Storm-flood dominated delta: a new type of delta in stormy oceans

Storms are increasingly identified as a key depositional process. Their deposits show unique sedimentary characteristics, distinct from deposits of fairweather wave processes, but are not currently separated in classic ternary deltaic classification schemes. The term ‘storm-flood-dominated delta’ is proposed for a new type of delta that more fully represents deltas dominated by storm-flood processes. This study describes several outcrop cliffs of the Gallup Formation, deposited during the Late Turonian to Early Coniacian in northwest New Mexico, that exemplify storm-generated facies and provide examples that can be used to generate a facies model for storm-flood-dominated deltas. Key identification criteria include extensive sharp-based planar to hummocky cross-stratified sandstone beds, commonly presenting as large-sized gutter casts. They are interbedded with mudstones that show low bioturbation intensity and normally and inversely graded beds, suggesting direct deposition from river plumes. Major types of gutter casts are classified based on geometry and dimension. The gutter casts are interpreted as storm channels of storm-flood-dominated deltas and formed by erosion and infill of submerged channels resulting from offshore-oriented downwelling currents that may include localized rip currents. These amalgamated channels are likely linked to multiple feeding rivers and distributary channels that ensured high sediment supply rates and highly efficient offshore-directed sediment transport through hyperpycnal flows. A four-component pyramidal classification scheme of deltaic deposition is employed to emphasize storms as a distinct process in contrast to the traditional classification that lumps storm and fair-weather processes in a single 'wave' end-member. The four component depositional model can be readily applied to interpret storm-dominated environments and provide new insights into depositional processes of the marine realm. Recognition of storm processes in deciphering depositional evolution will also help to better understand control mechanisms of sequence stratigraphy, such as relative sea-level changes, and to analyze sedimentary processes governing sediment transport in 'source to sink' systems.