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Abstract

A survey of tidal flat structure in the back-barrier area of Spiekeroog Island, Germany, southern North Sea, has provided a variety of morphologically distinct sediment bodies which, however, reveal only a limited number of definite structural units. These exhibit specific facies profiles visible in sediment cores, particularly in relief casts.

The most conspicuous structural and facies units, as seen from the margins at low water line towards the central parts of the tidal flats, are: (1) fine-sandy edges and spits where strong physical reworking by tidal currents results mainly in laminated stratification and complete absence of bioturbation; (2) edge-gully zones with channel fill structures; (3) trough channels with intense erosion of Mya arenaria shells originating from ancient colonies; (4) sandy tidal flats with ripple stratification showing sporadic and transient bioturbation by Arenicola marina; (5a) sandy tidal flats with ripple stratification, sporadic Arenicola bioturbation and sparsely spaced tubes of Lanice conchilega; (5b) sandy platforms with densely spaced tubes of Lanice conchilega and indistinct remnants of physical stratification; (6) Mytilus beds underlain by shell layers representing remnants of former Mytilus colonies; (7) muddy channel zones crossing the elevated watershed area of the back-barrier system totally bioturbated by Heteromastus filiformis, additionally populated by Mya arenaria; and (8) old semi-consolidated mud banks totally bioturbated and with shells of Mya arenaria in life position, representing ancient deposits in former watershed channel zones.

The time covered by the facies profiles increases from the marginal zones towards the central platforms of the tidal flats, that is, from several tides to several decades or even centuries. In addition to the succession of facies types in the horizontal direction, vertical successions are visible, as well, according to Walther's facies rule.

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