Bouma Sequences Research Articles

The thick-bedded tail of turbidite thickness distribution as a proxy for flow confinement: Examples from tertiary basins of central and northern Apennines (Italy)

This study reviews the thickness statistics of non-channelized turbidites from four tertiary basins of Central-Northern Apennines (Italy), where bed geometry and sedimentary character have been previously assessed. Though very different in terms of size and, arguably, character of feeder system, these basins share a common stratigraphic evolution consisting in transition from an early ponded to a late unconfined setting of deposition. Based on comparison of thickness subsets from diverse locations and stratigraphic heights within the studied turbidite fills, this paper seeks to answer the following questions: i) how data collection procedures and field operational constraints (e.g. measure location, outcrop quality, use of thicknesses data from single vs. multiple correlative sections, stratigraphic thickness of the study interval) can affect statistics of sample data? ii) how depositional controls of confined vs. unconfined turbidite basins can result in different thickness-frequency distributions?; and iii) is there in thickness statistics a ‘flow confinement’ signature which can be used to distinguish between confined and unconfined turbidites? Results suggest that: i) best practices of data collection are crucial to a meaningful interpretation of sample data statistics, especially in presence of stratigraphic and spatial trends of turbidite bed thickness; ii) a systematic bias against cm-thick Tcd Bouma sequence turbidites exists in sample data, which can result in the low-end tail of empirical thickness-frequency distributions to depart significantly from the actual distribution of turbidite thickness; and iii) thickness statistics of beds starting with a basal Ta/Tb Bouma division bear a coherent relationship to the transition from ponded to unconfined depositional settings, consisting in reduction of variance and mean and, consequently, parameters, or even type, of best fit model distribution. This research highlights the role of flow stripping, sediment by-pass and bed geometry in altering the initial thickness distribution of ponded turbidites and suggests how fully ponded mini-basins represent the ideal setting for further research linking turbidite thickness statistics and frequency distribution of parent flow volumes.

Provenance and depositional setting of Lower Silurian siliciclastic rocks on Hainan Island, South China: Implications for a passive margin environment of South China in Gondwana

Diverse models have been proposed for interpreting the early Paleozoic tectonic setting of the Cathaysia Block in South China. However, few of them undertook a comprehensive study of clastic sedimentary rocks in the Cathaysia Block. This study reports the petrography and sedimentary facies, the whole-rock geochemistry and Sm–Nd isotopic data, and the LA-ICP-MS U–Pb ages of detrital zircon from the Lower Silurian meta-siliciclastics on Hainan Island in order to constrain the early Paleozoic tectonic setting. The source rocks for the Lower Silurian meta-siliciclastics have suffered moderate to intense weathering, indicated by the weathering indices, like Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Weathering Index (W index). The Lower Silurian rocks have high Th/Sc (generally between 1.1 and 2.0) and Th/U (generally between 4 and 6) ratios, LREE-enriched REE patterns with negative Eu anomalies (avg. 0.55), and low εNd(t) values (−16.2 to −8.5), suggesting a continental margin basin receiving detritus mostly derived from a recycled-orogen or cratonic provenance. This Early Silurian turbiditic succession with rhythmic flysch and incomplete Bouma sequences also reveals a neritic to abyssal facies, continental shelf to continental slope depositional setting. The geochemical features like the pattern of REEs, the ratios of trace elements (e.g., Th/Sc, Th/Co, La/Sc, Cr/Th, La/Th), and the binary plots and ternary diagrams (e.g., A–CN–K and FMW diagram), demonstrate that the source-areas are dominated by felsic rocks. Together with the two-stage Nd depleted mantle model age (TDM-2) (ca. 2.3Ga), the detrital zircon U–Pb age populations with peaks mainly at ca. 1635Ma confirm that the source rocks are dominated by the Paleo- to Mesoproterozoic Baoban Group and ca. 1430Ma granitoids on Hainan Island. Linked to the early Paleozoic tectonic development of South China, the Lower Silurian siliciclastic rocks on Hainan Island can be interpreted as depositing in a passive continent margin.

