Vertical trends within the prograding Salt Wash distributive fluvial system, SW United States
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Abstract Progradation is an important mechanism through which sedimentary systems fill sedimentary basins. Although a general progradational pattern is recognized in many basins, few studies have quantified system scale spatial changes in vertical trends that record fluvial system progradation. Here, we provide an assessment of the spatial distribution of vertical trends across the Salt Wash distributive fluvial system ( DFS ), in the Morrison Formation SW , USA . The vertical distribution of proximal, medial and distal facies, and channel belt proportion and thickness, are analysed at 25 sections across approximately 80 000 km 2 of a DFS that spanned approximately 100 000 km 2 . The stratigraphic signature of facies stacking patterns that record progradation varies depending on location within the basin. An abrupt and incomplete progradation succession dominates the proximal region, whereby proximal deposits directly overlie distal deposits. A more complete succession is preserved in the medial region of the DFS . The medial to distal region of the DFS are either simple aggradational successions, or display progradation of medial over distal facies. Spatial variations in facies successions patterns reflects preservation changes down the DFS . A spatial change in vertical trends of channel belt thickness and proportion is not observed. Vertical trends in channel belt proportion and thickness are locally highly variable, such that systematic up‐section increases in these properties are observed only at a few select sites. Progradation can only be inferred once local trends are averaged out across the entire succession. Possible key controls on trends are discussed at both allocyclic and autocyclic scales including climate, tectonics, eustasy and avulsion. Eustatic controls are discounted, and it is suggested that progradation of the Salt Wash DFS is driven by upstream controls within the catchment.Keywords:
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Abstract It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.
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Abstract Since the end of the post‐glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south‐eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid‐Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid‐Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.
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Lower to Middle Permian strata in the Delaware Basin in southeast New Mexico and West Texas present a unique system to assess spatial and temporal variations in carbonate slopes as well as controls on variation. Detailed subsurface gamma-ray well-log mapping and visualization using approximately 8000 densely spaced well logs in the northern portion of the Delaware Basin aided in correlating and characterizing Wolfcampian and Leonardian (L1-L6 composite sequences) slope profiles. The Wolfcampian strata form irregular slope profiles ranging from 2 to 10°. L1-L2 composite sequences develop comparatively steeper aggradational to retrogradational slopes with maximum mapped gradients between 15 and 25°, platform to basin relief of 920–1100 m, and slope widths of 10–20 km. The L3-L6 composite sequences denote a shift to platform and margin progradation. However these intervals exhibit variable progradational profiles in the along the western slopes with maximum slope gradients between 2 and 10°, progradation aggradation ratios ranging approximately from 10 to 100, wider slope widths reaching over 30 km, and platform to basin relief ranging from 500 to 900 m while northern slopes exhibited more continuous steep gradients between 10 and 20°, progradation-aggradation ratios of 0–13, slope widths of 10–15 km, and average approximate platform to basin relief of 900 m. The broad shift from aggradation-retrogradation to progradation during the Leonardian correlates with low-order sea-level change previously noted from outcrop studies. The eustatic shift likely influenced platform accommodation, driving a regional transition to progradation during low-order highstand. The variation in Leonardian progradation rates is significantly influenced by underlying Pennsylvanian carbonate buildups generating large-scale inflections in the slope and toe-of-slope environments. The underlying feature generated lower platform to basin relief and wider slopes to the west, promoting progradation, and formed narrower slopes with greater platform to basin relief to the north, likely inhibiting rapid progradation. Bottom currents and subsequent drift deposition along the western slopes also changed slope angle and geometry, promoting progradation over and around the features in subsequent units. Also, bottom currents possibly swept sediments from toe of slope environments in the north where drifts aren't present, inhibiting progradation and driving aggradation. The documented spatial and temporal variations in slope profiles may exert significant influence on transport, routing, and confinement of platform-top and slope-derived sediments to the deeper basin, imparting control on basinal stratigraphic architecture. As such, this type of characterization should provide a predictive tool for assessing reservoir and non-reservoir architecture in the slope and re-sedimented basinal deposits in both the Permian Basin and other analogous basins.
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Late Jurassic (Kimmeridgian) to Early Cretaceous (Valanginian) carbonate platforms in the western Atlantic exhibit both progradational and aggradational growth modes. Their development is reflected in seismic shelf edge and depositional facies geometries. These geometric relationships, together with paleotologic and diagenetic data, permit interpretation of the primary controlling factors on platform evolution. Atlantic platform growth and development during the Late Jurassic relative to rise in sea level vary directly with the siliciclastics. Carbonate platforms receiving siliciclastics prograde; those which do not, aggrade. This variation in platform evolution is found in equivalent-age platforms from other western Atlantic basins and as successive platform growth stages within the Baltimore Canyon basin. The switch from progradation to aggradation follows the termination of siliciclastics influx during flooding of an exposed platform. Platform height is maintained or decreases in the prograded stage but increases in the aggraded stage of platform growth associated with rising sea level. Aggraded platform growth demands progressively larger volumes of carbonate sediment as platform height increases. Progradation is thus limited by an increase in carbonates required for vertical platform growth. Siliciclastic filling of basins adjoining carbonate platforms has several important implications. First, siliciclastics may bypass active carbonate margins. Second, their volumetric contribution tomore » basin fill can be sufficiently large to allow progradation of platforms that are otherwise limited to aggradational growth by their sedimentary carbonate budget.« less
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The world's deltas are at risk of being drowned due to rising relative sea levels as a result of climate change, decreasing supplies of fluvial sediment, and human responses to these changes. This paper analyses how delta morphology evolves over multi-decadal timescales under environmental change using a process-based model. Model simulations over 102 years are used to explore the influence of three key classes of environmental change, both individually and in combination: (i) varying combinations of fluvial water and sediment discharges; (ii) varying rates of relative sea-level rise; and (iii) selected human interventions within the delta, comprising polder-dykes and cross-dams. The results indicate that tidal asymmetry and rate of sediment supply together affect residual flows and delta morphodynamics (indicated by sub-aerial delta area, rates of progradation and aggradation). When individual drivers of change act in combination, delta building processes such as the distribution of sediment flux, aggradation, and progradation are disrupted by the presence of isolated polder-dykes or cross-dams. This suggests that such interventions, unless undertaken at a very large scale, can lead to unsustainable delta building processes. Our findings can inform management choices in real-world tidally-influenced deltas, while the methodology can provide insights into other dynamic morphological systems.
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