Can liquefied debris flows deposit clean sand over large areas of sea floor? Field evidence from the Marnoso‐arenacea Formation,Italian Apennines

The Marnoso‐arenacea Formation in the Italian Apennines is the only ancient rock sequence where individual submarine sediment density flow deposits have been mapped out in detail for over 100 km. Bed correlations provide new insight into how submarine flows deposit sand, because bed architecture and sandstone shape provide an independent test of depositional process models. This test is important because it can be difficult or impossible to infer depositional process unambiguously from characteristics seen at just one outcrop, especially for massive clean‐sandstone intervals whose origin has been controversial. Beds have three different types of geometries (facies tracts) in downflow oriented transects. Facies tracts 1 and 2 contain clean graded and ungraded massive sandstone deposited incrementally by turbidity currents, and these intervals taper relatively gradually downflow. Mud‐rich sand deposited by cohesive debris flow occurs in the distal part of Facies tract 2. Facies tract 3 contains clean sandstone with a distinctive swirly fabric formed by patches of coarser and better‐sorted grains that most likely records pervasive liquefaction. This type of clean sandstone can extend for up to 30 km before pinching out relatively abruptly. This abrupt pinch out suggests that this clean sand was deposited by debris flow. In some beds there are downflow transitions from turbidite sandstone into clean debrite sandstone, suggesting that debris flows formed by transformation from high‐density turbidity currents. However, outsize clasts in one particular debrite are too large and dense to have been carried by an initial turbidity current, suggesting that this debris flow ran out for at least 15 km. Field data indicate that liquefied debris flows can sometimes deposit clean sand over large (10 to 30 km) expanses of sea floor, and that these clean debrite sand layers can terminate abruptly.     

Shallow-water gravity-flow deposits, Chapel Island Formation, southeast Newfoundland, Canada

A remarkable suite of shallow-water, gravity-flow deposits are found within very thinly-bedded siltstones and storm-generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non-graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine-grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore-fluid pressure dissipation support the feasibility of such transitions. Raft-bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm^-1 and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm^-2 and a depositional slope estimate of 0.77°. The lithologies and sedimentary structures in member 2 indicate an overall grain-size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure included in situ liquefaction (‘disturbed bedding’), sliding and slumping. Raft-bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross-stratified sandstone directly overlies slide deposits and raft-bearing beds, linking sediment failure to the cyclical wave loading associated with large storms. The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud-rich, marine deltas. Details of deltaic gravity-flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies primarily carried out with side-scan sonar.     

Anatomy of a modern open-ocean carbonate slope: northern Little Bahama Bank

The open-ocean carbonate slope north of Little Bahama Bank consists of a relatively steep (4°) upper slope between water depths of 200 and 900 m, and a more gentle (1–2°) lower slope between depths of 900 and 1300+ m. The upper slope is dissected by numerous, small, submarine canyons (50–150 m in relief) that act as a line source for the downslope transport of coarse-grained carbonate debris. The lower slope is devoid of any well-defined canyons but does contain numerous, small (1–5 m) hummocks of uncertain origin and numerous, larger (5–40 m), patchily distributed, ahermatypic coral mounds. Sediments along the upper slope have prograded seaward during the Cenozoic as a slope-front-fill seismic facies of fine-grained peri-platform ooze. Surface sediments show lateral gradation of both grain size and carbonate mineralogy, with the fine fraction derived largely from the adjacent shallow-water platform. Near-surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well-lithified hardgrounds to patchily cemented nodular ooze to unlithified peri-platform ooze, controlled by lateral variations in diagenetic potential and/or winnowing by bottom currents. Submarine cementation stabilizes the upper part of the slope, allowing upbuilding of the platform margin, and controls the distribution of submarine slides, as well as the headward extent of submarine canyons. Where unlithified, sediments are heavily bioturbated and are locally undergoing dolomitization. Upper slope sediments are also ‘conditioned’eustatically, resulting in vertical, cyclic sequences of diagenetically unstable (aragonite and magnesian calcite-rich) and stable (calcite-rich) carbonates that may explain the well-bedded nature of ancient peri-platform ooze sequences. Lower slope sediments have prograded seaward during the Cenozoic as a chaotic-fill seismic facies of coarse-grained carbonate turbidites and debris flow deposits with subordinate amounts of peri-platform ooze. Coarse clasts are ‘internally’derived from fine-grained upper slope sediments via incipient cementation, submarine sliding and the generation of sediment gravity flows. Gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge-shaped apron of debris, parallel to the adjacent shelf edge, consisting of a complex spatial arrangement of localized turbidites and debris flow deposits. A proximal apron facies of thick, mud-supported debris flow deposits plus thick, coarse-grained, Ta turbidites, grades seaward into a distal apron facies of thinner, grain-supported debris flow deposits and thinner, finer grained Ta-b turbidites with increasing proportions of peri-platform ooze. Both the geomorphology and sedimentary facies relationships of the carbonate apron north of Little Bahama Bank differ significantly from the classic submarine fan model. As such, a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient, open-ocean carbonate slope sequences.     

