Character and distribution of hybrid sediment gravity flow deposits from the outer Forties Fan, Palaeocene Central North Sea, UKCS

Seven categories of event bed (1–7) are recognised in cores from hydrocarbon fields in the outer part of the Palaeocene Forties Fan, a large mixed sand-mud, deep-water fan system in the UK and Norwegian Central North Sea. Bed Types 1, 6 and 7 resemble conventional high-density turbidite, debrite and low-density turbidite, respectively. However the cores are dominated by distinctive hybrid event beds (Types 2–5; 81% by thickness) that comprise an erosively-based graded and structureless and/or banded sandstone overlain by an argillaceous sandstone or sandy-mudstone unit containing mudstone-clasts and common carbonaceous fragments. Many of the hybrid beds are capped by a thin laminated sandstone–mudstone couplet (the deposit of a dilute wake behind the head of the turbidity current). Different types of hybrid event bed Types are defined on the basis of the ratio of sandier lower part to upper argillaceous part of the bed, and the internal structure, particularly the presence of banding. Although the argillaceous and clast-rich upper divisions could reflect post-depositional mixing, sand injection or substrate deformation, they can be shown to be dominantly primary depositional features and record both a temporal (and by implication) spatial change from turbidite to debrite deposition beneath rheologically complex hybrid flows. Where banding occurs between lower sandy and upper argillaceous divisions, the flow may have passed through a transitional flow regime. Significantly, the often soft-sediment sheared and partly sand-injected argillaceous divisions are present in cores both close to and remote from salt diapirs and hence are not a local product of remobilisation around salt-cored topography. Lateral correlations between wells establish that sandy hybrid beds (Types 2, 3S) pass down-dip and laterally into packages dominated by muddier hybrid beds (Types 3M, 4) over relatively short distances (several km). Type 5 beds have minimal or no lower sandier divisions, implying that the debritic component outran the sandier component of the flow. The Forties hybrid beds are thought to record flow transformations affecting fluidal flows following erosion and bulking with mudstone clasts and clays that suppressed near-bed turbulence and induced a change to plastic flow. Hybrid beds dominate the muddier parts of sandying-upward, muddying-upward and sandying to muddying-upward successions, interpreted to record splay growth and abandonment, overall fan progradation, and local non-uniformity effects that either delayed or promoted the onset of flow transformations. The dominance of hybrid event beds in the outer Forties Fan may reflect very rapid delivery of sand to the basin, an uneven substrate that promoted flow non-uniformity, tilting as a consequence of source area uplift and extensive inner-fan erosion to create deep fan valleys. This combination of factors could have promoted erosion and bulking, and hence transformations leading to the predominance of hybrid beds in the outer parts of the fan.     

Hybrid sediment gravity flow deposits – Classification, origin and significance

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.     

Annot Sandstone in the Peïra Cava basin: An example of an asymmetric facies distribution in a confined turbidite system (SE France)

High-resolution physical stratigraphy and detailed facies analysis have been carried out in the foredeep turbidites of Annot Sandstone in the Peïra Cava basin (French Maritime Alps) in order to characterize the relationship between facies and basin morphology. Detailed correlation patterns are evidence of a distinction between a southern bypass-dominated region, coincident with a channel-lobe transition and a north-eastern depositional zone, represented by sheet-like basin plain. These depositional elements are characterized by three main groups of beds related to the downcurrent evolution of bipartite flows. These facies groups are: 1) pebbly coarse-grained massive sandstones with rip-up mudstone clasts and impact mudstone breccias (Type I and II beds) deposited by basal dense flows, 2) coarse-grained massive sandstone overlain by tractive structures (Type III and IV beds) indicating the bypass of overlying turbulent flows and 3) massive medium-grained and fine-grained laminated sandstones related to the deposition of high and low density turbidity currents (Type V and VI beds). Ponding and reflection processes, affecting the upper turbulent flows, can characterize all type beds, but especially the beds of the third group. As described in other confined basins of the northern Apennines (Italy), the lateral and vertical distribution of these type of beds, together with other important sedimentary characteristics, - such as the sandstone/mudstone ratio, bed thicknesses, amalgamation surfaces and paleocurrents - reveal that the deposition of the Annot Sandstone in the Peïra Cava basin was controlled by an asymmetric basin with a steep western margin. This margin favored, on the one hand, basal dense flow decelerations and impacts, as well as bypass and deflection of the upper turbulent flows towards the north east.     

