Sediment Gravity Flows Research Articles

The formation and development of avulsions and splays of submarine channel systems: Insights from 3D seismic data from the northeastern Bengal Fan

Avulsions and splays of submarine channels are important autogenic responses affecting sediment distribution in deep-water areas; however, their formation and development are still not well understood. Using high-resolution 3D seismic data and spectral decomposition red–green–blue (RGB) color blends, two seismically well-imaged submarine channel systems (SCS1 and SCS2) on the northeastern Bengal Fan are used to document the formation and development of avulsions and splays. The results suggest that avulsions occur mainly during the early evolution stage of submarine channel systems. SCS1 and SCS2 are respectively accompanied by one and four avulsions and accordingly produce two and five independent submarine channels. During this early phase, these channels collectively incise into the underlying stratigraphy but lack levee deposits. Splays, in contrast, develop consistently during the late evolution stage of the submarine channel systems. Three crevasse splays and three overbank splays develop on the overbank areas of SCS1 and SCS2. During that late stage, channel and levee deposits build over the site of former erosion, thus forming the aggradational phase of channel evolution. Crevasse and overbank splays are two different types of splays observed on this part of the Bengal Fan. The former are created by flow breaching of the adjacent levees, producing large-scale elongated shapes; they are related to relatively stable sediment gravity flows, a high-degree of channel instability, and steep levee topography. Overbank splays, however, are created by flow overtopping adjacent levees and produce small-scale, fan-like shapes; they are related to gradually-weaking sediment gravity flows, a low-degree channel unstability, and gentle levee topography.

Geological Controlling Factors of Low Resistivity Shale and Their Implications on Reservoir Quality: A Case Study in the Southern Sichuan Basin, China

At the Changning block and at the Luzhou block, the genetic mechanism of low-resistivity shale and its impact on reservoir quality are currently a hot topic on a world-wide scale. Shale with resistivity lower than 20 Ω·m is widely developed at the Wufeng-Longmaxi Formation in the Southern Sichuan Basin, bringing a considerable challenge for reservoir prediction using the electromagnetic method. This paper discusses the genetic mechanisms and reservoir qualities of three low-resistivity shale reservoir types in the Southern Sichuan Basin (the Changning block and Luzhou block). Three primary elements controlling low-resistivity shale distribution in the Southern Sichuan Basin have been deduced: widely distributed gravity flow deposits, poor structural preservation conditions and shale graphitization caused by Emeishan basalt. Specifically, (1) the shale reservoir with a resistivity <12 Ω·m was uniformly distributed with gravity flow deposits in the Southern Sichuan Basin. High clay mineral contents (especially illite) in gravity flow deposits increased cation exchange capacity and irreducible water saturation at shale reservoir, decreasing electrical resistivity. (2) The resistivity of the shale reservoir close to a complex fault-fracture zone was generally lower than 20 Ω·m, indicating that poor structural preservation conditions played an important role in the wide distribution of low-resistivity shale. The resistivity of the shale reservoir near NE-trending faults at the Changning block was significantly lower than that in other areas. (3) Emeishan basalt caused extensive shale graphitization at the west of the Changning block, which was limited at the Luzhou block. The shale resistivity at the Luzhou block was not affected by graphitization. Among three types of low-resistivity shale, type III was characterized by high quartz content, high TOC, high porosity, high gas content and low graphitization. Although the resistivity of type III is generally lower that 20 Ω·m, it is still a favorable exploration target in the Southern Sichuan Basin.

Characterization and evolution of a carbonate basin floor fan on a leeward side of an isolated carbonate platform, Sirte Basin, Libya

The recognition of carbonate basin floor fans and the interpretation of their formation and evolution are extremely crucial to the hydrocarbon exploration since they form important oil and gas reservoirs. There has been little research on the geometries and controlling factors of the carbonate basin floor fans from subsurface three-dimensional (3D) seismic and wireline log data. Most studies have been conducted on siliciclastic basin floor fans. Thus, this study characterizes the internal configuration and processes controlling the deposition of a lower Eocene carbonate basin floor fan system in the Sirte Basin in a sequence stratigraphic context through integrated 3D seismic and wireline log interpretations. The carbonate fan complex shows hummocky to undulated seismic geometries on vertical seismic sections and bounded by an unconformable sequence boundary at the base and a downlap surface at the top. It has an aerial extension of around 9 km from south to north along depositional dip and at least 8 km from east to west along strike. In well data, the fan system predominantly comprises limestone units intercalated with thin shale intervals. Log curves display an overall funnel-shaped trend, suggesting progradational stacking pattern. The vertical thickness of the fan complex varies from 120 to 165 m. The evolution of the carbonate fan was influenced by the interplay of accommodation space and sediment supply. A pre-existing topography, tectonic subsidence, and sea-level rise generated accommodation space for the deposition of the fan system, whereas sediment-gravity flows transported the fan sediments from the platform interior and margin to the basin floor. This study provides sedimentological and stratigraphic criteria to recognize and interpret the evolution of basin floor fans along isolated carbonate platforms from 3D seismic and wireline log data and may serve as an analog for ongoing and future studies on carbonate basin floor fans elsewhere.