Numerical simulation of turbidity current flow and sedimentation: II. Results and geological applications

A process-based, forward computer model of turbidity current flow and sedimentation, termed the TCFS model, has been developed to trace the downslope evolution of individual turbidity flows. Details of the model itself have been presented in a preceding paper. We here outline a series of tests of the TGFS model. The sensitivity tests of the TCFS model to general geological controls reveal the quantitative relationship between these controls and the behaviour of turbidity flows and the geometry and textural features of the resulting turbidites. Experimental turbidity currents on relatively steep slopes accelerate more rapidly and reach higher velocities than those on gentle slopes. Flows with larger initial volumes have higher initial velocities, travel further downslope, and form beds of greater thickness and downslope extent than smaller flows. Experimental high-concentration flows with suspended-sediment concentrations of 25% accelerate more rapidly and reach higher downslope velocities than dilute flows with 5% suspended sediment. The higher velocities and enhanced hindered-settling effects of the high-concentration flows lead to much greater transport distances and reduced vertical and lateral sediment size grading in the resulting turbidites. Beds formed by experimental high-concentration flows are massive or show coarse-tail grading whereas beds formed by low-concentration flows show distribution-grading. Experimental flows fed by coarse sediment sources tend to deposit the bulk of their suspended sediment loads on the proximal slope, resulting in more rapid flow deceleration and sedimentation than flows fed by silt-rich, fine-grained sediment sources. Turbidites formed by coarse-sediment flows tend to have a wedge-shaped geometry, with low downslope extent and high surface relief, whereas turbidites formed by fine-sediment flows tend to have a tabular geometry, with greater downslope extent and lower surface relief. A specific geological test of the TCFS model is based on studies of modern turbidity currents in Bute Inlet, British Columbia, Canada. With the input initial and boundary conditions estimated from Bute Inlet, the model predicts the downslope velocity evolution of turbidity currents comparable to those of modern and ancient turbidity flows measured in Bute Inlet. Model-calculated vertical and downslope grain-size properties of turbidites are similar to those exhibited by surface and cored Bute Inlet turbidites. Model flows tend to decelerate more rapidly than some stronger turbidity currents in the Bute Inlet system, and model beds tend to decrease in grain-size downslope more rapidly than observed bottom sediments. This is probably because the TCFS model flows lacked clay, which is abundant in Bute Inlet; they do not fully simulate turbulent mixing of suspended sediments; and they better represent the unsteady, depositional stage of turbidity-currents than the preceding stage of more-or-less steady-flow conditions. These tests demonstrate that the TCFS model provides a semi-quantitative method to study the growth patterns of submarine turbidite systems. It can serve as a predictive tool for analysing the facies architecture of ancient turbidite systems through simulating multi-depositional events by improving its erosion function, and the compatibility between its numerical components.     

