Sediment Gravity Flow Processes Research Articles

Integrated borehole image and rotary sidewall core data to support infrastructure-led appraisal: Capercaillie Field, Central North Sea

Abstract Quad 29 in the Central North Sea is a focus for bp, with a strategy to identify remaining hydrocarbon accumulations to tieback to existing infrastructure. Capercaillie was appraised in 2017 with well 29/04e-5 and sidetrack 29/04e-5z. A gas cap and thin oil rim were intersected within the siliciclastic Paleocene–Eocene Sele Formation. The reservoir sandstones were deposited in a deep marine setting by sediment-gravity flow processes. Within the context of a relatively detailed knowledge of the surrounding area, the reservoir data-operational risk-cost balance was addressed through acquisition of a full wireline logging suite, pressure data, latest-generation microresistivity images and targeted large-volume rotary sidewall cores (RSWCs), with the latter two favoured over whole core. Mature deepwater descriptive schemes were applied at the sidewall core (lithotype), bed (borehole image facies) and bed-stack (depositional package) scales. This hierarchical approach provided robust sedimentological data to underpin higher-order depositional models, which together were used as a framework to (1) constrain the reservoirs’ mineralogical and textural attributes, (2) establish the main controls on rock quality and (3) explain the resulting variations in porosity and permeability. This study demonstrates that the careful integration of data derived from the latest borehole imaging tools and large-volume RSWCs can be a successful means of characterizing reservoirs from a sedimentological, reservoir quality and reservoir architecture perspective in mature basins. Similar approaches to geological reservoir characterization in such settings are likely to be a common cornerstone of cost-effective development during the energy transition.

The effect of breached relay ramp structures on deep‐lacustrine sedimentary systems

AbstractFault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between en‐echelon fault segments, which become modified through progressive deformation, and may develop through‐going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep‐marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high‐quality subsurface data from the Early Cretaceous deep‐lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay‐ramp, an ideal example, formed during the syn‐rift and was later covered by a thick transitional and early post‐rift succession. Major transitional and early post‐rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo‐bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure‐bounding fault to the north. Reactivation of basin‐bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin‐located relay ramps on sedimentary systems from syn‐rift, transitional and into the early post‐rift phase. It suggests that these structures can become reactivated during post‐rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small‐scale lobes, with small, often line‐sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs.

Contrasting styles of siliciclastic flysch sedimentation in the Upper Cretaceous of the Silesian Unit, Outer Western Carpathians: sedimentology and genetic implications

This study reports on a new set of sedimentological data and related interpretations of the Santonian– Campanian siliciclastic deposits in the Western Flysch Carpathians based on natural outcrops in the uppermost Godula Formation and lowermost Istebna Formation. The rationale was to confront the characteristics of this flysch succession with current controversies and state of knowledge on deep-water clastic sedimentation. The sedimentological analysis of the field data allowed for multi-scale synthetic classifications of the depositional components in the investigated flysch. The hierarchical and practical nature of the suggested classification schemes allows for their application to similar deposits in other regions. The siliciclastic deposits are products of gravity-driven terrigenous sediment redeposition via submarine slumps, debris flows, and turbidity currents. Sediment reworking by tractional bottom currents is considered as an accompanying factor. Point-sourced turbiditic fan lobe fringes from the submarine piedmont ramp and linearly supplied debritic covers along the slope apron are proposed as dominant. The innovative linking between the textural-structural descriptive features of the deposits and the critical determinants of specific sediment gravity-flow processes and architectural elements of the deepwater clastic depositional systems is a significant contribution to this research field.

Deep to shallow-marine sedimentology and impact of volcanism within the Middle Triassic Palaeo-Tethyan Semantan Basin, Singapore