Differences of sedimentary triggers and depositional architecture of lacustrine turbidites from normal regression to forced regression: Eocene Dongying depression, Bohai Bay Basin, East China

Sedimentary processes of marine turbidite systems under sea-level changes have been the focus of deep-water sedimentology. Compared with well-studied marine basins, the products of deep-water sediment gravity flows responding to lake-level changes in closed lacustrine basins are still poorly understood. In this study, we integrate cores of 10 exploratory wells, logs of 280 development wells, 3D seismic data and geochemical elements from the Eocene Dongying Depression, Bohai Bay Basin, East China to investigate sedimentary mechanisms and depositional architecture of lacustrine turbidite systems under lake-level changes. The studied strata are divided into three parasequence sets (PSS4 ~ PSS2), corresponding to early highstand normal regression with ascending trajectories, late highstand normal regression with flat trajectories, and forced regression with descending trajectories, respectively. From early highstand normal regression to forced regression, the ratios of Fe/Mn and U/Th reveal that the humid climate shifted to the arid climate. More lithofacies clues indicate that turbidites in highstand normal regression were triggered by river floods, whereas counterparts in forced regression were related to sediment failures. There are different architectural features of turbidite systems in three regression stages. PSS4 developed a series of channel belts within which individual channel elements are aggraded vertically. PSS3 is composed of a channelized lobe formed by individual distributary channels migrating laterally. PSS2 is characterized by a suite of compensational stacking debrite tongues. Climate forcing is a crucial factor controlling depositional architecture of the turbidite systems. A relatively humid climate during highstand normal regression led to rising lake-level and frequent floods, which are conducive to the formation of hyperpycnal-fed channel-lobe systems. Nevertheless, during forced regression under an arid climate, the strong progradational clinothems were prone to failure and sediment remobilization, resulting in debrite tongue complexes. This study highlights depositional differences of lacustrine turbidites during regression stages under the control of climate. Meanwhile, it also provides a new predictive model for deep-water hydrocarbon exploration and production in lacustrine basins worldwide.

Reconsidering the glaciogenic origin of Gondwana diamictites of the Dwyka Group, South Africa

Abstract The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, as are diamictites which have been interpreted as in-situ or reworked tillites. These interpretations were tested by investigation of outcrops in formerly well-studied areas, throughout South Africa. Detailed analyses have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreted to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture combinations of fracturing and irregular abrasion associated with Quaternary glacial deposits, but mainly a mix of surface microtextures associated with multicyclical grains. The Dwyka Group diamictites warrant reinterpretation as non-glacial sediment gravity flow deposits.

Sedimentary characteristics and main controlling factors of the Middle-Upper Permian and Middle-Upper Triassic around Bogda Mountain of Xinjiang, NW China

Based on field geological survey, interpretation of seismic data and analysis of drilling and logging data, the evolution of geological structures, stratigraphic sedimentary filling sequence and sedimentary system around the Bogda Mountain were analyzed according to the idea of “structure controlling basin, basin controlling facies and facies controlling assemblages”. The tectonic evolution of the basin around the Bogda Mountain can be divided into nine stages. The Middle-Late Permian–Middle-Late Triassic was the development stage of intracontinental rift, foreland basin and inland depression basin when lake, fan delta and braided river delta sedimentary facies developed. Early intracontinental rifting, late Permian tectonic uplift, and middle-late Triassic tectonic subsidence controlled the shape, type, subsidence rate and sedimentary system evolution of the basin. The Bogda Mountain area was the subsidence center and deposition center of the deep water lake basin in the Middle Permian with mainly deep-water deposition and local gravity flow deposition. This area had tectonic inversion in the Late Permian, when the Bogda Mountain uplifted to form a low bulge and a series of fan delta sand bodies. In the Middle-Late Triassic, subsidence occurred in the Bogda low uplift, characterized by extensive development of braided river delta deposits.

