor, which produced distinct sedimentary structures, stacking patterns, and architectural units. The high-density hybrid event beds (HEBs) with both gravels and mud clasts overlaid by sandy turbidites were mainly induced by floodwater from the active river mouths of deltas, generating fast-moving debris flows and deceleration of turbidity currents. Damping of turbidity currents and deceleration of muddy debris flows due to high cohesion induced low-density turbidites were overlaid by HEBs, showing thin-bedded, lateral extended sandy beds on the basin floor.

Verification of gravity-flow models: case study from the Lower Eocene sediments (Tylmanowa site, SE Poland)

The thick-bedded, deep-water sandstone succession was described at the Tylmanowa site from the Polish Outer Carpathians. This part of the Carpathians is built mainly of the Upper Jurassic to Paleogene deep-water rocks. Succession at the Tylmanowa site is composed of massive, ripple-cross laminated, planar and trough cross-stratified, horizontally laminated and deformed sandstones as well as massive and horizontally laminated mudstones. All these sediments derived from gravity flows that prograde downslope from a basin margin towards the widespread abyssal plain. Exposed succession records the gradual transition from a decelerating debris flow to a turbidity current what is extraordinary in the recent investigations of deep-water sediments. The study succession has been compared with the widely known sediment models, such as: the classic Bouma Sequence (Bouma 1962), the high-density turbidite model (Lowe 1982), the fluxoturbidite model (Ślączka, Thompson 1981) and the hybrid event bed model (Haughton et al. 2009).

A Late Carboniferous–Early Permian back‐arc basin in the southeastern Central Asian Orogenic Belt: Constraint from sedimentological and geochronological investigations of the Shoushangou Formation in the central Inner Mongolia

The Solonker Suture Zone was formed during the closure process of the Paleo‐Asian Ocean (PAO). In order to better constrain its tectonic setting and evolution history in the Late Carboniferous–Early Permian, we conducted an integrated study of the Shoushangou Formation in the central Inner Mongolia, involving the sedimentological features, depositional age, and detrital zircon populations. Our results indicate that the Shoushangou Formation was deposited in ca. 320–280 Ma, overlain by the Dashizhai Formation with ages of ca. 280–274 Ma, and the 330‐ to 280‐Ma detrital age data almost serve as the entire age component of this formation. Also, its stratigraphic reconstruction shows a coarsening‐upward marine sequence upwards, which is characterized by sandstone turbidites with a Bouma sequence and the numerous tuff involvements in the lower strata, showing a deep marine environment and synchronous volcanism. Integrating previous data and our results, with a gradually deepening water depth and a geographic variation, detrital zircon age spectra from the Late Carboniferous–Early Permian sequence show distinct differences, and a marked shift of multiple resources to a single source occurred in the Late Carboniferous–Early Permian. These clues suggest an active marginal setting and a northward‐dipping subduction during this period in the north side of the PAO, and we further attribute such a source variation to the spreading of a marine back‐arc basin after the amalgamation of the Uliastai Active Continental Margin with the Northern Accretionary Orogen. The Shoushangou Formation was the direct depositional response to such a back‐arc basin.

