Neap–spring tide sequences of intertidal shoal deposits in a mesotidal estuary

TLDR

In the mesotidal Westerschelde estuary, the intertidal sandbank has been systematically studied for its hydrography, bedform distribution, and sedimentary structures, with particular attention to neap‑spring tidal variations. Sand‑waves and dunes respond differently to neap‑spring current changes, producing unidirectional cross‑bedding that forms tidal bundles arranged laterally to mirror neap‑spring periods, with internal structures indicating acceleration, full stage, and deceleration of the dominant tide. Full vortex structures are prominent near spring tide but fade at neap, while the subordinate tide sculpts pause planes that enclose the tidal bundles and can be either erosional or depositional mud.

Abstract

ABSTRACT In the mesotidal (tidal range 3·5–4·9 m) Westerschelde estuary (The Netherlands) the intertidal part of a sandbank was the subject of systematic observations of: (1) hydrographic properties, (2) the distribution and response pattern of various types of bedforms, and (3) the sedimentary structures produced. Ebb and flood usually differ considerably in strength, giving rise to a clear dominance of one over the other, which may change over the neap‐spring tide period. Parallel, long‐crested sand‐waves and irregular, short‐crested dunes have a different response to the neap‐spring variation in current velocity. Because one tide (usually the flood in our area) predominated over the other, the internal structure largely consists of unidirectional cross‐bedding, separated into a succession of tidal bundles, each formed during one tide. The tidal bundles are arranged in a lateral sequence reflecting neap‐spring tide periods and differing in character with location. Within the tidal bundle, reactivation, full vortex and slackening structures reflect acceleration, full stage and deceleration of flow respectively in the dominant tide. The full vortex structures tend to be well developed around spring tide but disappear towards neap tide. The subordinate tide carves ‘pause planes’ which enclose the tidal bundles. These pause planes are either erosional or depositional (mud).