The Middle Triassic Pulau Ayer Chawan Formation is a predominantly deep-marine, occasionally shallow-marine sedimentary succession, deposited in the Singapore sector of the Palaeo-Tethyan Semantan Basin. The formation provides an important record of the dynamic interplay between a siliciclastic sedimentary system and the products of an adjacent active volcanic arc. It is characterised by six sub-environments, including: deep-marine turbidite fan, deep-marine background sedimentation, subaqueous debris cone, shallow-marine, volcanically-sourced turbidite fan, and hyaloclastite mound or ridge. Turbidite fan deposits preserve the input of both siliciclastic and volcaniclastic sediments from the shelf, transported into the deep-marine environment by a suite of subaqueous sediment gravity flow processes, including: turbidity currents; mixed flow types (hybrid event beds); concentrated and hyper concentrated sediment gravity flows, and debris flows. Thick heterolithic successions of debrites were likely sourced through regular collapse of an unstable shelf. The presence of hybrid event beds, encountered within the deep-marine turbidite fans, supports a slope that was out-of-grade, and may have been actively retreating towards the hinterland. Together, these factors suggest regional-scale uplift of the eastern margins of the Semantan Basin during Triassic times, most likely facilitated through volcanic activity in the adjacent Palaeo-Tethys Sukhothai Arc. Evidence for contemporaneous, arc-related magmatism includes ubiquitous volcaniclastic sedimentary rocks within formation, including pyroclastic density current deposits and perhaps more-strikingly through the hyaloclastites of the Nanyang Member. The hyaloclastites formed through quenching of magmas delivered into the deep-marine setting from a series of sub-sea vents or mounds.

Controls on organic matter distributions in Eocene Lake Uinta, Utah and Colorado

The Green River Formation deposited in Eocene Lake Uinta in the Uinta and Piceance Basins, Utah and Colorado, contains the largest oil shale resource in the world with an estimated 1.53 trillion barrels of oil in place in the Piceance Basin and 1.32 trillion barrels in the Uinta Basin. The Douglas Creek arch, a slowly subsiding hinge-line between the two basins, created separate deep depocenters, one in each basin with shallow water conditions near the crest of the arch. Lake Uinta was a saline lake throughout its history with a lower saline to hypersaline layer (monimolimnion) and an upper less saline layer (mixolimnion). Most of the organic matter in the Green River Formation was derived primarily from algae that lived in the photic zone of the lake and is very hydrogen-rich and oil-prone. In many modern large and deep lakes, rates of organic matter production are highly variable due to differences in nutrient supply. However, cyclonic circulation often leads to winnowing out of organic and mineral matter in the mixolimnion leading to organic matter and fine-grained mineral matter being deposited in increasing amounts toward hydro-dynamically dead zones in the center of the circulation producing concentric bands of increasing organic matter content. Organic matter transport through the dense, hypersaline monimolimnion may have been facilitated by low density organic matter attaching to more dense clay mineral particles. Most of the oil shale intervals deposited in Lake Uinta display similar patterns in their organic matter distributions, increasing in very regular fashion toward the central areas of the lake’s two depocenters. This concentric feature is particularly prominent in the most laminated oil shale zones. Here, we propose that cyclonic circulation was present in Lake Uinta. Each basin appears to have had its own circulation currents, separated by shallow water conditions near the Douglas Creek arch, and one hydrodynamically dead zone in each basin. Sediment gravity flow processes were also very active in some strata of Lake Uinta, leading to the reworking and re-depositing of sediments. Two general types of sediment gravity flows are recognized: (1) organic-rich sediment gravity flows that reworked and may have concentrated organic-rich material closer to the two deep depocenters, and (2) sandstone- and siltstone-rich organic-poor mass movement deposits that originated on marginal shelves. Mass movements could have been triggered by various natural processes and/or possibly by the movement of dense brines that evolved on marginal shelves and moved along the bottom of the water column toward the deep part of the lake. The uppermost, poorly consolidated sediment layer was incorporated in sediment gravity flows as they moved, and in many cases sediment gravity flows scoured down significantly into the more consolidated underlying sediment producing large rip-up clasts of laminated sediments. Truncation of more than 100 ft occurs at the base of a sequence of sediment gravity flows in one well, indicating a significant incised channel. Coarser-grained sediment gravity flows terminated before reaching the lake’s deepest areas, forming thick concentric buildups of organically lean sediment near the base of the marginal slopes. Intervals dominated by organic-rich fine-grained sediment gravity flows have tightly concentric bands of increasing organic matter toward the deepest parts of the lake and can be organically richer than the richest laminated intervals. There is some evidence that the hydrodynamically quiet zones did not always correspond closely to the deepest areas of the lake, extending in some cases into some shallower areas.