GRAIN SIZE STATISTICAL PARAMETERS AND PALAEOFLOW VELOCITY MEASUREMENTS OF THE TERTIARY PINDOS FORELAND BASIN TURBIDITES

Pindos foreland is a tertiary turbiditic basin fill trending parallel to the external Hellenides (Aubouin, 1959). The basin is bounded to the east by the Pindos thrust and to the west by the Ionian thrust. Apart of these two major thrusts, minor thrusts separate the basin into linear narrow sub-basins, trending also parallel to the basin axis. For the grain size statistical analysis 35 sandstone samples were collected from sandstone beds in three sections: Metsovo, Amphilochia and Palaiopyrgos. The thickness of the beds ranges from 8 to 25 cm, and comprise Ta, Tb and Te Bouma sequence subdivisions. The samples were smashed in small pieces and then they were disaggregated using acetic acid solution. Then the samples were washed with deionized water and prepared for sieve analysis. The results of the sieve analysis were plotted in grain size cumulative diagrams in order to estimate the statistical parameters. Sorting, skewness and kyrtosis were calculated and also the samples were plotted in CM, FM and LM diagrams (Passega, 1957; Passega, 1964) The palaeoflow velocity measurements were estimated using Komar's model (1985). The results of the above analysis provided the following conclusions: a) Sorting values are decreasing at the top of Metsovo and Palaiopyrgos sections indicating an increase of the sediments immaturity, b) The asymmetry values range from positive to very positive with a trend to increase at he top in the sections of Metsovo and Amphilochia, which shows a dominance of the coarser fraction in the selected samples, c) C-M, F-M, L-M affirms that the sediment was transported to the deeper parts of the basin by turbidity currents. Mean flow velocities at the time of deposition range between 1,86 and 26,59 cm/sec. These values are very much in agreement with those proposed for low-density turbidity currents (&lt;25cm/sec) (Nelson and Nilsen, 1984). A similar velocity range is refereed also by Avramidis (1999) who studied the turbidites of the Middle Ionian zone. In Metsovo and Palaiopyrgos mean flow velocity values increase towards the top of the stratigraphy.

Oligocene storm-induced lacustrine deposits in the Yaxi Area of the Jiuxi Basin, northeastern margin of the Tibetan Plateau

Storm-induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow. Thin-bedded sandstone, siltstone, and interlayered mudstone deposits of the Oligocene Jianquanzi Member in the Yaxi Area of the Jiuxi Basin along the northeastern margin of the Tibetan Plateau were formed by shallow lacustrine storm-generated flows. Petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub-mature to immature. Based on investigation of drill cores, six storm-deposit lithofacies are identified, including massive or graded sandstone, parallel and trough cross-bedded sandstones, hummocky cross-stratified sandstone, wave-rippled sandstone and siltstone, massive or horizontal bedded mudstone, and structurally deformed sandstone and mudstone. In a vertical succession, these lithofacies occur as three fining-upward lithofacies associations, each of which represents different architectural elements (and depositional processes) of proximal to distal storm deposits. The relatively coarse proximal architectural element is formed by strong density flows and unidirectional-dominated combined flows; the transitional architectural element is dominated by unidirectional- and oscillatory-dominated combined flows to pure oscillatory flows, resembling Tb-e divisions of the classic Bouma sequence; the relatively fine distal architectural element is dominated by unidirectional-dominated combined flows with a very weak oscillatory flow component. The origin of storms responsible for these deposits is attributed to Asian winter monsoons associated with the paleoclimatic and paleogeographic setting.

Classification and sedimentary characteristics of lacustrine hyperpycnal channels: Triassic outcrops in the south Ordos Basin, central China

Subaquatic channels are known as active conduits for the delivery of terrigenous sediments into related marine and lacustrine basins, as well as important targets for hydrocarbon exploration. Compared to submarine channels, lacustrine subaqueous channels created by hyperpycnal flows are understudied. Using well-exposed outcrops collected from three different locations in the southern Ordos Basin, central China, morphologies and architecture of a channelized hyperpycnal system were studied and classified. Six facies associations represent sedimentary processes from strong erosion by bedload dominated hyperpycnal flows, to transitional deposition jointly controlled by bedload and suspended-load dominated hyperpycnal flows, finally to deposition from suspended-load dominated hyperpycnal flows. On the basis of channel morphologies, infilling sediments and sedimentary processes, the documented channels can be classified into four main categories, which are erosional, bedload dominated, suspended-load dominated, and depositional channels. In very proximal and very distal locations, erosional channels and depositional channels serve as two end-members, while in middle areas, bedload-dominated channels and suspended-load dominated channels are transitional types. Erosional channels, as a response to strong erosion from bedload dominated hyperpycnal flows on upper slope, were mainly filled by mud interbedded with thin sand beds. As flow energy decreases, bedload dominated channels develop on middle slopes, which are characterized mainly by under- to balanced sediment infillings with cross-bedded sandstones and/or minor massive sandstones. Compared to bedload dominated channels, suspended-load dominated channels mainly develop in deeper water, and were filled mainly by massive or planar-laminated sandstones. Depositional channels, as a response to suspended-load dominated hyperpycnal flows in deep-water areas, are characterized by thin-medium bed classical turbidites with Bouma sequences and thin- to thick massive sandstones. Such evolution patterns of hyperpycnal channel systems are ascribed to the progressive decrease in flow capacity of hyperpycnal flows, and provide an adequate explanation for the basinward channelization behavior of hyperpycnal systems.

