Quantitative analysis of variations in depositional sequence thickness from submarine channel levees



Abstract Thickness variations across-levee and downchannel in acoustically defined depositional sequences from six submarine channel-levee systems show consistent and quantifiable patterns. The thickness of depositional sequences perpendicular to the channel trend, i.e. across the levee, decreases exponentially, as characterized by a spatial decay constant, k. Similarly, the thickness of sediment at the levee crest decreases exponentially down the upper reaches of submarine channels and can be characterized by a second spatial decay constant, λ. The inverse of these decay constants has units of length and defines depositional length scales such that k−1 is a measure of levee width and λ−1 is a measure of levee length. Quantification of levee architecture in this way allowed investigation of relationships between levee architecture and channel dimensions. It was found that these measures of levee e-folding width and levee e-folding length are directly related to channel width and relief. The dimensions of channels and levees are thus intimately related, thereby limiting the range of potential channel-levee morphologies, regardless of allocyclic forcing. A simple sediment budget model relates the product of the levee e-folding width and e-folding length to through-channel volume discharge. A classification system based on the quantitative downchannel behaviour of levee architecture allows identification of a ‘mid-channel’ reach, where sediment is passively transferred from the through-channel flow to the levees as an overspilling flow. Downstream from this reach, the channel gradually looses its control on guiding turbidity currents, and the resulting flow can be considered as an unconfined or spreading flow.