Compensational Stacking Research Articles

Does fluvial channel-belt clustering predict net sand to gross rock volume? Architectural metrics and point-pattern analysis of a digital outcrop model

ABSTRACTSpatial point-pattern analyses (PPAs) are used to quantify clustering, randomness, and uniformity of the distribution of channel belts in fluvial strata. Point patterns may reflect end-member fluvial architecture, e.g., uniform compensational stacking and avulsion-generated clustering, which may change laterally, especially at greater scales. To investigate spatial and temporal changes in fluvial systems, we performed PPA and architectural analyses on extensive outcrops of the Cretaceous John Henry Member of the Straight Cliffs Formation in southern Utah, USA. Digital outcrop models (DOMs) produced using unmanned aircraft system-based stereophotogrammetry form the basis of detailed interpretations of a 250-m-thick fluvial succession over a total outcrop length of 4.5 km. The outcrops are oriented roughly perpendicular to fluvial transport direction. This transverse cross-sectional exposure of the fluvial system allows a study of the system's variation along depositional strike. We developed a workflow that examines spatial point patterns using the quadrat method, and architectural metrics such as net sand to gross rock volume (NTG), amalgamation index, and channel-belt width and thickness within moving windows. Quadrat cell sizes that are ∼ 50% of the average channel-belt width-to-thickness ratio (16:1 aspect ratio) provide an optimized scale to investigate laterally elongate distributions of fluvial-channel-belt centroids. Large-scale quadrat point patterns were recognized using an array of four quadrat cells, each with 237× greater area than the median channel belt. Large-scale point patterns and NTG correlate negatively, which is a result of using centroid-based PPA on a dataset with disparately sized channel belts. Small-scale quadrat point patterns were recognized using an array of 16 quadrat cells, each with 21× greater area than the median channel belt. Small-scale point patterns and NTG correlate positively, and match previously observed stratigraphic trends in the fluvial John Henry Member, suggesting that these are regional trends. There are deviations from these trends in architectural statistics over small distances (hundreds of meters) which are interpreted to reflect autogenic avulsion processes. Small-scale autogenic processes result in architecture that is difficult to correlate between 1D datasets, for example when characterizing a reservoir using well logs. We show that 1D NTG provides the most accurate prediction for surrounding 2D architecture.

Tectonics control on Quaternary sedimentary processes and basin infill from the coastal area to the basin plain: examples from the Capo d'Orlando Basin (Southeastern Tyrrhenian Sea)

Tectonic activity can affect every single segment of a sedimentary system and the various environments within a basin react in specific ways to tectonic forcing. However, the outcomes of tectonic activity on one environment have important consequences on the sedimentary processes in adjacent ones. In this paper, I evaluate the relationships between tectonic activity and sedimentary processes in several distinct environments, and in different times within the last sea- level cycle, in the submarine part of a single sedimentary system: the Capo d'Orlando Basin. The study area is located in the southeastern Tyrrhenian Sea along the northern Sicilian margin and is affected by active tectonics. A multibeam bathymetric coverage, chirp and regional sparker profiles are available over the study area. In the shelf, the Caronia Horst controls the thickness of the high stand systems tract and the character and the preservation potential of the coastal deposits of the transgressive systems tract. Coastal physiography, due to differential vertical movements, affects offshore current strength and pattern of sediment transport, causing the sand-mud line to move offshore. In the upper slope, tilting increases sediment instability and the accumulation of landslide deposits. Uplift, responsible for the local shallower depth of the shelf edge, favoured the connection between canyon heads and river mouths during the last sea-level low stand. More importantly, some of the canyon heads are still close to the coast and can receive sediment from rivers or offshore currents even during the present-day high stand. The Naso and the Calavà Channels have fault-controlled courses and an asymmetric profile, which controls the location of overbank flows, sediment failures and landslide deposition. In the basin plain, the continued activity of the major faults has created subtle topography and the turbidite lobes do not present a compensational stacking pattern but remain fixed in fault-controlled lows; differential compaction in the separate fault blocks also contribute to differences in turbidite thickness. Landslides are present at the base of the fault-controlled basin-bounding escarpments and punctuate the otherwise turbidite-dominated succession. The analysis of the Capo d'Orlando Basin illustrates the range of effects that tectonic structures have on sedimentary processes going from the continental shelf to the basin plain. More importantly, although in a qualitative way, this research shows how the tectonic influence on up-dip processes is also responsible for the nature of the depositional record in the deeper part of the basin.

