ALBERT

Description: Distributive submarine fans contain channel-lobe elements that compensationally stack to build a radially dispersive map pattern. The middle parts of some submarine fans contain juxtapositions of channel elements and lobe elements due to longitudinal and lateral shifts in their channel-lobe transition zones. This article uses an exceptionally well-exposed three-dimensional outcrop of the Ross Sandstone at Bridges of Ross (Ireland) to document the stratigraphic and plan-view manifestation of lateral juxtapositions of channel elements and lobe elements in submarine fans. Observations made herein compare favorably to those in seafloor studies of Navy Submarine Fan (offshore southern California, USA) by William Normark and others, indicating that these systems can be used as paired outcrop-seafloor analogs for distributive fans in which the channel-lobe transition zones are located in longitudinally variable positions. In addition, data from Bridges of Ross and Navy Submarine Fan are integrated to constrain a geometric model that predicts the fractional length of a fan that contains lateral juxtapositions of channel elements and lobe elements. Lateral juxtapositions of channel elements and lobe elements are important because they enhance vertical and lateral connectivity within subsurface reservoirs.

Description: Using a seismic database from the Qiongdongnan Basin in the South China Sea, this study demonstrates that shelf-edge trajectories and stratal stacking patterns are reliable, but understated, predictors of deep-water sedimentation styles and volumes of deep-water sand deposits, assisting greatly in locating sand-rich environments and in developing a more predictive and dynamic stratigraphy. Three main types of shelf-edge trajectories and their associated stratal stacking patterns were recognized: (1) flat to slightly falling trajectories with negative trajectory angles ( $${T}_{\mathrm{se}}$$ ) (–2° to 0°) and negative shelf-edge aggradation to progradation ratios ( $$\mathrm{d}y/\mathrm{d}x$$ ) (–0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief ( $${R}_{\mathrm{c}}$$ ) (150–550 m [492–1804 ft]) and negative differential sedimentation on the shelf and basin ( $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ ) (–0.6 to 0); (2) slightly rising trajectories with moderate $${T}_{\mathrm{se}}$$ (0°–2°) and medium $$\mathrm{d}y/\mathrm{d}x$$ (0–0.04), and associated progradational and aggradational stacking patterns with intermediate $${R}_{\mathrm{c}}$$ (250–400 m [820–1312 ft]) and intermediate $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ (0–0.6); and (3) steeply rising trajectories with high $${T}_{\mathrm{se}}$$ (2°–6°) and high $$\mathrm{d}y/\mathrm{d}x$$ (0.04–0.10) and associated dominantly aggradational stacking patterns with high $${R}_{\mathrm{c}}$$ (350–650 m [1148–2132 ft]) and high $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ (1–2). Each trajectory regime represents a specific stratal stacking patterns, providing new tools to define a model-independent methodology for sequence stratigraphy. Flat to slightly falling shelf-edge trajectories and progradational and downstepping stacking patterns are empirically related to large-scale, sand-rich gravity flows and associated bigger and thicker sand-rich submarine fan systems. Slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are associated with mixed sand/mud gravity flows and moderate-scale slope-sand deposits. Steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are fronted by large-scale mass-wasting processes and associated areally extensive mass-transport systems. Therefore, given a constant sediment supply, then $${T}_{\mathrm{se}}$$ , $$\mathrm{d}y/\mathrm{d}x$$ , $${R}_{\mathrm{c}}$$ , and $${A}_{\mathrm{s}}/{A}_{\mathrm{b}}$$ are all proportional to intensity of mass-wasting processes and to amounts of mass-transport deposits, and are inversely proportional to the intensity of sand-rich gravity flows and to amounts of deep-water sandstone. These relationships can be employed to relate quantitative characteristics of shelf-edge trajectories and stratal stacking patterns to deep-water sedimentation styles.

Description: :  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.