Sedimentology, stratigraphic occurrence and origin of linked debrites in the West Crocker Formation (Oligo-Miocene), Sabah, NW Borneo

The West Crocker Formation (Oligocene–Early Miocene), NW Borneo, consists of a large (>20 000 km²) submarine fan deposited as part of an accretionary complex. A range of gravity-flow deposits are observed, the most significant of which are mud-poor, massive sandstones interpreted as turbidites and clast-rich, muddy sandstones and sandy mudstones interpreted as debrites. An upward transition from turbidite to debrite is commonly observed, with the contact being either gradational and planar, or sharp and highly erosive. Based on their repeated vertical relationship and the nature of the contact between them, these intervals are interpreted as being deposited from one flow event which consisted of two distinct flow phases: fully turbulent turbidity current and weakly turbulent to laminar debris flow. The associated bed is called a co-genetic turbidite–debrite, with the upper debrite interval termed a linked debrite. Linked debrites are best developed in the non-channellised parts of the fan system, and are absent to poorly-developed in the proximal channel-levee and distal basin floor environments. Due to outcrop limitations, the genesis of linked debrites within the West Crocker Formation is unclear. Based on clast size and type, it seems likely that a weakly turbulent to laminar debris-flow flow phase was present when the flow event entered the basin. A change in flow behaviour may have led to deposition of a sand-rich unit with ‘turbidite’ characteristics, which was subsequently overlain by a mud-rich unit with ‘debrite’ characteristics. Flow transformation may have been enhanced by the disintegration and incorporation into the flow of muddy clasts derived from the upstream channel floor, channel mouth or from channel-levee collapse. Lack of preservation of this debrite in proximal areas may indicate either bypass of this flow phase or that the available outcrops fail to capture the debris flow entry point. Establishing robust sedimentological criteria from a variety of datasets may lead to the increasing recognition of co-genetic turbidite-debrite beds, and an increased appreciation of the importance of bipartite flows in the transport and deposition of sediments in deepwater environments.     

Autogenic controls on hybrid bed distribution in submarine lobe complexes

Hybrid beds, the deposits of sediment gravity flows that show evidence for more than one flow regime (turbulent, transitional and/or laminar), have been recognized as important components of submarine lobe deposits. A wide range of hybrid bed types have been documented, however, quantitative analysis of the stratigraphic and geographic distribution of these enigmatic bed types is rare. Here, extensive exposures integrated with research borehole data from Unit A of the Laingsburg Formation and Fan 4 of the Skoorsteenberg Formation, Ecca Group, South Africa, provide the opportunity to examine geographical and stratigraphic patterns over a range of hierarchical scales.For this purpose, >23,000 individual beds have been evaluated for deposit type and bed thickness. On average, hybrid beds make up < 5% of all events and <10% of the cumulative thickness. Lobe complex 1 (LC1) of Fan 4,Skoorsteenberg Formation, preserves a prominent geographical trend of hybrid beds becoming more prevalent towards the frontal fringes of a lobe complex (up to 33.2% of beds), whereas their proportion in proximal and medial lobe complex settings is <10%.Data from Unit A, Laingsburg Formation, show hybrid beds are less common in the basal (A.1) and top (A.6) subunits compared to A.2-A.5 in both core data sets. The bases and tops of some lobe complexes (A.2, A.3 and A.5.7) are observed to be slightly enriched in hybrid beds, whereas others (A.5.1, A.5.5 and A.6.1) show no hybrid beds in their bases, which does not conform to expected allogenically-driven distributions that predict more hybrid beds during the initiation of lobe complexes. Instead, the occurrence and distribution of hybrid beds in lobe complexes are interpreted to be controlled by autogenic processes, including flow transformation processes on the basin-floor meaning enrichment in frontal lobe fringe settings. Therefore, the 1D distribution of hybrid beds in lobe complexes reflects the dominant stacking pattern of lobes within a lobe complex, with enrichment at the base and top of lobe complexes due to overall progradational to retrogradational stacking patterns. Individual lobes show a wide range of hybrid bed distributions, due to stacking patterns of the component lobe elements. These findings highlight the importance of autogenic processes rather than allogenic controls in the distribution of hybrid beds, which has implications for reservoir evaluation and the assessment of lobe stacking patterns in 1D core data sets.     