Stratigraphy and depositional architecture of lobe complexes across a range of confinements: Examples from the Late Paleozoic Paganzo Basin, Argentina

Abstract The well-exposed, Late Paleozoic glacially-influenced Paganzo Basin offers a unique opportunity to compare and contrast the stratigraphy and depositional architecture of turbidite sheet/lobe complexes with distinct degrees of basin-scale confinements. These include sandstones deposited in narrow paleofjord (Quebrada Las Lajas), moderately narrow paleovalley (Quebrada Grande) and broad depression (Cerro Bola) that are part of the same synchronous transgressive-regressive succession recorded in the basin. This work emphasizes the importance of scales of observation and the importance in make the distinction between the degree of confinement experienced by individual flows and the degree to which the system as a whole is limited by the basin size and geometry. In Quebrada Las Lajas, sheet-like sandstones form lobe complexes largely reflecting the basin physiography. The succession was subdivided into two turbidite stages (Ts1 and 2), separated by a finer-grained interval (F1) that together represents lobe complexes and interlobe systems. Individual sandstone beds are laterally and longitudinally continuous and onlap the paleofjord walls, suggesting vertically-stacked beds. In Quebrada Grande paleovalley, the succession was subdivided into three stages (Ts1 to Ts3), representing lobe complexes, also separated by finer-grained packages (F1 and 2) ascribed to interlobe phases. Individual sandstone beds are truncated, thinning and pinching out, making them hard to correlate between logged sections. This depositional pattern may suggest that lateral stacking dominates during the construction of lobe complexes. At Cerro Bola, the succession was subdivided into five stratigraphic units (Ts1 to Ts5). These units also onlap the basin margin. Individual sandstone beds of Ts1 and Ts2 are similar to those recorded in Quebrada Grande area, also suggesting compensational stacking dominated systems. However, individual sandstone beds within Ts4 are thicker relative to the other units and more continuous longitudinally, reaching the basin margin thus pointing to larger volume flows ascribed to a vertically-stacked pattern. On the other hand, Ts3 and Ts5 correspond to the thinner sandstone beds in the succession and, as these intervals are as thick as the others, they may be attributed to lobe off-axis to fringe settings. In all three areas, the lobe complex geometry is determined by the basin-scale confinement and, at this scale, all of them are represented by more or less tabular, large sandstone bodies showing more similarities than differences. Each of these three successions record an upward increase in the sand-to-mud ratio, which may denote flow ponding or progradation of up-dip deltas that fed these systems.

Transport and deposition of mud in deep‐water environments: Processes and stratigraphic implications

AbstractDeep‐water mudstones are often considered as background sediments, deposited by vertical suspension fallout, and the range of transport and depositional processes are poorly understood compared with their shallow‐marine counterparts. This study presents a dataset from a 538·50 m thick cored succession through the Permian muddy lower Ecca Group of the Tanqua depocentre (south‐west Karoo Basin, South Africa). This study aims to characterize the range of mudstone facies, transport and depositional processes, and stacking patterns recorded in deep‐water environments prior to deposition of the Tanqua Karoo sandy basin‐floor fans. A combination of macroscopic and microscopic description techniques and ichnological analysis has defined nine sedimentary facies that stack in a repeated pattern to produce 2 to 26 m thick depositional units. The lower part of each unit is characterized by bedded mudstone deposited by dilute, low‐density turbidity currents with evidence for hyperpycnal‐flow processes and sediment remobilization. The upper part of each unit is dominated by more organic‐rich bedded mudstone with common mudstone intraclasts, deposited by debris flows and transitional flows, with scarce indicators of suspension fallout. The intensity of bioturbation and burrow size increases upward through each depositional unit, consistent with a decrease in physicochemically stressed conditions, linked to a lower sediment accumulation rate. This vertical facies transition in the single well dataset can be interpreted to represent relative sea‐level variations; the hyperpycnal stressed conditions in the lower part of the units were driven by relative sea‐level fall, and the more bioturbated upper part of the units represent backstepping, related to relative sea‐level rise. Alternatively, this facies transition may represent autogenic compensational stacking. The prevalence of sediment density flow deposits, even in positions distal or lateral to the sediment entry point, challenges the idea that deep‐water mudstones are primarily the deposits of passive rainout along continental margins.