浊流沉积研究的新进展:鲍马序列、海底扇的重新审视

本文在总结前人对浊流沉积研究的基础上,分析前人对浊流与浊积岩、浊流沉积与浊流相模式的对应关系之间的认识,并对鲍马序列进行重新审视。在海底扇研究过程中,鲍马序列已经不能充分反映浊流沉积的全过程。鲍马序列所反应的沉积模式其实是由碎屑流、浊流、底流等多种形式流体组合和改造后的结果,海底扇沉积模式不能笼统归结为浊流沉积作用的结果。在完善重力流、底流等沉积作用的同时,建立一个与沉积作用相互联系的深海沉积系统,以对深海研究提供更好地指导和预测。     

Clastic sediments associated with radiolarites (Tauglboden—Schichten, Upper Jurassic, Eastern Alps)

During the early Upper Jurassic, widespread deep-sea radiolarites were deposited in most parts of the Northern Limestone Alps. In the formation described (Tauglboden-Schichten), these pelagic sediments interfinger with local-source clastic material. Depending on the topography and the kind of material, either slides and slumps, mudflows, grain flows or turbidity currents operated and formed slump-folded beds, mud-flow breccias, fluxoturbidites or turbidites. A breccia had been traced over an area of 20 km². Its variation is described in terms of lithological columns, bed thicknesses, maximum grain sizes and grain orientations. It forms a tongue-shaped body, which was probably a part of a submarine fan. The fluxoturbidites of the proximal area grade distally on three sides into turbidites within 3–5 km. The clastic material consists of marls and limestones of Rhaetian and Jurassic age. It was probably derived from a tectonically uplifted palaeo-high by an interplay of tectonics and gravity. The clastics were deposited on submarine fans bordering this high. In its lithology the formation closely resembles certain marginal facies of flysch troughs.     

Eocene fan delta-submarine fan deposition in the Wagwater Trough, east-central Jamaica

ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.     

Delta-fed turbidites in a lacustrine rift basin: the Eocene Dongying depression,Bohai Bay Basin,East China

Sediment avalanche from delta ramp is one of the significant development mechanisms for a turbidite system in a lacustrine basin. To advance our understanding of deep-water sedimentary processes in a lacustrine delta ramp, delta-fed turbidites in the Eocene Dongying depression of the Bohai Bay Basin were studied using core data, 3-D seismic data and well log data. Sandy debris flows, muddy debris flows, mud flows, turbidity currents, slides, sandy slumps and muddy slumps were interpreted based on the identification of lithofacies. Data indicates that deep-water sedimentary processes in the study area were dominated by debris flows and slumps, which accounted for ～68% and 25% (in thickness) of total gravity flow deposits, respectively; turbidity-current deposits only accounted for ～5%. Mapping of turbidites showed that most were deposited after short-distance transportation (<20 km), restricted by the scale of deep-water areas and local topography. Channels, depositional lobes, debris flow tongues, muddy turbiditic sheets, slides and slumps were identified in a delta-fed ramp system. Slides and slumps were dominant at the base of slopes or at the hanging walls of growth faults with strong tectonic activity. Channels and depositional lobes developed in gentle, low-lying areas, where sediments were transported longer distances. Sand-rich sediment supply, short-distance transportation and local topography were crucial factors that controlled sedimentation of this ramp system. Channels generally lacked levees and only produced scattered sandstones because of possible hydroplaning of debris flow and unstable waterways. In addition to lobes, debris flow tongues could also be developed in front of channels. These findings have significant implications for hydrocarbon exploration of deep-water sandstone fed by deltas in a lacustrine basin.     

Glacially-influenced deep-marine sedimentation of the Late Precambrian Gaskiers Formation, Newfoundland, Canada

The Late Proterozoic Conception Group, exposed on the Avalon Peninsula in Newfoundland, Canada, is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites (the Gaskiers Formation) near the base. Massive and crudely-stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine-grained, thin-bedded turbidites. These diamictites are interpreted as submarine debris flow deposits. Disrupted diamictites form strongly deformed units that contain large, complexly folded rafts of mudstone and turbidite facies. These diamictite units are interpreted as submarine slumps. Diamictites contain glacially-striated and faceted clasts; clasts and matrix are predominantly of volcanic provenance. One outcrop shows interbedded volcanic agglomerate and diamictite, and volcanic bombs can also be identified. The interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below (Mall Bay Formation) and above (Drook Formation) indicate a deep marine slope setting of diamictite deposition. Diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone. The regional tectonic framework suggests the Conception Group accumulated in a deep, southward-opening ensialic rift basin with active but waning volcanic centres to the north. The Gaskiers Formation may be representative of other Late Precambrian glacially-influenced diamictite sequences that were deposited around the North Atlantic region and in Europe. These deep marine diamictite sequences characterized by debris flows, turbidites, and slump deposits, can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basinal settings.     