Sequence stratigraphy of an argillaceous,deepwater basin-plain succession: Vischkuil Formation (Permian), Karoo Basin,South Africa

The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudstones, separated by packages of graded sandstone and siltstone turbidites, and volcanic ash beds, and is an argillaceous precursor to a 1 km thick sand-prone basin floor fan to shelf succession. The Vischkuil Formation provides an insight into the process by which regional sand supply is initiated and for testing sequence stratigraphic principles in a basin plain setting. Regionally mapped 1–2 m thick hemipelagic mudstone units are interpreted as condensed drapes that represent the starved basin plain equivalents of transgressive systems tracts and maximum flooding surface on the coeval shelf (now removed during later uplift). The section above each mudstone drape comprises siltstone turbidites interpreted as highstand systems tract deposits and a surface of regional extent, marked by an abrupt grain size shift to fine sandstone. These surfaces are interpreted as sequence boundaries, related to abrupt increases in flow volume and delivery of sand grade material to the basin-plain. The interpreted lowstand systems tract comprises sandstone-dominated turbidites and is overlain by another hemipelagic mudstone drape. The upper Vischkuil Formation is marked by three 20–45 m thick debrites, with intraformational sandstone clasts up to 20 cm in diameter that can be mapped over 3000 km². In each case, debrite emplacement resulted in widespread deformation of the immediately underlying 3–10 m of silty turbidites. A sequence boundary is interpreted at the base of each deformation/debrite package. Six depositional sequences are recognised and the interfered energy shift across each successive sequence boundary and LSTs include a larger volume of sandstone increases up section. The lower two sequences thin to the NW and show NW-directed palaeocurrents. The four overlying sequences show a polarity switch in palaeocurrent directions and thinning, to the E and SE. Sequence 6 is overlain sharply by the 300 m thick sandstone dominated Fan A of the Laingsburg Formation. The LST debrites may indicate gradual development of major routing conduits that subsequently fed Fan A. The polarity shift from westward flowing turbidity currents to an eastward prograding deepwater to shelf system represents establishment of a long term feeder system from the west. Sand supply to the Karoo basin floor was established in an incremental, stepwise manner. Given the early post-glacial setting in an icehouse climate, glacio-eustatic sea-level changes are considered to have been the main control on sequence development.     

Submarine slope mudstone facies,cozy dell formation (Middle Eocene), California

A 120 m thick section of the Cozy Dell Formation (middle Eocene), southem California, consists of a graylaminated mudstone and a tanungraded mudstone; sandstone beds are associated with the laminated mudstone. Sedimentary structures, stratification sequences, foraminiferal distributions, and composition indicate that the ungraded mudstone is an upper slope hemipelagic deposit, while the laminated mudstone is an overbank deposit associated with shallow channels or gullies in which the sandstone beds were deposited. This depositional setting may be analogous to that of the modern Mississippi River delta front.     

Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy)

The Upper Cretaceous Bordighera Sandstone of NW Italy is a coarse-grained, sand-rich elongated turbidite system (ca. 15 × 45 km in outcrop) up to 250 m thick, interpreted to have been deposited in a trench setting. The siliciclastic succession interfingers with muddy calcareous turbidites, which become more abundant toward the lateral and distal domains. Bed type associations allow the distinction of a proximal channelized domain which transitions to a more distal lobe domain, characterized by abundant mudclast-rich sandstones and by bipartite and tripartite beds with a mud-rich middle or upper division (hybrid event beds). The transition between the proximal and distal domains occurs over a relatively limited spatial extent (ca. 5 km). The presence of lenticular bed-sets made up of coarse grained and mud-poor sandstones throughout the distal domain suggests that distributary channels were present, indicating sediment bypass further down-dip toward the most distal and not preserved parts of the system. Hybrid event beds - commonly associated with distal and marginal fan environments such as fan fringes - are present throughout the lobe domain and extend for up to ca. 30 km in down-dip distance. They are more abundant in the proximal and axial depositional lobe domain and their appearance occurs within a short basin-ward distance from the inferred channel-lobe transition zone. Flow expansion at the termination of the channelized domain and the enhanced availability of cohesive substrate due to the presence of intra-basinal muddy calcareous beds are interpreted as the key controls on the widespread occurrence of mudclast-rich and argillaceous sandstone beds. The abrupt appearance and the persistent occurrence of such beds across an extensive domain have implications for characterizing bed-scale (sub-seismic) heterogeneity of deep-water clastic hydrocarbon reservoirs.     

