Internal mouth-bar variability and differential preservation of coastal-process indicators in low-accommodation deltaic settings

<p>Mouth bars are fundamental architectural elements of deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial-, tide- and wave-dominated) is therefore paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low-accommodation systems because they are commonly characterized by a thin, condensed and top-truncated expression. In this study we analyze the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), which encompasses a fluvio-deltaic system along a ~450 km depositional dip-parallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ~25 km2 study area at the Tucumcari Basin margin. Analysis of facies distribution, depositional architecture and stratigraphic surfaces mapping reveals a 6–10-m-thick, sharp-based and sand-prone deltaic package, comprising several laterally-extensive (>1.4 km width) mouth bars. Within those, we distinguish four different along-strike sub-environments based on differences in grain size, sedimentary structures, bed thicknesses, and bioturbation indices; these are mouth bar axis, off-axis, lateral fringe to distal lateral fringe deposits, and overall reflect waning depositional energy with increasing distality from the distributary channel mouth. The interpreted mouth-bar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the lateral fringe and distal fringe environments. However, mouth-bar deposits amalgamate to form an extensive sand-rich sheet body throughout the study area, in which interflood mudstone to very-fine grained sandstone beds are nearly absent. This indicates a low accommodation/supply (A/S) setting, which promoted recurrent channel avulsion/bifurcation and thus reworking of mouth-bar fringe and distal-fringe sediments, where background coastal processes tend to be better recorded.</p><p>Trends in along-strike changes in sedimentary characteristics from axial to lateral environments are also recognized in other wave- and river-dominated deltaic settings as well, where axial components consist of higher energy facies associations resulting from high-density currents, whereas heterolithics become dominant towards the fringes, where there is an alternation of low- and high-density deposits combined with an increased recording of finer-grained facies associations. Complemented with our study, this suggests that internal hierarchy of mouth bars is evident and observed regardless of dominant coastal processes. Consequently, subdivision of mouth bars into different components can reduce complexity of models deriving from a myriad of facies subdivisions, and guide prediction of facies changes and sand distribution in future studies of proximal deltaic settings. Finally, results of this study evidence internal process-regime variability within mouth-bar components. This cautions against relying solely on the preserved deposits at one given location in a system to infer dominant and subordinate coastal processes (e.g. tidal indicators), with a consequent risk of underestimating the true mixed-influence nature of low-accommodation deltaic settings.</p>

Controls On Deep-Water Sand Delivery Beyond the Shelf Edge: Accommodation, Sediment Supply, and Deltaic Process Regime

ABSTRACT Stratigraphic models typically predict accumulation of deep-water sands where coeval shelf-edge deltas are developed in reduced-accommodation and/or high-sediment-supply settings. On seismic data, these relationships are commonly investigated on a small number of clinothems, with a limited control on their lateral variability. Advanced full-volume seismic interpretation methods now offer the opportunity to identify high-order (i.e., 4th to 5th) seismic sequences (i.e., clinothems) and to evaluate the controls on shelf-to-basin sediment transfer mechanisms and deep-water sand accumulation at these high-frequency scales. This study focuses on the Lower Barrow Group (LBG), a shelf margin that prograded in the Northern Carnarvon Basin (North West Shelf, Australia) during the Early Cretaceous. Thanks to high-resolution 3D seismic data, 30 clinothems (average time span of ∼ 47,000 years) from the D. lobispinosum interval (142.3–140.9 Ma) are used to establish quantitative and statistical relationships between the shelf-margin architecture, paleoshoreline processes, and deep-water system types (i.e., quantitative 3D seismic stratigraphy). The results confirm that low values of rate of accommodation/rate of sediment supply (δA/δS) conditions on the shelf are associated with sediment bypass, whereas high δA/δS conditions are linked to increasing sediment storage on the shelf. However, coastal process regimes at the shelf edge play a more important role in the behavior of deep-water sand delivery. Fluvial-dominated coastlines are typically associated with steep slope gradients and more mature, longer run-out turbidite systems. In contrast, wave-dominated shorelines are linked to gentle slope gradients, with limited development of turbidite systems (except rare sheet sands and mass-transport deposits), where longshore drift currents contributed to shelf-margin accretion through the formation of extensive strandplains. In this context, reduced volumes of sand were transported offshore and mud belts were accumulated locally. This study highlights that variations from fluvial- to wave-dominated systems can result in significant lateral changes in shelf-margin architecture (i.e., slope gradient) and impact the coeval development of deep-water systems (i.e., architectural maturity). By integrating advanced tools in seismic interpretation, quantitative 3D seismic stratigraphy represents a novel approach in assessing at high resolution the controls on deep-water sand delivery, and potentially predicting the type and location of reservoirs in deep water based on the shelf-margin architecture and depositional process regime.

Preparation and Morphological Studies of Epitaxial Layers of a Solid Solution Si1–xGex

The article shows the selection of the optimal process regime for the growth of epitaxial layers of Si1–xGex solid solutions from tin and gallium solution – melt on a Si<111> substrate, with the lowest dislocation densities that we experimentally achieved. An exponential relationship was found between the values of the dislocation density and the film thickness.With a smooth variable composition of the structure, respectively, by smoothly changing the lattiece parameters of the graded-gap solid solution, structurally perfect epitaxial layers Si1–xGex (0 < x < 1) were obtained

High-resolution correlations of strata within a sand-rich clinothem using grain fabric data, offshore New Jersey, USA

Trajectories of successive clinoform rollovers are widely applied to predict patterns of spatio-temporal sand distribution. However, the detailed internal architecture of individual clinothems is rarely documented. Understanding the textural complexities of complete topset-foreset-bottomset clinothem sequences is a key factor in understanding how and when sediment is transferred basinward. This study used high-resolution, core-based analyses of 267 samples from three research boreholes from quasi-coeval topset, foreset, and bottomset deposits of a single Miocene intrashelf clinothem recovered during Integrated Ocean Drilling Program (IODP) Expedition 313, offshore New Jersey, USA. Topset deposits were subdivided into three sedimentary packages based on grain character and facies analysis, consisting of upper and lower river-dominated topset process-regime packages separated by a middle wave- and storm-dominated process-regime package. Temporal variability in topset process regime exerts a quantifiable effect on grain character across the complete depositional profile, which was used here to correlate topset deposits with time-equivalent sedimentary packages in foreset and bottomset positions. River-dominated sedimentary packages have higher sand-to-mud ratios; however, the grain character of river-dominated sedimentary packages is texturally less mature than that of wave- and storm-dominated deposits. Differences in grain character between packages dominated by different process regimes increase basinward. The novel use of quantitative grain-character data allows intraclinothem time lines to be established at a higher resolution than is possible using chronostratigraphic techniques. Additionally, stratigraphic changes in grain character were used to refine the placement of the basal sequence boundary. These results challenge the idea that clinoform trajectories and stacking patterns are sufficient to describe spatio-temporal sand-body evolution across successive clinothems.