Submarine fans: A critical retrospective (1950–2015)

When we look back the contributions on submarine fans during the past 65 years (1950–2015), the empirical data on 21 modern submarine fans and 10 ancient deep-water systems, published by the results of the First COMFAN (Committee on FANs) Meeting (Bouma et al., 1985a), have remained the single most significant compilation of data on submarine fans. The 1970s were the “heyday” of submarine fan models. In the 21st century, the general focus has shifted from submarine fans to submarine mass movements, internal waves and tides, and contourites. The purpose of this review is to illustrate the complexity of issues surrounding the origin and classification of submarine fans. The principal elements of submarine fans, composed of canyons, channels, and lobes, are discussed using nine modern case studies from the Mediterranean Sea, the Equatorial Atlantic, the Gulf of Mexico, the North Pacific, the NE Indian Ocean (Bay of Bengal), and the East Sea (Korea). The Annot Sandstone (Eocene–Oligocene), exposed at Peira-Cava area, SE France, which served as the type locality for the “Bouma Sequence”, was reexamined. The field details are documented in questioning the validity of the model, which was the basis for the turbidite-fan link. The 29 fan-related models that are of conceptual significance, developed during the period 1970–2015, are discussed using modern and ancient systems. They are: (1) the classic submarine fan model with attached lobes, (2) the detached-lobe model, (3) the channel-levee complex without lobes, (4) the delta-fed ramp model, (5) the gully-lobe model, (6) the suprafan lobe model, (7) the depositional lobe model, (8) the fan lobe model, (9) the ponded lobe model, (10) the nine models based on grain size and sediment source, (11) the four fan models based on tectonic settings, (12) the Jackfork debrite model, (13) the basin-floor fan model, (14) supercritical and subcritical fans, and (15) the three types of fan reservoirs. Each model is unique, and the long-standing belief that submarine fans are composed of turbidites, in particular, of gravelly and sandy high-density turbidites, is a myth. This is because there are no empirical data to validate the existence of gravelly and sandy high-density turbidity currents in the modern marine environments. Also, there are no experimental documentation of true turbidity currents that can transport gravels and coarse sands in turbulent suspension. Mass-transport processes, which include slides, slumps, and debris flows (but not turbidity currenrs), are the most viable mechanisms for transporting gravels and sands into the deep sea. The prevailing notion that submarine fans develop during periods of sea-level lowstands is also a myth. The geologic reality is that frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more important in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., lowstand systems tract). Submarine fans are still in a stage of muddled turbidite paradigm because the concept of high-density turbidity currents is incommensurable.

Planktonic foraminifera and calcareous nannofossils record in the upper Campanian-Maastrichtian pelagic deposits of the Malatya Basin in the Hekimhan area (NW Malatya, eastern Anatolia)

The uppermost Cretaceous (upper Campanian-Maastrichtian) pelagic successions from the Malatya Basin (NW Malatya, eastern Anatolia) were studied by 688 samples, which were collected from five stratigraphic sections in the Hekimhan area. The pelagic deposits conformably overlie rudist bearing shallow-water limestones and are overlain conformably by Maastrichtian dolomites and unconformably by Paleocene-Eocene deposits, respectively.The pelagic successions in the Hekimhan area comprise the Kösehasan Formation at the base and the Zorbehan Formation at the top and reach up to 1100 m in thickness. The Kösehasan Formation rests over the neritic rudist-bearing limestones of the Güzelyurt Formation along a sharp contact and consists mainly of flysch-type sandstone-mudstone alternation with complete and partial Bouma sequences. The carbonate content of abundant planktonic foraminifera and nannoplankton-bearing 980-m-thick succession increases upwards and the formation passes gradually to the clayey limestones and marlstones of the Zorbehan Formation to the top. Occurrences of nannoplankton Lithraphidites quadratus Bramlette and Martini and Micula praemurus (Bukry) in the first beds of the Kösehasan Formation indicate that the age of the Kösehasan Formation and overlying Zorbehan Formation is of late Maasthrichtian. Another late Maastrichtian taxa Cribrosphaerella daniae Perch-Nielsen and Arkhangelskiella maastrichtiana Burnett are observed from the lowermost part of the succession. Maastrichtian planktonic foraminifera such as Contusotruncana walfischensis (Todd) and Globotruncanita pettersi (Gandolfi) were recorded through the successions. Although planktonic foraminifera are diverse and abundant particularly in the Kösehesan Formation, index late Maasthrichtian species were not encountered. Campanian and Santonian-Campanian planktonic foraminifera, e.g. Radotruncana calcarata (Cushman) and Globotruncanita elevata (Brotzen), obtained particularly from the lower part of the succession and calcareous nannofossils such as Broinsonia parca parca Bukry, Reinhardtites anthophorus (Deflanre) and Eiffellithus eximius (Stover) are interpreted as reworked from older strata. Trace fossils are common throughout the succession.Rareness of planktonic foraminifera and nannoplankton in the uppermost part of the succession (Zorbehan Formation) indicates maximum shallowing of the latest Maastrichtian sea in this part of the basin. Rare echinoids, bivalves and ammonites are observed in that part of the sequence.The obtained data indicate that sediment accumulation rate of the pelagic deposits is rather high and about 27.5 cm/ky for this part of the basin. Changes in thickness of the formations along short distances in the five stratigraphic sections analysed in this study should be related to the diachroneity of the depositional and erosional events.

