The elusive continental rise: Insights from residual bathymetry analysis of the Northwest Atlantic margin

Abstract

The morphology of divergent continental margins results from, amongst other things, rift processes, juxtaposition of oceanic, continental and transitional crust, and subsequent loading and shaping by sedimentary processes. The resulting morphologies have led to conventional classification of margin components into shelf, slope, rise and abyssal plain segments. Division of the inclined part of a continental margin into a slope and a rise; however, is challenging, due to complex geomorphologies and mixture of sedimentary processes. The rise, in particular, seems to elude specific criteria on which it might be uniquely identified, despite very specific criteria proposed in its original inception. This study seeks to develop a methodology on which the rise might be made more readily apparent. Solely diffusive behaviour of sediment flux should result in an exponential-shaped slope. This basic form is referred to as a graded shape. The continental rise, as a depositional body, conceptually should represent a departure from this shape, otherwise it cannot be morphologically distinguished from the continental slope. It is proposed that a graded slope represents the base-level condition for a margin that results from long-term diffusive sediment distribution due to a balance of processes of sediment input, transport, deposition, erosion and output. This study shows empirically that this base-level condition is mathematically predictable as an exponential decay curve with a constant exponent. To test this hypothesis, a 3D graded surface model slope that extends from the shelf break to the abyssal plain was produced for the Northwest Atlantic margin (NWAM) from Blake Outer Ridge to the northern tip of Labrador Sea. Removal of this model from measured bathymetry results in residual bathymetric anomalies that highlight graded, above-grade and below-grade segments of the margin. Nearly half of the margin by area (2 of 4.6?M?km2) fits the graded curve model to within +/? 100?m. Above-grade segments total 1.9?M?km2 and are mostly related to contourite deposition (e.g., Chesapeake Drift and Hatteras Outer Ridge) and submarine fan deposition (e.g., Laurentian Fan). Below grade segments (0.6?M?km2) are related to erosion (e.g., Blake Escarpment) or non-deposition (e.g., Flemish Cap). There do not appear to be any consistent criteria in the form or distribution of residual bathymetric anomalies and associated sedimentary properties that allow ready distinction of the continental slope from the continental rise. Chesapeake Drift and Hatteras Outer Ridge are the largest anomalies along the entire margin, and yet it was on these features that the original concept of the continental rise was developed. The complexity of the margin, highlighted by this residual bathymetry technique, indicates the inadequacy of dividing a margin into long established components of slope, rise and abyssal plain. Classification of the margin according to grade seems more informative in terms of understanding its geomorphology, sediment distribution and geologic history.