The role of saltwater and waves in continental shelf formation with seaward migrating clinoform

Abstract

Continental shelves have generally been interpreted as drowned coastal plains associated with the allogenic effect of sea-level variation. Here, without disputing this mechanism we describe an alternative autogenic mechanism for subaqueous shelf formation, driven by the presence of dissolved salt in seawater and surface waves. We use a numerical model describing flow hydrodynamics, sediment transport, and morphodynamics in order to do this. More specifically, we focus on two major aspects: 1) the role of saltwater in the subaqueous construction of continental shelves and 2) the transformation of these shelves into seaward-migrating clinoforms under the condition of repeated pulses of water and sediment input and steady wave effects, but no allogenic forcing such as sea-level change. In the case for which the receiving basin contains fresh water of the same density as the sediment-laden river water, the hyperpycnal river water plunges to form a turbidity current that can run out to deep water. In the case for which the receiving basin contains sea water but the river contains sediment-laden fresh water, the hypopycnal river water forms a surface plume that deposits sediment proximally. This proximate proto-shelf can then grow to wave base, after which wave-supported turbidity currents can extend it seaward. The feature we refer to is synonymous with near-shore mud belts.

Keywords: continental shelves; dissolved salt; hypopycnal flows; mud belts; wave base.

Fig. 1.

Simulation results of hypopycnal flow in case 1: (A) suspended sediment transport expressed by the excess density due to sediment, (B) dissolved salt transport expressed by the excess density due to salt, and (C) magnitude of flow velocity.

Fig. 2.

Simulation results of hyperpycnal flow in case 1: (A) suspended sediment transport expressed as excess density due to sediment and (B) magnitude of flow velocity.

Fig. 3.

Bed surface profiles developed by the hypo- and hyperpycnal flows of case 1 (SI Appendix, Table S1) after 4 h of simulation.

Fig. 4.

Computational result of shelf morphology development without an initial proto-shelf. The time interval between each line represents the time between four repeated pulse inputs, that is, 4(T_flood + T_wave). The blue points denote the rollover point, which is determined by the maximum curvature of shelf morphology, and the solid black line shows the time change of position of the rollover points.

Fig. 5.

Temporal change in modeled continental slope steepness corresponding to the case of Fig. 4.

Fig. 6.

(A–F) Development of sediment waves at the shelf rollover through three repeated pulse inputs.