The sequence of events around the epicentre of the 1929 Grand Banks earthquake: initiation of debris flows and turbidity current inferred from sidescan sonar

Continental slope sediment failures around the epicentre of the 1929 'Grand Banks' earthquake have been imaged with the SAR (Système Acoustique Remorqué) high-resolution, deep-towed sidescan sonar and sub-bottom profiler. The data are augmented by seismic reflection profiles, cores and observations from submersibles. Failure occurs only in water depths greater than about 650 m. Rotational, retrogressive slumps, on a variety of scales, appear to have been initiated on local steep areas of seabed above shallow (5–25 m) regional shear planes covering a large area of the failure zone. The slumps pass downslope into debris flows, which include blocky lemniscate bodies and intervening channels. Clear evidence of current erosion is found only in steep-sided valleys: we infer that debris flows passed through hydraulic jumps on these steep slopes and were transformed into turbidity currents which then evolved ignitively. Delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps provide a mechanism to produce a turbidity current with sustained flow over many hours.     

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.     

Status and Trends in Research on Deep-Water Gravity Flow Deposits

Deep-water gravity flows are one of the most important sediment transport mechanisms on Earth. After 60 years of study,significant achievements have been made in terms of classification schemes,genetic mechanisms,and depositional models of deep-water gravity flows. The research history of deep-water gravity flows can be divided into five stages: incipience of turbidity current theory; formation of turbidity current theory; development of deep-water gravity flow theory; improvement and perfection of deep-water gravity flow theory; and comprehensive development of deep-water gravity flow theory. Currently,three primary classification schemes based on the sediment support mechanism,the rheology and transportation process,and the integration of sediment support mechanisms,rheology,sedimentary characteristics,and flow state are commonly used.Different types of deep-water gravity flow events form different types of gravity flow deposits. Sediment slump retransportation mainly forms muddy debris flows,sandy debris flows,and surge-like turbidity currents. Resuspension of deposits by storms leads to quasi-steady hyperpycnal turbidity currents(hyperpycnal flows). Sustainable sediment supplies mainly generate muddy debris flows,sandy debris flows,and hyperpycnal flows. Deep-water fans,which are commonly controlled by debris flows and hyperpycnal flows,are triggered by sustainable sediment supply; in contrast,deep-water slope sedimentary deposits consist mainly of debris flows that are triggered by the retransportation of sediment slumps and deep-water fine-grained sedimentary deposits are derived primarily from finegrained hyperpycnal flows that are triggered by the resuspension of storm deposits. Harmonization of classification schemes,transformation between different types of gravity flow deposit,and monitoring and reproduction of the sedimentary processes of deep-water gravity flows as well as a source-to-sink approach to document the evolution and deposition of deep-water gravity flows are the most important research aspects for future studies of deep-water gravity flows study in the future.     

鄂尔多斯盆地南部三叠系延长组湖相重力流沉积细粒岩及其油气地质意义

随着页岩油气勘探开发和相关领域研究的不断深入,细粒沉积物的搬运和沉积已成为当前沉积学研究的热点问题之一,但中国中生代湖泊环境中的泥质重力流沉积尚未引起应有的关注。通过岩心观察、薄片鉴定等手段及综合研究,分析了鄂尔多斯盆地晚三叠世湖相泥质重力流沉积特征,探讨了其形成机制与成因分类。鄂尔多斯盆地三叠系延长组湖相泥页岩结构类型多样,发育泥质块体流沉积、泥质碎屑流沉积、泥质浊流沉积和泥质异重流沉积等多种重力流沉积类型。按照泥质含量将重力流划分为砂质重力流、泥质重力流和混合重力流3种亚类,并根据成因将重力流划分为滑塌体、碎屑流、浊流及异重流等4种亚类;结合成因和泥质含量,将重力流沉积共划分为12种类型。滑塌岩、碎屑岩分布于三角洲前缘斜坡脚附近;浊积岩、异重岩广泛分布于三角洲斜坡至沉积中心。认为泥质沉积物可以在强水动力条件下搬运-沉积;重力流沉积细粒物质在湖相沉积中占据很大的比例;泥质重力流对泥页岩中的碎屑物质、黏土矿物及有机质的搬运和沉积起到重要作用,因而对于页岩油气的生烃、储集性能和压裂工艺研究具有重要意义。     

