Hybrid Event Beds Research Articles

Hybrid event bed character and distribution in the context of ancient deep‐lacustrine fan models

AbstractHybrid event beds are texturally and compositionally‐diverse deposits preserved within deepwater settings. They are deposited by flows exhibiting ‘mixed behaviour’, forming complex successions of sandstone and mudstone, which are often challenging to predict. Hybrid event beds are documented in deep‐marine settings, where they have been thoroughly characterized, and are well‐known as effective fluid transmissibility barriers and baffles in reservoirs. By comparison, there are far‐fewer studies of hybrid event beds from deep‐lacustrine settings, where their character and distribution remains relatively under‐explored. In order to provide insights into these deposits, this study presents the detailed analysis of three‐dimensional seismic data, wireline logs and core from a series of ancient deep‐lacustrine fan systems in the North Falkland Basin. Results confirm that deep‐lacustrine hybrid event beds comprise the same idealized sequence of the ‘H1–H5’ divisions. However, in this study H3 ‘debrite’ units can be sub‐divided into ‘H3a–H3c’, based on: sharp or erosional intra‐H3 contacts, bulk lithology, mud‐content and discrete sedimentary textures. This study interprets the H3a–H3c sub‐units as the products of multiple flow components formed through significant rearward longitudinal flow transformation processes, during the emplacement of a single hybrid event bed. Hybrid event beds are observed within lobe fringes, where flow types, energies and transport mechanisms diversify as a result of flow transformation. The temporal context of hybrid event bed occurrences is considered in relation to stages of fan evolution, including: the Initiation; Growth (I); Growth (II); By‐pass; Abandonment; and Termination phases. Hybrid event beds are mainly found in either the initiation phase where flow interaction and erosion of initial substrates promoted mixed flow behaviour, or in the abandonment phase as facies belts retreated landward. The results of this study have important implications in terms of flow processes of hybrid event bed emplacement, in particular sub‐division of the H3 unit, as well as the prediction of hybrid event bed occurrence and character within ancient deep‐lacustrine fan settings, in general.

Depositional elements and evolution of gravity-flow deposits on Lingshan Island (Eastern China): An integrated outcrop-subsurface study

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution.

Depositional Models of Deep-Water Gravity-Flow in Lacustrine Basin and Its Petroleum Geological Significance—A Case Study of Chang 6 Oil Group in Heshui Area, Ordos Basin, China

Gravity-flow can carry a large number of sediments and organic matters from shallow water to deep lakes with its strong transporting energy, directly or indirectly facilitating the formation of deep-water tight reservoirs and shale reservoirs. Therefore, studying the genetic types, dynamic mechanisms, and depositional models of gravity-flow deposits is essential in the exploration of unconventional petroleum in large lacustrine basins. This research studied the genetic types, dynamic mechanisms, and sedimentary models of the gravity-flow deposits of the Chang 6 oil group in the Heshui Area, Ordos Basin, China, aiming to reveal its petroleum geological significance. Core observation, microscopic thin section identification, particle size analysis, and determination of rare earth elements were carried out. As a result, three types of gravity-flow deposits are detected, namely, slide-slump, sandy debris flow, and turbidity current. A certain slope gradient in bed form is the necessary geomorphic condition for gravity flow formation, and determines its development level, distribution range, and flow transformation efficiency. Sufficient provenance lays the material foundation and determines its depositional composition and development type. Other factors include earthquakes, volcanoes, and floods, which serve as triggering forces. In addition, fragmentation, liquefaction, and fluid mixing are the main dynamic mechanisms driving flow transformation. Based on the flow type of gravity flow, particle size characteristics, gravity-flow transformation relations, development mechanism, and spatial distribution pattern, we distinguished two depositional gravity-flow models, i.e., slump turbidite body and sublacustrine fan. Re-portrait the spatial distribution of deep-water gravity flow in the study area. From the perspective of sedimentology, explain the genesis of sand bodies in the northeast and southwest. The sandy debris flow in the middle fan braided channel microfacies of the sublacustrine fan sways the development of thick massive sand bodies in the study area. Hybrid event beds formed by the fluid transformation in a slump turbidite are the potential dessert area for deep-water tight oil and gas.

