Compensational Stacking Research Articles

Landsat imagery-based visualization of the geomorphological development at the terminus of a dryland river system

In this remote sensing-based study, we present the analysis of the geomorphological development at the low-gradient terminus of the modern Río Colorado dryland river system in the endorheic Altiplano Basin (Bolivia). Changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays, formation of new crevasse splays and in river path avulsion. Episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from ASTER global DEM (GDEM) remote sensing data. A time series of Landsat imagery for the period 1975–2001 was then used to analyze the river morphology changes after major peak discharge events. Compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system.

The Influence of A Lateral Slope On Turbidite Lobe Development On A Modern Deep-Sea Slope Fan (Villafranca Deep-Sea Fan, Tyrrhenian Sea)

Abstract In deep-sea fans, fan lobes form from the stacking of the deposits of turbidity currents and other sediment gravity flows beyond the channel mouth. Slopes lateral or oblique to the main direction of turbidity currents exiting channel confinement can profoundly affect flow behavior and the distribution of facies and lithology in fan lobes. In this paper, we report on the analysis of multibeam and CHIRP subbottom data aimed at the study of the effects of a lateral slope on turbidity-current behavior and resultant geomorphology and stratigraphy of a modern fan lobe. The Villafranca fan lobe, the subject of this study, develops on the 0.5° dipping seafloor of the Gioia intraslope basin in the southeastern Tyrrhenian Sea. The Villafranca is a transient fan lobe that has its downslope limit coinciding with a 200-m-high bathymetric step formed by the erosional flank of the Stromboli slope valley, perpendicular to the fan-lobe trend. A “lobe complex” hierarchical level is assigned to the Villafranca fan lobe. The lobe complex develops beyond the mouth of the Villafranca leveed channel, which trends N and is oblique to the regional NW-dipping slope. The depositional topography created by an adjacent channel levee wedge to the west is the cause of the observed obliquity. The western and the eastern “lobes” constitute the Villafranca lobe complex. Both lobes have mainly NW-trending channels, and therefore they do not conform to the classical divergent pattern of channels observed in deep-sea fan lobes. The channel trend is a result of the inability of flows to freely spread sideways, due to the presence of the lateral slope and the dip of the seafloor at the channel mouth. The western lobe has an up-dip apex with respect to the eastern one and is characterized by channels that nucleate on the downslope side of a channel-mouth bar and that die out distally before reaching the flank of the Stromboli slope valley. The eastern lobe, in contrast, has channels that are depositional upslope but on approaching the flank of Stromboli slope valley become mainly erosional or bypass features. The two lobes develop concomitantly, therefore an evolution due to compensational stacking with the two lobes being in different stages of development is here disregarded. It is therefore feasible that turbidity currents, which due to different initiating mechanisms have distinct magnitude and efficiency, deposit their load either in the eastern or in the western lobe. Another possibility is that single turbidity currents exiting the Villafranca channel mouth form two separate flow portions with different properties, which independently but simultaneously feed the eastern and the western lobe. In both cases, the effect of the lateral slope and the obliquity between the dip of the slope and the trend of the channel mouth are here considered as the primary factors controlling the differences in depositional style of the two lobes.

Depositional architecture and evolution of progradationally stacked lobe complexes in the Eocene Central Basin of Spitsbergen

