Tidal deposits and their sedimentary structures (Seven examples from Western Europe)

Abstract

Seven examples of tidal sedimentation ranging between modern and Devonian age are illustrated and briefly discussed. They cover known (modern intertidal and subtidal) and mostly undetermined ancient tidal subenvironments, and have been taken from: 1. The estuarine reach of a tidal river, subtidal, Holocene, Barendrecht excavation, The Netherlands. 2. An estuarine channel, subtidal, Holocene, Haringvliet excavation, The Netherlands. 3. A Lower Pleistocene (Tiglian), possibly subtidal estuarine succession, Hattem (Veluwe), The Netherlands. 4. A dune-bearing estuarine sand-bank, intertidal, modern Western Scheldt, The Netherlands. 5. The Lower Cretaceous (Lower Greensand) Woburn sands ± 50 km NW of London, England, being probably deposited in an open marine tidal environment. 6. The Oligocene (Tongrian) Kerkom- and Neerrepen sands SE of Brussels, Belgium, representing an as yet uncertain type of tidal subenvironment. 7. An Upper Devonian tidal succession belonging to the transgressive complex (Cork beds) overlying the Old Red, W. of Cork, Eire. The following features were considered to be diagnostic for these (and other?) tidal deposits (a) vectorial bimodality of the cross-stratification, (b) common joint occurrence at different proportions of largescale and smallscale structured units in super- of juxtaposition. (c) Usually poorly developed sequential regularity with occasional occurrence of fining upward sequences. (d) Unidirectional cross-stratified sets displaying several kinds of features resulting from the intermittent and bidirectional character of the currents (discontinuity planes). (e) Fairly common occurrence of flaser- and/or lenticular bedding respectively consanguineous mud-sand interlaminations in smallscale-structured units. (f) Slight to intense bioturbation in several types of sandy and/or muddy units. At the present state of knowledge a reliable determination of the tidal sub-environment (e.g inshore, offshore, intertidal, subtidal) seems to be virtually impossible for ancient clastic deposits, except for a few cases. Establishment of detailed paleocurrent patterns may help in distinguishing open-sea tidal deposits from inshore ones.