Sedimentary structures associated with hybrid event beds from the Gull Island Formation (Pennsylvanian) of north Clare, Ireland

A previously unreported assemblage of sedimentary structures, including several types of ripple, chevron marks, ridge and furrows, plant-rich laminations, found in association with hybrid event beds, with related muddy debrites and the trace fossil Kouphichnium, is described from a short 10m section through the middle of the Gull Island Formation [GIFm] (Pennsylvanian) of northern County Clare, Ireland. The section exposes a sequence of thickening and coarsening-upwards gravity flow deposits and interbedded mudstone layers. The upper part of the section is dominated by thick, massive and amalgamated sandstone beds; the middle section contains thinner sandstone beds with debritic mudstone caps, while the lowermost section comprises thin beds with a range of sedimentary structures produced by low density, transitional flows influenced by mud content, including mud ripples. 'High-density' turbidity currents operating in a delta front setting are inferred to have transformed to hybrid event beds with dilute but mud-rich suspensions travelling further distally to emplace thin beds, with evidence for clay-induced transitional flow and an assemblage of sedimentary structures not previously described from the GIFm. Small-scale ripple foresets from the upper part of the lower section indicate a southerly component of palaeocurrent flow direction. Chevron marks show size and palaeo-flow direction variation, suggesting temporal changes in current flow. Ridge and furrows have previously been interpreted as being formed under the head of turbidity current; however, the scale of the ridge and furrows described here suggest that interpretation is unlikely and some other unknown mechanism is involved. The presence of Kouphichnium trackways is significant as Carboniferous examples of this ichnofossil are usually recorded from very shallow marine or marginal marine environments. This part of the GIFm in north Clare is interpreted as a shallowing, delta front lobe environment exhibiting an upward transition from marginal or distal lobe to proximal or axial lobe sub-environment. This study highlights the influence of flow transformation in a muddy delta front environment with a high level of sediment supply favouring sediment remobilisation and the generation of a distinctive suite of sedimentary structures.

Sedimentary Structures Associated With Hybrid Event Beds From the Gull Island Formation (Pennsylvanian) Of North Clare, Ireland

A previously unreported assemblage of sedimentary structures, including several types of ripple, chevron marks, ridge and furrows, plant-rich laminations, found in association with hybrid event beds, with related muddy debrites and the trace fossil Kouphichnium, is described from a short 10m section through the middle of the Gull Island Formation [GIFm] (Pennsylvanian) of northern County Clare, Ireland. The section exposes a sequence of thickening and coarsening-upwards gravity flow deposits and interbedded mudstone layers. The upper part of the section is dominated by thick, massive and amalgamated sandstone beds; the middle section contains thinner sandstone beds with debritic mudstone caps, while the lowermost section comprises thin beds with a range of sedimentary structures produced by low density, transitional flows influenced by mud content, including mud ripples. ‘High-density’ turbidity currents operating in a delta front setting are inferred to have transformed to hybrid event beds with dilute but mud-rich suspensions travelling further distally to emplace thin beds, with evidence for clay-induced transitional flow and an assemblage of sedimentary structures not previously described from the GIFm. Smallscale ripple foresets from the upper part of the lower section indicate a southerly component of palaeocurrent flow direction. Chevron marks show size and palaeo-flow direction variation, suggesting temporal changes in current flow. Ridge and furrows have previously been interpreted as being formed under the head of turbidity current; however, the scale of the ridge and furrows described here suggest that interpretation is unlikely and some other unknown mechanism is involved. The presence of Kouphichnium trackways is significant as Carboniferous examples of this ichnofossil are usually recorded from very shallow marine or marginal marine environments. This part of the GIFm in north Clare is interpreted as a shallowing, delta front lobe environment exhibiting an upward transition from marginal or distal lobe to proximal or axial lobe sub-environment. This study highlights the influence of flow transformation in a muddy delta front environment with a high level of sediment supply favouring sediment remobilisation and the generation of a distinctive suite of sedimentary structures.

Facies Analysis and Sedimentary Architecture of Hybrid Event Beds in Submarine Lobes: Insights from the Crocker Fan, NW Borneo, Malaysia

