THE STRATIGRAPHIC RECORD OF OUTCROPPING DEEP-WATER SLOPE CHANNEL DEPOSITS

Deep-water slope channel deposits preserve a variable record of sediment transfer across ancient basin margins. For example, levee complexes only provide a record of turbidity currents thick enough to be impacted by flow stripping, as smaller flows do not escape the channel. Sandstone-dominated channel fill tends to bias the stratigraphic record towards the preservation of the terminal phase of channel-filling processes at the expense of the record of potentially long-lived sediment bypass. Heterolithic intra-channel fill is common within channelized slope systems, but these deposits are often overlooked as stratigraphers have focused on sandstone-prone components.

We hypothesize that a more complete record of sediment transfer is preserved in heterolithic intra-channel fill deposits. To evaluate this, we consider heterolithic deep-water strata of the Cretaceous Tres Pasos Formation that crop out in the Magallanes Basin, Chilean Patagonia. The channelized Tres Pasos Formation comprises mudstone, very-fine- to very-coarse-grained sandstone, rare extrabasinal granules to pebbles, mudstone rip-up clast breccias and conglomerates, and sparse fine-grained chaotically bedded units. We interpret that the position of the Tres Pasos channels at a transitional, intraslope point across the Cretaceous basin-margin profile, i.e., down-dip of canyons and up-dip of turbidite depositional environments characteristic of the base of slope, preserves an inclusive perspective of channel-fill components. We provide recognition criteria of these channel-fill components, and assess that they are broadly applicable to slope channels in a range of submarine settings. We also observe distinctive, smaller-scale erosional surfaces and fill within channels that suggest hierarchy and predictable cyclicity of deposition from turbidity currents. Finally, we consider the temporal significance of submarine channel fill, and hypothesize on the relative longevity of channels as important conduits for sediment transfer to deeper water. This hypothesis bears on the meaning of the stratigraphic record of channelized turbidites, sheds light on the longevity and three-dimensional geometry of channels across the slope, and more clearly differentiates channels from common, more ephemeral scours on the modern seafloor.