Sedimentary characteristics and meticulous depiction of Palaeogene lacustrine gravity flow deposits in BZ25 area, Bohai Sea, China

Sandstone thickness and reservoir quality in the third member of Paleogene Shahejie formation (E2s3) of different wells in BZ25 oil field are significantly variable. The uncertain cognition of high quality reservoir distribution restricts the development progress. By inspiration and application of latest theory progress of gravity flow, we carried out studies on lithological characteristic, lithofacies combination, paleo-geomorphology, sand period and evolutionary model of E2s3 gravity flow in BZ25 area. We identified three facies types of gravity flows in E2s3 of BZ25 area including slump, sandy debris flow and turbidity current. The above three types of gravity flows formed five typical lithofacies combinations in BZ25 area. Based on research of paleo-geomorphology and lateral facies distribution, we conclude that the process of gravity flow deposition from slope to deep-water environment in BZ25 area can be recovered as “slump - sandy debris flow - turbidity current”, and the sandy debris flow body distributes as special tongue-like shape with two branches, while the turbidity current body is characterized by fan-shaped. The sand-rich sandy debris flow in BZ25 area can be divided into two periods. The east branch of second period of the sandy debris flow is less exploited, and its central body has great development potential thickness.

FACIES ANALYSIS AND DEPOSITIONAL MODEL OF THE MIDDLE-UPPER TRIASSIC SEMANTAN FORMATION, CENTRAL PAHANG, MALAYSIA

This study details the sedimentological analysis of the Middle-Upper Triassic Semantan Formation in the Jerantut-Temerloh-Kemayan region of central Pahang. Seven (F1-7) facies have been identified which are; F1) poorly sorted conglomerate, F2) pebbly sandstone, F3) structureless-to-parallel laminated sandstone, F4) wavy-to-ripple fine-to-medium laminated sandstone, F5) slumped thin-interbeded sandstone and shale , F6) interbedded sandstone and shale, and F7) shale, represent a subordinate part of the Semantan Formation. Examination of the succession of the vertical facies resulted in concession of genetic units (FA1-FA5) which are; FA1) deep channel complex, FA2) distal lobe, FA3) hybrid gravity flow deposit, FA4) channelised lobe and FA5) non-channelised lobe. It is believed that these five genetic units were deposited within four proposed laterally contiguous depositional environments which are; 1) Inner fan – deep channel-levee complex (represent by FA1), 2) Mid fan – channelised lobe (represented by FA5 and FA3), 3) Mid Fan – non-channelised lobe (represented by FA4 and FA3), and 4) Outer fan – distal lobe (represented by FA2). The Semantan Formation deep-water fan is analysed as a sand-rich fan system, based on its sediment types.

Asymmetric abundances of submarine sediment waves around the Azores volcanic islands

Sediment waves are common on the submarine flanks of volcanic islands, but the processes that form them are difficult to decipher from geophysical data alone. Here, we identify downslope-trending trains of sediment waves in multibeam sonar data from five islands of the Azores. Train abundances are derived by counting the train intersections with depth contours around each island, revealing that they are typically twice as common on the north slopes of the islands compared with their south slopes. To provide context, we also characterise the Holocene mobility of sediments on the shelves, ocean density structure (for evaluating internal wave critical gradients) and sediment lithologies in four cores from beneath the slopes, which contain shelf carbonates in some volcaniclastic turbidites. Holocene mobility is assessed using sidescan sonar data, which reveal shelf-crossing sand streams. Using results of a high-resolution wave model, wave-imposed shear stresses are found to have exceeded the stresses at threshold of motion for most of the modern shelf sediments during recent extreme wave conditions.Sediment waves could potentially arise from sedimentary flows, gravity mass movements (shallow slumping) or ocean internal waves. Sedimentary flow origins are favoured by streamwise changes of wavelengths and upslope bedform migrations suggested by seismic reflections. The asymmetry of abundances north and south of each island is also an important clue. For example, it is unlikely that systematically different sediment geotechnical properties occur on the two sides of the islands, hence no reason to expect greater mass movements on northern slopes. Internal waves originate mainly from the south and hence also do not explain the asymmetry. Primary volcaniclastic flows are likely to be equally important on both sides. Sedimentary flows originating from shelf sediment agitated during storms or from failed slope sediment appear to explain best the asymmetry. In that case, prevailing wind and waves approaching the islands from the northwest lead to more frequent sediment gravity flows because they lead to greater flux of sediment from coastal erosion towards northerly outer shelves and upper slopes, where it is prone to slope failure. Offshore-directed near-bed currents originating from northwesterly wind-driven surface currents also transport shelf sediment efficiently to the north, further aided by greater wave agitation on that side. The high abundances of wave trains on the north sides of the islands therefore favours origins involving sedimentary gravity flows associated with wind and wave effects.