Hybrid event beds represent the combined effect of multiple geological processes, which result in complex depositional geometries and distinct facies distribution in marine environments. Previous work on hybrid event beds highlights the classification, origin, and types of hybrid facies. However, in the present study, we discuss the development of hybrid event beds in submarine lobes with an emphasis on the analysis of proximal to distal, frontal to lateral relationships and evolution during lobe progradation. Detailed geological fieldwork was carried out in the classical deep-marine Late Paleogene Crocker Fan to understand the relationship between the character of hybrid bed facies and lobe architecture. The results indicate that hybrid facies of massive or structureless sandstone with mud clasts, clean to muddy sand, and chaotic muddy sand with oversized sand patch alternations (H1–H3) are well developed in proximal to medial lobes, while distal lobes mainly contain parallel to cross-laminated clean to muddy hybrid facies (H3–H5). Furthermore, lateral lobes have less vertical thickness of hybrid beds than frontal lobes. The development of hybrid beds takes place in the lower part of the thickening upward sequence of lobe progradation, while lobe retrogradation contains hybrid facies intervals in the upper part of stratigraphy. Hence, the development of hybrid beds in submarine lobe systems has a significant impact on the characterization of heterogeneities in deep-marine petroleum reservoirs at sub-seismic levels.

Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

ABSTRACTDeposits of sediment gravity flows in the Aberystwyth Grits Group (Silurian, west Wales, United Kingdom) display evidence that sole marks are suitable for reconstructing depositional processes and environments in deep-marine sedimentary successions. Based on drone imagery, 3D laser scanning, high-resolution sedimentary logging, and detailed descriptions of sole marks, an outcrop 1600 m long between the villages of Aberarth and Llannon was subdivided into seven lithological units, representing: a) mudstone-poor, coarse-grained and thick-bedded submarine channel fills, dominated by the deposits of erosive high-density turbidity currents with flute marks; b) mudstone-rich levee deposits with thin-bedded, fine-grained sandstones formed by low-density turbidity currents that scoured the bed to form flute marks; c) channel–lobe transition-zone deposits, dominated by thick beds, formed by weakly erosive, coarse-grained hybrid events, with pronounced mudstone-rich or sandstone-dominated debritic divisions and groove marks below basal turbiditic divisions, and with subordinate amounts of turbidites and debris-flow deposits; d) tabular, medium- to thick-bedded turbiditic sandstones with flute marks and mixed sandstone–mudstone hybrid event beds mainly with groove marks, interpreted as submarine lobe-axis (or off-axis) deposits; and e) tabular, thin- to medium-bedded, fine-grained, mainly turbiditic sandstones mostly with flute marks, formed in a lobe-fringe environment. Both lobe environments also comprised turbidites with low-amplitude bed waves and large ripples, which are interpreted to represent transient-turbulent flows. The strong relationship between flute marks and turbidites agrees with earlier predictions that turbulent shear flows are essential for the formation of flute marks. Moreover, the observation as part of this study that debris-flow deposits are exclusively associated with groove marks signifies that clay-charged, laminar flows are carriers for tools that are in continuous contact with the bed. A new process model for hybrid event beds, informed by the dominance of tool marks, in particular grooves, below the basal sand division (H1 division of Haughton et al. 2009) and by the rapid change from turbidites in the channel to hybrid event beds in the channel–lobe transition zone, is proposed. This model incorporates profound erosion of clay in the channel by the head of a high-density turbidity current and subsequent transformation of the head into a debris flow following rapid lateral flow expansion at the mouth of the channel. This debris flow forms the groove marks below the H1 division in hybrid event beds. A temporal increase in cohesivity in the body of the hybrid event is used to explain the generation of the H1, H2, and H3 divisions (sensuHaughton et al. 2009) on top of the groove surfaces, involving a combination of longitudinal segregation of bedload and vertical segregation of suspension load. This study thus demonstrates that sole marks can be an integral part of sedimentological studies at different scales, well beyond their traditional use as indicators of paleoflow direction or orientation.

Evolution from syn‐rift carbonates to early post‐rift deep‐marine intraslope lobes: The role of rift basin physiography on sedimentation patterns

