Page 234 - Sedimentology and Stratigraphy

Tide-dominated Clastic Shallow Seas 221

Fig. 14.6 The strata in the hillside are a
succession passing up from offshore
mudstones (bottom left), to thin-bedded
sandstone of the offshore transition zone
up to the cliff-forming shoreface sand-
stones.

Fig. 14.7 Sandwaves, sand ridges and sand ribbons in shallow, tidally influenced shelves and epicontinental seas.

The form of tidal deposits in shallow marine environ- is only matched in size by aeolian dunes and some
ments depends on the velocity of the tidal current. large bar forms in rivers. Individual sandwaves are
In areas of low velocity currents (ca. 50 cm s 1 ) isolated on the sea floor if the supply of sediment is
sand occurs in low relief sheets and patches that are low, but form amalgamated banks of sandwaves if
rippled on the surface. At low to moderate near- there is abundant sand supply to the shelf.
surface tidal current velocities (50 to 100 cm s 1 ) In shallow seas with higher velocity tidal currents
sandwaves are typical: these bedforms are a class of (over 100 cm s 1 ) sediment on the sea floor forms
large subaqueous dunes that have heights of at least sand ribbons elongated parallel to the flow direction
1.5 m and wavelengths ranging from 150 m to 500 m (Fig. 14.7). These ribbons are only a metre or so thick
(Fig. 14.7). The crests are straight to moderately but are up to 200 m wide and stretch for over 10 km
sinuous and the lee slope is a lower angle than in the flow direction. Areas of low sand supply are
most subaqueous bedforms at around 158 (Johnson characterised by isolated ribbons whereas higher
& Baldwin 1996). Migration of sandwaves in the sediment supply results in ribbons amalgamated
direction of the predominant tidal current generates into sand ridges. Very strong tidal currents (over
cross-stratification with sets that may be many metres 100 cm s 1 ) can sweep sand off the sea floor leaving
thick (Fig. 14.8). Cross-stratification on this scale is only patches of gravel and metre-deep furrows eroded
not generally seen in other marine environments and into the sea bed.