Vertical patterns of grain-size parameters of shoreface-connected ridges in the German Bight

Abstract

This report evaluates the vertical variation of grain-size parameters of shoreface-connected ridges off Spiekeroog Island (German Bight). It is based on eight, 1.5 to 2 m-long, vibrocores retrieved along a single transect from successive morphozones of two ridges. The ridges are situated in water depths of 12 to 18 m, have a relief of 2 to 5 m, and are 1 to 1.5 km wide. The colour and textural composition of the ridge sediments suggest a two-fold grain-size facies. The surficial facies is mainly composed of medium to coarse-grained, relatively poorly sorted, brownish to orange-brown sands, and is usually about 60 cm thick. The subsurficial facies consists of gray-coloured fine sands with discrete occurrences of coarser sand layers that are probably storm-deposited. In general, four vertical grain-size patterns are displayed in the cores: sharp (relative to facies transition), gradational, fluctuational and homogeneous. As observed for the surficial cross-ridge pattern, the subsurface sediments of the landward flank and trough of the outer ridge are in general coarser and more poorly sorted than those of the crest and seaward flank. Similarly, the subsurface sediments of the inner ridge crest are coarser and less sorted than their counterparts from the outer ridge crest. These observations suggest that ridge sedimentation processes have always been coherent in time, even prior to the deposition of the surficial facies. The latter is considered to reflect a vertical growth of the ridges. Its origin is probably related to the present-day supply of Pleistocene sediments from the inlet channels during ebb-storm-surge conditions, rather than an in-situ reworking process.