Sedimentation adjacent to atolls and volcano-cored carbonate platforms in the Mozambique Channel (SW Indian Ocean)

Recently acquired data from the Iles Eparses (southwestern Indian Ocean) reveal new information about the geomorphology, depositional processes, and sedimentary deposits on the slopes of atolls and atoll-like platforms. The deposits discussed here lie on the deepwater flanks of isolated, inactive volcanos that are capped by shallow, relatively flat carbonate platforms 45–210 km2 in area. Much of the slope geomorphology is controlled by the underlying volcanic edifice. Steep (~25–35°) upper slopes consist of outcrops of volcanic basement, smooth banks, failure scarps, and channels. Sedimentary features seen in the lower slope and proximal basin (2000–3500 m deep) consist of channels, levees, lobes, and mass transport deposits (MTDs). In places, channels terminate 13–18 km from the platform margin, ending in lobes up to 3.5 km across, a feature not often seen in modern carbonates. In the subsurface, MTDs are present near all platforms. Within MTDs, seismic character is variable, often consists of chaotic reflections indicative of sediment gravity flow processes. Subsurface units with organized (retro- or progradational) reflections are interpreted as turbidite lobes or MTDs with compressional features. Core taken within lobes and near the base of slopes reveal decimeter-scale turbidites and debrites composed primarily of graded and massive bioclastic grainstones and packstones with abundant neritic skeletal components, interbedded with hemipelagic aragonitic and clay-rich foraminiferal ooze. Slope depositional processes are therefore primarily gravity-driven and occur at different scales; i.e., bed-scale turbidites and muds may be remobilized and redeposited through slope failure and deposition of large MTDs. Dominant wind direction may also play a role in slope sedimentation: leeward slopes are generally less rugose and show increased sedimentation at the toe of the slope. This study thus provides new insight into depositional systems surrounding atoll-like carbonate platforms, and provides a new analogue for similar deposits in the geologic record.

Aggradational lobe fringes: The influence of subtle intrabasinal seabed topography on sediment gravity flow processes and lobe stacking patterns

AbstractSeabed topography is ubiquitous across basin‐floor environments, and influences sediment gravity flows and sediment dispersal patterns. The impact of steep (several degrees) confining slopes on sedimentary facies and depositional architecture has been widely documented. However, the influence of gentle (fraction of a degree) confining slopes is less well‐documented, largely due to outcrop limitations. Here, exceptional outcrop and research borehole data from Unit A of the Permian Laingsburg Formation, South Africa, provide the means to examine the influence of subtle lateral confinement on flow behaviour and lobe stacking patterns. The dataset describes the detailed architecture of subunits A.1 to A.6, a succession of stacked lobe complexes, over a palinspastically restored 22 km across‐strike transect. Facies distributions, stacking patterns, thickness and palaeoflow trends indicate the presence of a south‐east facing low angle (fraction of a degree) lateral intrabasinal slope. Interaction between stratified turbidity currents with a thin basal sand‐prone part and a thick mud‐prone part and the confining slope results in facies transition from thick‐bedded sandstones to thin‐bedded heterolithic lobe fringe‐type deposits. Slope angle dictates the distance over which the facies transition occurs (hundreds of metres to kilometres). These deposits are stacked vertically over tens of metres in successive lobe complexes to form an aggradational succession of lobe fringes. Extensive slides and debrites are present at the base of lobe complexes, and are associated with steeper restored slope gradients. The persistent facies transition across multiple lobe complexes, and the mass flow deposits, suggests that the intrabasinal slope was dynamic and was never healed by deposition during Unit A times. This study demonstrates the significant influence that even subtle basin‐floor topography has on flow behaviour and depositional architecture of submarine lobe complexes. In addition, we present a new aggradational lobe fringe facies associations and recognition criteria for subtle confinement in less well‐exposed and subsurface basin fills.