Description: Well-exposed three-dimensional fluvial outcrops of the high net-sand content middle Wasatch Formation in Three Canyon, Uinta Basin, Utah, were used to create and develop a new methodology for describing the architecture of fluvial systems. The methodology builds on the works of Campbell, Jackson, Allen, and Miall, and addresses sedimentary processes, scale, and temporal context for reservoir and non-reservoir bodies. The methodology developed herein is a three-level hierarchical framework that classifies meso- and macroscale architecture of fluvial systems. The three-level hierarchy contains, from smallest to largest: stories, elements, and archetypes. Eight story types provide the foundational building blocks of this framework and account for sedimentation in both channel-belt and floodplain-belt elements, including (1) downstream accreting; (2) laterally accreting; (3) erosionally-based fine-grained fill; (4) fine-grained fill associated with laterally accreting; (5) levee; (6) splay; (7) crevasse or overbank channels; and (8) floodplain fines. Two types of elements are recognized: (1) channel belt and (2) floodplain belt. An archetype consists of a channel-belt element and its genetically related floodplain-belt elements. Two distinct upward-stacking patterns differentiate braided and meandering archetypes. In deconstructing the evolution of archetypes, three distinct associations between channel-belt elements and their adjacent splays are documented: (1) unassociated splays; (2) associated coeval splays; and (3) associated non-coeval splays.Width and thickness for stories, channel-belt elements, and archetypes are documented providing dimensional constraints for analog high-net-sand-content fluvial systems. Additionally, this methodology provides object-based models with shape-defined reservoir and nonreservoir geobodies that realistically compare to fluvial systems.

Description: Weakly confined channel systems are common in low-relief minibasins on continental margins and are important hydrocarbon reservoirs. They are characterized by channels that diverge in the proximal part of the basin and converge because of topographic confinement in the distal part of the basin. The Morillo 1 member, in the Ainsa Basin, Spain, is an excellent outcrop analog of a weakly confined submarine channel system. Data from the Morillo 1 member are used to quantitatively document how reservoir characteristics vary laterally and longitudinally in weakly confined submarine channel reservoirs. The key axis-to-margin patterns are the proportions of channel elements, channel complexes, channel-complex sets, reservoir facies, and net sand content; static connectivity decreases laterally from the axis to the margins of the system. The key longitudinal patterns in the updip area are channel elements that have levees, are spatially dispersive, and have a radially divergent map pattern. In the downdip area, channel elements are spatially focused and have uniform orientations, and the proportion of channel elements does not change along the longitudinal profile. However, the size of channel elements, percentage of reservoir facies, and connectivity of channel elements are higher in the downdip area. Patterns identified herein are significant because they cannot be resolved using subsurface or sea-floor data. Results of this study can therefore be used to reduce uncertainty in the interpretation of subsurface data, provide input to constrain rule-based forward stratigraphic models, and provide input to constrain reservoir models.

Description: I appreciate Higgs' (2009) discussion of Pyles' (2008) article on the Carboniferous Ross Sandstone of western Ireland (Figure 1), and I am eager to provide a follow-up herein. In his analysis, Higgs challenges the long-established interpretation of the Ross Sandstone both in terms of its depositional environment (submarine fan) and tectonic setting (structurally confined basin). Higgs interprets the Ross Sandstone as being deposited in a large, shallow, freshwater equatorial lake located in a broad foreland basin. He uses this interpretation to argue that the Ross Sandstone is not a suitable outcrop analog for structurally confined submarine fans especially those in northern Gulf of Mexico salt withdrawal basins. Higgs concludes his discussion with a reinterpretation of several of the Earth's best studied submarine-fan outcrops and suggests that they too are lake deposits based on their gross similarities to the Ross Sandstone. This reply examines each of Higgs' criticisms and alternate interpretations and compares them with existing data in the Ross Sandstone. This analysis shows that Higgs' interpretations are inadequately justified. Figure 1 Geologic map of western Ireland and north–south cross section though the Ross Sandstone showing regional stacking patterns and the correlation of condensed sections (goniatite-bearing shale layers and marine bands). Geologic map modified from Pyles (2008). See the work of Pyles (2008) for sources of data used in cross sections. VE = vertical exaggeration. Higgs (2009, p. 1705) proposes that the Ross Sandstone contains evidence for “less-than-marine salinity.” Higgs cited the following observations to support this interpretation: (1) marine fossils are confined to a few thin goniatite-rich layers, and (2) trace fossils are rare and no Nereites ichnofacies are reported. Higgs reasons that because lakes do not contain marine body fossils or Nereites ichnofacies, the Ross Sandstone must therefore have been deposited in a lake. Both of these observations are …