Compositional differences between Besko and Otryt sandstones of the Krosno Beds (Oligocene) in Eastern part of the Silesian Tectonic Unit [abstract

The aim of this research was to quantitatively and qualitatively analyse under microscope sandstones representing two lithofacies called Otryt and Besko (or Lesko) facies of the Krosno Formation (Oligocene), Silesian Tectonic Unit in the E part of the Polish Outer Carpathians. Four samples were taken into consideration — two from each lithofacies. Special attention was paid to structural and textural characteristics and mineral composition that reflects mainly provenance. The results have been compared and analysed between and within facies, in terms of source and transport of the material, mineralogical and textural maturity and sedimentary processes that affect the deposition. Ślączka & Unrug (1972) and Shideler at al. (1976) concluded that mineralogical variations exhibited by sandstones in Krosno Formation were attributed to both regional (provenance) and local (sorting) effects. The samples have been taken from the northern (Otryt) and southern (Besko) limbs of the Otryt Syncline. They represent different sources of the material - Silesian Ridge in NW in case of Besko facies and Maramures Massif in the the SE in case of Otryt facies (Wendorff 1981). The result of the analysis confirm statement about two different sources. Transport and deposition mechanism results in considerable difference in grain size and its distribution. Besko facies sandstones display classic Bouma sequence at structures (T b - c ) indicative of deposition from normally diluted, waning turbidity currents, with incremental deposition layer-by-layer (Talling 2012) and this results in very fine grained sandstones and in better sorting. Deposits of that lithofacies appear much relatively uniform and well organized (Wendorff 1987). By contrast, sedimentary features of the Otryt facies suggest deposition from rapidly decelerating high density turbidity currents — “sandy debris flows” (Talling 2012), conditions and mechanisms of transportation and deposition must have been very variable (Wendorff 1987). These sandstones are medium grained, but the bottom sample is characterized by coarse tail caused by rapid deposition. The detrital material of the top sample was deposited also of high density turbidity current, however slightly less dense than that of the bottom sample. The diversity of mineral composition between Besko and Otryt lithofacies is very clear. There are probably three reasons for that — completely different source rocks, initially different maturity of the detritus involved in density flows and the depositional processes within the basin . Besko (Leslo) lithofacies is dominated by nonundulatory quartz. Other components are rather minor. The only difference between carbonate components is in the proportion of detrital calcite/limestone (or dolomite) to carbonate cement, which may have resulted from diagenetic processes. Carbonate cements are very rich in both samples. In Otryt facies mineral composition is more differentiated. Quartz predominates, but not as distinctly as in Besko facies. The most abundant and the coarsest are polycrystalline grains. Nonundulatory quartz is similarly abundant. Altered K-feldspars and metamorphic rock fragments are important however they occur less frequently. The bottom and top samples differ in quantity of coarse to very coarse metamorphic rock fragments and polycrystalline quartz — the bottom sample is richer in such grains. This is the result of grain-size sorting — coarser grains have been deposited first and this sorting process affects the mineral composition of the top-of-bed sample in which the coarse tail is absent and the proportion of metamorphic grains is less pronounced. There is very little cement in both samples. Next to high mineralogical maturity, Besko facies sandstones show also high textural and mineralogical maturity (presence of well-rounded grains and high quartz content). On the other hand, Otryt facies sandstones are immature both texturally and mineralogically, which results from both source area composition and rapid deposition. According to provenance diagrams (Dickinson 1983), source of the Besko facies is recycled orogen, but close to craton interior field. Whereas source of the Otryt facies should be recycled orogen too. Despite the fact that all samples are the same age and occur in one formation, they are totally different. In summary, differences in mineral composition and textural features, observed under the microscope, between the analysed samples of the Besko/Lesko and Otryt lithofacies turbidite sandstones reveal different mineral composition of the source areas and are comparable to the macroscopically observed sedimentary features. The latter indicate pronounced differences in the transportation and deposition mechanisms – from normally diluted turbidity currents to rapidly decelerating high sandy debris flows.