Petrography, provenance, diaganesis and depositional environment of Murree Formation in Jhelum Valley, Sub Himalayas, Azad Jammu and Kashmir, Pakistan

The clastic sediments of the Murree Formation of Miocene age are exposed in Jhelum valley areas of Azad Jammu and Kashmir Pakistan. Field observations revealed the cyclic deposition in the Murree Formation. The sandstone, siltstone, and shale constitute a single cycle within the formation. This single unit is divided into five different lithofacies which constitute the Bouma sequence in the Murree Formation. The Murree Formation shows faulted contacts with Panjal Formation and Nagri Formation in the study area. The modal mineralogy data obtained from the petrography of sandstone indicates that sandstone is litharenite and lithic greywacke. The mineralogical and textural data suggests that sandstone is compositionally mature and poorly to moderately sorted. The dominantly angular to sub angular quartz grains show nearness of the source area. Fractured and sutured quartz grain reveals tectonodiagentic changes that occurred in Murree Formation. The sandstone experienced diagenetic changes. The pressure solution and cementation reduced the primary porosity of sandstone. However, alteration of feldspar and fractures in grains have produced secondary porosity. The X-ray diffraction (XRD) of the shale samples indicates that shale of the Murree Formation is argillaceous and dominated by illite clay mineral. The illite crystallinity values indicate very low grade metamorphism of Murree Formation in core of Hazara Kashmir Syntaxis. The petrographic data suggests that the provenance of sandstone is recycled orogen. Quartz is of igneous and metamorphic origin. Feldspar (albite and microcline) composition suggests its derivation from acidic igneous rocks. The rock fragments of volcanics, slate, phyllite, and schist suggest igneous and metamorphic provenance. The petrographic data suggests that at the time of deposition of Murree Formation, igneous and low grade metamorphic rocks were exposed. However, presence of some clasts of carbonates indicates that sedimentary rocks were also exposed in the source region. The quartz content and clay minerals in the shale revealed that source region was igneous and metamorphic rocks. Cyclic deposition, lithofacies, and various sedimentary structures like cross bedding, ripple marks, and calcite concretions suggest that deposition of Murree Formation occurred in fluviatile environment by meandering river system having decreasing turbidity current.

Suprabenthic assemblages from the Capbreton area (SE Bay of Biscay). Faunal recovery after a canyon turbidity disturbance

In the Capbreton area, suprabenthic assemblages were sampled with a sledge towed over the bottom, at different sites located within the upper part of a ‘gouf-type’ canyon (8 hauls between 642m and 797m, on the axis of the thalweg or on flat perched flank terraces such as site K), on the northern adjacent open slope (2 hauls between 500 and 567m) and on the northern adjacent shelf margin (2 hauls between 151m and 158m). A multivariate analysis carried on the faunal data discriminated different assemblages in this area: a near-canyon shelf assemblage (55 species, mainly amphipods and decapods; 3496 ind./100m2, 40% mysids; dominant species: Nyctiphanes couchii, Leptomysis gracilis, Weswoodilla rectirostris, Anchialina agilis, Scopelocheirus hopei and Philocheras bispinosus); an open slope assemblage (111 species, mainly amphipods and isopods; 249 ind./100m2, 36% amphipods; dominant species: Disconectes phalangium, Munnopsurus atlanticus and Boreomysis arctica); a canyon E assemblage (129 species, mainly amphipods, mysids and cumaceans; 1172 ind./100m2, 58% amphipods; dominant species: Melphidippa sp. B, Chelator insignis); a canyon E’ assemblage (107 species, mainly amphipods and mysids; 507 ind./100m2, 73% amphipods; dominant species: Cleonardopsis carinata, Bonnierella abyssorum, Rhachotropis caeca and Arcturopsis giardi); and a temporary canyon assemblage at site K (34 species, mainly amphipods and mysids; 899 ind./100m2, 85% amphipods; dominant species: Tmetomyx similis, Caeconyx caeculus, Nebalia sp. A and Cleonardopsis carinata). Site K was sampled only four months after a turbidity event, detected on sediment cores (18cm thick Bouma sequence) taken during the same cruise and triggered by the violent storm (‘ouragan Martin’, wind up to 200km/h) which affected the French Atlantic coast on 27 December 1999. The corresponding suprabenthic assemblage showed evidence of deep structural changes after this catastrophic event, characterized by relative low values of species richness and diversity indices and by the exceptional dominance of opportunistic pioneer colonizers such as the scavenger Tmetonyx similis (61% of total density). Partial assemblage recovery was noticed 18 months after the turbidite deposition, attested by higher values of structural indices and by the inclusion of the corresponding sample within the climax canyon assemblage cluster.