A Neoproterozoic glacially influenced basin margin succession and ‘atypical’ cap carbonate associated with bedrock palaeovalleys, Mirbat area, southern Oman

ABSTRACTThe Ayn Formation of the Neoproterozoic Mirbat Group comprises <400 m of little‐deformed, glacially influenced basin margin deposits. These deposits are preserved in several palaeovalleys eroded in crystalline basement and overlain by a discontinuous cap carbonate. The Ayn Formation and the cap carbonate, which are superbly exposed along a 20 km SW–NE‐striking escarpment in south Oman, provide important insights into the processes operating on a basin margin during a Neoproterozoic glaciation and its demise. The Ayn Formation comprises units of glacimarine rain‐out diamictite and sediment gravity flow deposits, alternated with units of fluvial and deltaic sandstones and conglomerates, which may have formed by proglacial outwash. The stratigraphic evolution of the Ayn Formation indicates a highly active hydrological cycle during a phase of overall (glacio‐eustatic?) low stand when glaciers advanced into and receded upon bedrock valleys. The transgressive cap carbonate was deposited primarily in shallow marine or shallow lacustrine environments over palaeohighs during the deglaciation, and was partly reworked into deeper parts of the basin through sediment gravity flow processes. Locally, the cap carbonate transgresses over crystalline basement containing a network of fissures filled with carbonate originating from the cap. The δ13C isotopic composition of the cap carbonate varies systematically between −3.5 and +5.8‰ Pee Dee Belemnite standard, in common with other older Cryogenian examples.

Morphology and sedimentary processes on the continental slope off Pine Island Bay, Amundsen Sea, West Antarctica

Morphology and sedimentation on the Pine Island Bay continental margin, eastern Amundsen Sea, is investigated using swath bathymetry, parametric subbottom profiler (TOPAS) records, and sediment cores. A major cross-shelf trough extends to the continental shelf edge at 114°W, through which fast-flowing ice drained the West Antarctic Ice Sheet under full-glacial conditions. The slope adjacent to the trough mouth is 4°, whereas to the east and west it is steeper. A bulge in bathymetric contours beyond and east of the trough mouth indicates some slope progradation. Elsewhere, seismic-reflection profiles indicate major shelf aggradation and minor slope progradation. Networks of gullies and channels dominate the slope adjacent to Pine Island Bay. In front of the trough mouth, gullies at the shelf edge and uppermost slope connect directly with channels that continue to the continental rise. Other gullies connect indirectly with channels further downslope. TOPAS records indicate that gully-channel systems acted as conduits for coarse-grained sediment transfer. Cores comprise turbidites representing channel deposits and diamicton facies deposited by debris flows. Sandy turbidites interbedded with hemipelagic muds occur on the rise and abyssal plain. The West Antarctic Ice Sheet extended to the shelf edge during full glacials, delivering water-saturated till and turbid water directly to the continental slope along a line source. Gully-channel systems are probably a product of sediment gravity flows derived from the release of water-saturated till and turbid water along the ice-sheet margin, perhaps augmented subsequently by downslope flow of dense cold and saline water associated with sea-ice production during the Holocene. The gully-channel systems enable sediment to bypass the continental slope. Spatial variations in gully-channel system morphology along the Amundsen Sea continental slope are probably linked to differences in sediment gravity flow processes and the rate and composition of sediment delivered to the shelf edge from the ice sheet. Cored sediments on the slope and abyssal plain record the widespread presence of turbidite sands, indicating that downslope transfer of glacier-derived sediments has taken place. This supports our interpretation that submarine channels are derived from, and are conduits for, mass wasting. Submarine channels on the upper slope continue into the abyssal plain as far north as ∼67°S. They are separated by sediment drifts and sediment waves resulting from the interaction between downslope turbidity current processes and along-slope bottom currents. Similar deep-sea sedimentary processes operate along much of West Antarctica and the western side of the Antarctic Peninsula.

Neoproterozoic environmental change recorded in the Port Askaig Formation, Scotland: Climatic vs tectonic controls

