The Stratigraphic Incompleteness of Submarine Channels

Turbidity currents transport prodigious quantities of sediment across the world's oceans through submarine channels. These flows damage strategically important seafloor infrastructure and their deposits host major oil and gas reservoirs. We therefore need to understand these flows, but their very powerful nature makes direct monitoring challenging. Most studies to date focus on the deposits that turbidity currents leave behind in the sedimentological record. However, deposits of individual flows are likely to be reworked by successive flows, but it is unclear as to what extent. How complete is the stratigraphy of these deposits? What percentage of flow deposits are preserved in the rock record? Are some events better preserved than others, and if so, why? We address these questions by re-analysing the most detailed time-lapse mapping yet of a turbidity current system. This field dataset comes from the fjord-head Squamish Delta in British Columbia, Canada where Hughes Clarke (2016) collected 93 near-daily repeat surveys in 2011. These surveys revealed the seafloor response to more than 100 turbidity currents. Here we use temporal changes in seabed elevation to understand patterns of deposition and erosion. We calculate the total thickness of sediment deposited at each location, and document the percentage of sediment that is preserved (i.e. stratigraphic completeness) at multiple time-steps over the surveyed period. The average stratigraphic completeness across the delta near submarine channels is <1%, but this is highly spatially variable. Some levees record up to 40% completeness. The low value is largely due to upstream migrating bedforms that constantly rework previously emplaced sediments. Surprisingly, even at the terminal lobes, stratigraphic completeness is typically <5%. These results provide new insights into the evolution of submarine channels and why their deposits produce a highly incomplete record of submarine flows.