Development and mass movement processes of the north-eastern Storegga Slide

The Storegga Slide, which occurred ～8100 years ago, is one of the world's largest and best studied exposed submarine landslides. In this study we use novel geomorphometric techniques to constrain the submarine mass movements that have shaped the north-eastern Storegga Slide, understand the link between different forms of failure, and propose a revised development model for this region. According to this model, the north-eastern part of the Storegga Slide has developed in four major events. The first event (event 1) was triggered in water depths of 1500–2000 m. In this event, the surface sediments were removed by debris flows and turbidity currents, and deposited in the Norwegian Sea Basin. Loading of the seabed by sediments mobilised by the debris flows and turbidity currents resulted in the development of an evacuation structure. Loss of support associated with this evacuation structure, reactivation of old headwalls and seismic loading activated spreading in the failure surface of event 1 up to the main headwall (event 2). In some areas, spreading blocks have undergone high displacement and remoulding. Parts of the spreading morphology and the underlying sediment have been deformed or removed by numerous debris flows and turbidity currents (event 3). We suggest that the higher displacement and remoulding of the spreading blocks, and their removal by debris flows and turbidity currents, was influenced by increased pore pressures, possibly due to gas hydrate dissolution/dissociation or by lateral variability in the deposition of contourite drifts in palaoeslide scars. The fourth event entailed a large, blocky debris flow that caused localised compression and transpressive shearing in the southern part of the spreading area.     

Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman

ABSTRACT The Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope. The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite. Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer-lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner- to mid-fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid- to outer-fan lobes. Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea-level fluctuations or pulses of tectonic activity.     

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

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.     

Earthquake‐induced turbidites triggered by sea level oscillations in the Upper Cretaceous and Paleocene of Italy

Earthquakes are widely recognized as triggers for turbidites, submarine debris flows and slumps. In tectonically active areas, surprisingly small changes in stress can trigger seismic events, implying that past sea level changes may be important in controlling the timing of seismicity and the occurrence of turbidites. We apply this idea in an analysis of turbidites from the upper Cretaceous–Paleocene Scaglia Rossa Formation of the Umbria‐Marches region of Italy. These turbidites are composed of resedimented foraminiferal tests derived from fluidizing deep‐water (∼1500 m), pelagic sediments; seismic triggering is the most likely triggering mechanism given this setting and composition. The timing of these turbidites (and associated synsedimentary slumps), constrained by biostratigraphy and magnetostratigraphy, reveals an unusual, non‐random temporal pattern that appears to correlate well with proposed eustatic fluctuations. This correlation between turbidites and eustatic fluctuations leads us to suggest that stress and pore fluid pressure changes associated with changing sea level may trigger periods of increased seismicity in the geological past.     

'Linked' debrites in sand-rich turbidite systems – origin and significance

Abstract The outer parts of a number of small Late Jurassic sandy deep‐water fans in the northern North Sea are dominated by the stacked deposits of co‐genetic sandy and muddy gravity flows. Sharp‐based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast‐rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a post‐depositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water‐rich sand bed just deposited by a co‐genetic concentrated gravity current. As such, they are referred to as ‘linked debrites’ to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low‐friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low‐concentration gravity currents should be more important here. In fact, evidence for transport by low‐concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast‐rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast‐charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand‐rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.     

