Browsing by Subject "Hyperpycnal flows"

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Evidence for multiple styles of sediment gravity flows constructing a prodelta-to-shelf depositional environment : Coaledo Formation, southwest Oregon
(2022-05-05) Gonzalez, Nicole; Mohrig, David
Previous depositional models for marine storm beds focus mainly on generation by combined flow, but this mechanism alone does not account for both volume and distance of sand transport necessary to explain the deposits. Previous studies have shown that sediment gravity flows (SGFs) efficiently transport sediment from deltas into marine sedimentary basins. Suspended sand moved by rivers during floods can continue basinward via hyperpycnal flows, undercurrents that have a higher density than water in the receiving basin. Hyperpycnal flows produce deposits predominantly composed of mud-to-fine sand called hyperpycnites. Phenomenal exposure of the Lower Coaledo Formation along the southwest Oregon coast provides a superb opportunity to refine, revisit, and grow process-based interpretations for storm beds that construct a prodelta-to-shelf depositional environment. The Lower Member of the Coaledo Formation preserves world-class examples of small and large-scale hummocky cross-stratification (HCS) formed under conditions of wave-dominated combined flow. While storm-generated waves aid in reworking sand to produce a suite of bedforms, we propose the storm beds of the Coaledo were deposited from hyperpycnal flows. In-depth stratigraphic and sedimentological analysis of the fabric, texture, grain size, and sedimentary structures of the Coaledo identified three different styles of sediment gravity flows: (1) hyperpycnal flows (2) subaqueous debris flows, and (3) slumps of previously deposited delta related sediment. Two types of hyperpycnites are expressed in the stratigraphy: (A) wave-influenced hyperpycnites and (B) non-wave-influenced hyperpycnites. The occurrence of aggrading wave ripples and HCS suggests influence from storm events. However, hyperpycnites composed of planar-laminated beds with fine-grained leafy detritus are interpreted to have been deposited during a period of stormless activity. In other words, fluvial flooding and hyperpycnal flows can occur independent of marine storm events. Planar-laminated hyperpycnites include tidal rhythmites associated with flood and ebb cycles that caused flow velocities and suspended-sediment concentrations from the river to decrease as tides came in and increases to occur as tides went out. Process-oriented analysis coupled with 3D outcrop models are used to constrain a depositional model for the Coaledo storm deposits
Influence of surface gravity waves on sediment transport and deposition of hummocky cross-stratified sands on marine shelves
(2017-05) Arora, Khushboo; Goff, John A.; Wood, Lesli J.; Steel, Ronald J; Mohrig, David; Kim, Wonsuck; Southard, John
Hummocky cross-stratification (HCS) is widely interpreted in wave-influenced ancient shallow-marine deposits. Currently, debate is abundant surrounding HCS with differences in opinions driven partly by poor understanding of the nature and magnitude of processes that lead to HCS formation; lack of enough data to characterize HCS three-dimensional architecture, lateral variability and facies; and inadequate knowledge of how the presence of waves influences sediment movement on marine shelves. This dissertation focusses on filling these knowledge gaps by conducting a tripartite study involving numerical analysis of outcrop exposures of HCS-bearing shelf sands, geostatistical analysis of post-storm-generated bedforms on a modern shelf, and physical modeling of wave interaction with turbid hyperpycnal flows. Numerical analysis and estimated wave time periods from HCS measurements in Late Cretaceous marine transgressive strata in southeastern San Juan Basin, New Mexico, and in deposits of the Cape Sebastian Sandstone (CSS) in southwestern Oregon suggest that HCS in these deposits was more likely formed by decayed swell waves that had travelled long distances away from powerful storms, rather than by active storm waves. These studies suggest that the maximum sustainable wind speeds during greenhouse periods such as those that characterize the late Cretaceous could have been higher than those of modern storms. Study of modern storm deposits shows that sand ridge hydrodynamics affects grain-size variation across the large-scale bedforms (sand ridges and sorted bedforms), which in turn governs the variation in surficial bedforms (megadunes, hummocky bedforms, and 2.5 dimensional dunes). Results indicate that while large-scale bedforms are more current-driven, the small-scale bedforms are more likely to have formed under wave-dominated conditions. These results underscore the role of seafloor morphology on distribution of hummocky bedforms and associated structures on the marine shelf. Lastly, study of wave interaction with turbid hyperpycnal shelf flows shows that wave-generated turbulence helps maintain elevated density in the flows and hence maintains the driving forces of current movement. These elements enable the flow to sustain its energy despite low shelf gradients. These results highlight the contribution of waves in sediment transport across the shelf. Overall, this doctoral dissertation enhances our ability to predict paleo-conditions responsible for deposition of HCS in the geological record, improves our understanding of the role of waves in sand movement on and across shelves, and is the first research to document the relationship between hummocky bedforms and storm processes in the modern shelf.