Sedimentary model for mixed depositional systems: Conceptual and economic implications

Interactions between along-slope bottom currents and down-slope turbidity flows can create a myriad of features and deposits. Despite numerous efforts to differentiate contourites from turbidites and mixed deposits, reliable diagnostic criteria are still lacking from the stratigraphic and sedimentological viewpoints. The main aim of this study is to differentiate mixed, along-slope- and down-slope-generated elements from other deep-water deposits and marine settings. Four main study areas offshore Argentina, Nova Scotia, the Pacific margin of the Antarctic Peninsula and SW Portugal were chosen to investigate the link between their large- to small-scale morphological features at the seismic and facies scale. Large-scale features include down-slope elongated mounded drifts and wide submarine channels, whereas small-scale features comprise sediment waves, mass-transport deposits (MTDs), gullies and/or scarps. Across the sedimentary record, mixed deposits can be differentiated in view of the respective end members, turbidites and contourites, as well as the occurrence of bottom current reworked sands, transitional facies and other deep-water facies (e.g., hemipelagites or MTDs). These elements are linked to different types of interactions (such as synchronous, asynchronous or passive) across distinct physiographic domains.
Mixed systems can be placed in three main groups based on their location, dimensions, asymmetry, lateral migration, and spatial variability: 1) turbidite-dominated mixed systems, 2) hybrid systems, and 3) contourite-dominated mixed systems. Changes in turbidity or current velocity, and their frequency and duration, condition the provision of sediments and formation of turbidite deposits along the channels and channel margins. The persistence of bottom currents—in addition to their velocity, direction, and hydrodynamic fluctuations— are responsible for pirating and redistributing fine-grained particles, carried in suspension by the turbidity flows, and for building gradual transitions between turbidites and contourite facies, fine-grained contourites and even bottom current reworked deposits.
Morphometric measurements and scaling relationships between the four study cases highlight a relationship between the different morphological features and their tectonic and sedimentary background. Further controlling factors are held to be influential in their evolution at the short- to long-term; varying degrees of confinement, sediment supply, climatic fluctuations or orbital forcing can generate cyclic stacking patterns and affect their overall dimensions and lateral migration.