Stratigraphic architecture and hierarchy of fluvial overbank splay deposits

Splay deposits represent an important sand-prone component of the otherwise fine-grained stratigraphic record of fluvial overbank systems. This work presents a hierarchical approach to the classification and palaeoenvironmental interpretation of ancient preserved splay deposits supported by the analysis of the stratigraphic architecture of 11 exhumed examples from the Jurassic Morrison Formation and the Cretaceous Mesaverde Group (western USA) and analysis of the morphology of splays from nine modern fluvial systems. A hierarchical arrangement of splay deposits is proposed, categorized into lithofacies, beds, elements and complexes. Recognition criteria for each tier of the hierarchy include identification of bounding surfaces, thinning and fining trends of splay elements and complexes, and palaeocurrent variability. Progradational and compensational stacking trends control the stratal architecture of splay deposits, and these are influenced by the following factors: (1) the rate of local accommodation generation, which influences the erosive power of floodwaters and whether splay elements are laterally offset owing to compensational stacking; (2) the nature of the topographic confinement of the floodplain; (3) the preservation potential linked to migration direction of channel. Splay bodies can contribute volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby enhancing reservoir connectivity. Supplementary material: Thickness and facies types with global positioning system (GPS) coordinates, length and width v. thickness, and modern data and coordinates are available at https://doi.org/10.6084/m9.figshare.c.4461026

Slope-fan depositional architecture from high-resolution forward stratigraphic models

Submarine fans in tectonically active continental-slope basins are targets of petroleum exploration and production. These slope fans commonly comprise compensationally stacked sandy and muddy architectural elements, including mass-transport deposits, weakly confined to distributary channel-and-lobe deposits, and leveed-channel deposits. The lateral continuity and vertical connectivity of these architectural elements are important uncertainties in reservoir characterization that influence fluid-flow behavior during hydrocarbon production. Here, we use a simple forward stratigraphic model to simulate the stratigraphic patterns and illuminate the likely distribution of fine-scale, sub-seismic heterogeneity in a slope fan. We used published seismic-reflection horizons from the tectonically active Columbus basin, offshore Trinidad, to define the top and base of a Pleistocene submarine fan. We then simulated the stratigraphic evolution of the slope fan with a series of DionisosFlow™ forward stratigraphic models. All variables were kept constant during the simulations in order to test the hypothesis that the autogenic evolution of the surface topography alone, as a result of erosion and deposition, can produce compensational-stacking patterns common in submarine fans. A reference-case model is similar to the thickness trend of published isochron maps of the Trinidad slope fan. The reference-case model also produced patterns of compensational stacking. Varying the time step impacts the heterogeneity of the model. Shorter time steps are characterized by less sediment accumulation, which results in less sediment diversion during the subsequent time step, more gradual migration of channel deposits, shorter offset distances of depocenters, and shorter length-scale heterogeneity compared to longer time steps. Thus, a key characteristic of slope-fan deposits is autogenic compensational stacking, without any external forcing, which governs heterogeneity in these reservoirs. Furthermore, our results suggest that relatively simple diffusion-based models can produce realistic compensation patterns and future work will be focused on higher-resolution model calibration to seismic-reflection data and the influence of input variables on heterogeneity of channel-and-lobe deposits of slope fans.

Sedimentology, stratigraphic context, and implications of Miocene intrashelf bottomset deposits, offshore New Jersey

Drilling of intrashelf Miocene clinothems onshore and offshore New Jersey has provided better understanding of their topset and foreset deposits, but the sedimentology and stratigraphy of their bottomset deposits have not been documented in detail. Three coreholes (Sites M27–M29), collected during Integrated Ocean Drilling Program (IODP) Expedition 313, intersect multiple bottomset deposits, and their analysis helps to refine sequence stratigraphic interpretations and process response models for intrashelf clinothems. At Site M29, the most downdip location, chronostratigraphically well-constrained bottomset deposits follow a repeated stratigraphic motif. Coarse-grained glauconitic quartz sand packages abruptly overlie deeply burrowed surfaces. Typically, these packages coarsen then fine upwards and pass upward into bioturbated siltstones. These coarse sand beds are amalgamated and poorly sorted and contain thin-walled shells, benthic foraminifera, and extrabasinal clasts, consistent with an interpretation of debrites. The sedimentology and mounded seismic character of these packages support interpretation as debrite-dominated lobe complexes. Farther updip, at Site M28, the same chronostratigraphic units are amalgamated, with the absence of bioturbated silts pointing to more erosion in proximal locations. Graded sandstones and dune-scale cross-bedding in the younger sequences in Site M28 indicate deposition from turbidity currents and channelization. The sharp base of each package is interpreted as a sequence boundary, with a period of erosion and sediment bypass evidenced by the burrowed surface, and the coarse-grained debritic and turbiditic deposits representing the lowstand systems tract. The overlying fine-grained deposits are interpreted as the combined transgressive and highstand systems tract deposits and contain the deepwater equivalent of the maximum flooding surface. The variety in thickness and grain-size trends in the coarse-grained bottomset packages point to an autogenic control, through compensational stacking of lobes and lobe complexes. However, the large-scale stratigraphic organization of the bottomset deposits and the coarse-grained immature extrabasinal and reworked glauconitic detritus point to external controls, likely a combination of relative sea-level fall and waxing-and-waning cycles of sediment supply. This study demonstrates that large amounts of sediment gravity-flow deposits can be generated in relatively shallow (~100–200 m deep) and low-gradient (~1°–4°) clinothems that prograded across a deep continental shelf. This physiography likely led to the dominance of debris flow deposits due to the short transport distance limiting transformation to low-concentration turbidity currents.