ABSTRACTThe stratigraphic architecture of Early Jurassic strata exposed along a >10 km long transect in the Chachil Graben, an exhumed marine rift depocentre in the Neuquén Basin (Argentina), provides insights into the sedimentological and stratigraphic expression of the syn‐rift to post‐rift transition. A change from syn‐rift intrabasinal carbonate to post‐rift extrabasinal siliciclastic sedimentation is recorded, as well as variations in sediment supply and dispersal patterns across rift‐related topography. The late syn‐rift was marked by a transgression and development of a shallow‐marine carbonate system, including carbonate platform deposits perched on fault‐block highs and periplatform deposits accumulated in fault‐block lows, which overlies continental volcano‐sedimentary syn‐rift deposits. Differential subsidence and basin deepening induced retrogradation of the carbonate system, which was progressively drowned and overlain by organic‐rich calcareous mudstone that draped across rift structures at the onset of the early post‐rift. The first extrabasinal siliciclastic influx led to progradation of an early post‐rift intraslope lobe complex into the graben, which is associated with kilometre‐scale clastic injectites. The depositional architecture, facies distribution and pinch‐out style of intraslope lobes record the effects of an inherited compaction hinge, which acted as an oblique counterslope to sediment gravity flows. The occurrence of combined‐flow bedforms, widespread erosion, and limited facies segregation across lobes bearing different hybrid event bed types, is in sharp contrast to sedimentological characteristics of existing intraslope lobe models. Documentation of the syn‐rift to post‐rift transition stratigraphy permitted identification of changes in thickness and facies resulting from the passive infill of inherited topography with early post‐rift differential compaction. This architecture contrasts markedly with those developed during syn‐rift normal faulting. Furthermore, the influence of local inherited topography on the development of early post‐rift lobes is key to improve subsurface prediction of sandstone distribution and quality during assessment of hydrocarbon reservoirs and carbon storage sites.

Mud-forced turbulence dampening facilitates rapid burial and enhanced preservation of terrestrial organic matter in deep-sea environments

Hybrid event beds (HEBs) are tiered deep-water deposits that can include significant intervals of organic-rich muddy sandstone and sandy mudstone. They are emplaced by decelerating sediment gravity flows in which turbulence becomes extinguished due to increasing cohesion. Whilst several studies have addressed the distribution of organic matter (OM) in turbidites, the extent to which OM is segregated between the component HEB divisions has yet to be quantitatively addressed for clastic systems. Here we document bed scale fractionation of terrestrial OM in HEBs drawn from a range of deep-water sub-environments (basin floor sheets, outer fan fringes and mid-fan lobes) preserved in the Ross Sandstone Formation, western Ireland, a tropical Pennsylvanian deep-water fan complex. A suite of bulk geochemical techniques (TOC, Rock-Eval pyrolysis, δ13C isotopes, and XRF scanning) and petrographic analyses were applied to HEB-dominated core intervals retrieved from four behind-outcrop boreholes. Results shows that muddy sandstones (H3 divisions) of the HEBs have significantly higher OM (average TOC = 1.2 wt% and up to 2.7 wt%) and mud contents than co-genetic cleaner sandstones (H1 divisions; average TOC = 0.2 wt% and up to 0.7 wt%). Muddy caps (H5 divisions) to the event beds have higher mud but relatively low OM contents (average TOC = 0.7 wt% and up to 1 wt%) compared to H3 divisions, implying textural fractionation of OM components, greater burn-down linked to slower suspension settling and/or downward propagation of oxidation fronts. HEBs can dominate distal lobe stratigraphy and are thus an important but under-represented sink for terrestrial carbon with enhanced preservation of OM in H3 divisions on account of rapid en-masse deposition, high mud abundances offering enhanced protection, and where thick, emplacement beyond the reach of oxidation fronts descending from the sea floor. The present study has important implications for understanding how carbon is buried and distributed in deep-water successions.

Hybrid event beds generated by erosional bulking of modern hyperpycnal flows on the Choshui River delta front, Taiwan Strait

AbstractHyperpycnal flows are important agents for transporting detrital sediments from rivers to oceans. Previous studies often assumed that the deposits of flood‐controlled delta fronts would be dominated by graded sand with common hummocky cross‐stratification. This study documents, for the first time, hybrid event beds and plumite deposits inside the succession of a modern delta front. This delta in the Taiwan Strait is fed by the Choshui River, which is considered a highly efficient sediment transport system with individual floods with extremely high suspended sediment concentrations. The deltaic event beds recorded in the core were interpreted as hyperpycnal flow‐generated hybrid event beds and turbidites triggered by hypopycnal flows. Accelerator Mass Spectrometry 14C dating, grain‐size analysis and measurements of stable isotopic composition of organic matter were conducted to delineate the depositional process of the recognized beds. The internal subdivisions of the hybrid event beds were differentiated mainly based on sedimentary textures, including cohesive mud content, sand content and sorting. The disorganized portion (division H3) appears internally chaotic and contains large rafted substrate clasts but also displays an upward increase in dispersed cohesive mud from 35% (division H3a) to 50% (division H3b). In contrast, massive H1 divisions are characterized by much lower cohesive mud of ca 8%. The vertical arrangement between depositional facies allows the discrimination of three hybrid event bed types. The stable carbon isotopic composition of the organic matter reveals that the cohesive mud in each division of the hybrid event beds was sourced from marine substrate, rather than supplied by the original hyperpycnal flows. Therefore, the hybrid event beds are generated by energetic hyperpycnal flows, which can delaminate the muddy sea‐floor and incorporate large quantities of substrate fragments. The bulking of erosional hyperpycnal turbulent flow forces flow transformation and generates a more cohesive flow with a turbulent, dense head and trailing bipartite debris flow. The occurrence of hybrid event beds in modern shallow delta front settings is also an important novelty, implying a high density of hyperpycnal flows at the mouth of sediment‐laden rivers as well as the crucial impact of substrate entrainment on the development of hybrid event beds.