Application of imaging logging in reservoir geological research

With the ability to provide complete profiles of pseudocolor image formation, IL (imaging logging) measures directionally, thereby offering a possible alternative to drilling and coring. This paper, relying mainly on IL information but also referring to formation dip angle logging information, details sandstone lamina dips and identifies sedimentary body dips, and sums up a practical palaeo-provenance direction judgment method using heavy mineral data analysis. By identifying a fine sandstone reservoir through an IL lithological model, a bedding model and a rhyme model, a 1D pseudoresistivity curve (named the 'IL rhythm curve') is, according to IL image analysis, extracted to assist in the interpretation of sedimentary microfacies and to establish an IL single-well facies analysis profile, overcoming an observance difficulty of IL in silty sandstone regions and intuitively displaying whole-horizon sedimentary evolution. As for a glutenite formation, calibrating IL information by rock cores, a single-well facies sequence of IL is established by identifying the Bouma sequence, high-density turbidity succession, the middle fan braided channel, the outer fan mudstone and other microfacies sequences, finely dividing the sedimentary facies sequence about sedimentary stage.

Signatures of Late Neoproterozoic Gondwana assembly and Maronian glaciation in Lesser Himalaya: a palaeogeographical and stratigraphical approach

Abstract Stratigraphical and sedimentological analyses of Late Neoproterozoic successions in Lesser Himalaya are combined herein with palaeogeographical considerations and comparisons with equivalent successions in India and South China. The succession starts with the Hazara Formation, which contains complete and incomplete Bouma sequences suggesting its deposition in deep marine turbidite settings. The overlying Tanawal Formation, rich in massive sandstone, shale and siltstone, was deposited in shallow marine conditions, as indicated by the presence of parallel lamination, large scale tabular, trough cross- and hummocky cross-stratifications. The Tanawal Formation facies shift laterally from proximal (south-southeast) to distal (north-northwest). The glaciogenic Tanaki Boulder Bed, overlying the Tanawal Formation, was deposited during the Maronian glaciation. It is equivalent to the Blaini Formation of India, and to the Sinian diamictites of South China. The Abbottabad Formation of Cambrian age overlies the Tanaki Boulder Bed, and is composed of dolomite, chert nodules and phosphate-rich packages; similar successions are documented in India and South China at the same stratigraphical interval. The similarities of the Neoproterozoic successions of Lesser Himalaya (both in Pakistan and India) and South China suggests their possible proximity during the break-up of Rodinia and the assembly of the Gondwana Supercontinent.

Age and nature of the late Early Cretaceous Zhaga Formation, northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.

A Late Triassic gravity flow depositional system in the southern Ordos Basin

Abstract Based on the analysis of numerous drill cores and drilling data, lacustrine gravity flow depositional systems were analyzed comprehensively in the Chang6 and Chang7 oil members (Triassic Yanchang Formation) in the southern part of the Ordos Basin. The gravity flow depositional systems in these members are made up of slides, slumps, sandy debris flows, liquefied flows, turbidity current etc , forming well developed units in the study area. Successive beds characterized by, from bottom to top, massive bedding (MB), graded bedding (GB) and horizontal bedding (HB) form well developed sequences; parallel bedding (GB) and ripple bedding (RB) are rare. It turns out that the depositional sequences are quite different from turbidites with a Bouma sequence: (1) MB represents a sandy debris flow, (2) GB deposits in a turbidity current, (3) PB and RB are deposits that were reworked by bottom currents (traction flow), and (4) HB represents a deep-water environment rather than gravity flows. Deposits in the proximal part of the subaqueous lacustrine fan consist mainly of slides, slumps and massive sandy debris flows. Deposits at the middle part of the fan are characterized by an MB-GB-HB sequence of massive sandy debris flows, graded turbidites and horizontally bedded lacustrine mudstones. Deposits at the end of the subaqueous lacustrine fan were mainly graded turbidites and horizontally bedded lacustrine mudstones (GB-HB sequence). Sandy gravity flow deposits mainly developed on the delta front and in the basin plain, extending for dozens of kilometers. They directly cover the source rock in the Chang7 oil members, which has the advantage of near-source oil accumulation. The sandstones at the bottom of each sedimentary cycle are worth further exploration because of their good reservoir properties and high oil content.