Distal turbidite fan/lobe succession of the Late Oligocene Zuberec Fm. – architecture and hierarchy (Central Western Carpathians, Orava–Podhale basin)

AbstractA part of the Upper Oligocene sand-rich turbidite systems of the Central Carpathian Basin is represented by the Zuberec Formation. Sand/mud-mixed deposits of this formation are well exposed in the northern part of the basin, allowing us to interpret the turbidite succession as terminal lobe deposits of a submarine fan. This interpretation is based on the discrimination of three facies associations that are comparable to different components of distributive lobe deposits in deep-water fan systems. They correspond to the lobe off-axis, lobe fringe and lobe distal fringe depositional subenvironments, respectively. The inferences about the depositional paleoenvironment based on sedimentological observations are verified by statistical analyses. The bed-thickness frequency distributions and vertical organization of the facies associations show cyclic trends at different hierarchical levels that enable us to reconstruct architectural elements of a turbidite fan. First, small-scale trends correspond with shift in the lobe element centroid between successive elements. Differences in the distribution and frequency of sandstone bed thicknesses as well as differences in the shape of bed-thickness frequency distributions between individual facies associations reflect a gradual fining and thinning in a down-dip direction. Second, meso-scale trends are identified within lobes and they generally correspond to the significant periodicity identified by the time series analysis of the bed thicknesses. The meso-scale trends demonstrate shifts in the position of the lobe centroid within the lobe system. Both types of trends have a character of a compensational stacking pattern and could be linked to autogenic processes. Third, a largescale trend documented by generally thickening-upward stacking pattern of beds, accompanied by a general increase of the sandstones/mudstones ratio and by a gradual change of percentage of individual facies, could be comparable to lobe-system scale. This trend probably indicates a gradual basinward progradation of lobe system controlled by allogenic processes related to tectonic activity of sources and sea-level fluctuations.

Complementing coal seam gas facies modelling workflows with decompaction based processes

Differential compaction plays a key role in influencing the palaeogeographic organisation of many depositional systems. In the Jurassic Walloon Subgroup, Surat Basin, Eastern Australia, the process of compensational stacking contributes significantly to the complex coal layer architecture and is documented in mine exposure, borehole and seismic datasets. Despite this understanding, current best-practices do not formally consider the mechanics of compensational stacking when populating palaeogeography facies in coal seam gas (CSG) reservoir models. To address this limitation, a hybrid modelling workflow was developed in which numerical rules representing the process of differential compaction are used explicitly to condition an iterative workflow containing traditional geostatistical facies modelling algorithms. The workflow is facilitated by a newly developed open source plugin which allows grid decompaction in Schlumberger PETREL™ 2015. Application of the workflow was tested in a CSG production area containing closely spaced wellbores and a 3D seismic survey. In this area, facies models were constructed using both traditional geostatistical approaches and the newly developed hybrid methodology. Comparison of these models suggests that facies models constructed via unconstrained geostatistical approaches often result in unrepresentative realisations, inconsistent with coal seam architectures as observed in seismic and outcrop. The hybrid geostatistical-forward modelling approach developed during this study was better able to reproduce complex alluvial stacking patterns, particularly with respect to coal seam amalgamation, bifurcation and washout.

The influence of lateral topographic confinement on fluvial channel‐belt clustering, compensation and connectivity – lower Wasatch Formation and Dakota Sandstone, Utah, USA

AbstractFluvial channel‐belt clustering has recently been documented using quantitative metrics for systems dominated by autogenic controls. It has long been recognized that allogenic forcing (tectonic and eustatic controls) can lead to confinement of fluvial systems, resulting in clustering of channel belts. To date, no study has quantitatively documented the differences in channel‐belt clustering, compensational stacking of channel belts and interchannel‐belt connectivity in unconfined and confined systems. This study quantitatively compares world‐class outcrops of an unconfined fluvial system (Palaeocene lower Wasatch Formation) with outcrops of a confined fluvial system (Cretaceous Dakota Sandstone). Two new methods have been developed to quantitatively document channel‐belt clustering and intrachannel‐belt connectivity. These new methods, and other previously developed methods, are used to document an increase in channel‐belt clustering and intrachannel‐belt connectivity downdip in both systems. Additionally, it was found that channel belts within the unconfined system stack more compensationally than those in the confined system. These new methods and empirical relationships can be used for predicting intrachannel‐belt connectivity, and accurately modelling unconfined and confined fluvial systems in the subsurface.