Hybrid event bed distribution in a mixed siliciclastic-calcareous turbidite succession: a cross-current perspective from the Bordighera Sandstone, Ligurian Alps, NW Italy

Transitional flow deposits (TFDs) and hybrid event beds (HEBs) form part of the high-density sediment gravity flow spectrum. They record flow characteristics intermediate between fully turbulent and laminar flow states. The transformation from initially frictional into transitional or quasi-laminar cohesive flow behaviour results from enhanced detrital clay concentrations that dampen flow turbulence, triggering the development of an argillaceous sandstone interval that displays characteristics of a cohesive debris flow deposit. Since excess detrital clay proportions reduce primary porosity and permeability, the presence of TFDs and HEBs hinders turbidite sandstone reservoir properties. In applied scenarios, understanding their spatial distribution is crucial for avoiding well misplacement. Spatial distribution models for TFDs and HEBs largely document them as a characteristic feature of distal and lateral outer fan environments. These models are compared with results from facies analysis conducted on a dataset derived from the detailed investigation of three stratigraphic sections, acquired along a cross-current transect of the distal depositional domain of the Bordighera Sandstone (BGS). In its distal domain, the sand-rich Bordighera turbidite system is characterized by direct interbedding with calcareous fine-grained turbidites of the Sanremo Helminthoid Flysch, and features high proportions of HEBs and TFDs. Analysis of spatial trends in facies distribution allows the detailed allocation of intra- bed- to sandbody-scale heterogeneity to different axial and off-axis lobe sub-environments which are determined through quantitative sedimentological parameters. Results reveal that in the Bordighera turbidite system argillaceous sandstone bed types are not limited to lobe-fringe environments, but instead are primarily occurring in more proximal high net-to-gross zones of the largely unconfined terminal lobe domain. They account for more than 60% of stratigraphic thickness in axial zones (net-to-gross ratio = 0.87), whereas their proportions decrease towards off-axis lobe (36%; net-to-gross = 0.58) and marginal lobe to lobe fringe domains (23%; net-to-gross = 0.49).

Bed type and flow mechanism of deep water sub-lacustrine fan fringe facies: an example from the Middle Permian Lucaogou Formation in Southern Junggar Basin of NW China

Submarine or sub-lacustrine lobe deposits are important reservoirs, but the fan fringe deposits form heterogeneities within deep water fan deposits. Fan fringe facies records the complex sediment gravity flow types. By understanding of the bed types and flow mechanisms, we can identify the fan fringe deposit, which aids in the reconstruction of deep water fan and reservoir evaluations. The Jiucaiyuanzi and Dalongkou sections in the West Bogda Mountains preserve well-exposed 536-m and 171-m thick successions, respectively, of a deep water lacustrine depositional system from the Middle Permian Lucaogou Formation. Bed types of the Lucaogou Formation include high-density turbidite, low-density turbidite, incomplete Bouma-type turbidite, hybrid event beds, and slump deposits. The Lucaogou Formation is interpreted here as a fan fringe facies due to the thin bed thickness that characterize turbidites and hybrid event beds, as well as the predominance of the isolated sheet architecture. Previous studies suggest that these deposits were considered as deposited in a deep water setting due to the absence of wave-related structures. The presence of abundant mud clasts in massive medium-coarse grained sandstone beds reflects the significant erosional capability and interactions between high-density turbidity currents and lake floor. The fan fringe facies here contains amalgamated and thick-bedded homolithic facies (~ 30%) and thin-bedded heterolithic facies (~ 70%). The examination of the bed type is of wider significance for facies prediction and reservoir heterogeneity in the sub-lacustrine fan fringe facies.