Organic Matter Burial in Deep-Sea Fans: A Depositional Process-Based Perspective

Organic matter burial in the deep-sea fan sediments is an important component of the long-term carbon cycle. Although there is increasing recognition of the importance of organic matter in deep-sea sediments, a major focus has been on mudstones, commonly interpreted as the background sediments, deposited by pelagic or hemipelagic vertical suspension fallout in low-energy fan environments. Emerging evidence suggests that relatively coarse-grained sediment gravity flow deposits (e.g., turbidites and hybrid event beds) can also store a significant quantity of organic carbon, implying that a wide range of depositional processes can result in the concentration and enrichment of organic matter in submarine fans. However, the role of these processes on carbon burial is still not fully understood. This review aims to discuss the impact of three widely documented deep-sea depositional mechanisms/processes, namely vertical suspension settling, grain-by-grain (incremental aggradation), and the en-masse deposition on distribution, burial, and preservation of organic matter in deep-marine deposits. Organic matter accumulated from slowly settling suspension in mud caps (Te or H5 divisions of turbidites and hybrid beds, respectively) is prone to higher oxidation compared to the carbon buried in sandy components of turbidity currents (Ta-Tc units) and hybrid beds (H2/H3 divisions). The burial of organic matter in sandy parts of the deposits has important implications for understanding the fundamental physical processes that control carbon accumulation and preservation in deep-marine rock record.

Sedimentologic and stratigraphic criteria to distinguish between basin‐floor and slope mudstones: Implications for the delivery of mud to deep‐water environments

AbstractDeep‐water mudstones overlying basin‐floor and slope sandstone‐prone deposits are often interpreted as hemipelagic drapes deposited during sand starvation periods. However, mud transport and depositional processes, and resulting facies and architecture of mudstones in deep‐water environments, remain poorly understood. This study documents the sedimentology and stratigraphy of basin‐floor and slope mudstones intercalated with sandstone‐prone deposits of the Laingsburg depocentre (Karoo Basin, South Africa). Sedimentologic and stratigraphic criteria are presented here to distinguish between slope and basin‐floor mudstones, which provide a tool to refine palaeogeographical reconstructions of other deep‐water successions. Several mudstone units were mapped at outcrop for 2500 km2 and investigated using macroscopic and microscopic core descriptions from two research boreholes. Basin‐floor mudstones exhibit a repeated and predictable alternation of bedsets dominated by low‐density turbidites, and massive packages dominated by debrites, with evidence of turbulent‐to‐laminar flow transformations. Slope mudstones exhibit a similar facies assemblage, but the proportion of low‐density turbidites is higher, and no repeated or predictable facies organisation is recognised. The well‐ordered and predictable facies organisation of basin‐floor mudstones suggest local point sources from active slope conduits, responsible for deposition of compensationally stacked muddy lobes. The lack of predictable facies organisation in slope mudstones suggests deposition took place in a more variable range of sub‐environments (i.e. ponded accommodation, minor gully/channel‐fills, levees). However, regional mapping of three mudstone units evidence basinward tapering and similar thicknesses across depositional strike. This geometry is consistent with the distal part of basin margin clinothems, and suggests laterally extensive mud delivery across the shelf edge combined with along‐margin transport processes. Therefore, the sedimentology and geometry of mudstones suggests that mud can be delivered to deep‐water dominantly by sediment gravity flows through point source and distributed regionally, during periods of up‐dip sand storage. These findings challenge the common attribution of deep‐water mudstones to periods of basin‐floor sediment starvation.

Diagenetic and hydrothermal fluid influence on tight sandstone reservoir quality: Gravity-flow deposits from the Huangliu Formation, Ledong area, Yinggehai Basin, South China Sea