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.     

The Cengio submarine turbidite system of the Tertiary Piedmont Basin, Northwestern Italy

The Cengio sandstone member of the Tertiary Piedmont Basin in northwestern Italy has a conservatively estimated volume of 2.5 to 3 km³ (length: 6.4 km; width: 4.8 km; thickness: 170 m). It is interpreted as a sandstone-rich submarine fan deposit. The Cengio member consists of eight tabular depositional sandstone lobes that are 5- to 25-m thick. These lobes filled a submarine structural depression and onlap and/or pinch-out against bounding slope mudstones. The stacking of the lobe units was related to synsedimentary tectonism. Margin setting represents fan and/or source area     

The Cengio submarine turbidite system of the Tertiary Piedmont Basin,Northwestern Italy

The Cengio sandstone member of the Tertiary Piedmont Basin in northwestern Italy has a conservatively estimated volume of 2.5 to 3 km³ (length: 6.4 km; width: 4.8 km; thickness: 170 m). It is interpreted as a sandstone-rich submarine fan deposit. The Cengio member consists of eight tabular depositional sandstone lobes that are 5- to 25-m thick. These lobes filled a submarine structural depression and onlap and/or pinch-out against bounding slope mudstones. The stacking of the lobe units was related to synsedimentary tectonism.     

Three-dimensional forward stratigraphic modelling of the gravel-to mud-rich fan-delta in the slope system of Zhanhua Sag,Bohai Bay Basin,China

An important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es₃) in the Zhanhua Sag, Bohai Bay Basin. Seismic stratal slices reveal different characteristics of channels and fan-delta lobes between the south (slope break belt) and southwest (gentle slope) areas combined with lithology, wire-line logs and three-dimensional (3-D) seismic data in the southern slope of Zhanhua Sag. And an excellent analogue has been provided for understanding various key depositional evolution of fan-deltas in the slope system (from base to top: Es₃^L, Es₃^M and Es₃^U). The Sedsim, a three-dimensional stratigraphic forward modelling programme, is applied to simulate the evolution of fan-deltas in the southern slope break systems and southwestern gentle slope systems of the Zhanhua Sag by considering a number of key processes and parameters affecting the fan-deltaic deposition from 43 Ma to 38.2 Ma. Modelling results indicate that depositional types and scales evolved from the thickest medium-scale gravel- or sand-rich fan deltas (43 Ma ∼41.4 Ma, Es₃^L) to the thinnest small-scale mud-rich fan deltas and lacustrine mud (41.4 Ma ∼39.8 Ma, Es₃^M), and lastly to less thicker larger-scale mixed sand-mud fan deltas (39.8 Ma ∼38.2 Ma, Es₃^U). The types of slope system, sediment supply and lake-level change are three controlling factors for determining the source-to-sink architecture of the gravel-to mud-rich fan-deltas and sediment-dispersal characteristics. This study has demonstrated that the process-based modelling approach can be effectively used to simulate complex geological environments and quantify controlling factors.     