Shape difference of mud clasts depending on depositional facies: application of newly modified elliptic Fourier analysis to hybrid event beds

ABSTRACT Hybrid event beds are the deposits from sediment gravity flows that change their rheological behavior through their passage, entraining muddy sediments and damping turbulence. Muddy facies of hybrid event beds are often associated with abundant mud clasts which show a wide variety of size and shape. The variation of clast occurrence in hybrid event beds is expected to preserve the information of entrainment and transport processes of muddy sediments in submarine density currents. However, previous analyses of hybrid event beds have focused on describing the overall clast occurrence rather than the statistical size and shape analyses because traditional shape parameters are incapable of characterizing the complex shape of mud clasts. Here, a new quantitative grain-shape analysis of mud clasts is conducted and allows visualization of the spatial variation of clast size and shape, which suggests the wide variety of origin and transport systems of entrained mud clasts. This new method revises the traditional elliptic Fourier analysis, substituting Fourier power spectra (FPS) for traditional elliptic Fourier descriptors to overcome the mirror-wise shape problem. Further, principal-component analysis is shown to capture significant shape attributes more effectively than traditional shape parameters. The proposed method is applied to mud clasts in sediment-gravity-flow deposits in the lower Pleistocene Otadai Formation, central Japan. Results imply that there are distinctive shape and size differences of mud clasts that are strongly associated with depositional facies rather than the distance from the source. The clasts have a higher angularity than other facies in the debrite intervals in hybrid event beds. It is also shown that clasts in sandy, structureless facies have different characteristics in shapes based on elongation and convexity compared to laminated facies. Comparison between different shape-analysis methods demonstrates that none of the traditional methods are able to visualize these trends as effectively as the method presented herein. These results highlight the importance of the quantitative shape analysis of sediment grains and the effectiveness of FPS-based elliptic Fourier analysis.

An integrated process‐based model of flutes and tool marks in deep‐water environments: Implications for palaeohydraulics, the Bouma sequence and hybrid event beds

AbstractFlutes and tool marks are commonly observed sedimentary structures on the bases of sandstones in deep‐water successions. These sole structures are universally used as palaeocurrent indicators but, in sharp contrast to most sedimentary structures, they are not used in palaeohydraulic reconstructions or to aid prediction of the spatial distribution of sediments. Since Kuenen's famous 1953 paper, flutes and tool marks in deep‐water systems have been linked to turbidity currents, as reflected in the standard Bouma sequence taught to generations of geologists. Yet, these structures present a series of unaddressed enigmas. Detailed field studies in the 1960s and early 1970s observed that flutes are typically associated with thicker, more proximal beds, whilst tools are generally prevalent in thinner, more distal, beds. Additionally, flutes and tool marks are rarely observed on the same surfaces, and flutes are seen to change downstream from larger wider parabolic to smaller narrower spindle‐shaped forms. No model has been proposed that explains these field‐based observations. This contribution undertakes a radical re‐examination of the formative flow conditions of flutes and tool marks, and demonstrates that they are the products of a wide range of sediment gravity flows, from turbulent flows, through transitional clay‐rich flows, to debris flows. Flutes are not solely the product of turbulent flows, but can continue to form in transitional flows. Grooves are shown to be formed bydebris flows, slumps and slides, not turbidity currents, and in many cases the debris flows are linked to the debritic component of hybrid flows. Discontinuous tool marks, including skim (bounce) marks, prod marks and skip marks, are shown to be formed by transitional mud‐rich flows. Consequently, the observed spatial distribution of flutes and tool marks can be explained by a progressive increase in flow cohesivity downstream. This model of flutes and tool marks dovetails with models of hybrid flows that predict such a longitudinal increase in flow cohesivity. However, some deposits show grooves preferentially associated with BoumaTAbeds, and these are likely formed by flows transforming from higher to lower cohesion, and are present in basins where hybrid beds are absent or rare. The recognition that sole structures may haveno genetic linkto the later overlying turbidity current deposits, and can be formed by a wide range of flow types, indicates that the existing pictorial description of the Bouma sequenceis incorrect. A modified Bouma sequence is proposed here that addresses these points. In utilizing the advances in fluid dynamics since Kuenen's pioneering research, this study demonstrates that it is possible to use flutes and tool marks to interpret flow type at the point of formation, the nature of flow transformations, and the mechanics of the basal layer. These advances suggest that it is then possible to predict the nature of deposit type down‐dip. This new understanding, in combination with further testing in outcrop of the proposed relationships between sole marks and palaeohydraulics, opens up a wealth of possibilities for improving the understanding of deep‐water clastic environments, with implications for developing more complete facies models, assessing subaqueous geohazards and the resilience of seafloor infrastructure, and advancing our understanding of deep‐water sediments as archives of palaeoenvironmental change.

