Neap-spring cycles in a subrecent tidal channel fill (3665 BP) at Schoorldam, NW Netherlands

Abstract

Neap-spring tidal sequences are described from an intertidal channel fill. The channel fill consists of a larger-scale thinning-up and fining-up sequence of parallel laminations. On a smaller scale, more or less regular thickening-up to thinning-up sequences are formed, separated from each other by more or thicker clay laminae, peat detritus or Echinocardium spicules. Deeper lying sequences contain about 28 laminae; higher-up the sequences become more irregular and contain gradually a lesser number of laminae. These thickening-up to thinning-up sequences are interpreted as neap-spring tidal sequences formed by semi-diurnal flood tidal currents (hence ca. 28 laminae per 14 days). Three types have been found: (a) cm-thick sand/clay alternations, (b) ripple-like sand/mud alternations, and (c) mm-thick sand/clay alternations. These types show transitions to each other. Type (a) was deposited at higher current velocities and type (c) at lowest current velocities. Type (b) features are not linsen and flaser bedding but ripple-like features, which during flood only moved one cross-lamina, followed by one slackwater lamina (clay, peat or spicules). They are called “rippled tidal couplets”. The pattern of neap-spring sequences is compared with recent HW-water levels in the North Sea and related to Mean-High Water levels (MHW) and Mean Sea Level (MSL).