AbstractThe down‐dip portion of submarine fans comprises terminal lobes that consist of various gravity flow deposits, including turbidites and debrites. Within lobe complexes, lobe deposition commonly takes place in topographic lows created between previous lobes, resulting in an architecture characterized by compensational stacking. However, in some deep water turbidite systems, compensational stacking is less prominent and progradation dominates over aggradation and lateral stacking. Combined outcrop and subsurface data from the Eocene Central Basin of Spitsbergen provide a rare example of submarine fans that comprise progradationally stacked lobes and lobe complexes. Evidence for progradation includes basinward offset stacking of successive lobe complexes, a vertical change from distal to proximal lobe environments as recorded by an upward increase in bed amalgamation, and coarsening and thickening upward trends within the lobes. Slope clinoforms occur immediately above the lobe complexes, suggesting that a shelf‐slope system prograded across the basin in concert with deposition of the lobe complexes. Erosive channels are present in proximal axial lobe settings, whereas shallow channels, scours and terminal lobes dominate further basinward. Terminal lobes are classified as amalgamated, non‐amalgamated or thin‐bedded, consistent with turbidite deposition in lobe axis, off‐axis and fringe settings, respectively. Co‐genetic turbidite–debrite beds, interpreted as being deposited from hybrid sediment gravity flows which consisted of both turbulent and laminar flow phases, occur frequently in lobe off‐axis to fringe settings, and are rare and poorly developed in channels and axial lobe environments. This indicates bypass of the laminar flow phase in proximal settings, and deposition in relative distal unconfined settings. Palaeocurrent data indicate sediment dispersal mainly towards the east, and is consistent with slope and lobe complex progradation perpendicular to the NNW–SSE trending basin margin.

Quantifying the Hierarchical Organization of Compensation In Submarine Fans Using Surface Statistics

Abstract Stratigraphy is often interpreted within hierarchical, or scale-dependent, frameworks that subdivide deposits based on distinct jumps in characteristics such as duration of deposition or scale. While the interpretation is logically valid, few studies quantitatively demonstrate that the jumps exist. Rather, recent work has quantitatively shown some characteristics of stratigraphy to be fractal, or scale invariant. Compensational stacking, the tendency for sediment-transport systems to preferentially fill topographic lows, is a concept widely used in stratigraphic interpretation. Here we use the compensation index, a metric that quantifies the strength of compensational stacking in sedimentary deposits, to describe the architecture of stratigraphy exposed in outcrops of submarine-fan strata in the Carboniferous Ross Sandstone representing contrasting architectural styles: (1) predominantly lobe elements and (2) predominantly channel elements. In both datasets, the stratigraphic architecture is classified into hierarchical classes of beds, stories, and elements. Results are the following. First, at both sites we document statistically significant increases in the strength of compensation across larger hierarchical levels supporting the use of hierarchical interpretations of stratigraphy. It is therefore plausible for some characteristics of sedimentary systems to be hierarchical and others to be fractal. Second, we document that lobe elements stack more compensationally than channel elements. We interpret this pattern to document that compensation increases along a longitudinal transect through this distributive submarine fan.

Deltaic process and architectural evolution during cross-shelf transits, Maastrichtian Fox Hills Formation, Washakie Basin, Wyoming

The topset compartments of two Maastrichtian basin-scale clinothems are characterized, with focus on the function they played in constructing the Lance–Fox Hills–Lewis shelf-margin sedimentary prism in the Laramide Washakie Basin, south Wyoming. Approximately 1000 well logs were used to map the delta lobes and complexes on the Fox Hills shelf and to detail their depositional character, dimensions, and orientation as they autogenically shifted during transit from an inner-shelf to shelf-edge position. The regressive transits of the deltas initiated up to 40 km (25 mi) landward from the preexisting shelf-edge and preserved river and wave-dominated deltaic deposits that thicken and concentrate sand on the outer shelf. Tidally influenced deltas (now outcropping) also occur in localized areas along the paleoshelf edge, probably where wave influence was reduced along invaginated coastal segments. Net sandstone maps of the individual clinothem topsets show that (1) coeval delta lobes exist within each clinothem, suggesting multiple rivers; (2) delta lobes have a likely autogenic compensational stacking pattern; and (3) deltas thicken and storm-wave influence become dominant closer to the shelf edge. Our results support the ideas of (1) predictable increased wave influence and (2) change to strike-elongate architecture as deltas transit the shelf. In addition, along-strike changes in process dominance cause deltaic reservoirs to be highly variable in their orientation, external shape, and internal character. Some process changes are interpreted to be autogenic responses during overall shoreline progradation. The study also provides new data on delta-lobe and delta-complex thicknesses as well as on deltaic coastline versus shelf-edge progradation rates.