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.

High‐resolution X‐ray fluorescence profiling of hybrid event beds: Implications for sediment gravity flow behaviour and deposit structure

AbstractHybrid event beds form when turbidity currents that transport or locally acquire significant quantities of mud decelerate. The mud dampens turbulence driving flow transformations, allowing both mud and sand to settle into dense, near‐bed fluid layers and debris flows. Quantifying details of the mud distribution vertically in what are often complex tiered deposits is critical to reconstructing flow processes and explaining the diverse bed types left by mud‐bearing gravity flows. High‐resolution X‐ray fluorescence core scanning provides continuous vertical compositional profiles that can help to constrain mud distribution at sub‐millimetre scale, offering a significant improvement over discrete sampling. The approach is applied here to cores acquired from the Pennsylvanian Ross Sandstone Formation, western Ireland, where a range of hybrid event beds have been identified. Raw X‐ray fluorescence counts are calibrated against element concentrations and mineral abundances determined on coincident core plugs, with element and element log‐ratios used as proxies to track vertical changes in abundances of quartz, illite (including mica), chlorite and calcite cement. New insights include ‘stepped’ (to higher values) as opposed to ‘saw‐tooth’ vertical changes in mud content and the presence of compositional banding that would otherwise be overlooked. Hybrid event beds in basin floor sheets that arrived ahead of the prograding fan system have significantly cleaner sandy components than those in mid‐fan lobes. The latter may imply that the heads of the currents emerging from mid‐fan channels entrained significant mud immediately before they collapsed. Many of the H3 debrites are bipartite with a sandier H3a division attributed to re‐entrainment and mixing of a trailing debris or fluid mud flow (H3b) with sand left by the forward part of the flow. Hybrid event bed structure may thus partly reflect substrate interaction and mixing during deposition, and the texture of the bed divisions may not simply mirror those in the suspensions from which they formed.

Origin of mud in turbidites and hybrid event beds: Insight from ponded mudstone caps of the Castagnola turbidite system (north‐west Italy)

AbstractThe partitioning of different grain‐size classes in gravity flow deposits is one of the key characteristics used to infer depositional processes. Turbidites have relatively clean sandstones with most of their clay deposited as part of a mudstone cap or as a distal mudstone layer, whereas sand‐bearing debrites commonly comprise mixtures of sand grains and interstitial clay; hybrid event beds develop alternations of clean and dirty (clay‐rich) sandstones in varying proportions. Analysis of co‐genetic mudstone caps in terms of thickness and composition is a novel approach that can provide new insight into gravity flow depositional processes. Bed thickness data from the ponded Castagnola system show that turbidites contain more clay overall than do hybrid event beds. The Castagnola system is characterized by deposits of two very different petrographic types. Thanks to this duality, analyses of sandstone and mudstone composition allow inference of which proportion of the clay in each of the deposit types was acquired en route. In combination with standard sedimentological observations the new data allow insight into the likely characteristics of their parent flows. Clean turbidites were deposited by lower concentration, long duration, erosive, muddy turbidity currents which were more efficient at fractionating clay particles away from their basal layer. Hybrid event beds were deposited by shorter duration, higher‐concentration, less‐erosive sandier flows which were less efficient at clay fractionation. The results are consistent with data from other turbidite systems (for example, Marnoso‐arenacea). The approach represents a new method to infer the controls on the degree of clay partitioning in gravity flow deposits.

Facies variability within a mixed carbonate–siliciclastic sea-floor fan (upper Wolfcamp Formation, Permian, Delaware Basin, New Mexico)