Linked turbidite–debrite resulting from recent Sahara Slide headwall reactivation

The northwest African margin has been affected by numerous large-scale landslides during the late Quaternary. This study focuses on a recent collapse of the Sahara Slide headwall and characterises the resulting flow deposit. Core and seismic data from the base of the upper headwall reveal the presence of blocky slide debris, comprising heavily deformed hemipelagic slope sediments. The blocky slide debris spilled over a lower headwall 60 km downslope and formed a thick transparent debris flow unit. Cores recovered 200–250 km farther downslope contain a surficial turbidite that is interpreted to be linked to the headwall collapse event based on timing and composition. One core located approximately 200 km from the headwall scar (C13) contains debrite encased in turbidite. The debrite comprises sheared and contorted hemipelagic mudstone clasts similar as those seen in the vicinity of the Sahara Slide headwall, and lacks matrix. This debrite pinches out laterally within 25 km of C13, whereas the accompanying turbidite can be correlated across 700 km of the northwest African margin. The linked turbidite–debrite bed is interpreted to have formed through recent failure of the steep Sahara Slide headwall that either 1) generated both a debris flow and a turbidity current almost simultaneously, or 2) generated a debris flow which with entrainment of water and progressive dilution led to formation of an accompanying turbidity current.     

Seismic stratigraphy and hydrocarbon prospectivity of the Lower Cretaceous Knurr Sandstone lobes along the southern margin of Loppa High,Hammerfest Basin,Barents Sea

The Lower Cretaceous Knurr Sandstone deposited along the southern slope of Loppa High and overlain by the Kolje and Kolmule seals forms an attractive play in the Hammerfest Basin of the Barents Sea. Late Jurassic organic-rich Hekkingen shale directly underlies the Knurr Sandstone and acts as a source to provide effective charge. Three wells, 7120/2-2, 7122/2-1 and 7120/1-2, have proven the Knurr-Kolje play in structural traps, with an oil discovery in 7120/1-2. Prospectivity related to stratigraphic traps is, however, highly under-explored.In order to document and map the reservoir distribution and stratigraphic-trap fairway, the Lower Cretaceous sedimentary package containing the Knurr Sandstone is divided into a number of depositional sequences and systems tracts using key regional seismic profiles calibrated with logs. Mapping of the key surfaces bounding the Knurr sandstone has been carried out using all the seismic vintages available from Norwegian Petroleum Directorate (NPD).The thick massive nature of the sandstone (123 m in well 7122/2-1), sedimentary features characteristic of gravity flow deposits, high-resolution internal seismic reflections and stratal geometries (truncations and lapout patterns), and sequence stratigraphic position of the Knurr Sandstone on seismic profiles confirm that the lobes identified on the seismic section are gravity driven base of the slope lobes. These Knurr lobes and slope aprons were formed as a result of uplift of the Loppa paleo-high in the Late Jurassic to Early Cretaceous times which caused subaerial exposure and incision. The characteristic mounded, lobate geometry evident on the seismic can be mapped along the toe-of-slope and records multiple stacked lobes fed by multiple feeder canyons. Lateral partitioning and separation of the lobes along the toe-of-slope could potentially create stratigraphic traps. The existing 2D seismic coverage is, however, not sufficient to capture lateral stratigraphic heterogeneity to identify stratigraphic traps. 3D seismic coverage with optimum acquisition parameters (high spatial and vertical resolution, appropriate seismic frequency and fold, long offsets and original amplitudes preserved) can allow for the reconstruction of 3D geomorphologic elements to de-risk potential stratigraphic traps prior to exploratory drilling.     

Stratigraphic architecture and evolution of a deep-water slope channel-levee and overbank apron: The Upper Miocene Upper Mount Messenger Formation,Taranaki Basin

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

Evolution and depositional structure of earthquake-induced mass movements and gravity flows: Southwest Orphan Basin, Labrador Sea