An endemic ichnoassemblage from a late Miocene paleolake in SE Iceland

In Þórisdalur valley, a small relict of a sedimentary body was identified in southeastern Iceland. It probably represents a remnant of the deep, tectonically arranged paleolake (Late Miocene, 8–9Ma), and filled by volcaniclastic material from nearby, active volcanic centers. In the profile of tuffitic sandstone, siltstone and claystone, the ripple-bedding layers, molds and flute casts indicate periodic mass flow episodes. In the sedimentary profile, the characteristic arrangement of sediments is evident, showing features of the Bouma sequences. In the claystone layers, deposited during episodes of lowest kinetic energy, a specific ichnoassemblage was found, represented by Thorichnus ramosus igen. et isp. nov., T. corniculatus igen. et isp. nov., Mammillichnis jakubi isp. nov., Helminthoidichnites multilaqueatus comb. nov., Vatnaspor jachymi igen. et isp. nov. and five preliminarily identified trace fossils. The assemblage belongs to Mermia ichnofacies, the nonmarine representative of an ichnofacies, developed in a turbiditic environment; most of identified trace fossils are so far endemic.

THE AURIFEROUS SUBMARINE FANS SANDSTONES OF THE IONIAN ZONE (EPIRUS, GREECE)

During recent decades many studies have be done on the rocks that developed in the area of Western Greece and especially in Epirus, known in geoscientific literature as Ionian Zone of External Hellenides. These rocks have undergone geological research (basic geological mapping, research for hydrocarbons, metals and inert materials) and exploitation (inert materials). Recently, within the sedimentary succession of the Ionian zone submarine fans, in the region of Peta–Kompoti, in the prefecture of Arta, have been identified positions where sedimentary gold is present. Recently, positions where sedimentary gold is present, within the sedimentary rocks of the submarine fans, in the region of Peta–Kompoti, prefecture of Arta, have been identified. For the continuation and the practical application of these positive results, it was necessary to obtain a documented reference of geological parameters. After a series of new sampling and detailed analysis of the samples, this work presents the detected gold concentrations, illustrates the limits of grouping areas of interest as well as delineating and illustrating palaeogeographic factors that have contributed to the creation of gold-bearing formations. The highest gold concentration found was in the Ta Bouma sequence. The observation of constant indications of high gold values above background that suddenly increase in certain places up to 260 ppb, leads to the conclusion that in the broader area and within locations with equivalent formations procedure, there could be gold pockets of exploitable concentrations.

Storm-reworked shallow-marine fans in the Middle Triassic Baise area, South China

Shallow-marine fans have been analyzed for sedimentary characteristics and genetic mechanisms far less commonly than deep-marine fans. Based on outcrop observations, this study reveals that large-scale submarine fans in the Middle Triassic Baise area, south China, consisted of a braided channel-levee facies complex in the inner fans; sinuous channel-levee facies complex, overbank facies and crevasse splay facies in the medial fans; and sand sheet facies in the outer fans, which present different lithology assemblages and depositional successions. The lithofacies of dominant massive sandstone, secondary graded sandstone and laminated sandstone from bottom to top is similar to the Bouma sequence, indicative of gravity flow processes. However, the laminated sandstones mostly bear sedimentary structures of gutter casts, wave-generated ripple marks, hummocky cross-bedding, wave-generated cross-bedding and ripple bedding, and were thus attributed to frequent storm reworking processes. Moreover, discontinuous deposition recorded by abrupt sediment grain-size changes, distinct interfaces and divergent paleocurrent orientations occurred between the laminated sandstones and the underlying massive (or graded) sandstones. Accordingly, storm reworking processes together with abundant fossils of Daonella, Ammonite and Crinoidea indicate a shallow-marine paleo-environment. As a whole, this study provides a good example of large-scale shallow-marine fans and laminated sandstones resulted from storm reworking.