The snowball Earth hypothesis suggests that the Neoproterozoic was characterized by several prolonged and severe global glaciations followed by very rapid climate change to ‘hot house’ conditions. The Neoproterozoic Port Askaig Formation of Scotland consists of a thick succession of diamictite, sandstone, conglomerate and mudstone. Sedimentological and stratigraphic analysis of Port Askaig deposits exposed on the Garvellach Islands was carried out to establish the nature of Neoproterozoic palaeoenvironmental change preserved in this thick succession. Particular emphasis was placed on identifying and distinguishing between climatic and tectonic controls on sedimentation. Port Askaig Formation diamictite units are attributed to deposition by sediment gravity flow processes or ‘rainout’ of fine-grained sediment and ice-rafted debris in a glacially influenced marine setting. Associated facies record various depositional processes ranging from sediment gravity flows (conglomerate, massive sandstone and laminated mudstone) to deposition under other unidirectional currents (cross-bedded and horizontally laminated sandstone). The Port Askaig Formation is also characterized by abundant soft sediment deformation features that occur at discrete intervals and are interpreted to record episodic seismic activity. Stratigraphic analysis of the Port Askaig Formation on the Garvellach Islands reveals three phases of deposition. Phase I was dominated by sediment gravity flow processes and sedimentation was primarily tectonically controlled. Phase II was a transitional phase characterized by continued tectonic-instability, an increased supply of sand to the basin and the preservation of current-generated facies. In the third and final phase of deposition, the interbedded units of sandstone and diamictite are interpreted to reflect development of large sandy bedforms and ice margin fluctuations in a tectonically stable marine setting. Sedimentological and stratigraphic analysis of the Port Askaig Formation demonstrates that tectonic activity had a significant influence on development of the lowermost parts of the succession. Climatic influences on sedimentation are difficult to identify during such phases of tectonic activity but are more easily discerned during episodes of tectonic quiescence (e.g.,, Phase III of the Port Askaig Formation). The thick succession of diamictite interbedded with current-deposited sandstone preserved within the Port Askaig Formation is not consistent with deep freeze conditions proposed by the snowball Earth hypothesis.

The role of fluid- and sediment-gravity flow processes during deposition of deltaic conglomerates (Cardium Formation, Upper Cretaceous), west-central Alberta

ABSTRACT Most previous studies on conglomerates of the Carrot Creek Member (Cardium Formation) in the Cyn-Pem/Carrot Creek area of west-central Alberta have focused on understanding the stratigraphic context of these informally termed conglomerate However, a detailed understanding of the depositional mechanisms that controlled the spatial distribution of conglomerate facies, which in turn control reservoir quality and related production characteristics within and between different pools, is presently lacking. As a result, a number of important questions still remain. However, many of these issues can be resolved by differentiating distributary-channel from delta-front conglomerates, and also appreciating the sedimentological importance of the steep depositional gradient (>15°) on the delta front. Distributary-channel and delta-front conglomerates commonly consist of interstratified, matrix-rich and matrix-poor conglomerate. In distributary-channel conglomerates, this bipartite texture is most likely related to the spatial variability of gravel and sand deposition on the leeside of gravel-bed, subaqueous dunes and/or bars. Reservoir quality in these conglomerates was significantly reduced by a later infiltrated sand matrix. Marine conglomerates, on the other hand, were deposited on a steep delta-front by sediment-gravity processes. Planar-based, ungraded, matrix-supported conglomerate overlain abruptly by better-sorted, very coarse sandstone or granule/fine pebble conglomerate represent, respectively, en masse deposition from high-concentration sediment-gravity-flow dispersions and grain-by-grain gravity-driven transport and deposition. Most dispersions collapsed on the delta slope, but others flowed to the base of the slope and deposited their sediment in a granular jump. Much of the sand-sized sediment deposited at the mouths of the distributary-channel network was winnowed and remobilized by waves and transported alongshore and away from the delta front and, as a consequence, preserved the high-reservoir quality that characterizes delta front conglomerates. End_Page 426------------------------

Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland

Abstract The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions.

Depositional processes, triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain

Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites). During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds. Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east–west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope. Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls. Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault.

Slump-generated megachannels in the Pliocene–Pleistocene glaciomarine Yakataga Formation, Gulf of Alaska

Pliocene–Pleistocene megachannels, as much as 400 m deep and several kilometers wide, are exceptionally well exposed around the Gulf of Alaska coastal margin in glaciomarine sedimentary strata of the Yakataga Formation. In the Icy Bay region, stacked megachannels of the Yakataga Formation are filled predominantly with debris flow diamictites, crudely stratified conglomerates, and thinly bedded sandstone and mudstone turbidites. Detailed sedimentological and micropaleontological analyses of the megachannel deposits identified two distinct facies associations that represent channel margin and central channel fill depositional settings. All facies types indicate deposition by sediment gravity-flow processes and high rates of sediment supply in a tectonically active setting. Foraminiferal biofacies suggest deposition in inner neritic to upper bathyal water depths and vigorous transport of sediment from shallow to deeper water. The Yakataga Formation megachannels probably formed as slump scars, produced by the retrogressive failure and downslope slumping of rapidly deposited outer shelf–upper slope sediments. The intimate association of megachannels with thrust faults and folds suggests that initial slumping was triggered by tectonic tilting and high-magnitude earthquakes. Megachannels have a relatively immature form and were probably short lived and filled rapidly with sediment gravity-flow deposits. Megachannels of the Yakataga Formation provide a well-exposed analog for megachannels formed in other tectonically active settings or passive margin settings characterized by high sedimentation rates. There is little evidence to suggest that megachannel development is systematically related to episodes of sea-level lowstand, and a sequence stratigraphic context is difficult to identify.