The tight sandstone of gravity-flow deposits in Miocene Mbr 2 of the Huangliu Fm, Ledong area is one of the focuses future exploration in the Yinggehai Basin. Reservoirs have complex sedimentary and diagenesis processes, experienced multi-stage fluid influence. However, the influence of different diagenesis on reservoir quality, and distribution patterns of different diagenetic facies in Mbr 2 of the Huangliu Fm are still short in research. In this paper, a variety of methods, such as thin sections analysis, electron microscopy (SEM), electron probe micro-analyzer (EPMA), cathode luminescence (CL), x-ray diffraction measurement (XRD), in situ carbon and oxygen isotopes composition of cements (SIMS), fluid inclusion homogenization temperature, and high-pressure mercury injection (HPMI), are used to analyze reservoir quality, diagenetic evolution stages, diagenesis, fluid filling processes, etc. Studies have shown that there are obvious differences in diagenetic evolution process, including carbonate cementation, organic acid and hydrothermal dissolution, etc. Which ultimately leads to large differences in physical properties. Although sandstone texture has an impact on reservoir quality, strong compaction and early carbonate cementation were the main reasons for reservoir densification. Early calcite was precipitated in alkaline seawater, and later ferrocalcite was formed by the recrystallization of calcite and polyframboidal pyrites and euhedral pyrites formed under organic acid charged. After hydrothermal fluid charged, dolomite, iron dolomite, cement pyrites and dispersed granular pyrites precipitated. Besides, illite filled the throat. The dissolution of organic acids and late CO2-rich hydrothermal fluids improved the quality of reservoirs. According to the difference of diagenetic process and the quality of the reservoirs, Mbr 2 of the Huangliu Fm is divided into 4 types of diagenetic facies. High-quality reservoirs are mostly developed in H2-Ⅰ, H2-Ⅱ, and H2-Ⅴ sand formation near faults. Early carbonate cements were less developed, which could support primary intergranular pores. Feldspar, rock fragments, and early carbonate cements were strongly dissolved. It includes two diagenetic facies, low cementation - strong dissolution reservoirs and slightly cementation - weaken dissolution reservoirs. On the contrary, tight reservoirs mostly develop in H2-Ⅲ, H2-IV, and H2–V sand formation far away from faults, including late compacted tight reservoirs and early carbonate cementation tight reservoirs. The research results are helpful to analyze the diagenetic evolution of tight sandstone reservoirs, and conducive to the study of similar gravity-flow reservoir quality with the influence of hydrothermal fluids.

Facies and systems tracts at high-resolution in an organic-rich mudstone: The Duvernay Formation, Kaybob area, Alberta, Canada

In organic-rich mudstones, vertical sequences of sedimentary facies record the depositional responses to complex and varying controls: development of accommodation, redox conditions and multiple sources of detrital and biogenic sediments whose flux varies with relative sea-level, and reworking by bottom currents. In this study of the Upper Devonian Duvernay Formation in the Western Canada Sedimentary Basin, we utilize dense core and well log control to characterize lithofacies and develop a sequence stratigraphic model for the formation, accounting for paleogeographic features whose effects on facies varied during three complete 3rd order sea level cycles. We further identify features characteristic of mudstone sequence boundaries, describe how they vary stratigraphically and geographically, and interpret processes responsible for their formation. These observations provide a framework for interpreting vertical successions in mudstones from lower density datasets.Five lithofacies are identified, deposited by combinations of suspension settling, sediment-gravity flows, and bottom currents under anoxic to fully oxygenated bottom water conditions. Lithofacies distribution was strongly affected by proximity to sediment sources, bottom topography, and sea level cyclicity. Relatively coarse-grained, carbonate-rich, bioturbated, and organic-poor facies are common close to large carbonate platforms and reefs; silt- and sand-rich facies deposited by bottom currents and sediment gravity flows are common on the slopes of carbonate platforms; hemipelagic, fine grained, biosiliceous, organic-rich facies are common in distal areas of the basin.The Duvernay was deposited during a 2nd order transgression and early 2nd order highstand comprising three 3rd order depositional sequences and nine 4th order depositional sequences. Third order transgressive deposits are characterized by upward trends of increasingly fine-grained, biosiliceous, organic-rich facies. Third order highstand deposits are characterized by increasingly coarse-grained, carbonate-rich, bioturbated, organic-poor facies. Third order lowstand deposits are characterized by detrital clay-rich, bioturbated facies. The expressions of 3rd order sequence boundaries vary with geographic position in the basin and stratigraphic positions within the 2nd order depositional sequence. In the 2nd order transgression, sequence boundaries are expressed as scoured surfaces with coarse overlying lags that represent sediment starvation and reworking. In the 2nd order highstand, sequence boundaries are underlain by coarse beds with soft sediment-deformed bases deposited during a forced regression.

Characteristics of the Sediment Gravity Flow Triggered by Wave-Induced Liquefaction on a Sloping Silty Seabed: An Experimental Investigation

The sloping silty sediments in estuarine deltas are frequently affected by extreme storms, and they are prone to liquefaction instability. The unstable liquefied sediments of the slopes can subsequently form a sediment gravity flow (SGF), which can seriously endanger offshore engineering facilities. To better understand the characteristics and mechanism of wave-induced liquefied sediment gravity flow (WILSGF), a flume experiment was conducted to reproduce the formation, movement, and deposition processes of the WILSGF and analyze their controlling factors using natural silty sediment samples collected from the Yellow River Delta in China. The results show that the mass of the WILSGF comes from the sediment in the liquefied layer, and the movement of the WILSGF in these experiments was significantly affected by the wave orbital velocity and the relative outflow position. At the initial stage of the formation of the WILSGF, the phase and amplitude of the WILSGF were the same as those of waves, and the maximum velocity of the WILSGF reached 2.39 cm/s. The velocity of the WILSGF decreased continuously with the downward evolution of the liquefaction interface. When the liquefaction depth reached its maximum value, there was no WILSGF. We also found that the median particle size of the WILSGF was greater than that of the original seabed due to wave-induced seabed coarsening and the intrusion of ambient water. This study has guiding significance for in-depth understanding and prediction of the geological hazards caused by WILSGF.

