Evolution and architecture of an overbank in an ocean-facing canyon-fill.

Submarine canyon-fills comprise substantial volumes of thin-bedded successions deposited by sediment gravity flows that are either stripped or overspill from adjacent channels into highly confined, topographically complex overbank settings. Here, we document the Punta Baja Formation, a rare example of an exhumed canyon-confined overbank succession with good 3D constraints from the Mesozoic Peninsular Ranges Forearc, Mexico. High-resolution sedimentary logging and drone-captured photogrammetric models reveal that the overbank was a highly dynamic environment, where different bed types point to a variety of flow transformations and complex topographical interactions that evolved through time. The lower overbank is characterised by variable bed thicknesses, grain-sizes and palaeocurrent directions, which point to a wide range of unfiltered flows that overspilled from channels. Thick sandstone beds contain distinct hummock-like bedforms, representing high energy combined flows that repeatedly deflected and reflected against the high relief canyon margin, suggesting complete confinement within the conduit. Locally, thinner beds are disrupted by slides, debrites and scour surfaces on the canyon floor. As the canyon system matured, constituent channels migrated laterally and aggraded. Here, the character of the overbank changes, developing distinct fining- and thinning-upward packages that decay in thickness and grain-size away from the channel axis. Packages are more mud-rich and beds contain mixed grain-size bedforms indicating smaller magnitude, rapidly decelerated transitional flows that failed to interact with the canyon margin. In more quiescent parts of the upper overbank, beds containing rhythmic bundles of silt-rich, mud-draped bedforms are interpreted to be the product of sediments reworked by internal tides. This is the first detailed study of fine-grained fills in an ancient ocean-facing submarine canyon. Canyon-confined overbanks offer a more diverse fill, with flow transformations that demonstrate a complex balance between erosion and deposition, and an absence of discrete internal levée or depositional terrace elements that have been identified in more distal confined overbank settings.