Palaeoenvironment of Eocene prodelta in Spitsbergen recorded by the trace fossilPhycosiphon incertum

Ichnological, sedimentological and geochemical analyses were conducted on the Eocene Frysjaodden Formation in order to interpret palaeoenvironment prodelta sediments in the Central Basin of Spitsbergen. Phycosiphon incertum is the exclusive ichnotaxon showing differences in size, distribution, abundance and density, and relation to laminated/bioturbated intervals. Large P . incertum mainly occur dispersed, isolated and randomly distributed throughout the weakly laminated/non-laminated intervals. Small P . incertum occur occasionally in patches of several burrows within laminated intervals or as densely packed burrows in thin horizons in laminated intervals or constituting fully bioturbated intervals that are several centimetres thick. Ichnological changes are mainly controlled by oxygenation, although the availability of benthic food cannot be discarded. Changes in oxygenation and rate of sedimentation can be correlated with the registered variations in the Bouma sequence of the distal turbiditic beds within prodeltal shelf sediments. Keywords: Trace fossils; Phycosiphon ; Eocene; prodelta; turbiditic sediments; Spitsbergen (Published: 20 October 2014) Citation: Polar Research 2014, 33 , 23786, http://dx.doi.org/10.3402/polar.v33.23786

Depositional environments and hydrocarbon potential of the Miocene deposits of Zakynthos Island

Structure, sedimentary features, and biostratigraphy were taken into account to provide additional knowledge into the depositional conditions that influenced the sedimentary sequence along the southern coast of Zakynthos. This succession is exposed from Keri village to Ag. Sostis peninsula and appears to have been influenced by intense tectonic activity. The active tectonics resulted in the formation of at least four coarsening upward cycles. The co-occurrence of Bouma sequence subdivisions and slump horizons within the studied sediments suggest a deep-sea depositional environment. Grain size analysis indicates a regional swallowing upward trend. TOC and CaCΟ3 contents are presented with both positive and negative correlations while the TOC values indicate that the study area contain samples with source rock potential.

Fault rate controls on carbonate gravity-flow deposits of the Liassic of Central High Atlas (Morocco)

The aim of this work is to describe and to explore linkages between fault activity and gravity-related sedimentary deposits during the early rifting of a carbonate depositional system. The study area, located at the southern edge of Central High Atlas (Morocco), hosts 900 m thick Liassic gravity-related sedimentary deposits, preserved at the hanging wall of one of the possibly inherited, reactivated Hercynian faults (Tizi n'Firest fault). This study documents gravity-flow deposits constrained by a high-resolution biostratigraphic framework (ca. 100 ka). These gravity-flow deposits evolve over time. Unstratified conglomerate beds are limited to the Sinemurian and are preserved near the fault. Incomplete, ungraded Bouma sequence beds are preserved preferentially at the transition between the Sinemurian and the Carixian while complete, graded Bouma sequences are recorded during the Carixian and Domerian. Fine-grained turbidite beds are observed dominantly during the Domerian. Incomplete, graded Bouma sequences are observed all along the section. Our work suggests that fault activity over time, associated to rifting, has led to this diversity of gravity-flow deposits. Fault activity plays a key role in the initiation of gravity flows, but also influences the type of gravity-flow deposits, by controlling the amount of sediment available, the sediment grain-size, the slope morphology and the basin deepening.

Lithofacies Characters and Significance of the Submarine Fan of the Liufengguan Group in Qinling

