Architecture of a coarse-grained channel-levée system: the Rosario Formation, Baja California, Mexico

Seafloor images of coarse-grained submarine channel-leveée systems commonly reveal complex braid-plain patterns of low-amplitude bedforms and zones of apparent bypass; however, mechanisms of channel evolution and the resultant channel-fill architecture are poorly understood. At Playa Esqueleto the lateral relationships between various elements of a deep-marine slope channel system are well-exposed. Specifically, the transition from gravel-dominated axial thalwegs to laterally persistent marginal sandstones and isolated gravel-filled scours is revealed. Marginal sandstones pass into a monotonous thin-bedded succession which built to form relatively low-relief leveées bounding the channel belt; in turn, the leveées onlap the canyon walls. Three orders of confinement were important during the evolution of the channel system: (i) first-order confinement was provided by the erosional canyon which confined the entire system; (ii) confined leveées built of turbidite sandstones and mudstones formed the second-order confinement, and it is demonstrated that these built from overspill at thalweg margins; and (iii) third-order confinement describes the erosional confinement of coarse-grained thalwegs and scours. Finer-grained sediment was transported in suspension and largely was unaffected by topography at the scale of individual thalwegs. Facies and clast analyses of conglomerate overlying channel-marginal scours reveal that they were deposited by composite gravity flows, which were non-cohesive, grain-dominant debris flows with more fluidal cores. These flows were capable of basal erosion but were strongly depositional; frictional freezing at flow margins built gravel leveées, while the core maintained a more fluidal transport regime. The resultant architecture consists of matrix-rich, poorly sorted leveées bounding better-sorted, traction-dominated cores. The planform geometry is interpreted to have consisted of a low-sinuosity gravel braid-plain built by accretion around mid-channel and bank-attached bars. This part of the system may be analogous to fluvial systems; however, the finer-grained sediment load formed thick suspension clouds, probably several orders of magnitude thicker than the relief of braid-plain topography and therefore controlled by the leveées and canyon wall confinement.