What factors control the development of moats and their associated contourite drifts?

The interaction of sedimentary systems with oceanographic processes in deep-water environments is still not well understood, despite its importance for improving paleoreconstructions and for understanding source-to-sink sediment transport. The aim of this study is to improve our understanding of sediment deposits formed by the action of bottom currents (contourites), and specifically elongated depressions that are called ‘moats’. Moats are common incisions oriented alongslope formed by the main core of the current and are usually associated with a mounded, elongated sediment deposit (contourite drift) adjacent to one of the moat sides. Based on seismic/sub-bottom profiler and bathymetric data, we analyse the morphology and stratigraphy of moats in different parts of the world ocean. Moats show a large variability in terms of size and shapes. The length can be over 95 km, width (horizontal distance between slope and drift crest) can be over 26 km, relief (vertical distance between drift crest and deepest point of the moat) can be over 480 m and the relief-width ratio can be over 0.1. Even though Coriolis force has a great influence on the general wind and ocean circulation, it seems to have only a small or no influence on the moat morphology. However, over 100 measurements show that the slope angle at which the moat forms influences the drift angle and the relief-width ratio. Higher slope angle leads to high drift angle and relief-width ratio. Possibly steep slopes focus and narrow the main core of the current. The gradient of speed between inside and outside the moat (i.e. on the contourite drift) is higher in a narrow jet. Thus, a narrow jet can lead to higher drift angle and thus a higher relief-width ratio. Three types of stratigraphic stacking pattern can be distinguished based on the location at which the seismic reflections terminate. Seismic reflections from the drift in Type A reflections follow the moat morphology and onlap at the slope side. Going from drift to moat in Type B, the slope angle of the seismic reflections increases and reflections downlap at the bottom of the moat. In Type C seismic reflections are truncated at the drift side and show only a slight dip towards the moat. Within one moat, the stratigraphic type can change. These different types cannot alone be explained by variations in slope angle and thus other factures like current velocity and sediment supply have to be considered.