Sea-floor fans containing both carbonate and siliciclastic detritus are rarely recognized in the rock record but are common within the Permian upper Wolfcamp Formation in the Delaware Basin in southeast New Mexico and west Texas. One such compositionally mixed fan has been studied in detail as part of an unconventional oil and gas exploration and development program in southeast New Mexico. Three cores were collected through an approximately 100-m (∼350-ft)-thick interval that defines the structurally controlled fan. Cores collected represent the frontal to distal fringe, off-axis, and lateral fringe parts of the fan. Unlike siliciclastic fans where axial facies are characterized more by turbidites, the axis, off-axis, and lateral fringe cores in the Wolfcamp are dominated by carbonate debrites. Coarse carbonate deposition decreases toward the frontal fringe areas where core facies are composed of mixed carbonate–siliciclastic mud-rich hybrid event beds and background sedimentation. The core through the lateral fringe differs from the off-axis core in that the debrites in the lateral fringe are thinner and commonly rheologically stratified with finer grained debrites sitting directly on top of coarse-grained debrites, suggesting a genetic link in their formation. The axial facies appear to be dominated by thick amalgamated debrites. Changes in carbonate content from axis to frontal fringe appear to be gradual but rapid from axis to lateral fringe over a 3-km (2-mi) distance. The carbonate-rich part of the fan trends at least 55 km (34 mi) in a northeast-southwest direction and extends 18 km (11 mi) northwest-southeast across.

Formation of detrital clay grain coats by dewatering of deep-water sands and significance for reservoir quality

ABSTRACTWell-developed detrital clay grain coats are observed in deep-marine sandstones of the Upper Cretaceous Springar Formation of the Vøring Basin in the Norwegian Sea. The detrital clay coats form thin and compact rims on individual sand grains and meniscus-shaped bridges between grains. These well-developed coats are found in high-density turbidites and proximal hybrid event beds with common to pervasive dewatering structures deposited in proximity to the base of a syndepositionally active basin high. Here, in one exploration well, detrital clay grain coats are common throughout a sandstone package 100 m thick. High-density turbidites and proximal and distal hybrid event beds drilled in mid- to distal-fan settings unaffected by seismically resolved seafloor topography show common dewatering features, but have only scattered detrital clay coats confined to individual dewatering pipes or dish structures. Hence, we propose that intense sediment dewatering has the potential to form detrital clay coats in deep-marine sandstones by a combination of elutriation and reorganization of clays during fluid escape from sediment bodies with pore fluid pressures significantly higher than the hydrostatic pressure. In submarine fan systems, deposition of sediment with coeval trapping of large volumes of interstitial pore fluid is most likely to occur where gravity flows undergo rapid deceleration in response to an abrupt decrease in confinement or gradient. Such environments include the channel–lobe transition and settings in proximity to seabed topography.The investigated sandstones are quartz arenites and subarkoses, with minor to moderate volumes of quartz cement (up to 6%). However, strongly to completely quartz-cemented intergranular pore space is observed where detrital clay coats or matrix does not cover quartz grains in the deepest part of the studied formation. Modeling of quartz cementation predicts that most intergranular macroporosity in the lower part of the Springar Formation would be quartz cemented if the sandstones were free of detrital clays. Based on our observations and modeling results we propose that intense sediment dewatering has the potential to form detrital clay coats, which can be important for retaining porosity in deeply buried sandstones and in basins with high present or past heat flow.

Trigger mechanisms of gravity flow deposits in the Lower Cretaceous lacustrine rift basin of Lingshan Island, Eastern China

Gravity flows were commonly developed in subaqueous, steep slopes in lacustrine basins, which show distinct sedimentary records due to different flow behaviors and trigger mechanisms. Outcrops of the Lower Cretaceous deposits on the Lingshan Island are well exposed to illustrate flow transformations between turbidity currents and debris flows in shallow water and basin floor in a lacustrine basin. Outcrop sections were carefully measured to characterize sedimentary records of the gravity flows. Facies associations include laminated mudstone (FA1), deformed silty mudstone (FA2), massive very-fine sandstone with rip-up mud clasts (FA3), graded very-fine sandstone with (partial) Bouma sequences (FA4), massive gravelly sandstone with rip-up mud clasts (FA5), gravelly sandstones with partial Bouma sequences (FA6), and cross-stratified pebbly coarse-grained sandstones (FA7). Trigger mechanisms for the gravity flow include mainly plunging of hyperpycnal river discharges and earthquakes, which both induced failure of delta front/slope and the consequent debris flows/turbidity currents. Flow transformations occurred both at the shallow water and basin floor, which produced distinct sedimentary structures, stacking patterns, and architectural units. The high-density hybrid event beds (HEBs) with both gravels and mud clasts overlaid by sandy turbidites were mainly induced by floodwater from the active river mouths of deltas, generating fast-moving debris flows and deceleration of turbidity currents. Damping of turbidity currents and deceleration of muddy debris flows due to high cohesion induced low-density turbidites were overlaid by HEBs, showing thin-bedded, lateral extended sandy beds on the basin floor.