A series of submarine canyons on the southwest slope of Orphan Basin experienced complex failure at 7–8 cal ka that resulted in the formation of a large variety of mass-transport deposits (MTDs) and sediment gravity flows. Ultra-high-resolution seismic-reflection profiles and multiple sediment cores indicate that evacuation zones and sediment slides characterize the canyon walls, whereas the canyon floors and inner-banks are occupied by cohesive debris-flow deposits, which at the mouths of the canyons on the continental rise form large, coalescing lobes (up to 20 m thick and 50 km long). Erosional channels, extending throughout the length of the study area (<250 km), are observed on the top of the lobes. Piston cores show that the channels are partially filled by poorly sorted muddy sand and gravel, capped by inversely to normally graded gravel and sand. Such deposits are interpreted to originate from multi-phase gravity flows, consisting of a lower part behaving as a cohesionless debris flow and an upper part that was fully turbulent.The Holocene age and the widespread synchronous occurrence of these failures indicate a large magnitude earthquake as their possible triggering mechanism. The large debris-flow deposits on the continental rise originated from large failures on the upper continental slope, involving proglacial sediments. Retrogression of these failures led to the eventual failure of marginal sandy till deposits on the upper slope and outer shelf, which due to their low cohesion disintegrated into multi-phase gravity flows. The evacuation zones and slide deposits on the canyon walls were triggered either by the earthquake, or from erosion of the canyon walls by the debris flows. The slides, debris-flows, and multi-phase gravity flows observed in this study are petrographically different, indicating different sediment sources. This indicates that not all failures lead through flow transformation to the production of a multi-phase gravity flow, but only when the sediment source contains ample coarse-grained material. The spatial segregation of the slide, debris-flow, and multi-phase gravity-flow deposits is attributed to the different mobility of each transport process.     

Turbidite facies in an ancient subduction complex: Torlesse terrane,New Zealand

The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns.     

Current titles in marine geology

The asymmetrical Astoria Fan (110 × 180 km) developed off the Columbia River and Astoria submarine canyon during the Pleistocene. Morphology, stratigraphy, and lithology have been outlined for a Pleistocene turbidite, and a Holocene hemipelagic sedimentary regime to generate geologically significant criteria for comparison with ancient equivalent deposits. Both gray silty clay of the Late Pleistocene and olive-gray clay of the Early Holocene are interrupted by turbidites. The few deeply incised fan valleys of the more steeply sloping upper fan contain thick, muddy and very poorly sorted sand and gravel beds that usually have poorly developed internal sedimentary structures. The numerous shallower fan valleys and distributaries of the flatter middle and lower fan contain thick, clean, and moderately sorted medium to fine sands that are vertically graded in texture, composition and well-developed internal sedimentary structures. Tuffaceous turbidites (containing Mazama ash, 6600 B.P.) can be traced as thick deposits (ca. 30–40 cm) throughout the Astoria Channel system and as thin correlative interbeds (ca. 1–2 cm) in interchannel areas. Similarly, sand/shale ratios are high throughout the fan valleys and the middle and lower fan areas of distributaries, but are low in the upper-fan interchannel areas.These depositional trends indicate that high-density turbidity currents carry coarse traction loads that remain confined in upper but not lower fan valleys. Fine debris selectively sorts out from channelized flows into overbank suspension flows that spread over the fan and deposit clayey silt. A high content of mica, plant fragments, and glass shards (if present) characterizes deposits of the overbank flows, a major process in the building of upper fan levees and interchannel areas.In the Late Pleistocene, turbidity currents funneled most coarse-grained debris through upper channels to depositional sites in middle and lower fan distributaries that periodically shifted, anastomosed and braided to spread sand layers throughout the area. At this time, depositional rates were many times greater (>50 cm/1000 years) than in the Holocene (8 cm/1000 years).During the Holocene rise of sea level, the shoreline shifted, the Columbia River sediment was trapped, and turbidity-current activity slackened from one major event per 6 years in the Late Pleistocene, to one per 1000 years in the Early Holocene, to none since the Mt. Mazama eruption (ca. 6600 B.P.). Turbidites became muddier and deposited as thick beds within main channels, in part explaining Holocene deposition rates three times greater there (25 cm/1000 years) than in interchannel regions. Turbid-layer debris, funneled through channel systems and trapped from flows off the continental terrace, also contributed to rapid sedimentation in valleys; however, less than 2% of the suspended sediment load of the Columbia River has been trapped in fan valleys during the Holocene.By the Late Holocene, continuous particle-by-particle deposition of hemipelagic clay with a biogenous coarse fraction was the predominant process on the fan. These hemipelagites contain progressively more clay size and less terrigenous debris offshore, and are finer grained, richer in planktonic tests and dominated by radiolarians compared to the foraminiferal-rich Pleistocene clays. The hemipelagic sedimentation of interglacial times, however, is insignificant compared to turbidite deposition of glacial times.