A seismic investigation into the geometry and controls upon alluvial architecture in the Walloon Subgroup, Surat Basin, Queensland

ABSTRACTThe sedimentology of the Walloon Subgroup (WSG) has been extensively studied; however, gaps exist in our understanding of the succession's alluvial architecture and the mechanisms controlling its complex internal organisation. Successful coal-seam gas development in the Surat Basin requires the construction of predictive facies models, which in turn necessitates a fulsome understanding of the geometry and controls on the spatial and temporal distribution of alluvial sub-environments. To improve our models of WSG facies, this study employs an open-source high-resolution 3D seismic dataset available on the western limb of the Surat Basin. Integration of core, wireline and seismic data has resolved the geometries of four discrete alluvial architectural elements, representing simple channel, channel belt, crevasse splay complexes and peat-mire sub-environments. Channel belts were found to be 1600–2000 m wide, simple channels 400–800 m in width and crevasse splays averaging 3.5 × 5.5 km. Coal bodies mapped from seismic attribute extractions were found to be 4.2 km2 on average. The high-resolution dataset has also yielded insight into the geological controls governing the spatial and temporal distribution of these sub-environments, explaining, in part the mechanisms responsible for the complex internal distribution of facies within the WSG. In places within the study area, the WSG's sedimentary organisation appears to be initiated by the rejuvenation of deep-seated tectonic features, the expression of which is propagated upward via the mechanics of compensational stacking.

Spatial Analysis of Channel-Belt Stacking Patterns: Metrics To Discriminate Between Local and Regional Controls On Deposition In the Fluvial John Henry Member of the Straight Cliffs Formation, Southern Utah, U.S.A.

The John Henry Member (JHM) of the Straight Cliffs Formation exposed along the Kaiparowits Plateau provides a record of ∼ 6 My of coastal-plain to marginal-marine deposition. To better understand the mechanisms influencing the alluvial architecture of the JHM, a detailed quantitative characterization of the stratigraphic organization and stacking patterns of fluvial channel-belt sandstone bodies was performed. Techniques of point pattern analysis were applied on three datasets consisting of 136, 228, and 55 channel belts collected from two outcrops in Bull Canyon and one outcrop in Rock House Cove; both localities are in the southwestern Kaiparowits Plateau. The results reveal two major trends in the spatial organization of channel belts through time, starting with a trend of decreasing channel-belt clustering from the lower to middle stratigraphic interval, followed by a reversed trend of increasing channel-belt clustering in the upper interval. Furthermore, up-section changes in channel-belt clustering exhibit strong similarities across the three outcrops as indicated by a correlation analysis based on dynamic time warping. Trends of consistent channel-belt clustering in Bull Canyon and Rock House Cove are interpreted to be mainly the product of changing allogenic conditions influencing autogenic processes. Correlation of clustering trends with reconstructed paleo-hydraulic conditions and previously measured architectural parameters suggests that significant channel-belt clustering likely reflects the combined influence of two major factors: 1) low aggradation rates driven by reduced accommodation and 2) high avulsion frequency where channels are deposited downstream of a backwater-mediated nodal avulsion point. A scale-dependent clustering analysis using the K-function indicates that channel belts are clustered at small length scales (∼ 10 to 50 m) and random or uniform at larger length scales (> 60 m). Small-scale clustering is interpreted to be the product of autogenic avulsion reoccupation, where abandoned channels that are preserved as topographic lows serve as attractors to new avulsion paths and lead to channel persistence. Conversely, the large-scale pattern of randomness and/or uniformity reflects periodic large-scale shifts in the local depocenter as a result of avulsion-driven compensational stacking of channel belts.

Coupling Between Shelf-Edge Architecture and Submarine-Fan Growth Style In A Supply-Dominated Margin