Upper miocene shallow-water turbidites from western Tuscany

A sequence of graded ophiolitic sandstones, 120 m thick (Sanguigna Formation), outcrops within the Messinian Evaporite Group in a limited area near Gabbro, Fine Basin, western Tuscany.The formation lies between fine-grained sediments laid down under a thin water-cover. The graded beds show, on the other hand, many features typical of proximal turbidites, such as amalgamated layers, scour-and-fill structures, clay flakes and lumps, top-truncated Bouma sequences. Grain-size analyses suggest that they were deposited from high-density turbidity flows.The petrographic composition and the grain fabric indicate that the feeding was from the northeast across the Fine Basin. From the inferred dimensions and depth of the basin, the mean slope should have been less than 1°. The turbidity currents probably originated at a river mouth during flood stages.The Sanguigna graded beds are compared with occurrences of shallow-water turbidites.     

The hyperpycnite problem

Sedimentologic, oceanographic, and hydraulic engineering publications on hyperpycnal flows claim that(1) river flows transform into turbidity currents at plunge points near the shoreline,(2) hyperpycnal flows have the power to erode the seafloor and cause submarine canyons, and,(3) hyperpycnal flows are efficient in transporting sand across the shelf and can deliver sediments into the deep sea for developing submarine fans. Importantly, these claims do have economic implications for the petroleum industry for predicting sandy reservoirs in deep-water petroleum exploration. However, these claims are based strictly on experimental or theoretical basis, without the supporting empirical data from modern depositional systems. Therefore, the primary purpose of this article is to rigorously evaluate the merits of these claims.A global evaluation of density plumes, based on 26 case studies(e.g., Yellow River, Yangtze River, Copper River,Hugli River(Ganges), Guadalquivir River, Rio de la Plata Estuary, Zambezi River, among others); suggests a complex variability in nature. Real-world examples show that density plumes(1) occur in six different environments(i.e., marine,lacustrine, estuarine, lagoon, bay, and reef);(2) are composed of six different compositional materials(e.g., siliciclastic,calciclastic, planktonic, etc.);(3) derive material from 11 different sources(e.g., river flood, tidal estuary, subglacial, etc.);(4) are subjected to 15 different external controls(e.g., tidal shear fronts, ocean currents, cyclones, tsunamis, etc.); and,(5) exhibit 24 configurations(e.g., lobate, coalescing, linear, swirly, U-Turn, anastomosing, etc.).Major problem areas are:(1) There are at least 16 types of hyperpycnal flows(e.g., density flow, underflow, high-density hyperpycnal plume, high-turbid mass flow, tide-modulated hyperpycnal flow, cyclone-induced hyperpycnal turbidity current, multi-layer hyperpycnal flows, etc.), without an underpinning principle of fluid dynamics.(2) The basic tenet that river currents transform into turbidity currents at plunge points near the shoreline is based on an experiment that used fresh tap water as a standing body. In attempting to understand all density plumes, such an experimental result is inapplicable to marine waters(sea or ocean) with a higher density due to salt content.(3) Published velocity measurements from the Yellow River mouth, a classic area, are of tidal currents, not of hyperpycnal flows. Importantly, the presence of tidal shear front at the Yellow River mouth limits seaward transport of sediments.(4) Despite its popularity, the hyperpycnite facies model has not been validated by laboratory experiments or by real-world empirical field data from modern settings.(5) The presence of an erosional surface within a single hyperpycnite depositional unit is antithetical to the basic principles of stratigraphy.(6) The hypothetical model of "extrabasinal turbidites", deposited by river-flood triggered hyperpycnal flows,is untenable. This is because high-density turbidity currents, which serve as the conceptual basis for the model, have never been documented in the world's oceans.(7) Although plant remains are considered a criterion for recognizing hyperpycnites, the "Type 1" shelf-incising canyons having heads with connection to a major river or estuarine system could serve as a conduit for transporting plant remains by other processes, such as tidal currents.(8) Genuine hyperpycnal flows are feeble and muddy by nature, and they are confined to the inner shelf in modern settings.(9) Distinguishing criteria of ancient hyperpycnites from turbidites or contourites are muddled.(10) After 65 years of research since Bates(AAPG Bulletin 37: 2119-2162, 1953), our understanding of hyperpycnal flows and their deposits is still incomplete and without clarity.