The Straub method: a quantitative metric for detection of compensational stacking in channelized deposits

Straub et al. (2009) suggest a metric that allows the quantitative detection of compensational stacking in channelized deposits based on a model whereby the standard deviation of channelized deposit sedimentation rate obeys a power-law function within a given measurement time window. Here, we outline this Straub method and use it to analyze channel deposits stacked within ancient rocks exposed in terrestrial outcrops. A power-law exponent, termed the compensation index κ, can be an indicator of the degree of compensational stacking, where κ = 1.0 for perfect compensational stacking, κ = 0.5 for random stacking, and κ = 0 for anti-compensational stacking. The κ values estimated from stratigraphic cross-sections across submarine-fan successions of the lower Kiyosumi Formation of central Japan can account for the stacking patterns of the channel deposits, and indicate that a combination of the Straub method with the examination of stratigraphic cross-sections allows for an improved understanding of the compensational stacking of channel deposits.

Paleocene deep-water depositional systems in the North Sea Basin

Interpretation of 3D seismic reflection data supported by well data provides insights into the geometry of early Paleogene depositional systems along the eastern margin of the North Viking Graben. These deposits, which consist mainly of sandstones interbedded with claystones and siltstones, are interpreted to document deposition at the edge of a large base-of-slope to proximal basin floor fan system which was sourced from the eastern basin margin. Individual sandstone bodies are up to 80 m thick and occur within four sandstone-prone packages (DU1–4), and well-to-seismic ties indicate that the thicker sandstones (>10 m) are represented by channelized or sheet-like, high-amplitude anomalies. Both well and seismic data suggest that the sandstones are of limited lateral extent (<1–5 km), implying they were deposited in a series of channels. Channelization and compensational stacking of sandstones may have been at least partly controlled by differential compaction across previously deposited sandbodies. The study reveals that deep-water depositional patterns are more complex than is apparent from previous, lithostratigraphically-driven correlations and from regional isochron mapping. In particular, this study has implications for the controls on the distribution and reservoir architecture of deep-water sandstones.

Scale-dependent compensational stacking: An estimate of autogenic time scales in channelized sedimentary deposits

Recent studies show that paleoenvironmental (allogenic) signals preserved in the stratigraphic record may be contaminated or overprinted by internally generated (autogenic) sedimentation. This is problematic, but it is unclear over what temporal and spatial scales autogenic patterns are most prevalent. We propose that scale breaks in basin-fi lling trends can be used to identify the transition between allogenic and autogenic stratigraphy. Using data from numerical and physical experiments and an ancient outcrop, we explore how compensation, the tendency for sediment transport systems to preferentially fi ll topographic lows, varies with stratigraphic scale. Object-based models demonstrate the temporal scales at which stratigraphy changes from being partially infl uenced by autogenic processes to being completely determined by allo- genic forcings and suggest that this transition occurs at a time scale set by the maximum scale of surface roughness in a transport system divided by the long-term aggradation rate. This hypothesis is validated in a physical experiment where delta topography was monitored along fl ow-perpendicular transects at a high temporal resolution relative to channel kine matics. The strength of compensation in the experiment changes at the predicted time scale, where the maxi mum surface roughness is equal to the depth of the experimental channels. Above this compensation time scale deposits stack purely compensationally, but below this time scale deposits stack somewhere between randomly and deterministically. Similar scale-dependent stacking is also observed in the Ferris Formation (Cretaceous-Paleogene, Hanna Basin, Wyoming, United States). This study demonstrates that scale-dependent compensational stack- ing may be useful for isolating allogenic and autogenic stratigraphy in sedimentary basins.

Mechanisms Controlling the Clustering of Fluvial Channels and the Compensational Stacking of Cluster Belts