Sedimentary Environment and Enrichment of Organic Matter During the Deposition of Qiongzhusi Formation in the Upslope Areas—A Case Study of W207 Well in the Weiyuan Area, Sichuan Basin, China

For a more in-depth analysis on the marine chemical condition during the earliest Cambrian, mechanism of organic matter enrichment and exploration potential of shale gas resources, based on the petrology, organic geochemistry, and elemental characteristics of the W207 Well, this study discuss the Early Cambrian paleo-ocean environment and factors controlling of organic matter enrichment during the deposition of black strata Qiongzhusi Formation in the upslope areas, southwestern Sichuan Basin, Yangtze Block, China. The sedimentary cycles show that during the depositon of fine-grained Qiongzhusi Fm, interactive cycles of multiple deep-water and shallow-water shelf developed, and were controlled by the rise and fall in sea level. In particular, the slope turbidite (fan) and gravity flow sediments indicate that the shallow-water shelf facies were dominant in the upslope area (SW Sichuan Basin), where not in deep water for a long time, and the sedimentary thickness of the organic rich black shale was limited. Evidence from organic geochemistry indicates that the organic matter of the Qiongzhusi Fm from the W207 Well was mainly of type I kerogen, with a high degree of thermal evolution, little residual hydrocarbon, and low capacity for hydrocarbon generation. The redox parameters indicate that the marine environment on the upslope was medium restrictive, with some upwelling. The seawater had undergone a transformation according to anoxic–oxidic. Therefore, paleo-ocean productivity in the upslope area was generally low, and exhibited a prominent downward trend from bottom to top. Given its high risk of exploration, the authors suggest that the direction of shale gas resource evaluation should change to the intracratonic sag (downslope) area, which is characterized by deep-water shelf facies.

Application of μXRF analysis on the Upper Cretaceous Mancos Shale: A comparison with ICP-OES/MS

This study investigates the elemental signature of fine-grained sedimentary rock (mudstone) obtained using energy-dispersive micro-X-ray fluorescence (μXRF). A short section (28.5 cm) of Upper Cretaceous Mancos Shale was analysed with an Itrax μXRF core scanner at 0.2 mm intervals for comparison to elemental and petrographic composition. Fifteen subsamples were collected at 2 cm intervals for ICP-OES/MS analysis to obtain absolute concentrations of elements and oxides, and thin sections were used for petrographic and microfacies analysis. The data obtained from the Itrax core scanner show a high level of reproducibility between separate scans and moderate to strong Spearman-Rho correlation (R2 > 0.6) to ICP-derived concentrations for several oxides: Fe2O3, SiO2, Al2O3, K2O, TiO2, and MnO. Common elemental ratios used in palaeoenvironmental studies (e.g., Zr/Rb, Ca/Ti, Mn/Ti) also showed good correlation to ICP-derived data when the trends were compared. The elemental signature obtained from the Itrax also showed good relationships to the petrography and lithology, with the Si/Ti ratio accurately identifying quartz-rich layers, and the Ca/Ti and Mn/Ti ratios corresponding to increased carbonate content, which are of importance in the planning of hydraulic fracturing for unconventional resources. At core scale, the elemental trends in Zr/Rb, Ti, and Si were useful for detecting short-term, rapid patterns of sedimentary events, likely resulting from storm activity (e.g., tempestites, wave-enhanced sediment gravity flows). The high-sampling resolution data from μXRF core scanners can provide advantages over traditional geochemical techniques since data aliasing is minimized. Analysing these elements and ratios in longer core sections from the Mancos Shale may serve as a proxy for determining high-frequency climatic changes in the Cretaceous Interior Seaway and allow for improved correlations of intra- and inter-basinal events in other Cretaceous basins.

