VERTICAL PATTERNS OF GRAIN-SIZE PARAMETERS OF SHOREFACE-CONNECTED RIDGES IN THE GERMAN BIGHT

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 sing le 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 ridg e sediments suggest a two-fold grain-size facies. The surficial facies is mainly composed of medium to coarse-grained, relatively poorly sor ted, brownish to orange-brown sands, and is usually about 60 cm thick. The subsurficial facies consists of gray-coloured fine sands with dis crete occurrences of coarser sand layers that are probably storm-depos ited. In general, four vertical grain-size patterns are displayed in t he cores: sharp (relative to facies transition), gradational, fluctuat ional and homogeneous. As observed for the surficial cross-ridge patte rn, the subsurface sediments of the landward flank and trough of the o uter ridge are in general coarser and more poorly sorted than those of the crest and seaward flank. Similarly, the subsurface sediments of t he inner ridge crest are coarser and less sorted than their counterpar ts from the outer ridge crest. These observations suggest that ridge s edimentation processes have always been coherent in time, even prior t o the deposition of the surficial facies. The latter is considered to reflect a vertical growth of the ridges. Its origin is probably relate d to the present-day supply of Pleistocene sediments from the inlet ch annels during ebb-storm-surge conditions, rather than an in-situ rewor king process.