ON THE ORIGIN OF PALEOCURRENT COMPLEXITY WITHIN DEEP MARINE CHANNEL LEVEES

Complex patterns of temporal and spatial variation in paleoflow within individual turbidites have been inferred from studies of both recent and ancient submarine channel-levee sands and sandstones. Previous interpretations of these paleocurrents have invoked shifting patterns of meandering streamlines in essentially unconfined flows, and in some cases topographic deflection of flows as they spilled from the channel. However, other flow indicators, such as current ripples, suggest that overbank flow may exhibit more uniform flow directions. Here we re-examine the spatial and temporal evolution of overbank flow on submarine channel levees, using grain fabric data from the Cretaceous Rosario Formation and extant data from the recent Amazon channel. We integrate these observations with our theoretical understanding of the long-term evolution of overbank flow processes to propose a process-based model of evolution of overbank flow across submarine channel levees. Overbank flow over levees is inferred to be dominated by three main processes: (1) gravity forces that attempt to direct flow down the local maximum slope; (2) inherited momentum from the channelized flow, which in many cases is closer to the axial flow direction; (3) development over time of an overbank wedge of fluid that moves broadly parallel to the direction of the main channel. The interaction among these processes varies both in space and time, and collectively accounts for the observed paleoflow variations inferred from deposits. This study may allow better interpretation of depositional sub-environments within levee turbidites, both in core and in outcrop.