Spilling into confinement: Submarine canyon-confined overbank processes and architecture

Submarine canyon-confined overbanks comprise substantial volumes of thin-bedded sediment gravity flow deposits that were either stripped or overspilled from adjacent channels and offer more complete records of canyon evolution than axial deposits. The Punta Baja Formation, Mexico, comprises a rare exhumed canyon fill with kilometer-scale dip and strike exposures that permit detailed documentation by high-resolution sedimentary logging and photogrammetric models. The lower overbank is characterized by laterally variable sandstone bed thicknesses and grain sizes, indicating that turbulent flows of varying magnitudes overspilled from channels. Thick sandstone beds contain paleocurrent indicator reversals and hummock-like bedforms, representing high-energy combined flows that deflected and reflected against the canyon margin. In the upper overbank, as the canyon fill aggraded and widened, fining- and thinning-upward packages developed, which decay in thickness and grain size away from the axis. Mud-rich packages contain mixed-grain-size bedforms, where abrupt lateral changes between different types of bedform support unusually rapid lateral flow transformations from turbulent to laminar over a distance of ~200 m. Beds with rhythmic bundles of silt-rich, mud-draped bedforms support reworking by internal tides. This study demonstrates that bed-scale analysis of confined overbanks can provide vital, yet commonly overlooked, records of submarine canyon evolution. We show that canyon-confined overbank successions are more diverse than their basinward counterparts because erosional confinement and dynamic internal topography promote a wider range of flow types and magnitudes. These findings suggest the existing twofold interpretation of internal levee or terrace deposits may be oversimplified in proximal overbank settings, where such deposits are unlikely to develop as discrete subenvironments.