Page 230 - Sedimentology and Stratigraphy

Storm-dominated Shallow Clastic Seas 217

14.1.3 Shallow marine clastic environments structures. Not all sandy deposits occurring on mod-
ern shelves have been formed by processes occurring
The patterns and characteristics of deposition on in the present day: the sea-level rise in the past 10 kyr,
shelves and epicontinental seas with abundant terrige- the Holocene transgression, has drowned former
nous clastic supply are controlled by the relative impor- strand plain and barrier island ridges, along with
tance of wave, storm and tidal processes. The largest sands deposited in the shoreface, leaving them as
tidal ranges tend to be in epicontinental seas and inactive relics in deeper water.
restricted parts of shelves, although in some situations
the tidal ranges in narrow or restricted seaways
can be very small (11.2.2). Open shelf areas facing 14.2 STORM-DOMINATED SHALLOW
oceans are typically regions with a microtidal to CLASTIC SEAS
mesotidal regime and are affected by ocean storms.
Two main types, storm-dominated shelves and 14.2.1 Facies distribution across
tide-dominated shelves, can be recognised in both a storm-dominated shelf
modern environments and ancient facies: these are
end-members of a continuum and many modern Shoreface
and ancient shelves and epicontinental seas show
influence of both major processes (Johnson & Baldwin The shallower parts of the shelf and epicontinental sea
1996). The majority of modern shelves are storm- are within the depth zone for wave action (11.1) and
dominated (80%): the remainder are mainly tide- any sediment will be extensively reworked by wave
dominated (17%), with just a small number (3%) of processes. Sands deposited in these settings may pre-
shelves influenced mainly by ocean currents (Johnson serve wave-ripple cross-lamination and horizontal
& Baldwin 1996). These ocean-current-dominated stratification. Streaks of mud in flaser beds (4.8) depos-
shelves are generally narrow (less than 10 km) and ited during intervals of lower wave energy become
lie adjacent to strong geostrophic currents (11.4): more common in the deposits of slightly deeper
sandwaves and sand ribbons form on them, and as water further offshore (Fig. 14.1). Wave ripples are
such they are similar to tidal shelves, but the driving less common as the fair-weather wave base is
current is not of tidal origin. approached in the lower part of the shoreface. Within
The detailed characteristics of sands deposited on the shoreface zone sand ridges may be formed by
modern shelves can be determined directly only by flows generated by eddy currents related to storms
taking shallow cores that provide a limited amount and/or wave-driven longshore drift (Stubblefield
of information: indirect investigation by geophysical et al. 1984). These ridges occur in water depths of
techniques, such as shallow seismic profiles (22.2), 5 to 15 m and are oriented obliquely to the coastline
can also yield some information about the internal as oblique longshore bars. They are up to about 10 m

Fig. 14.1 Characteristics of a storm-dominated shelf environment.