Abstract:Field investigation and laboratory research on flysch of the Liufengguan Group in Qinling indicate the following: (1) Sandstone of the Liufengguan Group is categorized as feldspathic lithic graywacke with a minor amount of lithic graywacke in the QFR triangular diagram. Grain size ≤0.3 mm. Bedding plane structures such as groove casts and suspected flute casts can be found at the bottom of the sandstone. It is inferred that currents may have come from the southeast during deposition. Bedding structures such as ripple marks, graded bedding, parallel bedding, small‐scale cross bedding, climbing bedding, suspected convolute bedding, microlamination and sliding structures have also been observed, which are of indicative significance. It is thought that the Liufengguan Group has the sedimentary characteristics of bedding, bedding plane structures and lithological assemblages of deep‐sea low‐density turbidity current deposits. The vertical succession of the Bouma sequence in the inner fan subfacies zone is generally incomplete: the assemblage of Ta and Tabc is commonly seen; the succession of the middle fan subfacies zone is relatively complete; and divisions Te and Tb are common in the outer fan subfacies zone. (2) The flysh of the Liufengguan Group is a sequence of deep‐sea argillaceous‐arenaceous submarine fan deposits, in which the authors recognize the inner, middle and outer fan subfacies and also nine types of lithofacies: normal graded sandstone (A1), medium‐ to thick‐bedded, fine‐grained sandstone (A2), medium‐ to thick‐bedded and massive siltstone (A3), thin‐bedded, fine‐grained sandstone and mudstone (B1), irregular interbeds of thin‐bedded, fine‐grained sandstone and siltstone (B2), thin‐bedded, fine‐grained sandstone (C1), very thin‐bedded, fine‐grained sandstone (D1), olistostromes (E1) and deep‐sea mudstone (F). The inner fan consists of four microfacies: natural levee (A1), water channel (A2, A3) and olistostromc (E1); in the middle fan there also occur four microfacies, i.e., branch channel (B1), branch channel (B2), interdistributary bay (D1) and olistostrome. The outer fan is made up of the branch channel (C1) and sheet sand (D1) microfacies, which alternate vertically with sediments of deep‐sea plain subfacies (F). There occur fining‐ and thin ning‐upward channel deposits in the outer‐fan subfacies zone of the submarine fan of the Liufengguan Group observed in this study. The quartz content of the graywacke of the deposits is all higher than 40% and may reach as high as 60%. Therefore, on the basis of the aforementioned features, this flysh should be formed in a passive continental‐margin tectonic environment.

Porosity-Permeability and Textural Parameters of the Palaeogene Forearc Sedimentary Fill on Lemnos Island, NE Greece

The Palaeogene forearc sedimentary fill on Lemnos Island, NE Greece, was examined to determine reservoir characteristics and textural parameters. During this time interval the studied area was the site of accumulation of submarine fans that underlie shelf deposits, with tectonic activity responsible for the shallowing upward trend. Turbidites were deposited in both inner (slope fan facies) and outer parts (basin floor fan facies) of the submarine fan system and consist of alternating sandstone and mudstone beds. Sandstones occur in both complete and incomplete Bouma sequences while shelf deposits have been interpreted as storm-surge deposits on the deeper parts of shelves. The 'Mercury Porosimetry Technique' was carried out on 20 sandstone samples, while 30 sandstone samples were examined under a polarizing microscope for grain-size analysis. This porosity-permeability study suggests that these rocks can be considered as both oil and gas reservoirs. 'Slope' fan facies generally reveal the most efficient values, making them the most promising sub-environment for further hydrocarbon research. Most samples display two pore-size distributions suggesting major textural heterogeneity. Textural parameter analysis reveals that sorting was of great importance during sedimentation. Rocks are generally well to very well-sorted while samples with moderate sorting are also present. This fact can be both attributed to the restricted grain-size range and their possible great distance from the source area. This generally well-sorted sequence argues well for further hydrocarbon research in the Northeast Aegean Sea, since the higher the sorting the higher the porosity. Selected samples are generally very fine to fine grained, whereas medium-grained sandstones are extremely rare and mostly seen on 'slope' fan facies. The finest grained sandstones are the best sorted.

Emplacement of massive turbidites linked to extinction of turbulence in turbidity currents

