17 research outputs found
Tidal deposits and their sedimentary structures (Seven examples from Western Europe)
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.
The Oligocene (Tongrian) Kerkom- and Neerrepen sands
SE of Brussels, Belgium, representing areas 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 finingupward 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 (laser 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 determinetion
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. Establish=
ment of detailed paleocurrent patterns may help in distinguishing open-sea tidal deposits from inshore ones