The linkage between relative sea-level change, shelf-edge architecture, and evolution of Maastrichtian basin-floor fans in the Washakie Basin, Wyoming, has been investigated at the scale of lobes, lobe complexes, and submarine fans using 630 wireline logs. The basin-floor fan deposits of two adjacent clinothems form lobate shapes on the toe of slope and basin floor. The earlier lobe complexes of the two clinothems are only weakly developed (from no deposition to up to 3.9 km3 respectively in Clinothems 9 and 10), indicating small volumes of sandy sediment delivered to deep water. The lobe complexes (up to 6.4 km3 of each lobe complex) of Clinothem 9 aggraded with fixed slope channels and without strong basinward or lateral migration (40–170 m aggradation, 4–8 km progradation with 4 km lateral shift) and did so in concert with a highly aggradational shelf edge (50 m/100 ky with 5.5 km progradation) during a period of interpreted relative sea-level rise. In contrast, the deep-water lobe complexes (up to 11.5 km3 of each lobe complex) of Clinothem 10 prograded continuously for 15–18 km on the basin floor (with 60–210 m aggradation) coeval with a flattish shelf-edge progradation (25 km/100 ky with 25 m aggradation) and an interpreted minimal sea-level rise or stillstand. The depocenters of lobe complexes in Clinothem 10 switched laterally (7–14 km) by compensational stacking and slope-channel avulsions. During the late development of both clinothems, the deep-water lobe complexes became smaller (up to 1.9 and 6.1 km3 respectively in Clinothems 9 and 10) or retreated concurrently with shelf flooding. Washakie Basin deep-water fans thus evolved through stages of initiation, aggradation or progradation, and retreat of lobe complexes. The submarine-fan growth stages of these deep-water depocenters were surprisingly well linked to coeval changes in shelf-edge trajectory between successive, ca. 100 ky maximum flooding events on the shelf. We suggest that the close linkage of lobe-complex stacking pattern with shelf-edge behavior was because the Washakie Basin formed under greenhouse conditions with a continuously high, Laramide sediment discharge to the deep-water fans while the feeder deltas were at the shelf edge, despite significant sediment reworking of shelf-edge deltas by waves and tides.

Influence of deposit architecture on intrastratal deformation, slope deposits of the Tres Pasos Formation, Chile

Slope sediments on passive and active margins deform and fail across a broad range of scales ranging from loading and sediment remobilization near the sediment–water interface to submarine landslides and mass movements that incorporate significant volumes of slope deposits. Deformational styles are characterized by updip extension and downdip compressional features that occur above a detachment surface. Conditions for failure and deformation include the presence of weak layer(s) that serve as a detachment surface, competency contrasts that allow for detachment and downslope movement, deformation above a detachment surface, and a triggering mechanism(s) that initiates failure. Slope failure processes and products are well documented at scales resolvable by seismic-reflection surveys and in instances of extensive downslope failure, but the processes and products associated with intermediate-scale slope deformation are poorly understood.Intrastratal deformation is defined as stratigraphically isolated zones of deformation bounded above and below by concordant and undeformed strata. In this study, outcrop examples of intrastratal deformation from the Upper Cretaceous Tres Pasos Formation are used to elucidate the influence of depositional architecture on slope deformation. The facies distribution associated with compensational stacking of lobe deposits is shown to have a first-order control on the location and style of deformation. Detachment planes that form in mudstone deposits associated with lobe fringe and interlobe deposits are spatially limited and deformation is restricted to interbedded sandstone and mudstone associated with off-axial lobe positions. Downslope translation was arrested by stratigraphic buttresses associated with more sandstone-prone axial deposits. Emplacement of a regionally extensive mass transport deposit is interpreted as the triggering mechanism for contemporaneous intrastratal deformation of >60m of underlying stratigraphy. A vertical increase in ductile deformation through the deformation interval indicates the role of burial depth and compaction. Distinguishing synburial intrastratal deformation (10s of m below seafloor) from tectonic or at-seafloor deformation has important implications for interpretations of burial history, slope stability, and potential triggering mechanisms.

Research on the architecture of submarine-fan lobes in the Niger Delta Basin, offshore West Africa

As one of the most important reservoirs in submarine fan, submarine lobes are hot fields for the deep-water oil–gas exploration in the world. Although a large amount of researches have been carried out on the architecture model of submarine lobes throughout the world, some controversies still exist on aspects such as the differential distribution of composite patterns and the quantitative relationships among different sedimentary settings. This paper, which took an oilfield in the Niger Delta Basin offshore West Africa as an example and utilized abundant core, well-logging and seismic data comprehensively, established the semi-quantitative to quantitative architecture model for individual lobe and lobe complex.Results show that (1) A lobe complex was composed of multiple individual lobes as the compensational stacking pattern. According to the stacked relationship among individual lobes, four types of compensational stacking pattern were classified as the inordered type, the lateral migration type, the retrograding type and the prograding type. Influenced by the laterally confined degree of palaeotopography, the basin floor fan lobe complex was easily superimposed as inordered type, showing relatively large lateral width but relatively small thickness; the slope fan lobe complex was easily superimposed as lateral migration, retrograding and prograding types, showing relatively small lateral width but relatively large thickness. (2) Influenced by the laterally confined degree of palaeotopography, the basin floor fan individual lobes showed relatively large planar distribution area but relatively small thickness, while the slope fan individual lobes showed relatively small planar distribution area but relatively large thickness. An individual lobe was composed of multiple lobe stories by the way of aggradation–progradation pattern. From proximal to middle and distal part of individual lobes, the bottom mass-transport-deposits and the top branch channels disappeared gradually, and the amalgamated degree of the middle tabular sandbodies weakened as well with gradually developing of muddy interlayers. The formation of an individual lobe generally includes three stages as “rapid accumulation–progradation–aggradation”.