Modern and early Holocene ice shelf sediment facies from Petermann Fjord and northern Nares Strait, northwest Greenland

Based on sediment cores and geophysical data collected from Petermann Fjord and northern Nares Strait, NW Greenland, an Arctic ice shelf sediment facies is presented that distinguishes sub and pro ice shelf environments. Sediment cores were collected from sites beneath the present day Petermann Ice Tongue (PIT) and in deglacial sediments of northern Nares Strait with a focus on understanding the glacial and oceanographic history over the last 11,000 cal yr BP. The modern sub ice shelf sediment facies in Petermann Fjord is laminated and devoid of coarse clasts (IRD) due to strong basal melting that releases debris (debris filtering) from the basal ice at the grounding zone driven by buoyant subglacial meltwater and entrained Atlantic Water. Laminated sediments in the deep basin proximal to the gounding zone comprise layers of fine mud formed by suspension settling from turbid meltwater plumes (plumites) interrupted by normally graded very fine sand to medium silt layers with sharp basal contacts and rip-up clasts of mud, interpreted as turbidites. An inner fjord sill limits distribution of sediment gravity flows from the grounding zone to the deep inner fjord basin, such that sites on the inner sill and beyond the ice tongue largely only comprise plumites. Bioturbation and foraminiferal abundances increase with distance from the grounding zone. The benthic foraminiferal species, Elphidium clavatum is absent beneath the ice tongue, but dominant in the turbid meltwater influenced environment beyond the ice tongue. The very sparse IRD in sediments beneath the PIT and in the fjord beyond the PIT derives mainly from englacial debris in the ice tongue, side valley glaciers, rock falls from the steep fjord walls and sea ice.We use the modern ice shelf sediment facies characteristics to infer the past presence of ice shelves in northern Nares Strait using analyses of sediment cores from several cruises (OD1507, HLY03, 2001LSSL, RYDER19). On bathymetric highs, bioturbated mud with dispersed IRD overlies a 10–15 m thick, distinctly laminated silt and clay unit with rare coarse clasts and sparse foraminifera which forms a sediment drape of nearly uniform thickness. We interpret these laminated sediments to represent glaciomarine deposition by meltwater plumes emanating from ice streams that terminated in floating ice shelves. IRD layers, shifts in sediment composition (qXRD, MS and XRF) and faunal assemblage changes in the laminated unit document periods of ice-shelf instability sometimes, but not always, coupled with grounding zone retreat. Our deglacial reconstruction, including ice shelves, begins ∼10.7 cal ka BP, with confluent ice streams grounded in Hall Basin fronted by the Robeson Channel ice shelf. Ice shelf breakup and grounding zone retreat to relatively stable grounding zones at Kennedy Channel and the mouth of Petermann Fjord was accomplished by 9.4 cal ka BP when the Hall Basin ice shelf was established. This ice shelf broke up and reformed once prior to the final break up at 8.5 to 8.4 cal ka BP marking ice stream collapse, separation of Greenland and Innuitian ice sheets, and the opening of Nares Strait for Arctic-Atlantic throughflow. The Petermann ice shelf remained in Hall Basin until the Petermann Glacier retreated from the fjord mouth ∼7.1 cal ka BP. The resilience of these northern ice streams to strong early Holocene insolation and subsurface Atlantic Water advection is attributed to their northern aspect, buttressing by narrow passages, and high ice flux from the Greenland Ice Sheet (GIS).

Exploring the predictive power of seismic geomorphology to assess sedimentary characteristics of gravity-flow deposits: examples from offshore East and West Africa reservoirs

Abstract This research seeks to test the hypothesis that the rheology of submarine gravity-flows influences the planform geometry of associated fan lobe deposits. If this control exists, seismic geomorphology has the potential to predict the sedimentary characteristics of these deposits by analysing relatively simple seismic outputs such as time structure maps and amplitude maps. This predictive power could potentially aid the identification of prospective reservoirs through automated processes of seismic interpretation. In this work, Upper Cretaceous and Neogene deep-water reservoirs from East and West Africa with porosity in excess of 20% and multi-Darcy permeability were analysed. The overall external geometry of the reservoirs was described by integrating seismic amplitude extractions, structure and isochron maps derived from high-quality 3D seismic data. The maps were generated from detailed interpretation of the top and base reservoir reflections. Core photos of the reservoirs were integrated into the seismic project and their examination allowed the identification of dominant sedimentary facies, enabling a proposal for the classification of these deposits in terms of flow processes. The research indicates that the confinement of flows exerts an obvious control on the shape of the deposits and results in sedimentary bodies with marked elongation at the seismic scale. However, analysis of the reservoirs at the core scale indicated that they are dominated by massive, poorly sorted, medium- to coarse-grained sandstones, commonly with floating oversize clasts, as well as beds with disrupted laminae and sandy conglomerates. These characteristics may indicate ‘depositional freezing’ from high-density flows with yield strength, which may also have contributed to the limited lateral spread of the deposits analysed. The research also highlights the importance of the non-unique relationship between sedimentary processes and external geometries of the resulting deposits at the seismic scale. This means that deposits with similar geometries may have markedly different sedimentary characteristics. This work highlights the increasing importance of integrating detailed seismic geomorphology studies with an accurate examination of core data to validate models of gravity flow processes and test their predictive value.