Submarine turbidity currents are controlled by gravity acting on suspended sediments that pull water downslope along with them 1 . In addition to suspended sediments, turbidity currents also transport sediments at the base of the flow 2 , which causes the reorganization of basal sediments prior to the settling of suspended grains 3‐6 . However, as turbidity currents reach areas with minimal slope, they cross a fall-velocity threshold beyond which the suspended sediments begin to stratify the flow. This process extinguishes the turbulence near the bed 7,8 . Here we use direct numerical simulation of turbidity currents to show that this extinction of turbulence eliminates the ability of the flow to re-entrain sediment and rework the sediment at the base of the flow. Our simulations indicate that deposits from flows without basal reworking should lack internal structures such as laminations. Under appropriate conditions, then, sustained delivery of fine sediments will therefore result in the emplacement of massive turbidites. We suggest that this mechanism can explain field observations of massive deposits 9 that were emplaced gradually by dilute but powerful turbidity currents. We also conclude that turbulence in submarine turbidity currents is more fragile than river systems, and more sensitive to damping by the stratification of suspended sediment in the flow. Turbiditesareoftencharacterizedintermsofcompleteorpartial manifestations of the Bouma sequence 10 . Units Ta to Te in Fig. 1a correspond to a single flow event, with flow waning from bottom to top. Intervals Ta and Tb, tend to be sand. Interval Tb has parallel laminations, indicating bedload reworking as sediment settles 3 . Interval Ta is ‘massive’, that is, lacking sedimentary structures. Massive deep-sea turbidites are prominent features of channelized and unchannelized submarine fans. They are enigmatic in that they show little or none of the internal structure used to interpret emplacement mechanisms. Parallel laminations are seen in fluvial deposits as well as turbidites, and have been reproduced in the laboratory 36 . Their formation is due to the organization of sand grains into streaks according to size and orientation by bedload. Mechanisms for the emplacement of massive turbidites are more speculative. These units are common in the ancient rock record of deep-water deposits, as well as in deeper parts of modern continental margins. Individual event beds can be up to metres in thickness (Fig. 1b), and extend for tens to hundreds

Transport Mechanisms of Sand In Deep-Marine Environments--Insights Based On Laboratory Experiments--Reply

First of all we would like to thank Dr. Shanmugam for his interest in our study of transport mechanisms of sand in deep-marine environments (Breien et al. 2010). In order to answer the remarks of Shanmugam, we here provide some more details around the experiments, and we specify the intention of the study. The main objective of the experiments presented in our paper was to study transportation and depositional processes in sand-rich slurries run in water, and how these mechanisms respond to changing sand/clay ratios. We find some of the comments of Shanmugam (2011) to be more concentrated on the Bouma sequence and the properties of its divisions Ta–Te, than on the mechanism of sediment transport itself. We do not claim to have produced a complete Bouma sequence (cf. Bouma 1962; Walker 1965; Middleton and Hampton 1973); however, we rather see that our experiments indicate that similar graded-bed deposits may be produced by different transport mechanisms. We show that dense, laminar flows may transport sand through fluidization and incrementally produce sand deposits resembling deposits produced by turbulent turbidity currents. Thus, we regard the process observed in the laboratory as only one out of several mechanisms to transport and deposit clean sand in submarine basins. As part of a Ph.D. program (Breien 2009), 47 debris flow experiments were performed, prepared with five different compositions of varying sand/clay ratio (5, 10, 15, 20, and 25% clay by weight). Content of water and coal …

Depositional Characteristics and Distribution of Lake-Floor Fan of Paleogene Lower Member 3 of Shahejie Formation in Northwestern Part of Huanghekou Sag, Bohai Bay Basin

The lake-floor fan, as an important deposition of Paleogene in Huanghekou Sag, is characterized with both good physical character of reservoir and oil-bearing property, thus it has higher values for petroleum exploration. The lower Member 3 of Shahejie Formation, rich in sandstone, can be divided into lowstand system tract, extensive system tract and highstand system tracts based on seismic reflection signature, and core and logging data clearly. The sand bodies in the study area belonged to two kinds of lake-floor fan deposition formed under semi-deep-lake to deep-lake environment: deposition of the deep-water turbidite fan corresponded to lowstand system tract; deposition of the slump turbidite fan corresponded to extensive system tract and highstand system tract, which was formed by slumping fan delta front sand. Viewed from spatial distribution, the deep-water turbidite fan was mainly developed in the second-order fault downthrown of the basin, while the slump turbidite fan was distributed in the abrupt slope which located in the south of study area. The complex of lake-floor fan consists of various sediments’gravity flow deposits, including:(1)turbidity current deposit with characteristics of Bouma sequences; (2)mud-bearing sandy debris flow deposits with characteristics of dominated sand and mixed by mud; (3)sand-bearing muddy debris flow deposits with characteristics of dominated mud and mixed by sand; (4)gritty debris flow deposits with characteristics of massive gravel accumulation;(5)sandy slump deposits with characteristics of deformed sedimentary structure. During lower water level period(LSTs3L), most of the study area were erosion or sediment pass-by areas, and terrigenous clastic matters were directly transported into deep-water area under the second-order faulted belt, and then formed the deep-water turbidite fan. During higher water level period (ESTs3L, HSTs3L), the slump tuibidite fan, which resulted from various sediments’gravity flows, mainly developed in front of fan dalta in south. The sandstones of the lake-floor fan are the main reservoirs of oil in the northwestern part of the Huanghekou sag.