Controls on Early Permian shelf deglaciation in the Karoo Basin of South Africa

Abstract The Late Carboniferous to Early Permian glacigene Dwyka Group has at its top three types of transitions from diamictite to postglacial mudrock in the southern Karoo. The type 1 transition sequence consists of clast-poor carbonate-rich diamictite having a sharp contact with the overlying mudrocks. This diamicton accumulated slowly by rain-out and sediment gravity flow processes. During sedimentation the polar ice front was stabilised by a possible ice shelf. Collapse of the ice shelf caused rapid ice margin retreat. The type 2 transition sequence consists of clast-poor diamictite with sandstone bodies overlain by mudrock with ice-rafted debris or thin interbedded debris-flow diamictites. Deposition of the diamictite facies occurred during oscillations of a subpolar ice front with short stable periods of surface melting of the ice resulting in the formation of small subaqueous outwash fans. The type 3 transition sequence consists of bouldery diamictite interbedded with massive or sandstone-bearing diamictite overlain by mudrock with ice-rafted debris or thin interbedded debris-flow diamictites. The sequence represents highly unstable ice front conditions and fluctuating sea-levels with periods of rapid deposition probably by collapsing ice streams. Climatic conditions may have alternated between polar and subpolar. The type 1 transition sequence is predominant in the western part and the type 3 sequence in the eastern part of the Karoo Basin. Long-term controls on deglaciation was climatic warming as Gondwana drifted over the pole as well as changes in land-sea configuration. Superimposed on these were local controls such as relative sea-level changes, presence of deformable substrate and meltwater production.

Sedimentology of the Permian Radok Conglomerate in the Beaver Lake area of MacRobertson Land, East Antarctica

AbstractThe mid- to Upper Permian Radok Conglomerate, the lowermost formation of the Permo-Triassic Amery Group, crops out in the Beaver Lake area of the northern Prince Charles Mountains, East Antarctica. Outcrop is confined to a north-south elongate, fault-bounded corridor interpretedas a remnant of a continental extensional basin formed during Late Palaeozoic times. This basinforms a small part of the much larger Lambert Graben, a major continental rift system. The RadokConglomerate consists of interbedded conglomerates, argillaceous sandstones, siltstones, and minor, thin carbonaceous siltstones and coals. Textural, petrographic, palaeocurrent and other data suggestlocal derivation from Precambrian massifs to the immediate west, during a period of fault activity.The unit is a minimum of 400 m thick, the base being unexposed, and grossly fines upward. It isabruptly overlain by quartzo-feldspathic sandstone-dominated rocks of the Upper Permian Bainmedart Coal Measures. Seven recurrent lithofacies have been recognized with the Radok Conglomerate, and are interpreted as the products of poorly-confined stream flow, sheet flow and sediment gravity flow processes, suspension fallout in shallow standing water, and organic sediment accumulation in peat-forming wetlands. The unit as a whole is interpreted as having accumulated as a coarse alluvial apron along the western margin of a ?graben extensional trough. Similar, though poorly exposed, facies are exposed on the eastern margin of the basin and may reflect similar depositional systems. Towards the top of the Radok Conglomerate, typical Radok lithologies are interbedded with quartzo-feldspathic sandstones derived from the south, precursors of the overlying Bainmedart Coal Measures. Interference between transverse (Radok) and axial (Bainmedart) drainage is possibly related to progressive infilling of extensional topography, thereby allowing axially flowing rivers to avulse increasingly into the Beaver Lake region from the main Lambert Graben.