Relating the compensational stacking of debris-flow fans to characteristics of their underlying stratigraphy: Implications for geologic hazard assessment and mitigation

Compensational stacking is the tendency for sediment transport systems to fill topographic lows through avulsion. This article quantitatively relates, for the first time, compensational stacking patterns within debris fans to characteristics of their internal stratigraphy and discusses implications to geologic hazard assessment and mitigation. Three exceptionally well-exposed debris fans were selected in Colorado for quantitative stratigraphic analyses. In each fan, the cross-sectional stratigraphy was subdivided into discrete depositional units (debris-flow and stream-flow deposits). The bounding surfaces between the depositional units were used to analyze the compensation index (κcv) of the fans, which is a measure of their compensational or avulsion tendencies. In the measured datasets, κcv ranged from 0.63 to 1.03. Values close to 0.5 represent intermediate levels of compensation, whereas values approaching 1.0 reflect high levels of compensation. The compensational values (κcv) were statistically compared to some physical, observable characteristics of the fans including: (1) debris-flow size, (2) amount of stream-flow deposits, (3) debris-flow composition, and (4) longitudinal position on the fan. These parameters correlated, either positively or negatively, to κcv, supporting their use as proxies for assessing the degree of compensational stacking in settings where large-scale cross-sections of a fan are unavailable. Such empirical results can be used by geologists and engineers for avoidance and mitigation measures of land use on debris fans.

Quantitative Analysis of the Dimensions and Distribution of Channelized Fluvial Sandbodies Within A Large Outcrop Dataset: Upper Cretaceous Blackhawk Formation, Wasatch Plateau, Central Utah, U.S.A

Abstract Conceptual and quantitative models of fluvial stratigraphy typically argue that alluvial architecture is driven by a combination of allogenic processes (e.g., tectonic subsidence, sea level, sediment supply), and autogenic behaviors (e.g., avulsion). We test these models via analysis of a large (c. 200 m thick by 100 km depositional-strike extent) alluvial-to-coastal-plain succession that records shoreline progradation in response to decreasing tectonic subsidence (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, U.S.A.). Oblique aerial photographs and architectural panels of six nearly linear, nearly vertical cliff face “windows” were used to characterize the dimensions (apparent widths, thicknesses) and spatial distributions of channelized fluvial sandbodies. Two statistical measures, raster-based lacunarity and point-based L-function, are used to investigate whether the distribution of channelized sandbodies show significant regularity, randomness, or clustering. Over 490 channelized fluvial sandbodies are identified in the six cliff-face “windows.” From base to top of the Blackhawk Formation, sandbodies broadly increase in width and decrease in overall abundance per unit area. Clustering of sandbodies occurs relatively frequently in lower-coastal-plain strata ( 50 km from the coeval shoreline). A strong negative correlation between lacunarity and stratigraphic position is also observed, such that a wider range and greater variety of spatial gaps occurs between sandbodies in lower-coastal-plain strata relative to upper-coastal-plain and alluvial strata. Comparison with numerical modeling studies of fluvial stratigraphic architecture implies the predominance of an avulsion-generated pattern of sandbody distribution that includes an element of compensational stacking on the upper coastal plain and alluvial plain, for a range of distances from the coeval shoreline (c. 40–130 km) and tectonic subsidence rates (c. 40–700 m/ Myr). On the lower coastal plain (< 50 km from the coeval shoreline) localized clustering of channelized sandbodies is interpreted to have formed by avulsion of deltaic distributary channels downstream of delta-apex avulsion nodes, possibly modulated by low-amplitude (< 30 m) sea-level fluctuations.

Crevasse splay morphodynamics in a dryland river terminus: Río Colorado in Salar de Uyuni Bolivia

