Geology Reference
In-Depth Information
13
Shallow-Marine Tidal Deposits
Jean-Yves Reynaud and Robert W. Dalrymple
Abstract
Shallow-marine tidal deposits form on open shelves, and more specifically in
open-mouthed embayments and semi-enclosed epicontinental seas, where the
oceanic tide is amplified by resonance. They are also present in straits and seaways
where the tidal currents are accelerated by flow constriction. Complex interactions
of the tide with the seafloor and coastal topography bring about tidal asymmetry,
generating tidal-transport pathways with net, unidirectional transport of sediment
over long distances. Tidal currents are commonly capable of resuspending mud in
shallow-marine settings, but little is known about the role of tidal currents in the
deposition of muddy deposits in the offshore domain. The best-known shelf tidal
deposits are sandy and bioclastic transgressive 'lags' that mantle flooding sur-
faces. These lags are generally thin, but can reach thicknesses of 10-30 m in tidal-
current ridges and sand sheets. These deposits are composed of dominantly
well-sorted, cross-bedded sands with good reservoir properties. Careful architec-
tural analysis allows the distinction between the deposits of compound dunes,
tidal-current ridges and migrating sand sheets. The occurrence of shallow-marine
tidal deposits is sensitive to changes in sea level; paleotidal modeling has great
potential to help understanding their occurrence in space and time.
13.1
Introduction
water depths from as shallow as 10-20 m to as much as
150-200 m, and up to 400 m along some formerly gla-
ciated margins. The tides, which generally are created
in the open ocean, pass over the shelf on their way to
the coast, interacting with the seafloor as they go
(Wright et al.
1999
; Allen
1997
). Geomorphologically,
continental shelves and shallow-water seas are diverse.
Most continental shelves with significant tidal currents
occur on wide passive continental margins, because
tidal action typically increases as shelf width becomes
greater. Such shelves are commonly straight, with the
shoreline essentially parallel to the shelf break for sev-
eral hundred kilometers. Structural complexities in
continental margins create large-scale
embayments
,
Shallow-marine areas, which includes continental
shelves and shallow-water seas, are considered here to
extend from near the coast to the shelf break, spanning