Depositional Processes and Predictive Modelling of Deep Water Turbidite Sands of the Britannia Formation, Britannia Field, U.K. North Sea: ABSTRACT

The Britannia Field is a large gas condensate accumulation located 125 miles northeast of Aberdeen in the U.K. North Sea. The reservoir formation is Lower Cretaceous in age, and is composed of a wide variety of {open_quotes}deep{close_quotes} water turbidite sands. These were shed off surrounding highs and temporarily stored in a shelfal setting before being fed axially into the asymmetrical Wytch Ground Graben. Depositional compartmentalization resulted from sand input from both the east and west. The sands represent a localized regression associated with the Austrian Orogeny, set in an overall transgressive regime. Detailed core examination has resulted in the recognition of an unusual variety of facies types arising from sediment gravity flow processes that span the spectrum between turbidity current and debris flow. Sandy high density turbidity (HDT) flows give rise to {open_quote}conventional{close_quotes} HDT deposits. With the addition of mud, cohesion becomes an important factor, and its amount, nature, and distribution governs the behavior of the flow. The resulting flows are termed {open_quotes}Slurry flows,{close_quotes} and their deposits exhibit a spectrum of internal structures and properties that reflect the changing dynamics of clay/sand interactions. Understanding the depositional processes, within the framework of a chronostratigraphically sub-divided reservoir, has enabled predictive mapping ofmore » the facies distribution across the Field. Depositional pattern models are being tested by predrilling producing wells. Since there is an intimate link between lithotype, permeability, and gas saturation, the development of a predictive model has greatly increased the confidence in OGIP estimation.« less

Contrasting sediment gravity flow processes in the late Llandovery, Rhuddnant Grits turbidite system, Welsh Basin

AbstractThe Rhuddnant Grits turbidite system was deposited within an elongate, fault‐bounded trough in the late Llandovery (Telychian) Welsh Basin. Two groups of sandstones are identified within the system: high‐matrix sandstones and laminated sandstones. The high‐matrix sandstones are medium to very thick bedded, fine to very coarse‐grained muddy sandstones. The high‐matrix sandstone beds are almost entirely structureless and have several features indicative of deposition from high density turbidity currents, probably undergoing late stage debris flow behaviour (e.g. grain size discontinuities, inverse grading, floating clasts). The laminated sandstones are thin to very thin bedded, fine‐grained and have a distinctive mud/silt lamination. Tractional structures and convolution are common in these beds. They were probably deposited by slow moving, dilute turbidity currents. Dissimilar palaeocurrent vectors and estimates of flow properties from the two types of sandstone support the contrasting nature of the depositing flows. A coarsening and thickening upwards trend is identified in the laminated sandstones of the Rhuddnant Grits Formation. This trend is not reflected in the high‐matrix sandstone beds. Although the high‐matrix sandstones appear in packets or groups within the laminated sandstone background, they were otherwise deposited in an entirely random manner throughout the exposed system. This may suggest that the two types of sandstone are the result of different triggering mechanisms at source, or of contrasting flow properties developed early in the flow histories.

Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.

Slope readjustment: A new model for the development of submarine fans and aprons

In a new model proposed for the formation of submarine canyons and submarine fan-aprons, numerical simulations of basin fill are utilized to illustrate simple concepts of slope grading. This modeling suggests that erosional truncation, sediment bypass, and marine onlap of submarine fan-apron complexes are formed in response to changing basin physiography. Two kinds of basin margins are identified. (1) Progradational margins represent the basinward advance of graded depositional profiles formed when diffusive and sediment gravity-flow processes are in equilibrium with sediment supply, basin subsidence, and basin physiography. (2) Erosional margins form when upper slope gradients exceed an equilibrium grade, and are characterized by erosion, slumping, and sediment bypass to lower slope environments via sediment gravity-flow processes. Erosional margins are transformed into progradational margins when the bathymetric escarpment (i.e., an oversteepened margin) is buried by onlapping and aggrading fanapron deposits. Progradational margins can develop bathymetric escarpments and become erosional in response to a rapid rise in relative sea level, structural deformation of basin profiles (e.g., faulting), and/or a transition from carbonate to siliciclastic deposition. Whereas relative falls in sea level play a major role in bringing the source of sediment input to the shelf edge, the development of slope unconformities and onlapping submarine fan-apron systems is controlled primarily by slope- readjustment processes triggered by changing basin physiography.