High-resolution satellite imagery in combination with field investigation enables the analysis of splay morphological developments in time and space at the low-gradient terminus of the modern Río Colorado dryland river system in the Salar de Uyuni (Bolivia). Two sets (2004/2005, 2010/2011) of satellite images are used to visualize splay morphological developments and three types of crevasse splays are identified based on temporal changes: (1) new crevasse splays (NCS), refers to those splays that are not observed in the first set of images but can be observed on the second sets of images; (2) changing crevasse splays (CCS), representing those splays that are found on the first set of images but their geometry has changed between two satellite observation periods; and (3) inactive crevasse splays (ICS), referred to those splays that are found on the first set of images but on the second set of images they remain the same, or are partially overlapped by adjacent expanding crevasse splays. The density or frequency of these three types of crevasse splays has no correlation with the distance to the starting point of the river or the local slope. The cross-sectional area of the studied river steadily decreases downstream, and frequency analysis shows a downstream increase in the number of crevasse splays and suggests an exponential relationship with the river cross-sectional area. Development of crevasse splays is often attributed to topographic lows between adjacent crevasse splays, and they fill in the depression by compensational stacking e.g. Li et al. (2014). These amalgamated splays are capable of forming a large area of sheet sands and potentially provide an analogue for thin-bedded hydrocarbon reservoirs.

Sedimentological evolution of Sele Formation deep-marine depositional systems of the Central North Sea

Abstract The Paleocene–Eocene-aged Sele Formation is developed across the basinal region of the Central North Sea. The section comprises a number of deep-marine fan systems that expanded and contracted across the basin floor in response to relative sea-level changes on the basin margin and fluctuating sediment yield off the Scottish landmass modulated by climate and hinterland uplift. Persistent sediment entry points to the basin resulted in the development of discrete axial and transverse fan fairways with a geometry dictated by an irregular bathymetry sculpted by differential compaction across Mesozoic faults, halokinesis and antecedent fan systems. A high-resolution biostratigraphic framework has allowed the evolution of fan-dispersal systems in response to these effects to be tracked across the basin within four genetic sequences. The proximal parts of the fans comprised channel complexes of low sinuosity, high lateral offset, and low aggradation. The development of these systems in a bathymetrically confined corridor of the Central Graben ( c. 65 km wide), combined with high sediment supply, resulted in the eventual burial of any underlying relief. The behaviour of sand-rich reservoirs in this region is dominated by the permeability contrast between high-quality channel fairways and more heterolithic overbank regions, with the potential for early water breakthrough and aquifer coning in the channel fairways, and unswept volumes in overbank locations. Compartmentalization of compensationally stacked channel bodies occurs locally, with stratigraphic trapping caused by lateral channel pinch-outs, channel-base debrites, mud-rich drapes and abandonment fines. Towards the southern part of Quadrant 22, approximately 150 km down-palaeoflow, the systems became less confined and in this region are dominated by channel–lobe complexes, which continued to interact with an irregular bathymetry controlled by antecedent fans, mass-transport complexes and halokinesis in the form of rising salt diapirs. Reservoirs in this region are inherently stratigraphically compartmentalized by their heterolithic lithology and compensational stacking of lobes, and further complicated by structuration and instability induced by the diapiric or basement structures needed to generate a trapping structure in these settings.

Depositional architecture of sand-attached and sand-detached channel-lobe transition zones on an exhumed stepped slope mapped over a 2500 km2 area

The geomorphology and seismic stratigraphy of deep-water clastic systems from slope valleys through channel-levee systems to basin-floor fans have been observed and described in modern and ancient subsurface examples around the world. However, the distribution of sedimentary facies, grain size, and small-scale architectural elements remains poorly constrained. Extensive exposures (>2500 km 2 ) of four stacked deep-water composite sequences have been mapped from heterolithic channel-levee systems on the slope to sand-rich basin-floor deposits. The data set from Units C–F of the Fort Brown Formation in the Permian Laingsburg depocenter of South Africa permits a unique opportunity to document and compare their depositional architecture at a high resolution for tens of kilometers downdip. Isopach thickness maps indicate that compensational stacking across multiple stratigraphic scales occurs on the basin floor, whereas preferred axial pathways were present on the slope, leading to subvertical stacking patterns. Units C and D are sand-attached systems; slope valley systems are mapped to pass transitionally downslope through levee-confined channels to lobe complexes over distances of >30 km. The slope valley fills of Units E and F, however, are separated from their downdip sand-rich lobe complexes by a thin, sand-poor tract several kilometers in length and are termed sand detached. Locally, this sand-poor tract is characterized by a distinctive facies association of thin-bedded turbidites with numerous scours mantled with rip-up clasts, and a top surface that includes megaflutes and remobilized sediments. This assemblage is interpreted to indicate a widespread area of sand bypass. This unique data set provides an exploration-scale insight and understanding of how different segments of a prograding slope evolved over time in terms of gradient, physiography, and hence the degree to which sand was stored or bypassed to the basin floor, and the evolution from sand-attached to sand-detached systems. The development of sand-detached systems suggests that a steeper gradient formed, possibly related to developing underlying structure, that led to the development of a stepped slope profile. The study highlights that updip stratigraphic trapping at reservoir scale can occur with minor bathymetric changes.