Page 233 - Sedimentology and Stratigraphy

220 Shallow Sandy Seas

Storm-dominated shelf 14.3 TIDE-DOMINATED CLASTIC
SHALLOW SEAS
Scale Lithology MUD SAND GRAVEL Structures etc Notes
clay silt vf m vc gran pebb cobb boul 14.3.1 Deposition on tide-dominated
f
c
shelves
Foreshore: stratified
sands
Offshore sand ridges
Near shorelines that experience strong tidal currents
large sand ridges (14.2.1) are found on modern
shelves. The ridges form parallel to the shoreline in
water depths of up to 50 m and may be tens of metres
Shoreface: wave
rippled and swaley high, in places rising almost to sea level (Fig. 14.7).
cross-stratified
(SCS) sands They are typically a few kilometres wide, similar dis-
10s metres tances apart and extend for tens of kilometres as
straight or gently curving features, elongated parallel
to the tidal current. Between the sandy ridges there
may be thin layers of gravel on the sea floor, deposited
during an earlier, probably shallower phase of sedi-
mentation on the shelf, and left behind as a lag as
sand has been winnowed away by the currents (Plint
1988; Hart & Plint 1995). The sands are moderately
well sorted, medium grained but the deposits may
Offshore transition: include some mud occurring as clay laminae depos-
hummocky cross-
stratified (HCS) ited during slack phases of the tidal flow. Internal
sands interbedded
with bioturbated mud sedimentary structures are cross-lamination and
cross-bedding generated by the migration of ripples
and subaqueous dune bedforms over the surface of
the ridges. The resulting sandstone body preserved in
the stratigraphic record is likely to have a basal lag
and consist of stacks of cross-bedded and cross-
laminated sandstone up to 10 m thick, or more: the
primary sedimentary structures may be wholly or
partly destroyed by bioturbation.

Tidal sandwaves and sand ribbons
Currents generated by tides influence large areas of
shelves and epicontinental seas. These tidal currents
affect the sea bed tens of metres below sea level and
are strong enough to move large quantities of sand
in shallow marine environments. The effects of waves
Offshore:
bioturbated mud and storms are largely removed by tidal currents
reworking the material in macrotidal regimes and
only the tidal signature is left in the stratigraphic
record. In seas with moderate tidal effects the influence
of tides is seen in shallower water, but storm deposits
Fig. 14.5 A schematic graphic sedimentary log of a are preserved in the offshore transition zone in these
storm-dominated succession. mixed storm/tidal shelf settings.