Abstract Analysis of sand-body distribution reveals that fluvial channel sands in the Upper Cretaceous lower Williams Fork Formation in the central Piceance Basin, Colorado, USA, are not randomly distributed but are predictable in their spatial and stratigraphic position. Sand bodies are organized in channel-belt clusters that are compensationally stacked on different temporal scales. While channel clustering occurred over a shorter period (channel and channel-belt scale 10s ky to 100s ky), compensational stacking occurred over a much longer time scale (channel cluster belts, ~ 400 ky). The lower Williams Fork Formation consists of three distinct intervals of clustered channel belts, each ~ 400 ft (~ 122 m) thick, topped by a thin coal layer. The primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase. Short-lived changes in in basin accommodation caused by either changes in tectonic subsidence or high-frequency eustatic changes due to Milankovitch-band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle. Coals at the end of each sub-cluster span the entire study area, but their overall thickness is greater in the inter-cluster, floodplain-prone areas. The resulting differential early coal compaction is greatest in the inter-cluster areas and has a significant control on the subsequent cluster-belt position overlying the coal. Channel sands overlying the coals are concentrated in the previously low net/gross floodplain-prone inter-cluster areas resulting in compensationally stacked cluster belts. On the longest time scale (~ 1 to 1.5 My), which encompasses the entire lower Williams Fork, changes in channel geometries and sand-body thickness seem to be controlled mainly by long-term changes in eustasy, and autogenic processes are of lesser importance.

Triassic platform-margin deltas in the western Barents Sea

The Early to Middle Triassic in the Barents Sea was dominated by prograding transgressive–regressive sequences. Internal clinoform geometries indicate that sediments were derived from the Baltic Shield in the south and the Uralian Mountains in the east and southeast. These systems were formed in a large, relatively shallow epicontinental basin, where modest variations in relative sea-level relocated the shoreline significantly. This study shows the development of strike elongated depositional wedges that thicken just basinward of the platform-edge. Seismic facies and time-thickness maps show the position and development of platform-margin delta complexes within each sequence. Seismic clinoforms and trajectory analysis show significant lateral variation from the axis of the delta complex to areas adjacent to the main delivery system. Frequent toplap geometries are observed in proximity to coarse-grained deposits, while aggradation of seismic clinoforms characterizes areas laterally to the platform-margin deltas. Complex shifts in depocenters are revealed by large-scale compensational stacking pattern and relict platform breaks. Locally, relict breaks are created due to pre-existing paleo-topography. Platform-margin deltas can be identified by careful mapping of clinoform geometries, clinoform angles and trajectories. However, seismic analysis of prograding clinoform units indicate that the shoreline and delta complexes commonly are positioned landward of the platform-edge. Deposition of platform-margin deltas is sometimes caused by locally increased sediment supply during slightly rising relative sea-level, and occasionally caused by a regional drop in relative sea-level with significant shelf bypass. Development, position, thickness and facies distribution of platform deltas and platform-margin deltas of very broad low-relief basins, like the Triassic of the epicontinental Barents Sea basin, are strongly sensitive to changes in relative sea-level due to rapid emergence and submergence of wide areas, and to changes in position of major rivers supplying sand to the delta systems. In this respect, the depositional model of the present study deviates from models of clinoform successions obtained from small and narrow basins or siliciclastic platforms with high coarse-clastic sediment supply.

Quantifying clinothem geometry in a forced-regressive river-dominated delta, Panther Tongue Member, Utah, USA

This study presents a methodology for the quantitative description of small-scale delta clinothems. The quantitative bed-data analysis is based on three-dimensional virtual outcrop models generated by ground-based laser scanning (Light Detection and Ranging). A large number of clinothem bed measurements have been collected from the ancient forced regressive delta system of the Panther Tongue that crops out in Utah, USA. In river-dominated marginal marine environments, clinothems separated by clinoform surfaces represent the former position of the delta front as it prograded. Systematic collection of data from virtual outcrop models has allowed for accurate, spatially constrained measurement of individual bed thicknesses and the compilation of a detailed database on clinothems and associated clinoform geometries. Measurement locations were selected so that each measurement was 10 m down depositional dip from the previous one. The study area covers 5 km2 within which 2376 measurements were made from 50 separate clinothems in 320 different positions within the virtual outcrop. A bed taper parameter permitted the thinning of the clinothems to be described as a single number and thus allowed relative comparison between beds. Combined measurements were also used to calculate the average dip angle of the clinothems. Analysis of the vertical and lateral stacking of the clinothems has revealed a series of stratigraphic cycles which are termed bedsets (stream-mouth bars). The surfaces that bound the bedsets are unremarkable and it is unlikely that their significance would be recognized without this style of detailed analysis. A cyclic depositional pattern, interpreted as related to autocyclic processes and compensational stacking of mouth bars, is proposed as the origin of these packages. Mapped length/thickness trends constrain the spread of these variables, and can be used to constrain subsurface models of analogous hydrocarbon reservoirs.

