Evidence for multiple styles of sediment gravity flows constructing a prodelta-to-shelf depositional environment : Coaledo Formation, southwest Oregon

Gonzalez, Nicole
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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