Depositional evolution and models for a deep-lacustrine gravity flow system in a half-graben rifted sag, Beibuwan Basin, South China Sea

The Paleogene Liushagang Formation is part of the Fushan Sag, a continental lacustrine basin located at theSoutheastern margin of the Beibuwan Basin, South China Sea. Further understanding of the deep-water gravityflow deposits in this formation will be conducive to lithologic reservoir exploration in the sag. In this study,three members of the Liushagang Formation, SQEls3 SQEls2 and SQEls1, from old to young, are used withcore observation, well log data, and three-dimensional seismic data to identify four deep-lacustrine gravity flowlithofacies including their vertical and lateral relationships within the depositional system. The results are thenused to establish a deep-water gravity flow depositional model. Four types of gravity flow lithofacies developed inthe sag: sandy debrite, turbidite, sandy slump, and bottom-current deposits. Sand-rich sub-lacustrine fan depositswith typical turbidite channels were developed mainly in the western depression, whereas distal isolated lobesformed by sandy debrite flow deposits occurred mainly in the eastern depression. The results obtained in this studywill be helpful in the research of gravity flows in similar continental lacustrine environments.

Implications for carbonate mass-wasting complexes induced by volcanism from Upper Cretaceous Austin Chalk strata in the Maverick Basin and San Marcos Arch areas of south-central Texas, USA

The upper part of the Austin Chalk (AC-B1 subunit), as seen in a series of cores in areas of the Maverick Basin and San Marcos Arch in south-central Texas, contains a stratigraphic section of mass-wasting complexes resulting from penecontemporaneous volcanic-induced gravity-flow deposition. The types of gravity flows are variable in composition depending on sources of volcanic or carbonate sediments available and position of initiation of the flow on the volcanic mounds.The gravity flows occurred in a deeper-water drowned-shelf setting where argillaceous chalk-rich sediments were being deposited. The mafic volcanoes are part of the Balcones Igneous Province that was active during very Late Santonian to Early Campanian time. Some of the volcanoes formed mounds that created elevated areas in the photic zone or above sea level. In the shallow-water photic zones, carbonate factories formed as atolls and shoals. These carbonate factories were a source for shallow-water allochems, and along with the volcanic debris, constituted gravity flows that were likely triggered by volcanic-associated earthquakes. As gravity flows moved downslope, they eroded deeper-water, bottom sediments, which were incorporated into the flows. The two major transport mechanisms were debris flows and hyperconcentrated-density flows.The debrites appear as two types: those dominated by mud clasts and those dominated by cm-scale skeletal fragments such as inoceramid or disarticulated oyster shells. The clasts in the mud-clast-dominant debrites range in rigidity from very soft (i.e., easily deformed) to firm (i.e., resist deformation and preserved burrows). Since the mud clasts are composed of planktic foraminifers and calcispheres in a coccolith-hash matrix, they are assumed to have originated from the deeper flanks of the volcanic mounds. The rigidity of the mud clasts are likely related to initial (i.e., early) depth of burial of the unlithified mud.The skeletal-dominated debrites with disarticulated oyster shells appear to have been sourced from the upper slopes of the volcanic mounds and underwent little abrasion during debris-flow transportation. The inoceramid-fragment-dominated debrites may have been initiated lower on the volcanic mounds where these low-oxygen-tolerating bivalves would dominate the macrofauna.The hyperconcentrated-density-flow deposits are grain-rich in skeletal fragments with a mixture of shallow- and deep-water biota, suggesting some direct sourcing from shallow-water shoals and later incorporation of deeper-water sediments from the flanks of the volcanic mounds. These hyperconcentrated-density-flow deposits are positioned close to the base of the mounds.This distinct stratigraphic interval varies greatly from the rest of the Austin Chalk section in the Maverick Basin and San Marcos Arch areas. Deeper-water chalk deposition was the general background environmental setting, but it was disrupted by this period of volcanism that initiated numerous mass-wasting complexes punctuating the deeper-water chalk deposits. Within the gravity-flow deposits, volcanic components such as lapilli, ash, and glauconite-altered volcanic grains occurred along with both shallow- and deep-water carbonate allochems. Investigation of this stratigraphic unit within the Austin Chalk provides an analog for mass-wasting complexes associated with active volcanism and explains the variations in composition of the gravity-flow deposits.