Evolution, architecture and hierarchy of distributary deep‐water deposits: a high‐resolution outcrop investigation from the Permian Karoo Basin, South Africa

AbstractSea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent planform timeslices of distributive deep‐water systems but commonly only limited information on cross‐sectional architecture. Extensive outcrops in the Tanqua depocentre, south‐west Karoo Basin, provide these three‐dimensional constraints on lithofacies distributions, stacking patterns, depositional geometries and the stratigraphic evolution of submarine lobe deposits at a scale comparable with modern lobe systems. Detailed study (bed‐scale) of a single‐lobe complex (Fan 3) over a 15 km by 8 km area has helped to define a four‐fold hierarchy of depositional elements from bed through to lobe element, lobe and lobe complex. The Fan 3 lobe complex comprises six distinct fine‐grained sandstone packages, interpreted as lobes, which display compensational stacking patterns on a 5 km scale. Between successive lobes are thin‐bedded, very fine‐grained sandstones and siltstones that do not change lithofacies over several kilometres and therefore are identified as a different architectural element. Each lobe is built by many lobe elements, which also display compensational stacking patterns over a kilometre scale. Thickness variations of lobe elements can be extremely abrupt without erosion, particularly in distal areas where isopach maps reveal a finger‐like distal fringe to lobes. Lobe deposits, therefore, are not simple radial sheet‐dominated systems as commonly envisaged.

Compensational Stacking of Channelized Sedimentary Deposits

Abstract Compensational stacking, the tendency for sediment transport systems to preferentially fill topographic lows through deposition, is a concept widely used in the interpretation of the stratigraphic record. We propose a metric to quantify the degree of compensation by comparing observed stacking patterns in subsiding basins to what would be expected from uncorrelated random stacking. This method uses the rate of decay of spatial variability in sedimentation between picked depositional horizons with increasing vertical stratigraphic averaging distance. We present data from six sedimentary basins where this decay can be measured. The depositional environments range from river deltas to deep-water minibasins, and scales range from meters to 1.5 km in thickness. The decrease in standard deviation of sedimentation divided by subsidence with increasing vertical averaging distance is well described by a power law in each study basin. We term the exponent in this power law the compensation index, κ; its value is 0.5 for uncorrelated random stacking and 1.0 for perfect compensational stacking. Values less than 0.5 indicate anti-compensation, i.e., a tendency of depositional units to stack on top of one another. Parameters controlling the magnitude of κ include the frequency of system-scale avulsions and the temporal variability in deposition rates. Data describing the decay in the standard deviation of sedimentation/subsidence from the six studied basins collapse approximately onto a single power-law trend with κ = 0.75 when the measurement window is standardized by the mean channel depth of each system. Channel depth thus emerges as a fundamental length scale in stratigraphic architecture across environments. Although further study will likely reveal measurable variability in κ between depositional environments, the overall power-law collapse presented here suggests that a stacking behavior midway between purely random and perfect compensation is a good starting point in quantitatively estimating the stratigraphic arrangement of sedimentary deposits.

Stochastic surface-based modeling of turbidite lobes

Flow event deposits in turbidite lobes are modeled with stochastic surface-based simulation. This method honors the geometries and compensational stacking of flow event deposits. Flow event deposit geometries are based on a flexible lobe parameterization. Compensational stacking is the tendency of flow event deposits to fill topographic lows and to smoothing of topographic relief. The surface-based model may be conditioned to well data.Models of reservoir properties such as porosity and permeability are constrained by the resulting geometric models. This approach is applied in a geostatistical workflow to better integrate available geologic information. The resulting models may improve the accuracy of model reservoir response and account for the uncertainty in the heterogeneity of turbidite lobes.