Sedimentary facies analysis and depositional model of gravity-flow deposits of the Yanchang Formation, southwestern Ordos Basin, NW China

ABSTRACTDuring the deposition of the Chang-7 (Ch-7) and Chang-6 (Ch-6) units in the Upper Triassic, gravity flows were developed widely in a deep lake in the southwestern Ordos Basin, China. Based on cores, outcrops, well-logs and well-testing data, this paper documents the sedimentary characteristics of the gravity-flow deposits and constructs a depositional model. Gravity-flow deposits in the study area comprise seven lithofacies types, which are categorised into four groups: slides and slumps, debris-flow-dominated lithofacies, turbidity-current-dominated lithofacies, and deep-water mudstone-dominated lithofacies. The seven lithofacies form two sedimentary entities: sub-lacustrine fan and the slump olistolith, made up of three and two lithofacies associations, respectively. Lithofacies association 1 is a channel–levee complex with fining-/thinning-upward sequences whose main part is characterised by sandy debris flow-dominated, thick-bedded massive sandstones. Lithofacies association 2 represents distributary channelised lobes of sub-lacustrine fans, which can be further subdivided into distributary channel, channel lateral margin and inter-channel. Lithofacies association 3 is marked by non-channelised lobes of sub-lacustrine fans, including sheet-like turbidites and deep-lake mudstones. Lithofacies association 4 is represented by proximal lobes of slump olistolith, consisting of slides and slumps. Lithofacies association 5 is marked by distal lobes of slump olistolith, comprising tongue-shaped debris flow lobes and turbidite lobes. It is characterised by sandy debris flow, muddy debris flow-dominated sandstone and sandstone with classic Bouma sequences. Several factors caused the generation of gravity flows in the Ordos Basin, including sediment supply, terrain slope and external triggers, such as volcanisms, earthquakes and seasonal floods. The sediment supply of sub-lacustrine fan was most likely from seasonal floods with a high net-to-gross and incised channels. Triggered by volcanisms and earthquakes, the slump olistolith is deposited by the slumping and secondary transport of unconsolidated sediments in the delta front or prodelta with a low net-to-gross and no incised channels.

Deep-lacustrine sandy debrites and turbidites in the lower Triassic Yanchang Formation, southeast Ordos Basin, central China: Facies distribution and reservoir quality

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.

Middle Miocene reworked turbidites in the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea margin: Processes, genesis, and implications

Our understanding of reworked turbidites is still in its infancy, and their flow processes and genesis still remain understudied. Core data from the middle Miocene Zhujiang Formation in the Pearl River Mouth Basin allow us to differentiate reworked turbidites, yielding two main contributions. Firstly, reworked turbidites are distinguished from turbidites by the association of traction structures and tidal signatures, which occur in discrete units rather than forming a classic “Bouma Sequence” for turbidites. Sedimentological characteristics of reworked turbidites proposed here will help to obtain a robust set of diagnostic criteria for the recognition of deep-water non-turbidite deepwater units as reservoirs. Secondly, our results suggest that, in the down-slope direction, classic detritus carried in turbidity flows would synchronously be bidirectionally reworked by internal tides and waves, resulting in tidal signatures seen in the interpreted reworked turbidites. In the along-slope direction, upper parts of dilute turbidity currents would mix vertically with seawater, and muddy fines would be winnowed away by contour currents, whereas lower parts of dilute turbidity currents would probably drop their coarse particles, resulting in traction structures recognized in the documented reworked turbidites. Our work highlights the influence of bottom currents on the development and modification of turbidites and suggests that reworked turbidites were created by the combined action of down-slope transport and reworking and along-slope winnowing and sorting, helping to better understand flow processes and genesis of non-turbidite reservoirs with a great economic interest.