Page 235 - Sedimentology and Stratigraphy

222 Shallow Sandy Seas

14.3.2 Characteristics of tide-dominated
shallow-marine successions

Packages of cross-stratified sandstone that contain
a fully marine fauna and lack evidence for any sub-
aerial exposure are normally interpreted as the depos-
its of tidally dominated shallow seas (Fig. 14.10). In
water depths of tens of metres tides are the only
currents that can generate and maintain the large
subaqueous dune or sandwave bedforms: geostrophic
currents are generally too weak and storm-driven
currents are too short-lived and infrequent to create
these bedforms. Features of tidal sedimentation
(11.2.4) that may be present in these offshore tidal
Fig. 14.8 Large-scale cross-stratification formed by facies include mud drapes on some of the smaller scale
the migration of sandwaves in a tidally influenced shelf cross-bedding and reactivation surfaces within the
environment. sandwave cross-stratification (Allen 1982). There
may be evidence of different directions of tidal cur-
rents from within a unit of tidally deposited sand-
stones, but herringbone cross-stratification is
uncommon. Tidal currents on a shelf tend to follow
regular patterns (rotary tides: 11.2.3) that do not
undergo the direct reversals seen in estuarine and
coastal tidal settings. Erosion surfaces overlain by
gravel or shelly lags are found, representing higher
energy parts of the shelf or sea, but the distinct chan-
nels found in estuarine deposits are not seen. The
packages of cross-bedded sandstone are typically
tens of metres thick, sometimes amalgamated into
even larger units, and are lens-shaped on a scale of
kilometres.

14.4 RESPONSES TO CHANGE
IN SEA LEVEL

The processes of waves, storms and tides on a shelf are
related to the water depth and hence the character-
istics of shelf sediments are largely controlled by rela-
tive positions of the sea floor and the sea level.
Fig. 14.9 Bioturbated, cross-bedded sandstones deposited Consequently, any change in the relative sea level is
on a tidally influenced shelf. likely to have an effect on the sedimentation on a
shallow shelf area. For example, an increase in rela-
The offshore transition and offshore zones of tive sea level of 20 m in a nearshore area will result in
shelves and epicontinental seas are too deep for the a change from wave-influenced shoreface deposition
effects of the surface tidal currents to be felt and are to storm-influenced offshore-transition sedimentation.
sites for mud deposition and sands deposited by storm Conversely, a fall in relative sea level in the offshore
currents. Mud is also deposited in shallower areas that transition area may have the opposite effect, resulting
are not affected by tidal currents. Bioturbation is in shallower water over that part of the shelf that
common in these fine-grained deposits (Fig. 14.9). would now become part of the shoreface zone. The