Features of Sandy Debris Flows of the Yanchang Formation in the Ordos Basin and Its Oil and Gas Exploration Significance

Abstract:Sandy debris flow is a new genetic type of sand bodies, which has gained much attention in recent years and its corresponding theory is proved to be a significant improvement and even partial denial to the “Bouma Sequence” and “turbidite fan” deep‐water sedimentary theories to some point. Oil exploration researchers are highly concerned with sandy debris flows for its key role in controlling oil and gas accumulation processes. In this article, by applying sandy debris flows theory and combining a lot work of core, outcrop observation and analysis plus seismic profile interpretation, we recognized three types of sedimentary gravity flows that are sandy debris flows, classic turbidites and slumping rocks in chang‐6 member of Yanchang Formation in the deep‐water area of central Ordos Basin. Among the three types, the sandy debris flows are the most prominent and possesses the best oil bearing conditions. On the contrary, the classic turbidites formed by turbidity currents are limited in distribution; therefore, previous Yanchang Formation deep‐water sedimentary studies have exaggerated the importance of turbidite currents deposition. Further study showed that the area distribution of deep water gravity flow sand bodies in Yanchang Formation were controlled by the slope of the deep‐water deposits and the flows had vast distribution, huge depth and prevalent advantages for oil forming, which make it one of the most favorable new areas for Ordos Basin prospecting.

Outcrop Characterization of an Early Miocene Slope Fan System, Chelif Basin, Algeria

The study on outcrop characterization is very useful for geologists and oil finders. There is a complex slope fan system of early Miocene in Chelif Basin. We divided the superimposed basin into two main development stages: an expanding rifting stage (early Miocene) and depression-extrusion transition stage. Comparison of the outcrops with typical subsurface data sets (logs, core, and seismic reflection data) provided criteria for a detailed study of the stratigraphic and lithologic features of the slope fan system. Based on these research methods, a sedimentary model was built. The slope fan system was an integrated series comprising two parts, the lower (S1) and the upper (S2), from the sequence stratigraphy study. S1 was progradational but had both rolling transport and gravity transport attributes. Similar to the Bouma sequence, S1 can be divided into four parts, A, B, C and D, from bottom to top. S2 was retrogradational but comprised four internal-progradational cycles. Some layers comprise planar-laminated and climbing-ripple–laminated sandstone with many fossils. The upper part of S2 was a thick marl bed and an obvious flooding surface. Each part had different lithofacies and sedimentary interpretations. Some formations could be good reservoirs based on both self-attributes and outer properties. Considerable evidence suggested that the slope fan have great hydrocarbon potential.

Quartz arenites of the Cambro-Ordovician Kamouraska Formation, Quebec Appalachians, Canada: I. Deep-water depositional processes in a continental-slope environment

The Kamouraska Formation is an uppermost Cambrian – lowermost Ordovician quartz-arenite-dominated unit of controversial origin deposited on the southeastern slope of Laurentia bordering the Iapetus Ocean. It is exposed in the Quebec Appalachians on the south shore of the St. Lawrence Estuary. The formation consists of basal polymictic conglomerate and overlying massive sheet-like quartz arenite. The conglomerate beds are reversely and reversely to normally graded. The quartz arenite beds are generally massive, although they may show coarse-tail grading. Beds containing full or partial Bouma sequences are rare. Paleoflow directions from ripple-cross lamination, ripple marks on bed surfaces, and sole marks point towards southeast, south, and southwest. The clastic sediments of the Kamouraska were transported into the deep sea by sediment gravity flows that evolved from hyperconcentrated to concentrated density flows, and then to turbidity currents. The depositional environment is interpreted to have been a southwest-trending meandering submarine canyon. The exposed part of the canyon deposits is slightly oblique to the strike of slope. If correct, our interpretation establishes the preservation of continental-slope deposits in more distal deep-water siliciclastic sedimentary rocks of the Taconian orogen in Quebec, which traditionally have been interpreted as submarine-fan and (or) basin-plain deposits. The orientation of a canyon near parallel-to strike of the slope may have been controlled by syn-depositional growth faults. The coarsest hyperconcentrated flows, which deposited the conglomerate, were restricted to the deepest parts of the canyon during its early stages of development, whereas the concentrated density flows that deposited the massive quartz-arenite beds covered a wider area.