On sediment accumulation rates and stratigraphic completeness: Lessons from Holocene ocean margins

Stratigraphic completeness is a fundamental consideration when deciphering the mass accumulation history of sediments and the geologic record of earth and ocean processes. In this study, stratigraphic completeness was examined in the context of late Holocene sedimentary successions using published sediment accumulation rates for five ocean margin systems (Amazon shelf, Hudson estuary, northern California shelf, Mid-Atlantic slope, Santa Monica Bay). Plots of mass accumulation rate versus time span of averaging were used to determine how rates scale with measurement period, and to estimate levels of stratigraphic completeness for comparison within and among margin systems. Statistically significant inverse correlations between accumulation rate and time span of averaging are indicated for all but one of these systems—most of the sedimentary records examined are stratigraphically incomplete. At the 103-yr level of resolution, completeness is 20–48% for strongly tidal estuarine (Hudson estuary) and deltaic shelf (Amazon shelf) sites, 51–91% for accretionary shelves (northern California shelf) and slopes (Mid-Atlantic slope), and 85–100% for a sediment-starved slope (Santa Monica bay). Mass accumulation rates converge to a relatively narrow range (0.01–0.1 g cm−2 yr−1) at the 104-yr level of resolution, consistent with the notion that there are universal controls on sediment accumulation rate, i.e., rate of sea-level rise and sediment supply. Among sites on the Amazon and northern California shelves, within-system completeness varies by ∼10–20% on account of site-specific sedimentary processes that preferentially trap or disperse suspended sediment. Overall, stratigraphic completeness increases with water depth shelf-to-slope, yet depth is not a robust predictor of completeness in general owing to differences in strata-forming processes among shallow-marine environments. Significantly, completeness varies inversely with instantaneous deposition rate as the most sediment-rich systems tend to exhibit the most incomplete sedimentary records. The findings of this study emphasize the importance of considering time span and fidelity when interpreting the accumulation history of modern and Holocene sedimentary strata.